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5 Paclitaxel (Taxol®) chemistry and structure— Activity relationships
The Chemistry and Pharmacology of Taxol and its Derivatives V. Farina, editor 9 1995 Elsevier Science B.V. All rights reserved
165
5 PACLITAXEL (TAXOL| CHEMISTRY AND STRUCTURE ACTIVITY RELATIONSHIPS
-
Shu-Hui Chen Bristol-Myers Squibb Pharmaceutical Research Institute, P.O.Box 5100, Wallingford CT 06492-7660, U.S.A.
Vittorio Farina Boehringer Ingelheim Pharmaceuticals, 900 Ridgebury Road, Ridgefield, CT 06877, U.S.A.
5.1. I N T R O D U C T I O N
Studies on the chemistry of paclitaxel I (1.1.1, Figure 1 ) a n d its derivatives began more than 30 years ago, when this compound was isolated from the bark of T a x u s b r e v i f o l i a by Wani and Wall [1]. In these early years, in spite of the a n t i t u m o r activity displayed by the substance, interest in its chemistry was modest, as was the supply of the drug. The field, therefore, advanced very slowly, mostly through the efforts of Kingston [2]. The lowyielding process that afforded paclitaxel from the bark of the yew tree appeared to be a limitation to the development of the compound as a therapeutic agent. Later, the isolation of 10-deacetylbaccatin III, 1.1.2, in r a t h e r high yields from 1Taxol| is a registered trademark of the Bristol-Myers Squibb corporation. The generic name "paclitaxer' is used throughout this chapter.
166 the leaves of Taxus baccata [3] opened the doors to the first semisynthesis of paclitaxel[4] and a more potent analog, Taxotere @ (docetaxel) [5] (1.1.3, Figure 1) utilizing a renewable source and thereby alleviating the supply problem substantially. As both paclitaxel and Taxotere | advanced through the clinical studies, the potential of these agents became obvious and generated considerable excitement [6]. Both paclitaxel and 10-deacetylbaccatin III, through the efforts of the Bristol-Myers Squibb company, the co-developer of the drug, became available in large amounts both to in-house chemists and outside collaborators for structure-activity relationship studies. These developments in the U.S., and also the continued efforts of the French groups at Gif and Rhone-Poulenc, led to a flurry of discoveries in the chemistry of these complex diterpenoids [7]. Paclitaxel has been approved by the Food and Drug Administration (FDA) for the t r e a t m e n t of refractory advanced ovarian cancer and breast cancer in 1992 and 1994, respectively. Due to these exciting developments, paclitaxel research has continued unabated. Years of efforts from many synthetic groups [8] finally culminated into two independent total syntheses of paclitaxel [9,10]. The syntheses are quite long and are unlikely to contribute much to the SAR in this area, at least in the near future, but represent an exciting academic achievement. O R~ II NH
O
18 R'O~,
O OH 1/19
....7~-i.~,~, 7 1,~ -,,16
o~3\
OH
7 ......6A..~ ,o
HO
6
O
'~O 1.1.1 Taxo] R = Ph, R ' = Ac 1.1.3 Taxotere R = t-Boc, R ' = H
Me
O
....
HO
:
t~ OAc
o
Bz() 1.1.2 1.1.4
R = H 10-deacety] baccatin II] R = COCH 3 baccatin I I I
Figure 1: Some medicinally important taxanes and the paclitaxe] | numbering system
At the same time, the search for n a t u r a l taxanes possessing core modifications compatible with bioactivity has begun to produce encouraging
167 results, most notably the discovery by Klein that 9-dihydrotaxanes, prepared by semisynthesis, have good biological activity [11], as well as recent reports on the activity of the 14-OH taxanes [12,13]. Research on the chemistry of paclitaxel continues for obvious reasons: the poor water solubility of paclitaxel, its rather poor in vivo potency, and its tendency to rapidly induce multi-drug resistance make it a sub-optimal drug [14]. It is hoped t h a t some of the new analogs may display improved properties in one or more of the above areas. For the pharmaceutical houses, due to the anticipated large sales of paclitaxel, the i n t e r e s t in devising unique, p r o p r i e t a r y and p a t e n t a b l e analogs of paclitaxel is obviously very strong. In addition, its high cost of production is enticing medicinal chemists to try to u n d e r s t a n d which portions of the molecule are actually interacting with its biological target and which have only minor functions or serve as scaffolding for the pharmacophore. As SAR studies become more complete, the first efforts to design a totally synthetic paclitaxel surrogate are appearing [15]. It is conceivable that more efforts in rational analog design will be reported in the near future. This chapter reviews all the relevant chemistry and structure-activity relationship studies up to the end of October 1994. Due to the existence of previous reviews on the subject [7], although comprehensive, this review will especially stress most recent key developments in the chemistry of taxanes and their implications for the SAR. We focus only on the chemistry of the core. Modifications of the C-13 side chain, which is crucial to the activity, form the topic of Chapter 6. 5.2. CHEMISTRY
5.2.1. Reactions at C-1/C-2 Functionalization of the tertiary alcohol function at C-1 without affecting the rest of the core is no trivial matter. Accordingly, few derivatives with C-1 substitution are known. Chen et al. initially reported a C-1 xanthate derivative [16a]. Subsequent reports by Klein et al. on derivatives of 9-dihydrotaxol [17] demonstrated the tendency of baccatin to undergo a C-2->C-1 benzoyl shift under basic conditions, leading, after t r e a t m e n t with carbon disulfide and methyl iodide, to 1-benzoyl-2-xanthyl derivatives. A re-examination of the earlier work [16b] verified that this is indeed the case in the baccatin series also (Scheme 1). Thus, 2.1.3 represents the first example of a C-1 acylated baccatin
168 III derivative. C-2 deoxygenation led to the deoxy analog 2.1.4. Similar results have been reported by the Kingston group as well [16c]. C-1 substitution does not imperil C-13 side chain introduction for biological evaluation. As outlined in Scheme 2, xanthate 2.1.3 was desilylated and then selectively resilylated at C-7 to give 2.1.7./~cylation according to the method of Holton [18], employing ~-lactam 2.1.10 as the side chain source, gave the desired C-1 functionalized paclitaxel derivative 2.1.9, after a final desilylation [19]. In search of a suitable C-1 protecting group, Chen and co-workers were led to utilize the novel dimethylsilane (DMS) group. Apparently, introduction of the bulkier trimethylsilyl group was difficult. As illustrated in Scheme 1, DMS was successfully introduced to the C-1 position of baccatin derivative 2.1.2 to give 2.1.5 in almost quantitative yield. Selective removal of DMS from C1 was accomplished with tetrabutylammonium fluoride at 0 ~ [20].
.o ....,,co 2o. TESO. . . . . . . . . . HO BzO
HO ~c
2.1.1 R=H -~ i 2.1.2 R=TES J
MeS 2.1.3
l iii AcO TESO....
O OTES
AcO
~0
TESO. . . . . . . . . .
20TES
Me2HSiO BzO 2.1.5
2.1.4
Conditions: (i) 5 equiv TESC1, imidazole, DMF, rt, 87%; (ii) Nail, THF+CS2, 70 ~ then MeI, 56%; (iii) 2 equiv Bu3SnH, AIBN, PhMe, 100 ~ 92%; (iv) 3 equiv Me2HSiC1, imidazole, DMF, 0 ~ 97%; (v) Bu4NF, THF, 0 ~ 84% of 2.1.2. [TES=triethylsilyl]
169 The significance of DMS protection of the C-1 hydroxyl group in the selective deacetylation at C-4 will be discussed in section 5.2.2. C-1-Benzoyl-2-deoxybaccatin 2.1.4 was converted to the corresponding C7 silylated analog 2.1.12 via a desilylation/mono-silylation sequence. The paclitaxel side chain was then attached onto 2.1.12 using Holton's protocol, to give 1-benzoyl-2-debenzoyloxytaxol, 2.1.14, after standard deprotection (Scheme 3) [19]. [ S c h e m e 21
AcO ~., RO ....\ \ BzO
Bz,
O ,~ OR'
P h ~ O
iii
=
AcO
MeS2CO 2.1.3 R=R'=TES ~ i 2.1.6 R=R'=H 2.1.7 R=H, R'=TES ~ i i
....
OR
O TESO,",,
-
O OR
NH O
Ph ],-" N. oJBz
O
BzO =MeS2CO
OAc
"
2.1.8 R=TES ) i 2.1.9 R=H
2.1.10
Conditions: (i) Py, 48%HF, CH3CN, 5 ~ 2.1.3 to 2.1.6, 76%; 2.1.8 to 2.1.9, 84%; (ii) TESC1, imidazole, DMF, 0 ~ 86%; (iii) LiHMDS, THF, -45 ~ then 2.1.10, 86%. [ Scheme 3 /
AcO RO ....
O OR'
Bz" NH
AcO ~'
O
-i i i -~ p h ~ . O
O
,,,, BzO
2.1.4 R=R'=TES ~ i 2.1.11 R=R'=H "h ii 2.1.12 R=H, R'=TES
2.1.13 R=TES "h Jr i 2.1.14 R=H
Conditions: (i) Py, 48%HF, CH3CN, 5 ~ 2.1.4 to 2.1.11, 79%; 2.1.13 to 2.1.14, 87%; (ii) TESC1, imidazole, DMF, 91%; (iii) LiHMDS, THF, 0 ~ then 2.1.10, 83%.
170 Under acidic conditions, a skeletal rearrangement of the A ring occurs, perhaps initiated by carbonium ion formation at C-1. A representative example is shown in Scheme 4. Kingston prepared ring-contracted paclitaxel analog 2.1.16 by reacting derivative 2.1.15 with methanesulfonyl chloride, followed by desilylation [21]. Later, similar rearrangements were observed by Potier [22] and Chen [23] under a variety of acidic conditions. In order to assess the contribution of the C-2 benzoate moiety to binding, a selective procedure for debenzoylation at C-2 was needed. Kingston has described a number of protocols for deacylation reactions of baccatin III. Under basic conditions, C-10 is deacylated first and, in certain cases, even the C-4 acetate is more labile than the C-2 benzoate [24]. Special conditions are therefore needed for selective C-2 deacylation.
AcO ph/~~_
0
Bz.. NH 0
Bz" NH 0 0 ....
phiaL-
i "
2.1.15
oH
O,
III
-<
o 2.1.16
Conditions: (i) MsC1, NEt3, -15 ~ to 0 ~ 20%; (ii) HF, Py, THF, 0 ~ to rt, 55%.
The first succesful attempt to selectively cleave the C-2 ester in a polyacylated baccatin derivative was reported by Farina et al., and made use of tin alkoxides or oxides. The reaction was postulated to occur by prior pentacoordination of the tin reagent to the C-1 hydroxy group, leading to rapid C-2 debenzoylation. Unfortunately, this was followed by backside attack of the C-2 alkoxide onto C-20, leading to oxetane opening, with formation of novel tetrahydrofuran derivative 2.1.18 in good yield (Scheme 5) [25]. The selectivity for the C-2 ester over the other four acetate groups is remarkable. Especially diagnostic here, in the 1H NMR spectrum, was the geminal coupling constant of the C-20 hydrogens, which increased from 8.3 Hz in 2.1.17 to 11.6 Hz in 2.1.18, and suggested a ring-expansion reaction.
171
Scheme 5~ AcO
AcO,,
O
AcO
OAc i
ii
~
AcO,
Sl I l l
Iii
o
OBz 2.1.17
O
.o, ~ .
O 2.1.18
r,,,oOc.
Conditions: (i) Bu3SnOMe, LiC1, NMP, rt, 67%.
Similar t e t r a h y d r o f u r a n - c o n t a i n i n g baccatin derivatives were later described by other r e s e a r c h groups, which used alkaline or reducing conditions to cleave the C-2 ester [11, 22, 24]. For example, the Gif group found t h a t reduction of 7-triethylsilyl-10-deacetyl baccatin 2.1.19 with lithium aluminum hydride at room temperature for a short period of time gave a 3:1 mixture of debenzoylated product 2.1.20 and the rearranged baccatin 2.1.21 in 80% overall yield (Scheme 6) [22].
.o
....
~
HO"
.o
....
o 2.1.20
2.1.19
HO .... H O 2.1.21 Conditions: (i) LiAIH4, THF, rt; 2.1.20 (60%), 2.1.21 (20%).
172 Selective C-2 debenzoylation in the presence of C-4 and C-10 acetates was finally achieved under a variety of conditions (Scheme 7).
AcO
O
AcO
OTES iorii
T E S O .... _
.
0
O
OTES
= T E S O ....
or iii
"
HO
2.1.2 Conditions: (i) NaOMe, MeOH, rt, 25%; (ii) Red-A1, THF, 0 ~
H
O
OH
2.1.22
87%; (iii) KOBu-t, THF, rt,
69%. Chen et al. reported that, whereas methoxide-promoted debenzoylation of 2.1.2 gave low yields of 2.1.22 [26], use of Red-A1, which presumably also precoordinates at C-l, led to the desired product in high yield [27]. Potassium tbutoxide was later reported by Datta et al. to perform similar function [28]. More recently, Kingston reported a simple procedure to debenzoylate paclitaxel derivatives. Thus, 2.1.15 was treated with NaOH under phase-transfer conditions to afford 2.1.23 in fair yields. Reacylation with a variety of substituted benzoates afforded a number of C-2 modified paclitaxel analogs for SAR studies (Scheme 8) [29]. Simultaneous, high-yielding C-2/C-10 deacylation in a 13-keto baccatin derivative could be carried out with catalytic bicarbonate in methanol/water [30]. The selective C-2 debenzoylation in baccatin derivatives has allowed chemists to prepare a number of paclitaxel analogs modified at C2. The simplest modification at C-2 is actually obtained by hydrogenation under forcing conditions, affording C-2 cyclohexanoate derivatives [24, 31]. In addition to the already mentioned approach directly from paclitaxel [29], baccatin derivatives modified at C-2 have been prepared by DCC-mediated coupling; introduction of the side chain afforded novel paclitaxel C-2 esters (Scheme 9) and carbamates (Scheme 10) [27]. As shown in Scheme 10, the carbamate linkage in 2.1.27 was prepared by reacting 2.1.22 with p-nitrophenyl isocyanate and pyridine in benzene at 70 ~ Standard desilylation and selective C-7 resilylation, followed by side chain acylation gave the desired
173 carbamate 2.1.29 in low yield, due to the unavoidable competitive formation of the C-1/C-2 cyclic carbonate 2.1.30 [27].
BZ'NH 2.1.15
O
p h ~ / [ L_"
AcO
20TES
0 ....
OTES
-
HO
OH
2.1.23
ii, i i i
Bz.. Ph
AcO
O
NH O . OH
0 ....
HO 2.1.24
OH
O,
R.~--[~ 0
Conditions" (i) 2 N NaOH, Bu4NHSO4, C6H6, 43%; (ii) DCC, DMAP, ArCOOH, PhMe, 50 ~ (iii) 5% HC1, MeOH, rt.
The formation of carbonate 2.1.30 underscores the instability of the C-2 carbamate linkage toward nucleophilic attack by the neighboring C-1 hydroxyl group. Similar cyclic derivatives have been reported in other publications also. For example, treatment of 2.1.22 with phosgene afforded 2.1.31 in high yield. Other attemps to protect the diol function as ketal or acetal led to complex mixtures of products, in which 2.1.32 (Scheme 11) predominated. A C2 benzyl ether could not be prepared either [19]. Another derivative that was easily prepared was the cyclic thiocarbonate 2.1.33, shown in Scheme 11 [16a]. One of the strategies for generating the C-2 benzoate in the total synthesis of paclitaxel is to utilize these C-1/C-2 cyclic carbonates. Treatment with phenyllithium in tetrahydrofuran at -78~ desired C-2 esters.
yielded regioselectively the
174
AcO
.O OTES 2.1.10
TESO ....
2.1.22
_
,,,
~Ac
0~==:0
2.1.25
R Bz,
AcO NH
0
O OH
\
....<
~~o
R a) p-MeOC6H4b) p-NO 2C6H4c) c-C6H lid) CH3~-
OH HO
-
'
O~=:=0
2.1.26
R Conditions: (i) 4 equiv DCC, xs DMAP, RCOOH, PhMe; (ii) Holton's protocol [18].
AcO
O
.h2
2.1.22
.o'~_ ~;No
2.1.27 2.1.28
R1 =R2 =TES ii R1 =TES, R2 =H J
~----~ ~ Bz. iii
TESO,,,,
AcO NH
0
0 OH
\
,,,Ph
o~'. 2.1.10
....< R10
Bz 2.1.29 2.1.30
!o OR 2
R1 =H, IRe =CONHC6H4-pNO2 R1,Ra =-C(O)-
Conditions: (i)p-NO2C6H4N=C=O, py, C6H6, 70 ~ 86%; (ii) Py, 48%HF, CH3CN, 5 ~ then TESC1, imidazole, DMF, 0 ~ 62%; (iii) LiHMDS, THF, -40 ~ 2.1.10, then (ii), 2.1.29 (15%), 2.1.30 (50%).
175 As illustrated in Scheme 12, Nicolaou et al. demonstrated t h a t treating carbonate 2.1.34 with five equiv of PhLi gave 2.1.35 in good yield [30]. Similarly, Holton showed t h a t t r e a t m e n t of carbonate 2.1.36 with 2.1 equiv of PhLi afforded the corresponding C-2 benzoyl derivative 2.1.37 in excellent yield [9]. These derivatives were subsequently transformed into paclitaxel. Complete defunctionalization of C-2 can also be achieved through the intermediacy of 2.1.22. As shown in Scheme 13, treatment of 2.1.22 w i t h sodium hydride in THF/carbon disulfide afforded xanthate 2.1.38, usually accompanied by a small amount of cyclic thiocarbonate 2.1.33. Barton deoxygenation smoothly afforded the desired C-2 deoxy derivative 2.1.39, which was acylated as usual to afford the paclitaxel analog 2.1.41. Attempted Barton deoxygenation on 2.1.33 and related analogs, on the other hand, brought about a number of interesting skeletal rearrangements, which are described fully in section 5.2.9. AcO
O OTES
T E S O .... :
.
O
OH
2.1.22
AcO
AcO
20TES
iii
T E S O ....
O OTES
T E S O .... O
O
OTES
S
2.1.33
2.1.31 T E S O .... _
HO
0
,,
OH
c
2.1.32 Conditions: (i) COC12,py, CH2C12, 0 ~ 87%; (ii) Nail, CS2+THF, MeI, then 1 equiv Nail, 83%;
(iii) acidic or basic conditions.
176
AcO
O OTES
HQ
OBOM
TBSO"' O -
",Ac
O
0 I ii
2.1.34 I
i
AcO
O OTES
2.1.36
HO%
OBOM
TBSO .... HO
2.1.37
\
2.1.35
Taxol 1.1.1
Conditions: (i) 4 equiv PhLi, THF, -78 ~
80%; (ii) 2.1 equiv PhLi, THF, -78 ~
Ac..~,(3 i
2.1.22
OBz
O OTES .
= TESO ....
85%.
,.....
L,._ ~u
.
.
.
1/ h
/ v
R 2.1.38
R=OCS2Me "~ ii t
2.1.39 R=H
HO .... AcO.
~O OTES
BZ'NH
O
ph~~['-O_
AcO~,
,60 OH
....
oH
HO
HO 2.1.40
OAc
2.1.41
Conditions" (i) Nail, THF+CS2(5:I), then MeI, 2.1.38 (61%), plus 2.1.33 (21%); (ii) Bu3SnH, AIBN, PhMe, 100 ~ 89%; (iii) Bu4NF, THF, rt, 85% (iv) TESC1, imidazole, DMF, 0 ~ 87%; (v) LiHMDS, THF, -40 ~ then 2.1.10, then Bu4NF, THF, rt, 63% overall.
177 5.2.2. Reactions at C-4: Deacetylation and Reacylation Early work on C-4 deacetylation was reported by Kingston [24] and Potier [22]. As outlined in Scheme 14, t r e a t m e n t of 1.1.4 under forcing hydrogenation conditions gave 2.2.1 in good yield. Methanolysis gave several products, including 2.2.2 and 2.2.3. Finally, extensive hydrolysis gave completely deacylated product 2.1.20 in fair yield.
AcO
~O OH
AcO i, ii
HO.... NO
Bz
=
,~O OTES iii
HO....
o
HO
O
1.1.4
O
~=2.2.1
O IIII
.o
HJ~"
....
HOA~~cO O
O
"
O
HO% ,~O OTES
He/. . HO
Hb
~
2.1.20
Conditions:
(i) Pt/H2, 90%; (ii) TESC1, Py, 80%; (iii) 0.5 N NaOMe, MeOH, rt, 69%.
Thus, it seems that deacetylation of the more hindered C-4 acetate is faster than deacylation at C-2, which is somewhat surprising. Kingston has
178 postulated that this occurs because of an initial base-catalyzed intramolecular acetyl transfer from C-4 to C-13, followed by rapid deacetylation. In agreement with this, if the C-13 hydroxyl is protected, the order of reactivity is reversed. In any case, the difference in reactivity is not satisfactory under these basic conditions, and completely selective C-4 deacetylation cannot be carried out [24]. Similarly, Potier's group reported attempted selective deacylation of C-4 under a number of basic and reductive condition, but in every case mixtures of products were obtained [22]. A solution to this problem was formulated by Datta et al., who treated 7triethylsilyl baccatin III 2.2.4 with potassium t-butoxide, yielding the desired 4deacetyl derivative 2.2.5 in fair yield (Scheme 15). This was explained once again by assuming acetyl transfer from C-4 to the free hydroxyl at C-13. The use of the bulkier base presumably retards direct deacylation at C-10 and C-2, which was highly competitive with methoxide [28].
AcO
O OTES
AcO i
HO ....
/
[
,," HO .... O
~O
BzO
BzO
2.2.4
2.2.5
Conditions: t-BuOK, THF,-20~
O OTES
58%.
Kingston et al. applied this chemistry to the preparation of 4-deacetyl-10acetyltaxotere, 2.2.8 (Scheme 16) and its related paclitaxel analog [32]. The starting material, 2.2.6, was prepared by the above t-butoxide-promoted hydrolysis of 2.2.1 which, in this case (temperature is not specified by the authors) apparently led to both C-4 and C-10 deacylation, contrary by the report of Datta and coworkers [28]. Coupling with synthon 2.2.7 was followed by C-10 reacylation and final deprotection.
179
h/h/t/
H OIIII
Boc~NZO
~
H
2.2.7
2.2.6
Boc,, NH O
\
AcO
O
OH
OH
HO BzO 2.2.8 Conditions: (i) DCC, DMAP, PhMe, 90%; (ii) Ac20, DCC, 4-(pyrrolidino)pyridine, THF, 65%; (iii) HCOOH, rt, 46%.
A more general solution to C-4 deacylation, which does not depend on the presence of a free hydroxyl group at C-13, was reported by Chen et a/.[20]. Since Red-A1 reduction of baccatin III seems to take place at the C-2 ester because of coordination with the hydroxyl group at C-l, it was felt that blocking this functionality would redirect the r e a g e n t elsewhere in the molecule, and hopefully to C-4, perhaps by prior coordination with the oxetane oxygen, the most basic oxygen in the molecule. A suitable C-1 blocking group was therefore developed. The a l r e a d y m e n t i o n e d dimethylsilyl (DMS) was found to be optimal. Both its introduction and its removal could be carried out under mild conditions. As illustrated in Scheme 17, reduction of 2.1.5 with excess Red-A1 afforded the desired C-4 deacylated baccatin 2.2.9, together with a smaller a m o u n t of the C-4/C-10 di-deacylated baccatin 2.2.10. The undesired baccatin 2.2.10 was easily reacetylated in situ to 2.2.9 by the method of Greene [4]. A more efficient approach to C-4 deacylated derivatives utilized C-13 trimethylsilyl
derivative
2.2.11 as the s u b s t r a t e (Scheme 18). Baccatin
derivative 2.2.12, bearing three different types of silyl groups, was treated with Red-A1 and then quenched with a s a t u r a t e d solution of t a r t r a t e , giving the
180 desired C-4/C-13 diol 2.2.13 in acceptable yield [20]. Loss of the TMS group occurred during the acidic work-up, as planned. Derivative 2.2.13 is obviously an ideal substrate for side chain attachment and the preparation of C-4 modified taxanes.
T
SO ....
Me2HSiOJ B ~
H OA~~cO
2.1.5
AcO
OTES
TESO ....
!0
Me2HSiO
+
~
O OTES,
TESO .... ~
Me2HSi
BzO 2.2.9
Conditions:
HO
BzO
2.2.10
(i) Red-A1,THF, 0 ~ (3:1) 2.2.9/2.2.10 (84%); (ii) Py, AcC1, 5 ~ 84%.
I Scheme18 /
AcO TMSO,,,
n
O
us se
~o,7~. ~ ~d~o 2.2.11 R=H "h i 2.2.12 R=DMS
AcO ii
O
OTES
HO .... Me2HSiO BzO
(
o
2.2.13
Conditions: (i) Me2HSiC1, imidazole, DMF, 0 ~ 92%; (ii) Red-A1, THF, 0 ~ then Na,K tartrate, 50-60%.
181 Preparation of 4-deacetyltaxol can be carried out quite efficiently beginning with paclitaxel, and using the selective 2,4-deacylation reaction reported by Kingston et al. A s shown in Scheme 19, 2.2.16 is obtained in high overall yield by phase-transfer promoted deacylation followed by rebenzoylation [32]. Synthesis of a 4-deacylated taxane for biological evaluation was also reported by the Gif group, following similar chemistry [33]. As we have seen, several solutions exist for selective C-4 deacylation and, not surprisingly, a number of workers have recently focused on the reacylation (or other derivatization) of C-4 for SAR studies at this position. This operation turned out to be non-trivial. During the course of an extensive deacylation/acylation study on baccatin derivatives [24], Kingston found that selective benzoylation of 2.2.17 led to derivative 2.2.18. The hindered C-4 hydroxyl group in 2.2.18 could not be acetylated even under forcing conditions and only 2.2.19 was obtained.
BZ'NH O ph~ L ~ . . , O .... -
~sss 9
~
HO"
iii
Ace,,.
BZ'NH 0 ph~~"O. OH
i ph/~,~ -
o
2.2.14
ii
AcO
BZ'NH O 0 ....
O
ss ss ~
HO'# 2.2.15
OH
OH
....
2.2.16 Conditions: (i) Benzyltrimethylammonium methoxide, CH2C12; (ii) PhCO2H, DCC, DMAP,
PhMe; (iii) 5% HC1, MeOH (42% overall). However, direct acetylation of 2.2.17 under standard conditions gave tetraacetyl derivative 2.2.20 in modest yield, where the C-4 carbinol was successfully acetylated (Scheme 20). Given these observations, Kingston speculated that the
182 inaccessibility of C-4 hydroxyl group in 2.2.18 towards acetylation is due to the steric hindrance imposed by the bulky C-2 benzoate moiety. In any case, the yields reported are too low to draw any definite conclusions. Chen et al. reported a general solution for derivatization of the C-4 alcohol moiety, and this has led to the preparation of several C-4 modified paclitaxel analogs [19, 20, 34]. As shown in Scheme 21, the fully protected baccatin derivative 2.2.9 (obtained as in Scheme 17) has only the C-4 carbinol open for derivatization. Treatment under highly basic conditions with a variety of acyl chlorides afforded the C-4 analogs in high yields. These were all converted to the respective paclitaxel derivatives via the usual Holton procedure.
Aco..... o" ,~. o~ ~'ii
2.2.17
AcO
.... . o "
o
i i~,,~
AcO . . . . . .
2.2.18
AcO
O OTES
O
,,
.o"-~~o~o~~ 2.2.20
AcO .... "
HO
H
OBz
2.2.19
Conditions: (i) DCC, DMAP, PhCOOH, 14%; (ii) Ac20, CH2C12, DCC, rt, 24%.
THF, DCC, 55 ~
89%; (iii) Ac20,
183 The authors stress that the success of this protocol depends highly on the following two facts: (i) protection of the C-1 hydroxyl group as a dimethylsilyl ether (DMS) allows clean C-4 deprotonation without any complication; (ii) the choice of a strong base, such as LiHMDS or BuLi (weaker bases are ineffective), is crucial.
AcO O ~ F - ~ .'~ . ~ ~ "// OTES t
AcO O , ~ , ~, P ~// " OTES
i
so ....
z x__#~
/
-
~
ii
so,,,
,%
~
OBz O , ~ R 2.2.9
AcO ~, HO ....
O
//
2.2.21
OR'
Bz. 2.1.!0
,.... O OBz O , ~ R
=
O O
AcO NH
-
O
II
= P h ~ O
....
OH
H
'~'
o" BzO
O,~R
0 2.2.22 2.2.23
2.2.24
R'=H
R'=TES
O
iii
R "a, CH3; b, CC]3; c, CH2CH3; d, CH2CH2CH3; e, CH(CH3)2; f, (CH2)3CH3; g, (CH2)4CH3; h, Ph; i: p-NO2-C6H4;
Conditions: (i) LiHMDS, THF, 0 ~
then acyl chloride, 70-85%; (ii) Py, 48% HF, CH3CN, 5 ~
(iii) TESC1, imidazole, DMF, 0 ~
Similar strategies led to the successful preparation of C-4 carbonates (Scheme 22). Once again, the corresponding paclitaxel analogs were prepared without complications by Holton acylation of 2.2.27a,d.
184
AcO ~, i
2.2.9
TESO ....
AcO
O // OTES HO
,,
DMSd ~
H d WOBz
2.2.25
AcO
isssss
i i i i
~ ..-.~ O OBz OCO2R
Bz.
O
O OH
OCO2R
2.2.26
R'=H
2.2.27
R'=TES
~ iii
2.1.10 y
Ph
: OH
R: a, Me;b, Et; c, n-Pr; d,p-NO2-C6H4
O ....\
HO
2.2.28
&Bz OCO2R
(i) LiHMDS, THF, ROCOC1; (ii) Py, 48%HF, MeCN, 5 ~ (iii) TESC1, imidazole,
Conditions:
DMF, 0 ~
AcO
O OTES
TESO ....
i
DMSO
OBz O
2.2.25d
AcO~,
AcO
O~
DMSd ~ ~ N 0 2
NH O P h ~ O OR
I"11-~,,,.0 OBz OCONHR
AcO
O OH
i ....
HO Bz(3
OCONHR
2.2.32
R" a, n-Pr; b, cyclopropyl; c, cyclobutyl Conditions:
ii
2.2.29
Bz. 2.1.10
OBz OCONHR 2.2.30 R'=H iii 2.2.31 R'=TES
._~ TESO .... ~ , ,% . /, / ~ OTES ~ "
O
,~O OR'
HO ....
O
(i) RNH2, THF, rt; (ii) Py, 48% HF, MeCN, rt; (iii) TESC1, imidazole, DMF, 0 ~
185 To prepare C-4 carbamates (Scheme 23), these authors resorted to a stepwise strategy. Reactive carbonate 2.2.25d was the key intermediate. Its reaction with amines led to displacement of the p-nitrophenol moiety and produced a number of carbamate derivatives. Conversion to the paclitaxel analogs by standard Holton acylation was complicated by side reactions (vide infra), and afforded only modest yields of the desired 2.2.32. Scheme 24 shows the synthesis of the C-4 aziridine carbamate. It was found more practical here, instead of using aziridine as a nucleophile, to construct the 3-membered ring stepwise.
AcO
2.2.25d
NO ....
0 iii
,,,,
HdW
.-%1o
OBz OC020BHn-p-N02 2.2.33 R=H ~ ii 2.2.34 R=TES
AcO
0
AcO
HO ....
iv
tt o
Hd_= H.-%10 OBz 0 H 2.2.35
"~ N 0
HO ....
OBz 0
BZ'NH~v ~ 0 Ph
_ OH
,,,,
2.2.36
OH AcO
2.1.10
0
N~
0~
0
~-r 9
OH
0 .... HO
2.2.37
: OBz
0 O~
N~
Conditions" (i) Py, 48% HF, MeCN, rt; (ii) TESC1, imidazole, DMF, 0 ~ (iii) NH2(CH2)2OH, THF, rt; (iv) PPh3, DEAD, THF, rt, 50%.
186 In the event, p-nitrophenyl carbonate 2.2.33 was silylated at C-7, then quantitatively converted to 2.2.35 by t r e a t m e n t with ethanolamine. Standard Mitsunobu conditions gave aziridine 2.2.36. In this case, the paclitaxel side chain was introduced onto 2.2.36 in high yield to give the desired C-4 aziridine carbamate 2.2.37, after standard desilylation. A clue as to the problems associated with the Holton acylation of carbamates 2.2.31 was provided when acylation with modified lactam 2.2.40 was attempted under standard conditions. Surprisingly, acylation at the C-4 c a r b a m a t e nitrogen was observed as the major pathway, as outlined in Scheme 25. In two cases, the unwanted 2.2.39 predominated, while the desired C-4 carbamate analogs 2.2.38 were only minor products. This problem is then due to the acidic nature of the carbamate proton. The high-yielding acylation of the aziridine analog 2.2.36, which bears no such acidic proton, suggests that protection of the C-4 carbamates to remove the N-H function should result in a generally high-yielding C-13 acylation.
AcO ~,
B~ ..NH 0 i, ii 2.2.31a,b
AcO
O ~ OH
" -~0,
~-
III
2.2.~a,b
OH
HO
OBz 0 ~/--- NHR
HO..../
Hd ~OBz~O 2.2.39a,b
0
TESO,%
R ,,..[ ~ / Boc-N H
Conditions: (i) LiHMDS, THF, 2.2.40; (ii) Py, 48%HF, MeCN, rt.
,,,,,
oJ-' N. Boc 2.2.40
187 Recently, Georg and co-workers have also reported the synthesis of a C-4 modified analog of paclitaxel (2.2.43, Scheme 26). The approach relies on the intermediacy of cyclic carbonate 2.2.41, which is used as a protecting group for the C-1/C-2 diol system while performing C-4 acylation with a large excess of acylating agent. Although no further examples are reported, the procedure may have general utility [35].
BZ'NH Ph
O
"
_
AcO ~
O -//- OTES
0 ....
~ H
2.2.14
Ph
.
o,
,, - p.A_Ao:_
_
iii
~.-O Bz.
NH -
AcO
O OH
~
Ph/~~__ O.... OH
2.2.43
OH
O~H
O
0
OH
....
OTBS 2.2.41
BZ'NH
HO
LO OTES ~
O
_O OTES
0 ....
2.2.15 AcO~
BZ'NH
AcO
BZ.NH
OBz
0
2.2.42
I
=
?=o
20TES
II
Ph/~flL'" O.......... OTBS _
~0 HO
AcO
O
i v, v
/
Conditions: (i) t-BuOK, H20 (2 equiv), THF, -20 ~
75%; (ii) (Im)2CO (20 equiv), THF, 55 ~ C ; (iii) (iBuCO)20 (20 equiv), DMAP (20 equiv), CH2C12, rt, 67% overall; (iv) PhLi (10 equiv), THF,-78 ~ (v) Py, HF, 0 ~ to rt, 48% overall.
Kingston and co-workers recently reported a study aimed at deoxygenation of the C-4 position [36]. Deacylation of suitably protected
188 paclitaxel and C-1/C-2 protection gave 2.2.41 as in Scheme 26. Conversion to the xanthate proceeded uneventfully, and Barton deoxygenation geve the desired 4deacetoxy derivative. Final deprotection turned out to be difficult because of the competitive acid-catalyzed oxetane ring opening, which was reported to be faster than in the 4-acetoxy-bearing analog, in spite of the lack of anchimeric assistance by the ester group (see section 5.2.3). Nevertheless, HF in pyridine gave the target 2.2.45 in fair yield (Scheme 27).
AcO ~'
Bz. NH 0 i
p h ~ . J [ , . O _ ....
2.2.41
OTBS
AcO
BZ-NH 0 Ph
-
0 ....
0 ~O
2.2.44 ii, i i i
00TES
(~CS2Me
O O
sss S
2.2.45
(i) Nail, CS2, MeI, 90%; (ii) Bu3SnH, AIBN, PhMe, 90 ~ 80%; (iii) PhLi, THF, -78 ~ 60%; then HF, py, rt, 66%. Conditions:
5.2.3. The chemistry of the oxetane moiety In a study dealing with the reaction of paclitaxel with electrophilic reagents [21], Kingston found t h a t reacting paclitaxel with Meerwein's reagent (triethyloxonium tetrafluoroborate) gave, in low yield, 2.3.1, a compound where the oxetane ring had been opened. The presence of the diol moiety was subsequently confirmed by formation of acetonide 2.3.2. The A ring contraction reaction, already briefly mentioned in section 5.2.1, apparently also took place, due to the acidic conditions employed. Kingston noted that reaction of paclitaxel with acetyl chloride directly yielded both A-ring contraction and oxetane opening, in addition to C-5 acetylation (Scheme 28). The oxetane-opening chemistry was independently investigated by Gu~ritte-Voegelein [37], using baccatins as substrates.
189
BZ'NH Paclitaxel 1.1.1
0
phil_
0 .... -
sts
OH
OH 2.3.1
BZ.NH
O
ph~~Jl"O
AcO~, ....
.
2.3.2
iii
OAc
L O OH
OH
Paclitaxel 1.1.1
OBz
~k
OBz OAc
\
Bz. NH
AcO ~
O
P h ~ O , ,
'"O
/
O ... // uAc
'
OH 2.3.3 -
'
\
OBz
O
"'OAc OcH
Conditions: (i) Et3OBF4, CH2C12, 35-51%; (ii) Me2C(OMe)2, p-TsOH, CH2C12, 95%; (iii) MeCOC1, then H20, 68%.
Reaction of baccatin derivative 2.3.4 with anhydrous zinc chloride in toluene was found to give 2.3.5, again with an opened oxetane ring and a contracted A ring. Treatment of 2.3.4 with aqueous acid, on the other hand, gave a mixture of three products, i.e. 2.3.6, featuring an intact oxetane ring, in addition to regioisomers 2.3.7 a n d 2.3.8 [22] (Scheme 29). T r e a t m e n t of paclitaxel and baccatin derivatives with Lewis acid has been discussed in a rather comprehensive study [23]. Use of strong acids, such as boron halides or TMSBr gave, in addition to A-ring contraction, oxetane opening with complementary regiochemistry (Scheme 30), but weaker Lewis acids smoothly led to oxetane opening w i t h o u t concomitant A-ring transformations (Scheme 31). Once again, two regiochemical modes are possible for the oxetane solvolysis, and the ratio of 2.3.12 to 2.3.13 depends on the Lewis acid used [23].
190
Scheme 29
_
HO
TrocO ,~
O //
TrocO
O.Troc
....
O
= HO ....
,,,
0 HO
: AcO OBz
2.3.4
~
' \
OBz
OAc
2.3.5
ii
_
TrocO ~i,
O //-
TrocO
_OTroc
HO ....
O
HO .... 0
HO'\
",
OBz
OBz
Hd \
2.3.6
2.3.7 2.3.8
Conditions: (i) ZnC12, PhMe, 80 ~
R2
OR 1
RI=AC , R2=H R I=H, R2=Ac
50%; (ii) 1 N HC1, AcOH, rt, 2.3.6 (20%), 2.3.7 (30%), 2.3.8
(17%).
BZ'NH Ph
O
AcO
2
OH
: O .... ()Cbz 2.3.9
HO
0
ii
" AcO OBz BZ.N H
O
AcO
ph-~.~_ 2.3.10
R 1 =Ac, R 2 =H
2.3.11 R 1 =H, R 2 =Ac
O
-
O .... '"O R2
OCbz
\
OBz
Conditions: TMSBr, CH2C12, rt, 2.3.10, 79%; (ii) BBr3, CH2C12, rt, 2.3.11, 75%.
OR1
191
Scheme 31 /
~,, II O.H
AcO
az.
2.3.9
, ewi.
NH
0
acia
0 OCbz
Lewis acid SnC14 TiCl4 BF3
2.3.12 (%) 17 57 43
2.3.13 (%) 69 15 43
0
....
. . . . . . OBz
,,OR 2 OR 1
2.3.12 R1 =Ac, R2 =H 2.3.13 R1 =H, Re =Ac
A variety of paclitaxel and baccatin substrates were submitted to Lewis acid treatment, and similar products were obtained. Some qualitative kinetic information could be gleaned from these studies: for example, paclitaxel itself reacts very slowly with tin tetrachloride. Once the C-2' hydroxyl group is blocked as a carbonate, however, high reactivity is restored. Also, 7-epi derivatives react very sluggishly in comparison with their 7(~) counterparts. From all these observations, a mechanism can be proposed for the oxetane opening reaction that requires the anchimeric participation of the C-4 acetate group, and is consistent with proposals by Kingston [21] and Chen [23] (Scheme 32). The reaction is initiated by complexation of the Lewis acid with the oxetane oxygen, which is probably the most basic atom in the molecule. The acetoxy group is positioned for participation by backside attack onto C-5, leading to acetoxonium ion 2.3.15. Indeed, when the C-4 acetoxy group is not as readily available for nucleophilic displacement (it is hydrogen bonded to the C7 epi-hydroxyl group, and it apparently interacts with the C-2' hydroxyl group in paclitaxel), the overall reaction is sluggish at 0 ~ and only proceeds at room temperature. Intermediate 2.3.15 can isomerize to acetoxonium ion 2.3.17, via strained orthoester 2.3.16. Alternatively, 2.3.15 can be hydrolytically quenched to yield the hemiorthoester 2.3.18, which can further unravel to afford 2.3.13, bearing a C-5 acetoxy group. Similar quench of 3.2.17 leads to 2.3.12, featuring a C-20 acetoxy group [23]. As a consequence, the regioselectivity of the reaction is a reflection of the ratio of the two acetoxonium ions, 2.3.15 a n d
192 2.3.17, and this ratio seems to be highly dependent on the Lewis acid used, either due to steric or electronic reasons.
OH
OH OSnCl4- H27
oH// I
OH ..
.[,~
Me
T|
2.3.15
~O H
2.3.13
2.3.12
)=0 2.3.14
2.3.18
Me
+_SnCi4_ Me
"0
0-~
It o. ~
o.l H20 OSnCI4-
Oto 2.3.17 Me
OH Oe~:
M
2.3.19
H
F u r t h e r evidence supporting the above mechanism came from the observation that, when 2.3.9 was treated with tin tetrachloride for 2 h at 0 ~ an unusual product was isolated in c a . 20% yield. It was assigned structure 2.3.20 (Scheme 33) aider extensive NMR characterization [23]. The formation of 2 . 3 . 2 0 can be explained by assuming t h a t the conformation of ring C in 2.3.17 can flip to a boat, in which the 7-OH group can attack the C-20 methylene carbon, to lock the C ring permanently in a boat-like conformation. Compound 2.3.21 was isolated in low yield when 7,13diacetylbaccatin III was solvolyzed under similar conditions. The product is the result of a 1,2 hydride suprafacial shift from C-5 to C-4 in an intermediate of the type 2.3.17, and leads to inversion at C-4, as shown by NMR data.
193
Bz" NH O
AcO
O
AcO
O
p h ' ~ ' J l " O _ .... OCbz
AcO ....
O //
OAc
.~, O
'"O HO
OBz
OBz
OAc
2.3.21
2.3.20
Attempts to trap acetoxonium ions 2.3.15 and 2.3.17 with nucleophilic reagents in the presence of Lewis acids proved fruitless, but stirring baccatin III in ice-cold trifluoroacetic acid in the presence of a large excess of phenyl dimethylsilane produced acetals 2.3.22 and 2.3.23 in fair overall yield, suggesting that the two acetoxonium ions were indeed both present, at least under these particular conditions [23] (Scheme 34). As shown in Scheme 35, in order to f u r t h e r confirm the diol functionality present in structures 2.3.12 and 2.3.13, both compounds were cleaved using lead tetraacetate in acetonitrile. While 2.3.13 gave the expected product 2.3.24 in high yield, 2.3.12 cleanly produced the unexpected 2.3.25, the result of C-20 to C-7 acetyl migration followed by exocyclic cleavage. Endocyclic cleavage was only a minor product [23].
AcQ
O
i Baccatin III, 1.1.4
OH,
+
=HO ....
N O IIII
..
"OH
"" 0 -
OBz
O.~H
2.3.22
Conditions:
CF3CO2H, PhMe2SiH, 0 ~ 2.3.22(42%), 2.3.23(9%).
HO BzO 2.3.23
\
H
H
194
IScheme 35 /
AcO Bz. NH O 2.3.13
~,
= ph.,.-'L,,~O. .... OCbz
BZ'NH O -
Ph
"
:
OBz
AC!~ 0'"
OCbz 2.3.25
'"OAc HO
2.3.24
2.3.12
O OH
0
20Ac
i
sis
OH
HO
"
OBz
O
Conditions: (i) Pb(OAc)4, MeCN, rt, 71-80%.
The chemistry of the novel 2.3.20, featuring a bridged C ring, was also studied. As shown in Scheme 36, t r e a t m e n t of 2 . 3 . 2 0 u n d e r s t a n d a r d acetylation conditions only led to a C-5 acetate derivative, 2.3.26, via a formally i n t r a m o l e c u l a r transesterification process. Oxidation of 2.3.20 gave the expected C-5 keto analog. When 2.3.20 was treated with DAST, an interesting rearrangement took place, leading to another bridged analog, 2.3.29, via a 1,2 alkyl migration through 2.3.28 [19]. 5.2.4. Reactions at C-7 Epimerization of the C-7 hydroxyl group to the 7((~) isomer 2.4.1, presumably via a retroaldol/aldol sequence, was first described by Kingston in his a t t e m p t s to promote radical reactions at t h a t position [38]. A more convenient way to effect epimerization at C-7 is to treat paclitaxel with base, as shown in Scheme 37 [19]. The 7((z) isomer is apparently more stable than paclitaxel. The most direct way to assess the importance of the binding of the C-7 hydroxyl group within its biological target is to replace it with a hydrogen
195 atom. Thus, it is not surprising that several groups have engaged in research aimed at such deoxygenation reaction.
BZ"NH O 2.3.20
phil'_
AcO
2
O
0 ....
OCbz
HO
-
2.3.26
OAc
O Bz
AcO
O
O
BZ"NH O phil_
0 , ~ ::SF2NEt2
0 ....
HO
2.3.27
2.3.28
t ss
, co 2 P h ~ O .
= OBz
o\
....
OCbz 2.3.29
HO
OBz
(i) Ac20, Py, CH2C12, 77%; (ii) Jones reagent, Me2CO, rt, 82%; (iii) DAST, CH2C12, 0 ~ 74%. Conditions:
Bz" NH 0 Taxol 1.1.1
AcO ~,
0 //
.OH
i, orii, o r i i i
p h / ~ ~ _ . 0 .... 8N
o OBz 2.4.1
Conditions:
rt, 65-80%.
(i) AIBN, PhMe, reflux; (ii) DBU (2.5 equiv), PhMe, reflux, 84%; (iii) Nail, THF,
196 Initial attempts involving Barton type reaction on C-7 derivatives such as thionobenzoates [39], selenocarbonates, oxalates [19] proved fruitless. After this quite extensive search, both Kingston et al. [40] and Chen et al. [41] found an identical solution to the problem of C-7 deoxygenation via xanthates. Chen and Farina prepared both 7-deoxytaxol and 7-deoxytaxotere via 7-deoxybaccatin, as shown in Scheme 38.
AcO
0
,.,.
9
AcO O -~_~.~.
OCS2Me
iii
R O ....
0
0 HO
: OBz
OAc
HO
1.1.4 AcO
OAc
2.4.2 R=H 2.4.3 R=TES O
AcO
R O ....
-~
/~
O HO
: OBz
" OBz
2.4.4 R=TES 2.4.5 R=H
ii O
,9-L,
Ph
-
OAc
O .... O
OH HO TESO,
) iv
%
,,"
o•1
Ph
OBz
OAc
2.4.7 R=Bz 2.4.8 R=Boc
N. R
2.1.10 R=Bz 2.4.6 R=Boc
Conditions: (i) Nail, THF+CS2, MeI, 57%; (ii) TESC1, imidazole, DMF, rt, 74%; (iii) Bu3SnH, AIBN, PhMe, 110 ~ 83%; (iv) TBAF, THF, rt, 74%; (v) LiHMDS, THF, -40 ~ then 2.1.10 or 2.4.6; then 1 N HC1, CH3CN,-5 ~ 53% of 2.4.7; 77% of 2.4.8.
Formation of the C-7 xanthate was complicated by some competitive C-7 epimerization. Deoxygenation proceeded smoothly to afford 2.4.4. Deprotection and attachment of the side chains gave 2.4.7 and 2.4.8 [41]. Kingston produced a 7-deoxy derivative directly from 2'-protected paclitaxel using the same chemistry (Scheme 39) [40].
197 Incorporation of fluorine atom into biologically active molecules has become an i m p o r t a n t facet of medicinal research. Consequently, C-7 fluorinated paclitaxel was an interesting synthetic target. T r e a t m e n t of 2'protected paclitaxel derivative 2.3.9 with two equivalents of DAST led to 7(cz) fluoro analog 2.4.11, together with side product 2.4.13, which features a 7,19cyclopropane ring. If a larger excess of DAST was used, the already discussed A ring contraction also took place. The structure of 2.4.11 seems secure on the basis of NMR studies, including appropriate values of JH,F. Also, the structure of 2.4.13 was confirmed by NMR studies and, after cleavage of the side chain, by a single crystal X-ray of the corresponding baccatin analog (Scheme 40) [42, 43].
,,,
gz..
AcO
S H
O
OH
_~I
~O' ~
SMe ii
ph: ~ ~ _ O, OTES
2.4.7 -
HO
OBz
OAc
o
iii
o
OBz
2.4.9
2.4.10
Conditions: (i) Nail, CS2, THF, then MeI, 60%; (ii) Bu3SnH, AIBN, PhMe, refl.; (iii) dil. HC1,
49%.
gz.
i
2.3.9
=
AcO~ ~0 F NH 0
ph-2- _
1111
OR Bz.
AcO
O
NH 0
_
HO
: OBz
0
OAc
2.4.11 R=Cbz 2.4.12 R=H
ii
0 2.4.13 R=Cbz 2.4.14 R=H
ii
O.... OR HO
-
AcO
OBz Conditions: (i) 2 equiv DAST, CH2C12, rt, 2.4.11 (55%), 2.4.13 (32%); (ii) H2, Pd/C, EtOAc, 2.4.12 (88%), 2.4.14 (90%).
198 Standard cleavage of the Cbz group gave the modified 2.4.12 and 2.4.14 for biological evaluation. A subsequent study examined solvent effects in the DAST fluorination reaction [44]. When the reaction was run in THF/ether instead of dichloromethane, 2.4.11 was the major product, accompanied by 10-12% of a new side product, the very interesting 6,7-dehydrotaxol analog 2.4.15 [44, 45] (Scheme 41). An analogous derivative was later prepared by Kelly and co-workers, using a C-7 triflate and effecting its base-promoted elimination in high yields [46].
AcO~, gz.
2.3.9
NH
,O
O
= phil'_
0 ....~
/
~
+ 2.4.11
OR
HO
: OAc OBz 2.4.15 R=Cbz ~ ii 2.4.16
R=H
Conditions: (i) 2 eq. DAST, THF/Et20, 2.4.15 (10%), 2.4.11 (45%); (ii) H2, Pd/C, EtOAc, 87%.
Note that saturation of the C-6/C-7 double bond was not possible even under forcing conditions [19]. A separate study deals with the fluorination of 7epi-taxol derivatives [47]. As shown in Scheme 42, with this substrate the DAST reaction produces only cyclopropane derivatives, no fluorination being detectable in this case. Further treatment of 2.4.13 with DAST gave A ringcontracted product 2.4.18. The lack of fluorinated products in this case suggests that attack of fluoride at C-7 from the ~ face is too hindered. Instead, an unusual participation of the trans-diaxially positioned angular methyl group ensues, leading to protonated cyclopropane 2.4.20 (Scheme 43). Similar intermediates probably occur in 1,2 Meerwein rearrangements. The deactivating effect of the C-9 carbonyl was postulated to prevent the completion of the 1,2 shii~, therefore leading to an isolable cyclopropane derivative. When the substrate was the 7(~)OH group, direct methyl participation is electronically unfavorable, and in this case (Scheme 44) the reaction proceeds probably through a carbonium ion, a
199 common i n t e r m e d i a t e t h a t easily explains the formation, in addition to 2.4.13, of the fluorinated compound 2.4.11 and the olefinic analog 2.4.15.
AcO BZ'NH Ph
O :
O OH ---~
~ O ....
=
OCbz
O HO
OAc OBz
2.4.17
AcO BZ'NH
0
\
Ph 'o. --
O
)
BZ'NH
/~%.
Ill
phil'_
I
--
oc z
AcO ~'
O
O /J
Ollll
ocbz
~ " " A: H OU c OBz
2.4.13
~
o
ii
2.4.18
Conditions: (i) 2 equiv DAST, CH2C12, rt, 2.4.13 (28%), 2.4.18 (31%),.; (ii) 4 equiv DAST,
CH2C12, rt, 2.4.18 (81%).
Bz. NH
AcO
O
phil__
0
~
.... 0 DAST=
O ....
OCbz
O
HO 2.4.17
BZ'NH
O
AcO ~
"
OBz
OAc
O
ph/~/[L'O,,, OCbz ' 2.4.13
H 2.4.19 X=OSF2NEt2
-X-I H H H+
0
0 = HO
: OAc OBz 2.4.20
H
200
O 2.3.9
X
DAS~
O O
2.4.21
-X ._-
+
H X=OSF2NEt2
$ methyl participation H H
~+ F
O
H
2.4.22
_H§
O 2.4.20
2.4.11
_H+
2.4.15
2.4.13
Formation of the 7(~)-fluoro derivative seems to be prevented in each case by the steric hindrance to approach of fluoride from the top face of the molecule. Some recent results by Klein have indirectly confirmed this mechanism: as shown in Scheme 45, when the C-9 carbonyl is absent in the substrate, and the C-7 hydroxyl group is activated as a transient triflate, a 1,2 methyl shift is observed, leading to skeletal r e a r r a n g e m e n t and eventually to B-ringcontracted product 2.4.25. Interestingly, a small amount of cyclopropanecontaining product 2.4.26 was found here also. The authors suggest that the peculiar conformation of the C ring may be at the origin of this very unusual cyclopropanation reaction [48]. Oxidation of taxanes with various agents was studied by Kingston [49]. Treatment of paclitaxel with chromic acid yielded first the C-7 keto derivative 2.4.27. Reacting this ketone with DBU in CH2C12 at room t e m p e r a t u r e or simply c h r o m a t o g r a p h y on S i 0 2 caused oxetane ring opening via ~elimination, leading to 2.4.28. Saturation of the 5,6-double bond and subsequent reaction with warm methanol led to lactone 2.4.29 (Scheme 46). Others have found that t r e a t m e n t of 2.4.27 with DBU leads not to 2.4.28, but to isomeric
201 enone 2.4.30, where the strong base has catalyzed 1,2 acetyl shift from C-4 to C20 [19, 33].
AcO ~,
-
OH
AcO
OH
fOH -I
AcO ....
i
= AcO ....
J, 0
0 HO
OBz 2.4.23
OAc 2.4.24
OAc
AcO .... ~ - ~ ~ , , , ,
~
AcO
Me
~]~
+
AcO ....
'~
OH
"" 0
HO
: OBz 2.4.25
OAc
HO
OBz 2.4.26
OAc
Conditions: Tf20, CH2C12, Py, rt; 2.4.25 (56%); 2.4.26 (8%).
Esterification at C-7 is a rather straightforward operation. Kingston reported that paclitaxel is rapidly acetylated at C-2', but C-7 acetylation requires more forcing conditions, involving DMAP and DCC. Selective 2'deacetylation under mildly basic conditions afforded C-7 acetyltaxol [50]. Baccatin III yields 7-acetyl baccatin under standard acetylating conditions (Ac20, pyridine), the C-13 position being somewhat more difficult to acetylate. A C-7 carbonate derivative of baccatin was described in the same paper [51]. Using similar methods, other workers have reported the synthesis of more complex esters, including water-solubilizing moieties, to be used as possible paclitaxel prodrugs [52-54]. Synthesis of sulfonate esters has also been reported under standard conditions [45, 55]. The same paper also reports the preparation of C-7 carbonates. While simple unhindered chloroformates react smoothly at C-7, more substituted ones react only with difficulty, if at all [19]. The same study [45] also describes the stepwise preparation of C-7 carbamates, which turned out to be rather challenging, since typical one-step procedures failed due to the
202 hindered nature of this secondary hydroxyl group. Two methods for the preparation of carbamates are shown in Scheme 47.
AcO i Paclitaxel 1.1.1
0 //
BZ'NH Ph
0
0
~
: OH
/j
0
0 ....
" 0 OBz
2.4.27
0
AcO BZ'NH
/,0
0
iii Ph
OH
-
0 ....
9
OH 0
2.4.?,8 2.4.29
AcO
O O
Bz" NH O
Ph~O OH 2.4.30
.... HO
Ac
Conditions: (i) Jones reag., acetone, 50%; (ii) Si02, 70%; (iii) Pt/C, MeOH, H2, 77%.
In the first case, the C-7 chloroformate is formed in s i t u and immediately quenched with the required amine. In the second case, pnitrophenyl carbonate 2.4.33 is readily prepared and isolated. Treatment with amines then yielded the desired derivatives [45, 55]. Silylation at C-7 (usually the sturdy triethylsilyl group is used) represents the preferred protection procedure for the C-7 hydroxyl. Standard fluoride deprotection regenerates the hydroxyl group [21]. Silylation of the 7(a) epimer of paclitaxel is difficult because of intramolecular hydrogen bonding between the 7-OH group and the C-4 acetate moiety, but it has been recently achieved by operating in highly polar solvents [45].
203
AcO ~,
Bz" NH O
O // OH hth/tt
p h ~ / [ l ' - _ O .... OCbz
O OBz
2.3.9
AcO ~
Bz" NH O
O // .OCONHBu
p h / ' ~ ~ _ O.... OH
O OBz
2.4.31
AcO
O
Bz" NH O
OH iv
ph/~~_ O .... OAIIoc
O HO
OAc OBz
2.4.32
AcO
O
O 10/NO2 O.~O
Bz. NH O p h ' / ~ ~ _ O.... OAIIoc AcO
BZ'NH P h i l _ : O O''' ,-
O HO
v, v i
O
~
//
OH
2.4.33
_OCONHR 0
-
OBz
" OAc OBz
2.4.34 2.4.35
R=(CH2)3CO2H R=(CH2)2NMe 2
Conditions: (i) COC12, Py; (ii) n-BuNH2, 72% overall; (iii) H2, PcYC, EtOAc, 90%; (iv) pNO2C6H4-OCOC1, Py, CH3CN, 78%; (v) H2N-(CH2)3-CO2All,THF, rt, 93%; or H2N-(CH2)2NMe2, THF, rt, 91%; (vi) Pd2dba3, CH2C12,PPh3, triethanolamine, rt, 68% for 2.4.34; 65% for 2.4.35 [Alloc= allyloxycarbonyl].
204 5.2.5.Reactions at C-9/C- 10 The ketone function at C-9 in paclitaxel is exceedingly resistant to many reagents that traditionally attack the carbonyl group, and consequently it has escaped t r a n s f o r m a t i o n until recently, when Commerqon reported its reduction by electrochemical means [56]. As s h o w n in S c h e m e 48, w h e n T a x o t e r e | was reduced eletrochemically, 9(a)-dihydrotaxotere 2.5.1 and 9([~)-dihydrotaxotere 2.5.2 were produced with very little stereoselectivity. Treating 7-epi-taxotere 2.5.3 under identical conditions led only to the 9(~)-dihydro derivative 2.5.4. On the other hand, when the electrolytic reduction was carried out at -1.95 to -1.90 V in the presence of CaC12, which is presumed to alter the electron density at the C-9/C-10 hydroxyketone moiety by forming a tight complex, C-10 deoxygenation was achieved in modest yield (vide infra).
HQ RI R2 OH B~ Taxotere 1.1.2
0
phil_
0 .... OH
0
HO
: OAc OBz
2.5.1 R 1 =OH, R2 =H 2.5.2 R1 =H, R2 =OH
H B~ 7-epi-Taxotere 2.5.3
OHoH
H O
ii
....
6H 2.5.4
o HO
OBz
OAc
Conditions" (i) E= -1.85V (SCE Hg cathode), MeOH, NH3, NH4C1; 2.5.1 (40%) and 2.5.2 (24%); (ii) same as i, 63%.
Klein et al. reported the synthesis of 9(a)-dihydrotaxol from the naturally occurring 13-acetyl-9(cx)-dihydrobaccatin III [11, 57]. As depicted in Scheme 49, the synthesis began with the protection of the C-7/C-9 diol moiety of 2.5.5
as an acetonide. The crucial C-13 deacetylation was achieved
205 chemoselectively in acceptable yield by the use of n-butyllithium under carefully optimized conditions. The paclitaxel side chain was then readily attached onto 2.5.7 according to Holton's protocol, affording 2.5.8 in fair yield. Final removal of the dimethylketal protecting group from 2.5.8 then furnished the desired 9(r dihydrotaxol 2.5.10 [11] in modest overall yield. The Abbott group has also reported the acylation of 2.5.7 with a large variety of side chains, bearing modified C-3' substituents, from which useful and extensive SAR information has been obtained (vide infra) [58].
Me O-A< 'Me i
AcO
OH
AcO
OH
-
0 i
AcO . . . . . . . . .
= RO .... O
HO
O
OAc OBz
2.5.5
HO
OAc OBz
2.5.6 R=Ac~ 2.5.7 R=H J i i
Aco
Me
Bz"~ NH 0 Ph
:
~
=iii'iv
,,,Ph
o/~N-Bz
0 .... 0
OH
2.5.8
EEO,%
HO
2.5.9
: OAc OBz AcO ~,
Bz. NH
OH OH
O
p h / ~ ' v ~ O _ .... OH 2.5.10
s HO
0 OBz OAc :
Conditions: (i) Me2C(OMe)2, CSA, 97%; (ii) n-BuLi, THF, -44~ 46%; (iii) n-BuLi, then 2.5.9; (iv) 0.5% HC1 in EtOH, 67%; (v) CSA, MeOH, 56% [EE=Ethoxyethyl].
206 Datta et al. recently reported the first example of enolization of the C-9 keto group in a fully functionalized taxane [59]. As shown in Scheme 50, t r e a t m e n t of 2.5.11 with potassium t-butoxide led to carbonate 2.5.12 in fair yield. This was t h e n converted into the corresponding paclitaxel and Taxotere | derivatives for biological evaluation. Many efforts have been devoted to the complete deletion of functionality at C-10 in order to examine its effect on biological activity. The first synthesis of 10-deoxytaxol was unexpectedly achieved by Chen et al. during their attemps to fluorinate such position. The synthesis began with 10-deacetyltaxol 2.5.15, which was obtained by Lewis acid-promoted methanolysis of paclitaxel [23] (Scheme 51). Treatment with trichloroethyl chloroformate then gave the 2',7diprotected derivative 2.5.16. Subjection of 2.5.16 to Yarovenko's reagent (C1FHCCF2NEt2) in CH2C12 at room temperature surprisingly yielded dienone 2.5.17, together with a small amount of a C-12 fluorinated enone [60].
os
TrocO O ~L /7 OTroc
\
i HO ....
e/SSso
O
HO ....
OBz
/
OTroc
9....
O OBz
2.5.12
2.5.11
/h//h/v,v
Ph~,,CO2H B o c - N ~ ,~. O 2.2.7
RH~____/~L
OH
Ph- ~' v0 .... OH
0
HO
: OAc OBz
2.5.13 R= PhCO 2.5.14 R= t-BuOCO
Conditions: (i)t-BuOK, THF, -30 ~ to 0 ~ (58%); (ii) 2.2.7, DCC, DMAP, PhMe, 70 ~ 62%; (iii) HCO2H, rt (71%); (iv) PhCOC1, NaHC03 or (Boc)20, NaHCO3 (62-72%); (v) Zn, AcOH, MeOH, 60 ~ 65% for 2.5.13, 69% for 2.5.14.
207 This reaction is remarkable since Yarovenko's reagent is a fluorinating reagent, and dehydration products are rarely obtained. Removal of the protecting group from 2.5.17 was readily accomplished with zinc in a mixture of acetic acid and methanol, to afford 2.5.18 in high yield. Finally, 2.5.18 was found to undergo smooth catalytic hydrogenation to afford 10-deoxytaxol 2.5.19. Owing to its exciting biological activity, 10-deoxytaxol has been the subject of several investigations. For example, Chen at al. reported that defunctionalization at C-10 can be obtained by radical methods using 10thionocarbonates [16a]. As shown in Scheme 52, 10-deacetyl baccatin is an appropriate s t a r t i n g material. When C-7 is protected, thionocarbonate formation proceeds at C-10 vs. C-13 with excellent selectivity. Barton deoxygenation affords 2.5.22, and acylation according to Holton then affords 10deoxytaxol in good overall yield [19]. An analogous C-10 deoxygenation, utilizing a xanthate, was described by Kingston [61].
HO gz..
NH
O
OR
0
phil_
ii
O ....
OR HO 2.5.15 R=H 2.5.16 R=Troc) i
l
O " OBz
OAc
0
BZ'NH
0
phil__
0 ....
OR
0 BZ'NH 0 phi_: 0 .... OH 2.5.19
OBz 2.5.17 R=Troc,~ 2.5.18 R=H 2 i ii
OH
0 HO
iv
-OAc OBz
Conditions: (i) TrocC1, Py, CH2C12, 0 ~ 46%; (ii) Et2NCF2CHFC1, CH2C12, 47%; (iii) Zn, MeOH, AcOH, 40 ~ 81%; (iv) H2, PcYC,EtOAc, 68%.
208 Efforts have been made at deleting the C-10 acetoxy group directly from paclitaxel. Since the C-10 acetoxy group is a doubly activated moiety (i.e. allylic and cz-keto) its removal may be achieved in principle by a direct Barton deoxygenation reaction. Indeed, Chen et al. reported t h a t t r e a t m e n t of 7-epitaxol 2.4.1 with 6-8 equivalents of tributyltin hydride and AIBN in toluene at 100~ afforded directly the corresponding 10-deoxy derivative 2.5.23 in excellent yield (Scheme 53) [62]. When paclitaxel was treated under the same conditions, however, only 2.5.23 was obtained in 39% yield, together with some 7-epi-taxol and unreacted starting material, thus suggesting that the function at C-7 plays a role in this radical deoxygenation [62].
H
,~O OTES
R
O
OTES
i
HO ....
=
HO ....
O
O HO
OAc OBz 2.5.20 R=OC(S)OC6F5 ii 2.5.21 R=H ,,/ 0 OR
OBz 2.1.19
iii
BZ-NH ph N - : ~ _ .
0 ....
TESO~I'Ph OR O Bz 2.1.10
0
HO 2.5.22 R=TES 2.5.19
R=H
OBz J]
OAc
iv
Conditions: (i) n-BuLi, THF, -40 ~ then C6F5C(S)C1, -20 ~ 74%; (ii) Bu3SnH, AIBN, PhMe, 90 ~ 99%; (iii) n-BuLi, THF, -40 ~ then 2.1.10, 0 ~ (iv) dil. HC1, CH3CN, 0 ~ 76% overall from 2.5.21 to 2.5.19.
The first one-step synthesis of 10-deoxytaxol directly from paclitaxel was reported by Holton using SmI2 as the reducing agent [63]. Similar chemistry was also reported by other authors [64, 65]. Electrochemical conditions were
209 also applied successfully to the reduction of C-10 acetoxy moiety, as mentioned above [56]. Scheme 53 / gz.
7-epi-taxol 2.4.1
z~ NH
O_H
0
ph/~~_
0 ,, O
OH HO
: OBz
OAc
2.5.23 Conditions: (i) Bu3SnH, PhMe, AIBN, 100 ~
88%.
When the SmI2 deoxygenation was conducted with an excess of the reagent and for prolonged periods of time, 10-deoxygenation and C-9 reduction were reportedly achieved simultaneously. Taxotere | gave instead a mixture of two products, since in this case the C-10 hydroxy is not as good a leaving group as the acetoxy and direct carbonyl reduction can compete with (~-reduction. Once the C-9 carbonyl is reduced, C-10 deoxygenation cannot obviously occur, and 2.5.25 is produced, along with the expected 2.5.26 (Scheme 54) [64]. Using 9-dihydrobaccatin III as a starting material, Klein and coworkers reported, in preliminary form, the selectively deoxygenation at C-9 using t r a d i t i o n a l Barton chemistry, as well as C-7 and C-9 double deoxygenation. Their chemistry is highlighted by the elegant synthesis of 7,9,10-trideoxytaxol, 2.5.31, the analog with the most defunctionalized northern half prepared to date (Scheme 55) [17]. Details of this chemistry have not yet appeared. Selective C-10 deacetylation in paclitaxel is not a facile operation. Kingston et al. reported that t r e a t m e n t of paclitaxel with zinc bromide in methanol yielded 10-deacetyltaxol in low yield together with its C-7 epimer [21]. Chen et al. reported a C-10 deacetylation study in which several other Lewis acids were examined, without substantial improvements [23]. However, with the ready availability of 10-deacetyl baccatin III as a convenient source of bioactive taxanes, this synthetic operation is no longer synthetically important. Although acylation at C-10 is well precedented [4, 52] the first general approach to C-10 modified taxanes was reported only recently by Kant [66].
210 Ethers, esters, carbonates, carbamates, and sulfonates were all prepared in good yields under mild conditions (Scheme 56).
BZ'NH 0
\
OHoH
mh Oo....
Paclitaxel 1.1.1
!o HO
OBz
2.5.24
Boc..
.o,, 2"o.
NH 0
phil"_
O,
OH
o
2.5.25
i
Taxotere, 1.1.2
.
B~
0
phil_
+
OHoH
0 .... oH
HO 2.5.26
Conditions:
-
OBz
(i) SmI2, 83% for 2.5.24; 40% for 2.5.25; 50% for 2.5.26.
As outlined in Scheme 56, 2.1.19 was treated with 1.05 equiv of n-BuLi at -40 ~ in THF, followed by the addition of 1.2 equiv of the electrophile. Derivatives 2.5.32 were then directly acylated with the paclitaxel side chain (~-lactam method) for biological evaluation. 5.2.6. Reactions at C-13 Modification of the C-13 position is a critical operation that may profoundly affect the biological activity, due to the important role of the phenylisoserine side chain. The C-13 hydroxyl group of baccatin III is often silylated in order to protect it from functionalization during complex synthetic operations. Use of the TMS and
211 TES blocking groups has been exemplified widely t h r o u g h o u t the chapter. Acetylation in pyridine was described as requiring h a r s h conditions by several workers [51, 67], but more recently such acetylation could be carried out at room temperature in CH2C12 with acetic anhydride [23]. Carbonates have also been used at C-13 as protecting groups, especially the convenient Troc group [67]. Oxidation of 7-TES Baccatin III at C-13 using MnO2 affords the corresponding enone, which can be reduced back to the baccatin derivative with borane [68]. The various methods that have been discovered to introduce the phenylisoserine side chain at C-13 are described in Chapter 6. [ Scheme 55 SMe S AcO ~, AcO ....
:
OH
AcO ---~0 ~, --"
OH i _
sSess
ii
AcO ....
2.5.5
2.5.27 HO ~,
H O ....
OH
iii
9.....
HO .... ~
iv
HO
2.5.28
gz..
v
OH
2.5.29 NH
0
\
.... < Cbz-N~
0
2.5.30
HO BzO 2.5.31
Conditions: (i) LiHMDS, CS2, then MeI; (ii) Bu3SnH; (iii) MeLi, then CS2, MeI; (iv) Bu3SnH;
(v) 2.5.31, then H2, Pd/C, then (PhCO)20, aq. MeOH (no solvents, temp. nor yields).
212
Scheme 56 / HO
HO ....
0
//
RO
.OTES
r
HO....
0
/I
OTES
r
0
0
OBz
OBz
2.1.19
2.5.32 Electrophile
R
Yield(%)
AcC1 BzC1 n-BuCOC1
COCH3 COC6H5 COBu-n
c-C3H5COC1 MeOCOC1
COC3H5-c 78 CO2Me 75
M e2SO4 Me2NCOC1 PhNCO MeSO2C1
Me CONMe2 CONHPh SO2Me
90 85 75
85 72 78 68
Conditions: (i) n-BuLi, THF,-40 ~ then electrophile,-40 ~ to 0 ~
5.2.7. Reactions at C-14 Recently, Appendino et al. reported the isolation of 14([~)-hydroxy-10deacetylbaccatin 2.7.1 from the needles of T a x u s w a l l i c h i a n a Zucc [69]. Due to the presence of an additional hydroxyl group at the C-14 position, the new taxanes derived from 2.7.1 upon C-13 acylation can be expected to possess substantially improved water solubility vs. paclitaxel and docetaxel, and perhaps also better in vivo antitumor activity. With this in mind, two groups set out to prepare 14(~)hydroxytaxol and 14(~)-hydroxytaxotere, as well as a number of related analogues [12, 13, 70]. The relative reactivity of the four hydroxyl groups in 2.7.1 was independently studied by Kant and Ojima [12, 13]. It was found that the reactivity of these groups toward acylation decreases in the order C-7 > C-10 > C-14 > C-13. Therefore, the a t t a c h m e n t of the phenylisoserine side chain to the C-13 position requires appropriate protection at C-7, C-10 and C-14. Toward this end, 2.7.1 was converted
213 into the 7,10-diprotected derivative 2.7.2. C-1 and C-14 were t h e n protected as carbonate 2.7.3, orthoformate 2.7.4, or acetonide 2.7.5, as shown in Scheme 57.
,,," H
HO ~
0 /f
O_H
H ,,
TrocO ~,
0 //
OTroc
i
O,
0
HO
OH
: OBz
_
O~
OAc
OBz
TrocO
2.7.1
O
2.7.2
OTroc
HO ....
i i , or iii, or i v
0
O RO
OR'
: OAc OBz
2.7.3 R,R'=C(O) 2.7.4 R,R'=CH(OEt) 2.7.5
Conditions: (i) 4 equiv TrocC1, py, 80 ~ 55%; (ii) 2 equiv TrocC1, py, 80 ~ TsO)20, 92%; (iv) 2,2-Dimethoxypropane, (p-TsO)20, 89%.
HO
0
R,R'=C(Me) 2
75%; (iii) CH(OEt)3, (p-
OTES
i
ii
2.7.1
~
HO .... 0 HO
HO
O
OTES
HO ....
OH
O
y
EtO
O
OAc OBz 2.7.7
OAc
2.7.6 iii
O
" OBz
RO
20TES
= HO ....
or i v
0
Oyo EtO
OBz
OAc
2.7.8 R=Ac 2.7.9 R=TES
Conditions: (i) TESC1, imidazole, DMF, 92%; (ii) CH(OEt)3, PPTS, THF, 90%; (iii) LiHMDS, AcC1, THF, 0 ~ 75%; (iv) LiHMDS, TESC1, THF, 0 ~ 78%.
214 Alternatively, the C-7 hydroxyl group was protected as a triethylsilyl ether, affording 2 . 7 . 6 in high yield. Compound 2.7.6 was next converted to 1,14orthoformate 2.7.7. C-10 acetylation yielded 2.7.8, and silylation afforded 2.7.9 (Scheme 58). Using protected baccatin derivatives 2.7.4 and 2.7.9, 14(~)-hydroxy-taxotere 2.7.11 was readily obtained in good overall yield using the protocols illustrated in Scheme 59. In addition, cyclic carbonate analog 2.7.12 and acetonide 2.7.13 were also prepared in a similar fashion. Similarly, the coupling between 10-acetoxybaccatin derivative 2.7.8 with the appropriate ~-lactams 2.1.10 and 2.4.6 afforded the desired 14(~)-hydroxytaxol 2.7.14, and 10-acetyl-14(~)-hydroxytaxotere 2.7.15 for biological evaluation (Scheme 6O).
2.7.9
Kant i, i ~ iii
RO,,, 9
iII
Ph
2.7.4
N "Boc 2.4.6 R=TES 2.7.10 R=CH(Me)OEt
I
OJ--'
~,~ 0 II t-BuO ''~k" NH
HO O
I /
O
OH
\
OH 2.7.11
Ojima iv, v, vi
0 HO
O" B z
OH
OAc
O t-BuO"JJ~NH p h i - i l l _"
O
HO
O
OH
0 .... 0
RO
OR'
OBz
2.7.12 2.7.13
R,R'=CO R,R'=CMe 2
OAc
Conditions: (i) LiHMDS, THF, 0 ~ then 2.4.6, 75%; (ii) 10 N HC1, CH3CN, -5 ~ (iii) NH4OH, THF, 0 ~ 67% overall. (iv) NaHMDS, THF, -40 ~ then 2.7.10, then 0.5% HC1, EtOH, rt, 96%; (v) HCOOH, dioxane, rt, then THF, MeOH, NaHCO3, rt, 73%; (vi) THF, 0.5 N HC1, Zn, 0 ~ 73%.
215 I n t e r e s t i n g l y , as shown in Scheme 61, direct acylation of 2.7.2 u n d e r s t a n d a r d conditions yielded two novel C-14 side chain-bearing t a x a n e derivatives (2.7.16 and 2.7.17) in fair yield. This observation is in a g r e e m e n t with the finding t h a t the hydroxyl group at C-14 can be acylated more readily t h a n the one at C-13 [13]. Scheme 60 /
O
AcO
O
.
(i, i i ) or [i i i, i i) 2.7.8 TE SO,,,,
,,, Ph
oJ-' N. R
ph~/[l"O. OH
.... 0 HO
2.1.10 R=Bz 2.4.6 P~Boe
OH
2.7.14 R=Ph 2.7.15 R=t-OBu
: OBz
OAc
Conditions: (i) LiHMDS, THF, -40 ~ then 2.1.10, 65%; (ii) 10 N HC1, CH3CN, -5 ~ then NH4OH, THF, 0 ~ 55% for 2.7.14, 62% for 2.7.15; (iii) LiHMDS, THF, -40 ~ then 2.4.6, 75%.
,,"
HO
~, //O O.H
i, or ii, then iii 2.7.2
OH
I
~
N. R
2.7.18 R=Bz 2.7.10 R=Boc
RC(O)HN
O
O=z
2.7.16 R=Ph 2.7.17 R=t-OBu
Conditions: (i) NaHMDS, THF, -40 ~ then 2.7.18; (ii) NaHMDS, THF, -40 ~ then 2.1.10; (iii) Zn, AcOH, MeOH, 52% overall for 2.7.16; 50% overall for 2.7.17.
5.2.8. Skeletal r e a r r a n g e m e n t s This section will discuss baccatin,
some interesting skeletal r e a r r a n g e m e n t s
of
m o s t l y concerning the A and B rings. These r e a r r a n g e m e n t s are
usually initiated by radicals or carbonium ions. In addition, a few recent reactions t h a t are more properly classified as degradation reactions will also be discussed. A n u m b e r of radical-based deoxygenation reactions were carried out on baccatin III derivatives [16a].
In this connection, it was discovered t h a t formation of a
216 radical at the C-7 position of the baccatin core results in a complex skeletal rearrangement (Scheme 62). Tributyltin hydride-mediated deoxygenation of 2.8.1 gave 52% of the desired 7,10-dideoxy baccatin III 2.8.2 and 25% of its tetracyclic isomer 2.8.3 [16a]. Other radical conditions were examined, and the distribution of products characterized. When Ph3SnH was used as the reducing agent, in addition to 2.8.2 and its isomer 2.8.3 (ratio, c a . 3:1), methyl ether 2.8.4 was also isolated in 30% yield; when (TMS)3SiH was employed as the reducing agent, in addition to 2.8.3, two new products, enol acetate 2.8.5 and C-12 exomethylene derivative 2.8.6, were also obtained (Scheme 63). The formation of 2.8.3, 2.8.5 and 2.8.6 can be rationalized by invoking a cascade of radical rearrangements, as shown in Scheme 64.
O
Scheme 62 t O IIII
O
\
0
)CS2Me HO
HO ....(
+
0
HO
: OAc OBz O
: OAc OBz 2.8.1
0 Me
HO ....~
Conditions:
2.8.2
O HO
A c = OBz
2.8.3
(i) Bu3SnH, AIBN, PhMe, 80 ~ 2.8.2 (52%); 2.8.3 (25%).
The initially formed radical 2.8.7, a ~-keto radical, can isomerize, via alkoxy radical 2.8.8, to 2.8.9. This places the radical-bearing C-8 at a close distance with respect to the C-11/C-12 double bond, and a 5-exo cyclization to 2.8.10 takes place. Surprisingly, this radical is not quenched by the tin hydride, perhaps due to the hindered nature of the radical-bearing C-12. The major pathway for radical 2.8.10 is the remote intramolecular hydrogen abstraction of H-3, to provide 2.8.11. Radical 2.8.10 also suffers a disproportionation reaction to give the minor product 2.8.6 only when (TMS)3SiH was employed as the reducing agent. Radical 2.8.11 is evidently also sterically hindered toward direct reduction, and undergoes an unusual oxetane fragmentation reaction to give 2.8.12. The resulting a-alkoxy
217 radical is then trapped by tributyltin hydride to yield 2.8.3. When the reducing agent used is tributyltin deuteride, the product is specifically labeled only at the C-5 methoxy group [16a]. However, in the case of the (TMS)3SiH reduction, in addition to 2.8.3, alkoxymethyl radical fragmentation with loss of formaldehyde gives allylic radical 2.8.13, presumably due to slow trapping of 2.8.12 by the rather unreactive silane reagent. After this cascade of six sequential intramolecular reactions, radical 2.8.13 is finally quenched by the silane to give 2.8.5 (Scheme 64).
O i or i 2.8.1
"
i= H O ....
I +
O IIII
jO %
or i i i HO
OAc OBz
HO
2.8.2
OBz OAc 2.8.4
0
O
H,,,~~~L~ +
HO
OMe ....~
HO
HO":
+
c
I
: OBz 2.8.3
HO O
H
HO ....
~
= OBz 2.8.5
OAc
O HO
= OBz
2.8.6
Conditions: (i) Bu3SnH, AIBN, PhMe, 80 ~ (ii) Ph3SnH, AIBN, PhMe, 80 ~ (iii) (TMS)3SiH, AIBN, PhMe, 90 ~ The proposed mechanism for the C-7 methyl ether formation is shown in Scheme 65. Addition of the triphenyltin radical onto xanthate 2.8.1 leads to the unstable intermediate 2.8.14. Usually, this intermediate undergoes ~-scission to afford a carbon radical. However, in this case the highly reactive hydrogen donor triphenyltin hydride was able to trap 2.8.14 to form 2.8.15.
218
0
2.8.1
"H
~HO'"
= HO'"
0 Ho BzO
OAc
BzO
2.8.7 0 HO"
H OAc
2.8.8
H
0
~
ai
H Ola0a
-
H"
~
HO BzO
OAc
HO BzO
2.8.9
~
OBz
H OAc
2.8.10
0
H ,,~ , ? - - %.,~, ~ H17-HO ....(, ~ ~
2.8.6
0
)~
.w,,, ._ H','" - HO"'
H
OAc
OBz
2.8.11
.
CH20 ] T
HO .... ~
2.8.13
c 2.8.12
0
/N;,,"~
HO
+H" OCH-------~2.8.3
: OBz
/) "
+H"
~ 2.8.5
OAc
Elimination of triphenyltin thiomethoxide gave C-7 thionoformate 2.8.16. Further reduction by excess triphenyltin hydride gave thioacetal 2.8.18, which was finally converted to methyl ether 2.8.4 through another C-S bond cleavage reaction. The same study [16a] describes a series of complex radical rearrangements that arise via C-1 and C-2 carbon radicals. As shown in Scheme 66, treatment of cyclic thiocarbonate 2.8.19 with tributyltin hydride and AIBN failed to give either a C-1 or a C-2 deoxy
derivative. Instead, after t r e a t m e n t of the crude product
mixture with trifluoroacetic acid, two new products, 2.8.20 (major) and 2.8.21 (minor), were obtained. Inspection of the 1H-NMR spectra dearly showed that both
219 compounds were the results of skeletal rearrangements. Their structures were confirmed by X-ray crystallography.
SSnPh3 O O+ SMe + Ph3Sn"
+ H"
~ HO,,,
2.8.1
0
BzO
SSnPh3
S
2.8.14
O O~J~H
O O+SMe HO,,,'~
. Ph3SnSMe ~HO....
H O
HO BzO
O H
OAc O O -''~
2.8.15
+ Ph3Sn"
2.8.16
SSnPh3
~ ~ HO,,,
OBz
+ H" O HO BzO
OAc 2.8.17
~
. / ~ ~ ~ ~~SSnPh3
, ~
~"~
H O ....
+ Ph3SnH=__
0 \{/. H O .... .
'---f.-.4/__"H HO
BzO
2.8.18
OAc
O HO
" BzO
OAc
2.8.4
A mechanistic rationale for the observed products is presented in Scheme 67. The initial adduct resulting from addition of the tributyltin radical to 2.8.19 apparently leads to both of the two conceivable fragmentation products, radicals 2.8.22 and 2.8.23. It is likely that intermediate radical 2.8.22 is hindered by the presence of its neighboring tributyltin thiocarbonate residue at C-l, and consequently it is not rapidly trapped by tin hydride to yield the corresponding 2-deoxybaccatin derivative
220 as seen with C-2 xanthates. Instead, intramolecular processes, as already seen for the C-7 radical, take over and 2 . 8 . 2 2 undergoes a t h e r m o d y n a m i c a l l y unfavorable and quite unusual 4-exo cyclization to the cyclobutylcarbinyl radical 2.8.24. Since the cyclobutylcarbinyl radical is highly unstable, opening with concomitant ~-elimination leads to 2.8.26 and, after desilylation, 2.8.20. 00TES
[ Scheme 66 TESO,,,,~
0 -
"~0
S
0
HO ....
I
Ac 2.8.19
~ii 0
OH +
OH
HO .... 0
/ 2.8.20 Conditions:
6H oAo 2.8.2
(i) Bu3SnH, AIBN, PhMe, 100 ~ 77%; (ii) CF3CO2H, THF, H20; 2.8.20, 42%; 2.8.21,
8% overall. Radical formation at C-1 is also apparently taking place in a competitive fashion: radical 2.8.23 undergoes a cyclopropylcarbinyl r e a r r a n g e m e n t to yield, following hydrolytic deprotection, A ring-contracted product 2.8.21. In addition to radical-initiated ones, cationic r e a r r a n g e m e n t s are very common in taxane chemistry. We have already discussed the A-ring contraction reaction apparently initiated by C-1 carbonium ion formation (Scheme 4) [21]. Rearrangements initiated by C-7 carbonium ion formation lead to the already discussed cyclopropane derivatives (Scheme 42) [47, 48] and, in the case of 9dihydrotaxanes, to B-ring contracted analogs [48], as shown in Scheme 45. These B-ring contracted taxanes could be deprotected at C-13 and acylated under standard conditions for biological evaluation [48].
221 Whereas Lewis or Bronsted acid treatment of taxanes leads to A-ring contraction, as already discussed, recently Khuong-Huu et al. reported an alternative ring enlargement reaction that proceeds under acidic conditions. ES
TESO ....(X
/~'.._ .~. ~J~ 0,~6
Bu3Sn'/
/
S
OAc Bu3Sn"
2.8.19
i~ OTES TESO ....
T E S O , , , ~
o_ 9 OAc ~--SSnBu 3 O [ 2.8.22
Bu3SnS~/F6
~
~T-SSnBu3 O 2.8.24
O 1
2.8.23
TESO,,
OAc
TESO ....(k
I~'.. 4 . . L
.... -H/_ ~ ._-~o
Bu3SnS.~ ~ 2.8.25
OAc
O I +H
+"1 O OR RO ....
RO"' O
2.8.26 R=TES ) 2.8.20 R=H
2.8.27 R=TES) 2.8.21 R=H
222
Scheme 68 I AcO
AcO
OAc OAc
~,
OAc
.--
OAc
ii
HO ....
AcO ....
O
O OH
2.8.28 -
HO~'
OAc
OBz
AcO
HO
OH
2.8.29 OAc
S
OAc
OAc
/OAc
-
......
OH
,
OAc
....._
H 0 -
2.8.30
"'-0--/OH 2.8.31
-
HO ....
0
&o Ph
2.8.32
Conditions: (i) KCN, MeOH, rt, 65%; (ii) Camphorsulfonic acid, DMF, C6H6,reflux, 90%.
As shown in Scheme 68 [71], t r e a t m e n t of peracetylated baccatin derivative 2.8.28 with KCN in methanol led to deacylation at C-13, C-2 and C-4, affording 2.8.29. Tetrol 2.8.29 was w a r m e d overnight in DMF-benzene solution in the presence of a catalytic amount of camphorsulfonic acid, leading to r e a r r a n g e d product 2.8.31, which contains a 10-membered ring and a tetrahydrofuran ring, presumably via intermediate 2.8.30. A similar r e a r r a n g e m e n t of the oxetane to the tetrahydrofuran system was described in Schemes 5 and 6 (Section 6.2.1). It is unclear why the usual A-ring contraction does not occur here, and the authors do not offer any explanation.
223 However, it seems quite reasonable to explain this result by postulating that the Aring contraction requires the presence of the C-2 benzoate group and proceeds not via a C-1 carbonium ion, but via acyloxonium ions like 2.8.32. In the absence of an ester group at C-2, carbonium ion formation at C-1 is not a facile process and the pathway described above predominates. Another very complicated r e a r r a n g e m e n t , involving migration of C-2 benzoate and opening of the oxetane ring, is observed in the 14(~)-hydroxy baccatin series, and is outlined in Scheme 69 [72].
HO
20TES
HO....
0
" HO HphOh ~:~,~
HO~'
0 =~
OAc : H+ 2.7.6
~/
20TES
-- HO....
HO....
",,S
0
o.~o+ ~ O,~o~ Ph 2.8.34
~zo
o. o[2~ 2.8.35
I
0%. /? OTES Ho
.o
%./ _/? OTES ....
....
Bz
o, ~ H0~"~?o....--+h o. 0 2.8.86
BzO/
~
~
d"") ~
2.8.37
When 2.7.6 was refluxed in benzene in the presence of a catalytic amount of pyridinium p - t o l u e n e s u l f o n a t e (PPTS), rapid d i s a p p e a r a n c e of the s t a r t i n g
224 material took place, and 2.8.37 was isolated in 40% yield. A plausible mechanism for this r e a r r a n g e m e n t is also depicted in Scheme 69. Migration of the benzoate to C-14, presumably via C-1/C-2 and C-1/C-14 oxonium ions 2.8.33 and 2.8.34, triggers the Wagner-Meerwein A-ring contraction. This is accompanied by the already discussed C-4 assisted opening of the oxetane ring, eventually yielding the stable orthoformate 2.8.37 by trapping of 2.8.36 by the unacylated C-2 hydroxyl. A different type of A ring contraction, this time accompanied by B ring expansion, was recently reported by Appendino and co-workers [73]. As shown in Scheme 70, when 7-triethylsilyl-10-deacetylbaccatin 2.1.19 was treated with excess MnO2, cyclopentenone derivative 2.8.40 was obtained in low yield, in addition to the desired 13-keto product 2.8.38. The authors postulate the intermediacy of adiketone 2.8.39, which undergoes an a-ketol rearrangement. The driving force for this ring contraction may be the release of the angular strain due to the presence of four adjacent sp 2 centers in 2.8.39.
HO
O
OTES
H O .... O HO
OBz
O
: OAc
O HO
2.1.19
: OBz
": OAc
2.8.38
HO
0
OTES
O O O HO 0
OBz
OAc
2.8.40 Conditions: (i) MnO2, EtOAc, CH2C12,rt; 2.8.38 (40%)+ 2.8.40 (24%).
OBz
2.8.39
-: OAc
225 In order to assess the contribution of an i n t a c t A ring of the baccatin framework to the a n t i t u m o r activity, Ojima and co-workers prepared a novel class of nor-seco paclitaxel analogues 2.8.44 and 2.8.45, as shown in Scheme 71. The synthesis began with the oxidative cleavage of the A ring of 2.7.1, a n a t u r a l product isolated from T a x u s w a l l i c h i a n a Zucc [69], with periodic acid. This gives 2.8.41, which i m m e d i a t e l y cyclizes in situ to provide the h e m i k e t a l 2.8.42. The C-7 hydroxyl group of 2.8.42 was selectively protected as the triethylsilyl ether, and the aldehyde moiety was reduced with sodium cyanoborohydride to afford 2.8.43 in fair yield. Final side chain a t t a c h m e n t onto 2.8.43 was performed using the ~-lactam approach to provide the desired nor-seco paclitaxel and docetaxel analogs 2.8.44 and 2.8.45 [74] (Scheme 71).
HO
H 0'"'
O
HO
-
OHC
i
~
,'
~
o
HO
OH
:
"
0
OBz OAc
2.8.41
2.7.1
,4~
OHC
BzO C)Ac
OH
OTES
rO
O
i~, iii
HO
,,,..-
,
O O
O H()
HO
BzO (gAc
2.8A3
2.8.4,2
R'NH
iv, v
~ /0,,,, OEt 0
,,,Ph
)_, N.
_O. ~
- pha-2-_ OH
COR
2.7.18 R=Ph 2.7.10 R=OBu-t
BzO C)Ac frO
.....J
0
O.H
~
~
-.:'H HO
~
BzO C)Ac
2.8.44 R=Bz 2.8.45 R=t-Boc
Conditions: (i) H5IO6, 92%; (ii) TESC1, NEt3, DMAP, 76%; (iii) NaBH3CN, pH 6. 80%; (iv) NaHMDS, THF, -40 ~ 2.7.18, or 2.7.10; (v) 0.5% HCI, rt; then Bu4NF, THF, -10 ~ 82% for 2.8.44; 58% for 2.8.45.
226 Recently, Khuong-Huu and co-workers reported on the chemistry of oxidative cleavage of the B ring of baccatin at the C-1/C-2 and C-9/C-10 segments [75]. Since the reverse operation is often the key step in total syntheses aimed at the paclitaxel skeleton [8-10], the value of this work is in providing valuable materials for the study of these reductive cyclizations. The debenzoylation of C-2 was described in section 5.2.1. A typical product of these studies, 2.1.20, was protected at C-13 as a triethylsilyl ether, giving 2.8.46, which was then subjected to sodium metaperiodate oxidation, affording 2.8.47 in good yield (Scheme 72).
ES
Ro ....k
7."-,,_ ,L Hal"
_= - O H
2.8.46
R=TES J
HO 2.1.20 R=H
Conditions:
00TES
b
T SO,,,
0
-
OH i
2.8.47
(i) TESC1, py, rt, 100%; (ii) NaIO4, EtOH, pH 5 buffer, 75%.
The C-9/C-10 bond cleavage reaction is shown in Scheme 73. 10-Deacetylbaccatin III, 1.1.2, was selectively protected to yield 2.8.48 in two steps. The removal of the carbonate groups and the subsequent C-7 epimerization were achieved on treatment with zinc dust and DBU, respectively, providing 2.8.49 in excellent yield. Reduction of the C-9 keto moiety of 2.8.49 was effected remarkably well with BH3SMe2 in toluene, producing 9(~)-dehydrobaccatin derivative 2.8.50. The stereochemistry at C-9 was confirmed by NOE studies. Oxidative cleavage of the C-9/C-10 bond of 2.8.50 led to lactol-aldehyde 2.8.51, as a single isomer, in almost quantitative yield [75]. 5.2.9. Photochemistry No information on the photochemistry of paclitaxel was available until recently. During the course of the development of paclitaxel as a commercial antitumor drug, paclitaxel was subjected to a series of stability tests, including exposure to sunlight. In this test, traces of a paclitaxel isomer were isolated by s e m i p r e p a r a t i v e HPLC. After extensive NMR studies, the compound was
227 identified as the pentacyclic paclitaxel isomer 2.9.4 (Scheme 74), containing a new bond between C-3 and C-11 [76].
H
TrocO
OH
O OTroc
\ HO .... ( ~
Iii, i v
Lii
,~,,.
AcO ....
= O
0 HO
BzO
HO
OAc
" BzO
OAc
2.8.48
1.1.2
HO
AcO'"
AcO"'
OHoH
)
v, O
O HO
OAc
2.8.,50
2.8.49
o%IHoH OH
Aco,,,o .
" BzO
: BzO
2.8.51
OAc
Conditions: (i) TrocC1, py, 80 ~ 93%; (ii) Ac20, py, DMAP, rt, 100%; (iii) Zn, MeOH, reflux; (iv) DBU, PhMe, 80 ~ 80%; (v) BH3-SMe2, PhMe, 0 ~ 84%; (vi) NaIO4, EtOH, pH 5 buffer, rt, 97%.
Since very limited amounts of 2.9.4 could be produced by sunlight exposure, a more efficient method for further studies was highly desirable. An approach to this problem utilized a photochemical reactor. A very good conversion (55%) of paclitaxel into 2 . 9 . 4 was achieved. M e c h a n i s t i c a l l y , this r e m a r k a b l e photochemical transformation can be considered to follow in part the well-known oxa-di-~-methane r e a r r a n g e m e n t (Scheme 74). Nakanishi [77] was the first to describe a similar bond formation between C3 and C-11 in taxinine. However, taxinine differs from paclitaxel in that an enone chromophore is present at C-11/C-12/C-13, and it is undoubtedly the excitation of
228 this f u n c t i o n t h a t i n i t i a t e s the r e a r r a n g e m e n t . In the p r e s e n t case, the photoexcited moiety m u s t be the ~,y-unsaturated ketone, and the m e c h a n i s m shown in Scheme 74 was proposed.
AcO BzHN
O"
0
OH
hv
Paclitaxel 1.1.1
=
p h ~ . - J l_' - 0 .... OH 2.9.1
AcO~ BzHN
O"
" OBz
OAc
OH
0
Ph~O OH
H3-shift
.... 0 OBz
2.9.2
H
BzHN/~.flL_ H Ph
/
0
HO
,~
A
/.,,.., O H
O\1)..
~
Radical
_ O.... OH
Recombination
O : OAc OBz
HO
2.9.3
AcO zHm
o
-
0 OH
k H'
--
2.9.4
O IIII
OH
O HO
: OAc OBz
The excited state initiating the r e a r r a n g e m e n t m u s t be the TI(~,~*) of the C9 carbonyl group, which is r e p r e s e n t e d as the diradicaloid species 2.9.1, as postulated in the first step of the oxa-di-~-methane r e a r r a n g e m e n t . Diradicaloid 2.9.1 then r e a r r a n g e s to cyclopropylcarbinyl radical 2.9.2, and at this point the intramolecular hydrogen t r a n s f e r from C-3 to C-12 occurs, in the same vein as
229 described in section 5.2.8 w h e n discussing radical r e a r r a n g e m e n t s .
Finally,
t r a n s a n n u l a r bond formation in 2.9.3 leads to 2.9.4. Although the above m e c h a n i s m is reasonable on the basis of the literature, a more comprehensive study addresses the issue of w h e t h e r the C-9 keto group is directly excited, or w h e t h e r some of the a r o m a t i c groups in the molecule are involved in the a b s o r p t i o n and s u b s e q u e n t i n t r a m o l e c u l a r
energy transfer.
Confirmation of this possibility was sought by indirect m e a n s , i.e. each of the aromatic groups in the molecule were in t u r n deleted, and the effect on the yield of the pentacyclic products was examined [78].
R2
O
R2 I
,',H'"' ,
R1
//
O
al
hv ,..._
R 3 .....
R3
, O
O HO
OBz
HO
OAc
" OBz
OAc
2.2.4
R1 = OTES R 2 = OAc, R 3 = OH
2.9.6 (23%)
2.1.19
R1 = OTES R2= H, R3 = OH
2.9.7 (20%)
2.9.5
R1 = R2 = H
2.9.8
(21%)
1~ = OTES AcO
O
OTES
H O .... O HO
"
OAc O
2.9.9
As s h o w n
in
Scheme
75, b a c c a t i n
derivatives
were
studied
first.
Interestingly, the three baccatin III derivatives 2.2.4, 2.1.19 and 2.9.5, bearing a
230 benzoate group at C-2, when subjected to photolysis under standard reaction conditions (254 nm, Pyrex, 0.05 M in CC14, 20 h) cleanly gave the expected rearranged products 2.9.6, 2.9.7 and 2.9.8. In striking contrast to the above observations, attempted photolysis of 2.9.9 failed to produce any of the expected rearranged pentacyclic derivative. The above results suggest t h a t an aromatic ester moiety at C-2 is necessary for the photoisomerization to occur. The contribution
of the aromatic groups of the side chain to the
photochemistry of paclitaxel was next examined. The key substrate is the paclitaxel analog 2.1.26c, in which the C-2 benzoyl moiety has been replaced by the cyclohexanoyl ester.
i, ii 2.9.9
--
TESO,
AcO Ii
H Bz.N 0 = II P h ~ O OH
//
0
O.H
.... 0 -
0
2.1.26c 2.1.10 O AcO ~
H
Bz.N
o
/
HO
2.9.10
/I
0
OH
\IJl.
(5
OAc
~/N~O
Conditions: (i) LiHMDS, THF,-40 ~ 2.1.10, 85%; (ii) Py, 48%HF, CH3CN, 5 ~ 99%; (iii) hv, 254 nm, 40% of 2.9.10, 21%
of2.1.26c.
As shown in Scheme 76, the
photolysis of 2 . 1 . 2 6 c was performed as usual,
affording a 40% clean conversion to 2.9.10. The authors conclude that initial excitation of the 3'-N-benzoyl amide or the 3'-phenyl group in the side chain also serves to excite the C-9 keto function in an intramolecular fashion, since the
231 presence of the side chain restores the normal reaction mode absent in 2.9.9 [78]. Finally, to confirm t h a t photoinduced isomerization specifically requires a side chain endowed with an aromatic amide at C-3', analog
2.9.12 was prepared as
shown in Scheme 77. Under standard photolytic conditions, compound 2.9.12 was found, as expected, to be completely inert. This experiment clearly supports the hypothesis of an "antenna effect" between the C-3' benzamide (and/or the C-2 benzoate) and the C-9 ketone in this photochemical isomerization [78].
0 [~}/IL
AcO NH O
i, ii
/,O OH
2.9.9 --
Ill
O
2.9.11
0
Ok,~=O ~~/
2.9.12
Conditions: (i) LiHMDS, THF,-40 ~ then 2.9.11, 87%; (ii) Py, 48%HF, CH3CN, 5 ~ 94%.
Scheme 78
AcO
OH
o Paclitaxel 1.1.1
i or ii
= 2.9.4
o ....
OH 2.9.13
~
HO BzO: AcO
0
Conditions: (i) hv, PhMe, 300nm, 2.9.4 (31%); 2.9.13 (30%); (ii) hv, CCI4, 300 nm, 2.9.4 (41%); 2.9.13
(21%). The same study also reports confirmation of the t r a n s a n n u l a r bond formation by single crystal X-ray analysis of a typical r e a r r a n g e d product. Photophysical
232 studies on paclitaxel and the analogs described above may shed further light or the initial stages of this interesting photoisomerization. Photolysis of paclitaxel at different wavelengths also produced interestin~ results: at 300 nm, the new compound 2.9.13 was isolated in 30-40% yield, ii addition to 2.9.4 (20-30%) [78].
The formation of compound 2 . 9 . 1 3 can b~
rationalized by invoking the occurrence of a Norrish type I process after the photoisomerization step. The epimerization at C-7 may not be a photochemical event. Interestingly, only one configuration out of four possible at C-8 and (the former) C-10 was obtained. This stereoselectivity may be due to preference for the formation of the less strained pentacyclic ring system. 5.3. STRUCTURE-ACTIVITY RELATIONSHIPS A large array of paclitaxel analogs containing modifications within the diterpenoid core were evaluated in microtubule assembly or disassembly assays, as well as in vitro cytotoxicity assays against a number of tumor cell lines. Some of the more active analogs emerging from above in vitro assays were further tested in in vivo, usually in mice, against m u r i n e or h u m a n tumor xenografts. From the body of data discussed in this section, it is obvious t h a t an imperfect correlation exists among the several in vitro tests, and even more so between in vitro potency and in vivo efficacy. We briefly discuss the most common biological assays and try to highlight their differences.
(i) In Vitro Microtubule Assays Initial Rate of Tubulin Polymerization (Swindell-Horwitz-Ringel Method) [791: This assay determines the initial rate of tubulin polymerization. The assembly of microtubule protein (MTP) in the presence of paclitaxel or analogs is performed as follows: MTP (1.5 mg/mL) is incubated at 35 ~ with 15 pM paclitaxel or analogs (added as DMSO solution; 1% final DMSO concentration) in the absence of GTP. The assembly reactions are followed by turbidity measurements at 350 nm. The value for paclitaxel in this assay is used as an internal standard, and experiments with analogs are usually performed in parallel.
This
assay
is
kinetic
in
nature,
and
does
not
measure
thermodynamic affinity for the binding site (nor is it clear t h a t a correlation
233 between the two exists). In addition, it yields no information on the types of microtubules formed (i.e. length, shape, bundles etc.). It is therefore expected to be a rough measurement of activity, useful perhaps to eliminate the inactive compounds, but oi~en it does not correlate well with cytotoxicity. Extent of Microtubule Assembly (Himes Method) [80]" The aim of this assay is to determine the extent of assembly at different concentrations of the analogs and then calculate an ED50. The assembly reaction is done at 37 ~ in PEM buffer (0.1M PIPES, pH 6.9, 1 mM EGTA, and 1 mM MgSO4) at a protein concentration of 1 mg/mL (10 ~M) in the presence of paclitaxel or analogs and 0.5 mM GTP. The reaction is again monitored by the increase in the apparent absorbance at 350 nm. This method is different from the above in t h a t the kinetics of the process are not considered, and only thermodynamic factors are. These data may correlate better with cytotoxicity, which is usually measured over many hours or even days. Microtubule Disassembly (Potier Method) [52]" This assay was developed by Potier's group on the basis of the unusual stability of microtubules formed in the presence of paclitaxel. In this experiment, a solution of MTP (2 mg/mL) is assembled at 37 ~ in the presence of paclitaxel or analogs, followed thereafter by disassembly via lowering the temperature to 4 ~
The initial rate of microtubule disassembly in the presence of the
compound is monitored by the drop in turbidity at 400nm. The initial rate of microtubule disassembly of paclitaxel is set as a standard. This assay also measures a kinetic parameter, and it is not obvious t hat it should correlate well with cytotoxicity. It is also not evident t hat the correlation with assembly data will be good. EC(O.Ol) Expression (Long Method) [81]" The potencies of the different analogs are expressed as an effective concentration (EC0.01), which is defined as the analog concentration capable of inducing the tubulin polymerization at an initial rate of 0.01 OD/min at 37 ~ as measured at 350 nm. The rates of polymerization are determined at several concentrations and EC0.01 values calculated for each analog by interpolating the appropriate region of the polymerization curves obtained. This method is of
kinetic nature, but it yields data that are concentration based. In view of the poor predictive value of tubulin polymerization data, this review focuses on cytotoxicity results r a t h e r t h a n microtubule assemby or disassembly.
234 (ii) I n Vitro Cytoto~dcity Assays A number of murine or h u m a n cancer cell lines, such as P388, B16, HCT-116 and KB, have been used for the determination of in vitro cytotoxicity of paclitaxel and its analogs [82]. The in vitro IC50 value measures the drug concentration required for the inhibition of 50% cell proliferation, usually after prolonged (2-3 days) incubation. One must note that the success of the analog in this assay will depend on its stability within the culture medium as well as intracellularly; the ability to penetrate the cell, most likely by passive diffusion, is also critical, and highly ionic derivatives may not be bioactive because of poor lipophilicity. In addition, cells that express the m d r (multidrug resistance) phenotype may have more or less reduced sensitivity vs. paclitaxel and analogs. Unfortunately, this issue has not been widely addressed in the literature for paclitaxel analogs, although it is obviously very important, and will not be discussed here (for further details, see chapter 7). It is not clear whether cell lines that are resistant to paclitaxel may also be resistant to all its analogs. The choice of the cell line may be dictated by several factors, such as availability, ease of culture, attempted correlation with in vivo data, clinical relevance etc. One must be cautioned that cytotoxicity of each analog may vary substantially from one cell line to another, and may depend on the exact cell culture protocol employed.
(iii) In Vivo Assays The ultimate test of the efficacy of a drug in a model system is an in vivo assay, usually in mice. Obviously, the success of each analog will depend, in addition to cytotoxicity, on a n u m b e r of other factors, such as proper administration, biodistribution, metabolism, and systemic toxicity (or lack thereof) to the animal. Several protocols have been designed for paclitaxel and docetaxel; usually the more s t r i n g e n t tests involve the use of h u m a n xenografts and i.v. drug a d m i n i s t r a t i o n (i.e. distal t u m o r model), but i n t r a p e r i t o n e a l models are also commonly used. The drugs are usually evaluated in terms of their ability to delay tumor growth or prolong life span, as measured at the maximum tolerated dose (MTD, i.e. dose of drug that is not appreciably toxic to the animal) vs. a control untreated group [83]. Obviously,
235 one does not expect (and usually does not find) complete correlation between in vitro and in vivo results, and therefore one should resist making exaggerated
claims of potent activity based only on cell culture data. 5.3.1. Paclitaxel Analogs Modified at C-1/C-2 As oulined in section 5.2.1, derivatization at C-1 is very hard to achieve. Compound 2.1.9 (Figure 2), where a benzoate has m i g r a t e d to C-l, is essentially inactive in a tubulin polymerization assay [19]. Ring contraction due to solvolysis at C-1 leads to a number of interesting analogs, e.g. 2.1.16, which have tubulin-polymerizing activity, but have very poor activity against tumor cells [21].
BZ'NH 0 Ph~O oH
Bz.
AcO
0 OH
,
"'
~o AcO
NH 0
OCS2Me 0 OH
p h / ~ ~ _ 0 ....
2.1.16
OH
-
"\ BzO
Bz.NH 0 mh4... o.
2.1.9
AcO.2o. ,~
....
2.1.41
~
HO
Figure 2: Simple C-1/C-2 modified paclitaxel analogs
236
R'CO. NH O Ph~O
AcO
O OH
....
6. HO
-
RC(O)O Table 1: Cytotoxicity of Paclitaxel Analogs Modified at C-2 Cpd.
R
R'
IC50/IC50
Cell Line
Ref.
(paclitaxel) a 2.1.29
p-NO2-C6H4-NH
Ph
>200
HCT-116
27
2.1.26a
p-MeO-C6H4-
Ph
>20
HCT-116
27
2.1.26b
p-NO2-C6H4-
Ph
>100
HCT-116
27
2.1.26c
c-Hex
Ph
11
HCT-116
27
2.1.26c
c-Hex
Ph
56
P-388
31
2.1.26d
Me
Ph
>20
HCT-116
27
3.1.1
o-C1-C6H4-
Ph
0.01
P-388
29
3.1.2
m-C1-C6H4-
Ph
0.0014
P-388
29
3.1.3
p-C1-C6H4-
Ph
150
P-388
29
3.1.4
m-CN-C6H4-
Ph
0.33
P-388
29
3.1.5
m-N3-C6H4-
Ph
0.002
P-388
29
3.1.6
m-NH2-C6H4-
Ph
1,500
P-388
29
3.1.7
m-CF3-C6H4-
Ph
15
P-388
29
3.1.8
m-F-C6H4-
Ph
0.35
P-388
29
3.1.9
2-Furyl
Ph
25
UCLA-P3
84
3.1.10
2-Thienyl
Ph
4.2
UCLA-P3
84
3.1.11
2-Naphthyl
Ph
>1,000
UCLA-P3
84
3.1.12
c-Hex
t-BuO
1.1
B-16
64
3.1.13
c- H ex
t-BuO b
11
P-388
31
(a) Concentration of analog that inhibits cell proliferation by 50% divided by concentration of paclitaxel that achieves same result. (b) This compound is a 10-deacetyl derivative. These observations suggest t h a t the i n t a c t A-ring s u b u n i t is an i m p o r t a n t structural element for cytotoxicity. The C-2 benzoate clearly plays a role in the cytotoxicity of paclitaxel, since 2-deoxytaxol, 2.1.41, is essentially inactive [26].
237 Due to the important role of the C-2 substituent for proper binding, it is clear t h a t small modifications at this site may lead to optimization of the activity, and it is not surprising t h a t several groups have reported efforts in this direction (Table 1). Several conclusions may be derived from the results in the table, even though cytotoxicities are generally reported for different cell lines. First of all, introduction of p a r a substituent at the benzoate invariably leads to loss of activity (see 2.1.26a,b and 3.1.3). Thus, the fit of the benzoate within the binding site seems to be r a t h e r tight, unless complex conformational changes are engendered by the p a r a substitution. It is unclear whether an aromatic ester at C-2 is needed for activity. Both Chen et al. [27] and Ojima et al. [31] have shown t h a t reduction of the C-2 benzoate to a cyclohexanoate leads to substantial loss of activity (see 2.1.26c and 3.1.13). In contrast, Georg et al. [64], working in the 10-acetyltaxotere series (see 3.1.12), report no loss of activity on hydrogenation of the C-2 benzoate. These apparent discrepancies are very difficult to interpret. Smaller aliphatic C-2 esters [27] (see 3.1.26d) and heteroaromatic ones [84] (see 3.1.9 and 3.1.10) are also poorly active. The most productive modifications to date have been carried out by Kingston et al., who reported large increases in cytotoxicity for some C-2 oand m - s u b s t i t u t e d benzoates [29]. In addition to i m p r o v e m e n t s in the cytotoxicity vs. paclitaxel, these analogs are clearly more effective in promoting microtubule assembly in vitro. These compounds, especially 3.1.2 and 3.1.5, are therefore promising, although neither data vs. resistant cell lines nor i n vivo evaluation have been reported. 5.3.2. Paclitaxel Analogs Modified at C-4 Chen and co-workers have extensively explored the SAR at C-4. Table 2 shows some of the highlights. As with C-2, deacylation or deoxygenation at C-4 leads to complete loss of activity (see 2.2.16 and 2.2.45). Introduction of large groups is also deleterious (2.2.24h), but aliphatic esters slightly larger than acetyl lead to improved activity (2.2.24j). Small carbonates and carbamates at C-4 are also quite active, especially in conjunction with improved side chains (for details on improved side chains, see chapter 6).
238
a'~. NH
O
\
AcO
O OH
Iii1< OH HO
BzO
X
Table 2: Cytotoxicity of Paclitaxel Analogs Modified at C-4 Cpd.
R
R'
X
IC50/IC50
Ref.
(paclitaxel) a 2.2.45
Ph Ph
Bz Bz
OH H
>25 n.d. b
19, 32 36
2.2.24h
Ph
Bz
OBz
100
20
2.2.28a
Ph
Bz
OCO2Me
0.9
19
2.2.24j
Ph
Bz
OCO-c-Pr
0.4
20
3.2.1
Ph
Bz
3.9
19
4.5 d
35
2.2.1{}
Ok\ 2 - - N-~J ~
l-o 3.2.2
t- oc
O N.
l-o / - - 3.2.3
Ph
Bz c
",,1
OCO-i-Pr
(a) Measured in HCT-116 cells. (b) Very poor tubulin polymerization activity. (c) This compound is a 10-deacetyl derivative. (d) Measured in B-16 melanoma cells. Georg et al. have also reported a derivative modified at C-4, 3.2.3, which is slightly less active t h a n paclitaxel. Upon further exploration, modification of the C-4 function is likely to afford very potent derivatives. 5.3.3. Paclitaxel Analogs Modified at the Oxetane Ring Many of the derivatives described in section 5.2.3, in which the oxetane ring has been opened, were evaluated for their ability to polymerize tubulin [23]. None of t h e m displayed any measurable activity and they are therefore useless in defining the SAR at this locus. Most of them are missing an acetoxy group at C-4 which, as we discussed above, is crucial for binding. This is in agreement with Kingston's early results [2]. Thus, the oxetane seems to play
239 an essential role in the binding of paclitaxel to microtubules. It is not known whether the oxetane acts to rigidify ring C and point the C-4 acetoxy group in the appropriate direction for binding, or whether the oxetane oxygen itself is a binding element. The derivatives that would answer this question (e.g. the one bearing a cyclobutane ring in place of the oxetane) have not yet been described. 5.3.4. Analogs Modified at the C-7 Position C-7 Xylosyltaxol, 3.4.1 was isolated from the bark and leaves of T a x u s b a c c a t a , and shown to be more potent t h a n paclitaxel in the tubulin polymerization assay [3, 85]. Other analogs were prepared either from baccatin derivatives or from paclitaxel itself. Most of the analogs tested (Table 3) have in vitro activity comparable to paclitaxel and docetaxel. With the exception of large lipophilic substituents such as the silyl ethers (see 3.4.8 and 3.4.10), any modification seems well tolerated, with some analogs being slightly more cytotoxic than paclitaxel. It seems likely that the C-7 substituent is not engaged in significant interactions at the binding site, and t h a t chemical modification at this position will only serve to m o d u l a t e the activity (perhaps via altered solubility, metabolism, biodistribution). Some of the C-7 derivatives were tested in vivo. With the exception of the carbonate derivative 3.4.6, none of the
compounds compared favorably with paclitaxel at their MTD. Interestingly, even cyclopropane derivative 2.4.14 retains in vitro activity, in spite of the slight conformational alteration imparted to the C ring vs. paclitaxel. 5.3.5. Analozs Modified at the C-9 Carbonyl This section examines primarily the effect of reducing the C-9 carbonyl on the cytotoxicity. Some analogs carry also C-10 and/or C-7 modifications and are discussed here for the sake of convenience. As with the C-7 position, even with the small database available, it is clear t h a t most modifications, including complete defunctionalization, are well tolerated at C-9. None of the modifications effected led to complete loss of activity. The derivative 2.5.31, f e a t u r i n g a completely defunctionalized northern half, is only 5-6 times less active than paclitaxel. Compounds with a partially hydroxylated northern segment have activities of the same level as paclitaxel and docetaxel (Table 4).
240
R "NH O
AcO
O X
p h - - " L ~ _ O .... OH HO
" BzO
Table 3: Cytotoxicity of Paclitaxel Analogs Modified at C-7 Cpd.
R
X
In vitro
Cell
In vivo
IC50/IC50
Line
activityb
Ref.
paclitaxel a 3.4.1
Bz
([~)Xylosyl
n.d.c
-
3
3.4.2
Bz
([~)OAc
1.3
J774.2
85
3.4.3
Boc
(~)L-Phenyl
0.44
P-388
52
3.4.4
Bz
([~) L-Alanyl
2.3
B-16
86
3.4.5
Bz
([~)N,N-dimethyl
2.3
S-16
86
3.4.6
Bz
(~)OC02Et
3.4.7
Bz
3.4.8
Bz
3.4.8 2.4.1
alanyl d
glutaryl 1.5
HCT-116
289 (40)
([~)OCONHBu
1.0
HCT-116
157 (50)
([3)OMs
0.9
HCT-116
Bz
([~)OSiEt3
>20
HCT-116
Bz
(a)OH
0.5
HCT-116 126-154 (30-
-
-
45 45 45 45 45
32) 3.4.10
Bz
(a)OSiMe3
>20
HCT-116
2.4.7
Bz
2.4.8
Boc
H
1.0
HCT-116
157 (50)
45
H
0.4
HCT-116
156 (64)
45
2.4.16
Bz
2.4.14
Bz
A6-dehydro
1.2
HCT-116
161 (60)
45
A7,19-cyclopropa
2.0
HCT-116
2.4.12
Bz
156 (80)
45
(a)F
2.9
HCT-116
185 (132)
45
3.4.11
Boc
(a)F
1.2
HCT-116
147 (40)
45
-
45
(a) See Table 1 for definition. (b) I n v i v o data in unstaged M109 model. Values indicate T/C at the MTD (mg/Kg/inj., in parentheses). Paclitaxel gave T/C values of 183-276 at MTDs of 50-75 mg/Kg/inj. (c) IC50 0.4 v s . paclitaxel (1.0) in tubulin polymerization assay. (d) A C-10 deacetyl derivative.
241
R "NH 0 ph-~,~_
X
Y Z
0 ....
OH
0 HO
" BzO
OAc
Table 4: Paclitaxel Analogs Modified at the C-9 Carbonyl Cpd.
R
X
Y
Z
IC5o/IC5o (paclitaxel)a
Cell Line
Ref.
2.5.1
Boc
(~)OH
(a)OH
2.5.2
Boc
( [ 3 ) O H (~)OH
(~)OH
1.9b
P-388
56
([~)OH
2.0b
P-388
56
2.5.4
Boc
(~)OH
(a)OH
(a)OH
3.2b
P-388
56
2.5.10
Bz
3.5.1
Bz
([~)OAc
(a)OH
(~)OH
8-10
P-388
17
([~)OAc
H
(~)OH
0.5
P-388
17
3.5.2
Bz
([~)OAc
H
H
1
P-388
17
2.5.31 2.5.24
Bz Bz
H H
H (~)OH
H (~)OH
5-6 14
P-388 B-16
17 64
2.5.25
Boc
(~)OH (~)OH
B-16
64
Boc
(~)OH (~)OH
1.1
2.5.26
(~)OH H
1.8
B-16
64
(a) See Table 1. (b) This value is referenced to docetaxel, not paclitaxel. Reduction of the C-9 carbonyl yields active a or 13carbinols (see 2.5.1 and 2.5.2). A 10-deoxy-9-dihydroderivative is much less bioactive t h a n paclitaxel, b u t simply switching the side chain to the one from docetaxel restores the activity (see 2.5.24
vs.
2.5.26). These observations reinforce the notion t h a t the northern
half of the molecule does not intimately interact with the microtubule binding site. 5.3.6. Analogs Modified at the C-10 Position Table 5 shows some of the analogs t h a t bear modified C-10 substituents. Some also bear modifications at C-7 and are discussed here for the sake of convenience. Although it is well known t h a t introduction of polar esters or other functions at C-10 leads to loss of activity [52], minor modification with
242 relatively small substituents at this position have been shown to lead to active analogs [66].
X RHN
0
P h i _ _-
Oy
\
0 ....<
OH
0 HO BzO
OAc
Table 5: Paclitaxel analogs modified at C-10 [19, 60, 66] Cpd.
R
X
Y
IC50/IC50 paclitaxel a
3.6.1 2.5.19 3.6.2 3.6.3 3.6.4 3.6.5 3.6.6 3.6.7 3.6.8 3.6.9 3.6.10 3.6.11 3.6.12 3.6.13 3.6.14
Bz Bz Boc Bz Bz Boc Bz Bz Bz Bz Boc Bz Boc Bz Boc
=O H H H H H OCO-n-Bu OCO-c-Pr OCONMe2 OMe OMe OCO2Me OCO 2Me OCOPh OCOPh
(~)OH (~)OH (~)OH (a)OH H H (~)OH (~)OH (~)OH (~)OH (~)OH (~)OH (~)O H (~)OH (~)OH
14 1.5 0.5 1.0 7.2 3.5 1.4 1.0 0.4 5.0 0.5 1.2 0.6 0.9 0.9
(a) See Table 1. Cell line: HCT-116 in all experiments. While a 10-keto group leads to a substantial loss of activity (see 3.6.1), the functionality at C-10 can be completely removed without loss of activity (see 2.5.19, 3.6.2 and 3.6.3) [60, 63, 64, 65]. On the other hand, deletion of both the C-7 and C-10 functions leads to some drop in cytotoxicity (see 3.6.4 and 3.6.5). In general, functions including esters, carbonates, carbamates and ethers are all
243 conducive to good activity, i.e. comparable with paclitaxel. These observations once again support the theory that the functionalities in the northern half of the core are not involved in binding to the microtubule. 5.3.7. Analogs Modified at C- ll/C- 12 Very little information is available on the role of the bridgehead C-11/C12 double bond vs. the bioactivity of the taxanes, since all natural taxoids are endowed with such double bond, and it is very difficult to modify it chemically. Chen and co-workers have reported some unusual chemistry at C-10 that results in the formation of dienone systems at C-10->C-18 [60]. C-12 fluorinated derivatives, where the double bond has moved into conjugation with the C-9 carbonyl, were also obtained as side products. Biological evaluation of some of these compounds (see Figure 3) shows that migration of the C-11/C-12 double bond leads to some loss in activity. Ten-fold drops in cytotoxicity (vs. paclitaxel) are seen with dienones 2.5.18, 3.7.1 and 3.7.2. The fluorinated derivatives are also ten-fold less active than paclitaxel, except for derivative 3.7.3, which bears a ~-methyl grouop at C-12, and is over 100-fold less active [19]. It is likely that, due to the importance of the C-13 side chain in the binding process, its exact spatial positioning is crucial to the activity of these analogs. Even slight conformational changes in the A ring might simply alter the spatial relationship of the side chain vs. the other binding elements in the molecule (the C-2 and C-4 esters). 5.3.8. Analogs Modified at the C-14 Position The biological activity of analogs bearing a functionalized C-14 has been explored in a preliminary fashion by Kant et al. [12] and Ojima et al. [13, 70]. The 14-(~)OH analog of paclitaxel, 2.7.14 (Table 6) shows slightly reduced cytotoxicity vs. the parent drug. I n v i v o evaluation showed that this derivative is essentially devoid of antitumor activity [12]. Switching the side chain to the one found in docetaxel, as predicted, results in a slightly improved performance (see 2.7.15 and 2.7.11) [12]. Even a cyclic carbonate at C-14/C-1 is compatible with good activity, but only in the presence of the docetaxel side chain, the paclitaxel analog being remarkably less active (2.7.12 vs. 3.8.1). A C-1/C-14 acetonide (see 2.7.13) is deleterious to activity.
244
#
BzHN O
O
R
O ....~ OH
2.5.18 3.7.1 3.7.2
R=(~)OH R=(~)OCOCHFC1 R=(cz)OH
ICso/ICso(pacl) 9.5 9.5 10 O
BzHN O RI,,,,~..~ ~ , ~ o ....
Z'.. i
BzO 3.7.3 3.7.4 3.7.5
J,
OAc
RI= F; R2 =Me; 1~3 = (a)OH RI= Me; R2 =F; R3 = (a)OH RI= Me; R2 =F; R3 = (~)OH
Figure 3: Paclitaxel analogs modified at C-10/C-12 and their
IC5o/IC5o(pacl) 230 13.5 9.5 in vitro
cytotoxicity (HCT-116)
Analogs where the side chain was introduced at C-14 instead of C-13 were much less active than docetaxel [13, 70]. Not enough is known about the SAR at C-14 to draw final conclusions as to the involvement of this position at the binding site. Since most of the analogs in Table 6 have similar activity to paclitaxel, it is likely that the C-14 functionality does not perform a binding function, and therefore only minor changes in cytotoxicity can be realized by fine-tuning such functionality. 5.3.9. Misce!!.aneous Analogs Klein reported on the synthesis of novel paclitaxel derivatives featuring a contracted seven-membered B-ring, 3.9.1 and 3.9.2 (Figure 4 ) [ 4 8 ] . Interestingly, these compounds were of comparable activity to paclitaxel in the
245 in vitro P-388 cytotoxicity assay. More work needs to be done to assess the
potential of these unusual analogs.
FI~HN
PhA
O
\
2
OH
_- Lo .... OH
_ R40 OR 3 0 i =
O
Table 6: Cytotoxicity of Derivatives Modified at C-14. Cpd.
R1
R2
R3,R4
IC5o/IC50 paclitaxel a
Cell Line
Ref.
2.7.14
Bz
Ac
H,H
4.0
HCT-116
12
2.7.15 2.7.11 2.7.12
Boc Boc Boc
Ac H H
3.8.1
Bz
H
2.7.13
Boc
H
H,H H,H C=O C=O C(Me)2
1.0 1.0 1.0 17 7.5
HCT-116 HCT-116 A121 A121 A121
12 12 13, 70 13, 70 13, 70
(a) See Table 1. The Ojima group described the synthesis and biological evaluation of two novel nor-seco analogs of paclitaxel and docetaxel, 2.8.44 and 2.8.45 [74]. These compounds are 20-40-fold less potent than paclitaxel in a number of tumor cell lines. These results thus clearly indicate the importance of the A-ring for the proper binding of paclitaxel and docetaxel to their biological target. A pentacyclic paclitaxel derivatives (2.9.4), prepared by photochemical irradiation of paclitaxel by Chen et al. [76], failed to show any activity in the tubulin polymerization assay as well as in cytotoxicity assays. The core of this molecule is grossly distorted with respect to the one in paclitaxel, and no activity would be expected. A recent report from Commerqon et al. provides the first example of a C19 modification [88]. The fact that a C-19-hydroxylated docetaxel analog (3.9.3) exhibits slightly better activity than the parent drug in the tubulin disassembly
246 assay suggests that chemical modifications at C-19 may lead to useful derivatives. AcO R.N o ,, X,o.c o. 3.9.1 R=Bz 3.9.2 R=Boc OBz OAc RHN
O
0 _. // OH
\
~ ~OHo
,
O .
HO
BzHN "
Ogz OAc
AcO I ~, / H,,,
0
O
/? OH \l~ 2.9.4
o ....
HO
dR HO
BocHN
O
P h ~ O
~
/
O/OH
~ 3.9.3
~
~-~o
HO BzHN
OAc
....
o.
0
OBz OAc
AcO
O
I~,,,,,II
jOH ~oH'" 3.9.4
p h ~ - ~ ' O _ ....
o.
2.8.44 R=Bz 2.8.45 R=Boc
~ HO
~~o (DBz OAc
Figure 4: Miscellaneous paclitaxel and docetaxel derivatives
247 Finally, among the many docetaxel metabolites isolated, one (3.9.4) features a novel core functionalization, i.e. a hydroxyl group at C-6. Such hydroxylation leads to a 30-fold drop in activity vs. docetaxel, i.e. detoxification of the drug [89]. 5.3.10. Conclusion Although much work remains to be done in this area, a qualitative picture of the SAR of paclitaxel is beginning to emerge. At least three functional elements, i.e. the C-13 side chain (see chapter 6) and the C-2 and C-4 esters, are intimately involved in interactions at the binding site. It appears that the northern half of the molecule and the tetracyclic skeleton (including an intact oxetane) function essentially as a molecular scaffolding to hold these binding elements in the proper orientation. Some uncertainty still exists about a possible binding role for the oxetane oxygen and the C-1 hydroxyl group. Modifications of the essential functions may therefore lead (and in some cases this has been achieved in cell culture) to more potent paclitaxel analogs, through further optimization of the fit with the microtubule site, whereas modifications at the non-essential positions may modulate the activity by changing the physico-chemical parameters of the molecule or via other secondary effects. R~'EgENCIi~
.
.
.
.
.
7.
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o
.
10.
11. 12. 13. 14. 15. 16.
17.
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249 Science and Current Status; Georg, G.I.; Chen, T.T.; Ojima, I.; Vyas,
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165
5 PACLITAXEL (TAXOL| CHEMISTRY AND STRUCTURE ACTIVITY RELATIONSHIPS
-
Shu-Hui Chen Bristol-Myers Squibb Pharmaceutical Research Institute, P.O.Box 5100, Wallingford CT 06492-7660, U.S.A.
Vittorio Farina Boehringer Ingelheim Pharmaceuticals, 900 Ridgebury Road, Ridgefield, CT 06877, U.S.A.
5.1. I N T R O D U C T I O N
Studies on the chemistry of paclitaxel I (1.1.1, Figure 1 ) a n d its derivatives began more than 30 years ago, when this compound was isolated from the bark of T a x u s b r e v i f o l i a by Wani and Wall [1]. In these early years, in spite of the a n t i t u m o r activity displayed by the substance, interest in its chemistry was modest, as was the supply of the drug. The field, therefore, advanced very slowly, mostly through the efforts of Kingston [2]. The lowyielding process that afforded paclitaxel from the bark of the yew tree appeared to be a limitation to the development of the compound as a therapeutic agent. Later, the isolation of 10-deacetylbaccatin III, 1.1.2, in r a t h e r high yields from 1Taxol| is a registered trademark of the Bristol-Myers Squibb corporation. The generic name "paclitaxer' is used throughout this chapter.
166 the leaves of Taxus baccata [3] opened the doors to the first semisynthesis of paclitaxel[4] and a more potent analog, Taxotere @ (docetaxel) [5] (1.1.3, Figure 1) utilizing a renewable source and thereby alleviating the supply problem substantially. As both paclitaxel and Taxotere | advanced through the clinical studies, the potential of these agents became obvious and generated considerable excitement [6]. Both paclitaxel and 10-deacetylbaccatin III, through the efforts of the Bristol-Myers Squibb company, the co-developer of the drug, became available in large amounts both to in-house chemists and outside collaborators for structure-activity relationship studies. These developments in the U.S., and also the continued efforts of the French groups at Gif and Rhone-Poulenc, led to a flurry of discoveries in the chemistry of these complex diterpenoids [7]. Paclitaxel has been approved by the Food and Drug Administration (FDA) for the t r e a t m e n t of refractory advanced ovarian cancer and breast cancer in 1992 and 1994, respectively. Due to these exciting developments, paclitaxel research has continued unabated. Years of efforts from many synthetic groups [8] finally culminated into two independent total syntheses of paclitaxel [9,10]. The syntheses are quite long and are unlikely to contribute much to the SAR in this area, at least in the near future, but represent an exciting academic achievement. O R~ II NH
O
18 R'O~,
O OH 1/19
....7~-i.~,~, 7 1,~ -,,16
o~3\
OH
7 ......6A..~ ,o
HO
6
O
'~O 1.1.1 Taxo] R = Ph, R ' = Ac 1.1.3 Taxotere R = t-Boc, R ' = H
Me
O
....
HO
:
t~ OAc
o
Bz() 1.1.2 1.1.4
R = H 10-deacety] baccatin II] R = COCH 3 baccatin I I I
Figure 1: Some medicinally important taxanes and the paclitaxe] | numbering system
At the same time, the search for n a t u r a l taxanes possessing core modifications compatible with bioactivity has begun to produce encouraging
167 results, most notably the discovery by Klein that 9-dihydrotaxanes, prepared by semisynthesis, have good biological activity [11], as well as recent reports on the activity of the 14-OH taxanes [12,13]. Research on the chemistry of paclitaxel continues for obvious reasons: the poor water solubility of paclitaxel, its rather poor in vivo potency, and its tendency to rapidly induce multi-drug resistance make it a sub-optimal drug [14]. It is hoped t h a t some of the new analogs may display improved properties in one or more of the above areas. For the pharmaceutical houses, due to the anticipated large sales of paclitaxel, the i n t e r e s t in devising unique, p r o p r i e t a r y and p a t e n t a b l e analogs of paclitaxel is obviously very strong. In addition, its high cost of production is enticing medicinal chemists to try to u n d e r s t a n d which portions of the molecule are actually interacting with its biological target and which have only minor functions or serve as scaffolding for the pharmacophore. As SAR studies become more complete, the first efforts to design a totally synthetic paclitaxel surrogate are appearing [15]. It is conceivable that more efforts in rational analog design will be reported in the near future. This chapter reviews all the relevant chemistry and structure-activity relationship studies up to the end of October 1994. Due to the existence of previous reviews on the subject [7], although comprehensive, this review will especially stress most recent key developments in the chemistry of taxanes and their implications for the SAR. We focus only on the chemistry of the core. Modifications of the C-13 side chain, which is crucial to the activity, form the topic of Chapter 6. 5.2. CHEMISTRY
5.2.1. Reactions at C-1/C-2 Functionalization of the tertiary alcohol function at C-1 without affecting the rest of the core is no trivial matter. Accordingly, few derivatives with C-1 substitution are known. Chen et al. initially reported a C-1 xanthate derivative [16a]. Subsequent reports by Klein et al. on derivatives of 9-dihydrotaxol [17] demonstrated the tendency of baccatin to undergo a C-2->C-1 benzoyl shift under basic conditions, leading, after t r e a t m e n t with carbon disulfide and methyl iodide, to 1-benzoyl-2-xanthyl derivatives. A re-examination of the earlier work [16b] verified that this is indeed the case in the baccatin series also (Scheme 1). Thus, 2.1.3 represents the first example of a C-1 acylated baccatin
168 III derivative. C-2 deoxygenation led to the deoxy analog 2.1.4. Similar results have been reported by the Kingston group as well [16c]. C-1 substitution does not imperil C-13 side chain introduction for biological evaluation. As outlined in Scheme 2, xanthate 2.1.3 was desilylated and then selectively resilylated at C-7 to give 2.1.7./~cylation according to the method of Holton [18], employing ~-lactam 2.1.10 as the side chain source, gave the desired C-1 functionalized paclitaxel derivative 2.1.9, after a final desilylation [19]. In search of a suitable C-1 protecting group, Chen and co-workers were led to utilize the novel dimethylsilane (DMS) group. Apparently, introduction of the bulkier trimethylsilyl group was difficult. As illustrated in Scheme 1, DMS was successfully introduced to the C-1 position of baccatin derivative 2.1.2 to give 2.1.5 in almost quantitative yield. Selective removal of DMS from C1 was accomplished with tetrabutylammonium fluoride at 0 ~ [20].
.o ....,,co 2o. TESO. . . . . . . . . . HO BzO
HO ~c
2.1.1 R=H -~ i 2.1.2 R=TES J
MeS 2.1.3
l iii AcO TESO....
O OTES
AcO
~0
TESO. . . . . . . . . .
20TES
Me2HSiO BzO 2.1.5
2.1.4
Conditions: (i) 5 equiv TESC1, imidazole, DMF, rt, 87%; (ii) Nail, THF+CS2, 70 ~ then MeI, 56%; (iii) 2 equiv Bu3SnH, AIBN, PhMe, 100 ~ 92%; (iv) 3 equiv Me2HSiC1, imidazole, DMF, 0 ~ 97%; (v) Bu4NF, THF, 0 ~ 84% of 2.1.2. [TES=triethylsilyl]
169 The significance of DMS protection of the C-1 hydroxyl group in the selective deacetylation at C-4 will be discussed in section 5.2.2. C-1-Benzoyl-2-deoxybaccatin 2.1.4 was converted to the corresponding C7 silylated analog 2.1.12 via a desilylation/mono-silylation sequence. The paclitaxel side chain was then attached onto 2.1.12 using Holton's protocol, to give 1-benzoyl-2-debenzoyloxytaxol, 2.1.14, after standard deprotection (Scheme 3) [19]. [ S c h e m e 21
AcO ~., RO ....\ \ BzO
Bz,
O ,~ OR'
P h ~ O
iii
=
AcO
MeS2CO 2.1.3 R=R'=TES ~ i 2.1.6 R=R'=H 2.1.7 R=H, R'=TES ~ i i
....
OR
O TESO,",,
-
O OR
NH O
Ph ],-" N. oJBz
O
BzO =MeS2CO
OAc
"
2.1.8 R=TES ) i 2.1.9 R=H
2.1.10
Conditions: (i) Py, 48%HF, CH3CN, 5 ~ 2.1.3 to 2.1.6, 76%; 2.1.8 to 2.1.9, 84%; (ii) TESC1, imidazole, DMF, 0 ~ 86%; (iii) LiHMDS, THF, -45 ~ then 2.1.10, 86%. [ Scheme 3 /
AcO RO ....
O OR'
Bz" NH
AcO ~'
O
-i i i -~ p h ~ . O
O
,,,, BzO
2.1.4 R=R'=TES ~ i 2.1.11 R=R'=H "h ii 2.1.12 R=H, R'=TES
2.1.13 R=TES "h Jr i 2.1.14 R=H
Conditions: (i) Py, 48%HF, CH3CN, 5 ~ 2.1.4 to 2.1.11, 79%; 2.1.13 to 2.1.14, 87%; (ii) TESC1, imidazole, DMF, 91%; (iii) LiHMDS, THF, 0 ~ then 2.1.10, 83%.
170 Under acidic conditions, a skeletal rearrangement of the A ring occurs, perhaps initiated by carbonium ion formation at C-1. A representative example is shown in Scheme 4. Kingston prepared ring-contracted paclitaxel analog 2.1.16 by reacting derivative 2.1.15 with methanesulfonyl chloride, followed by desilylation [21]. Later, similar rearrangements were observed by Potier [22] and Chen [23] under a variety of acidic conditions. In order to assess the contribution of the C-2 benzoate moiety to binding, a selective procedure for debenzoylation at C-2 was needed. Kingston has described a number of protocols for deacylation reactions of baccatin III. Under basic conditions, C-10 is deacylated first and, in certain cases, even the C-4 acetate is more labile than the C-2 benzoate [24]. Special conditions are therefore needed for selective C-2 deacylation.
AcO ph/~~_
0
Bz.. NH 0
Bz" NH 0 0 ....
phiaL-
i "
2.1.15
oH
O,
III
-<
o 2.1.16
Conditions: (i) MsC1, NEt3, -15 ~ to 0 ~ 20%; (ii) HF, Py, THF, 0 ~ to rt, 55%.
The first succesful attempt to selectively cleave the C-2 ester in a polyacylated baccatin derivative was reported by Farina et al., and made use of tin alkoxides or oxides. The reaction was postulated to occur by prior pentacoordination of the tin reagent to the C-1 hydroxy group, leading to rapid C-2 debenzoylation. Unfortunately, this was followed by backside attack of the C-2 alkoxide onto C-20, leading to oxetane opening, with formation of novel tetrahydrofuran derivative 2.1.18 in good yield (Scheme 5) [25]. The selectivity for the C-2 ester over the other four acetate groups is remarkable. Especially diagnostic here, in the 1H NMR spectrum, was the geminal coupling constant of the C-20 hydrogens, which increased from 8.3 Hz in 2.1.17 to 11.6 Hz in 2.1.18, and suggested a ring-expansion reaction.
171
Scheme 5~ AcO
AcO,,
O
AcO
OAc i
ii
~
AcO,
Sl I l l
Iii
o
OBz 2.1.17
O
.o, ~ .
O 2.1.18
r,,,oOc.
Conditions: (i) Bu3SnOMe, LiC1, NMP, rt, 67%.
Similar t e t r a h y d r o f u r a n - c o n t a i n i n g baccatin derivatives were later described by other r e s e a r c h groups, which used alkaline or reducing conditions to cleave the C-2 ester [11, 22, 24]. For example, the Gif group found t h a t reduction of 7-triethylsilyl-10-deacetyl baccatin 2.1.19 with lithium aluminum hydride at room temperature for a short period of time gave a 3:1 mixture of debenzoylated product 2.1.20 and the rearranged baccatin 2.1.21 in 80% overall yield (Scheme 6) [22].
.o
....
~
HO"
.o
....
o 2.1.20
2.1.19
HO .... H O 2.1.21 Conditions: (i) LiAIH4, THF, rt; 2.1.20 (60%), 2.1.21 (20%).
172 Selective C-2 debenzoylation in the presence of C-4 and C-10 acetates was finally achieved under a variety of conditions (Scheme 7).
AcO
O
AcO
OTES iorii
T E S O .... _
.
0
O
OTES
= T E S O ....
or iii
"
HO
2.1.2 Conditions: (i) NaOMe, MeOH, rt, 25%; (ii) Red-A1, THF, 0 ~
H
O
OH
2.1.22
87%; (iii) KOBu-t, THF, rt,
69%. Chen et al. reported that, whereas methoxide-promoted debenzoylation of 2.1.2 gave low yields of 2.1.22 [26], use of Red-A1, which presumably also precoordinates at C-l, led to the desired product in high yield [27]. Potassium tbutoxide was later reported by Datta et al. to perform similar function [28]. More recently, Kingston reported a simple procedure to debenzoylate paclitaxel derivatives. Thus, 2.1.15 was treated with NaOH under phase-transfer conditions to afford 2.1.23 in fair yields. Reacylation with a variety of substituted benzoates afforded a number of C-2 modified paclitaxel analogs for SAR studies (Scheme 8) [29]. Simultaneous, high-yielding C-2/C-10 deacylation in a 13-keto baccatin derivative could be carried out with catalytic bicarbonate in methanol/water [30]. The selective C-2 debenzoylation in baccatin derivatives has allowed chemists to prepare a number of paclitaxel analogs modified at C2. The simplest modification at C-2 is actually obtained by hydrogenation under forcing conditions, affording C-2 cyclohexanoate derivatives [24, 31]. In addition to the already mentioned approach directly from paclitaxel [29], baccatin derivatives modified at C-2 have been prepared by DCC-mediated coupling; introduction of the side chain afforded novel paclitaxel C-2 esters (Scheme 9) and carbamates (Scheme 10) [27]. As shown in Scheme 10, the carbamate linkage in 2.1.27 was prepared by reacting 2.1.22 with p-nitrophenyl isocyanate and pyridine in benzene at 70 ~ Standard desilylation and selective C-7 resilylation, followed by side chain acylation gave the desired
173 carbamate 2.1.29 in low yield, due to the unavoidable competitive formation of the C-1/C-2 cyclic carbonate 2.1.30 [27].
BZ'NH 2.1.15
O
p h ~ / [ L_"
AcO
20TES
0 ....
OTES
-
HO
OH
2.1.23
ii, i i i
Bz.. Ph
AcO
O
NH O . OH
0 ....
HO 2.1.24
OH
O,
R.~--[~ 0
Conditions" (i) 2 N NaOH, Bu4NHSO4, C6H6, 43%; (ii) DCC, DMAP, ArCOOH, PhMe, 50 ~ (iii) 5% HC1, MeOH, rt.
The formation of carbonate 2.1.30 underscores the instability of the C-2 carbamate linkage toward nucleophilic attack by the neighboring C-1 hydroxyl group. Similar cyclic derivatives have been reported in other publications also. For example, treatment of 2.1.22 with phosgene afforded 2.1.31 in high yield. Other attemps to protect the diol function as ketal or acetal led to complex mixtures of products, in which 2.1.32 (Scheme 11) predominated. A C2 benzyl ether could not be prepared either [19]. Another derivative that was easily prepared was the cyclic thiocarbonate 2.1.33, shown in Scheme 11 [16a]. One of the strategies for generating the C-2 benzoate in the total synthesis of paclitaxel is to utilize these C-1/C-2 cyclic carbonates. Treatment with phenyllithium in tetrahydrofuran at -78~ desired C-2 esters.
yielded regioselectively the
174
AcO
.O OTES 2.1.10
TESO ....
2.1.22
_
,,,
~Ac
0~==:0
2.1.25
R Bz,
AcO NH
0
O OH
\
....<
~~o
R a) p-MeOC6H4b) p-NO 2C6H4c) c-C6H lid) CH3~-
OH HO
-
'
O~=:=0
2.1.26
R Conditions: (i) 4 equiv DCC, xs DMAP, RCOOH, PhMe; (ii) Holton's protocol [18].
AcO
O
.h2
2.1.22
.o'~_ ~;No
2.1.27 2.1.28
R1 =R2 =TES ii R1 =TES, R2 =H J
~----~ ~ Bz. iii
TESO,,,,
AcO NH
0
0 OH
\
,,,Ph
o~'. 2.1.10
....< R10
Bz 2.1.29 2.1.30
!o OR 2
R1 =H, IRe =CONHC6H4-pNO2 R1,Ra =-C(O)-
Conditions: (i)p-NO2C6H4N=C=O, py, C6H6, 70 ~ 86%; (ii) Py, 48%HF, CH3CN, 5 ~ then TESC1, imidazole, DMF, 0 ~ 62%; (iii) LiHMDS, THF, -40 ~ 2.1.10, then (ii), 2.1.29 (15%), 2.1.30 (50%).
175 As illustrated in Scheme 12, Nicolaou et al. demonstrated t h a t treating carbonate 2.1.34 with five equiv of PhLi gave 2.1.35 in good yield [30]. Similarly, Holton showed t h a t t r e a t m e n t of carbonate 2.1.36 with 2.1 equiv of PhLi afforded the corresponding C-2 benzoyl derivative 2.1.37 in excellent yield [9]. These derivatives were subsequently transformed into paclitaxel. Complete defunctionalization of C-2 can also be achieved through the intermediacy of 2.1.22. As shown in Scheme 13, treatment of 2.1.22 w i t h sodium hydride in THF/carbon disulfide afforded xanthate 2.1.38, usually accompanied by a small amount of cyclic thiocarbonate 2.1.33. Barton deoxygenation smoothly afforded the desired C-2 deoxy derivative 2.1.39, which was acylated as usual to afford the paclitaxel analog 2.1.41. Attempted Barton deoxygenation on 2.1.33 and related analogs, on the other hand, brought about a number of interesting skeletal rearrangements, which are described fully in section 5.2.9. AcO
O OTES
T E S O .... :
.
O
OH
2.1.22
AcO
AcO
20TES
iii
T E S O ....
O OTES
T E S O .... O
O
OTES
S
2.1.33
2.1.31 T E S O .... _
HO
0
,,
OH
c
2.1.32 Conditions: (i) COC12,py, CH2C12, 0 ~ 87%; (ii) Nail, CS2+THF, MeI, then 1 equiv Nail, 83%;
(iii) acidic or basic conditions.
176
AcO
O OTES
HQ
OBOM
TBSO"' O -
",Ac
O
0 I ii
2.1.34 I
i
AcO
O OTES
2.1.36
HO%
OBOM
TBSO .... HO
2.1.37
\
2.1.35
Taxol 1.1.1
Conditions: (i) 4 equiv PhLi, THF, -78 ~
80%; (ii) 2.1 equiv PhLi, THF, -78 ~
Ac..~,(3 i
2.1.22
OBz
O OTES .
= TESO ....
85%.
,.....
L,._ ~u
.
.
.
1/ h
/ v
R 2.1.38
R=OCS2Me "~ ii t
2.1.39 R=H
HO .... AcO.
~O OTES
BZ'NH
O
ph~~['-O_
AcO~,
,60 OH
....
oH
HO
HO 2.1.40
OAc
2.1.41
Conditions" (i) Nail, THF+CS2(5:I), then MeI, 2.1.38 (61%), plus 2.1.33 (21%); (ii) Bu3SnH, AIBN, PhMe, 100 ~ 89%; (iii) Bu4NF, THF, rt, 85% (iv) TESC1, imidazole, DMF, 0 ~ 87%; (v) LiHMDS, THF, -40 ~ then 2.1.10, then Bu4NF, THF, rt, 63% overall.
177 5.2.2. Reactions at C-4: Deacetylation and Reacylation Early work on C-4 deacetylation was reported by Kingston [24] and Potier [22]. As outlined in Scheme 14, t r e a t m e n t of 1.1.4 under forcing hydrogenation conditions gave 2.2.1 in good yield. Methanolysis gave several products, including 2.2.2 and 2.2.3. Finally, extensive hydrolysis gave completely deacylated product 2.1.20 in fair yield.
AcO
~O OH
AcO i, ii
HO.... NO
Bz
=
,~O OTES iii
HO....
o
HO
O
1.1.4
O
~=2.2.1
O IIII
.o
HJ~"
....
HOA~~cO O
O
"
O
HO% ,~O OTES
He/. . HO
Hb
~
2.1.20
Conditions:
(i) Pt/H2, 90%; (ii) TESC1, Py, 80%; (iii) 0.5 N NaOMe, MeOH, rt, 69%.
Thus, it seems that deacetylation of the more hindered C-4 acetate is faster than deacylation at C-2, which is somewhat surprising. Kingston has
178 postulated that this occurs because of an initial base-catalyzed intramolecular acetyl transfer from C-4 to C-13, followed by rapid deacetylation. In agreement with this, if the C-13 hydroxyl is protected, the order of reactivity is reversed. In any case, the difference in reactivity is not satisfactory under these basic conditions, and completely selective C-4 deacetylation cannot be carried out [24]. Similarly, Potier's group reported attempted selective deacylation of C-4 under a number of basic and reductive condition, but in every case mixtures of products were obtained [22]. A solution to this problem was formulated by Datta et al., who treated 7triethylsilyl baccatin III 2.2.4 with potassium t-butoxide, yielding the desired 4deacetyl derivative 2.2.5 in fair yield (Scheme 15). This was explained once again by assuming acetyl transfer from C-4 to the free hydroxyl at C-13. The use of the bulkier base presumably retards direct deacylation at C-10 and C-2, which was highly competitive with methoxide [28].
AcO
O OTES
AcO i
HO ....
/
[
,," HO .... O
~O
BzO
BzO
2.2.4
2.2.5
Conditions: t-BuOK, THF,-20~
O OTES
58%.
Kingston et al. applied this chemistry to the preparation of 4-deacetyl-10acetyltaxotere, 2.2.8 (Scheme 16) and its related paclitaxel analog [32]. The starting material, 2.2.6, was prepared by the above t-butoxide-promoted hydrolysis of 2.2.1 which, in this case (temperature is not specified by the authors) apparently led to both C-4 and C-10 deacylation, contrary by the report of Datta and coworkers [28]. Coupling with synthon 2.2.7 was followed by C-10 reacylation and final deprotection.
179
h/h/t/
H OIIII
Boc~NZO
~
H
2.2.7
2.2.6
Boc,, NH O
\
AcO
O
OH
OH
HO BzO 2.2.8 Conditions: (i) DCC, DMAP, PhMe, 90%; (ii) Ac20, DCC, 4-(pyrrolidino)pyridine, THF, 65%; (iii) HCOOH, rt, 46%.
A more general solution to C-4 deacylation, which does not depend on the presence of a free hydroxyl group at C-13, was reported by Chen et a/.[20]. Since Red-A1 reduction of baccatin III seems to take place at the C-2 ester because of coordination with the hydroxyl group at C-l, it was felt that blocking this functionality would redirect the r e a g e n t elsewhere in the molecule, and hopefully to C-4, perhaps by prior coordination with the oxetane oxygen, the most basic oxygen in the molecule. A suitable C-1 blocking group was therefore developed. The a l r e a d y m e n t i o n e d dimethylsilyl (DMS) was found to be optimal. Both its introduction and its removal could be carried out under mild conditions. As illustrated in Scheme 17, reduction of 2.1.5 with excess Red-A1 afforded the desired C-4 deacylated baccatin 2.2.9, together with a smaller a m o u n t of the C-4/C-10 di-deacylated baccatin 2.2.10. The undesired baccatin 2.2.10 was easily reacetylated in situ to 2.2.9 by the method of Greene [4]. A more efficient approach to C-4 deacylated derivatives utilized C-13 trimethylsilyl
derivative
2.2.11 as the s u b s t r a t e (Scheme 18). Baccatin
derivative 2.2.12, bearing three different types of silyl groups, was treated with Red-A1 and then quenched with a s a t u r a t e d solution of t a r t r a t e , giving the
180 desired C-4/C-13 diol 2.2.13 in acceptable yield [20]. Loss of the TMS group occurred during the acidic work-up, as planned. Derivative 2.2.13 is obviously an ideal substrate for side chain attachment and the preparation of C-4 modified taxanes.
T
SO ....
Me2HSiOJ B ~
H OA~~cO
2.1.5
AcO
OTES
TESO ....
!0
Me2HSiO
+
~
O OTES,
TESO .... ~
Me2HSi
BzO 2.2.9
Conditions:
HO
BzO
2.2.10
(i) Red-A1,THF, 0 ~ (3:1) 2.2.9/2.2.10 (84%); (ii) Py, AcC1, 5 ~ 84%.
I Scheme18 /
AcO TMSO,,,
n
O
us se
~o,7~. ~ ~d~o 2.2.11 R=H "h i 2.2.12 R=DMS
AcO ii
O
OTES
HO .... Me2HSiO BzO
(
o
2.2.13
Conditions: (i) Me2HSiC1, imidazole, DMF, 0 ~ 92%; (ii) Red-A1, THF, 0 ~ then Na,K tartrate, 50-60%.
181 Preparation of 4-deacetyltaxol can be carried out quite efficiently beginning with paclitaxel, and using the selective 2,4-deacylation reaction reported by Kingston et al. A s shown in Scheme 19, 2.2.16 is obtained in high overall yield by phase-transfer promoted deacylation followed by rebenzoylation [32]. Synthesis of a 4-deacylated taxane for biological evaluation was also reported by the Gif group, following similar chemistry [33]. As we have seen, several solutions exist for selective C-4 deacylation and, not surprisingly, a number of workers have recently focused on the reacylation (or other derivatization) of C-4 for SAR studies at this position. This operation turned out to be non-trivial. During the course of an extensive deacylation/acylation study on baccatin derivatives [24], Kingston found that selective benzoylation of 2.2.17 led to derivative 2.2.18. The hindered C-4 hydroxyl group in 2.2.18 could not be acetylated even under forcing conditions and only 2.2.19 was obtained.
BZ'NH O ph~ L ~ . . , O .... -
~sss 9
~
HO"
iii
Ace,,.
BZ'NH 0 ph~~"O. OH
i ph/~,~ -
o
2.2.14
ii
AcO
BZ'NH O 0 ....
O
ss ss ~
HO'# 2.2.15
OH
OH
....
2.2.16 Conditions: (i) Benzyltrimethylammonium methoxide, CH2C12; (ii) PhCO2H, DCC, DMAP,
PhMe; (iii) 5% HC1, MeOH (42% overall). However, direct acetylation of 2.2.17 under standard conditions gave tetraacetyl derivative 2.2.20 in modest yield, where the C-4 carbinol was successfully acetylated (Scheme 20). Given these observations, Kingston speculated that the
182 inaccessibility of C-4 hydroxyl group in 2.2.18 towards acetylation is due to the steric hindrance imposed by the bulky C-2 benzoate moiety. In any case, the yields reported are too low to draw any definite conclusions. Chen et al. reported a general solution for derivatization of the C-4 alcohol moiety, and this has led to the preparation of several C-4 modified paclitaxel analogs [19, 20, 34]. As shown in Scheme 21, the fully protected baccatin derivative 2.2.9 (obtained as in Scheme 17) has only the C-4 carbinol open for derivatization. Treatment under highly basic conditions with a variety of acyl chlorides afforded the C-4 analogs in high yields. These were all converted to the respective paclitaxel derivatives via the usual Holton procedure.
Aco..... o" ,~. o~ ~'ii
2.2.17
AcO
.... . o "
o
i i~,,~
AcO . . . . . .
2.2.18
AcO
O OTES
O
,,
.o"-~~o~o~~ 2.2.20
AcO .... "
HO
H
OBz
2.2.19
Conditions: (i) DCC, DMAP, PhCOOH, 14%; (ii) Ac20, CH2C12, DCC, rt, 24%.
THF, DCC, 55 ~
89%; (iii) Ac20,
183 The authors stress that the success of this protocol depends highly on the following two facts: (i) protection of the C-1 hydroxyl group as a dimethylsilyl ether (DMS) allows clean C-4 deprotonation without any complication; (ii) the choice of a strong base, such as LiHMDS or BuLi (weaker bases are ineffective), is crucial.
AcO O ~ F - ~ .'~ . ~ ~ "// OTES t
AcO O , ~ , ~, P ~// " OTES
i
so ....
z x__#~
/
-
~
ii
so,,,
,%
~
OBz O , ~ R 2.2.9
AcO ~, HO ....
O
//
2.2.21
OR'
Bz. 2.1.!0
,.... O OBz O , ~ R
=
O O
AcO NH
-
O
II
= P h ~ O
....
OH
H
'~'
o" BzO
O,~R
0 2.2.22 2.2.23
2.2.24
R'=H
R'=TES
O
iii
R "a, CH3; b, CC]3; c, CH2CH3; d, CH2CH2CH3; e, CH(CH3)2; f, (CH2)3CH3; g, (CH2)4CH3; h, Ph; i: p-NO2-C6H4;
Conditions: (i) LiHMDS, THF, 0 ~
then acyl chloride, 70-85%; (ii) Py, 48% HF, CH3CN, 5 ~
(iii) TESC1, imidazole, DMF, 0 ~
Similar strategies led to the successful preparation of C-4 carbonates (Scheme 22). Once again, the corresponding paclitaxel analogs were prepared without complications by Holton acylation of 2.2.27a,d.
184
AcO ~, i
2.2.9
TESO ....
AcO
O // OTES HO
,,
DMSd ~
H d WOBz
2.2.25
AcO
isssss
i i i i
~ ..-.~ O OBz OCO2R
Bz.
O
O OH
OCO2R
2.2.26
R'=H
2.2.27
R'=TES
~ iii
2.1.10 y
Ph
: OH
R: a, Me;b, Et; c, n-Pr; d,p-NO2-C6H4
O ....\
HO
2.2.28
&Bz OCO2R
(i) LiHMDS, THF, ROCOC1; (ii) Py, 48%HF, MeCN, 5 ~ (iii) TESC1, imidazole,
Conditions:
DMF, 0 ~
AcO
O OTES
TESO ....
i
DMSO
OBz O
2.2.25d
AcO~,
AcO
O~
DMSd ~ ~ N 0 2
NH O P h ~ O OR
I"11-~,,,.0 OBz OCONHR
AcO
O OH
i ....
HO Bz(3
OCONHR
2.2.32
R" a, n-Pr; b, cyclopropyl; c, cyclobutyl Conditions:
ii
2.2.29
Bz. 2.1.10
OBz OCONHR 2.2.30 R'=H iii 2.2.31 R'=TES
._~ TESO .... ~ , ,% . /, / ~ OTES ~ "
O
,~O OR'
HO ....
O
(i) RNH2, THF, rt; (ii) Py, 48% HF, MeCN, rt; (iii) TESC1, imidazole, DMF, 0 ~
185 To prepare C-4 carbamates (Scheme 23), these authors resorted to a stepwise strategy. Reactive carbonate 2.2.25d was the key intermediate. Its reaction with amines led to displacement of the p-nitrophenol moiety and produced a number of carbamate derivatives. Conversion to the paclitaxel analogs by standard Holton acylation was complicated by side reactions (vide infra), and afforded only modest yields of the desired 2.2.32. Scheme 24 shows the synthesis of the C-4 aziridine carbamate. It was found more practical here, instead of using aziridine as a nucleophile, to construct the 3-membered ring stepwise.
AcO
2.2.25d
NO ....
0 iii
,,,,
HdW
.-%1o
OBz OC020BHn-p-N02 2.2.33 R=H ~ ii 2.2.34 R=TES
AcO
0
AcO
HO ....
iv
tt o
Hd_= H.-%10 OBz 0 H 2.2.35
"~ N 0
HO ....
OBz 0
BZ'NH~v ~ 0 Ph
_ OH
,,,,
2.2.36
OH AcO
2.1.10
0
N~
0~
0
~-r 9
OH
0 .... HO
2.2.37
: OBz
0 O~
N~
Conditions" (i) Py, 48% HF, MeCN, rt; (ii) TESC1, imidazole, DMF, 0 ~ (iii) NH2(CH2)2OH, THF, rt; (iv) PPh3, DEAD, THF, rt, 50%.
186 In the event, p-nitrophenyl carbonate 2.2.33 was silylated at C-7, then quantitatively converted to 2.2.35 by t r e a t m e n t with ethanolamine. Standard Mitsunobu conditions gave aziridine 2.2.36. In this case, the paclitaxel side chain was introduced onto 2.2.36 in high yield to give the desired C-4 aziridine carbamate 2.2.37, after standard desilylation. A clue as to the problems associated with the Holton acylation of carbamates 2.2.31 was provided when acylation with modified lactam 2.2.40 was attempted under standard conditions. Surprisingly, acylation at the C-4 c a r b a m a t e nitrogen was observed as the major pathway, as outlined in Scheme 25. In two cases, the unwanted 2.2.39 predominated, while the desired C-4 carbamate analogs 2.2.38 were only minor products. This problem is then due to the acidic nature of the carbamate proton. The high-yielding acylation of the aziridine analog 2.2.36, which bears no such acidic proton, suggests that protection of the C-4 carbamates to remove the N-H function should result in a generally high-yielding C-13 acylation.
AcO ~,
B~ ..NH 0 i, ii 2.2.31a,b
AcO
O ~ OH
" -~0,
~-
III
2.2.~a,b
OH
HO
OBz 0 ~/--- NHR
HO..../
Hd ~OBz~O 2.2.39a,b
0
TESO,%
R ,,..[ ~ / Boc-N H
Conditions: (i) LiHMDS, THF, 2.2.40; (ii) Py, 48%HF, MeCN, rt.
,,,,,
oJ-' N. Boc 2.2.40
187 Recently, Georg and co-workers have also reported the synthesis of a C-4 modified analog of paclitaxel (2.2.43, Scheme 26). The approach relies on the intermediacy of cyclic carbonate 2.2.41, which is used as a protecting group for the C-1/C-2 diol system while performing C-4 acylation with a large excess of acylating agent. Although no further examples are reported, the procedure may have general utility [35].
BZ'NH Ph
O
"
_
AcO ~
O -//- OTES
0 ....
~ H
2.2.14
Ph
.
o,
,, - p.A_Ao:_
_
iii
~.-O Bz.
NH -
AcO
O OH
~
Ph/~~__ O.... OH
2.2.43
OH
O~H
O
0
OH
....
OTBS 2.2.41
BZ'NH
HO
LO OTES ~
O
_O OTES
0 ....
2.2.15 AcO~
BZ'NH
AcO
BZ.NH
OBz
0
2.2.42
I
=
?=o
20TES
II
Ph/~flL'" O.......... OTBS _
~0 HO
AcO
O
i v, v
/
Conditions: (i) t-BuOK, H20 (2 equiv), THF, -20 ~
75%; (ii) (Im)2CO (20 equiv), THF, 55 ~ C ; (iii) (iBuCO)20 (20 equiv), DMAP (20 equiv), CH2C12, rt, 67% overall; (iv) PhLi (10 equiv), THF,-78 ~ (v) Py, HF, 0 ~ to rt, 48% overall.
Kingston and co-workers recently reported a study aimed at deoxygenation of the C-4 position [36]. Deacylation of suitably protected
188 paclitaxel and C-1/C-2 protection gave 2.2.41 as in Scheme 26. Conversion to the xanthate proceeded uneventfully, and Barton deoxygenation geve the desired 4deacetoxy derivative. Final deprotection turned out to be difficult because of the competitive acid-catalyzed oxetane ring opening, which was reported to be faster than in the 4-acetoxy-bearing analog, in spite of the lack of anchimeric assistance by the ester group (see section 5.2.3). Nevertheless, HF in pyridine gave the target 2.2.45 in fair yield (Scheme 27).
AcO ~'
Bz. NH 0 i
p h ~ . J [ , . O _ ....
2.2.41
OTBS
AcO
BZ-NH 0 Ph
-
0 ....
0 ~O
2.2.44 ii, i i i
00TES
(~CS2Me
O O
sss S
2.2.45
(i) Nail, CS2, MeI, 90%; (ii) Bu3SnH, AIBN, PhMe, 90 ~ 80%; (iii) PhLi, THF, -78 ~ 60%; then HF, py, rt, 66%. Conditions:
5.2.3. The chemistry of the oxetane moiety In a study dealing with the reaction of paclitaxel with electrophilic reagents [21], Kingston found t h a t reacting paclitaxel with Meerwein's reagent (triethyloxonium tetrafluoroborate) gave, in low yield, 2.3.1, a compound where the oxetane ring had been opened. The presence of the diol moiety was subsequently confirmed by formation of acetonide 2.3.2. The A ring contraction reaction, already briefly mentioned in section 5.2.1, apparently also took place, due to the acidic conditions employed. Kingston noted that reaction of paclitaxel with acetyl chloride directly yielded both A-ring contraction and oxetane opening, in addition to C-5 acetylation (Scheme 28). The oxetane-opening chemistry was independently investigated by Gu~ritte-Voegelein [37], using baccatins as substrates.
189
BZ'NH Paclitaxel 1.1.1
0
phil_
0 .... -
sts
OH
OH 2.3.1
BZ.NH
O
ph~~Jl"O
AcO~, ....
.
2.3.2
iii
OAc
L O OH
OH
Paclitaxel 1.1.1
OBz
~k
OBz OAc
\
Bz. NH
AcO ~
O
P h ~ O , ,
'"O
/
O ... // uAc
'
OH 2.3.3 -
'
\
OBz
O
"'OAc OcH
Conditions: (i) Et3OBF4, CH2C12, 35-51%; (ii) Me2C(OMe)2, p-TsOH, CH2C12, 95%; (iii) MeCOC1, then H20, 68%.
Reaction of baccatin derivative 2.3.4 with anhydrous zinc chloride in toluene was found to give 2.3.5, again with an opened oxetane ring and a contracted A ring. Treatment of 2.3.4 with aqueous acid, on the other hand, gave a mixture of three products, i.e. 2.3.6, featuring an intact oxetane ring, in addition to regioisomers 2.3.7 a n d 2.3.8 [22] (Scheme 29). T r e a t m e n t of paclitaxel and baccatin derivatives with Lewis acid has been discussed in a rather comprehensive study [23]. Use of strong acids, such as boron halides or TMSBr gave, in addition to A-ring contraction, oxetane opening with complementary regiochemistry (Scheme 30), but weaker Lewis acids smoothly led to oxetane opening w i t h o u t concomitant A-ring transformations (Scheme 31). Once again, two regiochemical modes are possible for the oxetane solvolysis, and the ratio of 2.3.12 to 2.3.13 depends on the Lewis acid used [23].
190
Scheme 29
_
HO
TrocO ,~
O //
TrocO
O.Troc
....
O
= HO ....
,,,
0 HO
: AcO OBz
2.3.4
~
' \
OBz
OAc
2.3.5
ii
_
TrocO ~i,
O //-
TrocO
_OTroc
HO ....
O
HO .... 0
HO'\
",
OBz
OBz
Hd \
2.3.6
2.3.7 2.3.8
Conditions: (i) ZnC12, PhMe, 80 ~
R2
OR 1
RI=AC , R2=H R I=H, R2=Ac
50%; (ii) 1 N HC1, AcOH, rt, 2.3.6 (20%), 2.3.7 (30%), 2.3.8
(17%).
BZ'NH Ph
O
AcO
2
OH
: O .... ()Cbz 2.3.9
HO
0
ii
" AcO OBz BZ.N H
O
AcO
ph-~.~_ 2.3.10
R 1 =Ac, R 2 =H
2.3.11 R 1 =H, R 2 =Ac
O
-
O .... '"O R2
OCbz
\
OBz
Conditions: TMSBr, CH2C12, rt, 2.3.10, 79%; (ii) BBr3, CH2C12, rt, 2.3.11, 75%.
OR1
191
Scheme 31 /
~,, II O.H
AcO
az.
2.3.9
, ewi.
NH
0
acia
0 OCbz
Lewis acid SnC14 TiCl4 BF3
2.3.12 (%) 17 57 43
2.3.13 (%) 69 15 43
0
....
. . . . . . OBz
,,OR 2 OR 1
2.3.12 R1 =Ac, R2 =H 2.3.13 R1 =H, Re =Ac
A variety of paclitaxel and baccatin substrates were submitted to Lewis acid treatment, and similar products were obtained. Some qualitative kinetic information could be gleaned from these studies: for example, paclitaxel itself reacts very slowly with tin tetrachloride. Once the C-2' hydroxyl group is blocked as a carbonate, however, high reactivity is restored. Also, 7-epi derivatives react very sluggishly in comparison with their 7(~) counterparts. From all these observations, a mechanism can be proposed for the oxetane opening reaction that requires the anchimeric participation of the C-4 acetate group, and is consistent with proposals by Kingston [21] and Chen [23] (Scheme 32). The reaction is initiated by complexation of the Lewis acid with the oxetane oxygen, which is probably the most basic atom in the molecule. The acetoxy group is positioned for participation by backside attack onto C-5, leading to acetoxonium ion 2.3.15. Indeed, when the C-4 acetoxy group is not as readily available for nucleophilic displacement (it is hydrogen bonded to the C7 epi-hydroxyl group, and it apparently interacts with the C-2' hydroxyl group in paclitaxel), the overall reaction is sluggish at 0 ~ and only proceeds at room temperature. Intermediate 2.3.15 can isomerize to acetoxonium ion 2.3.17, via strained orthoester 2.3.16. Alternatively, 2.3.15 can be hydrolytically quenched to yield the hemiorthoester 2.3.18, which can further unravel to afford 2.3.13, bearing a C-5 acetoxy group. Similar quench of 3.2.17 leads to 2.3.12, featuring a C-20 acetoxy group [23]. As a consequence, the regioselectivity of the reaction is a reflection of the ratio of the two acetoxonium ions, 2.3.15 a n d
192 2.3.17, and this ratio seems to be highly dependent on the Lewis acid used, either due to steric or electronic reasons.
OH
OH OSnCl4- H27
oH// I
OH ..
.[,~
Me
T|
2.3.15
~O H
2.3.13
2.3.12
)=0 2.3.14
2.3.18
Me
+_SnCi4_ Me
"0
0-~
It o. ~
o.l H20 OSnCI4-
Oto 2.3.17 Me
OH Oe~:
M
2.3.19
H
F u r t h e r evidence supporting the above mechanism came from the observation that, when 2.3.9 was treated with tin tetrachloride for 2 h at 0 ~ an unusual product was isolated in c a . 20% yield. It was assigned structure 2.3.20 (Scheme 33) aider extensive NMR characterization [23]. The formation of 2 . 3 . 2 0 can be explained by assuming t h a t the conformation of ring C in 2.3.17 can flip to a boat, in which the 7-OH group can attack the C-20 methylene carbon, to lock the C ring permanently in a boat-like conformation. Compound 2.3.21 was isolated in low yield when 7,13diacetylbaccatin III was solvolyzed under similar conditions. The product is the result of a 1,2 hydride suprafacial shift from C-5 to C-4 in an intermediate of the type 2.3.17, and leads to inversion at C-4, as shown by NMR data.
193
Bz" NH O
AcO
O
AcO
O
p h ' ~ ' J l " O _ .... OCbz
AcO ....
O //
OAc
.~, O
'"O HO
OBz
OBz
OAc
2.3.21
2.3.20
Attempts to trap acetoxonium ions 2.3.15 and 2.3.17 with nucleophilic reagents in the presence of Lewis acids proved fruitless, but stirring baccatin III in ice-cold trifluoroacetic acid in the presence of a large excess of phenyl dimethylsilane produced acetals 2.3.22 and 2.3.23 in fair overall yield, suggesting that the two acetoxonium ions were indeed both present, at least under these particular conditions [23] (Scheme 34). As shown in Scheme 35, in order to f u r t h e r confirm the diol functionality present in structures 2.3.12 and 2.3.13, both compounds were cleaved using lead tetraacetate in acetonitrile. While 2.3.13 gave the expected product 2.3.24 in high yield, 2.3.12 cleanly produced the unexpected 2.3.25, the result of C-20 to C-7 acetyl migration followed by exocyclic cleavage. Endocyclic cleavage was only a minor product [23].
AcQ
O
i Baccatin III, 1.1.4
OH,
+
=HO ....
N O IIII
..
"OH
"" 0 -
OBz
O.~H
2.3.22
Conditions:
CF3CO2H, PhMe2SiH, 0 ~ 2.3.22(42%), 2.3.23(9%).
HO BzO 2.3.23
\
H
H
194
IScheme 35 /
AcO Bz. NH O 2.3.13
~,
= ph.,.-'L,,~O. .... OCbz
BZ'NH O -
Ph
"
:
OBz
AC!~ 0'"
OCbz 2.3.25
'"OAc HO
2.3.24
2.3.12
O OH
0
20Ac
i
sis
OH
HO
"
OBz
O
Conditions: (i) Pb(OAc)4, MeCN, rt, 71-80%.
The chemistry of the novel 2.3.20, featuring a bridged C ring, was also studied. As shown in Scheme 36, t r e a t m e n t of 2 . 3 . 2 0 u n d e r s t a n d a r d acetylation conditions only led to a C-5 acetate derivative, 2.3.26, via a formally i n t r a m o l e c u l a r transesterification process. Oxidation of 2.3.20 gave the expected C-5 keto analog. When 2.3.20 was treated with DAST, an interesting rearrangement took place, leading to another bridged analog, 2.3.29, via a 1,2 alkyl migration through 2.3.28 [19]. 5.2.4. Reactions at C-7 Epimerization of the C-7 hydroxyl group to the 7((~) isomer 2.4.1, presumably via a retroaldol/aldol sequence, was first described by Kingston in his a t t e m p t s to promote radical reactions at t h a t position [38]. A more convenient way to effect epimerization at C-7 is to treat paclitaxel with base, as shown in Scheme 37 [19]. The 7((z) isomer is apparently more stable than paclitaxel. The most direct way to assess the importance of the binding of the C-7 hydroxyl group within its biological target is to replace it with a hydrogen
195 atom. Thus, it is not surprising that several groups have engaged in research aimed at such deoxygenation reaction.
BZ"NH O 2.3.20
phil'_
AcO
2
O
0 ....
OCbz
HO
-
2.3.26
OAc
O Bz
AcO
O
O
BZ"NH O phil_
0 , ~ ::SF2NEt2
0 ....
HO
2.3.27
2.3.28
t ss
, co 2 P h ~ O .
= OBz
o\
....
OCbz 2.3.29
HO
OBz
(i) Ac20, Py, CH2C12, 77%; (ii) Jones reagent, Me2CO, rt, 82%; (iii) DAST, CH2C12, 0 ~ 74%. Conditions:
Bz" NH 0 Taxol 1.1.1
AcO ~,
0 //
.OH
i, orii, o r i i i
p h / ~ ~ _ . 0 .... 8N
o OBz 2.4.1
Conditions:
rt, 65-80%.
(i) AIBN, PhMe, reflux; (ii) DBU (2.5 equiv), PhMe, reflux, 84%; (iii) Nail, THF,
196 Initial attempts involving Barton type reaction on C-7 derivatives such as thionobenzoates [39], selenocarbonates, oxalates [19] proved fruitless. After this quite extensive search, both Kingston et al. [40] and Chen et al. [41] found an identical solution to the problem of C-7 deoxygenation via xanthates. Chen and Farina prepared both 7-deoxytaxol and 7-deoxytaxotere via 7-deoxybaccatin, as shown in Scheme 38.
AcO
0
,.,.
9
AcO O -~_~.~.
OCS2Me
iii
R O ....
0
0 HO
: OBz
OAc
HO
1.1.4 AcO
OAc
2.4.2 R=H 2.4.3 R=TES O
AcO
R O ....
-~
/~
O HO
: OBz
" OBz
2.4.4 R=TES 2.4.5 R=H
ii O
,9-L,
Ph
-
OAc
O .... O
OH HO TESO,
) iv
%
,,"
o•1
Ph
OBz
OAc
2.4.7 R=Bz 2.4.8 R=Boc
N. R
2.1.10 R=Bz 2.4.6 R=Boc
Conditions: (i) Nail, THF+CS2, MeI, 57%; (ii) TESC1, imidazole, DMF, rt, 74%; (iii) Bu3SnH, AIBN, PhMe, 110 ~ 83%; (iv) TBAF, THF, rt, 74%; (v) LiHMDS, THF, -40 ~ then 2.1.10 or 2.4.6; then 1 N HC1, CH3CN,-5 ~ 53% of 2.4.7; 77% of 2.4.8.
Formation of the C-7 xanthate was complicated by some competitive C-7 epimerization. Deoxygenation proceeded smoothly to afford 2.4.4. Deprotection and attachment of the side chains gave 2.4.7 and 2.4.8 [41]. Kingston produced a 7-deoxy derivative directly from 2'-protected paclitaxel using the same chemistry (Scheme 39) [40].
197 Incorporation of fluorine atom into biologically active molecules has become an i m p o r t a n t facet of medicinal research. Consequently, C-7 fluorinated paclitaxel was an interesting synthetic target. T r e a t m e n t of 2'protected paclitaxel derivative 2.3.9 with two equivalents of DAST led to 7(cz) fluoro analog 2.4.11, together with side product 2.4.13, which features a 7,19cyclopropane ring. If a larger excess of DAST was used, the already discussed A ring contraction also took place. The structure of 2.4.11 seems secure on the basis of NMR studies, including appropriate values of JH,F. Also, the structure of 2.4.13 was confirmed by NMR studies and, after cleavage of the side chain, by a single crystal X-ray of the corresponding baccatin analog (Scheme 40) [42, 43].
,,,
gz..
AcO
S H
O
OH
_~I
~O' ~
SMe ii
ph: ~ ~ _ O, OTES
2.4.7 -
HO
OBz
OAc
o
iii
o
OBz
2.4.9
2.4.10
Conditions: (i) Nail, CS2, THF, then MeI, 60%; (ii) Bu3SnH, AIBN, PhMe, refl.; (iii) dil. HC1,
49%.
gz.
i
2.3.9
=
AcO~ ~0 F NH 0
ph-2- _
1111
OR Bz.
AcO
O
NH 0
_
HO
: OBz
0
OAc
2.4.11 R=Cbz 2.4.12 R=H
ii
0 2.4.13 R=Cbz 2.4.14 R=H
ii
O.... OR HO
-
AcO
OBz Conditions: (i) 2 equiv DAST, CH2C12, rt, 2.4.11 (55%), 2.4.13 (32%); (ii) H2, Pd/C, EtOAc, 2.4.12 (88%), 2.4.14 (90%).
198 Standard cleavage of the Cbz group gave the modified 2.4.12 and 2.4.14 for biological evaluation. A subsequent study examined solvent effects in the DAST fluorination reaction [44]. When the reaction was run in THF/ether instead of dichloromethane, 2.4.11 was the major product, accompanied by 10-12% of a new side product, the very interesting 6,7-dehydrotaxol analog 2.4.15 [44, 45] (Scheme 41). An analogous derivative was later prepared by Kelly and co-workers, using a C-7 triflate and effecting its base-promoted elimination in high yields [46].
AcO~, gz.
2.3.9
NH
,O
O
= phil'_
0 ....~
/
~
+ 2.4.11
OR
HO
: OAc OBz 2.4.15 R=Cbz ~ ii 2.4.16
R=H
Conditions: (i) 2 eq. DAST, THF/Et20, 2.4.15 (10%), 2.4.11 (45%); (ii) H2, Pd/C, EtOAc, 87%.
Note that saturation of the C-6/C-7 double bond was not possible even under forcing conditions [19]. A separate study deals with the fluorination of 7epi-taxol derivatives [47]. As shown in Scheme 42, with this substrate the DAST reaction produces only cyclopropane derivatives, no fluorination being detectable in this case. Further treatment of 2.4.13 with DAST gave A ringcontracted product 2.4.18. The lack of fluorinated products in this case suggests that attack of fluoride at C-7 from the ~ face is too hindered. Instead, an unusual participation of the trans-diaxially positioned angular methyl group ensues, leading to protonated cyclopropane 2.4.20 (Scheme 43). Similar intermediates probably occur in 1,2 Meerwein rearrangements. The deactivating effect of the C-9 carbonyl was postulated to prevent the completion of the 1,2 shii~, therefore leading to an isolable cyclopropane derivative. When the substrate was the 7(~)OH group, direct methyl participation is electronically unfavorable, and in this case (Scheme 44) the reaction proceeds probably through a carbonium ion, a
199 common i n t e r m e d i a t e t h a t easily explains the formation, in addition to 2.4.13, of the fluorinated compound 2.4.11 and the olefinic analog 2.4.15.
AcO BZ'NH Ph
O :
O OH ---~
~ O ....
=
OCbz
O HO
OAc OBz
2.4.17
AcO BZ'NH
0
\
Ph 'o. --
O
)
BZ'NH
/~%.
Ill
phil'_
I
--
oc z
AcO ~'
O
O /J
Ollll
ocbz
~ " " A: H OU c OBz
2.4.13
~
o
ii
2.4.18
Conditions: (i) 2 equiv DAST, CH2C12, rt, 2.4.13 (28%), 2.4.18 (31%),.; (ii) 4 equiv DAST,
CH2C12, rt, 2.4.18 (81%).
Bz. NH
AcO
O
phil__
0
~
.... 0 DAST=
O ....
OCbz
O
HO 2.4.17
BZ'NH
O
AcO ~
"
OBz
OAc
O
ph/~/[L'O,,, OCbz ' 2.4.13
H 2.4.19 X=OSF2NEt2
-X-I H H H+
0
0 = HO
: OAc OBz 2.4.20
H
200
O 2.3.9
X
DAS~
O O
2.4.21
-X ._-
+
H X=OSF2NEt2
$ methyl participation H H
~+ F
O
H
2.4.22
_H§
O 2.4.20
2.4.11
_H+
2.4.15
2.4.13
Formation of the 7(~)-fluoro derivative seems to be prevented in each case by the steric hindrance to approach of fluoride from the top face of the molecule. Some recent results by Klein have indirectly confirmed this mechanism: as shown in Scheme 45, when the C-9 carbonyl is absent in the substrate, and the C-7 hydroxyl group is activated as a transient triflate, a 1,2 methyl shift is observed, leading to skeletal r e a r r a n g e m e n t and eventually to B-ringcontracted product 2.4.25. Interestingly, a small amount of cyclopropanecontaining product 2.4.26 was found here also. The authors suggest that the peculiar conformation of the C ring may be at the origin of this very unusual cyclopropanation reaction [48]. Oxidation of taxanes with various agents was studied by Kingston [49]. Treatment of paclitaxel with chromic acid yielded first the C-7 keto derivative 2.4.27. Reacting this ketone with DBU in CH2C12 at room t e m p e r a t u r e or simply c h r o m a t o g r a p h y on S i 0 2 caused oxetane ring opening via ~elimination, leading to 2.4.28. Saturation of the 5,6-double bond and subsequent reaction with warm methanol led to lactone 2.4.29 (Scheme 46). Others have found that t r e a t m e n t of 2.4.27 with DBU leads not to 2.4.28, but to isomeric
201 enone 2.4.30, where the strong base has catalyzed 1,2 acetyl shift from C-4 to C20 [19, 33].
AcO ~,
-
OH
AcO
OH
fOH -I
AcO ....
i
= AcO ....
J, 0
0 HO
OBz 2.4.23
OAc 2.4.24
OAc
AcO .... ~ - ~ ~ , , , ,
~
AcO
Me
~]~
+
AcO ....
'~
OH
"" 0
HO
: OBz 2.4.25
OAc
HO
OBz 2.4.26
OAc
Conditions: Tf20, CH2C12, Py, rt; 2.4.25 (56%); 2.4.26 (8%).
Esterification at C-7 is a rather straightforward operation. Kingston reported that paclitaxel is rapidly acetylated at C-2', but C-7 acetylation requires more forcing conditions, involving DMAP and DCC. Selective 2'deacetylation under mildly basic conditions afforded C-7 acetyltaxol [50]. Baccatin III yields 7-acetyl baccatin under standard acetylating conditions (Ac20, pyridine), the C-13 position being somewhat more difficult to acetylate. A C-7 carbonate derivative of baccatin was described in the same paper [51]. Using similar methods, other workers have reported the synthesis of more complex esters, including water-solubilizing moieties, to be used as possible paclitaxel prodrugs [52-54]. Synthesis of sulfonate esters has also been reported under standard conditions [45, 55]. The same paper also reports the preparation of C-7 carbonates. While simple unhindered chloroformates react smoothly at C-7, more substituted ones react only with difficulty, if at all [19]. The same study [45] also describes the stepwise preparation of C-7 carbamates, which turned out to be rather challenging, since typical one-step procedures failed due to the
202 hindered nature of this secondary hydroxyl group. Two methods for the preparation of carbamates are shown in Scheme 47.
AcO i Paclitaxel 1.1.1
0 //
BZ'NH Ph
0
0
~
: OH
/j
0
0 ....
" 0 OBz
2.4.27
0
AcO BZ'NH
/,0
0
iii Ph
OH
-
0 ....
9
OH 0
2.4.?,8 2.4.29
AcO
O O
Bz" NH O
Ph~O OH 2.4.30
.... HO
Ac
Conditions: (i) Jones reag., acetone, 50%; (ii) Si02, 70%; (iii) Pt/C, MeOH, H2, 77%.
In the first case, the C-7 chloroformate is formed in s i t u and immediately quenched with the required amine. In the second case, pnitrophenyl carbonate 2.4.33 is readily prepared and isolated. Treatment with amines then yielded the desired derivatives [45, 55]. Silylation at C-7 (usually the sturdy triethylsilyl group is used) represents the preferred protection procedure for the C-7 hydroxyl. Standard fluoride deprotection regenerates the hydroxyl group [21]. Silylation of the 7(a) epimer of paclitaxel is difficult because of intramolecular hydrogen bonding between the 7-OH group and the C-4 acetate moiety, but it has been recently achieved by operating in highly polar solvents [45].
203
AcO ~,
Bz" NH O
O // OH hth/tt
p h ~ / [ l ' - _ O .... OCbz
O OBz
2.3.9
AcO ~
Bz" NH O
O // .OCONHBu
p h / ' ~ ~ _ O.... OH
O OBz
2.4.31
AcO
O
Bz" NH O
OH iv
ph/~~_ O .... OAIIoc
O HO
OAc OBz
2.4.32
AcO
O
O 10/NO2 O.~O
Bz. NH O p h ' / ~ ~ _ O.... OAIIoc AcO
BZ'NH P h i l _ : O O''' ,-
O HO
v, v i
O
~
//
OH
2.4.33
_OCONHR 0
-
OBz
" OAc OBz
2.4.34 2.4.35
R=(CH2)3CO2H R=(CH2)2NMe 2
Conditions: (i) COC12, Py; (ii) n-BuNH2, 72% overall; (iii) H2, PcYC, EtOAc, 90%; (iv) pNO2C6H4-OCOC1, Py, CH3CN, 78%; (v) H2N-(CH2)3-CO2All,THF, rt, 93%; or H2N-(CH2)2NMe2, THF, rt, 91%; (vi) Pd2dba3, CH2C12,PPh3, triethanolamine, rt, 68% for 2.4.34; 65% for 2.4.35 [Alloc= allyloxycarbonyl].
204 5.2.5.Reactions at C-9/C- 10 The ketone function at C-9 in paclitaxel is exceedingly resistant to many reagents that traditionally attack the carbonyl group, and consequently it has escaped t r a n s f o r m a t i o n until recently, when Commerqon reported its reduction by electrochemical means [56]. As s h o w n in S c h e m e 48, w h e n T a x o t e r e | was reduced eletrochemically, 9(a)-dihydrotaxotere 2.5.1 and 9([~)-dihydrotaxotere 2.5.2 were produced with very little stereoselectivity. Treating 7-epi-taxotere 2.5.3 under identical conditions led only to the 9(~)-dihydro derivative 2.5.4. On the other hand, when the electrolytic reduction was carried out at -1.95 to -1.90 V in the presence of CaC12, which is presumed to alter the electron density at the C-9/C-10 hydroxyketone moiety by forming a tight complex, C-10 deoxygenation was achieved in modest yield (vide infra).
HQ RI R2 OH B~ Taxotere 1.1.2
0
phil_
0 .... OH
0
HO
: OAc OBz
2.5.1 R 1 =OH, R2 =H 2.5.2 R1 =H, R2 =OH
H B~ 7-epi-Taxotere 2.5.3
OHoH
H O
ii
....
6H 2.5.4
o HO
OBz
OAc
Conditions" (i) E= -1.85V (SCE Hg cathode), MeOH, NH3, NH4C1; 2.5.1 (40%) and 2.5.2 (24%); (ii) same as i, 63%.
Klein et al. reported the synthesis of 9(a)-dihydrotaxol from the naturally occurring 13-acetyl-9(cx)-dihydrobaccatin III [11, 57]. As depicted in Scheme 49, the synthesis began with the protection of the C-7/C-9 diol moiety of 2.5.5
as an acetonide. The crucial C-13 deacetylation was achieved
205 chemoselectively in acceptable yield by the use of n-butyllithium under carefully optimized conditions. The paclitaxel side chain was then readily attached onto 2.5.7 according to Holton's protocol, affording 2.5.8 in fair yield. Final removal of the dimethylketal protecting group from 2.5.8 then furnished the desired 9(r dihydrotaxol 2.5.10 [11] in modest overall yield. The Abbott group has also reported the acylation of 2.5.7 with a large variety of side chains, bearing modified C-3' substituents, from which useful and extensive SAR information has been obtained (vide infra) [58].
Me O-A< 'Me i
AcO
OH
AcO
OH
-
0 i
AcO . . . . . . . . .
= RO .... O
HO
O
OAc OBz
2.5.5
HO
OAc OBz
2.5.6 R=Ac~ 2.5.7 R=H J i i
Aco
Me
Bz"~ NH 0 Ph
:
~
=iii'iv
,,,Ph
o/~N-Bz
0 .... 0
OH
2.5.8
EEO,%
HO
2.5.9
: OAc OBz AcO ~,
Bz. NH
OH OH
O
p h / ~ ' v ~ O _ .... OH 2.5.10
s HO
0 OBz OAc :
Conditions: (i) Me2C(OMe)2, CSA, 97%; (ii) n-BuLi, THF, -44~ 46%; (iii) n-BuLi, then 2.5.9; (iv) 0.5% HC1 in EtOH, 67%; (v) CSA, MeOH, 56% [EE=Ethoxyethyl].
206 Datta et al. recently reported the first example of enolization of the C-9 keto group in a fully functionalized taxane [59]. As shown in Scheme 50, t r e a t m e n t of 2.5.11 with potassium t-butoxide led to carbonate 2.5.12 in fair yield. This was t h e n converted into the corresponding paclitaxel and Taxotere | derivatives for biological evaluation. Many efforts have been devoted to the complete deletion of functionality at C-10 in order to examine its effect on biological activity. The first synthesis of 10-deoxytaxol was unexpectedly achieved by Chen et al. during their attemps to fluorinate such position. The synthesis began with 10-deacetyltaxol 2.5.15, which was obtained by Lewis acid-promoted methanolysis of paclitaxel [23] (Scheme 51). Treatment with trichloroethyl chloroformate then gave the 2',7diprotected derivative 2.5.16. Subjection of 2.5.16 to Yarovenko's reagent (C1FHCCF2NEt2) in CH2C12 at room temperature surprisingly yielded dienone 2.5.17, together with a small amount of a C-12 fluorinated enone [60].
os
TrocO O ~L /7 OTroc
\
i HO ....
e/SSso
O
HO ....
OBz
/
OTroc
9....
O OBz
2.5.12
2.5.11
/h//h/v,v
Ph~,,CO2H B o c - N ~ ,~. O 2.2.7
RH~____/~L
OH
Ph- ~' v0 .... OH
0
HO
: OAc OBz
2.5.13 R= PhCO 2.5.14 R= t-BuOCO
Conditions: (i)t-BuOK, THF, -30 ~ to 0 ~ (58%); (ii) 2.2.7, DCC, DMAP, PhMe, 70 ~ 62%; (iii) HCO2H, rt (71%); (iv) PhCOC1, NaHC03 or (Boc)20, NaHCO3 (62-72%); (v) Zn, AcOH, MeOH, 60 ~ 65% for 2.5.13, 69% for 2.5.14.
207 This reaction is remarkable since Yarovenko's reagent is a fluorinating reagent, and dehydration products are rarely obtained. Removal of the protecting group from 2.5.17 was readily accomplished with zinc in a mixture of acetic acid and methanol, to afford 2.5.18 in high yield. Finally, 2.5.18 was found to undergo smooth catalytic hydrogenation to afford 10-deoxytaxol 2.5.19. Owing to its exciting biological activity, 10-deoxytaxol has been the subject of several investigations. For example, Chen at al. reported that defunctionalization at C-10 can be obtained by radical methods using 10thionocarbonates [16a]. As shown in Scheme 52, 10-deacetyl baccatin is an appropriate s t a r t i n g material. When C-7 is protected, thionocarbonate formation proceeds at C-10 vs. C-13 with excellent selectivity. Barton deoxygenation affords 2.5.22, and acylation according to Holton then affords 10deoxytaxol in good overall yield [19]. An analogous C-10 deoxygenation, utilizing a xanthate, was described by Kingston [61].
HO gz..
NH
O
OR
0
phil_
ii
O ....
OR HO 2.5.15 R=H 2.5.16 R=Troc) i
l
O " OBz
OAc
0
BZ'NH
0
phil__
0 ....
OR
0 BZ'NH 0 phi_: 0 .... OH 2.5.19
OBz 2.5.17 R=Troc,~ 2.5.18 R=H 2 i ii
OH
0 HO
iv
-OAc OBz
Conditions: (i) TrocC1, Py, CH2C12, 0 ~ 46%; (ii) Et2NCF2CHFC1, CH2C12, 47%; (iii) Zn, MeOH, AcOH, 40 ~ 81%; (iv) H2, PcYC,EtOAc, 68%.
208 Efforts have been made at deleting the C-10 acetoxy group directly from paclitaxel. Since the C-10 acetoxy group is a doubly activated moiety (i.e. allylic and cz-keto) its removal may be achieved in principle by a direct Barton deoxygenation reaction. Indeed, Chen et al. reported t h a t t r e a t m e n t of 7-epitaxol 2.4.1 with 6-8 equivalents of tributyltin hydride and AIBN in toluene at 100~ afforded directly the corresponding 10-deoxy derivative 2.5.23 in excellent yield (Scheme 53) [62]. When paclitaxel was treated under the same conditions, however, only 2.5.23 was obtained in 39% yield, together with some 7-epi-taxol and unreacted starting material, thus suggesting that the function at C-7 plays a role in this radical deoxygenation [62].
H
,~O OTES
R
O
OTES
i
HO ....
=
HO ....
O
O HO
OAc OBz 2.5.20 R=OC(S)OC6F5 ii 2.5.21 R=H ,,/ 0 OR
OBz 2.1.19
iii
BZ-NH ph N - : ~ _ .
0 ....
TESO~I'Ph OR O Bz 2.1.10
0
HO 2.5.22 R=TES 2.5.19
R=H
OBz J]
OAc
iv
Conditions: (i) n-BuLi, THF, -40 ~ then C6F5C(S)C1, -20 ~ 74%; (ii) Bu3SnH, AIBN, PhMe, 90 ~ 99%; (iii) n-BuLi, THF, -40 ~ then 2.1.10, 0 ~ (iv) dil. HC1, CH3CN, 0 ~ 76% overall from 2.5.21 to 2.5.19.
The first one-step synthesis of 10-deoxytaxol directly from paclitaxel was reported by Holton using SmI2 as the reducing agent [63]. Similar chemistry was also reported by other authors [64, 65]. Electrochemical conditions were
209 also applied successfully to the reduction of C-10 acetoxy moiety, as mentioned above [56]. Scheme 53 / gz.
7-epi-taxol 2.4.1
z~ NH
O_H
0
ph/~~_
0 ,, O
OH HO
: OBz
OAc
2.5.23 Conditions: (i) Bu3SnH, PhMe, AIBN, 100 ~
88%.
When the SmI2 deoxygenation was conducted with an excess of the reagent and for prolonged periods of time, 10-deoxygenation and C-9 reduction were reportedly achieved simultaneously. Taxotere | gave instead a mixture of two products, since in this case the C-10 hydroxy is not as good a leaving group as the acetoxy and direct carbonyl reduction can compete with (~-reduction. Once the C-9 carbonyl is reduced, C-10 deoxygenation cannot obviously occur, and 2.5.25 is produced, along with the expected 2.5.26 (Scheme 54) [64]. Using 9-dihydrobaccatin III as a starting material, Klein and coworkers reported, in preliminary form, the selectively deoxygenation at C-9 using t r a d i t i o n a l Barton chemistry, as well as C-7 and C-9 double deoxygenation. Their chemistry is highlighted by the elegant synthesis of 7,9,10-trideoxytaxol, 2.5.31, the analog with the most defunctionalized northern half prepared to date (Scheme 55) [17]. Details of this chemistry have not yet appeared. Selective C-10 deacetylation in paclitaxel is not a facile operation. Kingston et al. reported that t r e a t m e n t of paclitaxel with zinc bromide in methanol yielded 10-deacetyltaxol in low yield together with its C-7 epimer [21]. Chen et al. reported a C-10 deacetylation study in which several other Lewis acids were examined, without substantial improvements [23]. However, with the ready availability of 10-deacetyl baccatin III as a convenient source of bioactive taxanes, this synthetic operation is no longer synthetically important. Although acylation at C-10 is well precedented [4, 52] the first general approach to C-10 modified taxanes was reported only recently by Kant [66].
210 Ethers, esters, carbonates, carbamates, and sulfonates were all prepared in good yields under mild conditions (Scheme 56).
BZ'NH 0
\
OHoH
mh Oo....
Paclitaxel 1.1.1
!o HO
OBz
2.5.24
Boc..
.o,, 2"o.
NH 0
phil"_
O,
OH
o
2.5.25
i
Taxotere, 1.1.2
.
B~
0
phil_
+
OHoH
0 .... oH
HO 2.5.26
Conditions:
-
OBz
(i) SmI2, 83% for 2.5.24; 40% for 2.5.25; 50% for 2.5.26.
As outlined in Scheme 56, 2.1.19 was treated with 1.05 equiv of n-BuLi at -40 ~ in THF, followed by the addition of 1.2 equiv of the electrophile. Derivatives 2.5.32 were then directly acylated with the paclitaxel side chain (~-lactam method) for biological evaluation. 5.2.6. Reactions at C-13 Modification of the C-13 position is a critical operation that may profoundly affect the biological activity, due to the important role of the phenylisoserine side chain. The C-13 hydroxyl group of baccatin III is often silylated in order to protect it from functionalization during complex synthetic operations. Use of the TMS and
211 TES blocking groups has been exemplified widely t h r o u g h o u t the chapter. Acetylation in pyridine was described as requiring h a r s h conditions by several workers [51, 67], but more recently such acetylation could be carried out at room temperature in CH2C12 with acetic anhydride [23]. Carbonates have also been used at C-13 as protecting groups, especially the convenient Troc group [67]. Oxidation of 7-TES Baccatin III at C-13 using MnO2 affords the corresponding enone, which can be reduced back to the baccatin derivative with borane [68]. The various methods that have been discovered to introduce the phenylisoserine side chain at C-13 are described in Chapter 6. [ Scheme 55 SMe S AcO ~, AcO ....
:
OH
AcO ---~0 ~, --"
OH i _
sSess
ii
AcO ....
2.5.5
2.5.27 HO ~,
H O ....
OH
iii
9.....
HO .... ~
iv
HO
2.5.28
gz..
v
OH
2.5.29 NH
0
\
.... < Cbz-N~
0
2.5.30
HO BzO 2.5.31
Conditions: (i) LiHMDS, CS2, then MeI; (ii) Bu3SnH; (iii) MeLi, then CS2, MeI; (iv) Bu3SnH;
(v) 2.5.31, then H2, Pd/C, then (PhCO)20, aq. MeOH (no solvents, temp. nor yields).
212
Scheme 56 / HO
HO ....
0
//
RO
.OTES
r
HO....
0
/I
OTES
r
0
0
OBz
OBz
2.1.19
2.5.32 Electrophile
R
Yield(%)
AcC1 BzC1 n-BuCOC1
COCH3 COC6H5 COBu-n
c-C3H5COC1 MeOCOC1
COC3H5-c 78 CO2Me 75
M e2SO4 Me2NCOC1 PhNCO MeSO2C1
Me CONMe2 CONHPh SO2Me
90 85 75
85 72 78 68
Conditions: (i) n-BuLi, THF,-40 ~ then electrophile,-40 ~ to 0 ~
5.2.7. Reactions at C-14 Recently, Appendino et al. reported the isolation of 14([~)-hydroxy-10deacetylbaccatin 2.7.1 from the needles of T a x u s w a l l i c h i a n a Zucc [69]. Due to the presence of an additional hydroxyl group at the C-14 position, the new taxanes derived from 2.7.1 upon C-13 acylation can be expected to possess substantially improved water solubility vs. paclitaxel and docetaxel, and perhaps also better in vivo antitumor activity. With this in mind, two groups set out to prepare 14(~)hydroxytaxol and 14(~)-hydroxytaxotere, as well as a number of related analogues [12, 13, 70]. The relative reactivity of the four hydroxyl groups in 2.7.1 was independently studied by Kant and Ojima [12, 13]. It was found that the reactivity of these groups toward acylation decreases in the order C-7 > C-10 > C-14 > C-13. Therefore, the a t t a c h m e n t of the phenylisoserine side chain to the C-13 position requires appropriate protection at C-7, C-10 and C-14. Toward this end, 2.7.1 was converted
213 into the 7,10-diprotected derivative 2.7.2. C-1 and C-14 were t h e n protected as carbonate 2.7.3, orthoformate 2.7.4, or acetonide 2.7.5, as shown in Scheme 57.
,,," H
HO ~
0 /f
O_H
H ,,
TrocO ~,
0 //
OTroc
i
O,
0
HO
OH
: OBz
_
O~
OAc
OBz
TrocO
2.7.1
O
2.7.2
OTroc
HO ....
i i , or iii, or i v
0
O RO
OR'
: OAc OBz
2.7.3 R,R'=C(O) 2.7.4 R,R'=CH(OEt) 2.7.5
Conditions: (i) 4 equiv TrocC1, py, 80 ~ 55%; (ii) 2 equiv TrocC1, py, 80 ~ TsO)20, 92%; (iv) 2,2-Dimethoxypropane, (p-TsO)20, 89%.
HO
0
R,R'=C(Me) 2
75%; (iii) CH(OEt)3, (p-
OTES
i
ii
2.7.1
~
HO .... 0 HO
HO
O
OTES
HO ....
OH
O
y
EtO
O
OAc OBz 2.7.7
OAc
2.7.6 iii
O
" OBz
RO
20TES
= HO ....
or i v
0
Oyo EtO
OBz
OAc
2.7.8 R=Ac 2.7.9 R=TES
Conditions: (i) TESC1, imidazole, DMF, 92%; (ii) CH(OEt)3, PPTS, THF, 90%; (iii) LiHMDS, AcC1, THF, 0 ~ 75%; (iv) LiHMDS, TESC1, THF, 0 ~ 78%.
214 Alternatively, the C-7 hydroxyl group was protected as a triethylsilyl ether, affording 2 . 7 . 6 in high yield. Compound 2.7.6 was next converted to 1,14orthoformate 2.7.7. C-10 acetylation yielded 2.7.8, and silylation afforded 2.7.9 (Scheme 58). Using protected baccatin derivatives 2.7.4 and 2.7.9, 14(~)-hydroxy-taxotere 2.7.11 was readily obtained in good overall yield using the protocols illustrated in Scheme 59. In addition, cyclic carbonate analog 2.7.12 and acetonide 2.7.13 were also prepared in a similar fashion. Similarly, the coupling between 10-acetoxybaccatin derivative 2.7.8 with the appropriate ~-lactams 2.1.10 and 2.4.6 afforded the desired 14(~)-hydroxytaxol 2.7.14, and 10-acetyl-14(~)-hydroxytaxotere 2.7.15 for biological evaluation (Scheme 6O).
2.7.9
Kant i, i ~ iii
RO,,, 9
iII
Ph
2.7.4
N "Boc 2.4.6 R=TES 2.7.10 R=CH(Me)OEt
I
OJ--'
~,~ 0 II t-BuO ''~k" NH
HO O
I /
O
OH
\
OH 2.7.11
Ojima iv, v, vi
0 HO
O" B z
OH
OAc
O t-BuO"JJ~NH p h i - i l l _"
O
HO
O
OH
0 .... 0
RO
OR'
OBz
2.7.12 2.7.13
R,R'=CO R,R'=CMe 2
OAc
Conditions: (i) LiHMDS, THF, 0 ~ then 2.4.6, 75%; (ii) 10 N HC1, CH3CN, -5 ~ (iii) NH4OH, THF, 0 ~ 67% overall. (iv) NaHMDS, THF, -40 ~ then 2.7.10, then 0.5% HC1, EtOH, rt, 96%; (v) HCOOH, dioxane, rt, then THF, MeOH, NaHCO3, rt, 73%; (vi) THF, 0.5 N HC1, Zn, 0 ~ 73%.
215 I n t e r e s t i n g l y , as shown in Scheme 61, direct acylation of 2.7.2 u n d e r s t a n d a r d conditions yielded two novel C-14 side chain-bearing t a x a n e derivatives (2.7.16 and 2.7.17) in fair yield. This observation is in a g r e e m e n t with the finding t h a t the hydroxyl group at C-14 can be acylated more readily t h a n the one at C-13 [13]. Scheme 60 /
O
AcO
O
.
(i, i i ) or [i i i, i i) 2.7.8 TE SO,,,,
,,, Ph
oJ-' N. R
ph~/[l"O. OH
.... 0 HO
2.1.10 R=Bz 2.4.6 P~Boe
OH
2.7.14 R=Ph 2.7.15 R=t-OBu
: OBz
OAc
Conditions: (i) LiHMDS, THF, -40 ~ then 2.1.10, 65%; (ii) 10 N HC1, CH3CN, -5 ~ then NH4OH, THF, 0 ~ 55% for 2.7.14, 62% for 2.7.15; (iii) LiHMDS, THF, -40 ~ then 2.4.6, 75%.
,,"
HO
~, //O O.H
i, or ii, then iii 2.7.2
OH
I
~
N. R
2.7.18 R=Bz 2.7.10 R=Boc
RC(O)HN
O
O=z
2.7.16 R=Ph 2.7.17 R=t-OBu
Conditions: (i) NaHMDS, THF, -40 ~ then 2.7.18; (ii) NaHMDS, THF, -40 ~ then 2.1.10; (iii) Zn, AcOH, MeOH, 52% overall for 2.7.16; 50% overall for 2.7.17.
5.2.8. Skeletal r e a r r a n g e m e n t s This section will discuss baccatin,
some interesting skeletal r e a r r a n g e m e n t s
of
m o s t l y concerning the A and B rings. These r e a r r a n g e m e n t s are
usually initiated by radicals or carbonium ions. In addition, a few recent reactions t h a t are more properly classified as degradation reactions will also be discussed. A n u m b e r of radical-based deoxygenation reactions were carried out on baccatin III derivatives [16a].
In this connection, it was discovered t h a t formation of a
216 radical at the C-7 position of the baccatin core results in a complex skeletal rearrangement (Scheme 62). Tributyltin hydride-mediated deoxygenation of 2.8.1 gave 52% of the desired 7,10-dideoxy baccatin III 2.8.2 and 25% of its tetracyclic isomer 2.8.3 [16a]. Other radical conditions were examined, and the distribution of products characterized. When Ph3SnH was used as the reducing agent, in addition to 2.8.2 and its isomer 2.8.3 (ratio, c a . 3:1), methyl ether 2.8.4 was also isolated in 30% yield; when (TMS)3SiH was employed as the reducing agent, in addition to 2.8.3, two new products, enol acetate 2.8.5 and C-12 exomethylene derivative 2.8.6, were also obtained (Scheme 63). The formation of 2.8.3, 2.8.5 and 2.8.6 can be rationalized by invoking a cascade of radical rearrangements, as shown in Scheme 64.
O
Scheme 62 t O IIII
O
\
0
)CS2Me HO
HO ....(
+
0
HO
: OAc OBz O
: OAc OBz 2.8.1
0 Me
HO ....~
Conditions:
2.8.2
O HO
A c = OBz
2.8.3
(i) Bu3SnH, AIBN, PhMe, 80 ~ 2.8.2 (52%); 2.8.3 (25%).
The initially formed radical 2.8.7, a ~-keto radical, can isomerize, via alkoxy radical 2.8.8, to 2.8.9. This places the radical-bearing C-8 at a close distance with respect to the C-11/C-12 double bond, and a 5-exo cyclization to 2.8.10 takes place. Surprisingly, this radical is not quenched by the tin hydride, perhaps due to the hindered nature of the radical-bearing C-12. The major pathway for radical 2.8.10 is the remote intramolecular hydrogen abstraction of H-3, to provide 2.8.11. Radical 2.8.10 also suffers a disproportionation reaction to give the minor product 2.8.6 only when (TMS)3SiH was employed as the reducing agent. Radical 2.8.11 is evidently also sterically hindered toward direct reduction, and undergoes an unusual oxetane fragmentation reaction to give 2.8.12. The resulting a-alkoxy
217 radical is then trapped by tributyltin hydride to yield 2.8.3. When the reducing agent used is tributyltin deuteride, the product is specifically labeled only at the C-5 methoxy group [16a]. However, in the case of the (TMS)3SiH reduction, in addition to 2.8.3, alkoxymethyl radical fragmentation with loss of formaldehyde gives allylic radical 2.8.13, presumably due to slow trapping of 2.8.12 by the rather unreactive silane reagent. After this cascade of six sequential intramolecular reactions, radical 2.8.13 is finally quenched by the silane to give 2.8.5 (Scheme 64).
O i or i 2.8.1
"
i= H O ....
I +
O IIII
jO %
or i i i HO
OAc OBz
HO
2.8.2
OBz OAc 2.8.4
0
O
H,,,~~~L~ +
HO
OMe ....~
HO
HO":
+
c
I
: OBz 2.8.3
HO O
H
HO ....
~
= OBz 2.8.5
OAc
O HO
= OBz
2.8.6
Conditions: (i) Bu3SnH, AIBN, PhMe, 80 ~ (ii) Ph3SnH, AIBN, PhMe, 80 ~ (iii) (TMS)3SiH, AIBN, PhMe, 90 ~ The proposed mechanism for the C-7 methyl ether formation is shown in Scheme 65. Addition of the triphenyltin radical onto xanthate 2.8.1 leads to the unstable intermediate 2.8.14. Usually, this intermediate undergoes ~-scission to afford a carbon radical. However, in this case the highly reactive hydrogen donor triphenyltin hydride was able to trap 2.8.14 to form 2.8.15.
218
0
2.8.1
"H
~HO'"
= HO'"
0 Ho BzO
OAc
BzO
2.8.7 0 HO"
H OAc
2.8.8
H
0
~
ai
H Ola0a
-
H"
~
HO BzO
OAc
HO BzO
2.8.9
~
OBz
H OAc
2.8.10
0
H ,,~ , ? - - %.,~, ~ H17-HO ....(, ~ ~
2.8.6
0
)~
.w,,, ._ H','" - HO"'
H
OAc
OBz
2.8.11
.
CH20 ] T
HO .... ~
2.8.13
c 2.8.12
0
/N;,,"~
HO
+H" OCH-------~2.8.3
: OBz
/) "
+H"
~ 2.8.5
OAc
Elimination of triphenyltin thiomethoxide gave C-7 thionoformate 2.8.16. Further reduction by excess triphenyltin hydride gave thioacetal 2.8.18, which was finally converted to methyl ether 2.8.4 through another C-S bond cleavage reaction. The same study [16a] describes a series of complex radical rearrangements that arise via C-1 and C-2 carbon radicals. As shown in Scheme 66, treatment of cyclic thiocarbonate 2.8.19 with tributyltin hydride and AIBN failed to give either a C-1 or a C-2 deoxy
derivative. Instead, after t r e a t m e n t of the crude product
mixture with trifluoroacetic acid, two new products, 2.8.20 (major) and 2.8.21 (minor), were obtained. Inspection of the 1H-NMR spectra dearly showed that both
219 compounds were the results of skeletal rearrangements. Their structures were confirmed by X-ray crystallography.
SSnPh3 O O+ SMe + Ph3Sn"
+ H"
~ HO,,,
2.8.1
0
BzO
SSnPh3
S
2.8.14
O O~J~H
O O+SMe HO,,,'~
. Ph3SnSMe ~HO....
H O
HO BzO
O H
OAc O O -''~
2.8.15
+ Ph3Sn"
2.8.16
SSnPh3
~ ~ HO,,,
OBz
+ H" O HO BzO
OAc 2.8.17
~
. / ~ ~ ~ ~~SSnPh3
, ~
~"~
H O ....
+ Ph3SnH=__
0 \{/. H O .... .
'---f.-.4/__"H HO
BzO
2.8.18
OAc
O HO
" BzO
OAc
2.8.4
A mechanistic rationale for the observed products is presented in Scheme 67. The initial adduct resulting from addition of the tributyltin radical to 2.8.19 apparently leads to both of the two conceivable fragmentation products, radicals 2.8.22 and 2.8.23. It is likely that intermediate radical 2.8.22 is hindered by the presence of its neighboring tributyltin thiocarbonate residue at C-l, and consequently it is not rapidly trapped by tin hydride to yield the corresponding 2-deoxybaccatin derivative
220 as seen with C-2 xanthates. Instead, intramolecular processes, as already seen for the C-7 radical, take over and 2 . 8 . 2 2 undergoes a t h e r m o d y n a m i c a l l y unfavorable and quite unusual 4-exo cyclization to the cyclobutylcarbinyl radical 2.8.24. Since the cyclobutylcarbinyl radical is highly unstable, opening with concomitant ~-elimination leads to 2.8.26 and, after desilylation, 2.8.20. 00TES
[ Scheme 66 TESO,,,,~
0 -
"~0
S
0
HO ....
I
Ac 2.8.19
~ii 0
OH +
OH
HO .... 0
/ 2.8.20 Conditions:
6H oAo 2.8.2
(i) Bu3SnH, AIBN, PhMe, 100 ~ 77%; (ii) CF3CO2H, THF, H20; 2.8.20, 42%; 2.8.21,
8% overall. Radical formation at C-1 is also apparently taking place in a competitive fashion: radical 2.8.23 undergoes a cyclopropylcarbinyl r e a r r a n g e m e n t to yield, following hydrolytic deprotection, A ring-contracted product 2.8.21. In addition to radical-initiated ones, cationic r e a r r a n g e m e n t s are very common in taxane chemistry. We have already discussed the A-ring contraction reaction apparently initiated by C-1 carbonium ion formation (Scheme 4) [21]. Rearrangements initiated by C-7 carbonium ion formation lead to the already discussed cyclopropane derivatives (Scheme 42) [47, 48] and, in the case of 9dihydrotaxanes, to B-ring contracted analogs [48], as shown in Scheme 45. These B-ring contracted taxanes could be deprotected at C-13 and acylated under standard conditions for biological evaluation [48].
221 Whereas Lewis or Bronsted acid treatment of taxanes leads to A-ring contraction, as already discussed, recently Khuong-Huu et al. reported an alternative ring enlargement reaction that proceeds under acidic conditions. ES
TESO ....(X
/~'.._ .~. ~J~ 0,~6
Bu3Sn'/
/
S
OAc Bu3Sn"
2.8.19
i~ OTES TESO ....
T E S O , , , ~
o_ 9 OAc ~--SSnBu 3 O [ 2.8.22
Bu3SnS~/F6
~
~T-SSnBu3 O 2.8.24
O 1
2.8.23
TESO,,
OAc
TESO ....(k
I~'.. 4 . . L
.... -H/_ ~ ._-~o
Bu3SnS.~ ~ 2.8.25
OAc
O I +H
+"1 O OR RO ....
RO"' O
2.8.26 R=TES ) 2.8.20 R=H
2.8.27 R=TES) 2.8.21 R=H
222
Scheme 68 I AcO
AcO
OAc OAc
~,
OAc
.--
OAc
ii
HO ....
AcO ....
O
O OH
2.8.28 -
HO~'
OAc
OBz
AcO
HO
OH
2.8.29 OAc
S
OAc
OAc
/OAc
-
......
OH
,
OAc
....._
H 0 -
2.8.30
"'-0--/OH 2.8.31
-
HO ....
0
&o Ph
2.8.32
Conditions: (i) KCN, MeOH, rt, 65%; (ii) Camphorsulfonic acid, DMF, C6H6,reflux, 90%.
As shown in Scheme 68 [71], t r e a t m e n t of peracetylated baccatin derivative 2.8.28 with KCN in methanol led to deacylation at C-13, C-2 and C-4, affording 2.8.29. Tetrol 2.8.29 was w a r m e d overnight in DMF-benzene solution in the presence of a catalytic amount of camphorsulfonic acid, leading to r e a r r a n g e d product 2.8.31, which contains a 10-membered ring and a tetrahydrofuran ring, presumably via intermediate 2.8.30. A similar r e a r r a n g e m e n t of the oxetane to the tetrahydrofuran system was described in Schemes 5 and 6 (Section 6.2.1). It is unclear why the usual A-ring contraction does not occur here, and the authors do not offer any explanation.
223 However, it seems quite reasonable to explain this result by postulating that the Aring contraction requires the presence of the C-2 benzoate group and proceeds not via a C-1 carbonium ion, but via acyloxonium ions like 2.8.32. In the absence of an ester group at C-2, carbonium ion formation at C-1 is not a facile process and the pathway described above predominates. Another very complicated r e a r r a n g e m e n t , involving migration of C-2 benzoate and opening of the oxetane ring, is observed in the 14(~)-hydroxy baccatin series, and is outlined in Scheme 69 [72].
HO
20TES
HO....
0
" HO HphOh ~:~,~
HO~'
0 =~
OAc : H+ 2.7.6
~/
20TES
-- HO....
HO....
",,S
0
o.~o+ ~ O,~o~ Ph 2.8.34
~zo
o. o[2~ 2.8.35
I
0%. /? OTES Ho
.o
%./ _/? OTES ....
....
Bz
o, ~ H0~"~?o....--+h o. 0 2.8.86
BzO/
~
~
d"") ~
2.8.37
When 2.7.6 was refluxed in benzene in the presence of a catalytic amount of pyridinium p - t o l u e n e s u l f o n a t e (PPTS), rapid d i s a p p e a r a n c e of the s t a r t i n g
224 material took place, and 2.8.37 was isolated in 40% yield. A plausible mechanism for this r e a r r a n g e m e n t is also depicted in Scheme 69. Migration of the benzoate to C-14, presumably via C-1/C-2 and C-1/C-14 oxonium ions 2.8.33 and 2.8.34, triggers the Wagner-Meerwein A-ring contraction. This is accompanied by the already discussed C-4 assisted opening of the oxetane ring, eventually yielding the stable orthoformate 2.8.37 by trapping of 2.8.36 by the unacylated C-2 hydroxyl. A different type of A ring contraction, this time accompanied by B ring expansion, was recently reported by Appendino and co-workers [73]. As shown in Scheme 70, when 7-triethylsilyl-10-deacetylbaccatin 2.1.19 was treated with excess MnO2, cyclopentenone derivative 2.8.40 was obtained in low yield, in addition to the desired 13-keto product 2.8.38. The authors postulate the intermediacy of adiketone 2.8.39, which undergoes an a-ketol rearrangement. The driving force for this ring contraction may be the release of the angular strain due to the presence of four adjacent sp 2 centers in 2.8.39.
HO
O
OTES
H O .... O HO
OBz
O
: OAc
O HO
2.1.19
: OBz
": OAc
2.8.38
HO
0
OTES
O O O HO 0
OBz
OAc
2.8.40 Conditions: (i) MnO2, EtOAc, CH2C12,rt; 2.8.38 (40%)+ 2.8.40 (24%).
OBz
2.8.39
-: OAc
225 In order to assess the contribution of an i n t a c t A ring of the baccatin framework to the a n t i t u m o r activity, Ojima and co-workers prepared a novel class of nor-seco paclitaxel analogues 2.8.44 and 2.8.45, as shown in Scheme 71. The synthesis began with the oxidative cleavage of the A ring of 2.7.1, a n a t u r a l product isolated from T a x u s w a l l i c h i a n a Zucc [69], with periodic acid. This gives 2.8.41, which i m m e d i a t e l y cyclizes in situ to provide the h e m i k e t a l 2.8.42. The C-7 hydroxyl group of 2.8.42 was selectively protected as the triethylsilyl ether, and the aldehyde moiety was reduced with sodium cyanoborohydride to afford 2.8.43 in fair yield. Final side chain a t t a c h m e n t onto 2.8.43 was performed using the ~-lactam approach to provide the desired nor-seco paclitaxel and docetaxel analogs 2.8.44 and 2.8.45 [74] (Scheme 71).
HO
H 0'"'
O
HO
-
OHC
i
~
,'
~
o
HO
OH
:
"
0
OBz OAc
2.8.41
2.7.1
,4~
OHC
BzO C)Ac
OH
OTES
rO
O
i~, iii
HO
,,,..-
,
O O
O H()
HO
BzO (gAc
2.8A3
2.8.4,2
R'NH
iv, v
~ /0,,,, OEt 0
,,,Ph
)_, N.
_O. ~
- pha-2-_ OH
COR
2.7.18 R=Ph 2.7.10 R=OBu-t
BzO C)Ac frO
.....J
0
O.H
~
~
-.:'H HO
~
BzO C)Ac
2.8.44 R=Bz 2.8.45 R=t-Boc
Conditions: (i) H5IO6, 92%; (ii) TESC1, NEt3, DMAP, 76%; (iii) NaBH3CN, pH 6. 80%; (iv) NaHMDS, THF, -40 ~ 2.7.18, or 2.7.10; (v) 0.5% HCI, rt; then Bu4NF, THF, -10 ~ 82% for 2.8.44; 58% for 2.8.45.
226 Recently, Khuong-Huu and co-workers reported on the chemistry of oxidative cleavage of the B ring of baccatin at the C-1/C-2 and C-9/C-10 segments [75]. Since the reverse operation is often the key step in total syntheses aimed at the paclitaxel skeleton [8-10], the value of this work is in providing valuable materials for the study of these reductive cyclizations. The debenzoylation of C-2 was described in section 5.2.1. A typical product of these studies, 2.1.20, was protected at C-13 as a triethylsilyl ether, giving 2.8.46, which was then subjected to sodium metaperiodate oxidation, affording 2.8.47 in good yield (Scheme 72).
ES
Ro ....k
7."-,,_ ,L Hal"
_= - O H
2.8.46
R=TES J
HO 2.1.20 R=H
Conditions:
00TES
b
T SO,,,
0
-
OH i
2.8.47
(i) TESC1, py, rt, 100%; (ii) NaIO4, EtOH, pH 5 buffer, 75%.
The C-9/C-10 bond cleavage reaction is shown in Scheme 73. 10-Deacetylbaccatin III, 1.1.2, was selectively protected to yield 2.8.48 in two steps. The removal of the carbonate groups and the subsequent C-7 epimerization were achieved on treatment with zinc dust and DBU, respectively, providing 2.8.49 in excellent yield. Reduction of the C-9 keto moiety of 2.8.49 was effected remarkably well with BH3SMe2 in toluene, producing 9(~)-dehydrobaccatin derivative 2.8.50. The stereochemistry at C-9 was confirmed by NOE studies. Oxidative cleavage of the C-9/C-10 bond of 2.8.50 led to lactol-aldehyde 2.8.51, as a single isomer, in almost quantitative yield [75]. 5.2.9. Photochemistry No information on the photochemistry of paclitaxel was available until recently. During the course of the development of paclitaxel as a commercial antitumor drug, paclitaxel was subjected to a series of stability tests, including exposure to sunlight. In this test, traces of a paclitaxel isomer were isolated by s e m i p r e p a r a t i v e HPLC. After extensive NMR studies, the compound was
227 identified as the pentacyclic paclitaxel isomer 2.9.4 (Scheme 74), containing a new bond between C-3 and C-11 [76].
H
TrocO
OH
O OTroc
\ HO .... ( ~
Iii, i v
Lii
,~,,.
AcO ....
= O
0 HO
BzO
HO
OAc
" BzO
OAc
2.8.48
1.1.2
HO
AcO'"
AcO"'
OHoH
)
v, O
O HO
OAc
2.8.,50
2.8.49
o%IHoH OH
Aco,,,o .
" BzO
: BzO
2.8.51
OAc
Conditions: (i) TrocC1, py, 80 ~ 93%; (ii) Ac20, py, DMAP, rt, 100%; (iii) Zn, MeOH, reflux; (iv) DBU, PhMe, 80 ~ 80%; (v) BH3-SMe2, PhMe, 0 ~ 84%; (vi) NaIO4, EtOH, pH 5 buffer, rt, 97%.
Since very limited amounts of 2.9.4 could be produced by sunlight exposure, a more efficient method for further studies was highly desirable. An approach to this problem utilized a photochemical reactor. A very good conversion (55%) of paclitaxel into 2 . 9 . 4 was achieved. M e c h a n i s t i c a l l y , this r e m a r k a b l e photochemical transformation can be considered to follow in part the well-known oxa-di-~-methane r e a r r a n g e m e n t (Scheme 74). Nakanishi [77] was the first to describe a similar bond formation between C3 and C-11 in taxinine. However, taxinine differs from paclitaxel in that an enone chromophore is present at C-11/C-12/C-13, and it is undoubtedly the excitation of
228 this f u n c t i o n t h a t i n i t i a t e s the r e a r r a n g e m e n t . In the p r e s e n t case, the photoexcited moiety m u s t be the ~,y-unsaturated ketone, and the m e c h a n i s m shown in Scheme 74 was proposed.
AcO BzHN
O"
0
OH
hv
Paclitaxel 1.1.1
=
p h ~ . - J l_' - 0 .... OH 2.9.1
AcO~ BzHN
O"
" OBz
OAc
OH
0
Ph~O OH
H3-shift
.... 0 OBz
2.9.2
H
BzHN/~.flL_ H Ph
/
0
HO
,~
A
/.,,.., O H
O\1)..
~
Radical
_ O.... OH
Recombination
O : OAc OBz
HO
2.9.3
AcO zHm
o
-
0 OH
k H'
--
2.9.4
O IIII
OH
O HO
: OAc OBz
The excited state initiating the r e a r r a n g e m e n t m u s t be the TI(~,~*) of the C9 carbonyl group, which is r e p r e s e n t e d as the diradicaloid species 2.9.1, as postulated in the first step of the oxa-di-~-methane r e a r r a n g e m e n t . Diradicaloid 2.9.1 then r e a r r a n g e s to cyclopropylcarbinyl radical 2.9.2, and at this point the intramolecular hydrogen t r a n s f e r from C-3 to C-12 occurs, in the same vein as
229 described in section 5.2.8 w h e n discussing radical r e a r r a n g e m e n t s .
Finally,
t r a n s a n n u l a r bond formation in 2.9.3 leads to 2.9.4. Although the above m e c h a n i s m is reasonable on the basis of the literature, a more comprehensive study addresses the issue of w h e t h e r the C-9 keto group is directly excited, or w h e t h e r some of the a r o m a t i c groups in the molecule are involved in the a b s o r p t i o n and s u b s e q u e n t i n t r a m o l e c u l a r
energy transfer.
Confirmation of this possibility was sought by indirect m e a n s , i.e. each of the aromatic groups in the molecule were in t u r n deleted, and the effect on the yield of the pentacyclic products was examined [78].
R2
O
R2 I
,',H'"' ,
R1
//
O
al
hv ,..._
R 3 .....
R3
, O
O HO
OBz
HO
OAc
" OBz
OAc
2.2.4
R1 = OTES R 2 = OAc, R 3 = OH
2.9.6 (23%)
2.1.19
R1 = OTES R2= H, R3 = OH
2.9.7 (20%)
2.9.5
R1 = R2 = H
2.9.8
(21%)
1~ = OTES AcO
O
OTES
H O .... O HO
"
OAc O
2.9.9
As s h o w n
in
Scheme
75, b a c c a t i n
derivatives
were
studied
first.
Interestingly, the three baccatin III derivatives 2.2.4, 2.1.19 and 2.9.5, bearing a
230 benzoate group at C-2, when subjected to photolysis under standard reaction conditions (254 nm, Pyrex, 0.05 M in CC14, 20 h) cleanly gave the expected rearranged products 2.9.6, 2.9.7 and 2.9.8. In striking contrast to the above observations, attempted photolysis of 2.9.9 failed to produce any of the expected rearranged pentacyclic derivative. The above results suggest t h a t an aromatic ester moiety at C-2 is necessary for the photoisomerization to occur. The contribution
of the aromatic groups of the side chain to the
photochemistry of paclitaxel was next examined. The key substrate is the paclitaxel analog 2.1.26c, in which the C-2 benzoyl moiety has been replaced by the cyclohexanoyl ester.
i, ii 2.9.9
--
TESO,
AcO Ii
H Bz.N 0 = II P h ~ O OH
//
0
O.H
.... 0 -
0
2.1.26c 2.1.10 O AcO ~
H
Bz.N
o
/
HO
2.9.10
/I
0
OH
\IJl.
(5
OAc
~/N~O
Conditions: (i) LiHMDS, THF,-40 ~ 2.1.10, 85%; (ii) Py, 48%HF, CH3CN, 5 ~ 99%; (iii) hv, 254 nm, 40% of 2.9.10, 21%
of2.1.26c.
As shown in Scheme 76, the
photolysis of 2 . 1 . 2 6 c was performed as usual,
affording a 40% clean conversion to 2.9.10. The authors conclude that initial excitation of the 3'-N-benzoyl amide or the 3'-phenyl group in the side chain also serves to excite the C-9 keto function in an intramolecular fashion, since the
231 presence of the side chain restores the normal reaction mode absent in 2.9.9 [78]. Finally, to confirm t h a t photoinduced isomerization specifically requires a side chain endowed with an aromatic amide at C-3', analog
2.9.12 was prepared as
shown in Scheme 77. Under standard photolytic conditions, compound 2.9.12 was found, as expected, to be completely inert. This experiment clearly supports the hypothesis of an "antenna effect" between the C-3' benzamide (and/or the C-2 benzoate) and the C-9 ketone in this photochemical isomerization [78].
0 [~}/IL
AcO NH O
i, ii
/,O OH
2.9.9 --
Ill
O
2.9.11
0
Ok,~=O ~~/
2.9.12
Conditions: (i) LiHMDS, THF,-40 ~ then 2.9.11, 87%; (ii) Py, 48%HF, CH3CN, 5 ~ 94%.
Scheme 78
AcO
OH
o Paclitaxel 1.1.1
i or ii
= 2.9.4
o ....
OH 2.9.13
~
HO BzO: AcO
0
Conditions: (i) hv, PhMe, 300nm, 2.9.4 (31%); 2.9.13 (30%); (ii) hv, CCI4, 300 nm, 2.9.4 (41%); 2.9.13
(21%). The same study also reports confirmation of the t r a n s a n n u l a r bond formation by single crystal X-ray analysis of a typical r e a r r a n g e d product. Photophysical
232 studies on paclitaxel and the analogs described above may shed further light or the initial stages of this interesting photoisomerization. Photolysis of paclitaxel at different wavelengths also produced interestin~ results: at 300 nm, the new compound 2.9.13 was isolated in 30-40% yield, ii addition to 2.9.4 (20-30%) [78].
The formation of compound 2 . 9 . 1 3 can b~
rationalized by invoking the occurrence of a Norrish type I process after the photoisomerization step. The epimerization at C-7 may not be a photochemical event. Interestingly, only one configuration out of four possible at C-8 and (the former) C-10 was obtained. This stereoselectivity may be due to preference for the formation of the less strained pentacyclic ring system. 5.3. STRUCTURE-ACTIVITY RELATIONSHIPS A large array of paclitaxel analogs containing modifications within the diterpenoid core were evaluated in microtubule assembly or disassembly assays, as well as in vitro cytotoxicity assays against a number of tumor cell lines. Some of the more active analogs emerging from above in vitro assays were further tested in in vivo, usually in mice, against m u r i n e or h u m a n tumor xenografts. From the body of data discussed in this section, it is obvious t h a t an imperfect correlation exists among the several in vitro tests, and even more so between in vitro potency and in vivo efficacy. We briefly discuss the most common biological assays and try to highlight their differences.
(i) In Vitro Microtubule Assays Initial Rate of Tubulin Polymerization (Swindell-Horwitz-Ringel Method) [791: This assay determines the initial rate of tubulin polymerization. The assembly of microtubule protein (MTP) in the presence of paclitaxel or analogs is performed as follows: MTP (1.5 mg/mL) is incubated at 35 ~ with 15 pM paclitaxel or analogs (added as DMSO solution; 1% final DMSO concentration) in the absence of GTP. The assembly reactions are followed by turbidity measurements at 350 nm. The value for paclitaxel in this assay is used as an internal standard, and experiments with analogs are usually performed in parallel.
This
assay
is
kinetic
in
nature,
and
does
not
measure
thermodynamic affinity for the binding site (nor is it clear t h a t a correlation
233 between the two exists). In addition, it yields no information on the types of microtubules formed (i.e. length, shape, bundles etc.). It is therefore expected to be a rough measurement of activity, useful perhaps to eliminate the inactive compounds, but oi~en it does not correlate well with cytotoxicity. Extent of Microtubule Assembly (Himes Method) [80]" The aim of this assay is to determine the extent of assembly at different concentrations of the analogs and then calculate an ED50. The assembly reaction is done at 37 ~ in PEM buffer (0.1M PIPES, pH 6.9, 1 mM EGTA, and 1 mM MgSO4) at a protein concentration of 1 mg/mL (10 ~M) in the presence of paclitaxel or analogs and 0.5 mM GTP. The reaction is again monitored by the increase in the apparent absorbance at 350 nm. This method is different from the above in t h a t the kinetics of the process are not considered, and only thermodynamic factors are. These data may correlate better with cytotoxicity, which is usually measured over many hours or even days. Microtubule Disassembly (Potier Method) [52]" This assay was developed by Potier's group on the basis of the unusual stability of microtubules formed in the presence of paclitaxel. In this experiment, a solution of MTP (2 mg/mL) is assembled at 37 ~ in the presence of paclitaxel or analogs, followed thereafter by disassembly via lowering the temperature to 4 ~
The initial rate of microtubule disassembly in the presence of the
compound is monitored by the drop in turbidity at 400nm. The initial rate of microtubule disassembly of paclitaxel is set as a standard. This assay also measures a kinetic parameter, and it is not obvious t hat it should correlate well with cytotoxicity. It is also not evident t hat the correlation with assembly data will be good. EC(O.Ol) Expression (Long Method) [81]" The potencies of the different analogs are expressed as an effective concentration (EC0.01), which is defined as the analog concentration capable of inducing the tubulin polymerization at an initial rate of 0.01 OD/min at 37 ~ as measured at 350 nm. The rates of polymerization are determined at several concentrations and EC0.01 values calculated for each analog by interpolating the appropriate region of the polymerization curves obtained. This method is of
kinetic nature, but it yields data that are concentration based. In view of the poor predictive value of tubulin polymerization data, this review focuses on cytotoxicity results r a t h e r t h a n microtubule assemby or disassembly.
234 (ii) I n Vitro Cytoto~dcity Assays A number of murine or h u m a n cancer cell lines, such as P388, B16, HCT-116 and KB, have been used for the determination of in vitro cytotoxicity of paclitaxel and its analogs [82]. The in vitro IC50 value measures the drug concentration required for the inhibition of 50% cell proliferation, usually after prolonged (2-3 days) incubation. One must note that the success of the analog in this assay will depend on its stability within the culture medium as well as intracellularly; the ability to penetrate the cell, most likely by passive diffusion, is also critical, and highly ionic derivatives may not be bioactive because of poor lipophilicity. In addition, cells that express the m d r (multidrug resistance) phenotype may have more or less reduced sensitivity vs. paclitaxel and analogs. Unfortunately, this issue has not been widely addressed in the literature for paclitaxel analogs, although it is obviously very important, and will not be discussed here (for further details, see chapter 7). It is not clear whether cell lines that are resistant to paclitaxel may also be resistant to all its analogs. The choice of the cell line may be dictated by several factors, such as availability, ease of culture, attempted correlation with in vivo data, clinical relevance etc. One must be cautioned that cytotoxicity of each analog may vary substantially from one cell line to another, and may depend on the exact cell culture protocol employed.
(iii) In Vivo Assays The ultimate test of the efficacy of a drug in a model system is an in vivo assay, usually in mice. Obviously, the success of each analog will depend, in addition to cytotoxicity, on a n u m b e r of other factors, such as proper administration, biodistribution, metabolism, and systemic toxicity (or lack thereof) to the animal. Several protocols have been designed for paclitaxel and docetaxel; usually the more s t r i n g e n t tests involve the use of h u m a n xenografts and i.v. drug a d m i n i s t r a t i o n (i.e. distal t u m o r model), but i n t r a p e r i t o n e a l models are also commonly used. The drugs are usually evaluated in terms of their ability to delay tumor growth or prolong life span, as measured at the maximum tolerated dose (MTD, i.e. dose of drug that is not appreciably toxic to the animal) vs. a control untreated group [83]. Obviously,
235 one does not expect (and usually does not find) complete correlation between in vitro and in vivo results, and therefore one should resist making exaggerated
claims of potent activity based only on cell culture data. 5.3.1. Paclitaxel Analogs Modified at C-1/C-2 As oulined in section 5.2.1, derivatization at C-1 is very hard to achieve. Compound 2.1.9 (Figure 2), where a benzoate has m i g r a t e d to C-l, is essentially inactive in a tubulin polymerization assay [19]. Ring contraction due to solvolysis at C-1 leads to a number of interesting analogs, e.g. 2.1.16, which have tubulin-polymerizing activity, but have very poor activity against tumor cells [21].
BZ'NH 0 Ph~O oH
Bz.
AcO
0 OH
,
"'
~o AcO
NH 0
OCS2Me 0 OH
p h / ~ ~ _ 0 ....
2.1.16
OH
-
"\ BzO
Bz.NH 0 mh4... o.
2.1.9
AcO.2o. ,~
....
2.1.41
~
HO
Figure 2: Simple C-1/C-2 modified paclitaxel analogs
236
R'CO. NH O Ph~O
AcO
O OH
....
6. HO
-
RC(O)O Table 1: Cytotoxicity of Paclitaxel Analogs Modified at C-2 Cpd.
R
R'
IC50/IC50
Cell Line
Ref.
(paclitaxel) a 2.1.29
p-NO2-C6H4-NH
Ph
>200
HCT-116
27
2.1.26a
p-MeO-C6H4-
Ph
>20
HCT-116
27
2.1.26b
p-NO2-C6H4-
Ph
>100
HCT-116
27
2.1.26c
c-Hex
Ph
11
HCT-116
27
2.1.26c
c-Hex
Ph
56
P-388
31
2.1.26d
Me
Ph
>20
HCT-116
27
3.1.1
o-C1-C6H4-
Ph
0.01
P-388
29
3.1.2
m-C1-C6H4-
Ph
0.0014
P-388
29
3.1.3
p-C1-C6H4-
Ph
150
P-388
29
3.1.4
m-CN-C6H4-
Ph
0.33
P-388
29
3.1.5
m-N3-C6H4-
Ph
0.002
P-388
29
3.1.6
m-NH2-C6H4-
Ph
1,500
P-388
29
3.1.7
m-CF3-C6H4-
Ph
15
P-388
29
3.1.8
m-F-C6H4-
Ph
0.35
P-388
29
3.1.9
2-Furyl
Ph
25
UCLA-P3
84
3.1.10
2-Thienyl
Ph
4.2
UCLA-P3
84
3.1.11
2-Naphthyl
Ph
>1,000
UCLA-P3
84
3.1.12
c-Hex
t-BuO
1.1
B-16
64
3.1.13
c- H ex
t-BuO b
11
P-388
31
(a) Concentration of analog that inhibits cell proliferation by 50% divided by concentration of paclitaxel that achieves same result. (b) This compound is a 10-deacetyl derivative. These observations suggest t h a t the i n t a c t A-ring s u b u n i t is an i m p o r t a n t structural element for cytotoxicity. The C-2 benzoate clearly plays a role in the cytotoxicity of paclitaxel, since 2-deoxytaxol, 2.1.41, is essentially inactive [26].
237 Due to the important role of the C-2 substituent for proper binding, it is clear t h a t small modifications at this site may lead to optimization of the activity, and it is not surprising t h a t several groups have reported efforts in this direction (Table 1). Several conclusions may be derived from the results in the table, even though cytotoxicities are generally reported for different cell lines. First of all, introduction of p a r a substituent at the benzoate invariably leads to loss of activity (see 2.1.26a,b and 3.1.3). Thus, the fit of the benzoate within the binding site seems to be r a t h e r tight, unless complex conformational changes are engendered by the p a r a substitution. It is unclear whether an aromatic ester at C-2 is needed for activity. Both Chen et al. [27] and Ojima et al. [31] have shown t h a t reduction of the C-2 benzoate to a cyclohexanoate leads to substantial loss of activity (see 2.1.26c and 3.1.13). In contrast, Georg et al. [64], working in the 10-acetyltaxotere series (see 3.1.12), report no loss of activity on hydrogenation of the C-2 benzoate. These apparent discrepancies are very difficult to interpret. Smaller aliphatic C-2 esters [27] (see 3.1.26d) and heteroaromatic ones [84] (see 3.1.9 and 3.1.10) are also poorly active. The most productive modifications to date have been carried out by Kingston et al., who reported large increases in cytotoxicity for some C-2 oand m - s u b s t i t u t e d benzoates [29]. In addition to i m p r o v e m e n t s in the cytotoxicity vs. paclitaxel, these analogs are clearly more effective in promoting microtubule assembly in vitro. These compounds, especially 3.1.2 and 3.1.5, are therefore promising, although neither data vs. resistant cell lines nor i n vivo evaluation have been reported. 5.3.2. Paclitaxel Analogs Modified at C-4 Chen and co-workers have extensively explored the SAR at C-4. Table 2 shows some of the highlights. As with C-2, deacylation or deoxygenation at C-4 leads to complete loss of activity (see 2.2.16 and 2.2.45). Introduction of large groups is also deleterious (2.2.24h), but aliphatic esters slightly larger than acetyl lead to improved activity (2.2.24j). Small carbonates and carbamates at C-4 are also quite active, especially in conjunction with improved side chains (for details on improved side chains, see chapter 6).
238
a'~. NH
O
\
AcO
O OH
Iii1< OH HO
BzO
X
Table 2: Cytotoxicity of Paclitaxel Analogs Modified at C-4 Cpd.
R
R'
X
IC50/IC50
Ref.
(paclitaxel) a 2.2.45
Ph Ph
Bz Bz
OH H
>25 n.d. b
19, 32 36
2.2.24h
Ph
Bz
OBz
100
20
2.2.28a
Ph
Bz
OCO2Me
0.9
19
2.2.24j
Ph
Bz
OCO-c-Pr
0.4
20
3.2.1
Ph
Bz
3.9
19
4.5 d
35
2.2.1{}
Ok\ 2 - - N-~J ~
l-o 3.2.2
t- oc
O N.
l-o / - - 3.2.3
Ph
Bz c
",,1
OCO-i-Pr
(a) Measured in HCT-116 cells. (b) Very poor tubulin polymerization activity. (c) This compound is a 10-deacetyl derivative. (d) Measured in B-16 melanoma cells. Georg et al. have also reported a derivative modified at C-4, 3.2.3, which is slightly less active t h a n paclitaxel. Upon further exploration, modification of the C-4 function is likely to afford very potent derivatives. 5.3.3. Paclitaxel Analogs Modified at the Oxetane Ring Many of the derivatives described in section 5.2.3, in which the oxetane ring has been opened, were evaluated for their ability to polymerize tubulin [23]. None of t h e m displayed any measurable activity and they are therefore useless in defining the SAR at this locus. Most of them are missing an acetoxy group at C-4 which, as we discussed above, is crucial for binding. This is in agreement with Kingston's early results [2]. Thus, the oxetane seems to play
239 an essential role in the binding of paclitaxel to microtubules. It is not known whether the oxetane acts to rigidify ring C and point the C-4 acetoxy group in the appropriate direction for binding, or whether the oxetane oxygen itself is a binding element. The derivatives that would answer this question (e.g. the one bearing a cyclobutane ring in place of the oxetane) have not yet been described. 5.3.4. Analogs Modified at the C-7 Position C-7 Xylosyltaxol, 3.4.1 was isolated from the bark and leaves of T a x u s b a c c a t a , and shown to be more potent t h a n paclitaxel in the tubulin polymerization assay [3, 85]. Other analogs were prepared either from baccatin derivatives or from paclitaxel itself. Most of the analogs tested (Table 3) have in vitro activity comparable to paclitaxel and docetaxel. With the exception of large lipophilic substituents such as the silyl ethers (see 3.4.8 and 3.4.10), any modification seems well tolerated, with some analogs being slightly more cytotoxic than paclitaxel. It seems likely that the C-7 substituent is not engaged in significant interactions at the binding site, and t h a t chemical modification at this position will only serve to m o d u l a t e the activity (perhaps via altered solubility, metabolism, biodistribution). Some of the C-7 derivatives were tested in vivo. With the exception of the carbonate derivative 3.4.6, none of the
compounds compared favorably with paclitaxel at their MTD. Interestingly, even cyclopropane derivative 2.4.14 retains in vitro activity, in spite of the slight conformational alteration imparted to the C ring vs. paclitaxel. 5.3.5. Analozs Modified at the C-9 Carbonyl This section examines primarily the effect of reducing the C-9 carbonyl on the cytotoxicity. Some analogs carry also C-10 and/or C-7 modifications and are discussed here for the sake of convenience. As with the C-7 position, even with the small database available, it is clear t h a t most modifications, including complete defunctionalization, are well tolerated at C-9. None of the modifications effected led to complete loss of activity. The derivative 2.5.31, f e a t u r i n g a completely defunctionalized northern half, is only 5-6 times less active than paclitaxel. Compounds with a partially hydroxylated northern segment have activities of the same level as paclitaxel and docetaxel (Table 4).
240
R "NH O
AcO
O X
p h - - " L ~ _ O .... OH HO
" BzO
Table 3: Cytotoxicity of Paclitaxel Analogs Modified at C-7 Cpd.
R
X
In vitro
Cell
In vivo
IC50/IC50
Line
activityb
Ref.
paclitaxel a 3.4.1
Bz
([~)Xylosyl
n.d.c
-
3
3.4.2
Bz
([~)OAc
1.3
J774.2
85
3.4.3
Boc
(~)L-Phenyl
0.44
P-388
52
3.4.4
Bz
([~) L-Alanyl
2.3
B-16
86
3.4.5
Bz
([~)N,N-dimethyl
2.3
S-16
86
3.4.6
Bz
(~)OC02Et
3.4.7
Bz
3.4.8
Bz
3.4.8 2.4.1
alanyl d
glutaryl 1.5
HCT-116
289 (40)
([~)OCONHBu
1.0
HCT-116
157 (50)
([3)OMs
0.9
HCT-116
Bz
([~)OSiEt3
>20
HCT-116
Bz
(a)OH
0.5
HCT-116 126-154 (30-
-
-
45 45 45 45 45
32) 3.4.10
Bz
(a)OSiMe3
>20
HCT-116
2.4.7
Bz
2.4.8
Boc
H
1.0
HCT-116
157 (50)
45
H
0.4
HCT-116
156 (64)
45
2.4.16
Bz
2.4.14
Bz
A6-dehydro
1.2
HCT-116
161 (60)
45
A7,19-cyclopropa
2.0
HCT-116
2.4.12
Bz
156 (80)
45
(a)F
2.9
HCT-116
185 (132)
45
3.4.11
Boc
(a)F
1.2
HCT-116
147 (40)
45
-
45
(a) See Table 1 for definition. (b) I n v i v o data in unstaged M109 model. Values indicate T/C at the MTD (mg/Kg/inj., in parentheses). Paclitaxel gave T/C values of 183-276 at MTDs of 50-75 mg/Kg/inj. (c) IC50 0.4 v s . paclitaxel (1.0) in tubulin polymerization assay. (d) A C-10 deacetyl derivative.
241
R "NH 0 ph-~,~_
X
Y Z
0 ....
OH
0 HO
" BzO
OAc
Table 4: Paclitaxel Analogs Modified at the C-9 Carbonyl Cpd.
R
X
Y
Z
IC5o/IC5o (paclitaxel)a
Cell Line
Ref.
2.5.1
Boc
(~)OH
(a)OH
2.5.2
Boc
( [ 3 ) O H (~)OH
(~)OH
1.9b
P-388
56
([~)OH
2.0b
P-388
56
2.5.4
Boc
(~)OH
(a)OH
(a)OH
3.2b
P-388
56
2.5.10
Bz
3.5.1
Bz
([~)OAc
(a)OH
(~)OH
8-10
P-388
17
([~)OAc
H
(~)OH
0.5
P-388
17
3.5.2
Bz
([~)OAc
H
H
1
P-388
17
2.5.31 2.5.24
Bz Bz
H H
H (~)OH
H (~)OH
5-6 14
P-388 B-16
17 64
2.5.25
Boc
(~)OH (~)OH
B-16
64
Boc
(~)OH (~)OH
1.1
2.5.26
(~)OH H
1.8
B-16
64
(a) See Table 1. (b) This value is referenced to docetaxel, not paclitaxel. Reduction of the C-9 carbonyl yields active a or 13carbinols (see 2.5.1 and 2.5.2). A 10-deoxy-9-dihydroderivative is much less bioactive t h a n paclitaxel, b u t simply switching the side chain to the one from docetaxel restores the activity (see 2.5.24
vs.
2.5.26). These observations reinforce the notion t h a t the northern
half of the molecule does not intimately interact with the microtubule binding site. 5.3.6. Analogs Modified at the C-10 Position Table 5 shows some of the analogs t h a t bear modified C-10 substituents. Some also bear modifications at C-7 and are discussed here for the sake of convenience. Although it is well known t h a t introduction of polar esters or other functions at C-10 leads to loss of activity [52], minor modification with
242 relatively small substituents at this position have been shown to lead to active analogs [66].
X RHN
0
P h i _ _-
Oy
\
0 ....<
OH
0 HO BzO
OAc
Table 5: Paclitaxel analogs modified at C-10 [19, 60, 66] Cpd.
R
X
Y
IC50/IC50 paclitaxel a
3.6.1 2.5.19 3.6.2 3.6.3 3.6.4 3.6.5 3.6.6 3.6.7 3.6.8 3.6.9 3.6.10 3.6.11 3.6.12 3.6.13 3.6.14
Bz Bz Boc Bz Bz Boc Bz Bz Bz Bz Boc Bz Boc Bz Boc
=O H H H H H OCO-n-Bu OCO-c-Pr OCONMe2 OMe OMe OCO2Me OCO 2Me OCOPh OCOPh
(~)OH (~)OH (~)OH (a)OH H H (~)OH (~)OH (~)OH (~)OH (~)OH (~)OH (~)O H (~)OH (~)OH
14 1.5 0.5 1.0 7.2 3.5 1.4 1.0 0.4 5.0 0.5 1.2 0.6 0.9 0.9
(a) See Table 1. Cell line: HCT-116 in all experiments. While a 10-keto group leads to a substantial loss of activity (see 3.6.1), the functionality at C-10 can be completely removed without loss of activity (see 2.5.19, 3.6.2 and 3.6.3) [60, 63, 64, 65]. On the other hand, deletion of both the C-7 and C-10 functions leads to some drop in cytotoxicity (see 3.6.4 and 3.6.5). In general, functions including esters, carbonates, carbamates and ethers are all
243 conducive to good activity, i.e. comparable with paclitaxel. These observations once again support the theory that the functionalities in the northern half of the core are not involved in binding to the microtubule. 5.3.7. Analogs Modified at C- ll/C- 12 Very little information is available on the role of the bridgehead C-11/C12 double bond vs. the bioactivity of the taxanes, since all natural taxoids are endowed with such double bond, and it is very difficult to modify it chemically. Chen and co-workers have reported some unusual chemistry at C-10 that results in the formation of dienone systems at C-10->C-18 [60]. C-12 fluorinated derivatives, where the double bond has moved into conjugation with the C-9 carbonyl, were also obtained as side products. Biological evaluation of some of these compounds (see Figure 3) shows that migration of the C-11/C-12 double bond leads to some loss in activity. Ten-fold drops in cytotoxicity (vs. paclitaxel) are seen with dienones 2.5.18, 3.7.1 and 3.7.2. The fluorinated derivatives are also ten-fold less active than paclitaxel, except for derivative 3.7.3, which bears a ~-methyl grouop at C-12, and is over 100-fold less active [19]. It is likely that, due to the importance of the C-13 side chain in the binding process, its exact spatial positioning is crucial to the activity of these analogs. Even slight conformational changes in the A ring might simply alter the spatial relationship of the side chain vs. the other binding elements in the molecule (the C-2 and C-4 esters). 5.3.8. Analogs Modified at the C-14 Position The biological activity of analogs bearing a functionalized C-14 has been explored in a preliminary fashion by Kant et al. [12] and Ojima et al. [13, 70]. The 14-(~)OH analog of paclitaxel, 2.7.14 (Table 6) shows slightly reduced cytotoxicity vs. the parent drug. I n v i v o evaluation showed that this derivative is essentially devoid of antitumor activity [12]. Switching the side chain to the one found in docetaxel, as predicted, results in a slightly improved performance (see 2.7.15 and 2.7.11) [12]. Even a cyclic carbonate at C-14/C-1 is compatible with good activity, but only in the presence of the docetaxel side chain, the paclitaxel analog being remarkably less active (2.7.12 vs. 3.8.1). A C-1/C-14 acetonide (see 2.7.13) is deleterious to activity.
244
#
BzHN O
O
R
O ....~ OH
2.5.18 3.7.1 3.7.2
R=(~)OH R=(~)OCOCHFC1 R=(cz)OH
ICso/ICso(pacl) 9.5 9.5 10 O
BzHN O RI,,,,~..~ ~ , ~ o ....
Z'.. i
BzO 3.7.3 3.7.4 3.7.5
J,
OAc
RI= F; R2 =Me; 1~3 = (a)OH RI= Me; R2 =F; R3 = (a)OH RI= Me; R2 =F; R3 = (~)OH
Figure 3: Paclitaxel analogs modified at C-10/C-12 and their
IC5o/IC5o(pacl) 230 13.5 9.5 in vitro
cytotoxicity (HCT-116)
Analogs where the side chain was introduced at C-14 instead of C-13 were much less active than docetaxel [13, 70]. Not enough is known about the SAR at C-14 to draw final conclusions as to the involvement of this position at the binding site. Since most of the analogs in Table 6 have similar activity to paclitaxel, it is likely that the C-14 functionality does not perform a binding function, and therefore only minor changes in cytotoxicity can be realized by fine-tuning such functionality. 5.3.9. Misce!!.aneous Analogs Klein reported on the synthesis of novel paclitaxel derivatives featuring a contracted seven-membered B-ring, 3.9.1 and 3.9.2 (Figure 4 ) [ 4 8 ] . Interestingly, these compounds were of comparable activity to paclitaxel in the
245 in vitro P-388 cytotoxicity assay. More work needs to be done to assess the
potential of these unusual analogs.
FI~HN
PhA
O
\
2
OH
_- Lo .... OH
_ R40 OR 3 0 i =
O
Table 6: Cytotoxicity of Derivatives Modified at C-14. Cpd.
R1
R2
R3,R4
IC5o/IC50 paclitaxel a
Cell Line
Ref.
2.7.14
Bz
Ac
H,H
4.0
HCT-116
12
2.7.15 2.7.11 2.7.12
Boc Boc Boc
Ac H H
3.8.1
Bz
H
2.7.13
Boc
H
H,H H,H C=O C=O C(Me)2
1.0 1.0 1.0 17 7.5
HCT-116 HCT-116 A121 A121 A121
12 12 13, 70 13, 70 13, 70
(a) See Table 1. The Ojima group described the synthesis and biological evaluation of two novel nor-seco analogs of paclitaxel and docetaxel, 2.8.44 and 2.8.45 [74]. These compounds are 20-40-fold less potent than paclitaxel in a number of tumor cell lines. These results thus clearly indicate the importance of the A-ring for the proper binding of paclitaxel and docetaxel to their biological target. A pentacyclic paclitaxel derivatives (2.9.4), prepared by photochemical irradiation of paclitaxel by Chen et al. [76], failed to show any activity in the tubulin polymerization assay as well as in cytotoxicity assays. The core of this molecule is grossly distorted with respect to the one in paclitaxel, and no activity would be expected. A recent report from Commerqon et al. provides the first example of a C19 modification [88]. The fact that a C-19-hydroxylated docetaxel analog (3.9.3) exhibits slightly better activity than the parent drug in the tubulin disassembly
246 assay suggests that chemical modifications at C-19 may lead to useful derivatives. AcO R.N o ,, X,o.c o. 3.9.1 R=Bz 3.9.2 R=Boc OBz OAc RHN
O
0 _. // OH
\
~ ~OHo
,
O .
HO
BzHN "
Ogz OAc
AcO I ~, / H,,,
0
O
/? OH \l~ 2.9.4
o ....
HO
dR HO
BocHN
O
P h ~ O
~
/
O/OH
~ 3.9.3
~
~-~o
HO BzHN
OAc
....
o.
0
OBz OAc
AcO
O
I~,,,,,II
jOH ~oH'" 3.9.4
p h ~ - ~ ' O _ ....
o.
2.8.44 R=Bz 2.8.45 R=Boc
~ HO
~~o (DBz OAc
Figure 4: Miscellaneous paclitaxel and docetaxel derivatives
247 Finally, among the many docetaxel metabolites isolated, one (3.9.4) features a novel core functionalization, i.e. a hydroxyl group at C-6. Such hydroxylation leads to a 30-fold drop in activity vs. docetaxel, i.e. detoxification of the drug [89]. 5.3.10. Conclusion Although much work remains to be done in this area, a qualitative picture of the SAR of paclitaxel is beginning to emerge. At least three functional elements, i.e. the C-13 side chain (see chapter 6) and the C-2 and C-4 esters, are intimately involved in interactions at the binding site. It appears that the northern half of the molecule and the tetracyclic skeleton (including an intact oxetane) function essentially as a molecular scaffolding to hold these binding elements in the proper orientation. Some uncertainty still exists about a possible binding role for the oxetane oxygen and the C-1 hydroxyl group. Modifications of the essential functions may therefore lead (and in some cases this has been achieved in cell culture) to more potent paclitaxel analogs, through further optimization of the fit with the microtubule site, whereas modifications at the non-essential positions may modulate the activity by changing the physico-chemical parameters of the molecule or via other secondary effects. R~'EgENCIi~
.
.
.
.
.
7.
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54. 55. 56. 57.
58. 59. 60. 61. 62. 63. 64.
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