- Email: [email protected]
Synthetic studies in the 5-thio-d -xylopyranose seriespart 3: Preparations ofO-phenyl andC-(4-hydroxyphenyl) 5-thio-d -xylopyranosides
TETRAHEDRON Tetrahedron 54 (1998) 13783-13792
Pergamon
SYNTHETIC STUDIES IN THE 5-THIO-D-XYLOPYRANOSE SERIES part 3: PREPARATIONS OF O-PHENYL AND C-(4-HYDROXYPHENYL) 5-THIO-D-XYLOPYRANOSIDES Michel Baudry a, V~ronique Barberousse b, Yolande Collette b, G~rard Descotes a Joaquim Pires c, Jean-Pierre Praly a,,, Soth Samreth b a Laboratoire de Chimie Organique II associ6 au CNP.S, Universit6 Claude-Bernard Lyon I CPE-Lyon, 43 Boulevard du 11 Novembre 1918. F-69622 Villeurbanne, France b Laboratoires Fouruier SA, Centre de Recherche. 50 roe de Dijon. F-21121 Daix, France c SYNKEM, Division de Plasto SA, 47 rue de Ixmgvic, F-21301 Chen6ve, France
Received 9 March 1998; accepted 8 September 1998
Abstract: Treatment of the ~-O-acetyl 5-thio-a-D-xylopyranosyl l-O-trichloroacefmidate 1 and phenol in the presence of BF3 • OEt2 at low temperature led to the corresponding Ophenyl (4:-30 % yield) and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides (5:,-,40 % yield) as a result of competitive O-glyco~idation and aromatic electrophilic substitution. At higher temperatures, O-phenyl 5-thio-~D-xylopyranoside was shown to rearrange readily to give C(4-hydroxyphenyl) 5-thio-~D-xylopyranoside as the thermodynamically favoured product. In the presence of zinc oxide, the sugar a-bromide 2 and phenol led also to a mixture of 4 (-20 % yield) and 5 (--40 % yield). Compound 5 was prudueed from 2 on treatment with dibutyltin diphenoxide and from the I-O-trimethylsilyl sugar derivative 3 and trimethylsilyloxybenzenc, when reacted in the presence of trimethylsilyl triflate in dichloromethane (35 % yield). Under these conditions, a tetrahydrothiophene derivative was observed ( 6 : - 5 % yield). However, the sulfur transannular participation pathway which accounted for its formation became predominant when the a-bromide 2 and phenol we~ treated for several hours with zinc chloride, thus affording 6 in a 42 % yield. © 1998 Elsevier Science Ltd. All rights reserved. After various O- and S-aryl 5-thio-D-xylopyranosides were found to be orally active antithrombotic drugs, t a large array of analogs has been prepared for pharmacological studies. 2 As a result of our involvement in related synthetic investigations, we recently reported first on the radicalmediated bromination of 2,3,4-tri-O-acetyl-5-thio-D-xylopyranosyl bromide as a route to 5-thio-Dxylopyranonolactone derivatives. 3 Then, we focused on the synthesis of various C-aryl 5-thio-Dxylopyranosides in order to explore new routes to structural analogs of the known active compounds in this series. It was found that the 2,3,4-tri-O-acetyl-5-thio-D-xylopyrunosyl cations produced from either the a-trichloroacetimidate 14 or the a-bromide 24.5 reacted readily with different heterocyclic aromatic compounds to afford the corresponding C-hetaryl 5-thio-D-xylopyranosides 6 by electrophilic substitution. A similar behaviour was observed with electron-rich polyphenol derivatives which also led efficiently to
C-aryl
5-thio-D-xylopyranosides, 7 in
particular when
the
more
reactive
trichloroacetimidate 1 was transformed at low temperatures, thus avoiding further reactions triggered by sulfur intraannular participation. 6-9 We now describe further experiments aiming at the preparation of O-phenyl and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides by different approaches applied to phenol and either 1, 2 or the 1-O-trimethylsilyl 5-thio-a-D-xylopyranc~se 3.10
E-mail: jean-pierre.praly@univ-lyon 1.fr; Fax: 33-(0)4-78-89-89-14
0040-4020/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved. PH: S0040-4020(98)00848-5
13784
M. Baudr3, et al. / Tetrahedron 54 (1998) 13783-13792
When electrophilic substitution of phenol was attempted by treatment of the atrichioroacetimidate 1 in the presence of boron trifluoride etherate at -78"C, a complex product distribution was observed. In contrast to the case of polyphenol derivatives which led essentially to !
C-aryl 5-thio-D-xylopyranosides,7 both O-phenyl and C-(4-hydroxyphenyl) analogs were produced, the latter being found somewhat more abundant under the applied reaction conditions. The formation of both types of 5-thio-D-xylopyranosides can be easily rationalized by invoking the lower reactivity of phenol towards electrophilic substitution as compared to electron-rich polyphenol derivatives7 and heteroaromatic compounds,6 in keeping with related precedents reported in the literature.ll-13 However, in order to get a better insight into the reaction pathways (t-->C-rearrangement, anomerization), the reaction of 1 with phenol at different temperatures and the reactivity of each anomer of the O-phenyl 5-thio-D-xylopyranoside 4 were investigated in more details.
AcO-'~--....~S~
1
C6H5OH AcO"t'-..-.~S~
BF,.O CCI3
+
AcO,-..~.....~& A C O ~ H
4
5
Table 1:BF3 " OEt2-catalyzed reaction of 1 with phenol: influence of temperature on product distribution a T°C
4
~Ct ratio b
yield, % -78 -50 -20 40
30 33 40 16
5
~ot ratio b
yield. % 8/2 8/2 8/2 0/10
41
40 30 49
8/2 7/3 7/3 8/2
a T h e ratio 1 / C 6 H 5 O H ! B F 3 • O E t 2 was 1 / 2 / 0.1 ; b estimated by 1H N M R .
These experiments showed that, in the -78 to -20°C temperature range, O-phenyl and C-aryl 5-thio-D-xylosidation occurred competitively, the balance being in favour of the latter class of compounds (except for the experiment run at -20"C which show some discrepancy, Table 1), the 3anomer of 5 being predominant in each case. 6.7 This was expected on the basis of the preferred formation of 1,2-trans glycopyranosides from acetylated glycosyl donors and the higher thermodynamic stability of C-aryl &D-glycopyranosides.14.15 Whereas the combined 5-thio-Dxylopyranosides formed amounted to --65-70% whatever the reaction temperature, the experiment carried out at 40"C showed clearly a decreased proportion of the remaining O-phenyl 5-thio-Dxylopyranoside which contained only the more stable a-anomer.16 Conversely, a higher proportion of the C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides
was observed, with a somewhat higher
proportion of the #-anomer. These changes could be explained by the occurrence of a O -->Crearrangement which should be easier for 4p.
13785
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
To check this hypothesis, solutions of each anomer of 4 in CD2C12 containing a catalytic amount of BF3 . OEI2 and kept at controlled temperatures were examined by IH NMR. Since 413 remained unchanged after 30 min at temperature below -30°C (Table 2), it is reasonable to propose, in accordance with the similar proportion of 4 and 5 formed from 1 at either -78 or -50"C (Table 1), that compounds 5 arised from direct electrophilic substitution of phenol, even at low temperature (scheme). Anomerization and O-->C-rearrangement occurred around room temperature as shown by the presence of 4 a and 513 in the medium when 413 was exposed briefly tt, acidic conditions (Table 2). The fact that only 4c~ was present after 12 h in addition to 513 suggested that 4 a was much less reactive under the applied conditions than its /~-counterpart. 16 This was confirmed by applying the same protocol to 4 a which produced only 513 in detectable amount after 12 h at 27"C. This reactivity pattem and the higher thermodynamic stability of the C-5-thio-/3-D-xylopyranoside 513 paralleled observations collected from
analogs
of
natural
sugars.
Therefore,
it
is
established
that
C-5-ta,lo-D-
xylopyranosidation of phenol occurred in competition with the formation of O-5-thio-D-xylosides, even at low temperature (-78°C) within 30 min. Provided a /3-anomeric configuration, these latter rearranged readily around normal temperature to produce the more stable ~configurated C-5-thio-Dxylopyranoside (scheme). This and our previous observations 6.7 showed the higher reactivity of the 5-thiosugar studied since trichloroacetimidates of natural sugars and electron-rich aromatic compounds have been shown to lead to C-glycosides around room temperature. 17
Table 2: Transformation of 4 catalyzed by BF3 • OEt2 in CD2CI 2 : product distribution estimated by IH NMR Substrate
41~
" " " 4a " "
"
T
Time
*C
h
4~
Product distribution
4a
5~
-64 -30 27 27
0.5 0.5 0.5 12
100 100 50 0
0
0
0 15 30
0 35 70
-64 -30
0.5 0.5
27 27
0.5 12
0 0 "4)
100 100 -100
0 0 "43
0
~70
~30
13786
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
BF 3 , OEt 2 IA cO--'k-'-~iS +
AcO~S~,
A cO"~"----IS~
A cO "~"---.~...-~S~ 1
+= co- oo/ o
AcO ""&"'----IS~
°
°
4c~ 0 . ~
5~
.
.
.
.
.
.
0 OH
A cO ~ ' ~ - ~ ' ~ J AcO
,, AcO"k""-'tS~
tem
AcO-'-k----..--~S\
~co~.~..~o~~~co AcO ~ 4~ 5~
Acu
/~'~OH
~J
'1
t
tt
4~J
Scheme: Proposed reaction pathways for the BF3 • OEt 2 catalyzed reaction of 1 with phenol
The a-bromide 2 was reacted with phenol under three different sets of conditions, namely in the presence of either zinc oxide, or zinc chloride or dibutyltin diphenoxide. Use of ZnO 18,19 has already been shown
to produce
selectively C-aryl
5-thio-D-xylopyranosides
derived
from
polyphenols (resorcinol, phloroglucinol) by heating for - l h in acetonitrile in the presence of molecular sieves, v Since the analogous C-aryl 5-thio-D-xylopyranosides were not produced when either anisole or p-dimethoxybenzene were used as the aromadc compound, the formation of O-aryl intermediates prone to a O-->C-rearrangement was assumed to account for our observations. In contrast to the selectivity found in the case of polyphenols, reaction of 2 and phenol in the presence of ZnO in acetonitrile led to a complex mixture after 15 h at 50°C. In spite of the prolonged reaction time, O-phenyl and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides were observed ( 4 : 2 0 %, $: 40 % isolated yields) in keeping with the expected lower reactivity of phenol towards electrophilic substitution. When the bromide 2 was reacted with phenol in chloroform in the presence of ZnCI2
13787
M. Baud~. et al. / Tetrahedron 54 (1998) 13783-13792
(2/phenol/ZnCl2 : 1/~2)
for 12 h at room
temperature, besides minor amounts of 4 and 5, compound
6 was isolated as the major product. It was deacetylated uneventfully under Zempl6n conditions to afford 7. Their structures, established on the basis of NMR data and in particular a H , C - C O S Y correlation recorded with 7, showed a ring restriction explained by a sulfur intraannular participation mechanism.6.7. 9
AoO'~'~SXAcO.~I~,.,~,~C.H5OH~__~
S
O H
AcOBr ZnCI2
CHCF3 AcO ¢ Argon
2
MeONa (cat.).__
MeOH 6
OH
HO
OH
7 OH
As another route to 413, we tried to glycosylate the dibutyltin diphenoxide with the a-bromide 2 according to a method described by Ogawa et al. 2° This procedure exploited the enhancement of the nucleophilicity of hydroxyl groups by alkylstannylation. The diphenoxide, prepared according to a standard method, 21 was engaged in the glycosylation step in the presence of SnCl4. In contrast to the fact that the O-phenyl glucoside was produced in a 89 % yield 2° when this method was applied to natural sugar derivatives, surprisingly, in our case, the C-xyloside was obtained in a 59% yield. This can be again explained by the enhanced reactivity of 5-thio sugars due to a more efficient stabilization of a proximal positive charge at C-l by the sulfur atom, thus favouring the O-->C-rearrangement of intermediate O-5-thio-D-xylopyranosides. The obtained compound 513 was deacetylated to afford 8.
n-Bu
SnCi4 AcO-~---~S~
~OH
AcO 513
2
59 %
Since 1-O-deacetylation of 1,2,3,4-tetra-O-acetyl-5-thio-D-xylopyranose could be selectively achieved with hydrazine acetate to afford 2,3,4-tri-O-acetyl-5-thio-D-xylopyranose,4. 6 this compound was subjected to the Mitsunobu reaction in the presence of phenol. 22 The results were disappointing, so we turned our attention to another glycosidation method involving trimethylsilyl derivatives. 23 Hence, encouraged by the excellent yields and stereoselectivities reported for the preparation of O-aryl glycosides of
natural
sugars 23
by
means
of
l-O-trimethylsilyl
sugar
derivatives 24 and
trimethylsilylphenols, 23 this approach was applied to 3, obtained by adaptation of a reported method, 25 and trimethylsilyloxybenzene. 26 When the reaction was performed in dichloromethane at -500C for a short time (< 3 h) in the presence of trimethylsilyl triflate in either catalytic or stoechiometric amounts, no transformation was visible. The same conclusion was reached when
13788
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
attempting the reaction at 20"C for several hours with catalytic amounts of trimethylsilyl triflate, whereas alarger quantity of this reagent (1 eq, 15 min) afforded 5 in low yield (25%) accompanied by polar compounds visible on TLC plates. These unidentified compounds were also produced when the reaction was carried out with trimethylsilyl triflate (1 eq), trimethylsilyloxybenzene (1.2 eq) at controlled temperature (-50 to 20"C) for 4 h. However, under these conditions, 5 could be recovered in a 35 % yield.
OSiMe3
S
0 AcO /SiMe3TMSOTI C H2CI2
OH
3
5
35%
t]/ct 88/12
AcO /
OH
6 -5%
In summary, treatment of the tri-O-acetyl 5-thio-a-D-xylopyranose 1-O-trichloroacetimidate 1 and phenol in the presence of BF3 • OEt2 at low temperature led to the corresponding O-phenyl (4: -30% 3field) and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides (5: -40% yield) as a result of competitive O-glycosidation and aromatic electrophilic substitution, in keeping with the conclusion of independent
studies
using
4-cyanobenzenethiol
and
2,3,4,6-tetra-O-acetyl-5-thio-a-D-
glucopyranosyl bromide. 9 At higher temperatures, O-phenyl 5-thio-o-D-xylopyranoside was shown to
rearrange
readily
to
give
C-(4-hydroxyphenyl)
5-thio-o-D-xylopyranoside as
the
thermodynamically favoured product. 14.15 The easier acid-catalyzed (O-->C)-rearrangement of 41~ as compared to 4et is consistent with the generally observed higher reactivity of 0-D-glycosides, less stable than their a-counterparts due to the anomeric effect. 27 In the presence of zinc oxide, the sugar a-bromide 2 and phenol led also to a mixture of 4 (~20% yield) and 5 (--40% yield). Compound 5 was produced from 2 on treatment with dibutyltin diphenoxide and from the 1-O-trimethylsilyl sugar derivative 3 and trimethylsilyloxybenzene, when reacted in the presence of trimethylsilyl triflate in dichloromethane (59 and 35% yield, respectively). Under these conditions, a tetmhydrothiophene derivative was observed (6: -5% yield). However, the sulfur transannular participation pathway which accounted for its formation became predominant when the a-bromide 2 and phenol were treated for several hours with zinc chloride, thus affording 6 in a 42% yield.
ACKNOWLEDGMENT One of us (M. B.) gratefully thank the Association Nationale de la Recherche Technique and SY NKEM (groupe Foumier) for financial support (convention CI FRE).
M. Baudry et aL / Tetrahedron 54 (1998) 13783-13792
EXPERIMENTAL
General methods. The general methods were similar to those previously indicated. 2,3,4-Tri-O-acetyl-l-O-trimethylsilyl-5-thio-a-D-xylopyranose 3. Hexamethyldisilazane (0.90 g, 5.6 mmol), trimethylsilyl chloride (0.72 g, 6.63 mmol) and pyridine (0.65 g, 8.3 mmol) were successively added, with stirring under a nitrogen atmosphere to a solution of 2,3,4-tri-O-acetyl-5thio-a-D-xylopyranose (1.613 g, 5.52 mmol) in anhydrous dichloromethane (25 mL) at room temperature. A precipitate was immediately observed. After stirring for 45 min at room temperature, the reaction medium was filtered through a bed of celite and the organic phase was concentrated under reduced pressure. The crude residue was purified by chromatography on silica gel with ethyl acetatehexane 1:3 (v/v) as the mobile phase to yield 3 (1.79 g, 4.90 retool, 89% yield), white crystals; m.p. 100-101"C; [Ct]D+188* (c 0.6, CH2C12). 1H NMR (200 MHz, CDCt3) 6 5.53 (t, 1H, J3A 9.6, H-3), 5.13-4.99 (m, 3H, H-I, H-2 et H-4), 3.04 (dd, 1H, J4.sa 11.4, Jsa.Se 12.9, H-5a), 2.70 (dd, 1H,
Ja.se4.4, H-5e), 2.04 (s, 6H, acetyl), 2.03 (s, 3H, acetyl), 0.18 (s, 9H, H-SiMe3). 13C NMR (50 MHz, CDCI 3) 6 170.1, 169.9, 169.9 (acetyl), 76.1, 73.3, 72.2, 70.2 (C-I, C-2, C-3, C-4), 24.9 (C-5), 20.9, 20.8, 20.7 (acetyl), 0.0 (C-SiMe-3).
Anal.: Calcd for C14H2407SSi (364.49): C, 46.13; H, 6.64; S, 8.80; found: C, 46.51; H, 6.48; S, 9.17.
Phenyl 2,3, 4-tri-O-ace~l-5-thio-D-xylopyranoside 4 and 4-( 2,3, 4-tri-O-acetyl-5-thio-Dxylopyranosyl)phenol 5: from 1 : To a solution containing 1 (0.31 g, 0.71 mmol) and phenol (0.134 g, 1.42 mmol) in anhydrous dichloromethane (4 mL) at -78"C, boron trifluoride etherate (9 IaL, -43.07 mmol) was added with stirring under an inert atmosphere. After approximately 0.5 h, triethyt amine (2.1 mmol) was added, then the medium was taken up in chloroform and the organic phase was successively washed with 5% NaOH solution and water. After drying (MgSO4), the organic phase was filtered through a bed of celite, then concentrated under reduced pressure. The obtained residue was applied to a column of silica gel eluted with chloroform-methanol 14:1 (v/v) as the mobile phase to afford two fractions. Each of these fractions were subjected to another chromatography with ethyl acetate-hexane 1:1 (v/v) to yield 4 (0.077 g, 0.21 retool, 30% yield, r/a--84/16) and 5 (0.108 g, 0.29 mmol, 41% yield,/~/c~=77/23). Following the same protocol, additional experiments were run at -50, -20 and 40"C (Table 1). from 2 and zinc oxide: Zinc oxide (0.78 g, 9.58 mmol) and commercially available 13 X type molecular sieve (5.3 g) were introduced into a flask which was connected to a vacuum pump before heating to 80"C for 30 rain. After cooling down to room temperature, anhydrous acetonitrile and phenol (2.5 g, 26.56 retool) were introduced into the flask and the suspension was stirred for 30 rain at 50"C under nitrogen. After addition of 2 (1.7 g, 4.79 mmol), the reaction was shown by TLC to be complete within 15 h at 50"C. The reaction medium was centrifugated, the solids were separated and thoroughly rinsed with ethyl acetate whereupon the organic phases were combined prior to concentration under reduced pressure. The obtained residue was taken up in chloroform. The organic phase was washed twice with 10% NaOH, then water. Drying (MgSO4), filtration through a bed of celite and concentration of the solution led to a residue which was applied to a column of silica gel eluted with ethyl acetate-hexane 1:1.5 (v/v) to afford 4 (0.35 g, 0.94 mmoi, 20% yield,/~/a=20/80) and 5 (0.696 g, 1.89 mmol, 40% yield, ~ct--64/36). The anomers of 4 were resolved by
13789
13790
M. Baudr)' et al. / Tetrahedron 54 (1998) 13783-13792
chromatography using a column of silica gel eluted with ethyl acetate-hexane 1:1 (v/v). 413 was crystallized from diethyl ether. 513 was obtained by selective crystallization from diethyl ether. from 2 and dibutvltin dipl~enoxide: After stirring for 30 min, under an inert atmosphere, a solution of the a-bromide 2 (2 g, 5.6 mmol) and dibutyltin diphenoxide (2 g, 4.8 mmol) in a 1:1 anhydrous mixture of toluene and acetonitrile (10 mL) containing 4 A molecular sieves, tin tetrachtoride (0.56 mL, 4.7 mmol) was added. After heating for 5 h, the solids were removed by filtration. The filtrate was diluted with ethyl acetate. The organic phase was washed with 1N HCI then water, before drying and concentration under reduced pressure. The residue was purified by chromatography on silica gel, with toluene-ethyl acetate 5:1 v/v as the mobile phase to give 513( 1.05 g, 59% yield). from 3. A solution of 3 (0.10 g, 0.27 mmol) and trimethylsilyloxybenzene (0.07 g, 0.41 mmol) in anhydrous dichloromethane was cooled down to -50"C under argon. After addition of trimethylsilyl triflate (0.061 g, 0.27 mmol), the temperature was allowed to rise to +20°C and the medium was stirred for 4 h. Then, triethylamine (400 l.tL) was added and the medium was concentrated under reduced pressure. The obtained residue was purified by silica gel chromatography with ethyl acetatehexane 1:1.5 (v/v) as the mobile phase to give the C-5-thio-D-xyloside 5 (0.036 g, 0.096 mmol, 35% yield, ffa=88/12). Phenyl 2,3,4-tri-O-ace~l-5-thio-~-D-.u,lopyranoside 413. white crystals; m.p. 153-1550C (diethyl ether); [a]D-69° (c 0.5, CH2C12). 1H NMR (200 MHz, CDCI3) 6 7.31 (dd, 2H, Jarom 8.4, Jarom. 7.5, H-arom.), 7.09-7.01 (m, 3H, H-arom.), 5.53 (t, 1H, J2.3 9.1, J3.4 9.1, H-3), 5.19 (d, 1H, JI.2 9.0, H-I), 5.17-5.06 (m, 2H, H-2, H-4), 2.97 (dd, IH, Ja.se 3.3, J5a.Se 13.7, H-5e), 2.67 (dd, IH, J4.5a 10.1, H-5a), 2.05 (s, 6H, acetyl), 2.02 (s, 3H, acetyl), t3C NMR (50 MHz, CDCI 3) 6 169.8, 169.8, 169.4 (acetyl), 157.0 (C-arom.), 129.7 (2 C-H-arom.), 123.1 (C-arom.), 116.6 (2 C-H-arom.), 80.1 (C-I), 74.2, 72.4, 72.2 (C-2, C-3, C-4), 27.4 (C-5), 20.8, 20.6, 20.6 (acetyl). Anal.: Calcd for C17H2007S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 55.12; H, 5.43; O, 30.22; S, 8.93. Phenyl 2,3,4-tri-O-ace~.l-5-thio-ct-D-x),lopyranoside
4~t. colourless oil; [a] D +294* (c 1.3,
CH2C12). IH NMR (200 MHz, CDCI3) 6 7.33 (t, 2H,
Jarom. 7.4, H-arom.), 7.06 (t, IH, Jarom. 7.4,
H-atom.), 7.00 (d, 2H, Jarom. 7.4, H-arom.), 5.73 (t, 1H, J3.4 9.9, H-3), 5.51 (dd, 1H, J1.2 3.0, J1.se 1.0, H-I), 5.27 (dd, 1H, J2.3 10.2, H-2), 5.16 (ddd, IH, J4.se 4.6, H-4), 2.99 (dd, IH, J4.5a 11.3, H-5a), 2.71 (ddd, 1H, Jsa.se 13.1, H-5e), 2.07 (s, 3H, acetyl), 2.06 (s, 3H, acetyl), 2.03 (s, 3H, acetyl). 13C NMR (50 MHz, CDCI3) 6 170.3, 169.9, 169.8 (acetyl), 155.9 (C..arora.), 129.6 (2 C-Harom.), 122.8 (C-arom.), 116.9 (2 C-H-arom.), 77.1 (C-I), 74.7, 72.9, 70.2 (C-2, C-3, C-4), 25.3 (C-5), 20.8, 20.7, 20.7 (acetyl). Anal.: Calcd for C17H2007S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 55.39; H, 5.46; O, 30.°-3; S, 8.63. 4-(2,3,4-Tri-O-acetyl-5-ttu'o-#-D-xylopyranosyl)phenol 513. white crystals; m.p. 197-198°C (diethyl ether); [~t]D-9"(c 0.6, CH2C12). IH NMR (200 MHz, CDCI3) 6 7.19 (d, 2H, J~om. 8.6, Harom.), 6.75 (d, 2H, Jarom. 8.6, H-arom.), 5.46 (d, 1H, "/2'.3' 9.0, H-2'), 5.27 (s, 1H, OH-phenol), 5.18 (ddd, 1H, J4'.5'a 9.9, H-4'), 5.12 (t, 1H, "/3'.4'9.0, H-3'), 3.89 (d, 1H, J~'.2' 10.6, H-I'), 2.92 (dd, IH, J4'.5'e 4.7, H-5'e), 2.80 (dd, 1H, Js'a.5'e 13.4, H-5'a), 2.05 (s, 3H, acetyl), 2.02 (s, 3H, acetyl), 1.73 (s, 3H, acetyl). 13C NMR (50 MHz, CDCI3) 6 170.1, 170.0, 169.9 (acetyl), 156.1 (C-
M. Baud~. et al. /Tetrahedron 54 (1998) 13783-13792
13791
arom.), 129.5 (2 C-l-l-arom.), 126.9 (C-arom.), 115.6 (2 C-H-arom.), 75.4, 74.6, 72.9 (C-2', C-3', C4'), 49.0 (C-I'), 31.2 (C-5'), 20.9, 20.6, 20.3 (acetyl). Anal.: Calcd for C17H2007 S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 55.51; H, 5.52; O, 30.39; S, 8.71. 4-(2,3,4-Tri-O-ace~.l-5-thio-a-D-xylopyranosvl)phenol
5c~. This
compound
obtained
by
crystallization from enriched mother liquors was spoiled by trace amount of 513. colourless oil; 1H NMR (200 MHz, CDCI3) ~ 7.41 (d, 2H, Jarom 8.6, H-arom.), 6.80 (d, 2H, Jarom 8.6, H-arom.), 6.54 (s, 1H, OH-phenot),5.58 (t, 1H, "/3'.-)' 8.4, H-3'), 5.38 (dd, IH, J2'.3' 8.8, H-2'), 5.14 (ddd, 1H, J4'.5'a 9.2, H-4'), 4.40 (d, 1H, J]'.2' 4.5, H-I'), 2.88 (dd, IH, J4'.5'e 4.2, H-5'e), 2.70 (dd, 1H, JS'a.5'e 13.8, H-5'a), 2.11 (s, 3H, acetyl), 2.06 (s, 3H, acetyl), 2.00 (s, 3H, acetyl). I3C NMR (50 MHz, CDCI3) 6 170.4, 170.3, 170.3 (acetyl), 155.7 (C-arom.), 130.2 (2 C-H-arom.), 128.5 (C-
arom.), 115.6 (2 C-H-arom.), 73.4, 71.5, 70.0 (C-2', C-3', C-4'), 44.0 (C-I'), 27.5 (C-5'), 21.0, 20.9, 20.9 (acetyl). Anal.: Caled for C17H2007S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 54.98; H, 5.47; O, 30.10; S, 8.51.
3(S),4(S)-3,4-Diaceto.w-2(R)-[bis-(4-hydro.~,phenyl)methyl]tetrahydrothiophene 6. Zinc chloride (0.768 g, 5.63 mmol) was melted by heating in a Schlcnk tube connected to a vacuum line. After cooling down to room temperature under an inert atmosphere, a solution of 2 ( 1 g, 2.82 mmol) and phenol (0.53 g, 5.63 mmol) in anhydrous chlorolorm (10 mL) was added. Thc rcaction mixturc was stirred at room temperature for 12 h, then filtered through a bed of celite and neutralized by adding tnethylaminc. The solution was evaporated under vacuum and the obtained residue was applied to a column of silica gel eluted with ethyl acetate-hexane 1:1 (v/v) to give 6 in admixture with other compounds. Two successive chromatographic separations with ethyl acetate-hexane 3:2 (v/v) thcn chloroform-methanol 14:1 (v/v) gave pure 6 (0.48 g, 1.19 mmol, 42% yield), clear pink oil; [Ct]D-27* (c 0.5, CH2CI2). IH NMR (200 MHz, CDCI3) 67.14 (d, 2H, J8.6, H-arom.), 7.08 (d, 2H, J8.6, H-arom.), 6.74 (d, 2H, J8.6, H-arom.), 6.69 (d, 2H, J8.7, H-arom.), 5.60-5.40 (s, 2H, OH-phenol), 5.35-5.25 (m, 2H, H-3', H-4'), 4.13 (dd, 1H, J2'.3' 5.0, H-2'), 4.03 (d, 1H, J]'.2' 11.2, H-I'), 3.15 (dd, 1H, J4.5.a 5.3, Js'a.5'b 11.4, H-5'a), 2.85 (dd, IH, J4'.5'h 6.0, H-5'b), 2.08 (s, 3H, acetyl), 1.73 (s, 3H, acetyl). 13C NMR (50 MHz, CDCI3) ,5 170.5, 170.5 (acetyl), 154.6 (2
C-arom.), 135.0, 134.4 (C-arom.), 129.1, 129.1, 128.9, 128.9, 115.5, 115.5, 115.5, 115.5 (C-Harom.), 79.8, 78.0 (C-3', C-4'), 56.0 (C-2'), 51.8 (C-I'), 31.4 (C-5'), 21.0, 20.5 (acetyl). 3(S),4(S)-3,4-Dihydroxy-2(R)-[bis-(4-hydro.~,phenyl)nzethyl]tetrahydrothiophene
7.
Deacetylation of 6 (0.205 g, 0.51 mmol) in dr3' methanol was earned out under Zempl6n conditions by addition of 3 drops of a sodium methoxide solution in methanol (-1M) followed by stirring at room temperature for 12 h. Purification by chromatography on silica gel with dichloromethanemethanol 4:1 (v/v) led to 7 (0.146 g, 0.46 mmol, 90% yield), clear pink oil; [et]D +44" (c 0.6, CHCI3)" IH NMR (200 MHz, MeOD) 6 7.11 (d, 4H, J8.6, H-atom.), 6.68 (d, 2H, J 8.6, H-arom.), 6.66 (d, 2H, J8.6, H-arom.), 4.19 (d, 1H, Jl'.2' 10.1, H-I'), 4.16 (m, 1H, J4'.5'a 4.9, H-4'), 3.93 (dd, IH, J2'.3' 3.5, H-2'), 3.85 (t, 1H, J3'.4' 3.5, H-3'), 3.05 (dd, 1H, Js'a.5'b 11.2, H-5'a), 2.65 (dd, 1H, J4'.5'b 4.3, H-5'b). 13C NMR (50 MHz, MeOD) 6 155.5 (2 C-atom.), 135.8, 135.3 (C-
arom.), 129.4, 129.4, 129.0, 129.0, 115.0, 115.0, 114.7, 114.7 (C-H-arom.), 80.5 (C-3'), 78.9 (C-
13792
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
4'), 57.2 (C-2'), 54.7 (C-I'), 35.0 (C-5'). A H,C-COSY con'elation, obtained from a MeOD solution, led to unambiguous assignments of the carbon resonances numbered as for 5 and 6 according to the parent thiosugar. 4-(5-Thio-O-D-xylopyranosyl)phenol
8. To a suspension of 4-(2,3,4-tri-O-acetyi-5-thio-fl-D-
xylopyranosyi)phenol 513(1 g, 2.7 mmol) in dry methanol (80 mL), sodium methoxide in methanol was added (1.2 mL of a 19 % methanolic solution, 4 mmol) under an inert atmosphere. After stirring for 3 h, the reaction mixture was neutralized with an acidic resin (Ambertite IR 120 H ÷) before filtration and concentration under reduced pressure. The residue was purified by chromatography on a column of silica gel eluted with chloroform-methanol 19:.1 v/v and crystallization from methanoldiethyl ether to give 8 (170 mg, 26% yield), mp 266 *C; [c~]D22+36* (c 0.38, CH3OH). IH NMR (300 MHz, DMSO) 69.3 (broad s, 1H, OH-phenol), 7.06 (d, 2H, H-arom.), 6.66 (d, 2H, H-arom.), 3.55 (d, 1H, Jr,2' 10.1, H-I'), -3.55 (m, 1H), 3.0 (m, 1H), 2.5 (m, 3H). Anal.: Calcd for C 11H1404S: C, 54.54; H, 5.82; found: C, 54.78; H, 5.95.
REFERENCES 1 Samreth, S.; Barberousse, V.; Bellamy, F.; Horton, D.; Masson, P.; Millet, J.; Renaut, P.; Sepulchre, C.; Theveniaux, J. Actual. Chim. ThOr.- 21 ~me sgrie, 1994, 21, 23-33. 2 Bellamy, F.; Barberousse, V.; Martin, N.; Masson, P.; Millet, J.; Samreth, S.; Sepulchre, C.; Theveniaux, J.; Horton, D. Eur. J. Med. Chem., 1995, 30, 101-115 and references therein. 3 Baudry, M.; Bouchu, M.-N.; Descotes, G.; Praly, J.-P.; Bellamy, F. Carbohydr. Res., 1996, 282, 237-246. 4 Samreth, S.; Millet, J.; Bellamy, F.; Bajgrowicz, J.; Barberousse, V.; Renaut, P. Eur. Pat. Appl., EP 365397; Chem. Abstr.: 199 I, 114, 122,964k. 5 Whistler, R.L.; Van Es, T.J.J. Org. Chem., 1963, 28, 2303-2304. 6 Baudry, M.; Barberousse, V.; Descotes, G.; Faure, R.; Pires, J.; F'raly, J.-P. Tetrahedron, 1998, 54, 7431-7446. 7 Baudry, M.; Barberousse, V.; Descotes, G.; Pires, J.; Praly, J.-P. Tetrahedron, 1998, 54, 7447-7456. 8 AI-Masoudi, N. A. L. ; Hughes, N. A. J. Chem. Soc. Perkin Trans. I, 198 7, 2061-2067. 9 Boz6, E.; Boros, S.; Kuszmann, J. Carbohydr. R es., 1997, 304, 271-280. 10 Baudry, M. Thesis, University of Lyon, November 8 th, 1994. 11 Matsumoto, T.; Katsuki, M.; Suzuki, K. Tetrahedron Let/., 1988, 29, 6935-6938. 12 Mahling, J.-A.; Schmidt, R.R. Synthesis, 1993, 325-328. 13 Briner, K.; Vasella, A. Heir. Chim. Acta, 1990, 73, 1764-1778. 14 Schmidt, R.R.; Hoffmann, M. TetrahedronLett., 1982, 23, 409-412. 15 Hosoya, T.; Ohashi, Y.; Matsumoto, T.; Suzuki, K. Tetrahedron Lett., 1996, 37, 663-666. 16 Yamaguchi, M.; Horiguchi, A.; Fukuda, A.; Minami, T. J. Chem. Soc., Perkin Trans. I, 1990, 1079-108°,. 17 Castro-Palomino, J.C.; Schmidt, R.R. LiebigsAnn., 1996, 1623-1626 and references therein. 18 a- Gurudutt, K.N.; Rao, L.J.M.; Rao, S.; Srinivas, S. Carbohydr. Res., 1996, 285, 159165. b-Kalvoda, L. Coll. Czeck. Chem. Comm. 1973, 38, 1679-1692. 19 Nishizawa, M.; Garcia, D.M.; Yamada, H. Synlett, 1992, 797-799. 20 Ogawa, T.; Matsui, M. Carbohydr. Res., 1976, 51, C13-C18. 21 Rees, R.G.; Webb, A.F.J. Organometal. Chem., 1968, 12, 239-240. 22 Lubineau, A.; Meyer, E.; Place, P. Carbohydr. Res., 1992, 228, 191-203. 23 Tietze, L.-F.; Fischer, R.; Guder, H.-J. Tetrahedron Lett., 198 2, 23, 4661-4664. 24 Mukaiyama, T.; Matsubara, K. Chem. Lett., 1992, 1041-1044 and references therein. 25 Nashed, E.M.; Glaudemans, C.P.J.J. Org. Chem., 1989, 54, 6116-6118. 26 Olah, G.A.; Husain, A.; Gupta, B.G.B.; Salem, G.F.; Narang, S.C.J. Org. Chem., 198 I, 46, 5212-5214. 27 Yuasa, H.; Kamata, Y.; Hashimoto, H. Angew. Chem. Int. Ed. Engl., 1997, 36, 868-870.
Pergamon
SYNTHETIC STUDIES IN THE 5-THIO-D-XYLOPYRANOSE SERIES part 3: PREPARATIONS OF O-PHENYL AND C-(4-HYDROXYPHENYL) 5-THIO-D-XYLOPYRANOSIDES Michel Baudry a, V~ronique Barberousse b, Yolande Collette b, G~rard Descotes a Joaquim Pires c, Jean-Pierre Praly a,,, Soth Samreth b a Laboratoire de Chimie Organique II associ6 au CNP.S, Universit6 Claude-Bernard Lyon I CPE-Lyon, 43 Boulevard du 11 Novembre 1918. F-69622 Villeurbanne, France b Laboratoires Fouruier SA, Centre de Recherche. 50 roe de Dijon. F-21121 Daix, France c SYNKEM, Division de Plasto SA, 47 rue de Ixmgvic, F-21301 Chen6ve, France
Received 9 March 1998; accepted 8 September 1998
Abstract: Treatment of the ~-O-acetyl 5-thio-a-D-xylopyranosyl l-O-trichloroacefmidate 1 and phenol in the presence of BF3 • OEt2 at low temperature led to the corresponding Ophenyl (4:-30 % yield) and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides (5:,-,40 % yield) as a result of competitive O-glyco~idation and aromatic electrophilic substitution. At higher temperatures, O-phenyl 5-thio-~D-xylopyranoside was shown to rearrange readily to give C(4-hydroxyphenyl) 5-thio-~D-xylopyranoside as the thermodynamically favoured product. In the presence of zinc oxide, the sugar a-bromide 2 and phenol led also to a mixture of 4 (-20 % yield) and 5 (--40 % yield). Compound 5 was prudueed from 2 on treatment with dibutyltin diphenoxide and from the I-O-trimethylsilyl sugar derivative 3 and trimethylsilyloxybenzenc, when reacted in the presence of trimethylsilyl triflate in dichloromethane (35 % yield). Under these conditions, a tetrahydrothiophene derivative was observed ( 6 : - 5 % yield). However, the sulfur transannular participation pathway which accounted for its formation became predominant when the a-bromide 2 and phenol we~ treated for several hours with zinc chloride, thus affording 6 in a 42 % yield. © 1998 Elsevier Science Ltd. All rights reserved. After various O- and S-aryl 5-thio-D-xylopyranosides were found to be orally active antithrombotic drugs, t a large array of analogs has been prepared for pharmacological studies. 2 As a result of our involvement in related synthetic investigations, we recently reported first on the radicalmediated bromination of 2,3,4-tri-O-acetyl-5-thio-D-xylopyranosyl bromide as a route to 5-thio-Dxylopyranonolactone derivatives. 3 Then, we focused on the synthesis of various C-aryl 5-thio-Dxylopyranosides in order to explore new routes to structural analogs of the known active compounds in this series. It was found that the 2,3,4-tri-O-acetyl-5-thio-D-xylopyrunosyl cations produced from either the a-trichloroacetimidate 14 or the a-bromide 24.5 reacted readily with different heterocyclic aromatic compounds to afford the corresponding C-hetaryl 5-thio-D-xylopyranosides 6 by electrophilic substitution. A similar behaviour was observed with electron-rich polyphenol derivatives which also led efficiently to
C-aryl
5-thio-D-xylopyranosides, 7 in
particular when
the
more
reactive
trichloroacetimidate 1 was transformed at low temperatures, thus avoiding further reactions triggered by sulfur intraannular participation. 6-9 We now describe further experiments aiming at the preparation of O-phenyl and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides by different approaches applied to phenol and either 1, 2 or the 1-O-trimethylsilyl 5-thio-a-D-xylopyranc~se 3.10
E-mail: jean-pierre.praly@univ-lyon 1.fr; Fax: 33-(0)4-78-89-89-14
0040-4020/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved. PH: S0040-4020(98)00848-5
13784
M. Baudr3, et al. / Tetrahedron 54 (1998) 13783-13792
When electrophilic substitution of phenol was attempted by treatment of the atrichioroacetimidate 1 in the presence of boron trifluoride etherate at -78"C, a complex product distribution was observed. In contrast to the case of polyphenol derivatives which led essentially to !
C-aryl 5-thio-D-xylopyranosides,7 both O-phenyl and C-(4-hydroxyphenyl) analogs were produced, the latter being found somewhat more abundant under the applied reaction conditions. The formation of both types of 5-thio-D-xylopyranosides can be easily rationalized by invoking the lower reactivity of phenol towards electrophilic substitution as compared to electron-rich polyphenol derivatives7 and heteroaromatic compounds,6 in keeping with related precedents reported in the literature.ll-13 However, in order to get a better insight into the reaction pathways (t-->C-rearrangement, anomerization), the reaction of 1 with phenol at different temperatures and the reactivity of each anomer of the O-phenyl 5-thio-D-xylopyranoside 4 were investigated in more details.
AcO-'~--....~S~
1
C6H5OH AcO"t'-..-.~S~
BF,.O CCI3
+
AcO,-..~.....~& A C O ~ H
4
5
Table 1:BF3 " OEt2-catalyzed reaction of 1 with phenol: influence of temperature on product distribution a T°C
4
~Ct ratio b
yield, % -78 -50 -20 40
30 33 40 16
5
~ot ratio b
yield. % 8/2 8/2 8/2 0/10
41
40 30 49
8/2 7/3 7/3 8/2
a T h e ratio 1 / C 6 H 5 O H ! B F 3 • O E t 2 was 1 / 2 / 0.1 ; b estimated by 1H N M R .
These experiments showed that, in the -78 to -20°C temperature range, O-phenyl and C-aryl 5-thio-D-xylosidation occurred competitively, the balance being in favour of the latter class of compounds (except for the experiment run at -20"C which show some discrepancy, Table 1), the 3anomer of 5 being predominant in each case. 6.7 This was expected on the basis of the preferred formation of 1,2-trans glycopyranosides from acetylated glycosyl donors and the higher thermodynamic stability of C-aryl &D-glycopyranosides.14.15 Whereas the combined 5-thio-Dxylopyranosides formed amounted to --65-70% whatever the reaction temperature, the experiment carried out at 40"C showed clearly a decreased proportion of the remaining O-phenyl 5-thio-Dxylopyranoside which contained only the more stable a-anomer.16 Conversely, a higher proportion of the C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides
was observed, with a somewhat higher
proportion of the #-anomer. These changes could be explained by the occurrence of a O -->Crearrangement which should be easier for 4p.
13785
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
To check this hypothesis, solutions of each anomer of 4 in CD2C12 containing a catalytic amount of BF3 . OEI2 and kept at controlled temperatures were examined by IH NMR. Since 413 remained unchanged after 30 min at temperature below -30°C (Table 2), it is reasonable to propose, in accordance with the similar proportion of 4 and 5 formed from 1 at either -78 or -50"C (Table 1), that compounds 5 arised from direct electrophilic substitution of phenol, even at low temperature (scheme). Anomerization and O-->C-rearrangement occurred around room temperature as shown by the presence of 4 a and 513 in the medium when 413 was exposed briefly tt, acidic conditions (Table 2). The fact that only 4c~ was present after 12 h in addition to 513 suggested that 4 a was much less reactive under the applied conditions than its /~-counterpart. 16 This was confirmed by applying the same protocol to 4 a which produced only 513 in detectable amount after 12 h at 27"C. This reactivity pattem and the higher thermodynamic stability of the C-5-thio-/3-D-xylopyranoside 513 paralleled observations collected from
analogs
of
natural
sugars.
Therefore,
it
is
established
that
C-5-ta,lo-D-
xylopyranosidation of phenol occurred in competition with the formation of O-5-thio-D-xylosides, even at low temperature (-78°C) within 30 min. Provided a /3-anomeric configuration, these latter rearranged readily around normal temperature to produce the more stable ~configurated C-5-thio-Dxylopyranoside (scheme). This and our previous observations 6.7 showed the higher reactivity of the 5-thiosugar studied since trichloroacetimidates of natural sugars and electron-rich aromatic compounds have been shown to lead to C-glycosides around room temperature. 17
Table 2: Transformation of 4 catalyzed by BF3 • OEt2 in CD2CI 2 : product distribution estimated by IH NMR Substrate
41~
" " " 4a " "
"
T
Time
*C
h
4~
Product distribution
4a
5~
-64 -30 27 27
0.5 0.5 0.5 12
100 100 50 0
0
0
0 15 30
0 35 70
-64 -30
0.5 0.5
27 27
0.5 12
0 0 "4)
100 100 -100
0 0 "43
0
~70
~30
13786
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
BF 3 , OEt 2 IA cO--'k-'-~iS +
AcO~S~,
A cO"~"----IS~
A cO "~"---.~...-~S~ 1
+= co- oo/ o
AcO ""&"'----IS~
°
°
4c~ 0 . ~
5~
.
.
.
.
.
.
0 OH
A cO ~ ' ~ - ~ ' ~ J AcO
,, AcO"k""-'tS~
tem
AcO-'-k----..--~S\
~co~.~..~o~~~co AcO ~ 4~ 5~
Acu
/~'~OH
~J
'1
t
tt
4~J
Scheme: Proposed reaction pathways for the BF3 • OEt 2 catalyzed reaction of 1 with phenol
The a-bromide 2 was reacted with phenol under three different sets of conditions, namely in the presence of either zinc oxide, or zinc chloride or dibutyltin diphenoxide. Use of ZnO 18,19 has already been shown
to produce
selectively C-aryl
5-thio-D-xylopyranosides
derived
from
polyphenols (resorcinol, phloroglucinol) by heating for - l h in acetonitrile in the presence of molecular sieves, v Since the analogous C-aryl 5-thio-D-xylopyranosides were not produced when either anisole or p-dimethoxybenzene were used as the aromadc compound, the formation of O-aryl intermediates prone to a O-->C-rearrangement was assumed to account for our observations. In contrast to the selectivity found in the case of polyphenols, reaction of 2 and phenol in the presence of ZnO in acetonitrile led to a complex mixture after 15 h at 50°C. In spite of the prolonged reaction time, O-phenyl and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides were observed ( 4 : 2 0 %, $: 40 % isolated yields) in keeping with the expected lower reactivity of phenol towards electrophilic substitution. When the bromide 2 was reacted with phenol in chloroform in the presence of ZnCI2
13787
M. Baud~. et al. / Tetrahedron 54 (1998) 13783-13792
(2/phenol/ZnCl2 : 1/~2)
for 12 h at room
temperature, besides minor amounts of 4 and 5, compound
6 was isolated as the major product. It was deacetylated uneventfully under Zempl6n conditions to afford 7. Their structures, established on the basis of NMR data and in particular a H , C - C O S Y correlation recorded with 7, showed a ring restriction explained by a sulfur intraannular participation mechanism.6.7. 9
AoO'~'~SXAcO.~I~,.,~,~C.H5OH~__~
S
O H
AcOBr ZnCI2
CHCF3 AcO ¢ Argon
2
MeONa (cat.).__
MeOH 6
OH
HO
OH
7 OH
As another route to 413, we tried to glycosylate the dibutyltin diphenoxide with the a-bromide 2 according to a method described by Ogawa et al. 2° This procedure exploited the enhancement of the nucleophilicity of hydroxyl groups by alkylstannylation. The diphenoxide, prepared according to a standard method, 21 was engaged in the glycosylation step in the presence of SnCl4. In contrast to the fact that the O-phenyl glucoside was produced in a 89 % yield 2° when this method was applied to natural sugar derivatives, surprisingly, in our case, the C-xyloside was obtained in a 59% yield. This can be again explained by the enhanced reactivity of 5-thio sugars due to a more efficient stabilization of a proximal positive charge at C-l by the sulfur atom, thus favouring the O-->C-rearrangement of intermediate O-5-thio-D-xylopyranosides. The obtained compound 513 was deacetylated to afford 8.
n-Bu
SnCi4 AcO-~---~S~
~OH
AcO 513
2
59 %
Since 1-O-deacetylation of 1,2,3,4-tetra-O-acetyl-5-thio-D-xylopyranose could be selectively achieved with hydrazine acetate to afford 2,3,4-tri-O-acetyl-5-thio-D-xylopyranose,4. 6 this compound was subjected to the Mitsunobu reaction in the presence of phenol. 22 The results were disappointing, so we turned our attention to another glycosidation method involving trimethylsilyl derivatives. 23 Hence, encouraged by the excellent yields and stereoselectivities reported for the preparation of O-aryl glycosides of
natural
sugars 23
by
means
of
l-O-trimethylsilyl
sugar
derivatives 24 and
trimethylsilylphenols, 23 this approach was applied to 3, obtained by adaptation of a reported method, 25 and trimethylsilyloxybenzene. 26 When the reaction was performed in dichloromethane at -500C for a short time (< 3 h) in the presence of trimethylsilyl triflate in either catalytic or stoechiometric amounts, no transformation was visible. The same conclusion was reached when
13788
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
attempting the reaction at 20"C for several hours with catalytic amounts of trimethylsilyl triflate, whereas alarger quantity of this reagent (1 eq, 15 min) afforded 5 in low yield (25%) accompanied by polar compounds visible on TLC plates. These unidentified compounds were also produced when the reaction was carried out with trimethylsilyl triflate (1 eq), trimethylsilyloxybenzene (1.2 eq) at controlled temperature (-50 to 20"C) for 4 h. However, under these conditions, 5 could be recovered in a 35 % yield.
OSiMe3
S
0 AcO /SiMe3TMSOTI C H2CI2
OH
3
5
35%
t]/ct 88/12
AcO /
OH
6 -5%
In summary, treatment of the tri-O-acetyl 5-thio-a-D-xylopyranose 1-O-trichloroacetimidate 1 and phenol in the presence of BF3 • OEt2 at low temperature led to the corresponding O-phenyl (4: -30% 3field) and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides (5: -40% yield) as a result of competitive O-glycosidation and aromatic electrophilic substitution, in keeping with the conclusion of independent
studies
using
4-cyanobenzenethiol
and
2,3,4,6-tetra-O-acetyl-5-thio-a-D-
glucopyranosyl bromide. 9 At higher temperatures, O-phenyl 5-thio-o-D-xylopyranoside was shown to
rearrange
readily
to
give
C-(4-hydroxyphenyl)
5-thio-o-D-xylopyranoside as
the
thermodynamically favoured product. 14.15 The easier acid-catalyzed (O-->C)-rearrangement of 41~ as compared to 4et is consistent with the generally observed higher reactivity of 0-D-glycosides, less stable than their a-counterparts due to the anomeric effect. 27 In the presence of zinc oxide, the sugar a-bromide 2 and phenol led also to a mixture of 4 (~20% yield) and 5 (--40% yield). Compound 5 was produced from 2 on treatment with dibutyltin diphenoxide and from the 1-O-trimethylsilyl sugar derivative 3 and trimethylsilyloxybenzene, when reacted in the presence of trimethylsilyl triflate in dichloromethane (59 and 35% yield, respectively). Under these conditions, a tetmhydrothiophene derivative was observed (6: -5% yield). However, the sulfur transannular participation pathway which accounted for its formation became predominant when the a-bromide 2 and phenol were treated for several hours with zinc chloride, thus affording 6 in a 42% yield.
ACKNOWLEDGMENT One of us (M. B.) gratefully thank the Association Nationale de la Recherche Technique and SY NKEM (groupe Foumier) for financial support (convention CI FRE).
M. Baudry et aL / Tetrahedron 54 (1998) 13783-13792
EXPERIMENTAL
General methods. The general methods were similar to those previously indicated. 2,3,4-Tri-O-acetyl-l-O-trimethylsilyl-5-thio-a-D-xylopyranose 3. Hexamethyldisilazane (0.90 g, 5.6 mmol), trimethylsilyl chloride (0.72 g, 6.63 mmol) and pyridine (0.65 g, 8.3 mmol) were successively added, with stirring under a nitrogen atmosphere to a solution of 2,3,4-tri-O-acetyl-5thio-a-D-xylopyranose (1.613 g, 5.52 mmol) in anhydrous dichloromethane (25 mL) at room temperature. A precipitate was immediately observed. After stirring for 45 min at room temperature, the reaction medium was filtered through a bed of celite and the organic phase was concentrated under reduced pressure. The crude residue was purified by chromatography on silica gel with ethyl acetatehexane 1:3 (v/v) as the mobile phase to yield 3 (1.79 g, 4.90 retool, 89% yield), white crystals; m.p. 100-101"C; [Ct]D+188* (c 0.6, CH2C12). 1H NMR (200 MHz, CDCt3) 6 5.53 (t, 1H, J3A 9.6, H-3), 5.13-4.99 (m, 3H, H-I, H-2 et H-4), 3.04 (dd, 1H, J4.sa 11.4, Jsa.Se 12.9, H-5a), 2.70 (dd, 1H,
Ja.se4.4, H-5e), 2.04 (s, 6H, acetyl), 2.03 (s, 3H, acetyl), 0.18 (s, 9H, H-SiMe3). 13C NMR (50 MHz, CDCI 3) 6 170.1, 169.9, 169.9 (acetyl), 76.1, 73.3, 72.2, 70.2 (C-I, C-2, C-3, C-4), 24.9 (C-5), 20.9, 20.8, 20.7 (acetyl), 0.0 (C-SiMe-3).
Anal.: Calcd for C14H2407SSi (364.49): C, 46.13; H, 6.64; S, 8.80; found: C, 46.51; H, 6.48; S, 9.17.
Phenyl 2,3, 4-tri-O-ace~l-5-thio-D-xylopyranoside 4 and 4-( 2,3, 4-tri-O-acetyl-5-thio-Dxylopyranosyl)phenol 5: from 1 : To a solution containing 1 (0.31 g, 0.71 mmol) and phenol (0.134 g, 1.42 mmol) in anhydrous dichloromethane (4 mL) at -78"C, boron trifluoride etherate (9 IaL, -43.07 mmol) was added with stirring under an inert atmosphere. After approximately 0.5 h, triethyt amine (2.1 mmol) was added, then the medium was taken up in chloroform and the organic phase was successively washed with 5% NaOH solution and water. After drying (MgSO4), the organic phase was filtered through a bed of celite, then concentrated under reduced pressure. The obtained residue was applied to a column of silica gel eluted with chloroform-methanol 14:1 (v/v) as the mobile phase to afford two fractions. Each of these fractions were subjected to another chromatography with ethyl acetate-hexane 1:1 (v/v) to yield 4 (0.077 g, 0.21 retool, 30% yield, r/a--84/16) and 5 (0.108 g, 0.29 mmol, 41% yield,/~/c~=77/23). Following the same protocol, additional experiments were run at -50, -20 and 40"C (Table 1). from 2 and zinc oxide: Zinc oxide (0.78 g, 9.58 mmol) and commercially available 13 X type molecular sieve (5.3 g) were introduced into a flask which was connected to a vacuum pump before heating to 80"C for 30 rain. After cooling down to room temperature, anhydrous acetonitrile and phenol (2.5 g, 26.56 retool) were introduced into the flask and the suspension was stirred for 30 rain at 50"C under nitrogen. After addition of 2 (1.7 g, 4.79 mmol), the reaction was shown by TLC to be complete within 15 h at 50"C. The reaction medium was centrifugated, the solids were separated and thoroughly rinsed with ethyl acetate whereupon the organic phases were combined prior to concentration under reduced pressure. The obtained residue was taken up in chloroform. The organic phase was washed twice with 10% NaOH, then water. Drying (MgSO4), filtration through a bed of celite and concentration of the solution led to a residue which was applied to a column of silica gel eluted with ethyl acetate-hexane 1:1.5 (v/v) to afford 4 (0.35 g, 0.94 mmoi, 20% yield,/~/a=20/80) and 5 (0.696 g, 1.89 mmol, 40% yield, ~ct--64/36). The anomers of 4 were resolved by
13789
13790
M. Baudr)' et al. / Tetrahedron 54 (1998) 13783-13792
chromatography using a column of silica gel eluted with ethyl acetate-hexane 1:1 (v/v). 413 was crystallized from diethyl ether. 513 was obtained by selective crystallization from diethyl ether. from 2 and dibutvltin dipl~enoxide: After stirring for 30 min, under an inert atmosphere, a solution of the a-bromide 2 (2 g, 5.6 mmol) and dibutyltin diphenoxide (2 g, 4.8 mmol) in a 1:1 anhydrous mixture of toluene and acetonitrile (10 mL) containing 4 A molecular sieves, tin tetrachtoride (0.56 mL, 4.7 mmol) was added. After heating for 5 h, the solids were removed by filtration. The filtrate was diluted with ethyl acetate. The organic phase was washed with 1N HCI then water, before drying and concentration under reduced pressure. The residue was purified by chromatography on silica gel, with toluene-ethyl acetate 5:1 v/v as the mobile phase to give 513( 1.05 g, 59% yield). from 3. A solution of 3 (0.10 g, 0.27 mmol) and trimethylsilyloxybenzene (0.07 g, 0.41 mmol) in anhydrous dichloromethane was cooled down to -50"C under argon. After addition of trimethylsilyl triflate (0.061 g, 0.27 mmol), the temperature was allowed to rise to +20°C and the medium was stirred for 4 h. Then, triethylamine (400 l.tL) was added and the medium was concentrated under reduced pressure. The obtained residue was purified by silica gel chromatography with ethyl acetatehexane 1:1.5 (v/v) as the mobile phase to give the C-5-thio-D-xyloside 5 (0.036 g, 0.096 mmol, 35% yield, ffa=88/12). Phenyl 2,3,4-tri-O-ace~l-5-thio-~-D-.u,lopyranoside 413. white crystals; m.p. 153-1550C (diethyl ether); [a]D-69° (c 0.5, CH2C12). 1H NMR (200 MHz, CDCI3) 6 7.31 (dd, 2H, Jarom 8.4, Jarom. 7.5, H-arom.), 7.09-7.01 (m, 3H, H-arom.), 5.53 (t, 1H, J2.3 9.1, J3.4 9.1, H-3), 5.19 (d, 1H, JI.2 9.0, H-I), 5.17-5.06 (m, 2H, H-2, H-4), 2.97 (dd, IH, Ja.se 3.3, J5a.Se 13.7, H-5e), 2.67 (dd, IH, J4.5a 10.1, H-5a), 2.05 (s, 6H, acetyl), 2.02 (s, 3H, acetyl), t3C NMR (50 MHz, CDCI 3) 6 169.8, 169.8, 169.4 (acetyl), 157.0 (C-arom.), 129.7 (2 C-H-arom.), 123.1 (C-arom.), 116.6 (2 C-H-arom.), 80.1 (C-I), 74.2, 72.4, 72.2 (C-2, C-3, C-4), 27.4 (C-5), 20.8, 20.6, 20.6 (acetyl). Anal.: Calcd for C17H2007S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 55.12; H, 5.43; O, 30.22; S, 8.93. Phenyl 2,3,4-tri-O-ace~.l-5-thio-ct-D-x),lopyranoside
4~t. colourless oil; [a] D +294* (c 1.3,
CH2C12). IH NMR (200 MHz, CDCI3) 6 7.33 (t, 2H,
Jarom. 7.4, H-arom.), 7.06 (t, IH, Jarom. 7.4,
H-atom.), 7.00 (d, 2H, Jarom. 7.4, H-arom.), 5.73 (t, 1H, J3.4 9.9, H-3), 5.51 (dd, 1H, J1.2 3.0, J1.se 1.0, H-I), 5.27 (dd, 1H, J2.3 10.2, H-2), 5.16 (ddd, IH, J4.se 4.6, H-4), 2.99 (dd, IH, J4.5a 11.3, H-5a), 2.71 (ddd, 1H, Jsa.se 13.1, H-5e), 2.07 (s, 3H, acetyl), 2.06 (s, 3H, acetyl), 2.03 (s, 3H, acetyl). 13C NMR (50 MHz, CDCI3) 6 170.3, 169.9, 169.8 (acetyl), 155.9 (C..arora.), 129.6 (2 C-Harom.), 122.8 (C-arom.), 116.9 (2 C-H-arom.), 77.1 (C-I), 74.7, 72.9, 70.2 (C-2, C-3, C-4), 25.3 (C-5), 20.8, 20.7, 20.7 (acetyl). Anal.: Calcd for C17H2007S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 55.39; H, 5.46; O, 30.°-3; S, 8.63. 4-(2,3,4-Tri-O-acetyl-5-ttu'o-#-D-xylopyranosyl)phenol 513. white crystals; m.p. 197-198°C (diethyl ether); [~t]D-9"(c 0.6, CH2C12). IH NMR (200 MHz, CDCI3) 6 7.19 (d, 2H, J~om. 8.6, Harom.), 6.75 (d, 2H, Jarom. 8.6, H-arom.), 5.46 (d, 1H, "/2'.3' 9.0, H-2'), 5.27 (s, 1H, OH-phenol), 5.18 (ddd, 1H, J4'.5'a 9.9, H-4'), 5.12 (t, 1H, "/3'.4'9.0, H-3'), 3.89 (d, 1H, J~'.2' 10.6, H-I'), 2.92 (dd, IH, J4'.5'e 4.7, H-5'e), 2.80 (dd, 1H, Js'a.5'e 13.4, H-5'a), 2.05 (s, 3H, acetyl), 2.02 (s, 3H, acetyl), 1.73 (s, 3H, acetyl). 13C NMR (50 MHz, CDCI3) 6 170.1, 170.0, 169.9 (acetyl), 156.1 (C-
M. Baud~. et al. /Tetrahedron 54 (1998) 13783-13792
13791
arom.), 129.5 (2 C-l-l-arom.), 126.9 (C-arom.), 115.6 (2 C-H-arom.), 75.4, 74.6, 72.9 (C-2', C-3', C4'), 49.0 (C-I'), 31.2 (C-5'), 20.9, 20.6, 20.3 (acetyl). Anal.: Calcd for C17H2007 S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 55.51; H, 5.52; O, 30.39; S, 8.71. 4-(2,3,4-Tri-O-ace~.l-5-thio-a-D-xylopyranosvl)phenol
5c~. This
compound
obtained
by
crystallization from enriched mother liquors was spoiled by trace amount of 513. colourless oil; 1H NMR (200 MHz, CDCI3) ~ 7.41 (d, 2H, Jarom 8.6, H-arom.), 6.80 (d, 2H, Jarom 8.6, H-arom.), 6.54 (s, 1H, OH-phenot),5.58 (t, 1H, "/3'.-)' 8.4, H-3'), 5.38 (dd, IH, J2'.3' 8.8, H-2'), 5.14 (ddd, 1H, J4'.5'a 9.2, H-4'), 4.40 (d, 1H, J]'.2' 4.5, H-I'), 2.88 (dd, IH, J4'.5'e 4.2, H-5'e), 2.70 (dd, 1H, JS'a.5'e 13.8, H-5'a), 2.11 (s, 3H, acetyl), 2.06 (s, 3H, acetyl), 2.00 (s, 3H, acetyl). I3C NMR (50 MHz, CDCI3) 6 170.4, 170.3, 170.3 (acetyl), 155.7 (C-arom.), 130.2 (2 C-H-arom.), 128.5 (C-
arom.), 115.6 (2 C-H-arom.), 73.4, 71.5, 70.0 (C-2', C-3', C-4'), 44.0 (C-I'), 27.5 (C-5'), 21.0, 20.9, 20.9 (acetyl). Anal.: Caled for C17H2007S (368.40): C, 55.43; H, 5.47; O, 30.40 S, 8.70; found: C, 54.98; H, 5.47; O, 30.10; S, 8.51.
3(S),4(S)-3,4-Diaceto.w-2(R)-[bis-(4-hydro.~,phenyl)methyl]tetrahydrothiophene 6. Zinc chloride (0.768 g, 5.63 mmol) was melted by heating in a Schlcnk tube connected to a vacuum line. After cooling down to room temperature under an inert atmosphere, a solution of 2 ( 1 g, 2.82 mmol) and phenol (0.53 g, 5.63 mmol) in anhydrous chlorolorm (10 mL) was added. Thc rcaction mixturc was stirred at room temperature for 12 h, then filtered through a bed of celite and neutralized by adding tnethylaminc. The solution was evaporated under vacuum and the obtained residue was applied to a column of silica gel eluted with ethyl acetate-hexane 1:1 (v/v) to give 6 in admixture with other compounds. Two successive chromatographic separations with ethyl acetate-hexane 3:2 (v/v) thcn chloroform-methanol 14:1 (v/v) gave pure 6 (0.48 g, 1.19 mmol, 42% yield), clear pink oil; [Ct]D-27* (c 0.5, CH2CI2). IH NMR (200 MHz, CDCI3) 67.14 (d, 2H, J8.6, H-arom.), 7.08 (d, 2H, J8.6, H-arom.), 6.74 (d, 2H, J8.6, H-arom.), 6.69 (d, 2H, J8.7, H-arom.), 5.60-5.40 (s, 2H, OH-phenol), 5.35-5.25 (m, 2H, H-3', H-4'), 4.13 (dd, 1H, J2'.3' 5.0, H-2'), 4.03 (d, 1H, J]'.2' 11.2, H-I'), 3.15 (dd, 1H, J4.5.a 5.3, Js'a.5'b 11.4, H-5'a), 2.85 (dd, IH, J4'.5'h 6.0, H-5'b), 2.08 (s, 3H, acetyl), 1.73 (s, 3H, acetyl). 13C NMR (50 MHz, CDCI3) ,5 170.5, 170.5 (acetyl), 154.6 (2
C-arom.), 135.0, 134.4 (C-arom.), 129.1, 129.1, 128.9, 128.9, 115.5, 115.5, 115.5, 115.5 (C-Harom.), 79.8, 78.0 (C-3', C-4'), 56.0 (C-2'), 51.8 (C-I'), 31.4 (C-5'), 21.0, 20.5 (acetyl). 3(S),4(S)-3,4-Dihydroxy-2(R)-[bis-(4-hydro.~,phenyl)nzethyl]tetrahydrothiophene
7.
Deacetylation of 6 (0.205 g, 0.51 mmol) in dr3' methanol was earned out under Zempl6n conditions by addition of 3 drops of a sodium methoxide solution in methanol (-1M) followed by stirring at room temperature for 12 h. Purification by chromatography on silica gel with dichloromethanemethanol 4:1 (v/v) led to 7 (0.146 g, 0.46 mmol, 90% yield), clear pink oil; [et]D +44" (c 0.6, CHCI3)" IH NMR (200 MHz, MeOD) 6 7.11 (d, 4H, J8.6, H-atom.), 6.68 (d, 2H, J 8.6, H-arom.), 6.66 (d, 2H, J8.6, H-arom.), 4.19 (d, 1H, Jl'.2' 10.1, H-I'), 4.16 (m, 1H, J4'.5'a 4.9, H-4'), 3.93 (dd, IH, J2'.3' 3.5, H-2'), 3.85 (t, 1H, J3'.4' 3.5, H-3'), 3.05 (dd, 1H, Js'a.5'b 11.2, H-5'a), 2.65 (dd, 1H, J4'.5'b 4.3, H-5'b). 13C NMR (50 MHz, MeOD) 6 155.5 (2 C-atom.), 135.8, 135.3 (C-
arom.), 129.4, 129.4, 129.0, 129.0, 115.0, 115.0, 114.7, 114.7 (C-H-arom.), 80.5 (C-3'), 78.9 (C-
13792
M. Baudry et al. / Tetrahedron 54 (1998) 13783-13792
4'), 57.2 (C-2'), 54.7 (C-I'), 35.0 (C-5'). A H,C-COSY con'elation, obtained from a MeOD solution, led to unambiguous assignments of the carbon resonances numbered as for 5 and 6 according to the parent thiosugar. 4-(5-Thio-O-D-xylopyranosyl)phenol
8. To a suspension of 4-(2,3,4-tri-O-acetyi-5-thio-fl-D-
xylopyranosyi)phenol 513(1 g, 2.7 mmol) in dry methanol (80 mL), sodium methoxide in methanol was added (1.2 mL of a 19 % methanolic solution, 4 mmol) under an inert atmosphere. After stirring for 3 h, the reaction mixture was neutralized with an acidic resin (Ambertite IR 120 H ÷) before filtration and concentration under reduced pressure. The residue was purified by chromatography on a column of silica gel eluted with chloroform-methanol 19:.1 v/v and crystallization from methanoldiethyl ether to give 8 (170 mg, 26% yield), mp 266 *C; [c~]D22+36* (c 0.38, CH3OH). IH NMR (300 MHz, DMSO) 69.3 (broad s, 1H, OH-phenol), 7.06 (d, 2H, H-arom.), 6.66 (d, 2H, H-arom.), 3.55 (d, 1H, Jr,2' 10.1, H-I'), -3.55 (m, 1H), 3.0 (m, 1H), 2.5 (m, 3H). Anal.: Calcd for C 11H1404S: C, 54.54; H, 5.82; found: C, 54.78; H, 5.95.
REFERENCES 1 Samreth, S.; Barberousse, V.; Bellamy, F.; Horton, D.; Masson, P.; Millet, J.; Renaut, P.; Sepulchre, C.; Theveniaux, J. Actual. Chim. ThOr.- 21 ~me sgrie, 1994, 21, 23-33. 2 Bellamy, F.; Barberousse, V.; Martin, N.; Masson, P.; Millet, J.; Samreth, S.; Sepulchre, C.; Theveniaux, J.; Horton, D. Eur. J. Med. Chem., 1995, 30, 101-115 and references therein. 3 Baudry, M.; Bouchu, M.-N.; Descotes, G.; Praly, J.-P.; Bellamy, F. Carbohydr. Res., 1996, 282, 237-246. 4 Samreth, S.; Millet, J.; Bellamy, F.; Bajgrowicz, J.; Barberousse, V.; Renaut, P. Eur. Pat. Appl., EP 365397; Chem. Abstr.: 199 I, 114, 122,964k. 5 Whistler, R.L.; Van Es, T.J.J. Org. Chem., 1963, 28, 2303-2304. 6 Baudry, M.; Barberousse, V.; Descotes, G.; Faure, R.; Pires, J.; F'raly, J.-P. Tetrahedron, 1998, 54, 7431-7446. 7 Baudry, M.; Barberousse, V.; Descotes, G.; Pires, J.; Praly, J.-P. Tetrahedron, 1998, 54, 7447-7456. 8 AI-Masoudi, N. A. L. ; Hughes, N. A. J. Chem. Soc. Perkin Trans. I, 198 7, 2061-2067. 9 Boz6, E.; Boros, S.; Kuszmann, J. Carbohydr. R es., 1997, 304, 271-280. 10 Baudry, M. Thesis, University of Lyon, November 8 th, 1994. 11 Matsumoto, T.; Katsuki, M.; Suzuki, K. Tetrahedron Let/., 1988, 29, 6935-6938. 12 Mahling, J.-A.; Schmidt, R.R. Synthesis, 1993, 325-328. 13 Briner, K.; Vasella, A. Heir. Chim. Acta, 1990, 73, 1764-1778. 14 Schmidt, R.R.; Hoffmann, M. TetrahedronLett., 1982, 23, 409-412. 15 Hosoya, T.; Ohashi, Y.; Matsumoto, T.; Suzuki, K. Tetrahedron Lett., 1996, 37, 663-666. 16 Yamaguchi, M.; Horiguchi, A.; Fukuda, A.; Minami, T. J. Chem. Soc., Perkin Trans. I, 1990, 1079-108°,. 17 Castro-Palomino, J.C.; Schmidt, R.R. LiebigsAnn., 1996, 1623-1626 and references therein. 18 a- Gurudutt, K.N.; Rao, L.J.M.; Rao, S.; Srinivas, S. Carbohydr. Res., 1996, 285, 159165. b-Kalvoda, L. Coll. Czeck. Chem. Comm. 1973, 38, 1679-1692. 19 Nishizawa, M.; Garcia, D.M.; Yamada, H. Synlett, 1992, 797-799. 20 Ogawa, T.; Matsui, M. Carbohydr. Res., 1976, 51, C13-C18. 21 Rees, R.G.; Webb, A.F.J. Organometal. Chem., 1968, 12, 239-240. 22 Lubineau, A.; Meyer, E.; Place, P. Carbohydr. Res., 1992, 228, 191-203. 23 Tietze, L.-F.; Fischer, R.; Guder, H.-J. Tetrahedron Lett., 198 2, 23, 4661-4664. 24 Mukaiyama, T.; Matsubara, K. Chem. Lett., 1992, 1041-1044 and references therein. 25 Nashed, E.M.; Glaudemans, C.P.J.J. Org. Chem., 1989, 54, 6116-6118. 26 Olah, G.A.; Husain, A.; Gupta, B.G.B.; Salem, G.F.; Narang, S.C.J. Org. Chem., 198 I, 46, 5212-5214. 27 Yuasa, H.; Kamata, Y.; Hashimoto, H. Angew. Chem. Int. Ed. Engl., 1997, 36, 868-870.