Stereospecific thio-claisen rearrangement of S-crotylic α-hydroxy ketene dithioacetals. Creation of three contiguous stereogenic centres.

Tetmhedron Vol 49.No 15.n, 3131-31421993 RmtedIIIGreatBntm

0040-4020/93 $6OO+00 0 1993PerganonPressLJd

Stereospecific Thio-Claisen Rearrangement of S-Crotyfic a-Hydroxy Ketene Dithioacetals. Creation of three Contiguous Stereogenic Centres.

PierreBeslin*

andSt&phane Penio

Laboratorre de Chume des Compos&s Tluo-orgamques (ASSOCK!mcCNRS), ISIURA,Utuversuk de CAEN, 6, Boulevard du iUarMal Jum, 14050 Caen, France

(Received rn Belgium 4 February 1993)

Abstract AN four dlastereolsomenc S-crotylrc a-hydroxy ketene drthroacetals (ZE’, ZZ’, EE and EZ’) were prepared umqurvocallyfrom S-methyl or S-crotyl (2 or E) &hydroxy drthroesters by a tandem crs-deprotonatlon with WA and S-alkylatwn These dtthioacetals underwent, ln a rt$uxmg cyclohexane solution, an easy ttio-G&en rearrangement into dlthroesters, contamrng three contiguous ctiral centres The rearrangement 1sstereospeclfic Furthermore each of thefour system led to the formation of a different major drastereolsomer, thus makng all of the four possrble isomers (arm-anti, syn-syn. an&syn and syn-ann) accessible A rehatronshlpbetween the main component configuration and the startmg duluoacetal geometry has been ruled out The observed stereospecrficrty ongrnates from two lndependant stereocontrols, an internal and an external one Theformer 1s in aggreement wtth the classuzal internal control obtamed with a 13 31 srgmatroprc shrji The latter ISa result of an asymmetric rnductronbut surprrsmngty,ISdependent on the S-crotybc double bond geometry All the results were ratlonalrsed by transrtlon state models and the cortfiguratronsproven by chemical correlanons tratiormatlon rnto known esters and Swern oxidation Introduction The Chusen rearrangement has been used extensively for the stereoselechve construction of cychc and acyclic frameworks 1 Several types of stereocontrol may be involved The internal controlof the stereochemrstry results In either stereospeclflcltyl 2 and/or a 1,3 or 1,4-chlrahty transfer* 3 More recently, choral centres directly attached to the pencychc array on the terminal carbons have also been used to induce asymmetry48, thus gvmg the posslblhty of an external control Some of these latter results have been compded by Kahn and Hehres and interpreted in terms of stereoelectromc control In a chelated version of the anlomc enolate Clausen rearrangement, the stereochermstry was governed by an external hydroxy substituted centre on carbon 1 and hence aldols with a (C&p) anti configuration were stereoselectlvely formed (Scheme 1) 6It IS worth notmg that for the startmg dlamons only (EE’) and (EZ’) systems are available

o,L’--o/“

&d$/+

;;;;“‘”

_

1

EE’and EZ dlanlons

3) CYNz

schemeI

3131

R'

IA p

OMe

[-]

3132

P BERLINand S PERRIO

The correspondmg sulphur analogue, usmg ketene dlthmacetals, do not suffer such hrmtauons, due to the avadabhty for use of both double bond geometries. Thus, we have prewously reported a &astereoselecave asymmetnc induction VIU the thto-Claisen rearrangement of the neutral S-ally1 a-hydroxy ketene dlthloacetals 7a.bThis resulted mamly m formation of syn a-ally1 p-hydroxy dthloesters, independent of the geometry of the ketene double bond (Scheme 2. Rz= H)

d SMe

-

Me

scheme 2 More recently with ketene dlthloacetals mcludmg two vlcmal centres, a quasi total stereocontrol on three contiguous centres has been observed (Scheme 2, RZ=CH3) 7~Interestmgly. dlastereoselecnvlty has been also reported by P Metznm m a parallel mveshganon on similar systems.* In this case, the asymmemc mductlon was governed by a C-substituted asymmetnc centre on carbons 1 or 6 or an 0-subsatuted asymmemc centre on carbon 6 (see Scheme 2 for the numerotation of the carbons on the pencychc nucleus)

We now &sclose our new results concerning the thm-Clalsen rearrangement of S-crotyhc a-hydroxy ketene dlthloacetals 13-16 mto dlastereolsomenc a-ally1 p-hydroxy p’-methyl Qthloesters 17-20 (Scheme 3) This 1sanother method for the creation of three conaguous stereogemc centres 7cIn such a rearrangement, there IS a combmed use of a classic internal control and an external control The relanve configuranon (CaCP’) correlates with the two double bond geomemes of the pencychc nucleus and the (CuCfl) configuration depends on the external control by the chual hydroxy centre For this purpose, all of the four dlastereolsomenc S-crotyhc a-hydmxy ketene dtthmacetals are needed vvlth pure geomemc mtegr~ty

Owmg to the particular reactlvlty of dlthmesters. namely the S-alkylatlon of the correspondmg thmenolates with retention of configuration,9 the access to each of the desired dlastereolsomenc ketene drthloacetals has been successfully achieved From our prehmmary results, deprotonatlon of P-hydroxy Qthmesters occurs readrly and gives pure “IX” thloenolates 7 Their m-situ S-alkylanon gives ketene dlthloacetals with retention of the double bond geometry Thus the deprotonatlon of S-methyl P-hydroxy dlthloesters 1-4 followed by an S-alkylaaon Hrlth (E) or Q-crotyl bromide opens the route to (Z)-ketene dlthloacetals (formation of (ZE’) and (ZZ’) &astereolsomers10 respecuvely) (Scheme 4) The correspondmg (EE’)and (EZ’) isomers are then avalruble by a similar procedure deprotonation of (E) or (Z) S-crotyl phydroxy Qthmesters 6-12 and S-alkylawn with methyl itide (Scheme 4)

3133

S-Crotyhc a-hydroxy ketene dlthtoacetals

R’

2) Ha0

6.8 and 10 (Zgc=m%y)

13,,. - ls,o

7.9.11 an4 12 Qgeaxmy)

R’.MFE&~R.~BI~ scheme

13=. . 16,~

4

Results The synthesis of all four pure &astereolsomenc S-crotyhc ketene d~duoacetals was achieved usmg the chermstry described (Scheme 4) Startmg dithloesters l-4 have been prepared by an aldol condensanonll between S-methyl dxtioacetate and ethanal. propanal, lsopropanal and mmethylacetaldehyde Q-D~thmesters 6,s and 10 and (E)-&thmesters 7,9,11 and 12 have been formed from S-crotyl hduoacetates 52 and SE respecttvely and the same aldehydes A deprotonatton of d~thmcsters 1-4 with LDA at -78°C and a subsequent S-alkylanon by pure (2) or (Ii)-crotyl bromde afforded quantitanvely and respectively pure ketene &thioacetals 132~*-16zz* and 13~~-16~~ By a same stereoselective deprotonation of Q-dlthmesters 6,8 and 10 and (E)-dIthlocsters 7,9,11 and 12, followed by an ln-situ S-methylation, ketene dtthloacetals 13EZ~-&Z~ and &E’-16EE’ Were obtained

z!% tlh)

T

=‘V : :

R’

W

Gee t N mehy o’d__

a? MC 13 zz’ Is

5 6

,Ec 14 z

i

g

z

25 5

1;

365

63 ; 24

445 11

8

LPr 15 zz’ El? m

13 16 12

13 14 15

Zi? t&l 16 zz’ m

15 28 24

777 29 5 3

27 &5s 72 215 2:5 75:s

45 a

9 10 11 12

3 17 375 1

l!

2(

Table I Dmstereoselecnvq of the Rearrangement

P. BESUNand S PERRIO

3134

The 1HNMR spectra for each patr of (0 and (Z)-ketene d~tiuoacetals exhlbtted the previously observed difference of chermcal shift G(CH=)E > G(CH=)z and allowed us to ascertam the geomemc punty of the ketene 7 The rearrangement of these S-crotyhc ketene dtthmacetals into akiols 17-20 occured more slowly than that of the S-allybc analogues 7 several monthes at room temperature are required for the reacuon to go to completton Hence, It was run at 80 “C us a cyclohexane solution without any change m the &astereoisomenc dtsmbutlon The rearrangement rimes were rangtng from 2 5 to 28 hours (Table 1) All the four possible dlastereolsomew &thloesters 17-20, detected m HPLC analysis, were umformly formed with quite acceptable yield except III the case of Rt = tBu (Table 1, Enmes

Table

2

Stereospecrftcrty

of the

However the dtastereospeclficlty level IS better with (ZE’), (ZZ’) and @Z’) geomemes relative to the (IX) geometry. So the StereospecIficIty mcreases with the number of double bonds ~th a Q geometry (Table 1, Compare enmes 2,6, 10 wtth 4. 8, 12 and 1.5.9 and 3,7, 11). From the HPLC analysis, the four dlastereoisomers a b, c and d are eluted m two well separated series. Each constitute a couple of (a + b) and (c + d) isomers with the am-anti, antt-syn, syn-syn and synantt configuraaon respecttvely By purdicaaon by MPLC of each crude ~lnxt~ of d~~tereotsomenc aldols 1720, dlthmesters 17a antr-arm. 17b anti-syn, 19c syn-syn and 19d syn-antt have been isolated as pure products The others have been collected as rmxtures of syn-syn and w-anti isomers (17~ + 17d), (18~ + lSd), (2Oc+ 20d) and (anti-antr + antmyn) isomers (Ha + 18b), (19a + 19b), (20a + 20b) respectively Configuration Assignment The configurattons have been established only with the &astereolsomers 17a-d (R1 = Me) by a chermcal correlation with the analoguous esters previously described by Kurth et al w Thus the 95 5 rmxture of 17a and 17b were reactedwtth CuCl&uO m methanol 13Methanolysts resulted in a 95 5 rmxture of two diastereolsomenc esters (Scheme 5)

ma

cdl-antf 21aRlb

2lbanthyn -95

5

scheme5

These two esters were identified as the known esters with an anti-ant2 configuration for the major dlastereolsomer (95%) and antmyn configuration for the mmor one (5%) Thus an anti-arm and anti-syn configurauon have been assigned to the precursors 17a and 17b respecttvely A sumlar methanolysls applied to the mixture of dlastereolsomers 17~ and 17d faded Nevertheless, 17~ and 17d must have a syn (CcQ) configuranon The syn or anh (CaCe’) configuration has been unambiguously deternuned after Swem oxldatlot@ of B-hydroxy dlthloesters 17a-d into syn and ant8 p-ox0 dlthloesters (Scheme 6) Durmg this process the

S-Crotyhc a-hydroxy ketene drthroacetals

3135

asymmetry at the carbon in the /3-posrtton has been lost. The oxrdahon has been successtvely apphed to pure Isomers 17a and 17b and two Wfereut rmxtures of the other isomers 17c and 17d- a 96 4 rmxture of 17c+17d. obtamed from the rearrangement of dtthroacetal 132,. (Table 1, Entry 2) and a 4 96 mrxture issued from 13Ep (Table 1; Entry 4)

scheme 6

Oxrdahon of antr-arm 17a gave pure antI p-0x0 Qthtoester 22 and oxrdahon of am-syn 17b pure syn p-ox0 dtthroester 23 (Scheme 6) On the other hand. the 96 4 nuxture of (17c+17d) was converted m a 15 85 mrxture of (22+23) and the 4 96 mrxture provtded a 90 10 mixture of (22+23) So a syn-syn and a syn-antr configuratron has been assrgned to 17c and 17d mspectrvely The same trends m the drastereorsomenc ratro are observed with dtthmesters 18-20 as shown tn the table 1 The same configuratron assrgnements were then extended to drastereoisomers Ma-d, 19a-d and 2Oad So all four dtastereorsomenc rearranged dtthtoesters 17,18,19 and 20 may be formed predommantly by a Judicious choice of the stamng S-crotyhc ketene dtthtoacetal (Table 1) Dlscussion

The (C&V) configuraaons result from the internal control of the Clarsen rearrangement and correlate to the double bond geometnes The level of thrs control, quantrfied by the (anzr-syn + syn-syn) (am-am + synanti) ratro for each dtastereiosomenc mixture 17-20. IS quite hrgh except for (EE’)-ketene dtthtoacetals (see Table 3 for the partrcular case of &thmacetals 13)

Ez 4 96 I Table 3 Internal Control with Ketene Dlthloacetals I3

I

Changing only one of the geomemes of the hexadremc system induces a (GxC~Y)relattve configurahon mversron Conversely, changing the geometry of both double bonds gtves retennon of (Cc&a’) configuration Usual correlations between (CaCg’) contiguratrons and double bond geometries are effecnve and may be rattonahxed by the classrc pseudo chlur transmon state 11s

al Contro[ Obvrously, a (Z) geometry for the S-crotyltc double bond favoures a syn (C&a) contigurahon and a (E)geometry favoures an antt (C&p) configuranon It is noteworthy that, m our precedent related study, Sallyhc ketene dtthroacetals always rearranged into syn (C&g) drastereotsomers 7 So this difference for the major (CaCg) configuratton ongurates from the unroductmn of the methyl group on the termmal carbon of the S-allyltc cham Thts behavtour contrasts wtth Kurth’s results obtained with the dtamomc analogue wtth

3136

P BESLINand S. PERRIO

oxygenated precursors (Scheme 1) These results showed the exclustve formanon of wm (C&e) p-hydroxy esters, ngardlcss of bond geometry Accordmg to these cormlatlons between the mayor(CC&# umfigurat~onand the S~hc double bond geometry, a transltlon state model. smular to that proposed for S-allyhc a-hydroxy ketene dlthloacctal rearrangement. IS not suitable for a total mterpretaUon and can only be used to explain a syn (C&e) configurahon. Correspondmgly, with (ZZ’) and (EZ’) Qthloacetals a Sl-approach (on the bottom face) fits adequately with the observed (C&p) syn configuration of the rearranged &thmesters (Scheme 7)

TransIUon state B EZ

geometry

scheme 7 When the crotyhc double bond of the Qthloacetal possesses (E) geometry, an opposite asymmetnc mductlon IS observed which probably IS due to the approach mode dffenng In a St-approach pictured ar transmon state C, severe stenc mteracnons between the termmal methyl group and the encumbered asymmemc centres dlsfavour the C-C smgle bond formatlon (Scheme 8) The stereochermcal facial &fferenclatlon must then occur VWa Re approach (on the top face) as pictured m transmon state model D with the OH group lymg in an “made” position’6 and the hydrogen in an “outnde”posltlon 16 The approach occurs anti to the alkyl group Rl on the less congested face Such a conformation has been recently put forward m a stereocontrol interpretanon of some electrophdic reacaon results 1’

SMe

lbnsltlon Mate C (dlsfavoured) ZF geometry

lkansltIon state D

scheme 8

3142

P Besm and S FERRIO

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2

3

4

65

I 8 9

10 11 12 13 14 15 16 17

18 19 20 21

22 23 24

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