Pergamon

0040-4039(95)00410-6

Tetrahedron Letters, Vol. 36, No. 17, pp. 298%2990, 1995 Elsevier Science Ltd

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Stereoselective Synthesis of Substituted Tetrahydrofurans via Selenoetherification of 2-Silyl-3-Alkenols. A Study of Allylic Stereocontrol.

Yannick Landais,* Denis Planchenault and Val6ry Weber

Institut de Chimie Organique, Universitd de Lausanne Colldge Prop~deutique, 1015 Lausanne-Dorigny, ~itzerland.

Abstract: 5-endo-trig selenoetherifications of substituted 2-silyl-3-alkenols gave tri- or tetrasubstituted 2,4-cis tetrahydrofurans in moderate to good yields with excellent diastere0selectivities. Opposite stereoselectivities were found with an analogous ailylic diol. A transition state model has been proposed to rationalize this stereochemical outcome.

The 5-exo-trig selene and haloetherification of substituted alkenols is one of the most efficient and most widely used approaches for the synthesis of polysuhstituted tetrahydrofurans. 1 Conversely, the 5-endo-trig process has been much less studied hut recent activity in this field has however demonstrated that excellent stereoselectivities can be obtained during this process. 2 The fact that 5-endo-trig processes are disfavoured according to Baldwin rules 3 is probably at the origin of the lack of interest in this mode of ring closure. However, Knight recently suggested that the cationic nature of the reaction implied that this process should not he seen as an exception to these rules. 2a-b As a part of our ongoing interest in the development of the potential of ct-silylcarhonyl compounds, 4 we report herein a new 5-endo-trig selenoetherification of 2-silyl-3-alkenols, such as 3. We demonstrate that the silicon group on the chiral allylic centre not only controls efficiently the stereochemistry on the vicinal prochiral centre (allylic or 1,2-stereocontrol) but can also be used as a masked hydroxyl group, hence giving an easy and stereocontrolled access to the 3-hydroxytetrahydrofuran skeleton (Scheme 1).

('--[Si] E + .N/2 Ref4d [Si] LIAIH4 .,,,,~ . , ~ / O H R ' ~ - ~ C O 2 R' ~ R ~ " ~ C O 2 R' - R ~ R

1 2 3 4

Scheme1

Recent work in our group showed that rhodium mediated decomposition of vinyldiazoesters 1 in the presence of silane, led to good yields of allylsilanes of type 2. 4d We now report that mild reduction of the latter with LiAIH 4 affords the corresponding alcohols 3 in 80-90% yield. Cyclization of 3a-f was then carried out under kinetic conditions using PhSeCI (1.5 eq.) and K2CO 3 (1.5 eq.) in ether at temperatures ranging from -60C to room temperature over 2 hours. This gave the trisubstituted tetrahydrofurans 4a-f in low to good yields, depending on the substrates, but m all cases as a unique diastereoisomer (i.e. 2,4-cis) (1H NMR). The stereochemistry assigned, using nee experiments, was confirmed by comparison of the deselenylated products 5a-d with authentic samples prepared by an alternative method (Scheme 2). 4b We noticed that the course of the selenoetherification was strongly dependent on the olefinic substitution pattern. The stabilization of the developing positive charge by the aromatic ring in 3a-d favours the 5-endo-trig pathway. Conversely, the

2987

2988

absence of such a stabilization in 3e and 3f leads mainly to SE' reaction.5 The amount of tetrahydrofuran formed with 3e however indicates that a CH 3 group is still able to stabilize efficiently the nascent positive charge, probably through C-H bond hyperconjugation. 6

PhSe SiMe2 R" 8iMe2R"

R ~ O H K2CO:. -S0"C to R~ benzene, A ?d

3a, R:' Ph; R": H; R": Ph 3b, R: Ph; R',: H; R" : 6-Methylthienyl 3c, Rm Ph; R's Me; R": Ph 3d, R: Ph; R',, H; R"= OiPr 3e, R,, Me; R',, H; R": Ph 3f, R : Et; R' : H; R"g Ph

kl, (70% yield) 4b, (72%) 4c, (6O%) 4d, (74%) 4e, (44%) 4f, (17%)

Scheme 2

5a-d, (75-90% yield)

Introduction of a fourth ehiral centre on the tetrahydrofuran skeleton was achieved starting from an ot- silylketone (i.e. 6) 4d using the silicon group to control the stereochemistry on both vieinal prochiral centres (double 1,2-stereocontrol). Reduction of the carbonyl group, using DIBAH in ether at -100C, afforded solely the syn-13-hydroxysilane 7. 4b,7 Selenoetherification under the same conditions as above, then gave the desired tetrabydrofuran 8 as one diastereoisomer in 60% yield (Scheme 3). hOe experiments confirmed the 2,4-cis stereochemistry assigned previously for the trisubstituted tetrabydrofurans.

PhMe28. i PhMe2S. i PI1~e SiMe2Ph

Ph Ether ~ Ph Ether, O 7 OH K2COs Ph 8

6 63% yield, >98% d.e. 60% yield, >98% d.e.

Scheme 3

The methodology has also been extended to dienylsilane 9, which gave the corresponding tetrahydrofuran 10 in 60% yield as a sole diastereoisomer. The presence of an olefinic fragment on C-2 allows for further functionalization and could therefore circumvent the problems encountered with less reactive substrates 3e-f (Scheme 4).

PhMe 2 S i PhS,Cl, K2CO3

Ph Ether, -80"C, lh then O'C, 2h

9

Scheme 4

PhSe SiMe2Ph

Ph 10

60% yield, >98% d.c.

Further investigations on this 5-endo-trig selenoetherification have also been performed on the corresponding allylie alcohol 11 (Scheme 5). 8 We observed a much lower stereoseleetivity than that found with the allylsilanes (up to 45% d.c.), and more interestingly a reverse of the sense of this diastereoselectivity in favour of the 2,4- trans tetrabydrofuran 12.

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PhSe OH OH

Ph OH K : ~ 4rC to RT ph~ott benzw~ relk: phto~ ~ m y ~ d :h, 07%

11 12 (major), 48% d.e. 13

Scheme 5

Rationalization of the stereochemical outcome of our 5-endo-trig cyclization may be illustrated by the transition state conformations drawn in Scheme 6. The anticipated antiperiplanar addition of PhSe + and the hydroxyl group across the double bond restricts the number of reactive conformations in the transition state. 2a.9 The bulky silyl group is likely to prefer the less crowded outside position (the hydrogen occupying the inside position) for steric and electronic reasons, l Both factors predict the same stereoselectivity, i.e. the 2,4-cis stereochemistry, thus explaining the exceptional level of 1,2-stereocontrol obtained through conformation A. 11 On the other band, selenoetherification of allylic alcohol 11, led to a low and reversed diastereoselectivity, electronic and steric factors predicting opposite diastereoselectivities. This might be explained using again the conformations A and B. It is reasonable to assume that the OH will occupy the outside position in conformation A for steric reasons, leading to the minor 2,4-cis isomer. The major isomer 2,4-trans 12 would thus be formed through conformation B, presumably for electronic reasons. The overlapping of one of the lone pairs of oxygen with the 7c orbital of the olefin would raise the energy of the latter making the conformation B more reactive towards electrophiles. 12 A complexation of the incoming PhSe + cannot be ruled out and might also add to the former effect. 6 Such a reversal of the sense of the stereochemical outcome has already been observed for the 5- exo-trig cyclization of some homoaUylsilanes and their corresponding alcohols 4b and also for acyclic stereocontrol arising from electrophilic reactions on allylsilanes and allylic alcohols. 13

+ PhSe

X H.. / .... R

H- A ~+ PhSe

X A r i e l (L.. 4) ~ [ ~ X - . M e a l (not obsesS) X-OH(minor) X-OH(major)(L~lZ)

Scheme 6

Finally, the C-Si bond was converted into the corresponding C-OH bond with retention of configuration using different procedures, depending on the nature of the substituents on silicon. With PhMe2Si, we employed the method of Fleming 14 (Hg(OAc) 2, CH3CO3H) which gave the expected 2,4-cis tetrahydrofuran in 50% yield. Since these conditions may sometimes be incompatible with other functionalities, 15 we introduced the 5- methylthienyl group which was oxidized using a milder two-step procedure. The nucleophilic displacement of the thienyi moiety was easily carried out using a fluorine source (TBAF), 16 producing the corresponding Si-F bond which was then oxidized using H202 and KF in DMF (Scheme 7). This procedure finally gave the expected hydroxytetrahydrofuran in yields comparable to those obtained using Fleming's conditions.

SiMe 2 R' OH

c o ~ Ph Ph

SiMe~R'

R',' Ph

R'" ~ , ~

conditions

Hg(OAc) ~, AcO24"1, A,c;O, RT, 4h (50% yieid)

1) n-Bu4NF, DMF, RT, 30 rain 2) H~):, KF, DMF, 70"C, 1Sh (% yield)

Scheme 7

2990

In summary, we have presented here a highly stereoseleetive synthesis of tri- and tetrasubstituted tetrahydrofurans from primary and secondary 13-hydroxysilanes, respectively. We also found that 2,4-trans and 2,4-cis-hydroxytetrahydrofurans could be readily obtained starting from homoallylic alcohols having respectively a hydroxyl or a silicon group at the allylic position. Finally, it is also worth noting that further modifications of the tetrahydrofuran skeleton could easily be achieved using the residual PhSe group. 9

Acknowledgements. The authors gratefully acknowledge the Swiss National Science Foundation for generous support. We thank Mr. L. Ducry for preliminary studies and Prof. M. Malacria (Universit~ PARIS VI) for a preprint of his work and fruitful discussions. We are grateful to Dr. I.M. Eggleston for helpful suggestions.

REFERENCES AND NOTES

1. For reviews on the subject, see: (a) Bartlett, P.A. Asymmetric Synthesis, Morrison, J.D., Eds.; Academic Press: New York, 1984; vol 3, pp. 411-454; (b) Boivin, T.L.B. Tetrahedron, 1987, 43, 3309-3362; (c) Cardillo, G.; Orena, M. Ibid., 1990, 46, 3321-3408 and references cited therein.

2. For recent reports on 5-endo-trig cyclizations, see : (a) Barks, J.M; Knight, D.W.; Seaman, C.J.; Weingarten, G.G. Tetrahedron Lett., 1994, 35, 7259-7262; (b) Barks, J.M.; Knight, D.W.; Weingarten, G.G.J. Chem. Soc., Chem. Commun., 1994, 719-720; (c) Evans, R.D.; Magee, J.W.; Schauble, J.H. Synthesis, 1988, 862-868; (d) Kang, S.H.; Lee, S.B. Tetrahedron Lett., 1993, 34, 7579-7582; (e) Mihelich, E.D.; Hite, G.A.J. Am. Chem. Soc., 1992, 114, 7318-7319; (f) Lipshutz, B.H.; Barton, J.C. ibid., 1992, 114, 1084-1086; (g) Bedford, S.B.; Bell, K.E.; Fenton, G.; Hayes, C.J.; Knight, D.W.; Shaw, D. Tetrahedron Lett., 1992, 33, 6511-6514; (h) Kang, S.H.; Hwang, T.S.; Kim, W.J.; Lim, JK. ibid., 1990, 31, 5917-5920; (i) Kang, S.H.; Lee, S.B. ibid, 1993, 34, 1955-1958.

3. Baldwin, J.E.J. Chem. Soc., Chem. Commun., 1976, 734-736. 4. (a) Andrey, O.; Landais, Y.; Planchenault, D. Tetrahedron Left., 1993, 34, 2927-2930; (b) Andrey, O.;

Landals, Y. ibid., 1993, 34, 8435-8438; (c) Landals, Y.; Planchenault, D. ibid., 1994, 35, 4565-4568; (d) Landals, Y.; Planchenault, D.; Weber, V. ibid., 1994, 35, 9549-9552.

5. Fleming, I.; Dunogu6s, J.; Smithers, R. Org. React., 1989, 37, 57-575. 6. Kahn, S.D.; Pau, C.F.; Chamberlin, A.R.; Hehre, W.J.J. Am. Chem. Soc., 1987, 109, 650-663 and

references cited therein. 7. The anti diastereoisomer can be obtained via an alternative route, See: Le Bideau, F.; Gilloir, F.; Nilsson,

Y.; Aubert, C.; Malacria, M. Tetrahedron Lett., 1995, 36, (in press). 8. The diol 11 was prepared by reduction of the corresponding c~-hydroxyester (NaBH4, EtOH, RT, lh, 91%

yield). The stereochemistries of 12 and 13 were assigned unambiguously using difference nOe experiments. 9. Paulmier, C. Selenium Reagents andlntermediates in Organic Synthesis; Pergamon Press: Oxford, 1986. 10. An overlapping between the a-C-Si bond and the LUMO formed by the olefin and the incoming

electrophile might be at the origin of this conformational preference. 11. Our transition state models are in good agreement with the predictive ones made by D.W. Knight. 2a 12. (a) Kahn, S.D.; Hehre, W.J. Tetrahedron Lett., 1985, 26, 3647-3650; (b) Chamberlin, A.R.; Mulholland,

R.L., Jr.; Kahn, S.D.; Hehre, W.J.J. Am. Chem. Soc., 1987, 109, 672-677; (c) Houk, K.N.; Moses, S.R.; Wu, Y.-D.; Rondan, N.G.; Jiiger, V.; Schohe, R.; Fronczek, F.R. ibid., 1984, 106, 3880-3882; (d) Tamaru, Y.; Harayama, H.; Bando, T. J. Chem. Soc., Chem. Commun., 1993, 1601-1602.

13. Fleming, I. Pure & Appl. Chem., 1988, 60, 71-78 and references cited therein. 14. Fleming, I.; Sanderson, P.E.J. TetrahedronLett., 1987, 28, 4229-4232. 15. In some cases, the PhMe2Si group cannot be oxidized in the presence of double bonds, due to the

incompatibility of this function with the electrophilic conditions required for the oxidation. Other solutions to this problem have been proposed recently, see: (a) Fleming, I.; Winter, S.B.D. Tetrahedron Left., 1993, 34, 7287-7290; (b) Norley, M.C.; Kocienski, P.J.; Failer, A. Synlett, 1994, 77-78.

16. A similar approach has been proposed by Stork with the furyldimethylsilyl group, see: Stork, G. Pure & Appl. Chem., 1989, 61, 439-442. 5-Methyithienyl group was generally found to be more stable with our reaction conditions. We gratefully acknowledge Prof. G. Stork (Columbia University) for providing us with experimental details concerning the nucleophilic displacement of the furyl group.

(Received in France 31 January 1995; accepted 28 February 1995)