Asymmetric synthesis of 2,3-disubstituted oxepanes via acetalization–cyclization of an enantioenriched functionalized allylsilane with aldehydes

Abstract

According to the protocol for the acetalization–intramolecular allylsilane cyclization, a new enantioenriched allylsilane, (R)-(E)-7-(dimethylphenylsilyl)undec-5-en-1-ol, in the presence of a variety of aldehydes provided enantioenriched trans-2,3-disubstituted oxepanes stereoselectively.

References

1. For the five-membered ring formation, see: P. Mohr, Tetrahedron Lett., 1993, 34, 6251; T. Oriyama, A. Ishiwata, T. Sano, T. Matsuda, M. Takahashi and K. Koga, Tetrahedron Lett., 1995, 36, 5581; T. Sano and T. Oriyama, Synlett, 1997, 716.
2. For the six-membered ring formation, see: I. E. Markó and A. Mekhalfia, Tetrahedron Lett., 1992, 33, 1799; I. E. Markó and D. J. Bayston, Tetrahedron, 1994, 50, 7141; I. E. Markó, M. Bailey, F. Murphy, J. P. Declercq, B. Tinant, J. Feneau-Dupont, A. Krief and W. Dumont, Synlett, 1995, 123.
3. M. Suginome, T. Iwanami and Y. Ito, J. Org. Chem., 1998, 63, 6096.
4. For a review of the reactions of chiral allylsilanes, see: C. E. Masse and J. S. Panek, Chem. Rev., 1995, 95, 1293.
5. K. Matsumura, S. Hashiguchi, T. Ikariya and R. Noyori, J. Am. Chem. Soc., 1997, 119, 8738; K.-J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya and R. Noyori, Angew. Chem., Int. Ed. Engl., 1997, 36, 285 The absolute configuration (S) of 3 was assigned by an analogy with these reports.
6. M. Suginome, A. Matsumoto and Y. Ito, J. Am. Chem. Soc., 1996, 118, 3061; M. Suginome, T. Iwanami, A. Matsumoto and Y. Ito, Tetrahedron: Asymmetry, 1997, 8, 859.
7. The absolute configuration (R) was assigned by analogy with previous reports. See ref. 5 and 6. Selected data for (R)-1: δ_H(CDCl₃) 0.24 (s, 3H), 0.25 (s, 3H), 0.82 (t, J 6.9, 3H), 1.04–1.68 (m, 13H), 1.97–2.05 (m, 2H), 3.62 (t, J 6.6, 2H), 5.10–5.25 (m, 2H), 7.31–7.37 (m, 3H), 7.43–7.51 (m, 2H); δ_C(CDCl₃)–5.1, –4.5, 13.9, 22.3, 26.0, 28.5, 31.4, 32.4, 32.5, 62.9, 127.6, 128.5, 128.8, 131.5, 134.1, 138.4; ν_max(neat)/cm^–1 3356, 2968, 2940, 2864, 1432, 1250, 1114 (calc. for C₁₉H₃₂OSi 304.2222, found 304.2221); [α]²⁰_D–8.38 (c 3.1, benzene).
8. The trans stereochemistry in the seven-membered ring was determined on the basis of the ¹H NMR coupling constant between the 2- and 3-protons in the ring. Compound 6a exhibited a coupling constant of 9.6 Hz, whereas that for trans- and cis-2-(phenylsulfonylmethyl)-3-(phenylmethyl)oxepane were reported as 8.5 and 2.6 Hz, respectively. See: P. L. López-Tudanca, K. Jones and P. Brownbridge, Tetrahedron Lett., 1991, 32, 2261.
9. The enantiomeric excesses of 6, except for 6a, were determined after derivatization to the corresponding 2-substituted 3-oxepanecarboxylic acid by RuO₂-catalyzed oxidative C = C bond cleavage in the presence of NaIO₄(CCl₄, MeCN, H₂O). The 3,5-dinitrophenylanilides were subjected to chiral HPLC with Sumichiral OA columns indicated below [compound, column, solvent (a ratio of hexanes–1,2-dichloroethane–ethanol)]: (6b, OA-4400, 50∶15∶1); (6c, OA-4500 × 3, 15∶15∶1); (6d, OA-4400, 15∶5∶1); (6e, OA-4900, 15∶5∶1); (6f, OA-4500, 15∶5∶1); (6g, OA-4600, 15∶5∶1). The absolute configurations were assigned by analogy with the stereochemical outcome for the six-membered ring formation reported previously. See ref. 3.
10. The slight decrease in the enantiomeric excesses may be attributed to a minor contribution ‘syn attack’ of the electrophiles on the allylsilane moieties during the cyclization in addition to the normal ‘anti attack’. For a discussion on the ‘anti’vs.‘syn’ attack in the reaction of allylsilanes with electrohiles, see: M. J. C. Bucke, I. Fleming and S. Gil, Tetrahedron Lett., 1992, 33, 4479.