Control of diastereofacial selectivity in the nucleophilic epoxidation of γ-oxygenated α,β-unsaturated sulfones

Abstract

Epoxidation of γ-oxygenated-α,β-unsaturated sulfones 6–10 with either lithium or potassium tert-butyl peroxide proceeds with moderate to high diastereoselectivity to give mixtures of the syn-2-(phenylsulfonyl)oxiranes 11–15 and the corresponding anti-2-(phenylsulfonyl)oxiranes 16–20. In all cases examined, the presence of a free hydroxy group leads to the syn-oxirane with moderate to excellent stereoselectivity. For relatively unbulky substituents at the γ-position (methyl and propyl), the presence of a silyl protecting group leads to the formation of syn-oxiranes as the major product, while a methoxyethoxymethyl protecting group leads to the anti-oxirane in excess. However, when the γ-substituent is large (isopropyl), the anti-products are obtained with both silyl and methoxyethoxymethyl protection. Transition states are proposed to account for these results. X-Ray crystallographic results for compounds 12a, 19c and 22 are also reported.