S N 2′ Reactions between Lithiated Carbon Nucleophiles and Silylated Vinyloxiranes – Effects of Salts and Solvents on the Stereocontrol

(α,β‐Epoxy‐γ,δ‐alkenyl)‐ tert ‐butyldimethylsilanes 1 display, a priori, three electrophilic centres and two acidic protons. Depending on the configurations of their oxirane rings, the title compounds react towards lithiated bases either through metallation or S N 2′ reactions. In this paper we demonstrate that the trans ‐2‐( tert ‐butyldimethylsilyl)‐3‐vinyloxiran 1a reacts regioselectively with primary, secondary and tertiary butyllithium in S N 2′ fashion, which allows the formation of α‐silylated allylic alcohols with diastereomeric ratios of over 7 to 1 in favour of the Z olefins. A study of the effect of temperature, time, addition of salt and polarity of the solvent on the diastereoselectivity of the reaction is described and the results are interpreted in terms of different mechanistic models. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)