ANTI-SELECTIVE REACTION OF ALPHA-SULFENYL ACETALS WITH SILYLATED CARBON NUCLEOPHILES - SCOPE, LIMITATION, AND MECHANISM

In the presence of a Lewis acid, alpha-sulfenyl acetals 1 reacted with various silylated carbon nucleophiles 2 to give anti adducts (anti-3) with high diastereoselectivity. The stereochemistry was only slightly affected by the reaction conditions, such as temperature, solvent, and Lewis acid. However, the structure of substrate 1 and the kind of nucleophile 2 had considerable effect on the stereochemical course of the reaction. Almost exclusive anti selectivity was attained when 1,1-dimethoxy-2-(tert-butylthio)propane (1b) was used as a substrate or when ketene silyl acetal 2c was employed as a nucleophile. The mechanism of this reaction is essentially S(N)2, although the S(N)1 process participates to a various extent, depending on the structure of substrate 1. The usefulness of this anti-selective reaction was exemplified by the easy transformation of anti-3o to synthetically valuable allylic alcohol anti-6 without any loss of stereochemical information. The reaction of alpha-(benzyloxy)acetal 4 with 2 was also investigated. It gave a syn-rich mixture of diastereomers with lower selectivity.