SULFONIUM YLIDES - APPLICATION TO THE SYNTHESIS OF OPTICALLY-ACTIVE EPOXIDES

The transfer of benzylidene groups from S-benzylsulfonium salts obtain ed from several optically active thiolanes and thianes to aldehydes an d ketones yields optically active epoxides with high enatiomeric purit y. In contrast, methylene transfer via the same auxiliaries leads to i nactive epoxides. It is proposed that the difference in behaviour betw een these systems is best explained by assuming a 2+2 cycloaddition me chanism rather than the more commonly proposed nucleophilic anti-perip lanar addition for the reaction of a sulfur ylide with a carbonyl comp onent.