The S-alkylation of sulphides by an activated carbohydrate epimine under acidic catalysis: the formation of α-acetamido-sulphides. Part 4. Reactions with dithioacetals and monothioacetals

Abstract

The tetra-acetylepimine (1) has been found to react with dithioacetals and monothioacetals under acidic catalysis to produce monoalkylated sulphonium salts, which collapse via methylenesulphonium and methyleneoxonium ions, respectively, to afford sulphides. Cyclic dithioacetals and monothioacetals are efficient reagents for the introduction of ω-mercapto- and -hydroxy-alkylthio-substituents. Although the N-acetyl-activating group in (1) permits the stereospecific introduction of such substituents, its replacement by a basic amino-acid activating group leads to a mixture of 7S- and 7R-epimers. The mechanisms of these reactions are discussed.