S-alkylation of sulphides by an activated carbohydrate epimine under acidic catalysis: formation of α-acetamido-sulphides. Part 1

Abstract

Alkanethiols do not participate in the epimine ring-opening of methyl 2,3,4-tri-O-acetyl-6,7-acetylepimino-6,7,8-trideoxy-1-thio-D-erythro-α-D-galacto-octopyranoside (I), catalysed by acetic acid, whereas dialkyl sulphides react via an α-acetamido-sulphonium salt to give α-acetamido-sulphides. Dialkyl disulphides also are alkylated, giving dialkyl(alkylthio)sulphonium salts, which are cleaved by nucleophilic attack on the sulphenyl sulphur atom to give α-acetamido-sulphides, and offer advantages over dialkyl sulphides in which bulky alkyl groups reduce the nucleophilic reactivity of the sulphide sulphur atom. Exceptions are di-t-butyl disulphide and diallyl disulphide, which yield α-acetamido-disulphides. Diethyl trisulphide is alkylated on the termnal sulphide sulphur atom, giving the ethylthio- and not the ethyldithio-product. The mechanisms of these reactions are discussed.