S-alkylation of sulphides by an activated carbohydrate epimine under acidic catalysis: formation of α-acetamido-sulphides. Part 3. Further investigation of the reaction with ω-(alkylthio)alkanethiols

Abstract

The reaction between methyl 2,3,4-tri-O-acetyl-6,7-acetylepimino-6,7,8-trideoxy-1-thio-D-erythro-α-D-glacto-octopyranoside (1) and ω-(methylthio)alkanethiols, in the presence of acetic acid, gives the 7(S)-methylthioderivative (6)via an intermediate sulphonium salt which collapses by neighbouring group participation of the thiol substituent, and the 7(S)-[ω-(methylthio)alkyl]thio-derivatives (7). The latter are formed, not by methyl group transference at the sulphonium salt stage, but by direct alkylation of the thiol sulphur atom, due to the enhancement of its nucleophilic reactivity by the methylthio-substituent in the β-, γ-, or δ-position. Similar activation of the thiol group is afforded by an oxygen substituent, but not by a second thiol substituent.