Lewis acid induced rearrangement of 2,3-epoxy sulfides; Regiospecific trapping of thiiranium ion intermediates with nitrogen heterocycles and amides. Use of imines as nucleophilic equivalents for the selective monoalkylation of primary amines

The Lewis acid induced conversion of 2,3-epoxy sulfides into the corresponding 3-trimethylsilyloxy-1,2-thiiranium ions is described. Such intermediates react with silylated nitrogen heterocycles and amides regiospecifically to form 1-substituted-3-hydroxy-2-thioethers in good to moderate yields with full stereochemical control. Exclusive N-alkylation is observed. When simple primary amines are used as nucleophiles, only products of polyalkylation are isolated. However, imines can be effectively used as the nucleophilic equivalent of a primary amine, the initially formed iminium ions being readily hydrolysed by aqueous base to liberate a secondary amine, the product of overall selective monoalkylation of a primary amine.