Abstract Methylations of heterocyclic systems, such as benzimidazole, naphth [2, 3-c] imidazole, imidazo [4, 5-c]-pyridine, purine, pyridine, pyrimidine, pyridazine and s-triazolo [4, 3-b] pyridazine, which bore SH, NH and/or OH groups, were carried out with dimethylformamide dimethyl acetal to give the corresponding S-, N-and/or O-methyl derivatives in high yields. Selective methylation of some compounds containing both SH ...