Various heterocyclic acetic esters and nitriles of the pyridine, thiazole, and imidazole series react with 3-methylthio-4-phenyl-1, 2-dithiolium perchlorate to form fused pyridinethiones or dithiolylidene esters. These esters or nitriles also react with trichloromethanesulfenyl chloride to form fused isothiazoles.