Reaction of benzazole derivatives with dimethyl acetylenedicarboxylate. Crystal and molecular structures of [1,4]thiazino[4,3-a]benzimidazole derivatives

The reaction of (1H-benzimidazol-2-ylthio)acetonitrile with dimethyl acetylenedicarboxylate (DMAD) has been studied. In benzene, THF or DMF the major product is a [1,3]thiazolo[3,2-a]benzimidazole derivative (5a) but in ethanol the major product is either a [1,3]thiazino[3,2-a]benzimidazole or a [1,3]thiazepino[3,2-a]benzimidazole derivative. (1-Methyl-1H-benzimidazol-2- ylthio)acetonitrile reacted with DMAD in DMF to give three products in very low yield; two of these compounds were shown to be [1,4]thiazino[4,3-a] benzimidazoles by X-ray crystallographic analysis. The analogous (benzo-1,3-thiazol-2-ylthio)acetonitrile reacts with DMAD in aqueous methanol to give a complex product, one compound of which is a pyrido[1,2-b] [1,3]benzothiazole derivative. The mechanisms of these novel annelation processes are discussed.