Studies on Heteropentalenes. II. Cycloaddition of Imidazo[2,1-b]thiazoles, Thiazolo[3,2-a]benzimidazole, and Imidazo[2,1-b]benzothiazole with a Reactive Acetylenic Ester

Cycloaddition of imidazo[2,1-b]thiazoles with dialkyl acetylenedicarboxylate follows dual courses depending on the polarity of the solvent, affording pyrrolo[2,1-b]thiazoles in an aprotic nonpolar solvent, or imidazo[1,2-a]pyridines and imidazo[3,4-a]pyridines, together with thiophenes, in an aprotic polar solvent. Thiazolo[3,2-a]benzimidazole and imidazo[2,1-b]benzothiazole were also found to react with the acetylenedicarboxylate to give pyrido[1,2-a]benzimidazole and pyrrolo[2,1-a]benzothiazole, respectively. The reactions would be accounted for in light of the Diels-Alder reaction in an aprotic nonpolar solvent and the 1,4-dipolar cycloaddition in an aprotic polar solvent.