Studies on Heteropentalenes. II. Cycloaddition of Imidazo[2,1-<i>b</i>]thiazoles, Thiazolo[3,2-<i>a</i>]benzimidazole, and Imidazo[2,1-<i>b</i>]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
Cycloadditions of imidazo[2,1-b]thiazole and thiazolo[3,2-a]benzimidazole with dimethylacetylenedicarboxylate follow dual courses producing pyrrolo[2,1-b]thiazoles in an aprotic nonpolar solvent or imidazo[1,2-a]pyridines in an aprotic polar solvent.