Unsymmetrical polyheteropolyene: a versatile building block for the preparation of pyrrolo[2,1-b]thiazoles and 2H-thiopyrano[2,3-b]pyridines

The synthesis of two classes of bisheterocyclic compounds, pyrrolothiazoles and thiopyranopyridines, is reported. The 4-dimethylamino-2-dimethylaminomethylenamino-1-thiabuta-1,3-diene 1 is used like a useful building block for the chemoselective synthesis of these heterocycles. Indeed, synthon 1 can react as thiazabutadiene or thiabutadiene form to afford either five- or six-membered ring monocyclic azadienes, themselves being precursors to the bicyclic structures. Semi-empirical calculations were undertaken to explain this efficient chemoselectivity.