From thiourea to bicyclic structures: an original route to imidazo[2,1-b]thiazoles, 5H-thiazolo[3,2-a]pyrimidines, 7H-imidazo[2,1-b][1,3]thiazines,and 2H,6H-pyrimido[2,1-b][1,3]thiazines

We report an example of an efficient regioselective synthesis of biheterocyclic compounds using thiourea as starting material. In fact, N,N'-bis(dimethylaminomethylene)thiourea (1), easily prepared by double condensation of N,N-dimethylformamide dimethyl acetal with thiourea, can be reacted with haloketones or acrylic dienophiles to give thiazolic (2) and thiazinic (3) diazadienes, respectively, themselves undergoing cyclization reactions to yield imidazo2,1-b]thiazoles, 5H-thiazolo3,2-a]pyrimidines, 7H-imidazo2,1-b]1,3]thiazines, and 2H,6H-pyrimido2,1-b]1,3]thiazines without any regioisomeric ambiguity. This straightforward route represents an original and unambiguously regioselective pathway to these valuable heterocycles.