Cyclisation reactions of some pyridazinylimidoylketenes

Flash vacuum pyrolysis (FVP) of aminopyridazinone derivatives of Meldrum's acid at 600 degrees C (0.02 Torr) results in generation of an imidoylketene intermediate followed by cyclisation. In the case of the 5-amino derivatives, the products are pyrido[2,3-d]pyridazines, whereas the 4-amino compounds lead to mixtures of pyrido[2,3-d]pyridazines and pyrrolo[3,2-c]pyridazines. The feasibility of the 1,5-sigmatropic shift of a chlorine atom, required for the formation of two of the pyrido[2,3-d]pyridazines, was supported by the corresponding reaction of a corresponding 2,6-dichloroaniline derivative. The feasibility of the decarboxylation mechanism required for the formation of the pyrrolo[3,2-c]pyridazines, was supported by related processes in the FVP reactions of model compounds and by DFT calculations.