Intramolecular 1,3-dipolar cycloaddition at the periphery of heterocyclic systems. Part 3.1 A facile hydrazone-azomethine imine isomerization at the periphery of pyridine and pyrido[,2-a]pyrimidine systems

Article abstract of DOI:10.1016/0040-4020(95)00949-3

The phenylhydrazones of 2-(alk-2-enylamino)-4-oxo-4H-pyrido[1,2-a]pyrimidine-3-carbaldehydes and 4-(alk-2-enylamino)-1,6-dimethyl-2-oxo-1,2-dihydropyridine-3-carbaldehydes underwent the thermally induced 1,3-dipolar cycloaddition leading to a mixture of the fused pyrazolidine and pyrazoline derivatives under extremely mild conditions. For the observed facile hydrazone-azomethine imine isomerization, we proposed an intramolecularlly assisted process similar to that for the facile oxime-nitrone isomerization in these systems; the intramolecular proton transfer from the the protonated alkenylamino and/or carbonyl moieties to the imine nitrogen facilitated the generation of the azomethine imine intermediates.