Effect of substituents on the cyclization pattern of 2-pyridylhydrazones

摘要

Homologous hydrazones formed by addition of substituted 2-hydrazinylpyridines to dimethyl acetylenedicarboxylate or dimethyl-2-oxoglutarate undergo under reflux in acetic acid different types of cyclizations depending on the kind of substitution. Whereas 3-chloro-hydrazinylpyridine leads finally to derivatives of the pyrido[2,1-c][1,2,4]triazinone skeleton, starting from 3-nitro-2-hydrazinylpyridine gives rise to regioisomeric pyrido[1,2-b][1,2,4]triazinone derivatives. The finding is discussed in terms of substituent effects on the reaction behaviour which requires an N-N cleavage to give access to these heterocycles.