Stereoisomerization in heterocyclic hydrazones derived from 2-acylpyridines and their oxidative cyclization with mercury(II) acetate and lead tetra-acetate to fused 1,2,4-triazoles and 1,2,3-triazolium systems

Hydrazones prepared by coupling 2-acylpyridines with arylhydrazines were isolated predominantly as E-isomers when the acyl substituent R was small (H or Me). When R was a phenyl group significant yields of Z-isomers, containing an intramolecular hydrogen bond, were also isolated. Oxidation reactions of these hydrazones were not influenced by the E- or Z-geometry of the substrate contrary to earlier reports. A common metallo intermediate with a Z-structure was encountered in the oxidations of the E- and Z-pyridine 2-carbaldehyde 2-pyridylhydrazone with mercuric acetate. In oxidations of a series of hydrazones with lead tetra-acetate the product controlling factor was the nature of the methine substituent R. For ketone derivatives (R ≠ H) oxidation to fused 1,2,3-triazolium systems occurred via a 5-exo-tet cyclization with E- and Z-hydrazone substrates. For aldehyde systems (R = H) the oxidation involved dehydrogenation to a nitrilimine and gave products by solvent addition or a 5-endo-dig cyclization to fused 1,2,4-triazolo systems.