Tautomerism of Aza heterocycles: V. Structure of the spontaneous transformation products of 5-methyl-2-phenyl-3,3a,4,5,6,7-hexahydro-2 H -pyrazolo-[4,3- c ]pyridin-3-one in crystal and solution

5-Methyl-2-phenyl-3,3a,4,5,6,7-hexahydro-2 H -pyrazolo[4,3- c ]pyridin-3-one exists as zwitterion with a proton localized on the nitrogen atom of the piperidine ring and negative charge delocalized over the pyrazololate fragment. The compound is stable in crystal but ustable in solution. Its chromatographic purification and attempts to recrystallize or synthesize by condensation of phenylhydrazine with alkyl 1-methyl-4-oxopiperidin-3-carboxylate on heating in alcohols or benzene lead to the formation of a complex mixture of products. Among these products, we isolated and identified 3a,3a′-methylenebis(5-methyl-2-phenyl-3,3a,4,5,6,7-hexahydro-2 H -pyrazolo[4,3- c ]pyridin-3-one), 1-methyl-3-(2-phenylhydrazinylidene)pyrrolidin-2-one, and 5-methyl-2-phenyl-3,5,6,7-tetrahydro-2 H -pyrazolo[4,3- c ]pyridine. The structure of methyl 3-(2-phenylhydrazinylidene)-4,5-dihydro-3 H -pyrrole-2-carboxylate and 3a,5-dimethyl-2-phenyl-3,3a,4,5,6,7-hexahydro-2 H -pyrazolo[4,3- c ]pyridin-3-one was proved by NMR spectroscopy. 3a,5-Dimethyl-2-phenyl-3,3a,4,5,6,7-hexahydro-2 H -pyrazolo[4,3- c ]pyridin-3-one was isolated as hydrochloride hydrate whose structure was determined by X-ray analysis.