Carbon‐13 nuclear magnetic resonance study of a new ring‐chain tautomerism of some pyridazine derivatives

The 13 C NMR spectra of a number of pyridazine derivatives have been recorded in DMSO‐ d 6 solution and analysed. Examination of the most diagnostic resonances, with particular emphasis on those arising from the pyridazine ring system, enabled the ready establishment of the presence of a ring‐chain tautomerism in 5‐( o ‐aminophenylcarbamoyl)pyridazine‐4‐carboxylic acid, methyl 5‐( o ‐aminophenylcarbamoyl)pyridazine‐4‐carboxylate, 5‐( o ‐aminophenylcarbamoyl)‐3,6,‐dimethylpyridazine‐4‐carboxylic acid and 5‐(2‐amino‐1,2‐dicyanovinylenecarbamoyl)pyridazine‐4‐carboxylic acid. This gave rise to 3′,4′‐dihydro‐3′‐oxospiro[pyridazine‐5(2 H ),2′(1 H )‐quinoxaline]‐4‐carboxylic acid, methyl 3′,4′‐dihydro‐3′oxospiro[pyridazine‐5(2 H ),2′(1′ H )‐quinoxaline]‐4‐carboxylate, 3′,4′‐dihydro‐3′‐oxo‐3,6‐dimethylspiro[pyridazine‐5( 2H ), 2′(1′ H )‐quinoxaline]‐4‐carboxylic acid and 5‐oxo‐2,3‐dicyano‐1,4,8,9‐tetraazaspiro[5.5]undeca‐2,7,10‐triene‐11‐carboxylic acid, respectively.