Ring–chain isomerism of some 4,5-disubstituted pyridazines involving heterospiro-compounds

Abstract

I.r., u.v., and ¹H n.m.r. spectral evidence demonstrated that 5-(o-aminophenylcarbamoyl)pyridazine-4-carboxylic acid (4), for which the zwitterionic structure (4a) appeared the most likely in the solid state, existed in dimethyl sulphoxide solution nearly exclusively as 3′,4′-dihydro-3′-oxospiro[pyridazine-5(2H), 2′(1′H)-quinoxaline]-4-carboxylic acid (18). The equilibrium (4a)⇄(18) was strongly influenced by the nature of the solvent. A study of the behaviour of compounds (5)–(10) enabled us to establish that the spirocyclisation critically depends on both the nature of the substituents on the pyridazine ring and on the nucleophilicity of the group bonded to the phenyl ring.