1H and 13C NMR Study of 1-Hydrazino-2,3-dihydro-1H-pyrazolo[1,2-a]pyridazine-5,8-diones and -1H-pyrazolo[1,2-b]phthalazine-5,10-diones and Their Ring-Chain Tautomerism

Treatment of 1-hydroxy-2,3-dihydro-1H-pyrazolo[1,2-a]pyridazine-5,8-diones and -1H-pyrazolo[1,2-b]phthalazine-5,10-diones with hydrazides produces corresponding acylhydrazino derivatives. In this work, ten new hydrazino derivatives were synthesized, and their properties were studied by 1H and 13C NMR spectroscopy. These compounds exhibited several types of structural variation, including ring-chain tautomerism, cis-trans isomerism with respect to the substituents in the pyrazole ring, and (E)/(Z) rotamerism with respect to the nitrogen−carbon hydrazide bond with partial double-bond character. In [D6]DMSO, 2,3-dihydro-1H-pyrazolo[1,2-a]pyridazine-5,8-dione derivatives were found mainly as linear tautomers, whereas 2,3-dihydro-1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives favored cyclic tautomers. For the latter compounds, a six-component equilibrium was found, consisting of a linear and of cis and trans cyclic tautomers, all of which had two rotamers, (E) and (Z). (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)