Synthesis of pyrazoles by treatment of 3-benzylchromones, 3-benzylflavones and their 4-thio analogues with hydrazine

The synthesis of pyrazoles 13-24 has been accomplished by treatment of 3-benzylchromones 1-5, 3-benzylflavones 6-12 and their 4-thio analogues 25-29 with hydrazine hydrate in hot pyridine. A plausible reaction mechanism for the formation of pyrazoles 13-24 is discussed. A H-1 NMR study in [D-6]DMSO allowed the presence of both pyrazole annular tautomers to be observed, due to the presence of intramolecular hydrogen bonds in each tautomer (OH--N and NH--O). GIAO/B3LYP/6-311++G** calculations were carried out on some model pyrazoles to provide a theoretical. basis for the NMR experimental observations. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)