Eur. J. Org. Chem. 12, 2825-2832 (2006)

Doi: 10.1002/ejoc.200600043

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 1H NMR study in [D6]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.