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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)