Synthesis of 4-aryl-3(5)-(2-hydroxyphenyl)pyrazoles by reaction of isoflavones and their 4-thio analogues with hydrazine derivatives

4-Aryl-3(5)-2-(hydroxyphenyl) pyrazoles have been prepared by the reaction of isoflavones and their 4-thio analogues with hydrazine hydrate and phenylhydrazine in hot pyridine. The reaction mechanism for the formation of these pyrazoles is discussed. All the new compounds have been fully characterized by NMR spectroscopy. In [D-6] DMSO, a H-1 NMR study allows observation of the presence of both pyrazole annular tautomers, due to the presence of intramolecular hydrogen bonds in each tautomer (OH center dot center dot center dot N and NH center dot center dot center dot O). Theoretical calculations have been carried out on tautomers and conformers of compounds 20 (3(5)-(2-hydroxy-4-methoxyphenyl)-5(3)-methyl-4-phenylpyrazole) and 21 (3(5)( 2-hydroxy-4-methoxyphenyl)-4-(2-methoxyphenyl)-5(3)-methylpyrazole), including the absolute shieldings (GIAO/B3 LYP/6-311++G**) of 21.