Synthesis of 4-Aryl-3(5)-(2-hydroxyphenyl)pyrazoles by Reaction of Isoflavones and their 4-Thio Analogues with Hydrazine Derivatives

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Contents Vol 60(12)

doi:10.1071/CH07268

Synthesis of 4-Aryl-3(5)-(2-hydroxyphenyl)pyrazoles by Reaction of Isoflavones and their 4-Thio Analogues with Hydrazine Derivatives*

Albert Lévai ^A ^E, Artur M. S. Silva ^B ^E, José A. S. Cavaleiro ^B, José Elguero ^C, Ibon Alkorta ^C, József Jekő ^D

^A Department of Organic Chemistry, University of Debrecen, PO Box 20, H-4010 Debrecen, Hungary.
^B Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal.
^C Instituto de Química Médica, C/ Juan de la Cierva, 3, E-28006 Madrid, Spain.
^D Department of Chemistry, College of Nyíregyháza, Sóstói u. 31/b, H-4400 Nyíregyháza, Hungary.
^E Corresponding authors. Email: alevai@puma.unideb.hu; artur.silva@ua.pt





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Australian Journal of Chemistry 60(12) 905–914 http://dx.doi.org/10.1071/CH07268
Submitted: 30 July 2007 Accepted: 13 September 2007 Published online: 4 December 2007

Abstract

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 [D₆]DMSO, a ¹H NMR study allows observation of the presence of both pyrazole annular tautomers, due to the presence of intramolecular hydrogen bonds in each tautomer (OH···N and NH···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.

^* Dedicated to Professor Gábor Tóth on the occasion of his 65th birthday.