3(5)‐(2‐Hydroxyphenyl)‐5(3)‐styrylpyrazoles: Synthesis and Diels−Alder Transformations

Abstract Reactions between cinnamoyl(2‐hydroxybenzoyl)methanes and hydrazine hydrate in acetic acid gave 3‐(2‐hydroxyphenyl)‐5‐styrylpyrazoles, while the corresponding reactions with phenylhydrazine yielded 5‐(2‐hydroxyphenyl)‐1‐phenyl‐3‐styrylpyrazoles as the major products and 3‐(2‐hydroxyphenyl)‐1‐phenyl‐5‐styrylpyrazoles as by‐products. The reaction mechanism of this transformation is discussed. The first cycloaddition reactions between ortho ‐benzoquinodimethane and either 3‐(2‐hydroxyphenyl)‐5‐styrylpyrazoles or 5‐(2‐hydroxyphenyl)‐1‐phenyl‐3‐styrylpyrazoles afforded 5‐[2‐(3‐aryl‐1,2,3,4‐tetrahydronaphthyl)]‐3‐(2‐hydroxyphenyl)pyrazoles or 3‐[2‐(3‐aryl‐1,2,3,4‐tetrahydronaphthyl)]‐1‐phenyl‐5‐(2‐hydroxyphenyl)pyrazoles, respectively. These cycloadducts were converted into the corresponding naphthylpyrazoles by oxidation with DDQ in dry 1,4‐dioxane. The structures of all new derivatives have been established by NMR spectroscopy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)