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Issue 10, 2005
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New hydroxy-pyrazoline intermediates, subtle regio-selectivity and relative reaction rate variations observed during acid catalyzed and neutral pyrazole cyclization

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Abstract

Arylhydrazines 4 and 1,3-dicarbonyl compounds 5 react to form pyrazoles by loss of water via hydrazone isomer pairs 6 and 7 which give rise to two possible regio-isomers. Occasionally, 3-hydroxy-3,4-dihydropyrazoles or hydroxy-pyrazolines 8 and 9 are observed as stable isolatable intermediates that can be fully characterized prior to loss of the second molecule of water that gives rise to pyrazoles 10 and 11. Fully characterized examples of intermediates of type 8 and 9 are relatively rare. We studied the reaction series where R = CH3, CHF2 and CF3 and Ar = Ph and 5-methanesulfonylpyridin-2-yl, (Scheme 2), and observed differences in properties between kinetic behavior and regio-isomerism depending on the degree of electron-withdrawing capability of the R and Ar substituents. The reaction conditions that caused cyclization to pyrazoles varied from direct condensation of the hydrazine and 1,3-dicarbonyl compounds, to reactions requiring catalytic quantities of sulfuric acid to sulfuric acid in excess. Unexpected regio-selectivity was observed in the case of R = CF3 that depended upon the reaction conditions.

Graphical abstract: New hydroxy-pyrazoline intermediates, subtle regio-selectivity and relative reaction rate variations observed during acid catalyzed and neutral pyrazole cyclization

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Publication details

The article was received on 11 Jan 2005, accepted on 11 Apr 2005 and first published on 20 Apr 2005


Article type: Paper
DOI: 10.1039/B500413F
Citation: Org. Biomol. Chem., 2005,3, 1844-1849
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    New hydroxy-pyrazoline intermediates, subtle regio-selectivity and relative reaction rate variations observed during acid catalyzed and neutral pyrazole cyclization

    T. Norris, R. Colon-Cruz and D. H. B. Ripin, Org. Biomol. Chem., 2005, 3, 1844
    DOI: 10.1039/B500413F

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