The annular tautomerism of the curcuminoid NH-pyrazoles?

New Journal of Chemistry Pub Date: 2008-12-02 DOI: 10.1039/B812018H

Abstract

The structures of four NH-pyrazoles, (E)-3,5-bis[β-(4-hydroxy-3-methoxyphenyl)-ethenyl]-1H-pyrazole (3), (E)-3(5)-[β-(4-hydroxy-3-methoxyphenyl)-ethenyl]-5(3)-methyl-1H-pyrazole (4), (E)-3(5)-[β-(4-hydroxy-3-methoxyphenyl)-ethenyl]-4,5(3)-dimethyl-1H-pyrazole (5) and (E)-3(5)-[β-(3,4-dimethoxyphenyl)-ethenyl]-4-methyl-5(3)-phenyl-1H-pyrazole (8), have been determined by X-ray crystallography. Compounds that have a phenol residue crystallize forming sheets that are stabilized by a complex pattern of hydrogen bonds between a unique tautomer (4), or by a 2 : 1 mixture of both tautomers (5) (these tautomers being identical in the case of 3). Pyrazole 8, which lacks OH groups, crystallizes in cyclic dimers that are stabilized by N–H?N hydrogen bonds. The tautomerism in solution and in the solid state was determined by 13C and 15N CPMAS NMR spectroscopy. For compounds 4, 5 and 8, the solid state results agree with those observed by crystallography; the most abundant tautomer in solution coincides with the tautomer present in the solid state (4 and 8) or with the most abundant tautomer in the crystal (5).

Graphical abstract: The annular tautomerism of the curcuminoid NH-pyrazoles
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