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Issue 7, 2013
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Synthesis, crystal structures and tautomerism in novel oximes based on hydroxyalkylpyrazolones

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Abstract

Two novel hydroxyethyl pyrazolones were obtained by condensation of 2-hydroxyethylhydrazine with ethyl acetoacetate or ethyl benzoylacetate, later nitrosation of the C4 carbon atom in the heterocyclic moiety, afforded two novel oxime-derivatives. The 1H and 13C NMR data show that 1-(2-hydroxyethyl)-3-methyl-5-pyrazolone (1) is largely present together with a very small amount of the 5-one tautomer in DMSO-d6; in contrast, 1-(2-hydroxyethyl)-3-phenyl-5-pyrazolone (2) is found as a single tautomer in CDCl3. Two diastereomers occur in DMSO-d6 in 1-(2-hydroxyethyl)-3-methyl-4-hydroxyimino-5-pyrazolone (3) while in 1-(2-hydroxyethyl)-3-phenyl-4-hydroxyimino-5-pyrazolone (4), a single species is present in CDCl3. Despite the polarity differences in the solvents used, remarkably the oxime tautomers proved to be more stable in both cases and DMSO-d6 only allows the equilibrium between E/Z diastereomers. Single crystal X-ray diffraction data of 1, 3 and 4 are presented, they show that 1 occurs as a keto-enamine tautomer, in clear contrast with the situation in solution; in the case of 3 and 4, the tautomeric equilibrium remains shifted to the oximes in the solid phase as the Z-diastereomer, exclusively. This finding is the opposite of what was described in a previous report for 1-alkyl-3-methyl-4-hydroximino-5-pyrazolone, where the E-diastereomer was present. Besides, the data show that the oxime motif is not affected by the substitution changes described in C3. IR data in the solid phase supports the crystal structures and suggest that 2 occurs as a similar tautomer as 1.

Graphical abstract: Synthesis, crystal structures and tautomerism in novel oximes based on hydroxyalkylpyrazolones

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Supplementary files

Article information


Submitted
11 Feb 2013
Accepted
25 Mar 2013
First published
26 Mar 2013

New J. Chem., 2013,37, 2002-2010
Article type
Paper

Synthesis, crystal structures and tautomerism in novel oximes based on hydroxyalkylpyrazolones

J. Belmar, J. Quezada, C. A. Jiménez, P. Díaz-Gallifa, J. Pasán and C. Ruiz-Pérez, New J. Chem., 2013, 37, 2002
DOI: 10.1039/C3NJ00163F

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