Reactions of glyoxals with hydrazones: a new route to 4-hydroxypyrazoles

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Abstract
N-Substituted hydrazones of aldehydes react with glyoxals under non-aqueous conditions to give N-substituted 4-hydroxypyrazoles; fourteen such products are described. In the presence of water, N-substituted 3-acyl-4-hydroxypyrazoles are produced from 2-oxoaldehyde hydrazones and glyoxals. It is shown that glyoxals combine in a 1:1 ratio with N-monosubstituted hydrazines to give 2-hydrazonoaldehydes as the kinetically controlled products. At room temperature or lower, these hydrazonoaldehydes rearrange to the more stable 2-oxoaldehyde hydrazones and react with glyoxals to give N-substituted 3-acyl-4-hydroxypyrazoles; sixteen such products are described. This synthesis, involving easily accessible starting materials, opens up a new, and for certain derivatives exclusive, route to 4-hydroxypyrazoles.

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