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Chapter 3
Synthesis of trisubstituted pyrazoles/isoxazoles
3.5 Current research work
α-Oxoketene dithioacetals especially the dimethylthioacetals have recently
received considerable attention due to their synthetic importance for the construction
of a variety of alicyclic, aromatic and heterocyclic compounds. 44,45 Ketene
dithioacetals, in the presence of various regents, undergo different types of reactions
to yield other heterocyclic compounds, e.g., isoxazole, pyrazole, thiophenes,
pyrimidines, pyridines, etc. Consequently we were interested in surveying the
synthetic utility of ketene dithioacetals.
Recently, we have reported solution-phase library of pyrazoles and isoxazoles
functionalized with methyl, sulfone and carboxamide moieties in two steps with
excellent yield and chemical purity for medicinally interesting molecules. Water was
emerged as an efficient and green solvent in the condensation reaction of various
ketene dithioacetals with hydrazine hydrate or hydroxyl amine hydrochloride. 46
Figure-27
In an extension to this work, we describe here a novel synthesis of solvent free
library of trisubstituted pyrazoles (B) and isoxazoles (C) (Figure-27) by the reaction
of ketene dithioacetals with hydrazine hydrate and hydroxylamine hydrochloride
under microwave irradiations Thus, it has been found that reaction of substituted
acetoacetanilide derivatives with carbon disulfide in the presence of potassium
carbonate followed by the alkylation with methyl iodide gives the novel ketene
dithioacetals (A) (Figure 27), the structures of which have been established on the
basis of their elemental analysis and spectral data.
Department of Chemistry Saurashtra university, Rajkot-360005 95