Pyrazoles under Vilsmeier conditions

Hydrazones are also useful substrates in the preparation of pyrazoles. Reaction of N-monosubstituted hydrazones with nitroolefins led to a regioselective synthesis of substituted pyrazoles <060L3505>. lf/-3-Ferrocenyl-l-phenylpyrazole-4-carboxaldehyde was achieved by condensation of acetylferrocene with phenylhydrazine followed by intramolecular cyclization of the hydrazone obtained under Vilsmeier-Haack conditions <06SL2581>. A one-pot synthesis of oxime derivatives of l-phenyl-3-arylpyrazole-4-carboxaldehydes has been accomplished by the Vilsmeier-Haack reaction of acetophenone phenylhydrazones <06SC3479>. 

Under low temperature Vilsmeier conditions 4,4 -bipyrazol-3,3 -dione 86 was diformylated at the methyl group of one of the pyrazol-3-one rings that was converted into 3-chloropyrazole. The product 87 remained with one pyrazol-3-one ring intact. When the temperature was increased to 75 °C a furo[2,3-f 5,4-c]dipyrazole derivative was obtained (92BCJ1652) (Scheme 22). 

Reactions of Other Derivatives.—Condensation of 2-methyl-benzothiazoles with aromatic aldehydes may be carried out in 50% aqueous sodium hydroxide at room temperature in the presence of triethylbenzylammonium chloride. In some cases the intermediate carbinols are isolable under these conditions. 2-Methylbenzothiazoles undergo the Vilsmeier-Haack reaction to give products [1 R = C( CHOH)CHO] that can be converted into other heterocycles e.g., alkyl- or aryl-hydrazines furnish pyrazoles. ... 

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