Hydrazones Vilsmeier-Haack reaction

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>. 

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