Regioselective Synthesis of 5-Trifluoromethyl Pyrazoles by the (1 + 4) Cyclization of Phenylhydrazones with N-Aryl Trifluoroacetimidoyl Iodides.

Treatment of the phenylhydrazone of a methyl ketone (2) or cyclohexanone (3) with N-aryl trifluoroacetimidoyl iodide (1) in the presence of excess sodium hydride resulted in a [1+4] cyclization to give 5-trifluoromethyl pyrazoles (4, 5) regioselectively. The structure of products 4 or 5 was confirmed by the 13C NMR spectra.