Formal [4 + 1] Annulation of α-Arylhydrazonoketones and Dimethylsulfoxonium Methylide: One-pot Synthesis of Substituted Pyrazoles and Dihydropyrazoles.

A formal [4 + 1] annulation of readily available α-arylhydrazonoketones and trimethylsulfoxonium iodide in the presence of cesium carbonate is described involving a sequential Corey-Chaykovsky reaction and intramolecular nucleophilic cyclization process. Substituted pyrazoles were obtained exclusively from the reactions of α-arylhydrazono-β-oxo-amides and trimethylsulfoxonium iodide in moderate to good yields, whereas the reactions of α-arylhydrazono-β-oxo-ketone/α-arylhydrazono- β-oxo-ester afforded the corresponding dihydropyrazoles in good yields.