Regioselective Synthesis of 4-Nitro- or 4-Chloro-Tetrasubstituted Pyrazoles from Hydrazones and [beta]-Halo-[beta]-nitrostyrenes

We report an acid-catalyzed cycloaddition reaction of hydrazones with [beta]-bromo- or [beta]-chloro-[beta]-nitrostyrenes for the regioselective synthesis of 4-nitro- or 4-chloro-tetrasubstituted pyrazoles. Arising from a common 4-halo-4-nitropyrazolidine intermediate, the identity of the pyrazole product formed is dependent on the relative leaving group abilities of the halo and nitro substituents.