Convenient one-pot syntheses of pyrazoles from imines, a vilsmeier type reagent and hydrazine

A simple one-pot procedure for the regioselective synthesis of pyrazoles from readily available starting materials is described. Vilsmeier type reagent 1 reacts with imines 10 (via the corresponding tautomeric secondary enamines) in tetrahydrofuran to give enaminoimine hydrochlorides 11. Nonsymmetrical imines generally react preferentially with 1 at the sterically less hindered α-position. The enaminoimine hydrochlorides 11 are transformed in situ to the corresponding pyrazoles 12 in moderate to high yields by the addition of hydrazine.