Synthesis of trifluoromethylated pyrazolidines by [3 + 2] cycloaddition.

A highly efficient [3 + 2] cycloaddition between trifluoromethylated N-acylhydrazones and nitroolefins in the presence of potassium hydroxide under phase transfer catalysis is developed to afford potentially bioactive trifluoromethylated pyrazolidines, which can be further transformed into trifluoromethylated pyrazoles in good yields.