Beijing 10094, China

Introduction

Pyrazolidine and pyrazoline are highly important compounds due to their widely existence in natural products, bioactive and unique photophysical properties. 1–13 Therefore, many experimental and theoretical works have been dedicated to the efficient and stereoselective synthesis of pyrazolidine and pyrazoline compounds. 14–28 For instance, Kobayashi found that the Lewis acid chiral zirconium/BINOL complexes can be used as an efficient catalyst to synthesis en- tioselective pyrazolidine products via inter­ or intramolecular [3+2] cycloaddition between hydrazone and alkenes.22–24 Müller and List, however, using a chiral Brønsted­acid to generate enantiopure pyrazolidine derivative. 14 Later on, Rueping’s group provides a more general and enantioselective N­triflylphosphoramide catalyst for the intermolecular [3+2] cycloaddition between hydrazones and alkenes. 27 Houk et al. further employed experimental study and computational calculations to elucidate the mechanism of the N­triflylphosphoramide catalyzed [3+2] cycloaddition between hydrazones and alkenes. 21