Copper-Catalyzed Asymmetric Formal [3 + 2] Cycloaddition of Propargylic Acetates with Hydrazines: Enantioselective Synthesis of Optically Active 2‑Pyrazolines

A catalytic asymmetric [3 + 2] cycloaddition of hydrazines to bis-electrophilic C3 synthons generated from propargylic acetates, followed by an intramolecular 1,3-H migration, for the regio- and enantioselective construction of chiral 2-pyrazolines has been reported. By employment of copper catalysis in combination with a structurally rigid tridentate P,N,N-ligand, a variety of chiral 2-pyrazolines were obtained in good yields and with high enantioselectivities (up to 96% ee). A possible transition state has been proposed to explain the origin of the regio- and enantioselectivities.