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.