Fig. 2: Catalyst screening.

Reaction conditions: 1a (0.20 mmol), 2a (0.10 mmol), catalyst 6 (0.01 mmol, 10 mol%), and DBU (0.20 mmol) in DCM (0.5 mL) at -20 oC for 72 h. Isolated yields were based on 2a. The dr values were determined by 1H NMR analysis of the crude reaction mixtures. The ee values were determined by chiral HPLC analysis. iPr: isopropyl; tBu: tert-butyl; Ph: phenyl; Bn: benzyl; Ac, acetyl; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; DCM, dichloromethane. aThe reaction time was 24 h.