Asymmetric Construction of Chiral 2-Azetines and Axially Chiral Tetrasubstituted Allenes via Phosphine Catalysis.

Abstract

Chiral 2-azetines and allenes are highly valuable structural units in natural products and useful chemicals. However, enantioselective synthesis of both 2-azetines and allenes has been extremely challenging. Herein, we present asymmetric construction of chiral 2-azetines (70-98% yields and up to 96% ee) through chiral phosphine-catalyzed [2+2] annulation of yne-enones with sulfamate-derived cyclic imines. These 2-azetines were easily transformed into chiral allenes upon treatment with Et3SiH, BF3•Et2O and water at rt for 2 minutes. Based on the above transformations, a concise one-pot synthetic procedure combining [2+2] annulation of yne-enones and sulfamate-derived cyclic imines under phosphine catalysis and sequential reduction/isomerization/ring-opening reaction through Et3SiH, BF3•Et2O and water was thus set up, providing axially chiral tetrasubstituted allenes in satisfactory yields and enantioselectivities (56-90% yields and up to 91% ee).