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

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).