Copper Catalyzed Asymmetric [4 + 2] Annulations of D-A Cyclobutanes with Aldehydes

Copper catalyzed enantioselective [4 + 2] annulations of D-A cyclobutanes and aldehydes have been developed. In the presence of a side arm modified chiral bisoxazoline (SaBOX) ligand, the [4 + 2] annulations proceeded smoothly with a broad substrate scope. 22 examples were studied, leading to the corresponding products with various functional groups in 41%–99% yields with >99/1 dr and 90%–96% ee. The resulting product with two ester groups was mono-reduced, giving the corresponding product in excellent diastereoselectivity without loss of the enantiopurity.