Highly Enantioselective [2+2]-Cycloaddition Reactions Catalyzed by a Chiral Aluminum Bromide Complex

Enantioselective [2+2]-cycloaddition pathways to chiral cyclobutanes are rare and not generally utilized for synthesis. A new cycloaddition reaction of vinyl ethers with trifluoroethyl acrylate in the presence of a catalytic amount of chiral oxazaborolidine−AlBr3 complex is described which affords [2+2]-adducts with excellent yields and enantioselectivities. Applications of these adducts to the synthesis of synthetically valuable intermediates are also presented.