A Practical Catalytic Method for Preparing Highly Substituted Cyclobutanes and Cyclobutenes

Tf2NH, an organic acid, efficiently catalyzes (2 + 2) cycloaddition reactions of silyl enol ethers with acrylates or propiolate. The process affords highly substituted cyclobutanes in high yield with high stereoselectivity under practical and environmentally benign conditions. Application for the formation of terpene skeletons and multigram synthesis of four-membered carbocycles were demonstrated.