Catalytic stereoselective synthesis of cyclopentanones from donor-acceptor cyclopropanes and -generated ketenes.

A dual InBr-EtAlCl Lewis acidic system was found to be optimal for promoting the diastereoselective (3 + 2)-cycloaddition of donor-acceptor cyclopropanes with -generated ketenes to form cyclopentanones. The desired products were formed in good to excellent yields (70-93% for 16 examples) and with good to excellent diastereoselectivity and enantiospecificity.