Intramolecular [2 + 2] cycloaddition reactions of alkynyl ether derived ketenes. A convenient synthesis of donor-acceptor cyclobutanes

Mild thermolysis of tert-butyl alkynyl ethers furnishes aldoketenes, which undergo facile [2 + 2] cycloaddition reactions with pendant di- and trisubstituted alkenes. A wide variety of cis-fused cyclobutanones are produced in moderate to high diastereoselectivity and good to excellent yields by this method, and free hydroxyl groups are tolerated in the ene-ynol ether starting materials. Enol-ynol ethers also undergo efficient reaction to produce donor-acceptor cyclobutanes in high yields.