Enantioselective syntheses of spiroketals via a tandem reaction of Cu(I)-catalyzed cycloetherification and hydrogen-bond-induced [4 + 2] cyclization.

A tandem reaction consisting of a copper(I)-catalyzed cycloetherification and a hydrogen-bond-induced inverse-electron-demand oxa-DielsAlder cycloaddition was performed from chiral propargyl alcohol, generating several kinds of optically pure [5, 6] spiroketals in excellent stereoselectivities and yields. The investigation on mechanism found that the cyclization prompted by a hydrogen bond not only improved the efficiency but also determined the diastereoselectivity.