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-Diels–Alder
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.