Chiral N-Heterocyclic Carbene-Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates-Claisen Rearrangement

A combination of a chiral N-heterocyclic carbene catalyst and alpha,beta-unsaturated aldehyde leads to a catalytically generated alpha,beta-unsaturated acyl azolium, which participates in a highly enantioselective annulation to give dihydropyranone products. This full account of our investigations into the scope and mechanism of this reaction reveals the critical role of both the type and substitution pattern of the chiral triazolium precatalyst in inducing and controlling the stereochemistry. In an effort to explain why stable enols such as naphthol, kojic acid, and dicarbonyl are uniquely efficient, we have postulated that this annulation occurs via a Coates-Claisen rearrangement that invokes the formation of a hemiacetal prior to a sigmatropic rearrangement. Detailed kinetic investigations of the catalytic annulation are consistent with this mechanistic postulate.