Expanding the Chiral Monoterpene Pool: Enantioselective Diels-Alder Reactions of α-Acyloxy Enones.

An enantioselective Diels-Alder (DA) reaction of α-acyloxy enones has been developed to synthesize chiral oxidized cyclohexenes. Yttrium(III) triflate, in conjunction with a chiral pyridinebisimidazoline (PyBim) ligand, was found to catalyze the asymmetric [4 + 2] cycloaddition with a variety of dienes and α-acyloxy enone dienophiles. Using this method, terpinene-4-ol, a key intermediate in the synthesis of commercial herbicide cinmethylin, can be prepared in four steps from isoprene. A combination of kinetic data and NMR studies support a mechanism involving reversible binding of a dienophile to a yttrium catalyst followed by cycloaddition with a diene as the rate-determining step.