Lewis Acid Assisted Brønsted Acid Catalyzed Asymmetric Protonation Reaction

Significance Despite the increasing use of ­Brønsted acids in catalysis, application of these catalysts is often restricted due to lower reactivities compared to Lewis acid catalysts. The authors here use a Lewis acid assisted Brønsted acid (LBA) system to increase the reactivity of the Brønsted acid, and apply the catalyst nicely to the asymmetric protonation of silyl enol ethers of 2-substituted cyclic ketones. The Lewis base ­tolerance allows for a catalytic reaction, which was previously unattainable with LBA systems.