Chiral Brønsted acid from a cationic gold(I) complex: catalytic enantioselective protonation of silyl enol ethers of ketones.

A chiral Brønsted acid has been developed from a cationic gold(I) disphosphine complex in the presence of alcoholic solvent and applied to the enantioselective protonation reaction of silyl enol ethers of ketones. Various optically active cyclic ketones were obtained in excellent yields and high enantioselectivities, including cyclic ketones bearing aliphatic substrates at the α-position. Furthermore, the application of this Brønsted acid was extended to the first Brønsted acid-catalyzed enantioselective protonation reaction of silyl enol ethers of acyclic substrates, regardless of their E/Z ratio.