Chiral anion catalysis in the enantioselective 1,4-reduction of the 1-benzopyrylium ion as a reactive intermediate.

Anionic! Novel chiral anion catalysis of the enantioselective 1,4-reduction of the 1-benzopyrylium ion by a chiral phosphoric acid was accomplished with a Hantzsch ester as the reducing agent. The enantioselective reduction established is composed of a two-step consecutive transformation involving stereoablative loss of the hydroxy group from racemic 2H-chromen-2-ol derivatives to generate the achiral 1-benzopyrylium ion as a reactive key intermediate.