Deracemization of α-Aryl Hydrocoumarins via Catalytic Asymmetric Protonation of Ketene Dithioacetals

An unprecedented catalytic asymmetric protonation of ketene dithioacetals is described. Various racemic alpha-aryl hydrocoumarin derivatives are transformed into enantioenriched dithioacetal-protected hydrocoumarins in the presence of a chiral Bronsted acid catalyst. A newly developed phosphoric acid, featuring the 3,5-bis(pentafluorothio)phenyl (3,5-(SF5)(2)C6H3) substituent, is introduced. The obtained products can be easily converted into either hydrocoumarins or the corresponding chromans via simple hydrolysis or hydrogenation, respectively.