Deracemization of α-aryl hydrocoumarins via catalytic asymmetric protonation of ketene dithioacetals.

An unprecedented catalytic asymmetric protonation of ketene dithioacetals is described. Various racemic α-aryl hydrocoumarin derivatives are transformed into enantioenriched dithioacetal-protected hydrocoumarins in the presence of a chiral Brønsted acid catalyst. A newly developed phosphoric acid, featuring the 3,5-bis(pentafluorothio)phenyl (3,5-(SF(5))(2)C(6)H(3)) substituent, is introduced. The obtained products can be easily converted into either hydrocoumarins or the corresponding chromans via simple hydrolysis or hydrogenation, respectively.