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

Abstract

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.

Redox Non‐Innocence of Coordinated 2‐(Arylazo) Pyridines in Iridium Complexes: Characterization of Redox Series and an Insight into Voltage‐Induced Current Characteristics

“…The same group has also examined the deracemization of α‐aryl‐substituted hydrocoumarins 33 through the EP of ketene dithioacetals 31 catalysed by new fluorinated chiral phosphoric acids 34 and 35 (Scheme ) 24. Whereas catalyst 30 provided low ee values, new fluorinated catalysts 34 and 35 proved to be highly powerful, giving rise to 32 with excellent enantioselectivities of up to 98 %.…”

A Complete Switch of the Directional Selectivity in the Annulation of 2‐Hydroxybenzaldehydes with Alkynes

“…[16] To test whether the reaction could be readily scaled up, compound 3 n was synthesized on a 1.35 gram scale in 80 % yield. For example, compounds 3 n and 3 p can be readily transformed into the natural product (S)-equol and bioactive molecule 4, respectively (Scheme 5).…”

Deracemisation Processes Employing Organocatalysis and Enzyme Catalysis

“…Thus, in 2012, List et al. achieved the effective deracemisation of α‐aryl hydrocoumarins ( 35 ), compounds with biologically active properties, by using a three‐step procedure that comprises the dithioketal protection of the starting material, the asymmetric Brønsted acid‐catalysed cyclisation of the ketene dithioketal to the corresponding ortho ester and the final hydrolysis to obtain the enantiopure α‐substituted lactone . The asymmetric catalytic cyclisation of the ketene 36 was carried out in cyclohexane at room temperature in the presence of different chiral phosphoric acids.…”