Photochemical Deracemization of 3‐Substituted Oxindoles

Racemic 3-substituted oxindoles were successfully converted into enantiomerically pure or enriched material (up to 99% ee) upon irradiation at λ = 366 nm in the presence of a chiral benzo­phenone catalyst (10 mol%). The photochemical deracemization process allows to edit predictably the stereogenic center at carbon atom C3. Light energy compensates for the associated loss of entropy and enables the decoupling of potentially reversible reactions, i.e. a hydrogen atom transfer to (photochemical) and from (thermal) the carbonyl group of the catalyst. The major enantiomer is continuously enriched in several catalytic cycles. The obtained oxindoles were shown to be valuable intermediates for further transformations which proceeded with complete retention at the stereogenic center.