Combining incompatible processes for deracemization of a Praziquantel derivative under flow conditions

An efficient deracemization method for conversion of the racemate to the desirable ( R )‐enantiomer has been developed by coupling incompatible racemization and crystallization processes. By a library approach a derivative that crystallizes as a conglomerate in the solid phase has been identified. Racemization occurs via reversible hydrogenation over a palladium on carbon (Pd/C) packed column at 130 °C, whereas deracemization is achieved by alternating crystal growth/dissolution steps with temperature cycling between 5‐15 °C. These incompatible processes are combined by means of a flow system. Complete deracemization of the solid phase to the desired ( R )‐enantiomer (98% ee) takes place. Such an unprecedented deracemization by a decoupled crystallization/racemization approach can readily be turned into a practical process, and opens new opportunities for the development of essential enantiomerically pure building blocks that require harsh methods for racemization.