Chiral symmetry breaking of thiohydantoins by attrition-enhanced deracemization was performed. It was found that three thiohydantoins afforded conglomerate crystals, and their absolute structures were determined by single crystal X-ray structure analysis. The effect of substituents on the racemization rate via a ring-opening and -closing reaction was investigated experimentally and computationally. Attrition of the crystals of racemic thiohydantoins in the presence of a catalytic amount of base promoted deracemization, leading to optically active thiohydantoins with excellent enantiomeric purity (94–96% ee) without any external chiral source.

The highly enantioselective synthesis of p-anisoin was achieved via the benzoin condensation of a prochiral p-anisaldehyde using achiral NHC catalysts such as vitamin B1. In this reaction, p-anisoin crystallized as a conglomerate, and the deracemization of racemic p-anisoin under basic conditions was efficiently performed by Viedma ripening. Although the handedness of the enantioselective crystallization could not be controlled by spontaneous crystallization, it could be controlled by the coexistence of a catalytic amount of optically active valine. It was clarified that this is due to the asymmetric transformation of p-anisoin with enantiomeric valine in the mother liquor.

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