Preferential Enrichment
--Crystal Structure and Polymorphic Transition--

Rui TAMURA,* Hiroki TAKAHASHI and Takanori USHIO††

Graduate School of Human and Environmental Studies, Kyoto University; Yoshida Nihonmatsu-cho, Sakyou-ku, Kyoto-shi 606-8501 Japan
RIKEN; 2-1, Hirosawa, Wako-shi 351-0198 Japan
†† Taiho Pharmaceutical Co. Ltd.; 200-22, Aza Toyohara, Ooaza Motohara, Kamimawa-cho, Kodama-gun, Saitama 367-0241 Japan

We have discovered the first case of enantiomeric resolution by simple recrystallization of a series of racemic crystals, although in principle this sort of enantiomeric resolution was believed to be impossible for more than a century since the mechanical resolution of enantiomeric conglomerates by Pasteur and the discovery of the ``preferential crystallization'' technique by Gernetz. We have referred to this new phenomenon of enantiomeric resolution as the “preferential enrichment” in the mother liquor. By means of X-ray crystallographic analysis and construction of the binary melting point phase diagram, it has been found that the racemic crystals of the compounds, which show the preferential enrichment phenomenon, can be classified into a highly or fairly ordered mixed crystal composed of the two enantiomers, while those of the analogous compounds, which fail to show the phenomenon, are classified into a less ordered mixed crystal of the two enantiomers. By comparison of the presumed enantiomeric association mode in solution with the enantiomeric arrangement in the crystal, we propose a mechanism of the polymorphic transformation during crystallization closely associated with the preferential enrichment phenomenon.

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