“Solvent-induced chirality switching” in the enantioseparation of chlorine-substituted tropic acids via diastereomeric salt formation by (1R,2S)-(−)-2-amino-1,2-diphenylethanol (ADPE)

Abstract

The optical resolution of chlorine-substituted tropic acids (TAs) by diastereomeric salt formation with (1R,2S)-(−)-2-amino-1,2-diphenylethanol ((−)-ADPE) has been demonstrated. Efficient solvent-induced chirality switching was observed in the case of racemic 2-chlorotropic acid (rac-2-Cl-TA). The recrystallization of the diastereomeric salts of rac-2-Cl-TA from branched alcohols such as i-PrOH and s-BuOH afforded (R)-2-Cl-TA, whereas (S)-2-Cl-TA was obtained from linear alcohols such as MeOH, EtOH, and n-PrOH. The chirality switching mechanism was elucidated from the crystal structures of the diastereomeric salts, which revealed that the incorporation of solvent in the (S)-2-Cl-TA salt crystals played a key role in chirality switching by reinforcing the supramolecular structure of the salt. However, no chirality switching was achieved for racemic 3-chlorotropic acid and 4-chlorotropic acid. The position of the chlorine substituent plays an important role in the solvent-induced chirality switching effect.