Resolution Characteristics of Optically Active (1S,2S)-(+)-1-(p-Nitrophenyl)-2-Amino-1,3-Propanediol for trans-1,2-Cyclohexanedicarboxylic Acid.

The resolution behavior of chiral compounds containing two carboxyl groups remains an interesting subject due to their potential for forming complex stereospecific interactions. In this study, a representative for this group of compounds, trans-1,2-cyclohexanedicarboxylic acid (CHDA) was enantiomerically separated using optically active (1S,2S)-(+)-1-(p-nitrophenyl)-2-amino-1,3-propanediol ((SS)-ANP) as a resolving agent in ethanol solvent. Remarkably, regardless of the initial molar ratio of (SS)-ANP to CHDA (ranging from 1:3 to 3:1), only a cocrystal, di-(SS)-ANP·CHDA·ethanol was obtained. When the molar ratio was adjusted to 1:2, a pair of diastereomeric salts was formed, and (RR)-CHDA freed from the less soluble diastereomeric salt exhibited an enantiomeric excess (e.e.) exceeding 85%. Upon two cycles of recrystallization of the less soluble salts, the optical purity of the freed (RR)-CHDA reached 96.5 e.e.%. To elucidate the resolution mechanism, single crystals of both the less soluble salt 2(SS)-ANP·(RR)-CHDA·ethanol and the more soluble salt 2(RR)-ANP·(RR)-CHDA·ethanol were grown and structurally analyzed. The crystallographic data revealed that the hydrogen bonding in the less soluble salt was stronger than that in the more soluble salt, enhancing the stability of the less soluble salt, 2(SS)-ANP·(RR)-CHDA·ethanol.