Chiral ligand exchange high-speed countercurrent chromatography: mechanism and application in enantioseparation of aromatic α-hydroxyl acids

•Mechanism for chiral ligand exchange countercurrent chromatography was proposed.•The solvent systems with chiral ligand and transition metal ion were optimized.•Five racemates were separated by chiral ligand exchange countercurrent chromatography.•We report an alternative chromatographic method for enantioseparations.

This work concentrates on the separation mechanism and application of chiral ligand exchange high-speed countercurrent chromatography in enantioseparation of ten racemic aromatic α-hydroxyl acids, including mandelic acid, 2-chloromandelic acid, 4-methoxymandelic acid, 4-hydroxymandelic acid, α-methylmandelic acid, 4-hydroxy-3-methoxy-mandelic acid, 3-chloromandelic acid, 4-bromomandelic acid, α-cyclopentylmandelic acid and α-cyclohexylmandelic acid, in which five of the racemates were successfully enantioseparated by analytical apparatus with an optimized solvent system. The two-phase solvent system was composed of butanol-water (1:1, v/v) or hexane-n-butanol-water (0.5:0.5:1, v/v), to which N-n-dodecyl-l-proline was added in the organic phase as chiral ligand and cupric acetate was added in the aqueous phase as a transition metal ion. Various influence factors in high-speed countercurrent chromatography were optimized by enantioselective liquid–liquid extraction. The separation mechanism for chiral ligand exchange high-speed countercurrent chromatography was proposed based on the results of present studies. Successful enantioseparations of 72 mg of mandelic acid, 76 mg of 2-chloromandelic acid and 74 mg of 4-methoxymandelic acid were achieved individually with high resolution by preparative high-speed countercurrent chromatography. The HPLC purity of all enantiomers was over 96% with the recovery in the range of 82–90% from the collected fractions.