Enantioseparation of three isomeric α-(chlorophenyl)propanoic acid by countercurrent chromatography and investigation of chlorine substituent through characterization of inclusion interaction

Abstract

The influence of chlorine substituents in chiral separation of three racemic 2-(chlorophenyl)propanoic acids by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as a chiral additive were mainly investigated in the present paper, including 2-(2-chlorophenyl)propanoic acids, 2-(3-chlorophenyl)propanoic acids and 2-(4-chlorophenyl)propanoic acids. The influences of chromatographic conditions on the retention behavior were studied by enantioselective liquid-liquid extraction experiments using the methodology of response surface It was found that 2-(3-chlorophenyl)propanoic acids could be successfully chiral separated by countercurrent chromatography, while no resolution was achieved for racemic 2-(2-chlorophenyl)propanoic acids and 2-(4-chlorophenyl)propanoic acids under optimized separation conditions. The formation of 1:1 stoichiometric inclusion compounds between 2-(3-chlorophenyl)propanoic acids and HP-β-CD was determined by UV spectra measurements. The inclusion constants for 2-(3-chlorophenyl)propanoic acids and HP-β-CD were determined by the Benesi-Hildebrand equation. Meanwhile, the inclusion constants of 2-(3-chlorophenyl)-propanoic acid enantiomer and HP-β-CD were obtained by the pesudophase retention equation in countercurrent chromatography. Furthermore, the inclusion interactions of the three racemates with HP-β-CD were also investigated by the molecular docking. The results obtained from UV spectra measurements and molecular docking showed that the racemate with chlorine substituents in meta-position presented the highest enantiorecognition while the racemates with chlorine substituents in ortho-position had the lowest enantiorecognition. The above results further indicated that forming a stable inclusion complex between racemate and chiral selector is a prerequisite for a successful enantioseparation and at the same time, the difference in inclusion capacity between the two enantiomers is also essential for the enantioseparation.

Keywords: Countercurrent chromatography; Enantioseparation; Inclusion constant; Molecular docking.