Application and Comparison of High-Speed Countercurrent Chromatography and High Performance Liquid Chromatography in Preparative Enantioseparation of α-Substitution Mandelic Acids

This paper is mainly about extending research on application and comparison of preparative high-speed countercurrent chromatography (HSCCC) and preparative high performance liquid chromatography (HPLC) in chiral separations. Preparative enantioseparations of alpha-cyclopentylmandelic acid and alpha-methylmandelic acid by HSCCC and HPLC were compared using hydroxypropy-beta-cyclodextrin (HP-beta-CD) and sulfobutyl ether-beta-cyclodextrin (SBE-beta-CD) as the chiral mobile phase additives. In preparative HPLC the enantioseparation was achieved on the ODS C-18 reverse phase column with the mobile phase composed of a mixture of acetonitrile and 0.10 mol L-1 phosphate buffer at pH 2.68 containing 20 mmol L-1 HP-beta-CD for alpha-cyclopentylmandelic acid and 20 mmol L-1 SBE-beta-CD for alpha-methylmandelic acid. The maximum sample size for alpha-cyclopentylmandelic acid and alpha-methylmandelic acid was only about 10 mg and 5 mg, respectively. In preparative HSCCC the enantioseparations of these two racemates were performed with the two-phase solvent system composed of n-hexane-methyl tert.-butyl ether-0.1 molL(-1) phosphate buffer solution at pH 2.67 containing 0.1 mol L-1 HP-beta-CD for alpha-cyclopentylmandelic acid (8.5:1.5:10, v/v/v) and 0.1 mol L-1 SBE-beta-CD for alpha-methylmandelic acid (3:7:10, v/v/v). Under the optimum separation conditions, totally 250 mg of racemic alpha-cyclopentylmandelic acid could be completely enantioseparated by HSCCC with HP-beta-CD as a chiral mobile phase additive in a single run, yielding 114-116 mg of enantiomers with 98-99% purity and 89-92% recovery. But, no complete enantioseparation of alpha-methylmandelic acid was achieved by preparative HSCCC with either of the chiral selectors due to their limited enantioselectivity. In this paper, preparative enantioseparation by HSCCC and HPLC was compared from various aspects.