Chiral ligand exchange countercurrent chromatography: Equilibrium model study on enantioseparation of mandelic acid

Comparison of two different multiple dual-mode counter-current chromatograph for separation of ketoconazole enantiomers

In this work, two different multiple dual-mode (MDM) counter-current chromatography methods, conventional MDM and modified MDM elution modes, were compared for the chiral separation of the ketoconazole enantiomers. The biphasic solvent system which consisted of n-hexane: isobutyl acetate: 0.1 mol/L phosphate buffer (2:4:6, v/v) (pH = 8.5) was employed as stationary phase and mobile phase. And the hydroxypropyl-β-cyclodextrin (HP-β-CD) with a concentration of 100 mmol/L was dissolved in the phosphate buffer, as the chiral selector. Under two different methods, dual-mode (DM) elution was performed to determine the time of the transformed phase roles and multiple cycles were performed to isolate ketoconazole, respectively. The result indicated that the modified MDM elution had a significant improvement on the separation, increasing the resolution from 0.51 to 1.19, while the resolution was increased from 0.40 to 0.79 by the conventional MDM elution. Ultimately, baseline separation of ketoconazole enantiomers was essentially achieved by high-speed counter-current chromatography under optimized modified MDM separation conditions. The final recoveries of the two enantiomers, R-(K) and S-(K), were 92.5 % and 83.3 %, respectively, corresponding to enantiomeric excess values of 99.0 % and 97.0 %, as determined by HPLC.

An overview of recent progress in multiple dual-mode counter-current chromatography

Counter-current chromatography is a chromatographic separation and purification technique being developed. The development of different elution modes has significantly contributed to this field. Multiple dual-mode elution is a method developed based on dual-mode elution, which consists of a series of changing cycles of the phase role and the direction by switching between normal and reverse elution modes of counter-current chromatography. This dual-mode elution method takes full advantage of the liquid nature of stationary and mobile phases of counter-current chromatography and effectively improves the separation efficiency. So, this unique elution mode has gained extensive attention for separating complex samples. This review mainly describes and summarizes in detail its development, applications, and characteristics in recent years. Meanwhile, its advantages, limitations, and future outlook also have been discussed in this paper.

Enantioseparation of mandelic acid and substituted derivatives by high-performance liquid chromatography with hydroxypropyl-β-cyclodextrin as chiral mobile additive and evaluation of inclusion complexes by molecular dynamics

The enantioseparation and resolution mechanism of mandelic acid (MA), 4-methoxymandelic acid (MMA), and 4-propoxymandelic acid (PMA) were investigated by reversed-phase high-performance liquid chromatography (HPLC) with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) as a chiral mobile-phase additive and molecular dynamics simulation. The suitable chromatographic conditions for the enantioseparation of MA, MMA, and PMA were obtained. Under the selected chromatographic conditions, these enantiomers could achieve baseline separation. The results of thermodynamic parameter analysis revealed that the main driven forces for the enantioseparation of MA, MMA, and PMA could be van der Waals forces and hydrogen-bonding interactions and the chromatographic retention of these chiral compounds was an enthalpy-driven process. The results of the molecular simulation revealed that their chiral resolution mechanism on HP-β-CD was responsible for the formation of inclusion complexes of enantiomers with HP-β-CD with different conformations and binding energies. And the binding energy of HP-β-CD with (S)-isomer was larger than that with (R)-isomer, which is consistent with the experimental results of the first elution of (S)-isomer. Additionally, it is also confirmed that the interaction energies included the van der Waals energy (∆E_vdw ), electrostatic energy (∆E_elec ), polar solvation energy, and SASA energy (∆E_sasa ), and the separation factor (α) was closely connected with the disparity in the binding energies of optical isomers and HP-β-CD complexes. Meanwhile, from molecular dynamics simulation, it can be found that the ∆(∆E_binding ), (∆(∆E_binding ) = ∆E_binding,R  - ∆E_binding,S ) value was in order of MA-HP-β-CD complex > MMA-HP-β-CD complex > PMA-HP-β-CD complex, which was consistent with the order of Δ(ΔG) values obtained from van't Hoff plot. This indicated that the molecular dynamics simulation has predictive function for chiral resolution.

Recent progress in separation prediction of counter-current chromatography

As a liquid-liquid partition chromatography, counter-current chromatography has advantages in large sample loading capacity without irreversible adsorption, which has been widely applied in separation and purification fields. The main factors, including partition coefficient, two-phase solvent systems, apparatus, and operating parameters greatly affect the separation process of counter-current chromatography. To promote the applications of counter-current chromatography, it is essential to develop theoretical research to master the principles of counter-current chromatographic separations so as to achieve predictions before laborious trials. In this article, recent progress about separation prediction methods are reviewed from a point of the steady and unsteady state of the mass transfer process of counter-current chromatography and its mass transfer characteristics, and then it is divided into three aspects: prediction of partition coefficient, modeling the thermodynamic process of counter-current chromatography, and modeling the dynamic process of counter-current chromatography.

Chiral Separations by Countercurrent Chromatography

Chiral separations by countercurrent chromatography are mainly divided into two types: homogeneous chiral selector addition and interfacial chiral ligand exchange. In this chapter, we describe two methods for the enantioseparation of phenylsuccinic acid and α-hydroxy acids by high-speed countercurrent chromatography using hydroxypropyl-β-cyclodextrin and N-n-dodecyl-L-proline as chiral selectors for both above mentioned modes.

Application of pH-zone-refining countercurrent chromatography in the chiral separation of two β-adrenergic blocking agents

Two β-adrenergic blocking agents, 1-[(1-methylethyl)amino]-3-phenoxy-2-propanol (1) and 1-[(1-methylethyl)amino]-3-(3-methylphenoxy)-2-propanol (2; Toliprolol), were enantioseparated by pH-zone-refining countercurrent chromatography. A two-phase solvent system composed of chloroform containing 0.10 mol/L of di-n-hexyl l-tartrate/0.10 mol/L of boric acid aqueous solution (1:1, v/v) was selected, in which 20 mmol/L triethylamine was added in the organic phase as a retainer and 2 mmol/L HCl was added in the aqueous phase as an eluter. Fifty milligrams of each racemate was completely enantioseparated by pH-zone-refining countercurrent chromatography to yield each enantiomer with a purity of more than 98%, and the recovery of each separated enantiomer reached around 76-82%.