Recent progress on chiral extractants for enantioselective liquid-liquid extraction

As the demand for enantiopure compounds increases, chiral separation has become increasingly important in many fields. Enantioselective liquid-liquid extraction is an up-and-coming technology for enantiomeric separation because it is highly efficient and easy to be scaled up. The key factor for enantioselective liquid-liquid extraction is the development of novel chiral extractants with high enantiorecognition performance. With successful studies on catalytically active metal complexes as chiral extractants, novel chiral extractants can be screened and designed from the field of asymmetric catalysis. Chiral ionic liquids, sulfobutylether-β-cyclodextrins bonded magnetic nanoparticles and 2,2',3,3'-tetrahydro-1,1'-spirobi[indene]-7,7'-diol (SPINOL) based phosphoric acid host show unique potential ability in enantioselective liquid-liquid extraction and they deserve further study. Brief principles, extraction equipment and solvent systems in enantioselective liquid-liquid extraction are presented in the present paper, and recent progress in development of new chiral extractants in the past decade is mainly reviewed, including metal complexes, cyclodextrins, ionic liquids, tartrate acids and crown ethers.

Degree and distribution of substitution of hydroxypropyl-β-cyclodextrin in enantioselective liquid-liquid extraction and countercurrent chromatographic enantioseparation

Nine types of hydroxypropyl-β-cyclodextrin (HP-β-CD) with different degrees and distributions of substitution were synthesised, and nine racemates were selected to investigate the effect of different degrees and distributions of substitution of HP-β-CD on the enantioseparation factor. ¹H NMR and GC/MS were used to characterise the synthesised HP-β-CD. The degree and distribution of substitution had a significant influence on enantioselective liquid-liquid extraction and enantioseparation by countercurrent chromatography. For most of the tested racemates, increasing both the degree of substitution and distribution of substitution at the C-2 position for HP-β-CD would lead to an increasing enantioseparation factor; the optimal enantioseparation factor of 2-phenylbutyric acid, tropic acid, 2,3-diphenylpropionic acid, 2-(4-hydroxylphenyl) propanoic acid, and naproxen was increased to 1.77, 1.53, 1.67, 1.61, and 1.75, respectively. The enantioseparation of racemic naproxen, 2-(4-hydroxylphenyl) propanoic acid, and 2,3-diphenylpropionic acid by countercurrent chromatography was optimised using HP-β-CD with a degree of substitution of 16.5, and peak resolution was significantly improved to 1.03, 1.35, and 1.01, respectively.

Enantioseparation of N-methyl duloxetine, duloxetine and fluoxetine by countercurrent chromatography using anionic β-cyclodextrin as chiral selector

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Enantioselective liquid-liquid extraction of tryptophan enantiomers by a recyclable aqueous biphasic system based on stimuli-responsive polymers

The hydrophobic organic solvents/water biphasic system had been always used in the traditional enantioselective liquid-liquid extraction (ELLE). In recent years, aqueous biphasic systems (ABSs) are considered as a promising method used in the ELLE. In the present work, a recyclable ABS composed of a temperature-responsive polymer poly(MAH-β-CD-co-NIPAAm) (P_N-CD) and a pH-responsive polymer poly(AA-DMAEMA-BMA) (P_ADB) was employed in the enantioseparation of tryptophan enantiomers. The polymer P_N-CD acted as not only the phase-forming component but also the chiral selector, which can be recycled by changing the temperature. The polymer P_ADB can be used as the phase-forming component, which can also be recycled by adjusting the pH. The phase behaviors of this P_N-CD/P_ADB ABS had been studied. The influencing parameters were studied for this chiral separation process, including the polymer concentration, initial tryptophan concentration, extraction temperature, and system pH. The maximum separation factor (α) of 1.42 was obtained by one-step extraction under the optimal conditions. Meanwhile, the distribution coefficients of L-tryptophan (L-Trp) and D-tryptophan (D-Trp) were 2.79 and 1.96, respectively. This study develops a green and sustainable strategy for enantioseparation by using the ELLE.

Enantioseparation of three constitutional isomeric 2-(methylphenyl)propanoic acids by countercurrent chromatography

2-(3-Methylphenyl)propanic acid and 2-(4-methylphenyl)propanoic acid were successfully enantioseparated by countercurrent chromatography using hydroxypropyl-β-cyclodextrin (HP-β-CD) as a chiral selector. 2-(2-Methylphenyl)propanoic acid was also studied to compare the enantioseparation ability of three isomeric 2-(methylphenyl)propanoic acids. Totally 20 mg of 2-(3-methylphenyl)propanic acid and 20mg of 2-(4-methylphenyl)propanic acid were enantioseparated individually by countercurrent chromatography. Recovery for the (±)-2-(3-methylphenyl)propanic acid enantiomer was in the range of 85%-90% with 98.0%-98.8% purity and recovery for the (±)-2-(4-methylphenyl)propanic acid enantiomer was in the range of 80%-83% with 97.0%-98.0% purity. The enantioseparation factor in countercurrent chromatography for 2-(4-methylphenyl)propanic acid and 2-(3-methylphenyl)propanic acid were 1.31 and 1.26, and the peak resolution in HPLC reached 2.2 and 1.4. However, no enantioseparation could be found for 2-(2-methylphenyl)propanic acid. In addition, the inclusion complexes were investigated by UV spectrophotometer. The inclusion formation constant of inclusion complex between 2-(4-methylphenyl)propanic acid, 2-(3-methylphenyl)propanic acid, 2-(2-methylphenyl)propanic acid and HP-β-CD were determined as 121.73 mol/L, 78.12 mol/L and 53.18 mol/L, respectively. The present results showed that enantiorecognition was greatly affected by substituted positions of methyl group on the benzene ring. Combined with our previous results, the steric hindrance had a significant effect on inclusion interaction between HP-β-CD and racemic 2-(substitutedphenyl)propanoic acids. No enantiorecognition could be achieved for 2-(substitutedphenyl)propanoic acids with ortho-substituent group on benzene, while the influence of meta- and para- group on enantiorecognition varies with different substituent groups on benzene ring.

Enantioseparation of ondansetron by countercurrent chromatography using sulfobutyl ether-β-cyclodextrin as chiral selector

Ondansetron, a highly selective 5-hydroxytryptamine 3 receptor antagonist, was successfully enantioseparated by recycling countercurrent chromatography using sulfobutyl ether-β-cyclodextrin as chiral selector. Important factors for the enantioseparation were optimized, including different organic solvent, type of substituted β-cyclodextrin, pH of aqueous phase, concentration of chiral selector, and separation temperature. A biphasic solvent system composed of n-hexane: n-butyl acetate: 0.1 mol/L phosphate buffer solution pH 9.2 with 50 mmol/L of sulfobutyl ether-β-cyclodextrin (2.5:7.5:10, v/v/v) was selected. Under optimized separation conditions, 5 mg of ondansetron was enantioseparated using recycling countercurrent chromatography, yielding 1.2 and 1.5 mg of ondansetron enantiomers with 97.5 and 95.8% purity and the recovery reached 48-60%.

Enantioseparation of 2-(4-chlorophenyl)succinic acid by countercurrent chromatography and investigation of injection volume on resolution

2-(4-Chlorophenyl)succinic acid was successfully enantioseparated by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as chiral selector. A two-phase solvent system composed of n-hexane-ethyl acetate-0.1 mol/L phosphate buffer with pH 2.65 (5:5:10, v/v) was selected. Enantioselective liquid-liquid extraction was used to optimize the enantioseparation conditions. Meanwhile, the influence of injection volume on resolution in countercurrent chromatography was investigated and a linear relationship between the inflection point of injection volume and sample loading was tentatively obtained. The peak resolution will decrease significantly when the injection volume over the inflection point was used. In addition, it could be found that the smaller amount of sample loading, the larger impact of injection volume on resolution could be observed, which might serve as a good reference for the selection of sample volume in enantioseparations by countercurrent chromatography. Under optimized conditions, 20 mg of 2-(4-chlorophenyl)succinic acid racemate dissolved in 10 mL of aqueous phase was successfully enantioseparated by countercurrent chromatography. The recovery for both of the enantiomer of (±)-2-(4-chlorophenyl)succinic acid reached within 70-75% with a purity of 99.0%.

Chiral counter-current chromatography: A survey of its instrument, mechanism, procedure, and applications

The chiral separation by counter-current chromatography has made great progress in the past three decades. It has become increasingly popular in the field of chiral separation, and many applications have been introduced during the last years. This review mainly focuses on the current topics, applications, and trends in chiral separation by counter-current chromatography. It contains the development of modern counter-current chromatography apparatus, theory of counter-current chromatography, overview of applications of chiral counter-current chromatography enantioseparation, its current situation, and challenges. At last, some conclusions and perspectives also have been discussed in this review.

Stereoselective separation of (1S, 4S)-sertraline from medicinal reaction mixtures by countercurrent chromatography with hydroxypropyl-β-cyclodextrin as stereoselective selector

Four stereoisomeric components were produced during the synthesis of the antidepressant drug (1S, 4S)-sertraline hydrochloride due to the two chiral carbon centers in its chemical structure, including (1S, 4S), (1R, 4R), (1S, 4R), and (1R, 4S)-isomer. Stereoselective separation of the target isomer (1S, 4S)-sertraline from the medicinal reaction mixtures by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as the stereoselective selector was investigated. A biphasic solvent system composed of n-hexane/0.20 mol/L phosphate buffer solution with pH 7.6 containing 0.10 mol/L of hydroxypropyl-β-cyclodextrin (1:1, v/v) was selected for separation of cis-sertraline and trans-sertraline using reverse phase elution mode and (1S, 4S)-sertraline was separated with (1R, 4R)-sertraline using recycling elution mode. A fabricated in-house analytical countercurrent chromatographic apparatus was used for optimization of the separation conditions. Stationary phase retention and peak resolution were investigated for separation of cis-sertraline and trans-sertraline by the analytical apparatus.

Correlation in Chemical Metabolome and Endophytic Mycobiome in Cynomorium songaricum from Different Desert Locations in China

Cynomorium songaricum Rupr. is a valuable food and medicinal plant with functions, such as an increase in sexual function, mainly attributed to its complex secondary metabolites. However, the effect of internal microbes on metabolite production in C. songaricum is still largely unclear. In this study, the relationship between endophytes and differential secondary metabolites in C. songaricum from seven major producing regions of China were explored based on established methods of metabolomics and high-throughput sequencing. The results showed that there were 13 different marker metabolites, seven shared fungal OTUs, and numerous unshared OTUs among C. songaricum distributed at different locations in China and identified significant correlations between metabolites and endophytic fungi. Our study revealed that endophytic fungi may be one possible factor that can affect the plant secondary metabolite composition.

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%.

Separation of epimeric aromatic acid (-)-menthol esters by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as an additive

The separation of ten epimeric aromatic acid (-)-menthol esters by countercurrent chromatography with hydroxypropyl-β-cyclodextrin as the mobile phase additive was investigated, and methods for the analysis of all the epimeric esters by reversed-phase high-performance liquid chromatography were established. A biphasic solvent system composed of n-hexane/20-70% methanol containing 50 mmol/L of hydroxypropyl-β-cyclodextrin (1:1, v/v) was selected, which provided high separation factors for five of the epimeric esters, and successful separations by countercurrent chromatography were achieved. The complete separation of five pairs of epimeric ester was obtained with the purity being over 98% for each peak fractions, as determined by high-performance liquid chromatography. The recovery of each analyte from the eluted fractions reached around 80-88%.