Enantioseparation tuned by solvent polarity on a β‐cyclodextrin clicked chiral stationary phase

The efficient enantioseparation of 26 racemates has been achieved with the perphenylcarbamoylated cyclodextrin clicked chiral stationary phase by screening the optimum composition of mobile phase in high‐performance liquid chromatography. The chromatographic results indicate that both the retention and chiral resolution of racemates are closely related to the polarity of the mobile phases and the structures of analytes. The addition of alcohols can significantly tune the enantioseparation in normal‐phase high‐performance liquid chromatography. The addition of methanol and the ratio of ethanol/methanol or isopropanol/methanol played a key role on the resolution of flavonoids in ternary eluent systems. The chiral separation of flavonoids with pure organic solvent as mobile phase indicates the preferential order for chiral resolution is methanol>ethanol>isopropanol>n‐propanol>acetonitrile.     

Separation of chiral phosphorus compounds on the substituted β‐cyclodextrin stationary phase in normal‐phase liquid chromatography

The separation of enantiomers of a series of eighteen novel nitrogen mustard linked phosphoryl diamide derivatives was investigated on the prepared phenyl carbamate derivative β‐cyclodextrin bonded phase in normal‐phase HPLC. Some of the enantiomers could be separated in baseline. The chiral recognition mechanism was also suggested for the separation of chiral phosphorus organic compounds.     

Novel cyclodextrin chiral stationary phases for high performance liquid chromatography enantioseparation: Effect of cyclodextrin type

Three novel chiral stationary phases (CSPs) were prepared by regioselective chemical immobilization of mono(6^A-N-allylamino-6^A-deoxy)perphenylcarbamoylated (PICD) α-, β-, and γ-cyclodextrins (CDs) onto silica support via hydrosilylation. Their enantioseparation properties in high performance liquid chromatography (HPLC) were evaluated with a large spectrum of racemates including flavanone compounds, β-adrenergic blockers, amines and non-protolytic compounds. The effect of CD's cavity size on enantioseparation abilities was studied and discussed. The results indicated that CD's surface loading at silica support played an important role in the enantioseparation on these CSPs under normal-phase conditions while inclusion phenomena contributed the major driving force under reverse-phase conditions. As expected, α-PICD demonstrated the best resolutions towards flavonone and most aromatic alcohols under normal-phase conditions with the highest surface loading; while Fujimura's competitive inclusion model can be applied to explain the better enantioseparations towards β-adrenergic blockers, amines and non-protolytic compounds with α- and β-PICD CSPs. γ-PICD CSP showed superior enantioseparation ability for sterically encumbered analytes like flavanone compounds under both normal-phase and reversed phase conditions.     

Homochiral metal–organic framework used as a stationary phase for high‐performance liquid chromatography

Metal–organic frameworks are promising porous materials. Chiral metal–organic frameworks have attracted considerable attention in controlling enantioselectivity. In this study, a homochiral metal–organic framework [Co₂(D‐cam)₂(TMDPy)] (D‐cam = d ‐camphorates, TMDPy = 4,4′‐trimethylenedipyridine) with a non‐interpenetrating primitive cubic net has been used as a chiral stationary phase in high‐performance liquid chromatography. It has allowed the successful separation of six positional isomers and six chiral compounds. The good selectivity and baseline separation, or at least 60% valley separation, confirmed its excellent molecular recognition characteristics. The relative standard deviations for the retention time of run‐to‐run and column‐to‐column were less than 1.8 and 3.1%, respectively. These results demonstrate that [Co₂(D‐cam)₂(TMDPy)] may represent a promising chiral stationary phase for use in high‐performance liquid chromatography.     

Enantiomeric separation of oxybutynin by recycling high‐speed counter‐current chromatography with hydroxypropyl‐β‐cyclodextrin as chiral selector

Recycling high‐speed counter‐current chromatography was successfully applied to the preparative separation of oxybutynin enantiomers. The two‐phase solvent system consisted of n‐hexane, methyl tert‐butyl ether, and 0.1 mol/L phosphate buffer solution (pH = 5.0) with the volume ratio of 6:4:10. Hydroxypropyl‐β‐cyclodextrin was employed as the chiral selector. The influence of factors on the chiral separation process, including the concentration of chiral selector, the equilibrium temperature, the pH value of the aqueous phase were investigated. Under optimum separation conditions, 15 mg of oxybutynin racemate was separated with the purities of both the enantiomers over 96.5% determined by high‐performance liquid chromatography. Recovery for the target compounds reached 80–82% yielding 6.00 mg of (R)‐oxybutynin and 6.15 mg of (S)‐oxybutynin. Technical details for recycling elution mode were discussed.     

Application of bromoacetate‐substituted β‐CD‐bonded silica particles as chiral stationary phase for HPLC

Bromoacetate‐substituted [3‐(2‐O‐β‐cyclodextrin)‐2‐hydroxypropoxy]propylsilyl‐appended silica particles (BACD‐HPS), an important and useful synthetic intermediate for preparation of novel types of macrocycles‐capped β‐CD‐bonded silica particles including crown ether/cyclam/calix[4]arene‐capped β‐CD‐bonded silica particles, have been prepared and used as chiral stationary phase for HPLC. This synthetic stationary phase is characterized by means of elemental analysis. For the first time, the chromatographic behavior of BACD‐HPS was systematically evaluated with several disubstituted benzenes and some chiral drug compounds under both normal and RP conditions in HPLC. The results show that BACD‐HPS has excellent selectivity for the separation of aromatic positional isomers and chiral isomers of some drug compounds when used as stationary phase in HPLC.     

Chiral separation of cathinone derivatives using β‐cyclodextrin‐assisted capillary electrophoresis–Comparison of four different β‐cyclodextrin derivatives used as chiral selectors

In the past decade, more than 100 different cathinone derivatives slopped over entire Europe due to their enormous popularity. Generally, these novel psychoactive substances are easily available via the internet. This fact leads to various social problems, since cathinones are substances with consciousness‐changing effects and are mainly misused for recreational matters by their consumers. Cathinones possess a chiral center including two enantiomeric forms with potentially different pharmacological behavior. This fact makes analytical method development regarding their chiral separation indispensable. In this study, a chiral capillary zone electrophoresis method for the enantioseparation of 61 cathinone and pyrovalerone derivatives was developed by means of four different β‐cyclodextrin derivatives. As chiral selectors, native β‐cyclodextrin as well as three of its derivatives namely acetyl‐β‐cyclodextrin, 2‐hydroxypropyl‐β‐cyclodextrin, and carboxymethyl‐β‐cyclodextrin were used. The cathinone and pyrovalerone derivatives were either purchased in internet stores or seized by police. As a result, overall 58 of 61 studied substances were partially or baseline separated by at least one of the four chiral selectors using 10 mM of β‐cyclodextrin derivative in a 10 mM sodium phosphate buffer (pH 2.5). Furthermore, the method was found to be suitable for simultaneous enantioseparations, for enantiomeric purity checks and to differentiate between positional isomers. Moreover, an intra‐ and an interday validation was performed successfully for each chiral selector to prove the robustness of the method.     

柱前衍生化高效液相色谱手性固定相法拆分手性氰醇

利用正相高效液相色谱法在多糖衍生物手性固定相(OJ-H、OD-H、AD-H或AS-H)上成功地分离了一系列(31个)氰醇对映体的乙酰化或丙酰化衍生物,并拆分了3个脂肪族氰醇对映体的乙酰化衍生物。探讨了这些外消旋体在这四支手性柱上的色谱行为,通过考察流动相组成、流速、进样浓度和温度等因素对对映体拆分效果的影响,优化了色谱分离条件。方法已应用于脂肪酶催化转酯化拆分反应中手性氰醇衍生物的光学纯度的鉴定。     

Synthesis and application of a chiral ionic liquid functionalized β‐cyclodextrin as a chiral selector in capillary electrophoresis

A novel chiral ionic liquid functionalized β‐cyclodextrin, 6‐O‐2‐hydroxpropyltrimethylammonium‐β‐cyclodextrin tetrafluoroborate ([HPTMA‐β‐CD][BF₄]), was synthesized and used as a chiral selector in capillary electrophoresis. [HPTMA‐β‐CD][BF₄] not only increased the solubility in aqueous buffer in comparison with the parent compound, but also provided a stable reversal electroosmotic flow, and the enantioseparation of eight chiral drugs was examined in phosphate buffer containing [HPTMA‐β‐CD][BF₄] as the chiral selector. The effects of the [HPTMA‐β‐CD][BF₄] concentration and the background electrolyte pH were studied. Moreover, the chiral separation abilities of β‐CD and [HPTMA‐β‐CD][BF₄] were compared and possible mechanisms for the chiral recognition of [HPTMA‐β‐CD][BF₄] are discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of analyte lipophilicity on the resolution of α‐ and β‐amino acids on liquid chromatographic ligand exchange chiral stationary phases

Separation of the two enantiomers of racemic α‐ and β‐amino acids on two ligand exchange chiral stationary phases (CSPs) prepared previously by covalently bonding sodium N‐((S)‐1‐hydroxymethy‐3‐methylbutyl)‐N‐undecylaminoacetate or sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate on silica gel was studied with variation of the organic modifier (methanol) concentration in the aqueous mobile phase. In particular, the variation of retention factors with changing organic modifier concentration in the aqueous mobile phase was found to be strongly dependent on both the analyte lipophilicity and the stationary phase lipophilicity. In general, the retention factors of relatively lipophilic analytes on relatively lipophilic CSPs tend to increase with increasing organic modifier concentration in the aqueous mobile phases while those of less lipophilic or hydrophilic analytes tend to increase. However, only highly lipophilic analytes show decreasing retention factors with increasing organic modifier concentration in the aqueous mobile phase on less lipophilic CSPs. The contrasting retention behaviors on the two CSPs were rationalized by the balance of the two competing interactions, viz. hydrophilic interaction of analytes with polar aqueous mobile phase and the lipophilic interaction of analytes with the stationary phase.     

Enantioselective isolation of methyl jasmonate using permethyl‐β‐cyclodextrin HPLC

A method based on the use of HPLC for the enantioselective resolution of the four stereoisomers of methyl jasmonate (MJ) with no need for the previous formation of the diastereoisomers is developed. To that end, a Nucleodex‐β‐PM column as well as an optimization process considering different flow rates and mobile phase compositions were required. As a result, 0.8 mL/min and 55:45 methanol/water composition were the conditions selected to carry out the separation of the stereoisomers. Isolation of pure (–)‐ and (+)‐MJ was accomplished by collecting the HPLC fractions corresponding to their elution time. SPE was subsequently used to concentrate and change the solvent of the HPLC fractions collected. Chiral GC and polarimetry were additionally employed to evaluate the purity and optical rotation, respectively, of the enantiomers separated. The results found in this study are particularly relevant considering that MJ stereoisomers are not commercially available.     

Enantioseparation of chiral aromatic acids by multiple dual mode counter‐current chromatography using hydroxypropyl‐β‐cyclodextrin as chiral selector

This work concentrates on extending the utilization of multiple dual mode (MDM) counter‐current chromatography in chiral separations. Two aromatic acids, 2‐(6‐methoxy‐2‐naphthyl)propionic acid (NAP) and 2‐phenylpropionic acid (2‐PPA), were enantioseparated by MDM counter‐current chromatography using hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD) as chiral selector. The two‐phase solvent systems consisting of n‐hexane/ethyl acetate 0.1 mol/L phosphate buffer pH 2.67 containing 0.1 mol/L HP‐β‐CD (7.5:2.5:10 for NAP and 7:3:10 for 2‐PPA, v/v/v) were used. Conventional MDM and modified MDM were compared according to peak resolution under current separation mechanism. The influence of elution time after the first‐phase inversion and number of cycles for MDM were investigated. Peak resolution of NAP and 2‐PPA increased from 0.62 to 1.05 and 0.72 to 0.84, respectively, using optimized MDM conditions. Being an alternative elution method for counter‐current chromatography, MDM elution greatly improved peak resolution in chiral separations.     

HPLC enantioseparation of β2‐homoamino acids using crown ether‐based chiral stationary phase

RP high‐performance liquid chromatographic methods were developed for the enantioseparation of eleven unusual β²‐homoamino acids. The underivatized analytes were separated on a chiral stationary phase containing (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid as chiral selector. The effects of organic (alcoholic) and acidic modifiers, the mobile phase composition and temperature on the separation were investigated. The structures of the substituents in the α‐position of the analytes substantially influenced the retention and resolution. The elution sequence was determined in some cases: the S enantiomers eluted before the R enantiomers.     

Enantioseparation of citalopram analogues with sulfated β‐cyclodextrin by capillary electrophoresis

Capillary electrophoresis methods were developed for the enantiomeric separation of 27 citalopram analogues. Sulfated β‐cyclodextrin was the most broadly selective and useful chiral selector. The separations of most of the citalopram analogue compounds reported in this work have not been reported previously. Excellent enantiomeric separations were obtained for 26 out of 27 compounds, and most of the separations were achieved within 10 min. The effects of chemical parameters such as chiral selector types, buffer types, chiral selector and buffer concentrations, buffer pH and organic modifiers on the separation were investigated. The influence of analyte structure on separation also was examined and discussed.     

Gold nanoparticle decorated graphene oxide/silica composite stationary phase for high‐performance liquid chromatography

In the initial phase of this study, graphene oxide (GO)/silica was fabricated by assembling GO onto the silica particles, and then gold nanoparticles (GNPs) were used to modify the GO/silica to prepare a novel stationary phase for high‐performance liquid chromatography. The new stationary phase could be used in both reversed‐phase chromatography and hydrophilic interaction liquid chromatography modes. Good separations of alkylbenzenes, isomerides, amino acids, nucleosides, and nucleobases were achieved in both modes. Compared with the GO/silica phase and GNPs/silica phase, it is found that except for hydrophilicity, large π‐electron systems, hydrophobicity, and coordination functions, this new stationary phase also exhibited special separation performance due to the combination of 2D GO with zero‐dimensional GNPs.     

Space filling of permethylated β‐cyclodextrin by volatile hydrophobic and hydrophilic guests in polyethylene glycol

The inclusion of volatile organic compounds of various classes in the permethylated β‐cyclodextrin in an oligomeric solution of polyethylene glycol was investigated by inverse gas–liquid chromatography methods and by a stereochemical approach. It was established that the “guest–host” complexation process is influenced by the geometrical structure of the guest molecules, their chirality, and entropy factors.     

The degree of substitution affects the enantioselectivity of sulfobutylether‐β‐cyclodextrin chiral stationary phases

Three chiral stationary phases were prepared by dynamic coating of sulfobutylether‐β‐cyclodextrin (SBE‐β‐CD) with different degrees of substitution, onto strong anion‐exchange stationary phases. The enantioselective potential and stability of newly prepared chiral stationary phases were examined using a set of structurally different chiral analytes. Measurements were performed in RP‐HPLC. Mobile phases consisted of methanol/formic acid, pH 2.10, and methanol/10 mM ammonium acetate buffer, pH 4.00, in various volume ratios. SBE‐β‐CDs with degrees of substitution (DS) 4, 6.3, and 10 proved suitable for the enantioseparation of 14, 11, and 8 analytes, respectively. The SBE‐β‐CD DS 4 based chiral stationary phase enabled the enantioseparation of the nearly all basic and neutral compounds. Chiral stationary phases with higher sulfobutylether‐β‐cyclodextrin substitution (especially DS 10) yielded higher enantioresolution values for acidic compounds.     

高效液相色谱手性固定相法直接拆分外消旋雷诺嗪

在反相以及正相争件下，利用自制的涂敷型纤维素-三（3，5-二甲基苯基氨基甲酸酯）手性固定相直接拆分了外消旋雷诺嗪，并考察了不同流动相对手性拆分的影响，特别是醇类物质对拆分影响。结果表明，醇的立体结构、极性对雷诺嗪的手性拆分均有影响。实验结果显示无论在正相条件下还是在反相条件下，涂敷型纤维素-三（3，5-二甲基苯基氨基甲酸酯）手性固定相均可以很好的拆分外消旋体雷诺嗪。