Mono‐6A‐(4‐methoxybutylamino)‐6A‐β‐cyclodextrin as a chiral selector for enantiomeric separation

A new member of the family of methoxylalkylamino monosubstituted β‐cyclodextrins, mono‐6^A‐(4‐methoxybutylamino)‐6^A‐β‐cyclodextrin, has been developed as a chiral selector for enantioseparation in capillary electrophoresis. This amino cyclodextrin exhibited good enantioselectivities for 16 model acidic racemates including three dansyl amino acids at an optimum pH of 6.0. Excellent chiral resolutions over six were obtained for α‐hydroxy acids and 2‐phenoxypropionic acids with 3.0 mM chiral selector. The good chiral recognition for α‐hydroxyl acids was attributed to inclusion complexation, electrostatic interactions, and hydrogen bonding. The hydrogen‐bonding‐enhanced chiral recognition was revealed by NMR spectroscopy. The chiral separation of acidic racemates was further improved with the addition of methanol (≤10 vol%) as an organic additive.     

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography (CEC) is achieved with open-tubular capillaries (o-CEC), with packed capillaries (p-CEC) or with monolithic capillaries. In o-CEC, capillaries are coated with a thin film containing cyclodextrin derivatives, cellulose, proteins, poly-terguride or molecularly imprinted polymers as chiral selectors. In p-CEC, typical chiral HPLC stationary phases such as silica-bonded cyclodextrin or cellulose derivatives, proteins, glycoproteins, macrocyclic antibiotics, quinine-derived and 'Pirkle' selectors, polyacrylamides and molecularly imprinted polymers are used as chiral selectors. Chiral monolithic stationary phases prepared by in situ polymerization into the capillary were also developed for electrochromatographic enantiomer separation.     

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.     

Molecular Modeling of Enantioseparation of Phenylazetidin Derivatives by Cyclodextrins

β-Lactams are one of the most widely used types of antibiotics. As β-lactams are chiral, the enantiomeric separation of these compounds was investigated using cyclodextrins, frequently used as chiral separators. Molecular modeling methods were utilized in order to predict possible enantioseparation of four model compounds. Our results revealed that permethylated β-cyclodextrin is more likely to chirally separate the phenylazetidin derivates than the parent β-cyclodextrin. LC experiments using cyclodextrin as chiral stationary phase in most cases confirmed our prediction; however, more experiments and statistical evaluation of the results are needed in order to judge the prediction power of the molecular dynamic method.     

Use of various β‐cyclodextrin derivatives as chiral selectors for the enantiomeric separation of ofloxacin and its five related substances by capillary electrophoresis

A capillary electrophoretic method for the enantioseparation of ofloxacin and its five related substances (potential impurities, indicated as impurities B–F) was developed using β‐cyclodextrin derivatives as chiral selectors. To our knowledge, there are no previous studies about using capillary electrophoresis for the separation of impurities B–D. Six β‐cyclodextrin derivatives including cationic (piperidine‐ and cyclohexylamine‐), neutral (dimethyl‐ and hydroxypropyl‐), and anionic (carboxymethyl‐ and sulfated‐) β‐cyclodextrin derivatives were tested and operational parameters such as buffer pH and concentration of β‐cyclodextrin derivatives were investigated. The best resolutions were all obtained with anionic β‐cyclodextrin derivatives: ofloxacin, impurities C–F could be best resolved with carboxymethyl‐β‐cyclodextrin at satisfactory resolutions of 8.27, 9.98, 5.92, 8.49 and 6.78, respectively, while for impurity B, a particularly impressive resolution value, up to 21.38, was observed using sulfated‐β‐cyclodextrin. The enhancement of enantioseparation observed for the tested analytes using anionic β‐cyclodextrin derivatives might be due to some favorable interaction between selectors and analytes. Given the fact that the selection of chiral selector depends on the structures of analytes, with the help of structural similarities and differences of the analytes, the structure–separation relationship was further discussed.     

卡替诺尔和氟西汀对映体的高效毛细管电泳分离

考察了以羧甲基－β－（环糊精－β－CD）、β－环糊清（β－CD）、羟丙基－β－环糊精（HP－β－CD）、二甲基－β－环糊精（DM－β－CD)为手性选择剂,在50mmol/L醋酸三乙胺缓冲溶液中分离卡替诺尔和氟西汀对映体。该文还通过考察手性选择剂的浓度、背景电解质的酸度、背景电解质的类型等因素对映体手性分离的影响,对分离条件进行了优化,初步探讨了手性识别机理。实验结果表明：用约4mmol/L的CM－β－CD分离氟西汀和卡替诺尔对映体,能使对映体达到良好分离,不仅节约了分析成本,也简化了分析过程。     

Effect of fabrication strategy on the enantioseparation performance of β‐cyclodextrin‐functionalized polymethacrylate monoliths: A comparative evaluation

To evaluate the effect of the preparation strategy on the enantioseparation performance of β‐cyclodextrin‐functionalized monoliths, a series of β‐cyclodextrin‐functionalized organic polymeric monolithic columns were prepared through two‐step, single‐step, and one‐pot approaches, using the same cyclodextrin, linker–spacer, and crosslinker. Physicochemical characterization of the columns was carried out by determining the morphology, β‐cyclodextrin density, permeability, and chromatographic efficiency. For each type of monolithic column, the enantioresolution of 22 chiral compounds, including mandelic acid derivatives, nonsteroidal anti‐inflammatory drugs, N‐derivatized amino acids, and herbicides, was comparatively studied under optimum chromatographic conditions. The β‐cyclodextrin‐functionalized monolithic columns prepared through the one‐pot approach exhibited higher enantioresolution for most chiral compounds, and they have the advantage of good controllability and simple preparation. On the other hand, the enantioresolution obtained on columns prepared through the single‐step approach was quite unsatisfactory, and therefore the effect of using different linking spacers and crosslinkers was studied. A significant improvement of enantioresolution for 2‐chloro‐mandelic acid was obtained by using N ,N‐methylenebisacrylamide instead of ethylene dimethacrylate as the crosslinker in the single‐step preparation.     

Synthesis of the chiral selector heptakis(6‐O‐methyl)‐β‐cyclodextrin by phase‐transfer catalysis and hydrazine‐mediated transfer‐hydrogenation

The exhaustive primary‐side alkylation of cyclodextrins has never been achieved directly. The undesired and simultaneous derivatization of the secondary hydroxyl moieties generates intricate isomeric mixtures that are challenging to purify, analyse and characterize. The aim of this study was to develop a chromatography‐free and up‐scalable strategy towards the preparation of per‐6‐O‐methylated cyclodextrin and to test the compound as potential chiral selector. The target molecule was prepared according to a five‐step synthesis by using methyltriphenylphosphonium bromide as catalyst under heterogeneous conditions. The removal of benzyl moieties, used as temporary secondary‐side protecting groups, was attained by applying hydrazine‐carbonate in the presence of Pd/C. All the intermediates were obtained in high yields, thoroughly characterized and their purity was assessed by ad‐hoc developed HPLC methods. The per‐6‐O‐methylated β‐cyclodextrin showed promising chiral recognition ability as background electrolyte additive in cyclodextrin‐modified capillary electrophoresis using the recreational drug methylene‐dioxypyrovalerone as model compound. Additionally, a model for the inclusion geometry between the single isomer host and the selected drug was developed based on the extensive 2D NMR analysis. The versatility of the proposed synthetic strategy opens the way to the industrial production of homogeneously primary‐alkylated cyclodextrins and to their wide application in chiral separation of various drugs.     

Comparative analysis of chiral drugs in view of chemometrics

With the development of methods for obtaining chiral compounds as potential drugs, there is also need to develop analytical methods for the separation of both enantiomers. Keeping in mind that the physical and chemical properties of both enantiomers are identical, their different nature will only be revealed in a chiral environment that is appropriately designed. Physicochemical systems can be used to predict the differences in biological activity of both enantiomers. The complexity of the problem requires the use of additional tools, which are various chemometric methods. This paper reviews the application of chemometry in the analysis of chiral drugs and discusses the effects of a combination of chromatographic, electrophoretic, and spectroscopic analysis (UV-Vis absorption spectroscopy, and near-IR spectroscopy aided by cyclodextrin inclusion complexes) with chemometrics for improving the methods of enantioseparation (experimental design), explaining the mechanisms of behavior and chiral recognition (quantitative structure-enantioselective retention relationships) and indicating chiral purity (enantiomeric excess).     

Continuous free flow electrophoresis for preparative chiral separations of piperoxan using sulfated beta-cyclodextrin

Continuous free flow electrophoresis was investigated as a tool for the preparative chiral separation of piperoxan using a sulfated cyclodextrin chiral additive. In the absence of chiral additive, the sample stream was deflected cathodically. However, the presence of sulfated cyclodextrin in the run buffer caused anodic deflection and splitting of the sample stream into two streams, each enriched in one enantiomer. Although the sulfated cyclodextrin used was comprised of a mixture of homologues and isomers, this polydispersity did not seem to significantly impact band dispersion. Sample introduction rates ranged from approximately 0.9-7.2 mg h-1. Maximum resolution was 0.53, using an applied voltage of 220 V, buffer composition of 0.075% sulfated cyclodextrin, 7.6 mM citrate (pH 3), 4.5 degrees C.     

Liquid chromatography with mass spectrometry enantioseparation of pomalidomide on cyclodextrin‐bonded chiral stationary phases and the elucidation of the chiral recognition mechanisms by NMR spectroscopy and molecular modeling

A sensitive and validated liquid chromatography with mass spectrometry method was developed for the enantioseparation of the racemic mixture of pomalidomide, a novel, second‐generation immunomodulatory drug, using β‐cyclodextrin‐bonded stationary phases. Four cyclodextrin columns (β‐, hydroxypropyl‐β‐, carboxymethyl‐β‐, and sulfobutyl‐β‐cyclodextrin) were screened and the effects of eluent composition, flow rate, temperature, and organic modifier on enantioseparation were studied. Optimized parameters, offering baseline separation (resolution = 2.70 ± 0.02) were the following: β‐cyclodextrin stationary phase, thermostatted at 15°C, and mobile phase consisting of methanol/0.1% acetic acid 10:90 v/v, delivered with 0.8 mL/min flow rate. For the optimized parameter at multiple reaction monitoring mode 274.1–201.0 transition with 20 eV collision energy and 100 V fragmentor voltage the limit of detection and limit of quantitation were 0.75 and 2.00 ng/mL, respectively. Since enantiopure standards were not available, elution order was determined upon comparison of the circular dichroism signals of the separated pomalidomide enantiomers with that of enantiopure thalidomide. The mechanisms underlying the chiral discrimination between the enantiomers were also investigated. Pomalidomide‐β‐cyclodextrin inclusion complex was characterized using nuclear magnetic resonance spectroscopy and molecular modeling. The thermodynamic aspects of chiral separation were also studied.     

Chiral separation of highly negatively charged enantiomers by capillary electrophoresis

The separation of two highly negatively charged enantiomeric organic disulfates containing two chiral centers was investigated by capillary electrophoresis using cyclodextrin based chiral selectors added to the run buffer. The optimum separation for the enantiomers was achieved in less than 3 min at 25 degrees C with a run buffer of 10 mM glycine pH 2.4 and 5 mM QA-beta-CD, which is a positively charged quaternary ammonium beta-cyclodextrin derivative. The method resulted in baseline resolution, excellent linearity, and highly reproducible migration times allowing facile evaluation of the enantiomeric purity of the individual isomers. Detection limits for the enantiomeric pair were determined to be 0.3 ng/microl (S/N = 3). The nature of the selector-enantiomer interaction and a quantitative measurement of the apparent stability constants that governed chiral discrimination of the enantiomers with QA-beta-CD were also investigated by UV-Vis spectroscopy and electrospray ionization mass spectrometry.     

环糊精对手性药物伪麻黄碱分子识别的电喷雾飞行时间质谱研究

用电喷雾正交飞行时间质谱仪分别研究了以α－环糊精、β－环糊精和γ－环 糊精作为手性拆分剂对手性药物伪麻黄碱的分子识别效应,同时还分别研究了 Nozzle电的变化对α－环糊精和γ－环糊精的手性识别的影响。在质谱图中能明显 反映出三种手性拆分剂都具备很强的手性识别能力,随Nozzle电压的改变,三种手 性拆分剂双分别具有各自的手性识别特征。     

Fast separation of warfarin and 7‐hydroxywarfarin enantiomers by cyclodextrin‐assisted capillary electrophoresis

The enantioseparation of warfarin and its main metabolite has been achieved using several cyclodextrin types and buffers at different pH, including conditions that have not been attempted so far. Methyl‐β‐cyclodextrin, highly sulfated‐β‐cyclodextrin and highly sulfated‐γ‐cyclodextrin were the most efficient chiral selectors. The pH range, within which particular cyclodextrins support chiral separation, has been approximately determined for the first time. By shortening the effective capillary length to 10 cm, the time of analysis has been vastly reduced <2 min. Hence, baseline separations of warfarin and 7‐hydroxywarfarin enantiomers have been achieved in times unreported for those species until now. The established conditions are promising for the further development of new highly selective and fast methods involving warfarin, its derivatives, as well as the same cyclodextrin types.     

环糊精衍生物气相色谱手性固定相研究进展

评述近年环糊精衍生物气相色谱手性固定相的研究进展,对环糊精衍生物进行分类并总结了近年来其在GC手性分离上的应用,介绍环糊精衍生物的手性分离机制及纯度问题进展,展望环糊精衍生物作GC手性固定相的应用前景。     

Rapid development of the enantiomeric separation of beta-blockers by capillary electrophoresis using an experimental design approach.

A rapid method for determining the separation conditions for chiral resolution of eleven beta-blocking drug substances by capillary electrophoresis is described, using an experimental design approach. An acidic phosphate-triethanolamine buffer and an uncoated fused-silica capillary were used for all experiments. Several modified cyclodextrins were applied as chiral selectors: sulfobutyl ether beta-cyclodextrin (SBE-beta CD), dimethyl beta-cyclodextrin (DM-beta CD), carboxymethyl beta-cyclodextrin (CM-beta CD), and hydroxypropyl beta-cyclodextrin (HP-beta CD). Two different fractional factorial experimental designs were applied: (1) a design examining four factors at three levels (3(4-2)) and (2) one examining three factors at two levels (2(3-1)). The factors studied were: type of cyclodextrin, cyclodextrin concentration, pH of the background electrolyte and percentage of organic modifier. Enough resolution for the separation of the enantiomers and even for their quantification was reached. The same scheme is proposed when a fast chiral separation method needs to be developed for other drug families.     

Stereoisomer separation of flavanones and flavanone‐7‐O‐glycosides by means of nanoliquid chromatography employing derivatized β‐cyclodextrins as mobile‐phase additive

A nanoliquid chromatographic method for the stereoisomer separation of some flavanone aglycones and 7‐O‐glycosides has been proposed employing a C18 capillary column and a chiral mobile‐phase additive such as cyclodextrin. The chiral separation of eriodictyol, naringenin, and hesperitin was obtained by addition of carboxymethyl‐β‐cyclodextrin to the mobile phase, whereas eriocitrin, naringin, narirutin, and hesperidin diastereoisomers were resolved by using sulfobutyl ether‐β‐cyclodextrin. The influence of the composition of the mobile phase, the length of the capillary column, and the flow rate on the chiral recognition were investigated. At optimum conditions, baseline separation for the selected aglycones and glycosylated forms were achieved with a mobile phase consisting of 50 mM sodium acetate buffer pH 3 and 30% methanol containing 20 mM of carboxymethyl‐β‐cyclodextrin and 10 mM of sulfobutyl ether‐β‐cyclodextrin, respectively. Precision, linearity, and sensitivity of the method were tested. Limits of detection and quantification for the studied flavanone glycosides were in the range 1.3‐2.5 and 7.5‐12.5 µg/mL, respectively. The method was used for the determination of the diastereomeric composition of the flavanone‐7‐O‐glycosides in Citrus juices after solid‐phase extraction procedure.     

Capillary electrophoretic enantioseparation of basic drugs using a new single‐isomer cyclodextrin derivative and theoretical study of the chiral recognition mechanism

A novel single‐isomer cyclodextrin derivative, heptakis {2,6‐di‐O‐[3‐(1,3‐dicarboxyl propylamino)‐2‐hydroxypropyl]}‐β‐cyclodextrin (glutamic acid‐β‐cyclodextrin) was synthesized and used as a chiral selector in capillary electrophoresis for the enantioseparation of 12 basic drugs, including terbutaline, clorprenaline, tulobuterol, clenbuterol, procaterol, carvedilol, econazole, miconazole, homatropine methyl bromide, brompheniramine, chlorpheniramine and pheniramine. The primary factors affecting separation efficiency, which include the background electrolyte pH, the concentration of glutamic acid‐β‐cyclodextrin and phosphate buffer concentration, were investigated. Satisfactory enantioseparations were obtained using an uncoated fused‐silica capillary of 50 cm (effective length 40 cm) × 50 μm id with 120 mM phosphate buffer (pH 2.5–4.0) containing 0.5–4.5 mM glutamic acid‐β‐cyclodextrin as background electrolyte. A voltage of 20 kV was applied and the capillary temperature was kept at 20°C. The results proved that glutamic acid‐β‐cyclodextrin was an effective chiral selector for studied 12 basic drugs. Moreover, the possible chiral recognition mechanism of brompheniramine, chlorpheniramine and pheniramine on glutamic acid‐β‐cyclodextrin was investigated using the semi‐empirical Parametric Method 3.     

A high-throughput screening protocol for fast evaluation of enantioselective catalysts

A new high-throughput screening protocol that allows fast evaluation of enantioselective catalysts has been developed. The usefulness of norephedrine-derived beta-amino alcohols as catalysts for the enantioselective alkylation of prochiral aldehydes has been determined by simultaneous screening of three representative substrates. GC analysis of the crude product mixture using a selectively modified cyclodextrin as the chiral stationary phase avoids time-consuming workup procedures. The chemical yield, enantioselectivity, substrate specificity, and catalytic activity of the chiral catalysts as well as the induced absolute configuration have been determined in a single screening experiment and two short GC runs.     

Optimisation of the chlorthalidone chiral separation by capillary electrochromatography using an achiral stationary phase and cyclodextrin in the mobile phase

The separation of chlorthalidone enantiomers in capillary electrochromatography on an achiral stationary phase when adding a chiral selector, hydroxypropyl-β-cyclodextrin, to the mobile phase, was optimised. The goal was to investigate the feasibility of modelling retention times and resolution when during the optimisation procedure regular replacement of columns is required due to their fragility. Therefore, it is essential that the packing procedure delivers reproducible columns. The optimisation of an existing chlorthalidone separation was chosen as case study. The influence of two factors, chiral selector concentration and organic modifier content, on the responses was modelled. The experiments performed prior to modelling were defined by a central composite design. Results on different columns, obtained under identical experimental conditions, were found comparable and thus modelling was possible in situations where several columns were required to complete a design. A second-order polynomial model was built for both responses. Optimal separations were also predicted using Derringer’s desirability functions. The optimum was found at 33 mM cyclodextrin and 16% (v/v) acetonitrile on two types of columns (with different packing times) leading to a strong reduction in analysis time for an equally good separation compared to the initial conditions. Measured and predicted responses were found comparable, indicating that acceptable models were obtained.