Enantiomer separations by capillary GC on modified gyclodextrins

Three new chiral selectors, 6-tert-butyldimethylsilyl-2,3-diethyl-a-cyclodextrin, 6-tert-butyldimethylsilyl-2,3-diethyl- and dipropyl-β-cyclodextrin (TBDE-α-CD, TBDE-β-CD, TBDP-β-CD) were synthesized and tested as chiral stationary phases in capillary gas chromatography. TBDE-β-CD in particular showed a high enan-tioselectivity for test chiral compounds due to good solubility in a polar polysiloxane (OV-1701). Enantioselectivity obtained with TBDE-β-CD was compared with that of 6-tert-butyldimethylsilyl-2,3-di-O-methyl-β-cyclodextrin (TBDM-β-CD). Better enantiose-lectivity was obtained with TBDE-P-CD than with TBDM-β-CD for the test chiral compounds studied. This is probably due to greater effect of the increased hydrophobicity of TBDE-β-CD which favors inclusion of the analytes than the effect of increased steric hindrance. With TBDP-β-CD the less polar lactones are well separated due most likely to increased hydrophobicity of the propyl groups while the more polar are not well resolved. For TBDP-β-CD it is likely that the unfavorable steric hindrance is predominant over the favorable hydrophobicity of the propyl groups, thus hindering the formation of inclusion complexes of the alcohols with TBDP-β-CD. TBDE-α-CD was also a valuable chiral selector for the separation of small chiral molecules such as simple secondary alcohols and nitro-substituted alcohols.     

Enantiomeric Separation of β-Amino Alcohols by Capillary Electrophoresis Using DM-β-CD as Chiral Selector

Enantiomeric separations of several β-amino alcohol drugs, i.e., phenylephrine, epinephrine, norepinephrine, synephrine, and chlorprenaline were performed by capillary electrophoresis using DM-β-CD as a chiral selector. Five test solutes were baseline resolved in six minutes. The effects of DM-β-CD concentration, pH value, ionic strength of the buffer, and the type of β-CDs on resolution were investigated. The results indicated that DM-β-CD is suitable for enantiomeric separation of β-amino alcohols containing a phenyl group on the chiral atom. Enantiorecognition mechanisms for test solutes are also discussed.     

Comparison of different heptakis(6-O-alkyldimethylsilyl-2-3-di-O-ethyl)-β-cyclodextrins as chiral stationary phases in capillary GC

Three new β-cyclodextrin derivatives, heptakis(6-O-isopropyldi-methylsilyl-2,3-di-O-ethyl)-β-cyclodextrin, heptakis(6-O-thexyldi-methylsilyl-2,3-di-O-ethyl)-β-cyclodextrin, and heptakis(6-O-cy-clohexyldimethyl-2,3-di-O-ethyl)-β-cyclodextrin (IPDE-β-CD, TXDE-β-CD, and CHDE-β-CD), were synthesized and the enan-tioselectivities of these three CD derivatives and heptakis(6-O-tert-butyldimethylsilyl-2,3-di-O-ethyl)-β-cyclodextrin (TBDE-β-CD) were compared for GC separation of a range of chiral test com-pounds. In particular TXDE-β-CD showed much higher enentio-selectivity than TBDE-β-CD. Enentioselectivities of IPDE-β-CD and CHDE-β-CD are somewhat lower than that of TXDE-β-CD and CHDE-β-Cd are somewhat lower than that of TXDE-β-CD. These observations are indicative of significant effects of subtle changes in the structure of the 6-O-substituent on the enantioselec-tivity of the β-CD derivatives. The difference in enantioselectivities of the 6-O-substituted CD derivatives were explained in terms of relative contributions of the effects of hydrophobicity and steric hindrance of the substituent to the inclusion process. CHDE-β-CD showed the lowest enantioselectivity among the threederivatives. It is likely that the unfavorable steric hindrance of the bulky cyclo-hexyl group plays a greater role than the favorable hydrophobicity effect of the cyclohexyl group in the inclusion process in CHDE-β-CD. IPDE-β-CD showed lower selectivity than TXDE-β-CD and TBDE-β-CD. In the case of these CD derivatives having acyclic substituents the relative hydrophobicity of the substituent seems to be a dominant factor affecting the inclusion process. Isopropyl groups factor affecting the inclusion process. Isopropyl groups are less hydrophobic than thexyl and tert-butyl groups.     

Evaluation of a methacrylate-bonded cyclodextrins as a monolithic chiral stationary phase for capillary electrochromatography (CEC)-UV and CEC coupled to mass spectrometry

Glycidyl methacrylate-bonded β-cyclodextrin (GMA-β-CD) is synthesized as a new chiral monomer by direct chemical bonding with GMA using a fast and simple alternative procedure. Next, rigid and homogenous monolithic columns were prepared by polymerization of GMA-β-CD monomer with ethylene dimethacrylate (EDMA), in the presence of commonly used porogens and a charged achiral monomer to form a versatile chiral monolith. This is the first report in which a preparation procedure for a methacrylate-bonded CD is introduced for chiral separations in CEC. The degree of substitution of GMA-β-CD monomer and mobile-phase parameters were optimized to achieve the highest enantioselectivity and plate number. To evaluate the GMA-β-CD monolithic column, different classes of chiral compounds were screened. Under the optimized β-CD monolith phase and the optimum mobile-phase conditions, 30 neutral and basic chiral compounds and two acidic compounds could be separated. The high chemical and mechanical stability, homogenous microflow and no loss of material at the interface allows for the first time the feasibility of applying this polymer-based monolithic column for CEC coupled to ESI-MS. Compared with CEC-UV, CEC-ESI-MS showed higher sensitivity and lower resolution. However, resolution greater than 1.0 can still be obtained for majority of the select tested compound in CEC-ESI-MS with at least three out of seven compound providing Rs≥1.5. The results reinforce the potential of GMA-β-CD monolithic columns for chiral separations with high sensitivity in CEC-ESI-MS. Finally, using hexobarbital as the model chiral analyte, the monolithic column demonstrated excellent stability and reproducibility of retention time and enantioselectivity.     

Optimisation of a Headspace Solid-Phase Micro- Extraction Procedure for the Determination of 2,4,6-Trichloroanisole and Various Related Compounds in Cork Washing Waste Water by Use of Gas Chromatography-Mass Spectrometry

This paper reports the use of an anionic cyclodextrin, heptakis(2,3-di-O-methyl-6-O-sulfato)-β-cyclodextrin (HDMS-β-CD), for chiral separations of pharmaceutical enantiomers by nonaqueous capillary electrophoresis (NACE). Enantiomer resolution was affected mainly by HDMS-β-CD concentration and the acidity of the background electrolyte (BGE). The effects of capillary length and applied voltage on enantiomer resolution were also investigated. Results showed that in a methanol solution of 20 mM phosphoric acid, 10 mM sodium hydroxide, and 10 mM HDMS-β-CD, seven anticholinergic drugs were separated to baseline but no chiral separation was obtained for three other similar drugs. NACE is suitable for routine, rapid separation of the enantiomers of pharmaceutical compounds.     

3-位烯丙基及甲基衍生的β-环糊精固定相的合成及气相色谱性能研究

制备了一种新的β-环糊精衍生物固定相2,6-二-O-戊基-3-O-[(甲基)5(烯丙基)2]-β-CD,并对其气相色谱分离性能进行研究。实验显示,该固定相具有良好的柱表面性能和较强的色谱分离能力,对一些芳香族位置异构体的分离能力优于2,6-二-O-戊基-3-O-烯丙基-β-CD衍生物,对10余种手性物质显示出较好的选择性能,且对α-位取代的丙酸酯类手性分离效果明显优于杂环类β-CD衍生物。与2,6-二-O-戊基-3-O-烯丙基-β-CD固定相的分离性能比较表明,β-CD 3位羟基部分甲基基团的引入能增强对芳香族位置异构体的选择能力,但不能明显改变烯丙基衍生物的手性分离能力。     

Novel chiral stationary phases based on peptoid combining a quinine/quinidine moiety through a C9‐position carbamate group

By connecting a quinine or quinidine moiety to the peptoid chain through the C9‐position carbamate group, we synthesized two new chiral selectors. After immobilizing them onto 3‐mercaptopropyl‐modified silica gel, two novel chiral stationary phases were prepared. With neutral, acid, and basic chiral compounds as analytes, we evaluated these two stationary phases and compared their chromatographic performance with chiral columns based on quinine tert‐butyl carbamate and the previous peptoid. From the resolution of neutral and basic analytes under normal‐phase mode, it was found that the new stationary phases exhibited much better enantioselectivity than the quinine tert‐butyl carbamate column; the peptoid moiety played an important role in enantiorecognition, which controlled the elution orders of enantiomers; the assisting role of the cinchona alkaloid moieties was observed in some separations. Under acid polar organic phase mode, it was proved that cinchona alkaloid moieties introduced excellent enantiorecognitions for chiral acid compounds; in some separations, the peptoid moiety affected enantioseparations as well. Overall, chiral moieties with specific enantioselectivity were demonstrated to improve the performance of peptoid chiral stationary phase efficiently.     

基于非手性离子液体的毛细管电泳法拆分3种手性药物

建立了以非手性离子液体1-正丁基-3-甲基咪唑氯(［BMIM］Cl)为手性分离的添加剂、β-环糊精作为手性选择剂的毛细管区带电泳(CZE)分离扑尔敏、氯霉素前体和氧氟沙星3种对映体的方法,并与未添加［BMIM］Cl的CZE分离情况进行了对比研究。发现［BMIM］Cl对手性药物的拆分有协同作用,不仅能够增加对映体的分离度,还能有效地抑制毛细管内壁对样品分子的吸附作用,改善峰形。采用离子液体辅助手性选择剂(尤其是环糊精)的CZE改进方法,为其他毛细管电泳难以分离的手性药物的分离分析提供了新的方法。     

Simultaneous Chiral Separation of Four H1-Antihistamines by Capillary Zone Electrophoresis Using a Dual Cyclodextrin System

Capillary zone electrophoresis employing a dual cyclodextrin (CD) system, consisting of anionic sulfobutylether-β-CD and native β-CD, was developed for the simultaneous chiral separation of four H1-antihistamine racemates (brompheniramine, chlorpheniramine, cetirizine and promethazine). A cost-effective screening using different native and derivatized, neutral and ionized CDs as chiral selectors was performed to find suitable derivatives for the dual CD system. Under the optimized conditions consisting of 25 mM phosphate background electrolyte at pH 7.0, a combination of 15 mM SBE-β-CD and 10 mM β-CD as chiral selectors, +25 kV applied voltage and 20 °C system temperature, the baseline chiral separation of all racemates was accomplished in less than 8 min. The method proved to be suitable for routine analysis, since it provided satisfactory results during sensitivity, linearity and repeatability studies.

     

Electrophoretic separation strategy for chiral pharmaceuticals using highly-sulfated and neutral cyclodextrins based dual selector systems

The enantiomeric separation of chiral pharmaceuticals was investigated using dual systems with mixtures of cyclodextrin derivatives. The dual cyclodextrin systems, consisting of one highly-sulfated (α-, β-, and γ-HSCD) and one neutral cyclodextrin, i.e. either heptakis (2,3,6-tri-O-methyl)-β-CD (TMCD), heptakis (2,6-di-O-methyl)-β-CD (DMCD) or hydroxypropyl-β-CD (HPCD), are tested on 25 pharmaceutical compounds with different acid-basic properties (16 basics, 8 acids and 1 neutral). The influence on the separation of the type and concentration of neutral CD in highly-sulfated cyclodextrins-based dual selector systems, is investigated. For 11 of 16 basic compounds, a better separation is obtained with the CD mixtures compared to the use of only a highly-sulfated CD. Mixtures with TMCD give better results than those with DMCD and HPCD. Results showed that dual CD systems are useful to achieve and to optimise chiral separations of compounds not (sufficiently) separated with HSCDs alone. For example, ibuprofen was not resolved with α-, β- or γ-HSCD, but could be separated with the mixture 25 mM TMCD and 5% HS-β-CD. Based on the obtained results, a dual CD systems based separation strategy is defined.     

Enantioselective resolution of chiral aromatic acids by biphasic recognition chiral extraction

This paper reports a new chiral separation technology—biphasic recognition chiral extraction for the separation of aromatic acid enantiomers such as α-cyclohexyl-mandelic acid (CHMA) and naproxen (NAP). The biphasic recognition chiral extraction system was established by adding hydrophobic d(l)-isobutyl tartrate in 1,2-dichloroethane organic phase and hydrophilic β-cyclodextrin (β-CD) derivative in aqueous phase, which preferentially recognize the (R)-enantiomer and (S)-enantiomer, respectively. These studies involve an enantioselective extraction in a biphasic system, where aromatic acid enantiomers form complexes with the β-cyclodextrin derivative in the aqueous phase and d(l)-isobutyl tartrate in the organic phase, respectively. Factors affecting the extraction mechanism are analyzed, namely the influence of the concentrations of the extractants and aromatic acid enantiomers, the types of the extractants, pH, and temperature. The experimental results show that the biphasic recognition chiral extraction is of much stronger chiral separation ability than the monophasic recognition chiral extraction, which utilizes the cooperations of the forces of the tartrate and the β-CD derivative. Hydroxypropyl-β-cyclodextrin (HP-β-CD), hydroxyethyl-β-cyclodextrin (HE-β-CD), and methyl-β-cyclodextrin (ME-β-CD) have stronger recognition abilities for the (S)-aromatic acid enantiomers than those for (R)-aromatic acid enantiomers, among which HP-β-CD has the strongest ability. d-Isobutyl tartrate preferentially recognizes (R)-CHMA and (S)-NAP, while l-isobutyl tartrate preferentially recognizes (S)-CHMA and (R)-NAP. The maximum enantioselectivities of CHMA and NAP are 2.49 and 1.65, under conditions at which the pH values of the aqueous phases are 2.7 and 2.5, at the ratio of 2:1 of [isobutyl tartrate] to [HP-β-CD].     

Enantiomeric separation of racemic sulphoxides on chiral stationary phases by gas and liquid chromatography

Summary The enantiomeric resolution of seven racemic sulphoxides on chiral stationary phases has been investigated by gas and liquid chromatography. In gas chromatography the separations were performed on octakis-(2,6-di-O-pentyl-3-O-butyryl)-γ-cyclodextrin (FS Lipodex-E) and heptakis-(2,6-di-O-methyl-3-O-pentyl)-β-cyclodextrin (DMP-β-CD). Both stationary phases were suitable for separation of the enantiomers of the sulphoxides. With one exception for each series all racemetes could be resolved on both stationary phases; FS Lipodex-E was more enantioselective than DMP-β-CD, whereas the latter seemed more generally applicable. Liquid chromatographic separations with Chiralcel-OB as stationary phase were significantly improved by optimization of mobile phase composition and temperature. Resolution factors up to R_s=6 were achieved indicating that the improved separations could now be easily used for preparative purposes.     

On the use of dispersed nanoparticles modified with single layer beta-cyclodextrin as chiral selecor to enhance enantioseparation of clenbuterol with capillary electrophoresis

A new strategy for chiral separation by capillary electrophoresis employing modified-nanoparticles as chiral selector is described for clenbuterol analysis. Nanoparticles modified with β-cyclodextrin (β-CD) form a large surface area platform to serve as a pseudostationary chiral phase, which can be applied for the enhancement of the enantioseparation. The application of four kinds of nanoparticles was investigated (multi-walled nanotubes (MWNTs), polystyrene (PS), TiO₂ and Al₂O₃) modified with single layer β-CD as chiral selector in the enantioseparation of clenbuterol by capillary electrophoresis (CE). Successful clenbuterol enantioseparation could be achieved with the β-CD-modified MWNTs as chiral selector. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) confirmed the β-CD modification of the nanoparticles. The effects of nanoparticles, surfactant, chiral selector (β-CD) and run buffer were studied in relation to the enantiomeric separation of clenbuterol. This study opens attractive perspectives for the use of modified nanoparticles for chiral separational purposes in CE.     

Enantioseparation of solriamfetol and its major impurity phenylalaninol by capillary electrophoresis using sulfated gamma cyclodextrin

R-solriamfetol is a recently approved drug used for the treatment of excessive sleepiness associated with narcolepsy and sleep apnea. Herein, a capillary electrophoretic method was developed, enabling the simultaneous analysis of the API and its S-enantiomer in addition to the enantiomers of its major impurity phenylalaninol. Twenty-nine different cyclodextrins (CDs), including native, neutral, and charged ones were screened as potential chiral selectors, and the best results were obtained with sulfated CDs. Randomly sulfated-β-CD exhibited outstanding enantioresolution, the peaks of phenylalaninol enantiomers inserted between the two peaks of solriamfetol enantiomers, while sulfated-γ-CD (S-γ-CD) showed remarkable resolution values in a much shorter analysis time with the optimal enantiomer migration order. Among the single isomer sulfated CD derivatives, substituent dependent enantiomer migration order reversal could also be observed in the case of heptakis(6-O-sulfo)-β-CD (HS-β-CD) or heptakis(2,3-O-dimethyl-6-O-sulfo)-β-CD (HDMS-β-CD) with R-,S-solriamfetol, and heptakis(2,3-O-diacetyl-6-O-sulfo)-β-CD (HDAS-β-CD) resulting S-,R-solriamfetol migration order. The sulfated-γ-CD system was chosen for method optimization applying orthogonal experimental design. The optimized method (45 mM Tris-acetate buffer, pH 4.5, 4 mM S-γ-CD, 21°C, +19.5 kV) was capable for the baseline separation of solriamfetol and phenylalaninol enantiomers within 7 min. The optimized method was validated according to the ICH guidelines and successfully applied for the analysis of pharmaceutical preparation (Sunosi^® 75 mg tablet), thus it may serve as a routine procedure for the laboratories of regulatory authorities as well as in Pharmacopoeias.     

6-tert-Butyldimethylsilyl-2,3-dimethyl-α-, β-, and γ-cyclodextrins,dissolved in polysiloxanes,as chiral selectors for gas chromatography. Influence of selector concentration and polysiloxane matrix polarity on enantioselectivity

The enantioselectivity of three chiral selectors, 6-t-butyldimethyl-silyl-2,3-dimethyl-α-, β- and γ-cyclodextrin (TB-α-CD, TB-β-CD, TB-γ-CD), are compared and discussed for a range of chiral test compounds. TB-β-CD in particular offers high enantioselectivity for a variety of chiral compounds and has the special property of excellent solubility in different alkylpolysiloxanes, including the weakly polar variety, because of its weak self-association. To investigate the influence of the polarity of polysiloxane matrices this selector can be used at a wide range of concentrations in the most suitable polysiloxane matrices and at low separation temperatures without impairment of resolution by peak broadening and symmetry distortion.     

Separation of terbutaline enantiomers in capillary electrophoresis with neutral cyclodextrin-type chiral selectors and investigation of the structure of selector-selectand complexes using nuclear magnetic resonance spectroscopy

The major goal of this study was to determine the affinity pattern of the terbutaline (TB) enantiomers toward α-, β-, γ-, and heptakis(2,3-di-O-acetyl)-β-cyclodextrins and using NMR spectroscopy for the understanding of the fine mechanisms of interaction between the cyclodextrins (CD) and TB enantiomers. It was shown once again that CE in combination with NMR spectroscopy represents a sensitive tool to study the affinity patterns and structure of CD complexes with chiral guests. Opposite affinity patterns of TB enantiomers toward native α- and β-CDs were associated with significant differences between the structure of the related complexes in solution. In particular, the complex between TB enantiomers and α-CD was of the external type, whereas an inclusion complex was formed between TB enantiomers and β-CD. One of the possible structures of the complex between TB and heptakis(2,3-di-O-acetyl)-β-CD (HDA-β-CD) was quite similar to that of TB and β-CD, although the chiral recognition pattern and enantioselectivity of TB complexation with these two CDs were very different.     

Cyclodextrin derivatives for GC separation of racemic mixtures of volatile compounds. Part VIII: 2,6-di-O-methyl-3-O-pentyl-γ-cyclodextrin and 2,6-di-O-methyl-3-O-(4-oxo-pentyl)- and 2,6-di-O-pentyl-3-O-(4-oxo-pentyl)-β-and -γ-cyclodextrins

In pre vious papers, 2,6-di-O-methyl-3-O-pentyl-β-cyclodextrin (CD) was demonstrated to be successful in separating volatile compounds, while avoiding the drawbacks of 2,3,6-tri-O-methyl-O-methyl-β-CD in terms of column stability and operating temperature. Since a CD chiral selector of universal use has not yet been found, and at least two (or more) columns coated with different CD derivatives are therefore necessary for routine work, the performance of 2,6-di-O-methyl-3-O-pentyl-γ-CD, 2,6-di-O-methyl-3-O-(4-oxopentyl)-γ-CD, 2,6-di-O-pentyl-3-O-(4-oxo-pentyl)-β-CD, and 2,6-di-O-pentyl-3-O-(-4-oxo-pentyl)-γ-CD diluted in polysiloxanes for the separation of volatile compounds in aromas and essential oils will be illustrated; each column coated with each of the newly synthesized CD derivatives was evaluated by analyzing more than 150 different recemates with different structures.     

Chiral separation of amino acids by capillary electrophoresis with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate as chiral selector

Summary 3-[(3-cholamidopropyl)-dimethylammoniol-1-propane sulfonate (CHAPS) can be used as an effective chiral selector for the separation of dansyl-amino acids by capillary electrophoresis (CE). While CHAPS can serve as an chiral selector, better enantiomeric separation can be performed by using CHAPS not as the sole chiral selector but as one of a [CHAPS-SDS-cyclodextrin] three-component system. In this CHAPS-SDS-CD system, enantiomeric separations of the amino acids can be readily accomplished by judiciously adjusting the pH of the solution, concentrations of CHAPS and SDS, and the concentration and type of CD. All amino acids can be baseline resolved in less than 15 minutes with resolution as high as 2.01 at pH 6.5 with 50 mM of CHAPS and 75 mM of SDS. The resolution is also dependent on the size of the CD. Substantial increase in the resolution can be readily achieved by replacing β-CD with γ-CD. For example, theR _s for Leu was increased by four-folds (from 1.65 to 6.29) while the elution time still remains as short as 20 min when β-CD was replaced by γ-CD.     

Stereoisomeric flavor compounds,part LVIII: The use of heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin as a chiral stationary phase in flavor analysis

The characteristics of the new chiral stationary phase heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin are outlined and compared with permethyl- and perethyl-β-cyclodextrins.     

Preparation and evaluation of inclusion complexes of diacerein with β-cyclodextrin and hydroxypropyl β-cyclodextrin

The objective of this research was to improve the aqueous solubility, dissolution rate and, consequently, bioavailability of diacerein, along with avoiding its side effect of diarrhea, by complexation with β-cyclodextrin (β-CD) and HP-β-cyclodextrin (HP-β-CD). Phase solubility curve was classified as an A_N type for both the CDs, which indicated formation of complex of diacerein with β-CD and HP-β-CD in 1:1 stoichiometry and demonstrating that both CDs are proportionally less effective at higher concentrations. The complexes were prepared by kneading method and were evaluated to study the effect of complexation on aqueous solubility and rate of dissolution in phosphate buffer (pH 6.8). Based on the dissolution profile HP-β-CD was selected for preparing fast disintegrating tablet of diacerein which was compared with marketed formulation (MF-J). The HP-β-CD complex was probed for Fourier transform infrared spectroscopy, differential scanning calorimetry, and powder X-ray diffraction studies which evidenced stable complex formation and increase in amorphousness of diacerein in complex. In brief, the characterization studies confirmed the inclusion of diacerein within the non-polar cavity of HP-β-CD. HP-β-CD complex showed improved in vitro drug release profile compared to pure drug and similar to that of marketed formulation respectively.