Nonaqueous capillary electrophoretic separation of basic enantiomers using heptakis(2,3-dimethyl-6-sulfato)-beta-cyclodextrin.

The enantiomers of 40 basic analytes, mostly pharmaceuticals, were separated by nonaqueous capillary electrophoresis in acidic methanol background electrolytes using the sodium salt of heptakis(2,3-dimethyl-6-sulfato)-beta-cyclodextrin (HDMS-beta-CD). The effective mobilities, separation selectivities, and peak resolution values were determined as a function of the HDMS-beta-CD concentration in the 0-40 mM range and were found to follow the theoretical predictions of the charged resolving agent migration model (CHARM model). Fast, efficient enantiomer separations were achieved for a large number of both very hydrophobic and hydrophilic weak bases.     

2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl-beta-cyclodextrin, a useful stationary phase for gas chromatographic separation of enantiomers

Heptakis(2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin (2,3-MOM-6-TBDMS-beta-CD), synthesized by using methoxymethylchloride (MOM-Cl) as derivatization reagent, was used for capillary gas chromatographic separation of enantiomers. The new chiral stationary phase proved to be suitable for the enantiodifferentiation of volatiles from various chemical classes. Compared to the corresponding gamma-CD derivative (2,3-MOM-6-TBDMS-gamma-CD), the spectrum of compounds for which enantiomers could be separated was more limited and the enantioseparation achieved was generally less pronounced. Unusually high separation factors were observed for 2-alkyl esters of short chain acids (C2-C6). Phenomena underlying the enantioseparation of 2-pentyl acetate (alpha: 4.31; 35 degrees C) were investigated by determining thermodynamic parameters. Data show that only one enantiomer is retained significantly on the chiral stationary phase whereas the other one behaves like the hydrocarbons used as references.     

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.     

Influence of the nature of the electrolyte on the chiral separation of basic compounds in nonaqueous capillary electrophoresis using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin

The influence on the enantiomeric resolution of the nature of the cationic BGE component (sodium, ammonium or potassium) and that of the anionic component (chloride, formate, methanesulfonate or camphorsulfonate) as well as the concentration of heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin (HDMS-beta-CD), the selected chiral selector, was studied in nonaqueous capillary electrophoresis (NACE). For this purpose, two D-optimal designs with 33 and 26 experimental points were applied. Three beta-blockers (atenolol, celiprolol and propranolol) and three local anesthetics (bupivacaine, mepivacaine and prilocaine) were selected as basic model compounds. Both cationic and anionic BGE components were found to have a deep impact on the enantiomeric resolution of the investigated analytes but it is the cationic component that has shown the strongest influence. Indeed, in some cases, the change of the latter led to a complete loss of enantioresolution. Based on the observed results, two NACE systems were recommended, namely ammonium formate and potassium camphorsulfonate in a methanolic solution containing HDMS-beta-CD and acidified with formic acid, in order to separate efficiently the enantiomers of basic drugs.     

Determination of salbutamol enantiomers in human urine using heptakis(2,3-di-O-acetyl-6-O-sulfo)-beta-cyclodextrin in nonaqueous capillary electrophoresis

Nonaqueous capillary electrophoresis (NACE) was successfully applied to the resolution and the determination of salbutamol enantiomers in urine samples using heptakis(2,3-di-O-acetyl-6-O-sulfo)-beta-cyclodextrin (HDAS-beta-CD). After optimization of the electrophoretic parameters, namely the background electrolyte (BGE) composition and the HDAS-beta-CD concentration, salbutamol enantiomers were completely resolved using a BGE made up of 10 mM ammonium formate and 15 mM HDAS-beta-CD in methanol acidified with 0.75 M formic acid. Isoprenaline was selected as internal standard. Solid-phase extraction (SPE) was used for sample cleanup prior to the CE separation. Different sorbents involving polar, nonpolar interactions or dual retention mechanisms were evaluated and extraction cartridges containing both nonpolar and strong cation-exchange functionalities were finally selected. Salbutamol enantiomers recoveries from urine samples were determined. The method was then successfully validated using a new approach based on accuracy profiles over a concentration range from 375 to 7500 ng/mL for each enantiomer.     

Semi-preparative gas chromatographic separation of all-trans-perhydrotriphenylene enantiomers on a chiral cyclodextrin stationary phase

Enantiomers of all-trans-perhydrotriphenylene (PHTP) were separated by gas chromatography using heptakis(6-O-tert.-butyldimethylsilyl-2,3-di-O-methyl)-beta-cyclodextrin (TBDMS-beta-CD) as the chiral selector. Conditions for semi-preparative separations were established using a 2 m x 2 mm I.D. packed column and subsequently extended to a 1.8 m x 4 mm I.D. column which enabled separations on a mg scale. The column packing was TBDMS-beta-CD dissolved in SE-54 coated on Chromosorb P AW-DMCS 80-100 mesh. Optimization of the chromatographic conditions (oven temperature, carrier gas flow, and column load) with respect to better efficiency and peak retention resulted in a system capable of separating up to 10 mg of the racemate per day. Purities of separated enantiomers were determined by capillary gas chromatography. Yields and purities of the fractions obtained by single- and double-step separations are compared. Highly enriched enantiomers with purities of up to 99.6% (99.2% ee) were obtained by a single separation step.     

Synthesis and chromatographic properties of a novel chiral stationary phase derived from heptakis(6-azido-6-deoxy-2,3-di-O-phenylcarbamoylated)-beta-cyclodextrin immobilized onto amino-functionalized silica gel via multiple urea linkages

A novel chiral stationary phase (PPHCDN7) was prepared by immobilization of heptakis(6-azido-6-deoxy-2,3-di-O-phenylcarbamoylated)-β-cyclodextrin (PPHCD) onto the surface of amino-functionalized silica gel via multiple urea linkages derived from an extended application of the Staudinger reaction. A wide range of structurally divergent racemic drugs and other compounds were successfully separated into their enantiomers under both normal and reversed-phase conditions. β-Adrenergic blockers and racemic tertiary, secondary and primary amines were readily separated using a mixture of methanol and aqueous triethylammonium acetate buffer. The optimal pH value for the separation falls in the range of 4.65 to 6.30. With atropine and isoproterenol, good enantioseparations with separation factors of >5 were easily attainable.     

Comparison of the separation of aziridine isomers applying heptakis(2,3-di-O-methyl-6-sulfato)beta-CD and heptakis(2,3-di-O-acetyl-6-sulfato)beta-CD in aqueous and nonaqueous systems

Aziridines are attracting interest as protease inhibitors, which might be used, e.g., for treatment of parasitic diseases. Within the framework of greater projects dealing with the search of new selective protease inhibitors, a huge number of aziridines with two stereogenic centers will be synthesized. Thus, a fast and reliable screening method for the evaluation of the isomeric composition is needed. Robust baseline separations were obtained using heptakis(2,3-di-O-acetyl-6-sulfato)beta-CD (HDAS) in acidic methanol and sulfated beta-CD in acidic phosphate buffer. With HDAS the resolutions were higher and migration times shorter. Thus, the method will be used as a screening method for further isomeric mixtures of aziridines.     

Enantiomeric separation of basic compounds using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin in combination with potassium camphorsulfonate in nonaqueous capillary electrophoresis: optimization by means of an experimental design

The enantiomeric separation of a series of basic pharmaceuticals (beta-blockers, local anesthetics, sympathomimetics) has been investigated in nonaqueous capillary electrophoresis (NACE) systems using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin (HDMS-beta-CD) in combination with potassium camphorsulfonate (camphorSO3-). For this purpose, a face-centered central composite design with 11 experimental points was applied. The effect of the concentrations of HDMS-beta-CD and camphorSO3- on enantioresolution was statistically evaluated and depended largely on the considered analyte. The presence of camphorSO3- was found to be particularly useful for the enantioseparation of compounds with high affinity for the anionic CD. CamphorSO3- seems to act as a competitor, reducing the affinity for the CD, probably by ion-pair formation with these analytes. For compounds with lower affinity for HDMS-beta-CD, the combination of camphorSO3- and the CD appeared to have a favorable effect on enantioresolution only if the optimal CD concentration could be reached. On the other hand, for compounds characterized by a very low affinity for the anionic CD, the association of camphorSO3- and HDMS-beta-CD is always unfavorable. Finally, experimental conditions were selected by means of the multivariate approach in order to obtain the highest resolution (Rs) value for each studied compound.     

Liquid chromatographic separation of enantiomers of beta-amino acids using a chiral stationary phase

The enantiomers of both alpha-substituted beta-alanines and beta-substituted beta-alanines may be chromatographically separated using silica-bonded chiral stationary phases derived from N-acetylated alpha-arylalkylamines. The amino acids are chromatographed as alkyl esters of N-3,5-dinitrobenzoyl derivatives; separability factors range from 1.11 to 1.65 for nine alpha-substituted beta-alanines and from 1.08 to 1.20 for nine beta-substituted beta-alanines. The enantiomers of beta-aminoisobutyrate and beta-leucine, chiral beta-amino acids occurring in animal tissues and physiological fluids, are among those resolved. The enantiomers of R,S-beta-aminoisobutyrate and several related alpha-alkyl-beta-alanines were prepared by chromatographic resolution of diastereomeric dipeptides.     

Mechanistic study on the opposite migration order of clenbuterol enantiomers in capillary electrophoresis with beta-cyclodextrin and single-isomer heptakis(2,3-diacetyl-6-sulfo)-beta-cyclodextrin

Opposite migration order was observed for the enantiomers of the chiral beta2-adrenergic drug clenbuterol (CL) in capillary electrophoresis (CE) when resolved with native beta-cyclodextrin (beta-CD) and heptakis (2,3-diacetyl-6-sulfo)-beta-CD (HDAS-beta-CD). The possible mechanisms of the affinity reversal of the CL enantiomers depending on the structure of the CD were studied using 1H-nuclear magnetic resonance (1H-NMR) spectrometry and one-dimensional rotating frame nuclear Overhauser and exchange spectrometry (1-D ROESY). Significant differences were observed between the structure of the (+/-)-CL complexes with beta-CD and HDAS-beta-CD.     

Nonaqueous capillary electrophoresis method for the enantiomeric purity determination of S-timolol using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin: validation using the accuracy profile strategy and estimation of uncertainty

Nonaqueous capillary electrophoresis (NACE) was successfully applied to the enantiomeric purity determination of S-timolol maleate using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin (HDMS-beta-CD) as chiral selector. With a background electrolyte made up of a methanolic solution of 0.75 M formic acid, 30 mM potassium camphorsulfonate and containing 30 mM HDMS-beta-CD, the determination of 0.1% of R-timolol in S-timolol could be performed with an enantiomeric resolution of 8.5. Pyridoxine was selected as internal standard. The NACE method was then fully validated by applying a novel strategy using accuracy profiles. It is based on beta-expectation tolerance intervals for the total measurement error which includes trueness and intermediate precision. The uncertainty of measurements derived from beta-expectation tolerance intervals was estimated at each concentration level of the validation standards. To confirm the suitability of the developed and validated method, several real samples of S-timolol maleate containing R-timolol maleate at different concentrations were analysed and the results were compared to those obtained by liquid chromatography.     

HPTLC separation of aromatic α-amino acid enantiomers on a new histidine-based stationary phase using ligand exchange

Summary A new chiral ligand exchange selector for hydrophobic stationary phase modification has been synthesized by selective alkylation of L-histidine at the pyrrolic nitrogen atom in its imidazolic ring. Its performance was then tested on RP-HPTLC plates treated with copper acetate. Significant selectivity towards aromatic amino acid enantiomers, was observed. Chromatographic retention data were compared to available thermodynamic complex formation parameters for the relevant model systems in aqueous solution.Stationary and mobile phase effects on retention were studied by using different RP-HPTLC plates and various binary aqueous solvent mixtures.Optimized separation conditions for aromatic amino acid sample class are given.     

2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl-gamma-cyclodextrin: a new class of cyclodextrin derivatives for gas chromatographic separation of enantiomers

Octakis(2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl)-gamma-cyclodextrin (2,3-MOM-6-TBDMS-gamma-CD) was employed as stationary phase for capillary gas chromatographic separation of enantiomers. Selective introduction of the acetal function at positions 2 and 3 of the glucose units was achieved by reaction of 6-O-TBDMS-gamma-cyclodextrin with methoxymethyl chloride. 2,3-MOM-6-TBDMS-gamma-CD was shown to be a chiral stationary phase suitable for enantiodifferentiation of a broad spectrum of chiral volatiles from various chemical classes. A total of 125 pairs of enantiomers could be separated. Structural influences of the analytes on the enantioseparation were demonstrated. High alpha values up to 1.8 were observed for the hydroxyketone acetoin and some methyl branched ketones. Pronounced enantioseparations were also determined for cyclic pentenolone and furanone derivatives.     

Liquid chromatographic separation of the enantiomers of beta-amino acids on a ligand exchange chiral stationary phase

A liquid chromatographic ligand exchange chiral stationary phase (CSP) derived from (S)-leucinol was applied in the separation of the enantiomers of 12 beta-amino acids. The resolution was quite successful especially for the enantiomers of beta-amino acids containing aromatic functional group in the side chain. The chromatographic resolution behaviors were dependent on the organic modifier and Cu(II) concentration in aqueous mobile phase and the column temperature.     

High performance liquid chromatographic separation of dipeptide and tripeptide enantiomers using a chiral crown ether stationary phase

A chiral stationary phase (CSP) based on (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was evaluated for the direct resolution of the enantiomers of dipeptides and tripeptides. The type and concentration of the acid and the methanol content were optimized with regard to retention time and resolution using Ala-Phe as model peptide. A mobile phase consisting of 10 mM sulfuric acid in 70% aqueous methanol was applied to the separation of a set of 16 structurally diverse dipeptides and tripeptides. Generally, the configuration of the amino acid at the N-terminus determined the enantiomer elution order. With a few exceptions the LL- and LD-enantiomers interacted stronger with the CSP compared to the corresponding DD- or DL-enantiomers. The experimental conditions also allowed the simultaneous separation of all four stereoisomers of Ala-Phe. Addition of ammonium sulfate generally reduced retention times and enantiomer resolution. Addition of triethylamine as modifier led to an overall increase of the retention times while the resolution did not show a general trend, increasing in the case of Ala-Ala but decreasing in the case of Ala-Phe.     

Heptakis(6-amino-6-deoxy)-beta-cyclodextrin as a chiral selector for the separation of anionic analyte enantiomers by capillary electrophoresis

A hepta-substituted beta-cyclodextrin bearing seven amino groups, heptakis(6-amino-6-deoxy)-beta-cyclodextrin (per-6-NH2-beta-CD) was successfully used as a chiral selector for the enantioseparation of different anionic analytes. The running buffer pH and chiral selector concentration were the studied parameters crucial in achieving the maximum possible enantioresolution. Enantiomeric separation of a mixture of seven carboxybenzyl-amino acids was achieved in 24 min. Excellent resolution was obtained for carboxybenzyl-tryptophan (Rs = 11.2).     

Chiral separation of gamma-butyrolactone derivatives by gas chromatography on 2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl-beta-cyclodextrin

The chiral GC separation of 2-alkyl-2-keto-gamma-butyrolactone derivatives and their alcohol analogs using 2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl-beta-cyclodextrin (DIMETBCD) as chiral selector was discussed. The results, supported by the ketone preliminary molecular modelling calculations, suggest that the chiral recognition for DIMETBCD depends more on the geometry than on the polarity of the alkyl substituents on the butyrolactones. Hydrogen bonds and alkyl group steric effects should be an important function of the alcohol chiral recognition for DIMETBCD. Comparison of the retention times of the alcohol derivatives, in achiral and chiral stationary phases, suggests a specific structural effect for the cyclodextrin selctor.     

High-performance liquid chromatographic separation of position isomers using metal-organic framework MIL-53(Al) as the stationary phase

Metal-organic framework MIL-53(Al) was explored as the stationary phase for high-performance liquid chromatographic separation of position isomers using a binary and/or polar mobile phase. Baseline separations of xylene, dichlorobenzene, chlorotoluene and nitrophenol isomers were achieved on the slurry-packed MIL-53(Al) column with high resolution and good precision. The effects of mobile phase composition, injected sample mass and temperature were investigated. The separation of xylene, dichlorobenzene, chlorotoluene and nitrophenol isomers on MIL-53(Al) were controlled by entropy change.     

Enantiomeric separation of alcohols and amines on a proline chiral stationary phase by gas chromatography

A new chiral stationary phase for gas chromatography was prepared by covalently attaching a diproline chiral selector that has proven to be effective in liquid chromatography to a methylhydrosiloxane-dimethylsiloxane copolymer. With this new chiral stationary phase for GC, racemic aromatic alcohols could be resolved without derivatization. Racemic aromatic and aliphatic amines could also be resolved after derivatization of the amino groups with trifluoroacetic anhydride or isopropyl isocyanate. On this stationary phase, the isopropyl isocyanate derivatives of amines showed higher enantioselectivity than the trifluoroacetic anhydride derivatives. In both the enantiomeric separations of alcohols and derivatized amines, the aromatic racemic analytes showed higher enantioselectivities than their aliphatic analogs. Some of the alpha-amino and alpha-hydroxy aromatic acids could also be separated after derivatization to N-trifluoroacetyl methyl esters for amino acids or O-trifluoroacetyl methyl esters for hydroxyl acids.