Derivatized cyclodextrins for the separation of enantiomers in capillary electrophoresis

The use of cyclodextrin derivatives for the efficient separation of enantiomeric drugs is described. Hydroxypropylation, methylation or carboxymethylation of the cyclodextrin not only result in a better solubility of the cyclodextrin in aqueous solutions, but also favor, via additional hydrogen bonding, the stabilization of one of the cyclodextrin-analyte complexes. The influence of the background electrolyte on peak shape is also described here. Carboxymethylated cyclodextrin can be used in similar manner to uncharged cyclodextrins at low pH values (below 4). At pH values above 5, however, its charge also allows the separation of uncharged enantiomers as in a micellar-like system.     

Derivatives for separation of amino acid enantiomers

Summary An optimum gas chromatographic separation of all protein amino acids in one run on capillaries coated with Chirasil-Val is difficult to achieve. Overlap of enantiomers of different amino acids may occur because the relative retention times depend upon the overall polarity of the stationary phase, the film thickness and the actual temperature programm. Employment of different derivatives formed by esterification with isopropanol, n-propanol, isobutanol and n-butanol and by acylation with trifluoroacetic, pentafluoropropionic and heptafluorobutyric anhydrides yields patterns of relative elution of all amino acid enantiomers. Thus, even critical pairs of amino acid enantiomers can be separated or shifted in their relative retention times. All amino acid enantiomers can be separated and quantitatively estimated.     

Semi-preparative high-performance liquid chromatographic separation of trimedlure isomers

Summary Semi-preparative high-performance liquid chromatographic procedures on 5 m silica were developed for the isolation of gram quantities of eight trimedlure isomers (trans: A, B₁, B₂, and C;cis: V, W, X, and Y) for comparative biological evaluation and NMR studies. Isolations were made from an eight-component 21cis: trans-trimedlure mixture, a four-componentcis-trimedlure mixture, trimedlure-B₂:X and trimedlure-C:W epimerization merization mixtures, a trimedlure-B₁:Y:B₂ mixture, and a trimedlure-A concentrate.     

反应萃取手性分离苯基琥珀酸对映体动力学研究

通过恒界面池研究了羟丙基-β-环糊精(HP-β-CD)萃取苯基琥珀酸对映体(PSA)动力学.采用伴随化学反应的萃取理论获得萃取动力学.实验分别考察了搅拌速率、界面面积、对映体浓度和萃取剂浓度等条件对PSA对映体萃取动力学的影响.实验结果表明:HP-β-CD萃取PSA对映体的反应为快反应;对对映体反应是一级反应,对萃取剂反应是二级反应;R-PSA,S-PSA反应速率常数分别为3.4×10-2m6mol-2s-1,9.96×103m6mol-2s-1.这些数据对萃取过程的设计是很重要的.     

Gas chromatographic resolution of enantiomeric and diastereoisomeric amino acid esters on Chirasil-Val capillary column

Summary D,L-amino acids were derivatized with (+), (±)-2-butanol or N-trifluoroacetyl-L-prolyl chloride (L-TPC) and then chromatographed. Four optical isomers were separated on a Chirasil-Val capillary column. By this method, the concentration of optical impurities arising from the commercial optically active reagents can be determined. The observed abnormal elution orders of enantiomeric amino acid esters may be caused by a selective intermolecular force.     

高效液相色谱手性流动相添加法拆分奥昔布宁对映体

采用C18固定相,以羟丙基-β-环糊精为手性流动相添加剂,建立了奥昔布宁对映体的高效液相色谱拆分方法。考察了手性添加剂、有机极性调节剂、缓冲盐的种类和浓度以及流动相的pH值和流速及柱温等因素对对映体分离的影响。在最佳分离条件下,奥昔布宁对映体的分离度为1.54,检测限为1.0 ng。该方法简便,重复性好,比手性固定相法更加经济。     

Liquid chromatographic methods for separation,determination, and bioassay of enantiomers of etodolac: A review

The control of enantiomeric purity and determination of individual enantiomeric drug molecules remains the subject of importance for clinical, analytical, and regulatory purposes and to facilitate an accurate evaluation of the risks posed by them to human health. A large number of pharmaceuticals are marketed and administered as racemates. Etodolac is among such nonsteroidal anti‐inflammatory drugs. Overall literature reports on its enantioseparation are scanty. Liquid chromatography (LC) methods of enantioseparation of (±)‐etodolac, including certain unconventional ones, are well covered and discussed in this paper. Methods of direct approach without using chiral columns or chiral thin‐layer chromatography plate and of indirect approach using certain chiral derivatizing agents such as (S)‐naproxen and (S)‐levofloxacin are described. Most interesting aspects include establishment of structure and molecular asymmetry of chemically different types of diastereomeric derivatives using liquid chromatography with mass spectrometry (LC–MS), ¹H NMR spectroscopy and by drawing conformations in three dimensional views by using certain software. The methods provide chirality recognition even in the absence of pure enantiomers. Besides, recovery of pure enantiomers by detagging or via solubility difference of chiral inducing reagent and the analyte, without racemization at any stage, has been achieved. The limits of detection and quantification are much lower than the industry benchmarks.     

Gas chromatographic separation of the enantiomers of flumecinol and some related compounds

Flumecinol and related compounds can, without derivatization, be separated into their enantiomers by gas chromatography on cyclodextrin phases; permethylated β-cyclodextrin dissolved in OV-1701 can be particularly recommended. Thermodynamic data describing the different intensities of interaction of the individual enantiomers with the stationary phases were determined. The values measured imply different separation mechanisms for the compounds investigated.     

Capillary gas chromatographic separation of C10–C12 secondary phenylalkanes on modified cyclodextrin stationary phases

The separation of isomers and enantiomers of branched C₁₀-C₁₂ phenylalkanes by gas chromatography on fused silica capillary columns coated with some modified β- and γ-cyclodextrins was studied. It was shown that the separation of positional isomers of C₁₀-C₁₂ phynylalkanes on modified cyclodextrin capillary columns is not better than that on a column coated with modified polyethylene glycol. Differences were found in the enantioselectivity of modified β- and γ-cyclodextrins for the separation of C₁₀-C₁₂ secondary phenylalkane enantiomers. While alkylderivatives of β-CDs resolve enantiomers of 3-phenylalkanes, alkyl derivatives of γ-CD resolve enantiomers of 2-phenylalkanes. Since shape selectivity factors of modified cyclodextrins have indicated no inclusion of the considered solutes in cyclodextrin cavities, enantioselective interactions most probably occur on the outer sphere of cyclodextrins.     

Enantioselective separation of twelve pairs of enantiomers on polysaccharide‐based chiral stationary phases and thermodynamic analysis of separation mechanism

The enantioseparation of twelve pairs of structurally related 1‐aryl‐1‐indanone derivatives was studied in the normal‐phase mode using three different polysaccharide‐type chiral stationary phases, namely Chiralpak IB, Chiralpak IC, and Chiralpak ID. n‐Hexane/2‐propanol and n‐hexane/ethanol were employed as mobile phases. Among all the investigated chiral columns, Chiralpak IC exhibited the most universal and the best enantioseparation ability toward all the racemates, particularly with the mobile phase composed of n‐hexane/2‐propanol (90/10, v/v). Then the effects of column temperature on retention and enantioselectivity were examined in the range of 25–40°C. Satisfactory enantioseparation was obtained at ambient temperature. The natural logarithm of retention and separation factors (ln k and ln α) versus the reciprocal of absolute temperature (1/T) (Van't Hoff plots) were found to be linear for all racemates, indicating that the retention and separation mechanisms were independent of temperature in the range investigated. Then, the thermodynamic parameters (ΔΔH°, ΔΔS°, and ΔΔG°) were calculated from Van't Hoff plots. These values indicated that the solute transfer from the mobile to stationary phase was enthalpically favorable, and the process of enantioseparation was mainly enthalpy controlled. At last, the impact of small changes in molecular structures of the tested 1‐indanone derivatives on enantioseparation was also discussed.     

Liquid chromatographic resolution of racemic biotin

Summary The separation of the enantiomeric forms of biotin (vitamin H) using preparative chiral HPLC is described.     

Reversed-phase HPLC separation of Δ;2 and Δ;3 isomers of 7-ADCA and cephalexin monohydrate

Summary High-performance liquid chromatography was applied for the separation and determination of the Δ² and Δ³ isomers of 7-aminodeacetoxycephalosporanic acid (7·ADCA) and cephalexin monohydrate in their mixtures. Separation was performed on a column containing bonded octadecyl silica phase. The effects of pH (in the range of 3.0–7.6) on the investigated compounds were studied. Two organic modifiers, acetonitrile and methanol, were used to improve and accelerate separation. The applied procedure was very reliable for quantitative determination. Excellent correlation with the data of microbiological assay was found. The procedure was very convenient even when other impurities were present in the sample. This is an advantage with respect to microbiological assays.     

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.     

Application of molecular statistical calculations to the prediction of chromatographic separation of isomeric difluorobiphenyls

Molecular statistical calculations have been used for estimation of the possibility of chromatographic separation of difluorobiphenyl mixtures. GC-MS investigation of the mixture of difluorobiphenyls has been carried out. It is impossible to identify each isomer with only mass spectrometric data due to the similarity of their mass spectra. A chromato graphic column that makes adequate chromatographic separation possible has been selected based on the molecular statistical calculations. The identification of each isomer resulted from combination of the mass spectrometric investigation results and molecular statistical calculations.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 623–626, March, 1996.     

Direct separation of 2-hydroxy acid enantiomers by high-performance liquid chromatography on chemically bonded chiral phases

Summary The resolution of 2-hydroxy acid enantiomers by the principle of ligand exchange chromatography on chemically modified silica gel is described. The phases were synthezised by fixing L-amino acids via 2-glycidoxypropyl-trimethoxysilane to silica gel. The highest enantioselectivity for 2-hydroxy acids was obtained by using L-hydroxyproline as fixed ligand and Cu(II) as complexing agent. A number of aromatic as well as aliphatic 2-hydroxy acids could be resolved on this sorbent. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Determination of the interconversion energy barrier of 2,3-pentadienedioic acid enantiomers by HPLC. 2. On-column interconversion

In this paper, an HPLC method is used to determine the enantiomerization barrier of 2,3-pentadienedioic acid enantiomers. The racemate of 2,3-pentadienedioic acid was separated by HPLC on a chiral CHIROBIOTIC T column with a 90:10 (100:0.5:0.5 MeOH/HOAc/TEA)/H2O mobile phase. Peak areas of enantiomers prior to (A(+)0, A(-)0) and after the separation (A(+), A(-)), were used for calculation of the rate constants and the enantiomerization barrier, as determined by computer-assisted peak deconvolution of the peak clusters on the chromatograms. The kinetic equation for irreversible reactions was used to determine the apparent enantiomerization rate constants and the interconversion energy barrier. The dependence of the apparent enantiomerization barrier (deltaG1(app), deltaG-1(app)) on temperature was used to determine the apparent activation enthalpy (deltaH1(app), deltaH(-1)app) and entropy (deltaS1(app), deltaS-1(app)) for the interconversion of 2,3-pentadienedioic acid enantiomers, where the coefficients 1 and -1 designate the interconversions (+) --> (-) and (-) --> (+), respectively.     

Self-disproportionation of enantiomers of heterocyclic compounds having a tertiary trifluoromethyl alcohol center on chromatography with a non-chiral system

Self-disproportionation of enantiomers of heterocycles having a tertiary trifluoromethyl alcohol center on an achiral silica-gel stationary phase is discussed. During the chromatographic separation of an enantiomerically enriched mixture of 1-(3,4-dimethoxyphenethyl)-3-hydroxy-3-(trifluoromethyl)-6,7-dihydro-1H-indole-2,4(3H,5H)-dione (1) by eluting with ether on a non-chiral regular silica-gel significant enantiomeric enrichment was observed. Separation of non-racemic samples of 1 with enantiomeric excess values of 10-54% was carefully investigated: enantiomerically pure 1 with 99.9% ee was obtained by the use of 1 with at least 40% ee. A remarkable enantiomeric enrichment in the faster eluting fractions was also observed for compound 1 with only 30% ee to transform into 80% ee. Other enantiomeric mixtures of heterocyclic molecules containing a trifluoromethyl alcohol moiety at their quaternary carbon center were also examined from an SDE view point.     

Attempts to explain the self-disproportionation observed in the partial sublimation of enantiomerically enriched carboxylic acids

The partial sublimation of two carboxylic acids, the mandelic acid and ibuprofen has been studied. Many (RS) + (S) samples with various enantiomeric excesses (ee) have been slowly and partially sublimed at a low temperature and the sublimates have been condensed before analysis. About 1% of the starting material was sublimed in each experiment. The results are reproducible showing that the sublimation is under control. The ee of sublimates are comparable to the ee of the eutectic but also to those obtained by mixing the sublimates of two apparatuses used to sublime separately the racemate and the enantiomer. Thus, the sublimations of both carboxylic acids could be controlled by the saturated vapor pressure of the components ((RS) and (S)) or, as usually proposed, by the formation of a gas phase with a eutectic composition. In the case of mandelic acid, a definitive answer has been given by the partial sublimation of (S) + (R) solid mixtures where sublimates with a eutectic composition have been obtained and without any indication of the sublimation of a “kinetic conglomerate”. This study paves the way for future investigations on the slow and partial sublimation of enantioenriched compounds to determine how this latter occurs.