A new application of stopped-flow chiral HPLC: inversion of enantiomer elution order

A newly developed procedure to reverse the enantiomer elution order of compounds resolved on chiral stationary phases (CSPs) for HPLC is presented. The optimized analytical protocol is based on the effect of temperature on enantioselectivity and does not involve any changing in mobile phase composition or type of CSP. In essence, the approach entails variable temperature chromatography at two temperatures. The enantiomer separation is performed at a low column temperature, with stopping the flow prior to elution of the less retained enantiomer. Then, the column temperature is changed with the peaks trapped inside the column, followed by elution with the same mobile phase in reverse direction. Under these conditions, the more pronounced loss in free energy of binding for the more strongly bound enantiomer results in an inversion of the elution order. This procedure may be applied to each enantiomer pair that is separated by chiral HPLC under an appreciable enthalpy-control.     

Enantiomer discrimination by mass spectrometry: noncovalent interactions of an N-derivatized dipeptide with various cinchona alkaloid derivatives and comparison with enantioselective liquid-phase separations

The enantiomer discrimination properties of cinchona alkaloid derived chiral selectors (CSs) towards a dipeptide analyte are examined by electrospray ionization mass spectrometry. The complexes formed between the CSs and the analyte enantiomers owing to various noncovalent interactions are analyzed and the magnitudes of enantiomer discrimination are determined from the complexes mass spectrometric intensities. The influence of different structural features of the CSs on enantioselectivity is discussed. The enantiomer discrimination results obtained by mass spectrometry are compared with those from related liquid chromatography enantiomer separations. A certain coherence between the chromatographic and mass spectrometric enantioselectivities could be established and the enantiomer discrimination patterns, i.e., the relative binding strengths, were identical for the two techniques. Thus, the use of mass spectrometry as a screening tool in the development of new CSs for chromatographic applications seems feasible.     

脂肪酶催化作用下酰胺化反应的立体规则性研究

提出了在LipasefromCandidaAntrarctica(CAL)脂肪酶的催化作用下,含有手性中心C原子的胺类化合物酰胺化的异构选择性规则.与氨基相连的碳原子的立体结构对酰胺化反应有很大的影响.型胺类化合物呈R-型优先选择,型胺类化合物呈S-型优先选择.     

2D Ising Model for Adsorption-induced Enantiopurification of Racemates

Mechanisms for the spontaneous transformation of achiral chemical systems into states of enantiomeric purity have important ramifications in modern pharmacology and potential relevance to the origins of homochirality in life on Earth. Such mechanisms for enantiopurification are needed for production of chiral pharmaceuticals and other bioactive compounds. Previously proposed chemical mechanisms leading from achiral systems to near homochirality are initiated by a symmetry-breaking step resulting in a minor excess of one enantiomer via statistical fluctuations in enantiomer concentrations. Subsequent irreversible processes then amplify the majority enantiomer concentration while simultaneously suppressing minority enantiomer production. Herein, equilibrium adsorption of amino acid enantiomer mixtures onto chiral and achiral surfaces reveals amplification of surface enantiomeric excess relative to the gas phase; i. e. enantiopurification of chiral adsorbates by adsorption. This adsorption-induced amplification of enantiomeric excess is shown to be well-describe by the 2D Ising model. More importantly, the 2D-Ising model predicts formation of homochiral monolayers from adsorption of racemic mixtures or prochiral molecules on achiral surfaces; i. e. enantiopurification with no apparent chiral driving force.     

Stereoselective and Asymmetric-Selective Polymerization of α-Amino Acid N-Carboxyanhydride

The enantiomer selectivity in the propagation reaction of NCA was investigated by using suitable model reactions. Contrary to the assumption usually made, the enantiomer selectivity in the nucleophilic addition of chiral amines to NCA depended strongly on the structure of amine or NCA and the solvent. In the polymerization by an activated-NCA mechanism, the addition of activated NCA to NCA was found for the first time to be enantiomer-selective. In addition to this, the chiral penultimate unit was found to participate in the enantiomer selection. Structures of the transition states leading to the different types of enantiomer selection were proposed.     

Stereoelective polymerization of tert-butyl thiirane

The polymerization in bulk of racemic tert-butyl thiirane with a chiral initiator resulting from the reaction between diethylzinc and (? )3,3-dimethyl-1,2-butanediol produces an optically active polymer by preferential consumption of R enantiomer. The unreacted monomer is enriched in S enantiomer. The relative rate r of consumption of R enantiomer versus S enantiomer is as high as 2.8. Obtained polymer could be separated into two crystalline fractions: an optically active fraction, formed from regular sequences of R type enantiomeric units, and an optically inactive fraction which corresponds to a racemate. Experimental results are consistent with a stereospecific mechanism of addition, the two enantiomers being chosen by two different type of sites. The stereoelective process is due to an unequal number of these two types of sites.     

Chiral Phosphoric Acid Catalyzed Kinetic Resolution of Indolines Based on a Self‐Redox Reaction

A strategy for oxidative kinetic resolution of racemic indolines was developed, employing salicylaldehyde derivative as the pre‐resolving reagent and chiral phosphoric acid as the catalyst. The iminium intermediate, formed by the condensation reaction of an enantiomer of indoline with salicylaldehyde derivative, was hydrogenated by the same enantiomer of indoline to afford another enantiomer of indoline by a self‐redox mechanism. The oxidative kinetic resolution of 2‐aryl‐substituted indolines proceeded to give enantiomers in good yields with excellent enantioselectivities.     

Enantiomeric purity determination by NMR: proving the purity of a single enantiomer

The NMR spectra of separate samples of an analyte complexed with each enantiomer of a chiral solvating agent (CSA) give an accurate estimate of the chemical shifts of racemic analytes in the presence of a single enantiomer of the CSA. This effect allows a CSA-based chiral NMR method to be developed when only a single enantiomer of analyte is available. The ability to develop a method capable of discriminating between enantiomers in these circumstances is useful, for example, to resolve the question of whether racemization has occurred during the synthesis of a chiral molecule.     

Separation of the two enantiomers of T-3811ME by normal-phase HPLC using modified amylose as chiral stationary phase

A normal-phase enantioselective high-performance liquid chromatographic method is developed for the separation of the undesired enantiomer from T-3811ME and quantitaion of the undesired enantiomer at low levels using a Chiralpak AD-H column (4.6 x 150 mm) packed with modified amylose stationary phase. The 2% water-modified 2-propanol is used for the method development activities, including exploration of various organic modifiers, optimization of additive acid concentration, column screening, and column temperature optimization. The final optimized method separated the undesired enantiomer from T-3811ME and is proven to be robust, sensitive, linear, accurate, and precise.     

The racemate cage. Influence of p(1),n(1) salt occurrence on enantiomer separation processes. The case of trans-chrysanthemic acid

The occurrence of p(1),n(1) salt when accompanied by substrate self-association can have profound effects on enantiomer separation processes of non-racemic mixtures, impeding the complete recovery of the major enantiomer through formation of an inescapable racemate cage.     

Preparation of radioactive enantiomers of amino acids using labelled racemates only

It is shown from experiments on leucine, that is possible to obtain pure enantiomer tracers of amino acids by using radioactive racemates only. The resolution takes place in a single crystallization step after mixing the active racemate with the inactive enantiomer, due to an absolute stereoselection.     

Chiral recognition of thiaheterohelicenes by alkyl beta-D-pyranoside micelles. Influence of extension of helix

Chiral recognition of alkyl beta-D-pyranoside micelles toward [7] and [5] heterohelicenes possessing helical structures was investigated by 1H-NMR and CD (circular dichroism) spectroscopy. In dodecyl maltopyranoside micellar solution, P and M enantiomers of tetrathia[7]heterohelicene (7TH), which have rigid and stable helixes, manifested different chemical shifts in their 1H-NMR spectra due to differences in the diastereomeric interactions, implying that the micelles undergo stronger recognition toward the M enantiomer than the P enantiomer. On the other hand, in octyl glucopyranoside micellar solution, trithia[5]heterohelicene (5TH) and two kinds of its derivatives which are rapidly equilibrated between the enantiomers in solution, gave no distinctly resolved 1H-NMR peaks for either enantiomer even at a lowered temperature. However, these racemic [5]heterohelicenes in the micelles did develop induced CD absorptions owing to a displacement of the equilibrium, suggesting from the signs of their Cotton effects that the micelles prefer the M enantiomer to its antipode in conformity with the 1H-NMR results of 7TH.     

Case of enantiomer impurity identification by normal-phase chiral high-performance liquid chromatography with optical rotation and mass spectrometric detection

An impurity produced in the synthesis of compound I is separated and identified as its enantiomer II using normal-phase chiral high-performance liquid chromatography (HPLC) with UV absorbance, optical rotation (OR) and mass spectrometric (MS) detection. The results show that the impurity II and compound I have equal and opposite specific rotations, identical MS spectra and the same MS-MS fragmentation pattern, as required for enantiomers. The procedures presented demonstrate a novel combination of methods for enantiomer identification and characterization that do not require the preparation of individual enantiomer markers or even the racemic mixture, thus reducing the need for additional synthetic work.     

Method Development of Enantiomer Separations by Affinity Capillary Electrophoresis,Cyclodextrin Electrokinetic Chromatography and Capillary Electrophoresis-Mass Spectrometry

 Capillary electrophoresis (CE) has become a powerful tool for enantiomer separations during the last decade. Since 1993, the author has investigated enantiomer separations by affinity capillary electrophoresis (affinity CE) with some proteins and by cyclodextrin electrokinetic chromatography (CDEKC) with some charged cyclodextrins (CDs). Many successful enantiomer separations are demonstrated from our study in this review article. In the enantiomer separations by affinity CE, the deterioration of detection     

Highly enantioselective recognition of dicarboxylic acid substrates by the control of nonlinear responses

An allosteric host, in which the molecular information of the chiral guest was precisely introduced, exhibits unconventional enantioselectivity toward the target enantiomer even under the conditions of a -20% enantiomeric excess (ee) mixture within a certain concentration window. This is effected by incorporation of the structural information of the enantiomer into the host and the utilization of multiple equilibrium.     

石墨烯基环糊精传感器的构建及对组氨酸对映体的电位识别

以具备手性识别作用的环糊精聚合物为手性选择剂, 以石墨烯为手性放大材料, 通过层层修饰的方法构建了石墨烯基环糊精聚合膜电位识别传感器. 由所构建的传感器测得的组氨酸对映体的氧化峰电位存在明显的差异, 电位差值近100 mV, 且不同比例的组氨酸对映体混合溶液的氧化峰电位值与对映体过量值呈现良好的线性关系. 据此, 建立了一种通过电位同时识别组氨酸对映体两组分的新方法.     

Study of the parameters controlling the enantio-differentiating ability of asymmetrically modified solid catalysts for the hydrogenation of γ-ketoesters

The enantio-differentiating hydrogenation of γ-ketoesters was carried out over asymmetrically modified solid catalysts. The parameters affecting the enantiomer excess (ee) were investigated and the results were compared with those of the hydrogenation of methyl acetoacetate and 2-octanone reported in the literature. The highest value of enantiomer excess of 51% was attained for the hydrogenation of methyl 4-oxopentanoate over a tartaric acid (TA)-NaBr-modified reduced nickel catalyst prepared from nickel oxide. The amount of NaBr in the modification solution needed to be optimized according to the manufacturers of the nickel oxides. The addition of an appropriate amount of carboxylic acid to the reaction media increased the enantiomer excess of the hydrogenated products.     

An efficient multigram synthesis of the potent histamine H3 antagonist GT-2331 and the reassessment of the absolute configuration

GT-2331 is a potent histamine H(3) antagonist which has entered clinical trials. Efficient multigram syntheses of this compound and its enantiomer are described. The literature reports that GT-2331 is the dextrorotatory (+), more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole with the absolute configuration of (1R,2R)-1. However, we found that the dextrorotatory, more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole has the (1S,2S) absolute configuration. We suggest a reconsideration of the absolute configuration of GT-2331.     

The CGC enantiomer separation of 2-arylcarboxylic acid esters by using β-cyclodextrin derivatives as chiral stationary phases

Chiral 2-arylcarboxylic acid esters are important intermediates in preparation of enantioenriched 2-arylpropionic acids type Non-steroidal anti-inflammatory drugs (NSAIDs). Enantiomer separation of 2-arylcarboxylic acid esters is crucial for evaluation of the asymmetric synthesis efficiency and the enantiomer excess of chiral 2-arylcarboxylic acid derivatives. The capillary gas chromatography (CGC) enantiomer separation of 17 pairs of 2-arylcarboxylic acid esters enantiomers was conducted by using seven different β-cyclodextrin derivatives (CDs) as chiral stationary phases. It was found that for the 7 pairs of 2-phenylpropionates enantiomers, CDs with both alkyl and acyl substituents especially 2,6-di-O-pentyl-3-O-butyryl-β-cyclodextrin exhibited better enantiomer separation abilities than the other CDs examined. For the 7 pairs of 2-(4-substituted phenyl)propionates enantiomers, 2,3,6-tri-O-methyl-β-cyclodextrin possessed better enantiomer separation abilities than the other CDs. Among the 3 pairs of 2-phenylbutyrates enantiomers examined, only methyl 2-phenylbutyrate enantiomers could be separated by three CDs among the 7 CDs tested, while enantiomers of ethyl 2-phenylbutyrate and isopropyl 2-phenylbutyrate couldn't be separated by any of the 7 CDs tested. Besides the structures of CDs, the structures of 2-arylcarboxylic acid esters including different ester moieties, substituents of phenyl, and different carboxylic acids moieties in 2-arylcarboxylic acid esters also affected the enantiomer separation results greatly. The CGC enantiomer separation results of 2-arylcarboxylic acid esters on different CDs are useful for solving the enantiomer separation problem of 2-arylcarboxylic acid esters.     

Microseparation techniques for the study of the enantioselectivity of drug–plasma protein binding

Stereoselectivity in protein binding can have a significant effect on the pharmacokinetic and pharmacodynamic properties of chiral drugs. The investigation of enantioselectivity of drugs in their binding with human plasma proteins and the identification of the molecular mechanisms involved in the stereodiscrimination by the proteins represent a great challenge for clinical pharmacology. In this review, the separation techniques used for enantioselective protein binding experiments are described and compared. An overview of studies on enantiomer–protein interactions, enantiomer–enantiomer interactions as well as chiral drug–drug interactions, including allosteric effects, is presented. The contribution of individual plasma proteins to the overall enantioselective binding and the animal species variability in drug–plasma protein binding stereoselectivity are reviewed. Copyright © 2008 John Wiley & Sons, Ltd.