Enantiomer separation of dihydropyridine calcium antagonists with cyclodextrins as chiral selectors: structural correlation

Native and substituted cyclodextrins (CDs) were used as chiral selectors both in high-performance liquid chromatography and capillary electromigration separations (HPCE and MEKC). Chromatographic data of five dihydropyridine calcium antagonists obtained on three β-CD chiral stationary phases in reversed-phase mode were compared with those of capillary electrophoresis using β-CDs in the presence and absence of sodium dodecyl sulfate (SDS). Competition of separated compounds with SDS molecules for penetration into the CD cavity can limit their necessary interaction with the chiral selector and consequently even preclude enantiomer separation. Some insight into this problem can be brough about by comparing the experimental data with computer-aided energy minimization of CD-solute and CD-SDS inclusion complexes.     

Structural studies on the chiral selector capacity of cyclodextrin derivatives

Chromatographic separation of enantiomers to assure or enhance chiral purity is of considerable importance and can be achieved by the use of selectors of great structural variety. Cyclodextrins are an important and frequently used class, and they are multimodal selectors since multiple chiral interactions are possible by very different mechanisms. Here, the results of a preliminary examination on the possible value of computational molecular modeling approaches for the predictability of cyclodextrin selector effects for compounds that possess both geometrical and optical isomerism are presented. Interactions between various cyclodextrins and pyrethroic acids are modeled, interpreted, and compared to experimental capillary electrophoresis data.     

Role of capillary electrophoresis methods in the drug development process

In the past several years, capillary electrophoresis (CE) has generated considerable interest from pharmaceutical companies for control of both the chiral and achiral purity of bulk drugs and drug products. This paper evaluates the use of CE as: (1) a technique complementary to HPLC for the determination of peak homogeneity of a drug, (2) for determination of chiral purity, and (3) for determination of achiral purity. It would be greatly advantageous if CE could be used to determine both the chiral and achiral purity in a single assay. This investigation compares the results obtained for the separation of the enantiomers of duloxetine using several neutral cyclodextrins to those obtained using anionic cyclodextrins (sulfobutyl ether derivatives) as chiral selectors added to the separation buffer. In addition, it reports chiral separations obtained by using neutral cyclodextrins in a sulfonic acid-coated capillary column, which give a negatively charged capillary surface and electro-osmotic flow even in low pH buffers. The possible mechanism of separation is discussed. © 1996 Wiley-Liss, Inc.     

Enantioselective separations using capillary electrophoresis

The preconditions are outlined for enantioselective separations in capillary electrophoresis (CE) with chiral selectors as additives to the background electrolyte. Free solution capillary electrophoresis conditions are characterised by a single solution phase. Chiral separations are reviewed by selector type (chiral ligand exchange, cyclodextrins, crown ethers, glycoproteins) with the extensive studies on cyclodextrins grouped into sections on amino acids, pharmaceuticals, and speciality chemicals, optimisation, biological fluids, and quantitative aspects. In micellar electrokinetic capillary chromatography, enantioselective discrimination occurs by partition in a two-phase system, with a chiral micellar phase as selector. Optimum separation conditions can be readily predicted for a given selector–selectand combination, and absolute values of binding constants determined by CE. Advantages of CE in comparison with HPLC using a chiral stationary phase include robust, rapid assays and the use of small volumes of aqueous solutions; disadvantages include less favourable detection limits. © 1994 Wiley-Liss, Inc.     

Enantioselectivity of potentiometric sensors with application of different mechanisms of chiral discrimination

In the recent years, numerous successful applications of various chiral selectors in high performance separation methods have generated an increasing interest in the application of some of these compounds as electroactive species in potentiometric sensors. The objective of this work was to examine the enantioselectivy of several different sensors employing substituted cyclodextrins, example antibiotic teicoplanin and electrodeposited conductive polymers for various chiral analytes. Varying degrees of enantioselectivity were found for the ion-selective electrodes examined, depending on the chiral selector used and the target analyte.     

Separation of drug enantiomers by liquid chromatography and capillary electrophoresis, using immobilized proteins as chiral selectors

Proteins display interesting chiral discrimination properties owing to multiple possibilities of intermolecular interactions with chiral compounds. This review deals with proteins which have been used as immobilized chiral selectors for the enantioseparation of drugs in liquid chromatography and capillary electrophoresis. The main procedures allowing the immobilization of proteins onto matrices, such as silica and zirconia particles, membranes and capillaries are first presented. Then the factors affecting the enantioseparation of drugs in liquid chromatography, using various protein-based chiral stationary phases (CSPs), are reviewed and discussed. Last, chiral separations already achieved using immobilized protein selectors in affinity capillary electrochromatography (ACEC) are presented and compared in terms of efficiency, stability and reproducibility.     

Chiral separations on polysaccharide stationary phases using polar organic mobile phases

About 30% of a chemically diverse set of compounds were found to separate on four polysaccharide chiral stationary phases using polar organic mobile phases. No structural features appeared to correlate to successful separations. Titrations between normal and polar organic mobile phases suggested that separation mechanisms do not differ between these mobile phases. Attempts made to control retention met with varying degrees of success. Addition of hexane to alcohols had minor effects on retention although this was occasionally beneficial. Addition of water to alcohols increased retention. Addition of water to acetonitrile decreased retention. Addition of alcohol to acetonitrile also proved beneficial to the separation of some compounds. Loading studies performed to mimic preparative separations indicated that the benefits of polar organic mobile phases are largely due to increased solubility.     

Use of Sulfated Cyclofructan 6 and Sulfated Cyclodextrins for the Chiral Separation of Four Basic Pharmaceuticals by Capillary Electrophoresis

Sulfated cyclofructan 6 (S‐CF6) and sulfated cyclodextrins (S‐α‐, β‐, γ‐CDs) are highly selective chiral selectors for the enantioseparation of basic solutes. In this study, S‐CF6 was introduced for the enantiomeric separation of four basic pharmaceuticals (including tamsulosin, tiropramide, bupivacaine, and norephedrine) by capillary electrophoresis (CE), and the enantiomeric separation performance was compared with S‐α‐, β‐, γ‐CDs. The effects of the chiral selector type, chiral selector concentration, operating voltage, and column temperature were examined and optimized. Excellent resolutions were obtained for all solutes on these chiral selectors. Chirality 25:735–742, 2013. © 2013 Wiley Periodicals, Inc.     

Separation of the enantiomers of some racemic nonsteroidal aromatase inhibitors and barbiturates by capillary electrophoresis

High-performance capillary electrophoresis (HPCE) and micellar electrokinetic capillary chromatography (MECC) were applied to the resolution of racemic nonsteroidal antiaromatase drugs and intermediates. Successful results were obtained in both modes using α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), or 2,6-di-O-methyl-β-cyclodextrin (DM-β-CD) as chiral selectors. Depending on the structure of the solute, one of the cyclodextrins was generally better suited for resolution of the racemate. The basic solutes were analyzed under HPCE conditions, whereas the nonionizable compounds such as glutethimide (Doriden®) were analyzed in MECC mode. For the azole-type antiaromatase Fadrozole, both HPCE and MECC modes could be used to achieve the separation of the enantiomers. The influence of experimental factors such as pH, the presence of organic modifier, temperature, the micelle concentration, and the concentration of the chiral selector is also discussed on the basis of the results obtained with some chiral barbiturates. The possibility of analyzing the enantiomers directly in plasma samples was also demonstrated. © 1993 Wiley-Liss, Inc.     

Model of CE enantioseparation systems with a mixture of chiral selectors. Part I. Theory of migration and interconversion

Theory of equilibria, migration and dynamics of interconversion of a chiral analyte in electromigration enantioseparation systems involving a mixture of chiral selectors for the chiral recognition (separation) are proposed. The model assumes that each individual analyte-CS interaction is fast, fully independent on other interactions and the analyte can interact with CS in 1:1 ratio and that the analyte is present in the concentration small enough not to considerably change the concentration of free CSs. Under these presumptions, the system behaves as there was only one chiral selector with a certain overall equilibrium constant, overall mobility of analyte-selector complex (associate) and overall rate constant of interconversion in a chiral environment. We give the mathematical equations of the overall parameters. A special interest is devoted to the dynamics of interconversion. Interconversion in systems with mixture of chiral selectors is governed by two apparent rate constants of interconversion in the same way as in case of singe-selector systems. We propose the experimental design that allows to determine rates of interconversion in both chiral and achiral parts of the enantioseparation system separately. The approach is verified experimentally in the second part of the article.     

Enantioseparation of Tröger's Base Derivatives by Capillary Electrophoresis Using Cyclodextrins as Chiral Selectors

The enantioseparation of seven Tröger's base derivatives (TBs) was carried out by capillary electrophoresis using α‐, β‐, and γ‐cyclodextrins as chiral selectors and phosphate at 20 mmol/l concentration, pH 2.5, as background electrolyte. The method was optimized with respect to the concentration of chosen chiral selectors (0–50 mmol/l) and the amount of organic solvent (acetonitrile, 0–25 % (v/v)) in the electrolyte. The results indicate that all the studied variables, i.e., type of chiral selector, its concentration, and the amount of the added organic solvent, have a significant impact on the enantioseparation of the studied TBs. The best results for the majority of the separated TBs were obtained utilizing β‐cyclodextrin at 5 mmol/l concentration and with various amounts of acetonitrile added ranging from 5 to 15% (v/v) in the background electrolyte. For the two smallest studied TBs, γ‐cyclodextrin with 10% (v/v) acetonitrile also provided good resolution. Chirality 25:379–383, 2013. © 2013 Wiley Periodicals, Inc.     

Chiral Separation of the Clinically Important Compounds Fucose and Pipecolic Acid Using CE: Determination of the Most Effective Chiral Selector

In this study, simple electrophoretic methods were developed for the chiral separation of the clinically important compounds fucose and pipecolic acid. In recent years, these analytes, and particularly their individual enantiomers, have attracted considerable attention due to their role in biological functions and disorders. The detectability and sensitivity of pipecolic acid and fucose were improved by reacting them with fluorenylmethyloxycarbonyl chloride (FMOC‐Cl) and 5‐amino‐2‐naphthalene‐sulfonic acid (ANSA), respectively. The enantioseparation conditions were optimized by initially investigating the type of the chiral selector. Different chiral selectors, such as polymeric surfactants and cyclodextrins, were used and the most effective ones were determined with regard to resolution and analysis time. A 10‐mM β‐cyclodextrin was able to separate the enantiomers of ANSA‐DL‐fucose and the polymeric surfactant poly(sodium N‐undecanoyl‐LL‐leucine‐valinate) was able to separate the enantiomers of FMOC‐DL‐pipecolic acid, with resolution values of 3.45 and 2.78, respectively. Additional parameters, such as the concentration and the pH of the background electrolyte (BGE), the concentration of the chiral selector, and the addition of modifiers were examined in order to optimize the separations. The addition of the chiral ionic liquid D‐alanine tert‐butyl ester lactate into the BGE was also investigated, for the first time, in order to improve resolution of the enantiomers. Chirality 25:556–560, 2013. © 2013 Wiley Periodicals, Inc.     

Comprehensive strategy for chiral separations using sulfated cyclodextrins in capillary electrophoresis

This review focuses on the emerging role of sulfated cyclodextrins in the capillary electrophoretic (CE) separation of chiral analytes. Since being introduced as enantioselective agents for CE in 1995, these anionic additives have continued to demonstrate remarkable application universality. The broad spectrum of chiral compounds successfully separated using this approach includes acidic, basic, neutral, and zwitterionic species. This impressive array of analyte structures is derived from a growing diversity of compound classes including pharmaceuticals, plant extracts, biomarkers, herbicides, alkaloids, fungicides, and metal ions. Moreover, literature reports highlight the minimal optimization required to achieve a successful separation. Based on these findings, sulfated cyclodextrins appear to be well suited for the development of a more universal, comprehensive separation strategy for chiral compounds. This review explores this proposition by beginning with the structure and migration properties of sulfated cyclodextrins, using applications to highlight the separating power of this technique and ending with a pragmatic, comprehensive separation strategy.     

Synthesis and enantiomeric analysis of cyclotriphosphazene derivatives with one centre of chirality

A series of chiral cyclotriphosphazene compounds 2-9 in which the spiro 3-amino-1-propanoxy moiety provides the one centre of chirality have been synthesised and characterised by elemental analysis, MS, ¹H and ³¹P NMR spectroscopies. The enantiomers of newly synthesised compounds have been analysed by the changes in the ³¹P NMR spectra on addition of a Chiral Solvating Agent (CSA), (S)-(+)-2,2,2-trifluoro-1-(9′-anthryl)ethanol. HPLC methods have been developed for the enantiomeric separations of chiral cyclotriphosphazenes containing one centre of chirality. It is found that chiral HPLC gave a good resolution of enantiomers of the racemic compounds 2-9 with resolution factors between 2.49 and 7.50 making them good candidates for enantiomeric separations and determination of absolute configuration.     

Chiral recognition mechanisms with macrocyclic glycopeptide selectors

Macrocyclic glycopeptide selectors are naturally occurring antibiotics produced by microorganisms. They were found to be excellent chiral selectors for a wide range of enantiomers, including amino acids. Four selectors are commercialized as chiral stationary phases (CSP) for chromatography. They are ristocetin, teicoplanin, vancomycin, and the teicoplanin aglycone (TAG). The key docking interaction for amino acid recognition was established to be a charge-charge interaction between the anionic carboxylate group of the amino acid and a cationic amine group of the macrocyclic peptidic selector basket. The carbohydrate units are responsible for secondary interactions. However, they hinder somewhat the charge-charge docking interaction. The TAG selector is more effective for amino acid enantioseparations than the other CSPs. The "sugar" units are however useful allowing for chiral recognitions of other analytes, e.g., beta-blockers, not possible with the aglycone. Thermodynamic studies established that normal phase and reversed phase enantioseparations were enthalpy-driven. With polar waterless mobile phases used in the polar ionic mode, some separations were enthalpy-driven and others were entropy-driven. The linear solvation energy method was tentatively used to gain knowledge about the chiral recognition mechanism. It appeared to be a viable approach with neutral molecules but it failed with ionizable solutes. With molecular solutes and the teicoplanin CSP, the study showed a significant role of the surface charge-induced dipole interaction and steric effects. The remarkable complementary enantioselectivity effect observed with the four CSPs is discussed.     

Enantiomeric Separations of Ruthenium (II) Polypyridyl Complexes Using HPLC With Cyclofructan Chiral Stationary Phases

The enantiomeric separation of 21 ruthenium (II) polypyridyl complexes was achieved with a novel class of cyclofructan‐based chiral stationary phases (CSPs) in the polar organic mode. Aromatic derivatives on the chiral selectors proved to be essential for enantioselectivity. The R‐napthylethyl carbamate functionalized cyclofructan 6 (LARIHC CF6‐RN) column proved to be the most effective overall, while the dimethylphenyl carbamate cyclofructan 7 (LARIHC CF7‐DMP) showed complementary selectivity. A combination of acid and base additives was necessary for optimal separations. The retention factor vs. acetonitrile/methanol ratio plot showed a U‐shaped retention curve, indicating that different interactions take place at different polar organic solvent compositions. The separation results indicated that π–π interactions, steric effects, and hydrogen bonding contribute to the enantiomeric separation of ruthenium (II) polypyridyl complexes with cyclofructan chiral stationary phases in the polar organic mode. Chirality 27:64–70, 2015. © 2014 Wiley Periodicals, Inc.     

Chiral separation of bioactive cyclic Mannich ketones by HPLC and CE using cellulose derivatives and cyclodextrins as chiral selectors

The chiral separation of cyclic Mannich ketones of potential pharmaceutical interest is investigated using HPLC and CE. These Mannich ketones show a marked antibacterial and antifungal activity. In HPLC, stationary phases containing cellulose derivatives or beta-cyclodextrin were used and in CE different cyclodextrins, such as beta-CD, gamma-CD, carboxymethyl-beta-CD and succinyl-beta-CD were added to the background electrolyte as chiral selectors.     

手性技术与生物催化

简要介绍了手性,手性技术与生物催化的基本概念。手性,是指一个有机分子具有不对称性,形成两种空间排布方式不同的对映异构体。手性技术即生产手性化合物的技术,手性化合物的制备方法主要有手性源、外消旋体拆分、不对称合成等几种。生物催化,即利用酶或微生物等生物材料催化进行某种化学反应,被认为是手性化合物生产取得突破的关健技术。文章还介绍了生物催化外消旋体拆分、生物催化不对称合成等几种生产手性化合物的应用实例。     

The enduring legacy of Koji Nakanishi's research on natural products and bioorganic chemistry. Part 2. Inception and establishment of the ECD exciton chirality method in 1960s to 1970s: A marvel of Nakanishi's Japanese team

The electronic circular dichroism (ECD) exciton chirality method is very useful for determining the absolute configuration (AC) of chiral compounds. In the ECD spectroscopy, the chromophore-chromophore interaction, ie, exciton coupling, is very important. For example, Harada and Nakanishi first discovered in 1969 that chiral dibenzoates exhibit exciton split bisignate Cotton effects, from the sign of which the screw sense between two long axes of benzoate chromophores, ie, the AC of dibenzoate, can be determined. This method was named the dibenzoate chirality rule and has been successfully applied to various natural products to determine their ACs. During these studies, it was also found that this CD method was expanded to encompass other aromatic and olefin chromophores like naphthalene, diene, enone, etc. Therefore, the name of the dibenzaote chirality rule was changed to the CD exciton chirality method. In 1970s, there were heated controversies about the inconsistency between X-ray Bijvoet and CD exciton chirality methods, which was a shocking and serious problem in the community of molecular chirality research. Harada and coworkers synthesized the most ideal cage compound with two anthracene chromophores to connect X-ray Bijvoet and CD exciton chitality methods and proved that these two methods are consistent with each other.     

Chiral discrimination by HPLC and CE and antifungal activity of racemic fenticonazole and its enantiomers

Fenticonazole is a chiral antifungal agent, used in therapy as the racemic mixture. The investigation on the chirality of fenticonazole is reported in this study. rac-Fenticonazole was resolved by HPLC and by capillary electrophoresis (CE). The chiral stationary phase (CSP), used in HPLC, was Daicel OD-H, a commercial phase, which allowed the separate collection of the two enantiomers. The chiral selectors used for CE were some cyclodextrin derivatives. The analysis time required from CE was about the half the HPLC enantioseparation time. The biological activity of the rac-mixture and each individual enantiomer was tested against Cryptococcus neoformans and two Aspergillus nidulans strains. The minimum inhibitory concentration (MIC) evaluation showed that the eutomer was the enantiomer chromatographically more retained and had a longer migration time in the electrophoretic enantioseparation. The CD spectrum of the eutomer showed a positive Cotton effect.