Reviewing mobile phases used on Chiralcel OD through an application of data mining tools to CHIRBASE database

Chiral stationary phases and applications in gas chromatography

Chiral compounds are ubiquitous in nature and play a pivotal role in biochemical processes, in chiroptical materials and applications, and as chiral drugs. The analysis and determination of the enantiomeric ratio (er) of chiral compounds is of enormous scientific, industrial, and economic importance. Chiral separation techniques and methods have become indispensable tools to separate chiral compounds into their enantiomers on an analytical as well on a preparative level to obtain enantiopure compounds. Chiral gas chromatography and high-performance liquid chromatography have paved the way and fostered several research areas, that is, asymmetric synthesis and catalysis in organic, medicinal, pharmaceutical, and supramolecular chemistry. The development of highly enantioselective chiral stationary phases was essential. In particular, the elucidation and understanding of the underlying enantioselective supramolecular separation mechanisms led to the design of new chiral stationary phases. This review article focuses on the development of chiral stationary phases for gas chromatography. The fundamental mechanisms of the recognition and separation of enantiomers and the selectors and chiral stationary phases used in chiral gas chromatography are presented. An overview over syntheses and applications of these chiral stationary phases is presented as a practical guidance for enantioselective separation of chiral compound classes and substances by gas chromatography.

Role of Stereoselective Assays in Bioequivalence Studies of Racemic Drugs: Have We Reached a Consensus?

The existence of stereoselectivity in metabolism and drug disposition, coupled with the existence of genetic polymorphisms and modulation of enantiomeric kinetics via special delivery systems, provides some compulsion to assess bioequivalence using stereoselective data. However, examination of the literature suggests that nonstereoselective data are commonly used for the bioequivalence assessment of drug racemates.

Total syntheses of racemic and natural glycinol

Total syntheses of racemic and (-)-glycinol (1) are described. A Wittig reaction produced the isoflav-3-ene from which a Sharpless dihydroxylation introduced either the racemic or enantiomeric 6a-hydroxy group. A 5.5% overall yield of racemic material was obtained after 12 steps. A method was devised for a one-pot switch of protecting groups masking a sensitive resorcinolic para-functionality, and conditions were optimized to prompt spontaneous closure of the pterocarpanolic dihydrofuran upon subsequent exposure of its ortho-functionality. These improvements eliminated two steps and increased the overall yield to 9.8% during production of the natural enantiomer.

Simple method for the prediction of the separation of racemates with high-performance liquid chromatography on Whelk-O1 chiral stationary phase

A simple method for the prediction of whether or not a racemate can be separated on a Whelk-O1 chiral stationary phase has been developed. In this approach, molecules are represented by counting the number of atom types of the neighbors spheres of the chiral center. A decision tree is then used to decide based on a few of these atom count descriptors whether a given racemate can be separated. High values of correct prediction were obtained, namely with more than 94% for training sets and of about 90% for cross-validation results. The same rate of correct prediction was also obtained on an external data set. The descriptors can be rapidly and easily retrieved by just counting the atom types around the chiral center by inspecting the chemical diagram of the molecule. Furthermore, the decision tree model can be applied through the use of a small set of rules that eventually predicts whether or not a racemate is separated. Due to its computational simplicity, the procedure is of interest for experimentalists that need to make rapid assessment of the separation without having to program or input complex formulas.

Enantiomeric separation and determination of absolute stereochemistry of asymmetric molecules in drug discovery—Building chiral technology toolboxes

The application of Chiral Technology, or the (extensive) use of techniques or tools for the determination of absolute stereochemistry and the enantiomeric or chiral separation of racemic small molecule potential lead compounds, has been critical to successfully discovering and developing chiral drugs in the pharmaceutical industry. This has been due to the rapid increase over the past 10-15 years in potential drug candidates containing one or more asymmetric centers. Based on the experiences of one pharmaceutical company, a summary of the establishment of a Chiral Technology toolbox, including the implementation of known tools as well as the design, development, and implementation of new Chiral Technology tools, is provided.

Chiral resolution of the epoxyeicosatrienoic acids, arachidonic acid epoxygenase metabolites

An HPLC method for the chiral analysis of the four regioisomeric epoxyeicosatrienoic acids (EETs) is described. The cytochrome P450 arachidonic acid epoxygenase metabolites are resolved, without the need for derivatization, by chiral-phase HPLC on a Chiralcel OJ column. Application of this methodology to the analysis of the liver endogenous EETs demonstrates stereospecific biosynthesis and corroborates the role of cytochrome P450 as the endogenous arachidonic acid epoxygenase.

Analyte Templating: Enhancing the Enantioselectivity of Chiral Selectors upon Incorporation into Organic Polymer Environments

A simple strategy for preserving and enhancing the chiral recognition capacity of polymer-embedded chiral selectors is proposed, capitalizing on a temporary blockage of the receptor binding site with tightly binding analytes during the polymerization process. We demonstrate that the copolymerization of a quinine tert-butylcarbamate selector monomer with chiral (and achiral) 3,5-dichlorobenzoyl amino acids allows one to control to a certain extent the binding characteristics of the resultant polymeric chiral stationary phases. The structural and stereochemical requirements of the templating analytes for maximizing the chiral recognition capacity of the polymer-embedded selectors are probed. The chromatographic chiral recognition characteristics of the analyte-templated polymeric chiral stationary phases are analyzed with respect to binding capacities and affinities and compared to those obtained with a conventional silica-based surface-grafted reference material. Changes in substrate-specific enantioselectivity originating from analyte templating are also addressed.

Probability rule for chiral recognition

Molecular Chirality is of central interest in biological studies because enantiomeric compounds, while indistinguishable by most inanimate systems, show profoundly different properties in biochemical environments. Enantioselective separation methods, based on the differential recognition of two optical isomers by a chiral selector, have been amply documented. Also, great effort has been directed towards a theoretical understanding of the fundamental mechanisms underlying the chiral recognition process. Here we report a comprehensive data examination of enantio separation measurements for over 72000 chiral selector-select and pairs from the chiral selection compendium CHIRBASE. The distribution of alpha = k'(D)/k'(L) values was found to follow a power law, equivalent to an exponential decay for chiral differential free energies. This observation is experimentally relevant in terms of the number of different individual or combinatorial selectors that need to be screened in order to observe alpha values higher than a preset minimum. A string model for enantiorecognition (SMED) formalism is proposed to account for this observation on the basis of an extended Ogston three-point interaction model. Partially overlapping molecular interaction domains are analyzed in terms of a string complementarity model for ligand-receptor complementarity. The results suggest that chiral selection statistics may be interpreted in terms of more general concepts related to biomolecular recognition.

Polysaccharide-based chiral phase under polar organic mode of elution in the determination of the enantiomeric purity of emtricitabine an anti-HIV analogue nucleoside

This work reports the separation of FTC enantiomers using an amylose tris[(S)-1-phenylethylcarbamate] coated onto APS-Nucleosil (7 microm particle size, 500 A pore size, 20% w/w, 15 x 0.46 cm ID) chiral column under polar organic elution mode. Good enantioselectivity (alpha=1.9) with excellent enantioresolution (R(S)=3.3) was achieved by the use of methanol with 0.02% of triethylamine acetate as mobile phase. The method allows the accurate determination of as low as 0.2% of each enantiomer as an impurity. The validated method proved to be reliable and sensitive for the quantification of both enantiomers as impurity in different batches of emtricitabine and beta-D-(+)-FTC.

Enantiomeric resolution of a series of chiral sulfoxides by high-performance liquid chromatography on polysaccharide-based columns with multimodal elution

The enantiomeric resolution of a series of 20 asymmetric sulfoxides was systematically investigated by HPLC using multimodal elution with amylose trisR(S)-1-phenylethylcarbamate], amylose tris(3,5-dimethoxyphenylcarbamate) and amylose and cellulose tris(3,5-dimethylphenylcarbamate) phases. The sulfoxide series was composed of aromatic, olefinic and ketosulfoxides, sulfinyl acids and esters. This work has shown that enantioselectivity and enantioresolution of the polysaccharide-based columns can be achieved by changing the type and composition of the mobile phase, widening the applicability of these chiral phases.

Knowledge-based system for method development of chiral separations with capillary electrophoresis using highly-sulphated cyclodextrins

Method development for chiral separations is not easy because it requires experience and many experimental possibilities can be chosen. In order to help the analyst, a knowledge-based system (KBS) for the rapid determination of experimental parameters, which allow a baseline separation of enantiomers, has been developed. On the basis of own laboratory knowledge, completed with literature data, rules were defined and a KBS was built. Five different techniques are considered in this KBS. This paper describes the capillary electrophoresis (CE) section, in which a strategy has been defined based on the use of highly-sulfated cyclodextrins as chiral selectors. A structured representation of the knowledge and its implementation in Toolbook software is presented.

Enantioseparations by capillary electrochromatography

The review summarizes recent developments in enantioseparations by capillary electrochromatography (CEC). Selected fundamental aspects of CEC are discussed in order to stress those features which may allow the success of this technique in the competitive field of enantioseparations. In addition, the comparative characteristics of the different modes of chiral CEC and the stationary phases are presented. The effects of the characteristics of the stationary and liquid phases and operational conditions on the separation results are discussed. Finally, some future trends are briefly addressed.