Use Of Chiral Stationary Phases Research Articles

Enantioselective Analysis of Mexiletine Using Chromatographic Techniques: A Review

Background: Mexiletine belongs to the β-amino-aryl-ether group of pharmaceutical and applied in the diagnosis of antiarrhythmics, allodynia, and myotonic disorders. In its chemical structure, it possesses a chiral center and practiced in the form of the racemic mixture. The production and accessibility of mexiletine have accompanied with a meaningful development in awareness of its pharmacologic actions. But in clinical arrhythmias and binding experiments on cardiac sodium channels, the (R)-enantiomer of mexiletine is more potent than the (S)-enantiomer. Also, (S)-enantiomer is further active in the diagnosis of allodynia than the (R)-enantiomer. Methods: During the last two decades, chromatographic techniques such as HPLC, and GC coupled with mass spectrometry or field ionization detector was used for the stereoselective analysis of MEX enantiomers. Results: The direct enantioresolution deal with the use of chiral stationary phases (CSPs) with or no pre-derivatization which depend on a chromophoric entity in the racemates whereas indirect HPLC process involved the use of chiral derivatization reagents (CDR) for the synthesis of diastereomeric derivatives of racemates. Different techniques have their strengths and weaknesses. Conclusion: Regulation of enantiomeric purity and estimation of particular enantiomers of drug molecules stays an essential topic for therapeutic, diagnostic, and regulatory uses and to promote a precise assessment of the hazards to human health by false enantiomers. This review aims to offer a systematic survey of the analytical methods (chromatography based) used in the enantioselective analysis of MEX developed in the last two decades (the year 2000 onwards).

Liquid Chromatographic Enantioseparations Utilizing Chiral Stationary Phases Based on Crown Ethers and Cyclofructans.

Natural compounds can exist in different forms, where molecules possessing chirality play an essential role in living organisms. Currently, one of the most important tasks of modern analytical chemistry is the enantioseparation of chiral compounds, in particular, the enantiomers of compounds having biological and/or pharmaceutical activity. Whether the task is to analyze environmental or food samples or to develop an assay for drug control, well-reproducible, highly sensitive, stereoselective, and robust methods are required. High-performance liquid chromatography best meets these conditions. Nevertheless, in many cases, gas chromatography, supercritical fluid chromatography, or capillary electrophoresis can also offer a suitable solution. Amino acids, proteins, cyclodextrins, derivatized polysaccharides, macrocyclic glycopeptides, and ion exchangers can serve as efficient selectors in liquid chromatography, and they are quite frequently applied and reviewed. Crown ethers and cyclofructans possessing similar structural characteristics and selectivity in the enantiodiscrimination of different amine compounds are discussed less frequently. This review collects information on enantioseparations achieved recently with the use of chiral stationary phases based on crown ethers or cyclofructans, focusing on liquid chromatographic applications.

Chirality: An Important Phenomenon Regarding Biosynthesis, Perception, and Authenticity of Flavor Compounds.

Chirality plays an important role in flavor research. This will be outlined using selected examples from the key areas analysis, authenticity assessment, biogenesis, and odor perception. Developments of analytical techniques, in particular the use of chiral stationary phases in capillary gas chromatography, enabled the determination of naturally occurring configurations of chiral volatiles at trace levels. Thus, knowledge of pathways and enzymes involved in the biogenesis of chiral substances was acquired, and enantioselective analysis has become a prominent tool in the authenticity assessment of flavorings. Increasing information is also available on structure-odor relationships of chiral flavor compounds and the influence of their configurations on odor thresholds and odor properties. A substantial extension of these data sets and a further understanding of the role of chirality in the perception of aroma compounds is expected from studies on the enantioselectivity of odorant receptor systems. Developments in these areas will be put into a historical perspective, recent progress will be emphasized, and data gaps will be described.

Efficient preparative separation of 6-(4-aminophenyl)-5-methyl-4, 5-dihydro-3(2H)-pyridazinone enantiomers on polysaccharide-based stationary phases in polar organic solvent chromatography and supercritical fluid chromatography.

6-(4-Aminophenyl)-5-methyl-4,5-dihydro-3(2H)-pyridazinone is a key synthetic intermediate for cardiotonic agent levosimendan. Very few studies address the use of chiral stationary phases in chromatography for the enantioseparation of this intermediate. This study presents two efficient preparative methods for the isolation of (R)(-)-6-(4-aminophenyl)-5-methyl-4,5-dihydro-3(2H)-pyridazinone in polar organic solvent chromatography and supercritical fluid chromatography using polysaccharide-based chiral stationary phases and volatile organic mobile phases without additives in isocratic mode. Under optimum conditions, Chiralcel OJ column showed the best performance (α=1.71, Rs=5.47) in polar organic solvent chromatography, while Chiralpak AS column exhibited remarkable separations (α=1.81 and Rs=6.51) in supercritical fluid chromatography with an opposite enantiomer elution order. Considering the sample solubility, runtime and solvent cost, the preparations were carried out on Chiralcel OJ column and Chiralpak AS column (250× 20mm i.d.; 10µm) in polar organic mode and supercritical fluid chromatography mode with methanol and CO2 /methanol as mobile phases, respectively. By utilizing the advantages of chromatographic techniques and polysaccharide-based chiral stationary phases, this work provides two methods for the fast and economic preparation of (R)(-)-6-(4-aminophenyl)-5-methyl-4,5-dihydro-3(2H)-pyridazinone, which are suitable for the pharmaceutical industry.

Laboratory-Scale Preparative Enantioseparations of Pharmaceutically Relevant Compounds on Commercially Available Chiral Stationary Phases for HPLC.

In response to the outburst of research in the field of synthetic medicinal chemistry, enantioselective chromatography methods based on the use of chiral stationary phases (CSPs) found immediate acceptance as the elective choice for the analytical determinations of the enantiomeric purity of synthetic compounds. In contrast to an initial scepticism, also the preparative-scale applications are gaining increasing recognition as a powerful alternative to enantioselective synthesis for the supply of pure enantiomers of bioactive compounds. The increasing success of liquid chromatography methods has been made possible thanks to the development of highly efficient CSPs allowing the enantioresolution of practically all the chemical classes of chiral compounds. However, only few CSPs are really suitable for preparative- scale applications, being the loading capacity is the major concern for preparativescale enantioseparations. The cellulose- and amylose-based CSPs present the highest loading capacity and enantiodiscrimination power, which makes these CSPs the most versatile and applicable for preparative-scale applications in all the applicable elution modes (reversedphase, normal-phase, and with polar-organic or polar-ionic eluents). However, also other types of CSPs have been successfully employed at this regard (brush-type phases, polyacrylamide and cross-linked di-allyltartardiamide phases as well as cyclodextrin, and glycopeptide containing phases). Several instrumental methods exist for the determination of the absolute configuration of organic compounds in absence of known enantiopure reference standards. The most widely known are X-ray crystallography, followed by chirooptical methods [e.g., electronic and vibrational circular dichroism (ECD and VCD, respectively)] and nuclear magnetic resonance (NMR) spectroscopy. All these aspects will be treated in the review.

Ultrafast Chiral Chromatography as the Second Dimension in Two-Dimensional Liquid Chromatography Experiments.

Chromatographic separation and analysis of complex mixtures of closely related species is one of the most challenging tasks in modern pharmaceutical analysis. In recent years, two-dimensional liquid chromatography (2D-LC) has become a valuable tool for improving peak capacity and selectivity. However, the relatively slow speed of chiral separations has limited the use of chiral stationary phases (CSPs) as the second dimension in 2D-LC, especially in the comprehensive mode. Realizing that the recent revolution in the field of ultrafast enantioselective chromatography could now provide significantly faster separations, we herein report an investigation into the use of ultrafast chiral chromatography as a second dimension for 2D chromatographic separations. In this study, excellent selectivity, peak shape, and repeatability were achieved by combining achiral and chiral narrow-bore columns (2.1 mm × 100 mm and 2.1 mm × 150 mm, sub-2 and 3 μm) in the first dimension with 4.6 mm × 30 mm and 4.6 mm × 50 mm columns packed with highly efficient chiral selectors (sub-2 μm fully porous and 2.7 μm fused-core particles) in the second dimension, together with the use of 0.1% phosphoric acid/acetonitrile eluents in both dimensions. Multiple achiral × chiral and chiral × chiral 2D-LC examples (single and multiple heart-cutting, high-resolution sampling, and comprehensive) using ultrafast chiral chromatography in the second dimension are successfully applied to the separation and analysis of complex mixtures of closely related pharmaceuticals and synthetic intermediates, including chiral and achiral drugs and metabolites, constitutional isomers, stereoisomers, and organohalogenated species.

Resolution of enantiomers of bupropion and its metabolites by liquid chromatography.

Bupropion, a tricyclic aminoketone, is used primarily in the treatment of depression, the management of patients with bipolar and schizo-affective disorder, and the treatment of Parkinson's disease. Bupropion is marketed as a racemate, but the racemic mixture is known to have several disadvantages while the two isomers of bupropion and its metabolite differ significantly in their pharmacological activities. Therefore, the stereoselective determination of the drug enantiomers in pharamaceutical dosages, plasma or urine is of potential clinical and analytical importance. Different chromatographic methods have been employed for the separation of the two enantiomers. This is the first attempt to review the methods of enantiosepartion of bupropion using both direct and indirect approaches in both HPLC and TLC. The review presents a detailed discussion on the use of chiral stationary phase (based on polysaccharide, α1 acid glycoprotein and ovomucoid column) and chiral derivatizing reagents (based on isothiocyanate and cyanuric chloride) along with TLC separation of bupropion enantiomers using ligand exchange and impregnation methods. The focus is also on the separation mechanism for enantioresolution using the various methods described herein.

Lipase-immobilized magnetic chitosan nanoparticles for kinetic resolution of (R,S)-ibuprofen

Abstract Chitosan (CS)–poly[N-benzyl-2-(methacryloxy)-N,N-dimethylethanaminium bromide] coated magnetic nanoparticles were prepared by co-precipitation method via epichlorohydrin CS cross-linking reaction and were used in the kinetic resolution of (R,S)-ibuprofen by enantioselective esterification. Enzyme immobilized onto the surface of the new magnetic supports with the use of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC)/N-hydroxysulfo-succinimide sodium salt (sulfo-NHS) procedure demonstrated high catalytic activity that enabled us to obtain (S)-methyl ester of ibuprofen with high enantioselectivity (E = 50.6). The chiral compounds that resulted from the application of magnetic nanoparticles were analyzed with the use of chiral stationary phases. It should be emphasized that the main advantage of the support is the possibility to magnetically recovery and effective separation (even up to 5 s) from the reaction mixture with the use of magnet. The properties of magnetic particles allow for better optimization and may reduce the total costs of the esterification reaction of ibuprofen. Moreover, the application of lipase-immobilized magnetic supports enables to maintain high enantioselective activity after repeated use.

ChemInform Abstract: Recent Theoretical and Experimental Advances in the Electronic Circular Dichroisms of Planar Chiral Cyclophanes

AbstractReview: 63 refs.

Separation of triphenyl atropisomers of a pharmaceutical compound on a novel mixed mode stationary phase: A case study involving dynamic chromatography, dynamic NMR and molecular modeling

Analysis of atropisomers is of considerable interest in the pharmaceutical industry. For complex chiral molecules with several chiral centers hindered axial rotation can lead to formation of interconverting diastereomers that should be separable on achiral stationary phases. However, achieving the actual separation may be difficult as the on-column separation speed must match or be faster then the rate of isomer interconversion. Often, this requirement can be satisfied by using low-temperature conditions and by improving selectivity via use of chiral stationary phases. In the current study, we present an alternative approach utilizing an Obelisc R column, a novel mixed mode stationary phase that provided acceptable separation of triphenyl atropisomers inside a conventional HPLC temperature range. The separation was investigated under various chromatographic conditions. The interconversion chromatograms exhibited classic peak–plateau-peak behavior indicating the simultaneous atropisomer separation and interconversion. The elution profiles were integrated in order to deconvolute the peak areas of the “pure” (non-exchanged) and interconverted species; these data were used to obtain kinetic information. Analysis of retention data rendered thermodynamic information on the mechanism of retention and selectivity. Chromatographic kinetic data were complemented with variable-temperature NMR and molecular modeling studies, which provided additional support and insights into the energetics of the interconversion process.

Enantiomeric separation of 2‐arylpropionic acid nonsteroidal anti‐inflammatory drugs by HPLC with hydroxypropyl‐β‐cyclodextrin as chiral mobile phase additive

The enantio-separations of eight 2-arylpropionic acid nonsteroidal anti-inflammatory drugs (2-APA NSAIDs) were established using reversed-phase high-performance liquid chromatography with hydroxypropyl-beta-cyclodextrin (HP-beta-CD) as chiral mobile phase additive for studying the stereoselective skin permeation of suprofen, ketoprofen, naproxen, indoprofen, fenoprofen, furbiprofen, ibuprofen and carprofen. The effects of the mobile phase composition, concentration of HP-beta-CD and column temperature on retention and enantioselective separation were investigated. With 2-APA NSAIDs as acidic analytes, the retention times and resolutions of the enantiomers were strongly related to the pH of the mobile phase. In addition, both the concentration of HP-beta-CD and temperature had a great effect on retention time, but only a slight or almost no effect on resolutions of the analytes. Enantioseparations were achieved on a Shimpack CLC-ODS (150 x 4.6 mm i.d., 5 microm) column. The mobile phase was a mixture of methanol and phosphate buffer (pH 4.0-5.5, 20 mM) containing 25 mM HP-beta-CD. This method was flexible, simple and economically advantageous over the use of chiral stationary phase, and was successfully applied to the enantioselective determination of the racemic 2-APA NSAIDs in an enantioselective skin permeation study.

Supramolecular Systems-Based HPLC for Chiral Separation of Beta-Adrenergics and Beta-Adrenolytics in Drug Discovery Schemes

Increasing amount of data considering polymorphism, splice variants and various affinity states of beta-adrenoceptors has resulted in a new range of opportunities for enantiopure beta-adrenergic and beta-adrenolytic drug discovery and continuous development of reliable high-throughput screening procedures enabling tissue specific pharmacological evaluation of these drugs. Design and fast pharmacological profiling of single enantiomeric molecules combining beta-adrenoceptor affinity with other pharmacophores is also still challenging ability. As the use of chiral stationary phases in HPLC has particularly benefited from results of supramolecular chemistry, this review summarises recent achievements provided by this technique in deciphering of enatiorecognition phenomena affecting pharmacological selectivity of beta-adrenergics and beta-adrenolytics and modifying the efficiency of currently proposed beta-adrenoceptor-targeted therapies. Detailed characteristic of chiral separation performance of these drugs in the range of available supramolecular HPLC systems has also been presented.

Combination of HPLC “Inverted Chirality Columns Approach” and MS/MS Detection for Extreme Enantiomeric Excess Determination Even in Absence of Reference Samples. Application to Camptothecin Derivatives

An original, extremely sensitive and selective HPLC-MS/MS technique for the identification and determination of the minor enantiomer in nonracemic mixtures, even when only one enantiomer is available as reference, is described. The method is based on the so-called "inverted chirality columns approach" (ICCA) and consists of the use of chiral stationary phases (CSPs) available in both enantiomeric forms: in fact, inversion of the elution order for a pair of enantiomers is observed in response to the change in column chirality. This offers two key advantages: first, it is possible to demonstrate the potential enantioselectivity of the system by generating a virtual racemate, and second, it permits the choosing of the right column chirality for trace determination. Combination with MS/MS detection affords high specificity allowing not only high sensitivity (down to 0.0025% of the minor enantiomer) but also unequivocal peak identification in complex mixtures. Applications to semisynthetic derivatives of camptothecin, endowed with antitumor activity, are reported. Moreover, applicability of ICCA is not limited to this class of molecules but generates universal support. Its use might also be extended to other classes of compounds by using other CSPs, available in both enantiomeric forms.

The role of β-cyclodextrin and hydroxypropyl β-cyclodextrin in the secondary chemical equilibria associated to the separation of β-carbolines by HPLC

The use of cyclodextrins (CDs) in HPLC as mobile phase additives provides a flexible alternative for the separation of chemically related compounds because these separations can be performed on conventional columns and are economically advantageous over the use of chiral stationary phases. The present paper describes the influence of the presence of β-cyclodextrin (β-CD) and 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) on the separation of the β-carboline alkaloids norharmane, harmane and harmine. The nature of the stationary phase (reverse phases C1 and C18) affects the chromatographic separations and also the stability of the inclusion complexes that are developed. The changes in the proportion of the organic solvents at constant concentration of CDs (3 mM for β-CD and 15 mM for HPβ-CD) modify the retention factors (k′) for all alkaloids studied. The nature of the organic solvent in the mobile phase also changes the chromatographic parameters. The logarithm of the capacity factor (k′) is linearly increased with the proportion of water in the hydro-organic mobile phase (ethanolic or methanolic) but the slopes obtained vary depending on the CD added to the mobile phase. The role of competitive equilibria, i.e., chromatographic distribution and inclusion complexes formation is discussed.

Capillary electrochromatographic chiral separations with potential for pharmaceutical analysis

The use of capillary electrochromatography as a chiral separation technique for pharmaceutical applications is reviewed. Publications of the past 10 years that provide a potential practical application in pharmaceutical analysis are considered. Method development or validation, separation strategies, and potential routine analysis by the methods/applications cited are the main subjects on which we focused our attention. The indirect chiral separation method was only used once in CEC mode. In the direct chiral separations, the use of chiral stationary phases was obviously preferred over the use of chiral mobile phases with non-chiral stationary phases. Amongst the chiral stationary phases, those based on macrocyclic antibiotics and polysaccharide selectors were the most frequently used. Monolithic stationary phases also have several applications, but not so extended as those with packed capillary electrochromatography. The considered papers not only describe the applicability of the technique for relatively large sets of chiral analytes, they also showed that various types of stationary phases can be produced in-house in a simple manner. However, to survive as a mature separation technique, considerable time and effort are still needed to solve some disadvantages currently characterizing capillary electrochromatography.

Exhaustively substituted bile acids as chiral selectors for enantioselective chromatography: Aim, use and perspectives

The use of chiral stationary phases (CSPs) obtained from cholic and deoxycholic acid derivatives in the HPLC resolution of racemic compounds is presented. The CSPs containing arylcarbamoyl derivatives of bile acids show enantiodiscriminating capabilities depending on the electronic character of the aryl substituents: the CSPs obtained starting from heteroderivatized selectors, i.e. bile acid derivatives containing both π-acidic and π-basic arylcarbamoyl moieties, show enantiodiscriminating capabilities strongly dependent on the arrangement of the electronically different arylcarbamates on the cholestanic backbone. The CSPs obtained starting from deoxycholic acid derivatives possessing both arylamido and arycarbamoyl substituents show enantiodiscriminating capabilities restricted to the resolution of benzodiazepine derivatives. Again, the enantioresolution properties depend not only on the electronic nature of the aromatic substituents but also on their arrangement on the cholestanic backbone. The comparison among the different families of bile acid based CSPs allows us to find likeness and differences in the enantiorecognition mechanism exhibited by the different chiral selectors.

Enantiomer separation of small amounts of amino acids and hydroxy acids by HPLC using chiral stationary phases and achiral derivatizing reagents.

The enantiomer separation of small amounts of amino acids and hydroxy acids by HPLC using chiral stationary phases and achiral derivatizing reagents was examined. Racemic dabsyl amino acids and pyrene-l-carbonyl hydroxy acids were well resolved on chiral stationary phases [VI] and [V], and excellent values of detection limits were obtained. The joint use of chiral stationary phases and achiral derivatizing reagents was found to be very effective for the separation and determination of small amounts of amino acid and hydroxy acid enantiomers by HPLC.

Enantiomeric derivatization for biomedical chromatography.

Derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers are reviewed, with emphasis on the literature published in the last 10 years. Three main aspects of chiral derivatization are discussed. (a) Enantiomers containing suitable functional groups (amino, carboxyl, hydroxyl, epoxy, etc.) are transformed into covalently bonded diastereomeric derivatives using homochiral derivatizing agents. The diastereomers formed (esters, amides, urethanes, urea and thiourea, etc., derivatives) can be separated on achiral stationary phases. The derivatization reactions often afford further advantages, such as the improvement of chromatographic properties and the detectability of the solutes using UV and fluorimetric detectors. (b) Covalent but achiral derivatization is often necessary even with the use of chiral stationary phases enabling in principle direct enantioseparations (Pirkle-type columns, cyclodextrin-bonded phases, glycoprotein column and functionalized cellulose columns). The main goals of these derivatization reactions (which are analogous to those discussed above), are to introduce functional groups into the molecule of the enantiomers that improve the possibilities for chiral interactions or block functional groups to avoid non-specific interactions. (c) In the broader sense, the dynamic formation of diastereomers using chiral mobile phase additives (cyclodextrins, various reagents to form diastereomeric ion pairs, adducts, mixed metal complexes) can also be considered to be chiral derivatization reactions and is therefore briefly discussed also.

Biomedical applications of chiral liquid chromatography

Many drugs are racemic and therefore much effort has to be devoted towards the stereoselective synthesis of the most effective or less harmful component of a racemic mixture. High performance liquid chromatography will play an important role in the clinical analysis of racemic drugs in anticipation of regulations that are currently being discussed and are expected to be enforced by the end of this decade. In this review a number of methods for chiral resolution are outlined. These include the formation of diastereoisomers and the use of chiral stationary phases or chiral mobile phase additives.

CO2 Supercritical Fluid Chromatography with Chiral Stationary Phases: A Promising Coupling for the Resolution of Various Racemates

The use of chiral stationary phases (CSPs) in supercritical fluid chromatography (SFC) with carbon dioxide is reviewed. The state-of-the-art for capillary and packed columns is discussed. However, because most of the initial reports are about packed columns, we more thoroughly investigate the resolution of optical isomers on various Pirkle-type CSPs (DNBPG, thio-DNBTyr-A or -E, DACH-DNB, etc.), cyclodextrin CSPs (Cyclobond), and polymer CSPs (ChiralCel OB and ChiralPak OT(+)). Several discrepancies in the LC and SFC behavior of these CSPs show that carbon dioxide-alcohol and hexane-alcohol mixtures are not interchangeable as mobile phases: solubilities of apolar compounds are lower in carbon dioxide than in hexane and the solvation state of both solutes and CSPs differs. For several applications, carbon dioxide is found to act like methylene chloride in regard to stereoselectivity. As a rule, the SFC-CSP coupling is a promising technique for the resolution of racemates: It usually leads to higher resolutions per unit of time than LC and sometimes allows new types of chiral separations.