Direct Resolution of Enantiomers in Column Liquid Chromatography

New chiral high-performance liquid chromatographic methodology used for the pharmacokinetic evaluation of dexfenfluramine

A new chiral high-performance liquid chromatographic (HPLC) method utilizing ultraviolet (UV) detection has been developed for determining plasma and urinary concentrations of d-fenfluramine and its major metabolite d-norfenfluramine, while being able to determine the possible presence of l-fenfluramine after oral administration of enantiopure d-fenfluramine hydrochloride. Sensitivity, stability, and specificity were enhanced by derivatizing the extracted analytes with 3,5-dinitrophenylisocyanate while utilizing a Pirkle-type chiral recognition approach. In vitro and in vivo statistical data are analogous. Overall plasma inter-assay precision was less than 7% with a minimum quantitation limit of 10 ng/ml. Overall urine inter-assay precision was also less than 7% with a minimum quantitation limit of 25 ng/ml.

Chromatographic separation of the diastereomers of a dihydropyridine-type calcium channel antagonist as the bis-3,5-dinitrobenzoates

The diastereomeric components of the calcium channel antagonist RS-93522-004 are separated as the bis-3,5-dinitrobenzoate ester by reversed-phase high-performance liquid chromatography. The diastereomer ratio determination is shown to be precise, accurate and sensitive, and is not affected by the reaction yield of the derivatization with 3,5-dinitrobenzoyl chloride. The four individual stereoisomers of RS-93522-004 were independently shown to have equal reactivity toward 3,5-dinitrobenzoyl chloride.

Thin-layer chromatographic enantiomeric resolution via ligand exchange

A thin-layer chromatographic technique for the separation of proteinogenic and non-proteinogenic amino acids, dipeptides and alpha-hydroxy acids is described. Other examples are given from the field of alpha-methyl, N-alkyl and halogenated amino acids. The separation of the enantiomers is achieved, without derivatization, by means of ligand exchange on a reversed-phase silica gel as stationary phase, which is covered with a chiral selector (proline derivative). The resolution is so good that the respective enantiomers can be determined at trace levels (greater than or equal to 0.25%). The proposed method is simple, inexpensive and needs no sophisticated instruments.

Separation of fatty acid ester and amide enantiomers by high-performance liquid chromatography on chiral stationary phases

The enantiomeric resolution of a series of chiral fatty acid epoxide and alpha-substituted palmitic acid analogues was examined by high-performance liquid chromatography on chiral stationary phases. The compounds were chromatographed as ester or amide derivatives on commercially available stationary phases that consisted of (R)-N-(3,5-dinitrobenzoyl)phenylglycine either covalently or ionically bonded to aminopropylsilica gel. Factors affecting separation included hydrocarbon chain length of the fatty acid, the type of substituents attached to the chiral center, the type of derivative, and column temperature. Effects of sample size, mobile phase composition, column type, and flow-rate on the resolution and separation factor values were also explored.

A high-performance liquid chromatographic method for the enantiomeric analysis of the enzymatically synthesized coenzyme A ester of 2-tetradecylglycidic acid

The enantiomeric composition of an enzymatically synthesized sample of the coenzyme A ester of 2-tetradecylglycidic acid (TDGA-CoA) was determined by the use of high-performance liquid chromatography with a chiral stationary phase. The stationary phase was commercially available and consisted of (R)-N-(3,5-dinitrobenzoyl)phenylglycine covalently bonded to aminopropyl silica gel. Analysis was performed using the phenacyl derivative of 2-tetradecylglycidic acid (TDGA), obtained by mild hydrolysis of the TDGA-CoA followed by reaction of the extracted TDGA with phenacyl chloride. Chromatography showed the enantiomeric purity of TDGA-CoA, synthesized in a rat liver microsomal enzyme mixture over a 2-h period, to be a 15.6:1 ratio of the R:S enantiomers (88% ee). The result demonstrates the stereoselectivity of the long-chain fatty acid-coenzyme A synthetase for chiral fatty acid epoxide, TDGA.

Chromatographic resolution of enantiomers using albumin as complexing agent in the mobile phase

Enantiomers of carboxylic acids have been separated with albumin as a chiral complexing agent in the mobile phase. Stereoselective separation has been obtained for different types of acids, in some cases with very high separation factor, as shown for di-p-toluoyltartaric acid (alpha(s) = 5.8). The stereoselectivity and retention properties depend on pH and on the concentration of albumin in the mobile phase. Retention can also be regulated by modifying the nature of the solid phase as well as by the use of additives in the mobile phase. Acids with low molar absorptivity, or with absorbance in the same wavelength range as albumin, can be detected by an indirect technique based on the use of a cationic mobile phase additive, such as 1-ethylquinolinium, which has high UV-absorptivity at a wavelength remote from that of albumin.

Trends in Analytical Scale Separations

Although gas chromatography has been called a mature technology, advances are still being made in the development of faster columns as well as detectors which yield more information on solutes. In liquid chromatography short columns packed with 3-micrometer particles are already popular for fast analyses, while long narrow-bore columns are being developed for high-resolution separations. An increasing range of detection modes is enhancing the problem-solving capabilities of liquid chromatography. Electrophoresis continues to be of central importance to molecular biology. Computer-aided analysis and display of electropherogram patterns is helping researchers to better comprehend the wealth of data from two-dimensional electrophoresis. A new mode of two-dimensional electrophoresis is permitting separation of whole chromosomes containing as many as 3 million base pairs.

Stereoselective D- and L-amino acid analysis by high-performance liquid chromatography

This paper describes a gradient system for separating D- and L-isomers of Dns-amino acids by mixed chelate complexation through the addition of histidine methyl ester and copper sulfate to the mobile phase. Most of the biologically important amino acids were separated in a single analysis. With a simple solvent gradient consisting of increasing concentrations of acetonitrile in L-histidine methyl ester buffer all the common amino acids were resolved except cysteine and all optical isomers were resolved except those of threonine, alanine and proline. Analysis time was 90 min. Use of this system for determining non-protein amino acids in human cerebrospinal fluid showed the amino acids to be L-isomers, as expected. The pattern in fluid from a patient with bacterial meningitis was different from that of most of the others.

Recent advances in methods of direct optical resolution

Very great advances have been made in the field of direct optical resolution of organic compounds by chromatographic techniques. Chiral capillary gas chromatography now permits a determination of the enantiomeric composition of a few nanograms of a compound present in a mixture of many others. Coupled with high resolution mass spectrometry the technique will additionally permit structural elucidation; of great interest in pheromone research and related areas. Analytical separations of enantiomers are now also carried out by high-performance liquid chromatography (HPLC) methods based on a variety of principles. Basically, two main types are used, differing as to whether the mobile phase has to be a chiral medium or not. Two-dimensional HPLC, whereby compounds separated on a non-chiral column are progressively and automatically transferred to a chiral column for optical resolution, has been used successsfully for chiral amino acid separations. Many different chiral sorbents for preparative LC and HPLC resolutions have been prepared; some of these are now used in columns capable of producing pure enantiomers from a given racemate at a rate of the order of one gram/hour in continuous, automatic HPLC procedures. Apart from all important applications of these results of optical resolution technology, an increased knowledge of the underlying chiral recognition phenomena responsible for enantioselection has also been achieved.

Direct resolution of alpha-amino acid enantiomers by ligand exchange: stereoselection mechanism on silica packings coated with a chiral polymer

A chiral polymer easily obtained by treatment of a linear polyacrylamide with L-proline was adsorbed on a silica support treatment of a linear polyacrylamide with L-proline. After complexation with copper(II), and in spite of a rather low efficiency due to slow ligand exchange, such packing are very convenient for the resolution of amino acid enantiomers eluted in water (relative retention, k'L/k'D, higher than 2 may be observed). Other eluents may also be used. The chiral polymer was studied in solution by potentiometry and nuclear magnetic resonance spectroscopy (13C relaxation measurement). A model of the complex involved in the stereoselection is proposed which accounts for the chromatographic results obtained.

Separation of enantiomeric amines by ion-pair chromatography

A high-performance liquid chromatographic method for the separation of optical isomers (enantiomers) of amines is described. It is based on ion-pair chromatography with a chiral counter ion in a system with an organic mobile phase and an adsorbing stationary phase. The method has been applied to enantiomers of 1-aryloxy-3-isopropylamine-2-propanol derivatives (alprenolol, metoprolol, propranolol) which are completely resolved with (+)-10-camphorsulphonate as the counter ion. Studies of the influence of the counter-ion structure and the mobile phase composition are presented.