Direct enantiomeric separations by high performance liquid chromatography using cyclodextrins

Due to their identical chemical and physical properties, the separation of enantiomers has been considered one of the most difficult challenges in chemistry from both an analytical or a preparative viewpoint. With the development and commercialization of many new or improved chiral stationary phases and chiral additives, interest in enantiomeric separation by HPLC has grown tremendously in the last two decades. Cyclodextrins and modified cyclodextrins are widely used chiral selectors. They are used as either the chiral stationary phases, as chiral mobile phase additives, or as chiral counter-ions. This review describes the historical development of derivatized and underivatized cyclodextrins in HPLC and their various applications.

Preparation of silica gel-bonded amylose through enzyme-catalyzed polymerization and chiral recognition ability of its phenylcarbamate derivative in HPLC

Amylose was prepared by enzymatic polymerization of alpha-D-glucose 1-phosphate dipotassium catalyzed by a phosphorylase using two kinds of the primers derived from maltopentaose, and then it was chemically bonded to silica gel to be used as a chiral stationary phase (CSP) in high-performance liquid chromatography. In method I, maltopentaose was first lactonized and allowed to react with (3-aminopropyl)triethoxysilane to form an amide bond. Amylose chains with a desired chain length and a narrow molecular weight distribution were then constructed by the enzymatic polymerization. The resulting amylose bearing a trialkoxysilyl group at the terminal was allowed to react with silica gel for immobilization. In method II, maltopentaose was first oxidized to form a potassium gluconate at the reducing terminal. After the enzymatic polymerization was performed with the potassium gluconate, the amylose end was lactonized to be immobilized to 3-aminopropyl-silanized silica gel through amide bond formation. Two amylose-conjugated silica gels thus obtained were treated with a large excess of 3,5-dimethylphenyl isocyanate to convert hydroxy groups of amylose to corresponding carbamate residues. The CSP derived through method II was superior in chiral recognition to the CSP derived from method I and showed better resolving power and higher durability against solvents such as tetrahydrofuran compared with a coated-type CSP. Influences of degree of polymerization of amylose, the spacer length between amylose and silica gel, and mobile phase compositions on chiral recognition were investigated.

High-performance liquid chromatographic separation of an HIV-1 reverse transcriptase inhibitor and its enantiomer

This article describes the direct separation of an HIV-1 reverse transcriptase inhibitor and its enantiomer by HPLC on a silica-bonded polyacrylamide (ChiraSpher) column. The column selection was based on specific interactions between the individual enantiomers and the chiral stationary phase. The influence of some chromatographic conditions, such as concentration of the polar modifier in the mobile phase, column flow-rate and column temperature, on column performance was investigated. The separation was applied to the determination of the minor enantiomer in the bulk drug and as low as 0.3% of minor enantiomer was detectable.

Substituent effects on the enantioselective retention of anti-HIV 5-aryl-delta 2-1,2,4-oxadiazolines on R,R-DACH-DNB chiral stationary phase

A series of racemic 3-phenyl-4-(1-adamantyl)-5-X-phenyl- delta 2-1,2,4-oxadiazo lines (PAdOx) were directly resolved by HPLC using a Pirkle-type stationary phase containing N,N'-(3,5-dinitrobenzoyl)-1(R),2(R)-diaminocyclohexane as chiral selector. The more retained enantiomers have S configuration, as demonstrated by X-ray crystallography and circular dichroism measurements. The influence of aromatic ring substituents on enantioselective retention was quantitatively assessed by traditional linear free energy-related (LFER) equations and comparative molecular field analysis (CoMFA). In good agreement with previous findings, the results from this study indicate that the increase in retention (k') is favoured mainly by the phi-basicity and the hydrophilicity of solute, whereas enantioselectivity (alpha) can be satisfactorily modeled by electronic and bulk parameters or CoMFA descriptors. The LFER equations and CoMFA models gave helpful insights into chiral recognition mechanisms.

Computational studies on chiral discrimination mechanism of cellulose trisphenylcarbamate

The calculation of interaction energies between cellulose trisphenylcarbamate (CTPC) and enantiomers of (+/-)-trans-stilbene oxide (1) and (+/-)-trans-1,2-diphenylcyclopropane (2) was performed using QUANTA/CHARMm and MOLECULAR INTERACTION programs to gain an insight into the chiral recognition mechanism of phenylcarbamate derivatives of cellulose. The structure of CTPC was optimized with the CHARMm force field based on the proposed structure of CTPC by X-ray analysis. In chromatographic enantioseparation on CTPC, 1 was completely resolved (alpha = 1.46) and the (R,R)-(+)-isomer eluted first followed by the (S,S)-(-)-isomer, but 2 was not resolved (alpha approximately 1). The results of calculation of interaction energies between CTPC and the enantiomers 1 suggested that the most important adsorbing site of CTPC for 1 may be the NH protons of the carbamate moieties at the 3-position of glucose units, and the (S,S)-(-)-isomer of 1 may interact more closely than the (R,R)-(+)-isomer with CTPC. In contrast, there was little difference in the minimum interaction energies between the enantiomers 2. These calculations agreed with the observed results for the chromatographic resolution on CTPC.

(Z)-1,1-dichloro-2-(4-benzyloxyphenyl)-2,3-bis(4-methoxyphenyl)cyclopropane: the synthesis and enantiomeric separation of an antitumor agent

(Z)-1,1-dichloro-2-(4-benzyloxyphenyl)-2,3-bis(4-methoxyphenyl)cyc lopropane (5), a potential antitumor agent designed to treat breast cancer, was prepared in three steps. A stereospecific palladium-catalyzed cross coupling reaction which provided the intermediate (Z)-triaryl alkene 4 was a crucial step in the synthesis. Makosza phase transfer reaction on 4 gave the enantiomeric (Z)-dichlorocyclopropane derivatives 5 which were resolved by semipreparative HPLC on a chiral stationary phase consisting of amylose tris-3,5-dimethylphenyl carbamate coated on silica gel.

Resolution of chiral cannabinoids on amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase: effects of structural features and mobile phase additives

The separation of six pairs of chiral cannabinoids was achieved using a dimethylphenylcarbamate derivative of amylose, immobilized on silica gel (ChiralPak AD, Daicel), using 2-propanol and ethanol as the modifiers of n-hexane in the mobile phase. Good separation was achieved for most of the solutes in both solvent systems under various conditions. The chromatographic parameters of various cannabinoids in the two solvent systems were determined. The pairs differ from each other in small structural features such as the degree of saturation, position of a double bond and closure of a pyran ring. Therefore, a comparative study could give some clues regarding the mechanism of discrimination between the enantiomeric pairs on the chiral stationary phase. Preliminary measurements of limit of determination showed that it was possible to assess 99.9% enantiomeric purity of the cannabinoids, owing to the high efficiency of the separation. Enantiomers of two monoterpenes, used as intermediates or as starting materials in the chiral synthesis of cannabinoids, were also separated, hence the described procedure is capable of assessing whether the chiral centres in the molecules were sustained throughout the synthetic procedures.

Resolution of enantiomers of amino acids by HPLC

Application of HPLC as a prime tool in the area of enantiomeric resolution has opened doors of success and varied interest. Use of chiral reagents either indirectly (as derivatization reagent) or directly (added to stationary or mobile phase) has led to achieve resolution of a wide range of compounds. Amino acids, being important molecules with simple structure and easy availability, have been extensively studied. A bibliographic survey on HPLC resolution of amino acids and derivatives along with a brief discussion on general methods of enantiomeric separation has been presented.

Determination of the R,R- and S,S-enantiomers of vamicamide in human serum and urine by high-performance liquid chromatography on a Chiral-AGP column

An enantioselective liquid chromatographic assay for the determinations of the R,R- and S,S-enantiomers of vamicamide, a potent anticholinergic drug, in human serum and urine is described. Racemic vamicamide and internal standard were purified from biological fluids using a two-step extraction procedure involving diethyl ether and 0.1% phosphoric acid. The overall recoveries of racemic vamicamide and internal standard were greater than 80%. The purified samples were measured by high-performance liquid chromatography on a Chiral-AGP column with ultraviolet absorbance detection at 260 nm. The standard curves for the analytes were linear from 10 to 200 ng/ml in serum and from 0.25 to 50 micrograms/ml in urine. The quantification limit of both enantiomers was 10 ng/ml for serum and 250 ng/ml for urine. Both intra-day and inter-day accuracy and precision data showed good reproducibility of the method. The assay has been applied for the analysis of vamicamide enantiomers in serum and urine samples from a healthy volunteer.

Capillary electrophoresis quantitation of l-L-folinic acid in the presence of its inactive d-L-form

The diastereomers d-L and l-L-folinic acid were separated by capillary electrophoresis, using heptakis (2,3-di-O-methyl)-beta-cyclodextrin as a chiral active component in the background electrolyte. The method proved suitable for the quantitation of these compounds in one commercial pharmaceutical preparation.