Applicability of new chiral stationary phases in the separation of racemic pharmaceutical compounds by high-performance liquid chromatography

Syntheses of PDE3A inhibitor ORG9935 and determination of the absolute stereochemistries of its enantiomers by X-ray crystallography

Two synthetic methods were developed for the synthesis of PDE3A inhibitor ORG9935. The first one proceeds in six steps and 34% overall yield and the second one in five steps and an overall yield of 69% starting from commercially available starting material 5,6-dimethoxybenzo[b]thiophene-2-carboxylic acid (6). The enantiomers of ORG9935 were separated by chiral column chromatography and the absolute stereochemistry of the (+)-enantiomer, ORG20865 was determined by X-ray crystallography to possess the (S)-configuration. The (-)-enantiomer, ORG20864, was therefore assigned the (R)-stereochemistry. The biologically less active (+)-isomer ORG20865 was converted to racemic ORG9935 under basic conditions, which then can be separated again into the enantiomers. The crystal structure of ORG20865 is notable for having the highest Z' for any known pharmaceutical substance.

Enantioselective recognition of 2,3-benzodiazepin-4-one derivatives with anticonvulsant activity on several polysaccharide chiral stationary phases

The retention behaviour of racemic 1-(4-aminophenyl)-1,2,3,5-tetrahydro-7,8-methylendioxy-4H-2,3-benzodiazepin-4-one derivatives with anticonvulsant activity on several chiral stationary phases was investigated. The selective performances of six polysaccharide phases, namely, Chiralcel OA, OD, OF, OG, OJ and Chiralpak AD were studied and normal phase HPLC methods were optimized to separate the enantiomeric forms of this class of compounds. The chiral recognition mechanism between the analytes and the chiral selectors was discussed. A molecular modeling study was carried out with the aim to explore the enantioselective molecular recognition process with the Chiralcel OG stationary phase.

Modeling of the separation of two enantiomers using a microbore column

A microbore column packed with Chiralcel OB (cellulose tribenzoate coated silica) was used for the measurement of the single and competitive equilibrium-isotherm data of the 1-indanol enantiomers by frontal analysis. The amount of sample needed for the isotherm data acquisition was about 20 times less than that required with a conventional column. The data obtained were fitted to different single and competitive isotherm models. Both the single and the competitive data sets fitted best to the same Bilangmuir (BL) isotherm model with small differences in the numerical values of the parameters. The best fitted Bilangmuir single and competitive isotherm models were used to predict the overloaded experimental profiles of both pure enantiomers, of the racemic mixture, and of different enantiomeric mixtures. All the calculated profiles were in excellent agreement with the experimental ones. This agreement confirms that in many chiral separations, the competitive isotherms can be derived from data acquired from the mere racemic mixture with a sufficient accuracy for a correct prediction of the band profiles of all kinds of enantiomer mixtures, making possible the computer-assisted optimization of the experimental conditions.

Comparison of the binary equilibrium isotherms of the 1-indanol enantiomers on three high-performance liquid chromatography columns of different sizes

The competitive isotherm data for the enantiomers of 1-indanol were measured on three columns, a microbore column (15 cm x 0.1 cm), a conventional analytical column (15 cm x 0.46 cm), and a semi-preparative column (20 cm x 1.0 cm), packed with Chiralcel OB. The sets of isotherm data measured on each one of these three columns could be fitted well by a bi-Langmuir isotherm model. The experimental elution band profiles of mixtures of the 1-indanol isomers were recorded on the three columns. The isotherm model, combined with the equilibrium dispersive model of chromatography, gave calculated profiles that are in excellent agreement with the experimental profiles in all cases investigated. It was found that the value of the inner diameter of the column is an important parameter in the calculation of the isotherm parameters from the measured isotherm data. In order to use isotherm data obtained on one column to account for the phase equilibrium on another one, the inner diameters of these columns must be measured accurately. The diameters of the three columns were all slightly off their nominal value. Without correction, an important systematic error was made on the isotherm data obtained with the microbore column while only negligible errors were made on the data obtained with the other two columns. After due correction for this effect, the relative difference between the isotherm data for the microbore and the semi-preparative column is still, on the average, about 10%, a difference that might be explained by the limited precision of the measurement of the microbore column diameter. The relative difference between the isotherm data for the analytical and the semi-preparative columns was about 1%, a reasonable value, since the two columns came from different batches of the same packing material.

Cyclodextrin-mediated micellar electrokinetic chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the enantiomer separation of racemorphan in human urine

Micellar electrokinetic chromatography (MEKC) was successfully and conveniently applied to the chiral separation with the addition of cyclodextrins (CDs) as chiral selector to the running buffer. Chiral separation depended on the type of CD; in particular, beta-CD was effective for the chiral separation of racemorphan. We investigated the optimal conditions of type and concentration of CD as chiral selector for the routine enantiomeric separation of racemorphan with good reproducibility. The effects of other parameters such as buffer pH and detection wavelength were also investigated to obtain the optimum conditions for the enantiomeric separation of racemorphan. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used for confirmation of racemorphan. The optimal conditions for enantiomeric separation of the racemorphan were as follows: 50 mM borate buffer at pH 9.4 with 50 mM SDS, 10 mM beta-CD and 20% 1-propanol, 57 cm x 50 microns fused-silica capillary column, and UV detection at 192 nm. Based on the developed method, racemorphan in human urine was also separated and determined using solid-phase extraction and MEKC.

Validation of an LC-MS assay for the quantification of the enantiomers of Org 4428 in human plasma

A sensitive and selective liquid chromatographic mass spectrometric assay has been validated for the quantification of Org 4428 enantiomers in human plasma. The assay employs n-hexane extraction from alkalinized plasma, separation on a narrow-bore enantioselective normal phase Chiralpak AD column and APCI MS-MS detection. The lower limit of quantification is 0.5 ng ml-1 plasma for the individual enantiomers.

Degradation kinetics and isomerization of cefdinir, a new oral cephalosporin, in aqueous solution. 2. Hydrolytic degradation pathway and mechanism for beta-lactam ring opened lactones

Hydrolysis of cefdinir leads to pH-dependent isomerizations and beta-lactam ring-opening. Lactam ring opened gamma-lactones were produced as a mixture of four diastereoisomers based on the lactone methyl, and C-6 isomerizations in acidic to neutral solutions. Cefdinir and its 7-epimer were hydrolyzed to clarify the pathway leading to these lactones and the mechanism of C-6 epimerization with the aid of chiral separation techniques. Chiral separation using a bovine serum albumin column was employed to detect the beta-lactam ring opened products of cefdinir and its 7-epimer; the C-6 and C-7 isomerization was thereby observed; however, it was found to be pH-dependent at pH > or = 9. Optical activity detection applied to the lactones produced from cefdinir and its 7-epimer demonstrated that the corresponding peaks of these lactones were enantiomeric pairs. In addition, the smallest rate constant at pH 4 was observed for C-6 epimerization of the lactones, and it was found to proceed without deprotonation at C-6 by 1H-NMR spectroscopy. From the results of these studies, a plausible mechanism for C-6 epimerization has been proposed. Additionally, it was confirmed that two degradation pathways were involved during hydrolysis of cefdinir to the lactone.

Mirror images: the analysis of pharmaceutical enantiomers

Pharmaceutical enantiomers often exhibit different pharmacodynamic and pharmacokinetic properties. Stereospecific chromatographic assays are available to separate these stereoisomers. Therapeutic agents often contain chemical functional groups (e.g. amino, hydroxyl, carbonyl, and carboxylic acid). These can be reacted with enantiomerically pure reagents to give diastereoisomers suitable for analysis on achiral gas chromatographic (GC) and high performance liquid chromatographic (HPLC) columns. Alternatively, derivatized or underivatized drugs may be resolved on chiral chromatographic phases. A wide variety of GC (e.g. amino acid, cyclodextrin, and metal-complex) and HPLC (mobile phase additive, crown ether, pi-pi interaction and related phases, protein, cyclodextrin, polysaccharide, methacrylate and amide polymer, and ligand exchange) columns are commercially available. This article reviews the chromatographic separation of enantiomers.

Preparative chromatographic separation of enantiomers

The potential of the chromatographic separation of enantiomers on a preparative scale as a tool for the isolation of optically pure compounds is gaining increasing recognition. This review surveys the different chiral stationary phases (CSPs) used for preparative chromatography, emphasizing the advantages and drawbacks of each. The strategy to be followed for preparative separations is discussed and tables summarizing separations reported in the literature give an overview of practical applications. Cellulose triacetate has been used most frequently, probably because of its broad application range and its low production costs in comparison with more recently introduced CSPs. Nevertheless, the high efficiency of some of the novel CSPs is likely to contribute to the further development and expansion of the method.