Resolution Of Stereoisomers Research Articles

Advances in analysis and synthesis of myo-inositol-derivatives through resolution by crystallisation

A simple method for the preparation of both enantiomers of tetra- O-benzyl- myo-inositol is presented. This method is based on the resolution of stereoisomers by crystallisation. Starting with the known synthesis of four diastereomers using d-camphor dimethyl acetal as chiral auxiliary, one diastereomer is separated by crystallisation from methanol and is converted to d-1,4,5,6-tetra- O-benzyl- myo-inositol. A second synthetic route was carried out with the mother liquor of the crystallisation, which has so far been neglected in former sequences [ Am. Chem. Soc. 1992, 6361]. This approach leads to a non-racemic mixture of tetra- O-benzyl- myo-inositols, from which d-3,4,5,6-tetra- O-benzyl- myo-inositol can be separated by crystallisation from 1-propanol in its enantiopure form. The yield of this crystallisation step was determined by its eutectical point and the ratio of the diastereomers in the mother liquor of crystallisation step 1. For this purpose, an analytical HPLC separation for the fast elucidation of the diastereomeric ratio has been developed. With this analytical method, it is possible to optimise the ratio of the diastereomers by changing the reaction conditions.

Capillary electrophoresis of cytochrome P-450 epoxygenase metabolites of arachidonic acid. 2. Resolution of stereoisomers.

Each of the four regioisomers of epoxyeicosatrienoic acids (EETs) is a candidate for being an endothelial-dependent hyperpolarizing factor (EDHF). One regioisomer, 14,15-EET, stereospecifically blocks cyclooxygenases from converting arachidonic acid to prostaglandins and stereospecifically binds to cellular receptors. Both stereospecific actions emphasize the need to establish the tissue availability of the 14,15-EET enantiomers. The present work describes a method to quantitate picogram amounts of 14,15-EET enantiomers by capillary electrophoresis. The 14,15-EET enantiomers were baseline resolved (R = 1.3) using unsubstituted beta-cyclodextrin and 32% acetonitrile (v/v). When absorption at 194 nm was monitored using a photodiode array detector, 8 and 1 pg of underivatized 14,15-EET were readily quantitated and detected, respectively. Capillary electrophoresis accurately assessed chiral excesses up to 97:3 for either 14,15-EET enantiomer. Moreover, capillary electrophoresis with a photodiode array detector was sufficiently sensitive to detect and measure 14,15-EET enantiomers from murine liver. Thus, unlike chiral-phase high-performance liquid chromatography, capillary electrophoresis can be used to directly assess the chirality of trace amounts of underivatized eicosanoids.

Solvent selectivity in reversed-phase high-performance liquid chromatographic separation of isomers of polystyrene oligomers

Isotactic, syndiotactic and atactic isomers of oligomers of low-molecular-weight polystyrene were successfully resolved using a highly efficient C 18 column and and acetonitrile—methylene chloride gradient. Structural assignments for some of the collected trimer peaks were made by gas chromatography—mass spectrometry and 13C nuclear magnetic resonance analysis. After the initial separation, 27 different mobile-phase solvents, several reversed-phase bonded packings, and different column temperatures were tested in an effort to maximise the resolution of the stereoisomers. Peak separation( P i ) for each peak pair and chromatographic response function (CRF) for each chromatogram were calculated and compared. Only a few of the solvents tested produced stereoisomer separation. The sample solubility of polystyrene in the mobile-phase solvent appeared to be the best predictor for an optimum mobile phase. Only “weak” solvents, those solvents that could not easily dissolve polystyrene, gave good isomer separation when used as the mobile phase. Hansen's solubility model appears promising for optimizing the mobile-phase composition for separations of other solutes using reversed-phase high-performance liquid chromatography. A phenyl bonded phase, a C 8 bonded phase, and a synthesized perfluorinated bonded phase were compared to octadecylsilane. Only the C 8 column produced isomer separation. Decreasing the column temperature resulted in better resolution of the stereoisomers at the expense of longer retention times. Based on the high-performance liquid chromatographic data, it has been proposed that the mobile-phase affects both the conformations of the polystyrene stereoisomers and of the long-chain hydrocarbon bonded phases. The resolution of stereoisomers by capillary gas chromatography was found to be significantly less than that obtained by high-performance liquid chromatography with the C 8 and C 18 columns.

Preparative gas chromatography in the industrial rubber laboratory

Preparative gas chromatography has been a practical tool to prepare laboratory quantities of highly purified materials. Actual applications have included monomer purification, separation of azeotropic mixtures, resolution of stereoisomers and solvent purification. These separations have been achieved with a modified Prepmaster using a 2.5 inch internal diameter column. Scale-up of analytical resolution to the preparative column was maintained primarily by controlling the radial gas flow distribution. Special insert baffles, were made and installed within the column. Standard liquid phases were used depending upon the separation required. A critical step in the experimental operation is the trapping of the isolated peak. Fogging was prevented by using small water-cooled columns packed with glass beads to maintain the mass transfer rate greater than the heat transfer rate. Each system required somewhat different trapping conditions. Sample sizes ranged from 2 to 25 ml and daily throughput was 10–100 ml depending upon the difficulty of the separation. Material purity greater than 99.9% was usually achieved.