Separation of Stereoisomers

Diastereomers (or diastereoisomers) are stereoisomers that are not mirror images of each other. Diastereomers include molecules containing more than one asymmetric (chiral) center and geometric isomers. They have different physical and chemical 

Achiral An object that is superposable on its mirror image 

Anomers Diastereomers of glycosides, hemiacetals, or related cyclic forms of sugars, differing in configuration only at C-1 of an aldose, C-2 of a 2-ketose, etc. 

Atropisomers Result when complete rotation about a single bond in a molecule is prevented by the bulk of non-identical neighboring substituents, so that a pair of enantiomers is formed. 

Chirality The geometric property of a rigid object of being non-superposable on its mirror image such an object has no symmetry elements, such as a mirror plane, center of inversion, or a rotating-reflection axis. 

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The concept of using continuous chromatography for the separation of stereoisomers or optical isomers is very old and was probably proposed for the first time by Martin and Kuhn in 1941 [28]. The suggested implementation was different from today s SMB technology, though the basic concept is the same. The chromatographic media is moved continuously in a conveyor belt, the feed is injected continuously at a fixed point, and the pure enantiomers are recovered at fixed points. In the idea of Martin and Kuhn, benefits were taken from the possibility of modulating the adsorption of the products at different temperatures. 

Separation of Stereoisomers Osing Liquid Crystal Stationary Phases... 

Nyambaka H and Ryley J. 1996. An isocratic reversed-phase HPLC separation of stereoisomers of the provitamin A carotenoids (a- and 3-carotene) in dark green vegetables. Food Chem 55 63-72. 

Investigations on the stereochemistry of chiral semiochemicals may be carried out by (gas) chromatographic separation of stereoisomers using chiral stationary phases, e.g. modified cyclodextrins [32]. Alter natively, formation of diastereomers (e.g. Mosher s ester or derivatives involving lactic acid etc.) may be followed by separation on conventional achiral stationary phases. Assignment of the absolute configuration of the natural product will again need comparison with an authentic (synthetic) reference sample. 

Arki, A. et al.. High-performance liquid chromatographic separation of stereoisomers of P-amino acids and a comparison of separation efficiencies on CHIROBIOTIC T and TAG columns, Chromatographia, 60, S43, 2004. 

The key issue for synthesis of pure stereoisomers, in either racemic or enantiomerically pure form, is that the configuration at newly created chiral centers must be controlled in some way. This may be accomplished in several ways. An existing functional group may control the approach of a reagent by coordination. An existing stereocenter may control reactant conformation, and thereby the direction of approach of a reagent. Whatever the detailed mechanism, the synthetic plan must include the means by which the required stereochemical control is to be achieved. When this cannot be done, the price to be paid is a separation of stereoisomers and the resulting reduction in overall yield. 

R. W. Soutcr in Chromatographic Separations of Stereoisomers, CRC Press, Boca Raton, Florida 1985. 

Numerous HPLC analyses have been carried out on biologically active diastereoisomeric peptides (encephalins, endorphins, hormones) in order to isolate them so that their activity can be studied however, the separation of stereoisomers is not yet common in the field of food science. 

Analytical Properties y-Cyclodextrin (cyclooctylamalyose), reverse phase separation of stereoisomers of polycyclic aromatic hydrocarbons the substrate has eight glucose units and has a relative molecular mass of 1297 the cavity has a diameter of 0.59 nm, and the substrate has a water solubility of 23.2 g/ml Reference 13-28... 

On the basis of this discussion a particularly promising scheme for synthesizing chiral compounds with great isomer purity and high overall yield would be to start with a productive asymmetric synthesis (to assure high yield) and follow by a destructive one (to assure unlimited isomer purity of the desired product). Such a procedure has the essential advantage that under suitable conditions it leads to pure chiral compounds without cumbersome separation of stereoisomers. With proper choice of the reactants the procedure will also permit an effective recovery of the auxiliary chiral materials needed as chiral templates in the asymmetric synthesis. 

The separation of stereoisomers of 37 (R = R = CHS or CGH6) has been reported25 and a detailed discussion of the stereochemistry of the system has appeared.21... 

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