Separation techniques indirect resolution

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

TLC has also been used for the separation of diastereomeric derivatives of enantiomers, but this form of chromatography has not attained widespread use in indirect resolutions. Other chromatographic techniques, for example, supercritical fluid chromatography, capillary electrophoresis, countercurrent chromatography, etc., have not received much attention in indirect enantioseparation. 

The most popular thin layer chromatography (TLC) techniques for separation of enantiomers are described here 1) use of non-chiral phases for indirect resolution of optical isomers after derivatization to obtain the corresponding diastereoisomers and 2) direct resolution of enantiomers using chiral stationary phases or chiral mobile phases. Advantages and limits of all reported techniques are discussed. 

Coupled on-line techniques (GC-MS, LC-MS, MS/ MS, etc.) provide for indirect mixture analysis, while many of the newer desorption/ionisation methods are well suited for direct analysis of mixtures. DI techniques, applied either directly or with prior liquid chromatographic separations, provide molecular weight information up to 5000 Da, but little or no additional structural information. Higher molecular weight (or more labile) additives can be detected more readily in the isolated extract, since desorption/ionisation techniques (e.g. FD and FAB) can be used with the extract but not with the compounded polymer. Major increases in sensitivity will be needed to support imaging experiments with DI in which the spatial distribution of ions in the x — y plane are followed with resolutions of a few tens of microns, and the total ion current obtained is a few hundreds of ions. 

The advent of modern column LC in the 1970s rapidly led to the use of this chromatographic technique in the separation of enantiomers as dia-stereomeiic derivatives. Today, most of the reported new developments in indirect enantiomer resolutions use LC, and LC is particularly important in the resolution of chiral pharmaceuticals. 

Whilst the dipolar interactions are of considerable interest in view of the structural information they contain, the broadening they produce in NMR spectra usually obscures the weaker chemical shift and indirect spin coupling information which is of great utility to the chemist. Over the last fifteen years or so a great deal of effort has been devoted to developing techniques which separately or in appropriate combinations allow high resolution NMR spectra of solids to be observed. The basic principles behind the techniques are just those discussed in the previous section, i.e., the use of appropriate averaging techniques, and we deal with them in turn below. 

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