Enantiomers of polarized alkenes: chromatographic enrichment and thermal interconversion

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.

Recent advances in methods of direct optical resolution

Very great advances have been made in the field of direct optical resolution of organic compounds by chromatographic techniques. Chiral capillary gas chromatography now permits a determination of the enantiomeric composition of a few nanograms of a compound present in a mixture of many others. Coupled with high resolution mass spectrometry the technique will additionally permit structural elucidation; of great interest in pheromone research and related areas. Analytical separations of enantiomers are now also carried out by high-performance liquid chromatography (HPLC) methods based on a variety of principles. Basically, two main types are used, differing as to whether the mobile phase has to be a chiral medium or not. Two-dimensional HPLC, whereby compounds separated on a non-chiral column are progressively and automatically transferred to a chiral column for optical resolution, has been used successsfully for chiral amino acid separations. Many different chiral sorbents for preparative LC and HPLC resolutions have been prepared; some of these are now used in columns capable of producing pure enantiomers from a given racemate at a rate of the order of one gram/hour in continuous, automatic HPLC procedures. Apart from all important applications of these results of optical resolution technology, an increased knowledge of the underlying chiral recognition phenomena responsible for enantioselection has also been achieved.