Forword

Chirality Discrimination  -- Historical Background ---
 

  The word "chiral" comes from the Greek word cheir , which means "hand".   A pair of isomers representing mutual reflections are said to have handedness.   They are optically active and are called "enantiomers".   A mixture of equal parts of enantiomers, called a "racemate", obtained by some ordinary chemical reaction (in a non-chiral environment) from non-chiral precursors is optically inactive, because the rotation caused by a molecule of one enantiomer is neatly cancelled by the opposite rotation caused by a molecule of its antipode.   In "asymmetric synthesis" a non-chiral compound can be converted into a chiral compound by the introduction of a chiral molecule.
 

  The enantiomer ratio in the products of an asymmetric synthesis can be determined by measuring the optical rotation with a polarimeter and reference to optically pure standard material, but this is not sufficiently accurate. Chromatographic tetchniques based on chiral stationary phases have been developed, and this new methodology provides a high degree of accuracy for the measurement of "enantiomeric excess".   Efforts have been directed towards finding new types of chiral stationary phases, based on stereochemical considerations and on the technical evolution of high-performance liquid chromatography. Systematic studies on chiral stationary phases for liquid chromatography began in 1979. A molecular design derived from hydrogen bond interactions, which is now widely used for chiral separations by liquid chromatography, gave the first successful results. Since then, extensive research soon established the superiority of the method of determining optical purity directly. Liquid chromatography is now accepted not only as a useful analytical method but also as an essential technology for preparative chiral separations.
 

  Direct optical resolution by chromatography is associated with diasteremeric interactions between selector and selectand molecules.   Thus, molecular complexes dynamically formed in chromatographic columns serve to produce a means for expressing the enantioselectivity of a systems. In this respect, mechanistic considerations are indispensable for developing new chiral recognition systems. In recent years, capillary zone electrophopresis and micellar electrokinetic chromatography with chiral additives have become most important technologies for chiral separations.   It should be stressed that the elucidation of chiral recognition phenomena is of cardinal interest in stereochemistry. The probing of chiral molecules is finding extensive use in studies of supramolecular chemistry.

Ref.  S. Hara, T. Nakagawa, S. Terabe: J. Chromatogr., A, 694 (1995) 1-2.