Forword
Chirality Discrimination -- Historical Background
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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.