Chiral separations by LC, SPC and GPC

Chiral separations are of great importance in various fields:

• pharmacological (the activity of a therapeutic molecule can vary from one absolute configuration to another; the most tragic case was that of thalidomide used as a sedative in pregnant women, one form of which proved to be teratogenic);

• agrochemical (many herbicides and pesticides have one or more centers of asymmetry and one form may be more active than the other; its use would make it possible to reduce the quantities applied and thus pollution);

• aromas and fragrances...

So it's not surprising that controlling optical purity and studying the properties of enantiomers is becoming a necessity, particularly for drug molecules.

Various techniques exist for determining optical or enantiomeric purity, which can be divided into two groups depending on whether or not enantiomers are actually separated.

Non-separation methods not developed here include polarimetry (using the property of a chiral compound to rotate the plane of light polarization), NMR (a distinction is made between indirect methods involving the derivation of diastereomeric enantiomers and direct methods involving the use of a chiral solvent such as ( R) - (I) - 2,2,2-trifluoro-1-phenylethanol or an optically active reagent such as a lanthanide complex), isotope dilution, calorimetry and enzymatic techniques.

Separation methods include fractional recrystallization and chromatography. Fractional recrystallization has long been the technique of choice for preparing optically pure compounds. This method is based on differences in the thermodynamic behavior of the two antipodes of a racemate: in the presence of a chiral resolving agent, they give complexes with distinct solubilities. It has a number of drawbacks: it takes a long time to set up, and is not immune to errors resulting from different reaction speeds, racemization of the chiral reagent or racemization during the final chiral reagent release step.

The development of gas chromatography (GPC), followed by liquid chromatography (LPC) and, more recently, supercritical chromatography (SPC), has enabled us to develop increasingly efficient techniques. The physico-chemical properties of two enantiomers are identical, except when they are placed in an asymmetric environment. The latter can be obtained before the chromatographic column (method 1) or in the chromatographic column via the mobile phase (method 2) or stationary phase (method 3).

Method 1: Enantiomers are chemically transformed into diastereomers and then separated with achiral stationary and mobile phases....