Page 218 - Chiral Separation Techniques

196 7 Chiral Derivatization Chromatography

7.2.2 Type II: Selective Derivatization of One Compound

Despite the use of unichiral derivatizing agents, achiral derivatization also offers cer-
tain advantages for chromatographic enantioseparation. The newly introduced group
can enhance the sensitivity for UV or fluorimetric detection. In addition, the chro-
matographic properties can be positively influenced with, on occasion, only the
derivatization step making it possible to separate the enantiomers on a CSP. A fur-
ther benefit of the derivatization step may be its selectivity for a single compound;
in this way, derivatization may be used to measure one compound selectively, in a
complex mixture (Fig. 7-12).

Fig. 7-12. Type II: Selective derivatization of one compound.

The latter approach is used in the enantioselective determination of a Phase I
metabolite of the antihistaminic drug, terfenadine. Terfenadine is metabolized to
several Phase I compounds (Fig. 7-13), among which the carboxylic acid MDL
16.455 is an active metabolite for which plasma concentrations must often be deter-
®
mined. Although terfenadine can be separated directly on Chiralpak AD – an amy-
lose-based CSP – the adsorption of the metabolite MDL 16.455 is too high to per-
mit adequate resolution. By derivatizing the plasma sample with diazomethane, the
carboxylic acid is converted selectively to the methyl ester, which can be separated
in the presence of all other plasma compounds on the above-mentioned CSP Chiral-
®
pak AD [24] (Fig. 7-14). Recently, MDL 16.455 has been introduced as a new anti-
histaminic drug, fexofenadine.
The method described is of particular value in the determination in complex
matrices of metabolites or impurities which differ in the presence of functional
groups. Notable examples of this are assays of compounds with free amino or acidic
groups.