High-performance liquid chromatographic separation of the enantiomers of unusual α-amino acid analogues

The direct and indirect stereochemical resolution of the enantiomers of ring- and α-methyl-substituted phenylalanines and phenylalanine amides was attempted by high-performance liquid chromatographic methods. The direct separation was carried out on two chiral stationary phases, the crown-ether-based Crownpak CR(+), and the teicoplanin-based Chirobiotic T, while the indirect resolution was performed by applying pre-column derivatization with 2,3,4,6-tetra- O-acetyl-β- d-glucopyranosyl isothiocyanate (GITC) and Nα-(2,4-dinitro-5-fluorophenyl)- l-alanine amide (Marfey’s reagent, FDAA). The Chirobiotic T column was efficient in the separation of ring- and α-methyl-substituted phenylalanine analogues, but was ineffective for the amides of these analogues. The Crownpak CR(+) column separated the ring-substituted phenylalanines and amides, whereas the α-methylated analogues were coeluted. Of the two indirect methods, GITC derivatization seemed more effective than FDAA derivatization.