Direct high-performance liquid chromatographic enantioseparation of beta-methyl-substituted unusual amino acids on a quinine-derived chiral anion-exchange stationary phase

A quinine-derived chiral anion-exchange stationary phase was used for the d irect high-performance liquid chromatographic separation of the enantiomers of the N-protected unusual beta -substituted alpha -amino acids, beta -met hylphenylalanine, beta -methyltyrosine, beta -methyltryptophan, and beta -m ethyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid, The readily prepare d 2,4-dinitrophenyl and tert-butyloxycarbonyl derivatives were well separat ed, and in most cases the separation of all four stereoisomers of these bet a -methyl-alpha -amino acids could be obtained in one chromatographic run. The elution sequences of the enantiomers of the different derivatives were determined and revealed a dependence on the type of the N-protecting group. In this context, the effects of different protecting groups (acetyl, tert- butyloxycarbonyl, benzoyl, 3,5-dinitrobenzoyl, benzyloxycarbonyl, 3,5-dinit robenzyloxycarbonyl, 2,4-dinitrophenyl, and 9-fluorenylmethoxycarbonyl) on the chromatographic behavior were investigated.