Comparative studies between covalently immobilized and coated chiral stationary phases based on polysaccharide derivatives for enantiomer separation of N-protected alpha-amino acids and their ester derivatives.

Liquid chromatographic enantiomer separation of several N-benzyloxycarbonyl (CBZ) and N-tert-butoxycarbonyl (BOC) alpha-amino acids and their corresponding ethyl esters was performed on covalently immobilized chiral stationary phases (CSPs) (Chiralpak IA and Chiralpak IB) and coated-type CSPs (Chiralpak AD and Chiralcel OD) based on polysaccharide derivatives. The solvent versatility of the covalently immobilized CSPs in enantiomer separation of N-CBZ and BOC-alpha-amino acids and their ester derivatives was shown and the chromatographic parameters of their enantioselectivities and resolution factors were greatly influenced by the nature of the mobile phase. In general, standard mobile phases using 2-propanol and hexane on Chiralpak IA showed fairly good enantioselectivities for resolution of N-CBZ and BOC-alpha-amino acids and their esters. However, 50% MTBE/hexane (v/v) for resolution of N-CBZ-alpha-amino acids ethyl esters and 20% THF/hexane (v/v) for resolution of N-BOC-alpha-amino acids ethyl esters afforded the greatest enantioselectivities on Chiralpak IA. Also, liquid chromatographic comparisons of the enantiomer resolution of these analytes were made on amylose tris(3,5-dimethylphenylcarbamate)-derived CSPs (Chiralpak IA and Chiralpak AD) and cellulose tris(3,5-dimethylphenylcarbamate)-derived CSPs (Chiralpak IB and Chiralcel OD). Chiralpak AD and/or Chiralcel OD showed the highest enantioselectivities for resolution of N-CBZ-alpha-amino acids and esters, while Chiralpak AD or Chiralpak IA showed the highest resolution of N-BOC-alpha-amino acids and esters.