Effect of the Amide Connecting Tether Type of Pirkle-Concept Chiral Stationary Phases Derived from (S)-N-(3,5-Dinitrobenzoyl)Leucine on Enantiomeric Separations

Liquid chromatographic comparisons are made between a secondary amide linked chiral stationary phase (CSP 1) and a tertiary amide double-tethered CSP (CSP 2) derived from (S)-N-3,5-[dinitrobenzoyl(DNB)]leucine. For the enantioseparation of the anilide derivatives of N-9-fluorenylmethoxycarbonyl (FMOC), benzyloxycarbonyl (CBZ) and t-butoxycarbonyl (BOC) protected α-amino acids, CSP 2 shows performance superior to that of CSP 1. It is considered that the amide hydrogen of the connecting tether of CSP 1 serves as a nonproductive adsorption site, while the enhanced basicity of the carbonyl oxygen of the tertiary amide tethered CSP 2 is responsible for the increased enantioselectivity of these analytes. For the same reason, it is observed that CSP 2 provides better separation of the enantiomers of N-CBZ α-amino acids as ethyl ester, n-butylamide and diethylamide derivatives than CSP 1.