A CHIRAL RECOGNITION MECHANISM PROPOSED FOR RESOLVING PI-ACIDIC RACEMATES ON PI-ACIDIC CHIRAL STATIONARY PHASES DERIVED FROM (S)-LEUCINE

A chiral recognition mechanism which can rationalize the resolution of N-(3,5-dinitrobenzoyl)-alpha-amino amides on chiral stationary phases (CSPs) obtained from N-(3,5-dinitrobenzoyl)leucine amide derivatives has been proposed on the basis of the chromatographic resolution behav ior of various N-(3,5-dinitrobenzoyl)-alpha-amino acid derivatives and N-(various benzoyl)leucine N-propyl amides. The proposed chiral recog nition mechanism utilizes two hydrogen bonding interactions between th e CSP and the analyte and a pi-pi donor-acceptor interaction between t he N-(3,5-dinitrobenzoyl) groups of the CSP and the analyte. From the chiral recognition mechanism proposed, it has been concluded that the resolution of pi-acidic N-(3,5-dinitrobenzoyl)-alpha-amino acid deriva tives on pi-acidic CSPs derived from N-(3,5-dinitrobenzoyl)leucine ami de derivatives is not unusual, but is merely the extension of the reso lution of the pi-basic racemates on pi-acidic CSPs. However, the chrom atographic behavior of the resolution of N-(3,5-dinitrobenzoyl)phenylg lycine derivatives on CSPs derived from N-(3,5-dinitrobenzoyl)leucine amide derivatives is different from that of the resolution of other N- (3,5-dinitrobenzoyl)-alpha-amino acid derivatives. To rationalize this exceptional behavior, a second chiral recognition mechanism which uti lizes two hydrogen bonding interactions (which are different from thos e of the first chiral recognition mechanism) between the CSP and the a nalytes and a pi-pi donor-acceptor interaction between the N-(3,5-dini trobenzoyl) group of the CSP and the phenyl group of the analytes has been proposed to compete with the first chiral recognition mechanism. In this instance, it has been proposed that the separation factors and the elution orders of the resolution of N-(3,5-dinitrobenzoyl)phenylg lycine derivatives are dependent on the balance of the two competing c hiral recognition mechanisms.