Correlations between conformational isomerism and chromatographic diastereoselectivity of bis amino amide s-triazine derivatives

NMR spectroscopy was used to probe the conformational behavior of diastereomeric s-triazine derivatives containing two chiral amino amide substituents, in order to shed light onto the mechanism of chromatographic diastereoselectivity. Utilizing the amino hydrogen signals in the proton NMR spectrum, the population of the conformations caused by rotation about the bond between the amino nitrogen and aromatic carbon atoms could be observed. The population distribution between the three possible conformations was similar but not identical between the two diastereomers, with similar trends being observed for both bis alanine amide and bis valine amide derivatives. Based on a simple model in which it is assumed that adsorption to the hydrophobic RP-LC stationary phase occurs only for the conformations having both amino amide R-groups on the same side of the triazine ring plane, the different conformation populations between the two diastereomers obtained by NMR was consistent with the observed RP-LC elution order (L-L diastereomer followed by L-D). The predicted diastereoselectivity values from NMR data were compared to RP-LC diastereoselectivity values obtained using both C18 and polymeric columns, with both acetonitrile/water and DMSO/water mobile phases. Values obtained with the polymeric column were in better agreement with calculated values than those obtained with the C18 column, suggesting that the simple adsorption model used to calculate the diastereoselectivity is more relevant towards a simple hydrophobic polymeric surface rather than a more complex C18 stationary phase. This study indicates that proton NMR is a useful tool for studying the diastereoselective mechanism of these derivatives, due to the relatively slow C-N bond rotation caused by the significant sp(2) character of the amino nitrogen atoms.