4-Oxo-α-amino acids: a caution when determining absolute configurations by 1H NMR using MPA derivatization and Ba2+ complexation

A method for determining the absolute configurations of chiral amines based on changes in the 1H NMR spectra of their (R)-MPA amides on addition of Ba2+ was applied to both enantiomers of a series of γ-oxo-α-amino acid methyl esters. The chemical shift changes ΔδBa observed for these compounds were inconsistent with the recently proposed model relating them to the absolute configuration of the amino group. These results suggest that care must be taken when using this technique.