Enantiomeric discrimination in the NMR spectra of underivatized amino acids and α-methyl amino acids using (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

The compound (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (I) is a useful chiral NMR discriminating agent for underivatized amino acids and α-methyl amino acids. The α-methyl amino acids, when mixed with I in methanol, are protonated through a reaction with a carboxylic moiety of I and associate with the crown ether. Amino acids such as tryptophan, valine, alanine, and phenylglycine can be solubilized at suitable concentrations in methanol through the addition of deuterium chloride. The NMR spectra of mixtures of these amino acids with I show enantiomeric discrimination. Addition of ytterbium(III)nitrate to crown-substrate mixtures causes only small shifts and no discernible enhancements in enantiomeric discrimination in the NMR spectra of the substrates.