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

The compound (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (I) is a usefu l chiral NMR discriminating agent for underivatized amino acids and alpha-m ethyl amino acids. The alpha-methyl amino acids, when mixed with I in metha nol, are protonated through a reaction with a carboxylic moiety of I and as sociate with the crown ether. Amino acids such as tryptophan, valine, alani ne, and phenylglycine can be solubilized at suitable concentrations in meth anol through the addition of deuterium chloride. The NMR spectra of mixture s 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. (C) 2000 Elsevier Science Ltd. All rights reserv ed.