A new method proposed for the determination of absolute configurations of α‐amino acids

Abstract

Enantiopure alpha-amino acids were converted to 4-substituted 2-aryl- and 2-alkyl-5(4H)-oxazolones under partial racemization. These nonracemic mixtures were dissolved in CDCl(3), an equimolar amount of the chiral dirhodium complex Rh(II)(2)[(R)-(+)-MTPA](4) (MTPA-H = Mosher's acid) was added, and the (1)H NMR spectra of the resulting samples were recorded (dirhodium method). The relative intensities of (1)H signals dispersed by the formation of diastereomeric adducts allow to determine the absolute configuration (AC) of the starting alpha-amino acids. Binding atoms in the adducts were identified by comparing the (1)H and (13)C chemical shifts of the oxazolones in the absence and presence of Rh(II)(2)[(R)-(+)-MTPA](4). Thereby, information about the scope and limits of this method can be extracted. A protocol how to use this method is presented.