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.

MgI2‐chemoselective cleavage for removal of amino acid protecting groups: A fresh vision for peptide synthesis

“…1 H NMR (DMSO, 400 MHz) δ 3.10 (dd, 1H, J = 10.7 Hz, J = 13.7 Hz), 3.22 (dd, 1H, J = 4.4 Hz, J = 13.8 Hz), 4.63–4.69 (m, 1H), 7.16–7.20 (m, 1H), 7.25–7.29 (m, 2H), 7.33–7.34 (m, 2H), 7.42–7.46 (m, 2H), 7.50–7.54 (m, 1H), 7.80–7.82 (m, 2H), 8.72 (d, 1H, J = 8.2 Hz); C NMR (DMSO, 100 MHz) δ 36.34, 54.30, 126.43, 127.41, 128.26, 128.31, 129.13, 131.43, 133.99, 138.27, 166.48, 173.31; LC/MS r t : 1.53; MS (ESI+): m/z 284.2 ([M + H] + ); HRMS (ESI+) m/z: Calcd for [C 16 H 15 NO 3 + H] + : 270.1130, found: 270.1129. Spectral data were consistent with those previously reported in the literature …”

Adducts of rhodium(II) tetraacylates with methionine and its derivatives: 1H and 13C nuclear magnetic resonance spectroscopy and chiral recognition

“…3,4 The absolute configurations of a few a-amino acids have been determined by a 'dirhodium method', using enantiomerically pure rhodium salts of Mosher's acid (Rh 2 MTPA 4 ) as auxiliary reagents. 19 It has been mentioned that rhodium tetraacetate formed a 1:2 axial adduct with methionine via the sulfur atom. Cysteine, another amino acid bearing a sulfur function, irreversibly replaced all the l-acetates in the dirhodium salt producing an equatorial adduct.…”

Complexation of rhodium(II) tetracarboxylates with aliphatic diamines in solution: 1H and 13C NMR and DFT investigations

“…By this, one could determine either the optical purity of a compound [10][11][12][13][14][15][16][17][18][19][20] or, in certain cases, its absolute configuration. [21][22][23][24] For the proper application of dirhodium methods, it is essential to know the plausible adduct structures and the details of complexation processes.…”