Boc-Phenylglycine: The Reagent of Choice for the Assignment of the Absolute Configuration of α-Chiral Primary Amines by 1H NMR Spectroscopy

Abstract
The absolute configuration of an α-substituted primary amine can be easily determined by direct comparison of the 1H NMR spectra of the (R)- and the (S)-Boc-phenylglycine derivatives. A simplified model, based on extensive conformational analysis and NMR data, is presented that associates the spatial location of the substituents around the asymmetric carbon of the amine with the signs of ΔδR,S. The substituent with a negative ΔδR,S occupies the position of L1 in Figure 7 and that with a positive ΔδR,S the position of L2. From this model the absolute configuration (R/S) of the chiral center can be determined.The BPG amides give rise to much higher ΔδR,S values than the classical reagents MTPA and MPA and offer a greater guarantee for the correct assignment of the absolute configuration.