Assignment of the absolute configuration of alpha-chiral carboxylic acids by H-1 NMR spectroscopy

The prediction of the absolute configuration of a-chiral carboxylic acids f rom the H-1 NMR spectra of their esters with (R)- and (S)-ethyl 2-hydroxy-2 -(9-anthryl) acetate [(R)- and (S)-g-AHA, 5] is discussed. Low-temperature NMR experiments, MM, semiempirical, and aromatic shielding effect calculati ons allowed the identification of the main conformers and showed that, in a ll esters studied, conformer crp is the most stable. A simple model for the assignment of the absolute configuration from NMR data is presented, and i ts reliability is corroborated with acids 6-31 of known absolute configurat ion. In addition to 5, other auxiliary reagents with open (32-38) and cycli c (39-42) structures have also been studied, trans-(+)- and (-)-2-phenyl-1- cyclohexanol (41) was found to be particularly efficient and produced Delta delta(RS) values similar to those of 5.