Assignment of the absolute configuration of β-chiral primary alcohols by NMR:: Scope and limitations

The prediction of the absolute configuration of beta-chiral primary alcohols from the H-1 NMR spectra of their esters with (R)- and (S)-9-anthrylmethoxyacetic acids (9-AMA, 3) is discussed. Low-temperature NMR experiments, MM, semiempirical, ab initio, and aromatic shielding effect calculations allowed the identification of the main conformers and showed that, in all alcohols for which the calculated Delta E-ag (CVff) is in the range of 0.7-1.5 kcal/mol, conformer a/a is the most stable. A simple model for the assignment of the absolute configuration from NMR data is presented and its reliability corroborated with alcohols (8-20) of known configuration. Nevertheless, cyclic alcohols 21-23 have much higher Delta E-ag values (2.2-3.1 kcaV mel) due to their different conformational composition, and their absolute configuration cannot be reliably predicted by this method.