Assignment of the Absolute Configuration of β-Chiral Primary Alcohols by NMR: Scope and Limitations

Abstract

The prediction of the absolute configuration of β-chiral primary alcohols from the 1H 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 ΔEag (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 ΔEag values (2.2−3.1 kcal/mol) due to their different conformational composition, and their absolute configuration cannot be reliably predicted by this method.