Determination of the absolute configuration of alcohols by low temperature 1H NMR of aryl(methoxy)acetates

Abstract

Aromatic shielding effects in esters of ( R)- and ( S)- aryl(methoxy)acetic acids (AMAAs) differ by a factor Δδ RS , which, due to changes in rotamer populations, was seen to increase in 1H NMR spectra of esters of AMAAs 6 – 10 as NMR probe temperature ( T) was decreased. For example, Δδ RS for esters of 6 increased by more than 100% between T = 298 and T = 220−175 K (depending on the alcohol). By comparing Δδ RS values obtained from low temperature 1H NMR spectra of their esters with ( R)- and ( S)- AMAAs 6 – 10, the absolute configuration of chiral alcohols can be reliably assigned.