Are Both the (R)- and the (S)-MPA Esters Really Needed for the Assignment of the Absolute Configuration of Secondary Alcohols by NMR? The Use of a Single Derivative

The absolute configuration of a secondary alcohol can be deduced from the 1 H NMR spectra of a single methoxyphenylacetic ester derivative [MPA, either the ( R ) or the ( S )] recorded at two different temperatures. This new approach simplifies the current NMR-based methodologies, requiring just one derivatizing reaction, instead of two, and, correspondingly, half of the usual amount of sample. At low temperature, the relative population of the most stable sp conformer is increased and the resonances of the substituents of the alcohol (L 1 /L 2 ), located under the shielding cone of the phenyl ring, are shifted upfield. At the same time, those protons under the shielding cone in the less populated ap conformer are shifted downfield. In this way, the spatial location of L 1 /L 2 around the asymmetric center of the alcohol can be established comparing the 1 H NMR spectra both at room and low temperatures. Application of this finding to alcohols of known absolute configuration, including complex structures such as cis -androsterone, is presented.