Relative and Absolute Stereochemistry of Secondary/Secondary Diols: Low-Temperature ~1H NMR of Their bis-MPA Esters

摘要

Comparison of the room- and low-temperature ~1H NMR spectra of the bis-(R)- or bis-(S)-MPA ester derivative of an open chain sec,sec-1, 2-diol allows the easy determination of its relative stereochemistry and in some cases absolute configuration. If the diol is anti, its absolute configuration can be directly deduced from the signs of Δδ~(T1T2) for substituents R_1/R_2, but if the relative stereochemistry of the diol is syn, the assignment of its absolute configuration requires the preparation of two derivatives (both the bis-(R)- and bis-(S)-MPA esters), comparison of their room-temperature ~1H NMR spectra, and calculation of the Δδ~(RS) signs for the methines Hα(R_1) and Hα(R_2) and R_1/R_2 protons. The reliability of these correlations is validated with 17 diols of known absolute configuration used as model compounds.