1H NMR and molecular mechanics studies of epimeric ethyl esters of 20(R, S)‐hydroxy‐23‐norcholanoic acids

Proton chemical shifts for the C‐18, C‐21, CO2CH2CH3 and CO2CH2CH3 protons of epimeric ethyl esters of 20(R,S)‐hydroxy‐23‐norcholanoic acids were measured in deuteriochloroform and pyridine‐d5. The observed solvent shifts due to specific OH—pyridine hydrogen‐bonded complexes allowed the quantification of the epimeric mixtures by analysis of the pyridine‐d5 1H NMR spectra. The main features of the pyridine‐induced shifts are rationalized in terms of the preferred conformations for the 20R‐ and 20S‐epimers, which are predicted by molecular mechanics calculations.