H-1-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 H-1 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.