An NMR study of the conformational mobility of steroid estrogen 7α-methyl-8α analogues

All the signals in the H-1 and C-13 NMR spectra of some analogues 7 alpha-methyl-8 alpha- and 6-oxa-8 alpha-steroid estrogens were completely assigned. Considering the values of nuclear Overhauser effect and vicinal coupling constants, these steroids were shown to exhibit a fast, on the NMR time scale, conformational equilibrium arising due to the inversion of ring B. The conformer populations were obtained from a comparison of the experimental and theoretical values of the dihedral angles and the interproton distances. This conformational equilibrium was shown to depend on the nature of atom in position 6: for the 7 alpha-methyl-6-oxa-8 alpha analogues of the steroid estrogens, the population of the conformer with the pseudoaxial orientation of the 7 alpha-methyl group was observed to be decreased compared with the 7 alpha-methyl-8 alpha analogue.