A proton magnetic resonance nuclear Overhauser enhancement study. Application to vitamin D derivatives D2 and D3

Nuclear Overhauser effect difference experiments on vitamins D 2 and D 3 at 400 MHz allow an unambiguous assignment to be made for the H E and H Z protons bonded to the C-19 carbon. This assignment is especially important since the A ring, and hence the C-19 atom, undergoes a conformational interconversion. The results indicate the importance of proton nOe difference experiments in determining points of configuration and they indicate that the assignment of resonances based on allylic coupling constants in a molecule undergoing a conformational equilibrium can be incorrect.