Solution conformations of the A and seco B rings of vitamin D3, 1α,25-dihydroxyvitamin D3, 1α-hydroxyvitamin D 3, and dihydrotachysterol3 have been established by high resolution, 300-megahertz proton magnetic resonance spectroscopy. The A ring of these steroids is dynamically equilibrated between two chair conformers. For vitamin D3, 1α-hydroxyvitamin D3, and 1α,25-dihydroxyvitamin D3 the relative proportions of the two conformers are 1:1, whereas dihydrotachysterol3 exists principally as only one conformer. Thus, the substituent groups on the A ring may be either equatorially or axially oriented, and suggests a refinement of the existing topological model for vitamin D hormonal activity.
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