Conformational analysis of 1α,25-dihydroxyvitamin D 3 by nuclear magnetic resonance

Abstract

The conformational equilibrium of 1α,25-dihydroxyvitamin D 3 ( 3) was studied by variable temperature 1H and 13C NMR spectroscopy. At temperatures below −100°C, two conformations in a ratio of 55: 45 were observed by splitting of the signals for H-3α, H-4α, and H-6. A free enthalpy of interconversion was estimated to be ΔG 193°K ‡ , 39.6 kJ/mol from the 13C NMR spectrum at −80°C. From an analysis of the A-ring coupling constants in various solvents, we propose that the A-ring of 3 undergoes chair-chair interconversion in hydrophobic media and chair-half-chair interconversion in hydrophilic media.