Solvent effect on previtamin D conformational equilibrium and photoreactions

The effect of solvent polarity on the conformational equilibrium of previtamin D has been discussed in terms of the abundance (%) of its conformers calculated by molecular mechanics in conjunction with nuclear magnetic resonance experimental data available in the literature. It has been assumed that polar reaction media shift the conformational equilibrium towards the most non-strained cZc conformations with the OH group in pseudo-equatorial orientation. 7-Dehydrocholesterol (provitamin D) photoisomerization on irradiation at lambda = 254 nm has been studied in ethanol and hexane by UV spectroscopy. A consideration of the observed differences in combination with the calculations of the reaction kinetics provides an estimation of the quantum yields of previtamin D cis-trans isomerization and additional experimental evidence for the solvent-induced changes in the conformational equilibrium of previtamin D in favour of cZc forms in polar environments. (C) 1997 Elsevier Science S.A.