SOLVENT EFFECT ON PREVITAMIN-D CONFORMATIONAL EQUILIBRIUM AND PHOTOREACTIONS

The effect of solvent polarity on the conformational equilibrium of pr evitamin D has been discussed in terms of the abundance (%) of its con formers calculated by molecular mechanics in conjunction with nuclear magnetic resonance experimental data available in the literature. It h as been assumed that polar reaction media shift the conformational equ ilibrium towards the most non-strained cZc conformations with the OH g roup in pseudo-equatorial orientation. 7-Dehydrocholesterol (provitami n D) photoisomerization on irradiation at lambda = 254 nm has been stu died in ethanol and hexane by UV spectroscopy. A consideration of the observed differences in combination with the calculations of the react ion kinetics provides an estimation of the quantum yields of previtami n D cis-trans isomerization and additional experimental evidence for t he solvent-induced changes in the conformational equilibrium of previt amin D in favour of cZc forms in polar environments. (C) 1997 Elsevier Science S.A.