Synthesis, Conformational Analysis, and Biological-Activity of the 1-α,25-Dihydroxy-10,19-Dihydrovitamin D3 Isomers

The synthesis of the four 1α,25-dihydroxy-10,19-dihydrovitamin D3 stereoisomers (8-11) is described starting from 25-hydroxyvitamin D3(1c). Acetic acid-catalyzed cycloreversion of 1 α-hydroxylated 3,5-cyclovitamin D compound 14, produced by allylic oxidation of the intermediate cyclovitamin 13, afforded 1α,25-dihydroxyvitamin D3 3-acetate (16) and its 5E-isomer 17. Catalytic hydrogenation of 16 produced 10,19-dihydrovitamin acetates 20 and 21, whereas the same reaction of 17 resulted in the formation of 5E-isomers 22 and 23. The analogous saturation of the 10,19-double bond in 14 gave 10(S), 19- and 10(R), 19-dihydrocyclovitamins 18 and 19 which after cycloreversion with acetic acid yielded different stereoisomeric pairs of 10,19-dihydrovitamin acetates 21, 23, 20, and 22, respectively. The stereochemistry and solution conformations of the A-ring of the 3-acetates 20-23 and their parent alcohols 8-11 were studied using 1H NMR data. The A-ring chair population ratios of these stereoisomers were determined by the method of correlation of the observed coupling constants with the limiting values derived from cyclohexanol. The obtained results were confirmed by evaluation of interaction energies introduced by A-ring substituents and calculation of the free energy differences between the respective dihydrovitamin conformers. Conformational analyses of 10,19-dihydrovitamins were also carried out on model compounds 24-27 by using force-field calculations. Biological activity in vivo revealed that the 1 α,25-dihydroxy-10(S), 19-dihydrovitamin D3(9) followed by the 1α,25-dihydroxy-10(S), 19-dihydro-(5E)-vitamin D3(11) to be the most active, while the 10(R)-isomers 8 and 10 possessed little or no activity. In vitro, the compounds possessing the most equatorial 1-hydroxyl, i.e., the 10(R)-isomers, were found most active, and the least equatorial were the least active.