Inhibitors of the 25-hydroxylation of vitamin D3 in the rat

Two new sidechain-modified analogs of vitamin D₃, 25-azavitamin D₃ and 25-fluorovitamin D₃, were prepared; both compounds were found to inhibit the in vivo 25-hydroxylation of vitamin D₃ in the rat. 25-Azavitamin D₃ was chemically synthesized from a degradation product of stigmasterol by a six-step process. The desired carbon skeleton was efficiently assembled by alkylation of a suitably protected C-20 bromomethylpregnane with the enolate of N,N-dimethylacetamide (70%). The completion of the synthesis utilized the known photochemistry of steroidal 5,7-dienes to prepare the vitamin D triene system. In contrast, 25-fluorovitamin D₃ was prepared by direct vitamin modification. 25-Hydroxyvitamin D₃ 3-acetate was fluorinated with diethylaminosulfur trifluoride to give 25-fluorovitamin D₃ 3-acetate (59%); saponification provided the desired analog. When vitamin D-deficient rats on a low calcium diet were dosed with 3-³H]vitamin D₃ (0.05 μg), 10% of the dose was found in serum as 25-hydroxyvitamin D₃ 4 hr after administration. If 25-azavitamin D₃ (50 or 200 μg) was given 2 hr before the radiolabeled vitamin D₃, however, serum 25-hydroxyvitamin D₃ concentration was markedly reduced. 25-Fluorovitamin D₃ caused similar reduction when administered at much lower doses.