23 活性型ビタミンDの合成研究

抄録

Fucosterol 6 is abundant and the almost sole sterol in brown algae, irrespective of species. Recently we have found that fucosterol-24,28-epoxide 7 is converted to desmosterol 8 by a brief treatment with BF_3-etherate. Thus, 8 is now readily available as the starting compound for the synthesis of the active forms of vitamin D_3. Oxymercuration-demercuration of desmosterol acetate 10 gave 25-hydroxycholesterol 3-acetate 14 in 85% yield. Dehydration of 14 yielded a mixture of Δ^<24>- and Δ^<25>-olefine(10 and 15), from which 24,25-glycol 16 and 25,26-glycol 17 were obtained by treatment with OsO_4 in high yields. 1α,25-Dihydroxycholesterol 31 was synthesized by two methods. In the first synthesis of 31 from 14, Δ^5-double bond was protected by hydroboration to give 6α-ol 22, which was inverted to 6β-ol 23 in order to assure the selective dehydration to recover Δ^5-double bond. Introduction of 1α-ol function rest on epoxydation with H_2O_2 -NaOH of 3-oxo-1-ene system 25. The second synthesis of 31 was based on the recently reported Barton's procedure. Thus, 14 was dehydrogenated with DDQ, followed by epoxidation to give 38, whose reduction with Li in liq. NH_3 afforded 31(over-all yield from 6; 6%). These hydroxycholesterol derivatives were transformed into the active forms of vitamin D by the established method.