抄録
The thermal conversion of 1 α-hydroxyprevitamin D3 (1α-OH-previtamin D3) diacetate to 1 α-hydroxyvitamin D3 (1α-OH-vitamin D3) diacetate was investigated in five solvents. The fraction of 1α-OH-vitamin D3 diacetate was calculated from the HPLC peak areas (UV detection) of 1α-OH-previtamin D3 diacetate and 1α-OH-vitamin D3 diacetate. When 1α-OH-previtamin D3 diacetate was dissolved in ethanol, benzene, toluene, isopropyl ether, or n-hexane, and heated at 60°C, the yield of 1α-OH-vitamin D3 diacetate increased during the first 4h, and reached an equilibrium level after 8.5h. Differences in the ratio of 1α-OH-previtamin D3 diacetate to 1α-OH-vitamin D3 diacetate at thermal equilibrium, and in the rate of the thermal isomerization were observed among these five solvents. Molecular mechanics (MM) calculations were performed in order to estimate solvent effects on conformation for 1α-OH-previtamin D3 diacetate and 1α-OH-vitamin D3 diacetate. The sol-vent effect was treated by specifying a dielectric constant representative of each of the three solvents: ethanol (polar), n-hexane (nonpolar), and benzene (aromatic). The dielectric constants used were 24.3 for ethanol, 1.5 for n-hexane, and 2.3 for benzene. It is suggested that the confor-oration of 1α-OH-vitamin D3 diacetate is stabilized in polar solvent. However, the order of conformational stability when solvent effects are included in the calculations is: ethanol> benzene>n-hexane. This order does not follow the experimental results. The proton NMR chemical shifts of 1α-OH-vitamin D3 diacetate are different in deuterated n-hexane, ethanol, and benzene. The downfield shift of the C-6 vinyl proton of 1α-OH-vitamin D3 diacetate, when compared to the chemical shift in benzene, is 0.15 and 0.11ppm relative to the chemical shift in n-hexane and ethanol, respectively, and that of the C-7 proton was 0.30 and 0.33 ppm, respectively. No significant proton shift of 1α-OH-previtamin D3 diacetate is recorded in these three solvents. To account for the increased ratio of 1α-OH-vitamin D3diacetate to 1α-OH-previtamin D3 diacetate ratio in benzene, we suggest that 1α-OH-vitamin D3 diacetate may be stabilized via specific solute-solvent interactions in benzene.
