58(P25) 2級、3級水酸基からなる1、2-グリコールの絶対配置決定へのフコフラノサイド法の適用(ポスター発表の部)
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The fucofuranoside method is a new ^1H and ^<13>C NMR procedure for determining the absolute configuration of secondary hydroxyl group by derivatizing to β-D- and β-L-fucofuranosides. For the ^1H NMR, it utilizes the specific deshielding effect of the solvent pyridine molecule (pyridine-induced shift) associated to the chiral D- and L-fucofuranosyl moiety. In contrast, for the ^<13>C NMR, it utilizes the specific glycosidation shift observed for the D- and L-isomers. The rationalization of such Δδ vues derived is described in detail in our previous report (Tetrahedron, 1997, 53, 5973 and 1998, 54, 10987). For application, the glycoside must first be examined for two points. (a) Occurrence of sufficient NOE between the anomeric proton and α-proton. (b) Whether the glycoside is 1) symmetrical or 2) unsymmetrical (R or S). It shoud be stressed that in this case the "R/S" does not mean the chemical priority. It is the order of 1) fucofuranosyl group 2) sterically bulkier β-carbon 3) sterically less bulky β-carbon. When the chemical shift of the β-D-isomer was subtracted from the corresponding chemical shift of the β-L-isomer, the abolute configuration could be indicated from the simple systematic rule depicted in Fig. 1 for ^1H NMR and in Fig. 2 for ^<13>C NMR. Detection of the accurate chemical shift of individual protons from a pile of signals in the ^1H NMR spectra is generally difficult and troublesome. In the ^<13>C NMR method of the fucofuranoside method, in contrast, the necessary signals are only four, the anomeric carbon, α-carbon, and two β-carbons. When we compare Fig. 1 and Fig. 2, it is obvious that the ^<13>C NMR method is by far advantageous than the ^1H NMR method. The fucofuranoside method is applied for five steroidal 1,2-glycols having one hydroxyl group at C-5, by glycosidating its secondary hydroxyl group. It is because it was not clear whether or not the neighboring polar hydroxyl group cause some influences, resulting in the inappropriate glycosidic linkage conformation. However, the NOE's observed for these compounds indicated that the glycosidic linkages are not affected at all and the necessary conformation was retained. The vicinal C-5 hydroxyl group contributed only increasing the steric hindrance at C-5. As a result, these five 1,2-glycols showed normal Δδ values typical of symmetrically or unsymmetrically substituted secondary alcohols as shown in Fig. 3 and Fig. 4, thus showing the effectiveness of the fucofuranoside method for determining the absolute configuration of such cyclic secondary-tertiary 1,2-glycols. Another advantage of the present method is that the reagent, fucofuranose tetraacetate, is stable and not expensive. The disadvantage is that, for the compounds liable to dehydrate during the glycosidation reaction, its application is not suitable.
- 1999-09-01
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関連論文
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- 58(P25) 2級、3級水酸基からなる1、2-グリコールの絶対配置決定へのフコフラノサイド法の適用(ポスター発表の部)