88(P19) カイラルな3級アルコール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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In contrast to many for chiral secondary alcohols, there are little spectroscopic methods for determining the absolute configuration of tertiary alcohols. The recently devised new method, the fucofuranoside method, was applied to several chiral tertiary alcohols and the scope and limitation of the method is presented. The advantageous characteristics of the fucofuranoside method, compared with those existing methods such as MTPA (Mosher) determination, can be summarized as follows. 1) For the ^1H NMR analyses, it utilizes not the direct anisotropy of the substituent, but the strong anisotropy of pyridine, the solvent which solvates to the polar substituent. 2) As polar and chiral substituents, the β-D- and β-L-fucofuranosyl groups are used for their orientation effect (exo-anomeric effect) on the aglycon. 3) Its conformation is therefore more rigid than that of the ester linkages so that it is applicable to axial hydroxyl groups. 4) The specific non-bonded interactions due to the glycosidic linkages cause carbon chemical shifts remarkably different between the two diastereomeric glycosides. 5) This last fact enables the ^<13>C NMR to be used simultaneously for determining the configurations. The fucofuranoside method was found to be effective for the four simple tertiary alcohols, 3α-methyl-5α-cholestane-3β-ol (1), 3β-methyl-5α-cholestane-3α-ol (2), 3α-methyl-5β-cholestane-3β-ol (3) and 3β-methyl-5β-cholestane-3α-ol (4). For the Δδ values (δ^L-δ^D) of the protons and carbons of the glycosides, placing the furanosyl group in front and the methyl group down, the following generalizations were derived. (a) The Δδ_H, values are positive for the protons of the right segment and negative for the protons of the left segment. (b) The Δδ_C, value is positive for the right-hand β-carbon and negative for the left-hand β-carbon, but is negligible for the anomeric, methyl, and carbinyl carbons. The same procedure applied for 17α-methyltestosterone (5) and the double bond isomer (6) of steviol showed inconsistent results, indicating that different treatment is neccessary for such compounds in which the conformation of the two enantiomeric furanosyl substituents seems to be unsymmetrical.
- 天然有機化合物討論会の論文
- 1997-07-20
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