119(P-72) ねじれ舟型配座の糖供与体を用いたβ選択的O-グルコシル化反応(ポスター発表の部)
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概要
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We developed a β-selective O-glucosidation reaction by means of controlling the ring-conformations of the glycosyl donors. A thioglucoside 1 was chosen as the glucosyl donor of our preliminary investigations. The reactions with cyclohexylmethanol were examined. As a result, β-selective reaction (α/β〜1/9) was observed in CH_2Cl_2, toluene, ether, and hexane, respectively. The results suggested that the high β-selectivity was controlled by the ring-conformation of the glucosyl donor. ^1H NMR showed that 1 existed in the skew-boat form; the methyleneoxy group at the 5-position did not orient axially completely. Thus, the 1,3-diaxial repulsion that prevents the β-face attack of the glucosyl acceptor was decreased in comparison with the hindrance due to the axial 2-O-TIPS group. When the protecting group at the 6-position was changed to pivaroyl group, higher β-selectivity was observed. The ring conformation of the 6-O-piv compound (3) was comfirmed to be in the skew-boat form (^3S_1,) by the ^1H NMR analysis, and 3 reacted not only with the plimary alcohol but also with secondary and tertiary alcohols in highly β-selective manner. This method was applied for the β-selective introduction of a disaccharide unit into cholesterol. Full TBS protection of the disaccharide 7 gave the disaccharide donor 8. The ring conformation of the glucose moiety was ^3S_1, and the ring of xylose part was in ^1C_4. Using 8, highly β-selective introduction of the disaccharide unit was possible to give 9 in 56% yield (α/β =3/97). This example shows that the convergent synthesis became possible to prepare the 2-O-glycosyl β-O-glucosides such as 10.
- 天然有機化合物討論会の論文
- 2004-10-01
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