Stereoselective Synthesis of 2-<i>C</i>-Branched (Acetylmethyl) Oligosaccharides and Glycoconjugates: Lewis Acid-Catalyzed Glycosylation from 1,2-Cyclopropaneacetylated Sugars
作者:Qiang Tian、Liang Dong、Xiaofeng Ma、Liyan Xu、Changwei Hu、Wei Zou、Huawu Shao
DOI:10.1021/jo1016579
日期:2011.2.18
selectivity under both conditions. The stereoselectivities of glycosylation depend on the reactivity of donor sugars and Lewis acid catalyst, which effectively dictated the glycosylation pathways. The evidence suggests that galactosyl donors (e.g., 7) can undergo SN1 pathway with a strong Lewis acid (TMSOTf) and SN2 pathway under BF3·Et2O, whereas the glucosyl donors (e.g., 8 and 10) followed SN2 pathway.
在路易斯酸存在下,作为糖基供体的1,2-环丙烷乙酰化的糖与一系列糖基受体(单糖,氨基酸和其他醇)反应,生成寡糖和含有2- C-乙酰甲基糖的糖缀合物。当使用TMSOTf作为催化剂时,半乳糖基供体具有良好的出色的α选择性,而当BF 3 ·Et 2 O被用作催化剂时,半乳糖基供体具有中等至良好的β选择性。然而,葡糖基供体在两种条件下均产生β-排他性选择性。糖基化的立体选择性取决于供体糖和路易斯酸催化剂的反应性,这有效地决定了糖基化途径。有证据表明,半乳糖基供体(例如7)可以经历小号Ñ 1个通路与强路易斯酸酯(TMSOTf)和S Ñ 2通路BF下3 ·的Et 2 O,而葡糖供体(例如,8和10),接着小号ñ 2途径。立体选择性也相应于通过2- C-乙酰甲基和糖基化中的异头碳鎓中间体形成的C2'-乙缩醛中间体的形成。