Stereo selectivity of glycosylations of conformationally restricted mannuronate esters
Stereo selectivity of glycosylations of conformationally restricted mannuronate esters
复制标题
DOI：
10.1016/j.tet.2009.02.067
复制
发表时间：
2009-05-02
期刊：
影响因子：
2.1
通讯作者：
van der Marel, Gijsbert A.
中科院分区：
文献类型：
--
作者：
Codee, Jeroen D. C.;de Jong, Ana R.;van der Marel, Gijsbert A.
构象不受限制的甘露糖醛酸酯供体的糖苷化以高度P-选择性方式进行，而构象上受3,4-丁烷二甲基缩醛或2,3-异丙叉官能团限制的甘露糖醛酸酯供体的缩合提供α-选择性产物。我们假设这些缩合的立体化学结果的差异是由于形成氧碳鎓中间体的产物的不同构象造成的。由柔性甘露糖醛酸形成的β-连接产物被认为源自最有利的H-3(4)氧碳鎓离子，该离子无法从构象受限的供体中获得。尽管在3,4-丁烷二甲缩醛保护的甘露糖醛酸酯硫供体活化时形成α-三氟甲磺酸酯中间体，但这不是形成中间体的产物。异头三氟甲磺酸酯充当H-4(3)氧碳鎓离子的储存库，其被糖苷化以提供α-连接的甘露糖醛酸。 (C) 2009 Elsevier Ltd. 保留所有权利。
Glycosidation of conformationally unrestricted mannuronate ester donors proceeds in a highly P-selective fashion, whereas condensations of mannuronate ester donors, which are con formationally constrained by a 3,4-butanedimethylacetal or a 2,3-isopropylidene function, provide a-selective products. We hypothesize that the difference in stereochemical outcome of these condensations results from the different conformations of the product forming oxacarbenium intermediate. The formation of the beta-linked products from the flexible mannuronates is thought to originate from the most favorable H-3(4) oxacarbenium ion, which is not accessible from the conformationally restrained donors. Although an a-triflate intermediate is formed upon activation of the 3,4-butanedimethylacetal protected mannuronate ester thio donor, this is not the product forming intermediate. The anomeric triflate serves as a reservoir for the H-4(3) oxacarbenium ion, which is glycosidated to provide the a-linked mannuronates. (C) 2009 Elsevier Ltd. All rights reserved.