Regioselective Glycosylation of Non-protected Methyl Hexopyranosides Using the Stannylene Activation Method

点击翻译按钮获取中文摘要

Developing a new method for regioselective glycosylation of unprotected sugars is highly desired, since the conventional protection-deprotection method renders the overall synthetic scheme lengthy and impractical. Chemical reactions should be much more selective, rapid, and productive.In this context, very few precedents have revealed that the stannylene-mediated regioselective glycosylation of unprotected hexopyranosides makes it possible to provide one-pot assembly of some oligosaccharides. However, the previous methods were limited to afford α / β-(1 → 6)-linked disaccharides only, I.e., glycosylation of the primary hydroxyl group of hexoses is preferred, and the yields of the glycosylation reaction are not always high. Therefore, it is necessary to develop a novel regioselective glycosylation applicable to the secondary hydroxyl groups even in the presence of the primary hydroxyl one, giving for example, α / β-(1→3)-linked disaccharides.We have developed a novel methodology of regio- and stereoselective glycosylation for fully unprotected methyl hexopyranosides by the stannylene activation method. Ag(I)-silica alumina-promoted glycosylation of stannylated sugars in the one-pot process was shown to yield β-(1 → 6)-linked disaccharides predominantly. Furthermore, addition of some tetra butylammonium salts to the reaction medium provides a significant regioselectivity shift on the glycosylation of unprotected methyl β-D-galactopyranoside, affording glucosyl-β-(1 → 3) -galactoside in preference to the corresponding β-(1→6)-linked disaccharide.Since glycosyl β-(1→ 3)-galactoside moiety is well known as a core disaccharide unit of many immunologically relevant oligosaccharides, the method reported here would contribute to synthetic carbohydrate chemistry, which can be an alternative to the conventional "protection-deprotection" method.