Modification Of NBS-catalyzed Glycosylation And Synthesis Of Alkynyl Xylosides

To efficiently construct glycosidic bonds has been a long term goal of carbohydrate chemists.After more than a century of development,various glycosylation methods have been developed with advantages and disadvantages.There is no unified formula and the yield of the reaction is not always ideal in the synthesis of complex oligosaccharides.In this article,we have improve an NBS-activated glycosylation method.In the glycosylation process,only 0.5 equivalent of NBS/NIS is used in conjunction with AgOTf.Innitially,only half of the glycosylation donors should be activated,consequently,0.5 equivalents of p-toluenethioflate(TfOSTol)generated during the reaction are proposed to promote the remaining 0.5 equivalent ofglycosylation donor affording a complete glycosylation.This new method incorporate inexpensive NBS reagent and could successfully avoid electrophilic side reaction from excess amount of NBS reagent.Xylose is a pentose sugar.Natural D-xylose majorly exists in plants in the form of macromolecular polymerized xylan,which is widely present in plants and is the second most abundant sugar in nature.Xylose is also present in N-glycans.N-glycans are very important glycoconjugates.It is difficult to study N-glycans in mammals,but some N-glycans in plants have unique xylose residues.The study of N-glycans in plants may be instructive for animal N-glycan research.For example,unlike mammalian cells,N-glycans in plants have ?-1,2 xylose residues attached to galactose adjacent to the trimannose core.These differences potentially result in allergic reactions induced by the injection of plant glycoproteins into mammals.Therefore,the functional study of xylose may promote the production of mammalian glycoproteins in plantsBecause the N-glycans in some plant proteins contain xylose residues that are not found in animals,we chose several derivatized xylosides as tools for biological study The designed xylosides are functionalized with an alkynyl group,which can be metabolized and transferred into N-glycan in cells.Then,a fluorophore or other functional group could be specifically connected to xyloside via click chemistry to probe interested biological process.In the first chapter,I overviewed some glycosylation methods followed by the biological significance of xyloseIn the second chapter,I first introduced the glycosylation activated with 0.5 equivalent of NBS,and conducted glycosylation with several commonly used donors and acceptors,and obtained satisfactory results.I also used 0.5 equivalent of NIS to activate glycosylation,and we still got satisfactory results.Then I discussed the reaction mechanism of 0.5 equivalent of NBS/NIS.In the process of NBS/NIS activated glycosylation,0.5 equivalent of donor activated by NBS/NIS produced 0.5 equivalent of p-toluenesulfonate trifluoromethanesulfonate(TfOSTol)during the reaction,which further activated the remaining 0.5 equivalent of the donor,resulting in a complete conversion glycosylation product.In the third chapter,I selected three xylose derivatives containing alkynyl groups for the study of N-glycans with unique xylose residues.All three xylose derivatives contain an alkynyl structure and can be connected to the fluorescent group through click chemistry.After regioselective protection with TBDMS group,I was able to isolate three different xylosides,which were acetylated followed with introduction of propargyl group to furnish the desired compounds.However,due to the acyl transfer,I finally got only one xylose derivative.