FeCl3-catalyzed α-glycosidation of glycosamine pentaacetates

Synthesis and Characterization of Versatile O-Glycan Precursors for Cellular O-Glycomics

Protein O-glycosylation is a universal post-translational modification and plays essential roles in many biological processes. Recently we reported a technology termed cellular O-glycome reporter/amplification (CORA) to amplify and profile mucin-type O-glycans of living cells growing in the presence of peracetylated Benzyl-α-GalNAc (Ac₃GalNAc-α-Bn). However, the application and development of the CORA method are limited by the properties of the precursor benzyl aglycone, which is relatively inert to further chemical modifications. Here we described a rapid parallel microwave-assisted synthesis of Ac₃GalNAc-α-Bn derivatives to identify versatile precursors for cellular O-glycomics. In total, 26 derivatives, including fluorescent and bioorthogonal reactive ones, were successfully synthesized. The precursors were evaluated for their activity as acceptors for T-synthase and for their ability to function as CORA precursors. Several of the precursors possessing useful functional groups were more efficient than Ac₃GalNAc-α-Bn as T-synthase acceptors and cellular O-glycome reporters. These precursors will advance the CORA technology for studies of functional O-glycomics.

Iron(iii) chloride-catalyzed activation of glycosyl chlorides

Glycosyl chlorides have historically been activated using harsh conditions and/or toxic stoichiometric promoters. More recently, the Ye and the Jacobsen groups showed that glycosyl chlorides can be activated under organocatalytic conditions. However, those reactions are slow, require specialized catalysts and high temperatures, but still provide only moderate yields. Presented herein is a simple method for the activation of glycosyl chlorides using abundant and inexpensive ferric chloride in catalytic amounts. Our preliminary results indicate that both benzylated and benzoylated glycosyl chlorides can be activated with 20 mol% of FeCl3.

Regiospecific Synthesis of Three Quercetin O-β-Glucosides of N-Acetylglucosamine

The regiospecific synthesis of three quercetin O-β-glucosides of N-acetylglucosamine has been achieved in good yield. Selective di-and tri- O-benzylation of quercetin followed by O-glycosylation with 2-acetamido-3,4,6-tri- O-acetyl-2-deoxy-α-D-glucopyranosyl chloride under phase-transfer catalysis conditions yielded, after deacetylation and debenzylation, 3-, 3′- and 4′-glycosylated quercetin.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

A protocol for metal triflate catalyzed direct glycosylations with GalNAc 1-OPiv donors

Herein we report on the development of novel glycosylation methodology for the concise synthesis of naturally occurring glycoconjugate motifs containing N-acetylgalactosamine (GalNAc) from the cheaper and commercially available N-acetylglucosamine (GlcNAc). The stereoselective glycosylations proceed with catalytic amounts of a promoter and without the need for N-protection other than the biologically relevant N-acetyl group. Among the catalysts explored, both Bi(OTf)3 and Fe(OTf)3 were found to be highly active Lewis acids for this reaction. It was also found that other less reactive metal triflates such as those of Cu(II) and Yb(III) can be beneficial in glycosylation reactions on more demanding glycosyl acceptors. We have furthermore demonstrated that it is possible to control the anomeric stereoselectivity in the glycosylation via postglycosylation in situ anomerization to obtain good yields of α-galactosides. The present protocol was used to prepare important naturally occurring carbohydrate motifs, including a trisaccharide fragment of the naturally occurring marine sponge clarhamnoside.

Glycosylation with N-acetyl glycosamine donors using catalytic iron(iii) triflate: from microwave batch chemistry to a scalable continuous-flow process

Efficient glycosylation reactions of peracetylated β-d-N-acetyl glycosamines are described using catalytic iron(iii) triflate under microwave conditions or in a continuous flow process.

Straightforward glycosylation of alcohols and amino acids mediated by ionic liquid

Green glycosylation of functionalized alcohols and α-amino acids, using an ionic liquid as a recyclable solvent, was performed in one step directly from the unprotected monosaccharide under scandium triflate or ferric chloride catalysis. Pure α- and β-glycosides could be obtained after specific enzymatic hydrolysis.

Glycosylation of α-amino acids by sugar acetate donors with InBr3. Minimally competent Lewis acids

A simplified method for the preparation of Fmoc-serine and Fmoc-threonine glycosides for use in O-linked glycopeptide synthesis is described. Lewis acids promote glycoside formation, but also promote undesired reactions of the glycoside products. Use of 'minimally competent' Lewis acids such as InBr(3) promotes the desired activation catalytically, and with greatly reduced side products from sugar peracetates.