Efficient synthesis and enzymatic extension of an N-GlcNAz asparagine building block.

N-Azidoacetyl-d-glucosamine (GlcNAz) is a particularly useful tool in chemical biology as the azide is a metabolically stable yet accessible handle within biological systems. Herein, we report a practical synthesis of FmocAsn(N-Ac3GlcNAz)OH, a building block for solid phase peptide synthesis (SPPS). Protecting group manipulations are minimised by taking advantage of the inherent chemoselectivity of phosphine-mediated azide reduction, and the resulting glycosyl amine is employed directly in the opening of Fmoc protected aspartic anhydride. We show potential application of the building block by establishing it as a substrate for enzymatic glycan extension using sugar oxazolines of varying size and biological significance with several endo-ß-N-acetylglucosaminidases (ENGases). The added steric bulk resulting from incorporation of the azide is shown to have no or a minor impact on the yield of enzymatic glycan extension.

Protecting-group-free one-pot synthesis of glycoconjugates directly from reducing sugars.

The conversion of sugars into glycomimetics typically involves multiple protecting-group manipulations. The development of methodology allowing the direct aqueous conversion of free sugars into glycosides, and mimics of oligosaccharides and glycoconjugates in a high-yielding and stereoselective process is highly desirable. The combined use of 2-azido-1,3-dimethylimidazolinium hexafluorophosphate and the Cu-catalyzed Huisgen cycloaddition allowed the synthesis of a range of glycoconjugates in a one-step reaction directly from reducing sugars under aqueous conditions. The reaction, which is completely stereoselective, may be applied to the convergent synthesis of triazole-linked glycosides, oligosaccharides, and glycopeptides. The procedure provides a method for the one-pot aqueous ligation of oligosaccharides and peptides bearing alkyne side chains.