Modular Synthesis of Core Fucosylated N‐Glycans

A modular synthesis of complex-type N-glycans containing the core fucosyl motif was optimized. The core trisaccharide building block was protected by a methoxyphenyl group for convenient core fucosylation. The trisaccharide was obtained on a large scale from the glycosylation of the corresponding chitobiosyl azide with a glucosyl donor followed by intramolecular inversion. Improved methods were established for the synthesis of the monosaccharide building blocks and for their couplings. The inversion to the β-mannoside was accompanied by previously unnoticed side-reactions resulting in the hydrolytic ring-opening of the iminocarbonate intermediate. The benzylidene-protected core trisaccharide was elongated into a biantennary N-glycan heptasaccharide by two regio- and stereoselective couplings. The final fucosylation also gave some of the β anomer, which could be removed by HPLC to give an α1,6-fucosylated N-glycan octasaccharide.