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. A modular synthesis of core‐fucosylated N‐glycans was optimized, leading to the biantennary octasaccharide N‐glycan A. Several obstacles to the synthesis of functionalized core trisaccharide building block B were overcome, leading to an improved and efficient protocol for β‐mannosylation by intramolecular inversion.