Carbohydrates are diverse bio-macromolecules with highly complex structures that are involved in numerous biological processes. Well-defined carbohydrates obtained by chemical synthesis are essential to the understanding of their functions. However, synthesis of carbohydrates is greatly hampered by its insufficient efficiency. So far, assembly of long carbohydrate chains remains one of the most challenging tasks for synthetic chemists. Here we describe a highly efficient assembly of a 92-mer polysaccharide by the preactivation-based one-pot glycosylation protocol. Several linear and branched oligosaccharide/polysaccharide fragments ranging from 5-mer to 31-mer in length have been rapidly constructed in one-pot manner, which enables the first total synthesis of a biologically important mycobacterial arabinogalactan through a highly convergent [31+31+30] coupling reaction. Our results show that the preactivation-based one-pot glycosylation protocol may provide access to the construction of long and complicated carbohydrate chains.

Glycosylation is unarguably the most important reaction in the field of glycochemistry. Photo-initiated reactions hold extraordinary potential in organic synthesis. Herein we report a novel light-driven strategy for the activation of thioglycosides via the merger of a CF₃ radical pathway followed by the subsequent glycosylation with glycosyl acceptors. This protocol could efficiently activate thioglycosides, thereby greatly enhancing the substrate scope of reactions. In particular, the glycosylation reactions, which are completely inert under the existing photo-induced glycosylation conditions, proceeded smoothly in high yields. Many common protective groups were well tolerated under the coupling conditions. Both UV and visible light or even sunlight were used as the source of light for the reactions. The high efficiency of this light-driven glycosylation protocol was further highlighted by the rapid one-pot sequential assembly of oligosaccharides.