Fluoride Migration Catalysis Enables Simple, Stereoselective, and Iterative Glycosylation.

Challenges in the assembly of glycosidic bonds in oligosaccharides and glycoconjugates pose a bottleneck in enabling the remarkable promise of advances in the glycosciences. Here, we report a strategy that applies unique features of highly electrophilic boron catalysts, such as tris(pentafluorophenyl)borane, in addressing a number of the current limitations of methods in glycoside synthesis. This approach utilizes glycosyl fluoride donors and silyl ether acceptors while tolerating the Lewis basic environment found in carbohydrates. The method can be carried out at room temperature using air- and moisture-stable forms of the catalyst, with loadings as low as 0.5 mol %. These characteristics enable a wide array of glycosylation patterns to be accessed, including all C1-C2 stereochemical relationships in the glucose, mannose, and rhamnose series. This method allows one-pot, iterative glycosylations to generate oligosaccharides directly from monosaccharide building blocks. These advances enable the rapid and experimentally straightforward preparation of complex oligosaccharide units from simple building blocks.