Rapid assembly of branched mannose oligosaccharides through consecutive regioselective glycosylation: A convergent and efficient strategy.

A convergent and efficient strategy for the synthesis of high-mannose oligosaccharides is described wherein regioselective glycosylations between trichloroacetimidate donors and partially protected acceptors are employed to reduce the number of protection-deprotection steps. Two representative branched mannose oligosaccharides, a mannose heptasaccharide (Man7) and a mannose nonasaccharide (Man9) were constructed via (4+3) and (5+4) glycosylations, respectively. These mannose-containing oligosaccharides were obtained in nine steps in ~25% overall yield and >98% purity on 60-70 mg scales to demonstrate the effectiveness of the strategy.