Regio- and stereoselective synthesis of oligosaccharides with unprotected or lightly protected sugars as glycosyl acceptors

Regio- and stereoselective synthesis of oligosaccharides with unprotected or lightly protected gluco-, manno-, and rhamnose as glycosyl acceptors was achieved via sugar orthoester formation-rearrangement. alpha-(1-->6)-Linked mannose oligosaccharides, beta-(1-->6)-linked glucose oligosaccahrides, and 3,6-branched mannose and glucose oligosaccharides were conveniently synthesized with quite simplified procedure. Biologically important oligosaccharides such as phytoalexin elicitor glucohexaose, antitumor fragments of lentinan, and some repeating units of biologically important polysaccharides were prepared readily. Recent progress in regioselective synthesis of oligosaccharides with lightly protected glucosamine and galactose as acceptors was also outlined.