Combinatorial Synthesis of an Oligosaccharide Library by Using β-Bromoglycoside-Mediated Iterative Glycosylation of Selenoglycosides: Rapid Expansion of Molecular Diversity with Simple Building Blocks

A new method for constructing an oligosaccharide library composed of structurally defined oligosaccharides is presented based on an iterative glycosylation of selenoglycosides. Treatment of 2‐acyl‐protected selenoglycosides with bromine selectively generates β‐bromoglycosides, which serve as glycosyl cation equivalents in the oligosaccharide synthesis. Thus, the coupling of the bromoglycosides with another selenoglycoside affords the corresponding glycosylated selenoglycosides, which can be directly used to next glycosylation. The iteration of this sequence allows the synthesis of a variety of oligosaccharides including an elicitor active heptasaccharide. A characteristic feature of the iterative glycosylation is that glycosyl donors and acceptors with the same anomeric reactivity can be selectively coupled by activation of the glycosyl donor prior to coupling with the glycosyl acceptor. Therefore, same selenoglycosides can be used for both the glycosyl donors and the acceptors. This feature has been exemplified by a construction of an oligosaccharide library directed to elicitor‐active oligosaccharides. The library composed of stereochemically defined oligoglucosides with considerable structural diversity can be constructed starting from simple selenoglycosides. Glycosylation without any anomeric protection/deprotection could be achieved in the β‐bromoglycoside‐mediated reaction by using selenoglycosides both as glycosyl donors and acceptors. This glycoslylation provides new selenoglycosides, which can be directly used for next reaction step. Combinatorial glycosylation allows the rapid assembly of an oligosaccharide library with considerably structural diversity (see scheme).