Synthesis of Arabinoxylan Oligosaccharides by Preactivation-Based Iterative Glycosylations

A concise synthetic strategy has been developed for assembling densely substituted arabinoxylan oligosaccharides, which are valuable substrates for characterizing hemicellulose-degrading enzymes. The xylan backbone has been prepared by an iterative preactivation-based glycosylation approach with phenyl thioglycosides. The preactivation has been performed with in situ generated p-nitrobenzenesulfenyl triflate prior to addition of the acceptor. The glycosylation temperature was shown to have an important impact on the yield of the coupling. The arabinose substituents have been introduced in one high-yielding glycosylation with an N-phenyl trifluoroacetimidate donor. The strategy has been successfully employed for the synthesis of three heptasaccharides in seven steps and overall yields of 24-36% from the corresponding monosaccharide building blocks.