Towards highly convergent synthesis of plant-derived oligosaccharides
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Plant cell walls are made up of complex polysaccharides that perform a number of essential functions including control of cell growth, regulation of cell and tissue differentiation as well as providing mechanical support to individual cells. Xyloglucan, which is a hemicellulosic polysaccharide, forms extensive cross links with cellulose microfibrils and the xyloglucan-cellulose network is considered to be the major load bearing structure in plants. Structural modification and reorganization of this network is considered essential for the growth and expansion of plant cells. There is a need for the synthesis of homogenous xyloglucan structures that can be used in the generation of well defined molecular probes. These probes can be used to study the abundances, structures and localization of xyloglucans in the plant wall and help in correlating the biological significance of xyloglucan structures in different plant species. In chapters II and III of this dissertation we describe our efforts towards the synthesis of tetrasaccharide and hexasaccharide xyloglucan side-chain which contain galacturonic acid. These unique structures are present in the root hairs of Arabidopsis thaliana and have found to play an important role in the growth of the root hair. The chemical synthesis of these xyloglucan side-chains will help in the characterization of monoclonal antibodies which can be used in the identification as well as visualization of these structures in different plant species. The key step in the synthesis is the use of xylosyl 1,2-oxathiane donors for stereoselective formation of 1,2-cis glycosidic linkage. Sufficient deactivation of the xylosyl donors by electron-withdrawing protecting groups proved to be critical in obtaining good α-stereoselectivity. A late-stage post-glycosylation oxidation strategy was used to install the galacturonic acid in the molecule. To streamline the process of oligosaccharide synthesis, several groups are pursuing one-pot glycosylation strategies based on chemoselective, orthogonal or pre-activation protocols. But, the synthesis of branched oligosaccharides, using these methods cannot be readily accomplished. In chapter IV , we report the synthesis of novel protecting group that can be deprotected in situ in combination with triflic acid promoted glycosylations of trichloroacetimidate donors, paving the way for the synthesis of branched oligosaccharides in an one-pot procedure.