Plant cell walls are composed of an intricate network of polysaccharides and proteins that varies during the developmental stages of the cell. This makes it very challenging to address the functions of individual wall components in cells, especially for highly complex glycans. Fortunately, structurally defined oligosaccharides can be used as models for the glycans, to study processes such as cell wall biosynthesis, polysaccharide deposition, protein-carbohydrate interactions, and cell-cell adhesion. Synthetic chemists have focused on preparing such model compounds, as they can be produced in good quantities and with high purity. This Review contains an overview of those plant and algal polysaccharides that have been elucidated to date. The majority of the content is devoted to detailed summaries of the chemical syntheses of oligosaccharide fragments of cellulose, hemicellulose, pectin, and arabinogalactans, as well as glycans unique to algae. Representative synthetic routes within each class are discussed in detail, and the progress in carbohydrate chemistry over recent decades is highlighted.
We report the synthesis of linear and branched (1→4)-d-galactans. Four tetrasaccharides and one pentasaccharide were accessed by adopting a procedure of regioselective ring opening of a 4,6-O-naphthylidene protecting group followed by glycosylation using phenyl thioglycoside donors. The binding of the linear pentasaccharide with galectin-3 is also investigated by the determination of a co-crystal structure. The binding of the (1→4)-linked galactan to Gal-3 highlights the oligosaccharides of pectic galactan, which is abundant in the human diet, as putative Gal-3 ligands.
Herein, we report a synthetic strategy aiming at synthesizing the ten canonical carrageenan oligosaccharides from one single precursor. The key β‐(1→4)‐linked disaccharide was synthesized from commercially available galactose pentaacetate. The notoriously difficult formation of β‐(1→4)‐d‐galactan linkages was successfully optimized on the differentially substituted monosaccharides to afford the desired disaccharide in 55 % yield. Following a convergent strategy, two disaccharides were then glycosylated to form the fully protected α‐(1→4)‐linked tetrasaccharide backbone of the carrageenans. The careful selection of protecting groups provides the opportunity to access all ten carrageenan substructures identified in polysaccharides isolated from red algae. Here, we demonstrate how one such target oligosaccharide can be obtained in a protected form.
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