Pectin is one of the major plant cell wall components. It is constituted by highly complex and heterogeneous polysaccharides, which contribute to the modulation of several physiological processes such as cell growth and differentiation, cell-cell adhesion and cell wall support, and defense mechanisms.[1,2] Rhamnogalacturonan I (RG-I) is one of the structural domains of pectic polysaccharides, along with homogalacturonan, rhamnogalacturonan II, xylogalacturonan, and apiogalacturonan. Structurally diverse and complex, RG-I polysaccharides have a backbone consisting of [→2)-α-L-Rhap-(1→4)-α-D-GalA-(1→] disaccharide repeats, with numerous branching side-chains of galactans, arabinans, or arabinogalactans, generally found at the C-4 position of L-rhamnose residues.
Given the structural complexity of RG-I polysaccharides, well-defined synthetic RG-I related oligosaccharides are necessary for investigating enzymes involved in pectin biosynthesis and degradation, as well as for aiding structural analyses of the peptic polysaccharides. While several syntheses of fragments corresponding to the RG-I backbone have been described, the chemical synthesis of large structures with a branched backbone have not been previously reported. Herein, we present our most recent results on the synthesis of a RG-I heptasaccharide backbone by means of a [4+3] glycosylation and late-stage oxidation approach, and the installation of galactan side-chains of different lengths to furnish novel branched RG-I fragments (Figure 1).
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