Chemical synthesis of branched rhamnogalacturonan I oligosaccharides

The plant cell wall glycan pectin 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, support, and defense. Rhamnogalacturonan I (RG-I) is one of the structural domains of pectic polysaccharides, with a backbone consisting of [→2)-α-L-Rhap-(1→4)-α-D- GalA-(1→] disaccharide repeats and numerous branching side chains of galactans, arabinans, and arabinogalactans. Given the structural complexity of this polysaccharide, 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. Herein, we present the synthesis of branched RG-I oligosaccharides through a convergent approach where a heptasaccharide RG-I backbone was glycosylated with different length D-galactan side chains. The heptasaccharide glycosyl acceptor was prepared by means of a [4+3] glycosylation and late-stage oxidation approach, starting from six D-galactopyranoside and L-rhamnopyranoside building blocks. Glycosylations with D-(1→4)-galactan imidate glycosyl donors afforded, after deprotection, the desired branched RG-I fragments. As some of the largest and most complex RG-I structures ever synthesized, they will underpin studies of RG-I biosynthesis, structure and deconstruction.