RhamnogalacturonanI fragments

RG-I is a group of pectic polysaccharides which have a common backbone heteropolymer with repeating disaccharide unit a-D-GalpA-(1^2)-a-L-Rhap-(1^4). The diversity of this group is a result of the presence of heterogeneous side chain polysaccharides represented by linear and branched (1^5)-a-L-arabinofuranan, (1^4)-P-D-galactopyranan and arabinogalactan

Chemical synthesis of RG-I backbone fragments has been performed using two approaches similar to those highlighted above for the synthesis of homogalacturonan fragments. The first approach employs galacturonic acid derivatives as constituents of glycosyl acceptors and donors (Reiffarth

Figure 3.19 Model of rhamnogalacturonan- I structure, adapted from Scheller et al. (2007). 20-80% of Rhap residues of RG-I are substituted at C-4 with oligosaccharide side chains composed of a-l-Araf and p-d-Galp residues. The GalpA residues of the RG-I backbone may be acetylated at C-2 and/or C-3.

Figure 3.19 Model of rhamnogalacturonan- I structure, adapted from Scheller et al. (2007). 20-80% of Rhap residues of RG-I are substituted at C-4 with oligosaccharide side chains composed of a-l-Araf and p-d-Galp residues. The GalpA residues of the RG-I backbone may be acetylated at C-2 and/or C-3.

& Reimer 2008) and the second adopts post-glycosylation oxidation of galactopyranose to galacturonic acid residues (Maruyama et al. 2000).

According to the first approach, protected galacturonates have been applied directly as building blocks in the synthesis of tetrasaccharide RG-I backbone fragment 3.20.1 . This oligosaccharide has galacturonate as the terminal ) reducing) residue and it has been assembled using a convergent scheme starting from a rhamnopyranosyl donor and a galacturonate acceptor (Fig. 3.20) (Reiffarth & Reimer 2008).

The late stage oxidation approach takes advantage of the high efficiency with which neutral saccharides can be coupled compared to saccharides, which have uronic acids as their constituents. Thus in the synthesis of tet-rasaccharide 3.21.1, which has been performed using the same convergent strategy as shown in Fig. 3.20, the disaccharide and tetrasaccharide intermediate building block were synthesized in high yields (Fig. 3.21). After removal of all protecting groups only two primary OH groups remain. These groups have been selectively oxidized forming carboxylic acids and furnishing RG-I tetrasaccharide synthesis (Maruyama et al. 2000 ).

Figure 3.20 Application of galacturonic acid-based building blocks in convergent synthesis of RG-I tetrasaccharide fragments (Reiffarth & Reimer 2008).
Figure 3.21 Synthesis of RG-I fragment using the late stage oxidation approach (Maruyama et al. 2000).

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