Synthesis of an xylosylated rhamnose pentasaccharide, the repeating unit of the O-chain polysaccharide of the lipopolysaccharide of Xanthomonas campestris pv. begoniae GSPB 525 Original Research Article

A xylosylated rhamnose pentasaccharide, α-l-Rhap-(1→3)-[β-l-Xylp-(1→2)-]-α-l-Rhap-(1→3)-[β-l-Xylp-(1→4)]-l-Rhap, the repeating unit of the O-chain polysaccharide (OPS) of the lipopolysaccharides of Xanthomonas campestris pv. begoniae GSPB 525 was synthesized by a highly regio- and stereoselective way. Thus coupling of 1,2-O-ethylidene-β-l-rhamnopyranose (1) with 2,3,4-tri-O-benzoyl-α-l-rhamnopyranosyl trichloroacetimidate (2) to give (1→3)-linked disaccharide (3), subsequent benzoylation, deethylidenation, acetylation, 1-O-deacetylation, and trichloroacetimidation afforded the disaccharide donor 11. Condensation of 11 with 1 yielded 2,3,4-tri-O-benzoyl-α-l-rhamnopyranosyl-(1→3)-2-O-acetyl-4-O-benzoyl-α-l-rhamnopyranosyl-(1→3)-1,2-O-ethylidene-β-l-rhamnopyranose (12), and selective deacetylation of 12 yielded the trisaccharide diol acceptor 15. Coupling of 15 with 2,3,4-tri-O-benzoyl-α-l-xylopyranosyl trichloroacetimidate (16), followed by deprotection, gave the target pentasaccharide 19.