Synthesis of an α-linked dimer of the trisaccharide repeating unit of the exopolysaccharide produced by Pediococcus damnosus 2.6 Original Research Article

A hexasaccharide, β-d-Glcp-(1→3)-[β-d-Glcp-(1→2)]-α-d-Glcp-(1→3)-β-d-Glcp-(1→3)-[β-d-Glcp-(1→2)]-d-Glcp, the α-linked dimer of the trisaccharide repeating unit of the exopolysaccharide produced by Pediococcus damnosus 2.6, was synthesized as its methyl glycoside. Condensation of fully benzoylated α-d-glucopyranosyl trichloroacetimidate (1) with isopropyl 4,6-O-benzylidene-1-thio-β-d-glucopyranoside (2) selectively furnished (1→3)-linked disaccharide 3, and subsequent 2-O-acetylation, desulfation, and trichloroacetimidate formation afforded the disaccharide donor 6. Meanwhile, selective 3-O-coupling of methyl 4,6-O-benzylidene-α-d-glucopyranoside (8) with 3-O-allyl-2,4,6-tri-O-benzoyl-α-d-glucopyranosyl trichloroacetimidate (7), followed by coupling with 1 gave the trisaccharide 10. Removal of the benzylidene group of 10, benzoylation, and deallylation produced the trisaccharide acceptor 12. Condensation of 12 with 6 yielded a pentasaccharide mixture 13 with β and α isomers in a ratio of 2:1. Removal of the benzylidene group of 13, followed by benzoylation gave the pentasaccharide mixture 14. Selective 2‴-deacetylation of the isolated β-linked 14β with MeCOCl/MeOH/CH2Cl2 did not give the expected pentasaccharide acceptor, and serious decomposition occurred, indicating a large steric hindrance at C-2‴. Alternatively, 2,3-di-O-glycosylation of allyl 4,6-O-benzylidene-β-d-glucopyranoside (21) with 1 gave 22, then deallylation and trichloroacetimidate formation afforded the trisaccharide donor 24. Condensation of 12 with 24 furnished only the α-linked hexasaccharide 25, and its deprotection gave the free hexaoside 27.