Synthesis of α-lactosyl-(1→3)-l-glycero-α-d-manno-heptopyranoside, a partial oligosaccharide structure expressed within the lipooligosaccharide produced by Neisseria gonorrhoeae strain 15253 Original Research Article

The glycosyl donor, hepta-O-benzyl-β-lactosyl trichloroacetimidate (4) was prepared by treating hepta-O-benzyl-lactose with trichloroacetonitrile in the presence of potassium carbonate. The acceptor, methyl 2-O-benzyl-4,6-O-benzylidene-7,8-dideoxy-α-d-manno-oct-7-enopyranoside (8) was synthesized by hydrolysis of a 3,4-butane diacetal of methyl l-glycero-α-d-manno-oct-enopyranoside and subsequent benzylidenation. Glycosidation of the donor 4 with the acceptor 8 in 1,4-dioxane using Me3SiOTf as a promoter for 1 h at room temperature gave methyl (2,3,4,6-tetra-O-benzyl-β-d-galactopyranosyl)-(1→4)-(2,3,6-tri-O-benzyl-α-d-glucopyranosyl)-(1→3)-2-O-benzyl-4,6-O-benzylidene-7,8-dideoxy-α-d-manno-oct-7-enopyranoside (9) as a major product (59%). The oct-enopyranoside moiety of the trisaccharide 9 was converted to a heptopyranoside (80%) by oxidative cleavage with OsO4–NaIO4 and subsequent reduction. Hydrogenolysis of the resulting trisaccharide and subsequent acetylation gave the peracetate of α-lactosyl-(1→3)-Hep. Deacetylation of the peracetate afforded the title trisaccharide.