Synthesis of #alpha#-lactosyl-(1->3)-L-glycero-#alpha#-D-manno-heptopyranoside, a partial oligosaccharide structure expressed within the lipooligosaccharide produced by Neisseria gonorrhoeae strain 15253

摘要

The glycosyl donor, hepta-O-benzyl-#beta#-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-#alpha#-D-manno-oct-7-enopyranoside (8) was synthesized by hydrolysis of a 3,4-butane diacetal of methyl L-glycero-#alpha#-D-manno-oct-enopyranoside and subsequent benzylidenation. Glycosidation of the donor 4 with the acceptor 8 in 1,4-dioxane using Me_3SiOTf as a promoter for 1 h at room temperature gave methyl (2,3,4,6-tetra-O-benzyl-#beta#-D-galactopyranosyl)-(1->4)-(2,3,6-tri-O-benzyl-#alpha#-D-glucopyranosyl)-(1->3)-2-O-benzyl-4,6-O-benzylidene-7,8-dideoxy-#alpha#-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 OsO_4-NaIO_4 and subsequent reduction. Hydrogenolysis of the resulting trisaccharide and subsequent acetylation gave the peracetate of #alpha#-lactosyl-(1->3)-Hep. Deacetylation of the peracetate afforded the title trisaccharide.