Synthesis of the colitose determinant of Escherichiacoli O111 and 3,6-di-O-(α-D-galactopyranosyl)-α-D-glucopyranoside

The syntheses of two branched trisaccharides, which constitute important elements of enterobacterial lipopolysaccharides are described. The common structural feature of each trisaccharide is α-D-glucopyranoside, upon which branching occurs at the O-3 and O-6 positions. Selective blocking of this hexoside at O-2 and O-4 by persistent blocking groups was accomplished by benzylation of 1,6-anhydro-β-D-glucopyranoside (1). Acetolysis of the product 2 afforded a mixture of anomeric triacetates 3 from which the corresponding 3,6-di-O-acetyl-2,4-O-benzyl-α-D-glucopyranosyl chloride (4) was prepared. The α and β 8-methoxycarbonyloctyl glycosides 5 and 6 were obtained in the ratio ~2:1, when 4 was reacted with 8-methoxycarbonyloctanol. Transesterification of the α-glycoside 5 gave the aglyconic intermediate 7 from which each trisaccharide could be obtained in a one step reaction. Thus glycosylation of 7 with tetra-O-benzyl-α-D-galactopyranosyl chloride (8) or 2,4-di-O-benzyl-3,6-dideoxy-α-L-xylo-hexopyranosyl chloride (10) gave the trisaccharides 11 and 13. In both cases removal of blocking groups was achieved in a single hydrogenolysis step. Trisaccharide 12 represents a trisaccharide sequence present in the core saccharide of Salmonella lipopolysaccharides, while the colitose containing trisaccharide 14 is an essential element of the E. coli O111 O-antigen.