Mercury iodide as a catalyst in oligosaccharide synthesis.

The disaccharide 4-O-alpha-D-mannopyranosyl-alpha-L-rhamnopyranose and the trisaccharide 2-O-alpha-D-galactopyranosyl-4-O-beta-D-mannopyranosyl-alpha-L-rhamno pyranose determinants, which are analogs of the repeating unit of Salmonella sero-group A, B and D, have been synthesized using mercury(II) iodide as a catalyst in the glycosylation reaction. The reducing end of the di- and the trisaccharide was substituted with a linking arm for covalent attachment to a protein carrier. Reaction of 8-ethoxycarbonyloct-1-yl 2,3-di-O-benzoyl-alpha-L-rhamnopyranoside with acetobromomannose in the presence of mercury(II) iodide gave, after deprotection, the disaccharide in 49% yield. The trisaccharide was prepared by a block synthesis in which 6-O-acetyl-4-O-allyl-2-O-(6-O-acetyl-2-O-allyl-3,4-di-O-benzoyl-alpha-D- galactopyranosyl)-3-O-benzyl-alpha-D-mannopyranosyl bromide (21) and 8-methoxycarbonyloct-1-yl 2,3-O-cyclohexylidene-alpha-L-rhamnopyranoside were condensed in the presence of mercury(II) iodide. These conditions gave the trisaccharide (26) in 26% yield. The disaccharide 21 was prepared by mercury(II) iodide catalyzed condensation of the protected galactopyranosyl bromide (15) and 4-O-allyl-1,6-anhydro-3-O-benzyl-beta- D-mannopyranose followed by acetolysis and reaction with titanium tetrabromide.