Synthesis of α-Manp-(1→2)-α-Manp-(1→3)-α-Manp-(1→3)-Manp, the tetrasaccharide repeating unit of Escherichia coli O9a, and α-Manp-(1→2)-α-Manp-(1→2)-α-Manp-(1→3)-α-Manp-(1→3)-Manp, the pentasaccharide repeating unit of E. coli O9 and Klebsiella O3 Original

The tetrasaccharide repeating unit of Escherichia coli O9a, α-d-Manp-(1→2)-α-d-Manp-(1→3)-α-d-Manp-(1→3)-d-Manp, and the pentasaccharide repeating unit of E. coli O9 and Klebsiella O3, α-d-Manp-(1→2)-α-d-Manp-(1→2)-α-d-Manp-(1→3)-α-d-Manp-(1→3)-d-Manp, were synthesized as their methyl glycosides. Thus, selective 3-O-allylation of p-methoxyphenyl α-d-mannopyranoside via a dibutyltin intermediate gave p-methoxyphenyl 3-O-allyl-α-d-mannopyranoside (2) in good yield. Benzoylation (→3), then removal of 1-O-methoxyphenyl (→4), and subsequent trichloroacetimidation afforded the 3-O-allyl-2,4,6-tri-O-benzoyl-α-d-mannopyranosyl trichloroacetimidate (5). Condensation of 5 with methyl 4,6-O-benzylidene-α-d-mannopyranoside (6) selectively afforded the (1→3)-linked disaccharide 7. Benzoylation of 7, debenzylidenation, benzoylation, and deallylation gave methyl 2,4,6-tri-O-benzoyl-α-d-mannopyranosyl-(1→3)-2,4,6-tri-O-benzoyl-α-d-mannopyranoside (11) as the disaccharide acceptor. Coupling of 11 with (1→2)-linked mannose disaccharide donor 17 or trisaccharide donor 21, followed by deacylation, furnished the target tetrasaccharide and pentasaccharide, respectively.