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

The tetrasaccharide repeating unit of Escherichia coli O9a, alpha-D-Manp-(1 --> 2)-alpha-D-Manp-(1 --> 3)-alpha-D-Manp-(1 --> 3)-D-Manp, and the pentasaccharide repeating unit of E. coli O9 and Klebsiella O3, alpha-D-Manp-(1 --> 2)-alpha-D-Manp-(1 --> 2)-alpha-D-Manp-(1 --> 3)-alpha-D-Manp-(1 --> 3)-D-Manp, were synthesized as their methyl glycosides. Thus. selective 3-O-allylation of p-methoxyphenyl alpha-D-mannopyranoside via a dibutyltin intermediate gave p-methoxyphenyl 3-O-allyl-alpha-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-alpha-D-mannopyranosyl trichloroacetimidate (5). Condensation of 5 with methyl 4,6-O-benzylidene-alpha-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-alpha-D-mannopyranosyl-(1 --> 3)-2,4,6-tri-O-benzoyl-alpha-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. (C) 2002 Elsevier Science Ltd. All rights reserved.