Application of β-1,4-galactosyltransferase in the synthesis of complex branched-chain oligosaccharide mimics of fragments of the capsular polysaccharide of Streptococcus pneumoniae type 14

The chemoenzymic synthesis is described of beta-D-Galp-(1-->4)-beta-D-Glcp-(1-->6)-[beta-D-Galp-(1-->4)]-beta-D-GlcpNAc-(1-->O[CH2](3)O-->4)-beta-D-Glcp-(1-->OCH2CH=CH2) 32 and beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O[CH2](3)O-->4)-beta-D-Glcp-(1-->6)-[beta-D-Galp-(1-->4)]-beta-D-GlcpNAc-(1-->O[CH2](3)O-->4)-beta-D-Glcp-(1-->OCH2-CH=CH2) 33, representing hexa- and octasaccharide mimics of fragments of the Streptococcus pneumoniae type 14 polysaccharide. In a chemical approach the intermediate linear oligosaccharide mimics 30 and 31 were synthesized, wherein both terminal and non-terminal N-acetyl-beta-D-glucosamine residues were not yet galactosylated. The alkyl-bridged derivatives were found to be good acceptor substrates for bovine milk beta-1,4-galactosyltransferase. Reaction of the anomeric allyl functions with cysteamine under UV-irradiation gave the corresponding 3-(2-aminoethylthio)propyl glycosides 34 and 35, suitable for further coupling of the oligosaccharide mimics to protein carriers.