A new route for the synthesis of Streptococcus pneumoniae 19F and 19A capsular polysaccharide fragments avoiding the beta-mannosamine glycosylation step.

The recently described [Attolino, E.; Bonaccorsi, F.; Catelani, G.; D'Andrea, F. Carbohydr. Res. 2008, 343, 2545-2556.] beta-D-MaNAcp-(1-->4)-beta-D-Glcp thiophenyl glycosyl donor 3 was used in alpha-glycosylation reactions of OH-2 and OH-3 of the suitably protected p-MeO-benzyl alpha-l-rhamnopyranoside acceptors 7 and 8. Glycosylation of the axial OH-2 of 7 took place in high yield (76%) and with acceptable stereoselectivity (alpha/beta=3.4) leading to the protected trisaccharide alpha-11, corresponding to the repeating unit of Streptococcus pneumoniae 19F. The same reaction on equatorial OH-3 of acceptor 8 gave the trisaccharide alpha-15, a constituent of the repeating unit of S. pneumoniae 19A, but in lower yield (41%) and without stereoselection (alpha/beta=1:1.3). Utilizing the introduced orthogonal protection of OH-1 and OH-4'', the trisaccharide alpha-11 was transformed into a trisaccharide building block suitable for the synthesis of its phosphorylated oligomers.