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

摘要

SUMMARY The recently described (Carbohydr. Res. 2008, 43, 2545-2556) b-D-MaNAcp- (1→4)-b-D-Glcp thiophenyl glycosyl donor 3 was used in a-glycosylation reactions of OH-2 and OH-3 of the suitably protected p-MeO-benzyl a-L-rhamnopyranoside acceptors 7 and 8. The glycosylation of axial OH-2 of 7 took place in high yield (76%) and with good stereoselectivity (a/b = 3.4) leading to the protected trisaccharide a-11, corresponding to the repeating unit of Streptococcus pneumoniae 19F. The same reaction on equatorial OH-3 of acceptor 8 gave the trisaccharide a-15, constituent of the repeating unit of S. pneumoniae 19A, but in lower yield (41%) and without stereoselection (a/b = 1:1.3). Utilizing the introduced orthogonal protection of OH-1 and OH-4’’, the trisaccharide a-11 was transformed into a trisaccharide building block suitable for the synthesis of its phosphorylated oligomers.