Capsular polysaccharide of Streptococcus pneumoniae type 19F: synthesis of the repeating unit

A new and more versatile synthesis of beta-D-ManpNAc-(1-->4)-alpha-D-Glcp-(1-->2)-alpha-L-Rhap, the trisaccharide repeating unit of the Streptococcus pneumoniae type 19F capsular polysaccharide, is described. The present approach allows a simple access to different fragments containing the trisaccharide and the conjugation of the product(s) to a protein through the selective manipulation of the anomeric position at the reducing end and of the HO-4 function at the nonreducing end. The synthetic scheme shows an efficient application of the sulfoxide method for the stereoselective and high yielding formation of the glycosidic linkages.