Synthesis of Monomeric and Dimeric Repeating Units of the Zwitterionic Type 1 Capsular Polysaccharide from Streptococcus pneumoniae

Zwitterionic polysaccharides (ZPSs) from Bacteroides fragilis and Streptococcus pneumoniae display unique T‐cell activities. The first synthesis of a hexasaccharide representing two repeating units of the zwitterionic capsular polysaccharide from S. pneumoniae type 1 (Sp1) is reported. Key elements of the approach are stereoselective construction of 1,4‐cis‐α‐galactose linkages based on a reactive trichloroacetimidate donor that incorporates a 6‐O‐acetyl group, which may contribute to the high α selectivity in glycosylation. After assembly of the fully protected hexasaccharide from five monosaccharide synthons 2 – 4 , 24 and 25 , selective deprotection of the primary hydroxyl groups of the four galactose residues followed by oxidation to the corresponding uronic acids provides hexasaccharide 19 . The trisaccharide counterpart 1 was synthesized in similar fashion from three synthons, 2 – 4 . This approach employed both conventional and dehydrative glycosylation methodologies and avoids the use of poorly reactive uronic acid derived glycosyl donors and acceptors.