Syntheses of anomerically phosphodiester-linked oligomers of the repeatingunits of the Haemophilus influenzae types c and f capsular polysaccharides

Spacer-equipped dimers and trimers of the repeating units of the capsular p olysaccharide of Haemophilus influenzae type c, -4)-3-O-Ac-ss -D-GlcpNAc-(1 -->3)-alpha -D-Galp-(1-OPO3--, and type f, -3)-ss -D-GalpNAc-(1 -->4)-3-O- Ac-alpha -D-GalpNAc-(1-OPO3--, have been synthesized for use in immunologic al studies. H-Phosphonate chemistry was used for the formation of the inter glycosidic phosphate diester linkages. Two types of building blocks, a spac er glycoside disaccharide starting monomer (15 and 22) and an anomeric mono ester alpha -H-phosphonate disaccharide elongating monomer (12 and 27), wer e built up for each serotype structure from properly protected monosacchari de precursors using mainly thioglycosides as glycosyl donors. Stereospecifi city in the formation of the alpha -linked monoester H-phosphonate was poss ible in type c through crystallization of the pure alpha -anomer of the pre cursor hemiacetal from an alpha/ss -mixture, whereas in type f, the hemiace tal was isolated directly as exclusively the alpha -anomer. Subsequent phos phonylation using triimidazolylphosphine was performed without anomerizatio n. Formation of the anomeric phosphate diester linkages was performed using pivaloyl. chloride as coupling reagent followed by I-2/H2O oxidation of th e formed diester H-phosphonates. Original experiments afforded no diester p roduct at all, but optimization of the oxidation conditions (lowering the t emperature and dilution with pyridine prior to I-2 addition) gave the dimer s in good yields (71% and 81%) and, subsequently, after removal of a tempor ary silyl protecting group in the dimers, the trimers in fair yields (36% a nd 37%), accompanied by hydrolysis of the dimer phosphate linkage. One-step deprotection through catalytic hydrogenolysis efficiently afforded the tar get dimer (30 and 36) and trimer structures (32 and 39). The synthetic sche me allows for further elongation to give higher oligomers.