Four differently substituted trimers of the capsular polysaccharide (CPS) repeating unit have been synthesized in order to investigate the dependence on oligosaccharide size, acetylation and mode of phosphorylation of glycoconjugate vaccines against Neisseria meningitidis group A. A spacer-contg. starting monomer, a H-phosphonate elongating monomer and a 6-O-phosphorylated H-phosphonate cap monomer have been synthesized and coupled together to afford, after deprotection, the target trimer structures differing in their acetylation and phosphorylation substitution pattern. [on SciFinder (R)]