Divergent Synthesis and Antigenicity Evaluation of\nCore Oligosaccharides of the Lipopolysaccharides from <i>Acinetobacter\nbaumannii</i> SMAL and ATCC 19606

<i>Acinetobacter baumannii</i> poses a serious threat\nto human health. Pathogenic bacterial lipopolysaccharides (LPSs) are\npotent immunogens for the development of antibacterial vaccines. To\ninvestigate the antigenic properties of <i>A. baumannii</i> LPS, five well-defined core oligosaccharide fragments from the LPS\nof <i>A. baumannii</i> SMAL and ATCC 19606 were synthesized.\nA divergent synthesis strategy based on orthogonally protected α-(2\n→ 5)-linked Kdo dimer <b>6</b> was developed. Selective\nexposure of different positions in this key precursor and then elongation\nof sugar chains via stereocontrolled formation of both 1,2-<i>trans</i> and 1,2-<i>cis</i>-2-aminoglycosidic linkages\npermitted the efficient synthesis of the targets. The synthetic route\nalso highlights a 4-<i>O</i> and then 7-<i>O</i> glycosylation sequence for assembly of the novel 4,7-branched Kdo\nframework. Antigenicity assay using the glycan microarray technique\ndisclosed that tetrasaccharide <b>3</b> featuring both 4,7-branch\nand α-(2 → 5)-Kdo-Kdo structural elements was a potential\nantigenic determinant.