Blockwise Approach to Fragments of the O-Specific Polysaccharide of <i>Shigella </i><i>f</i><i>lexneri</i> Serotype 2a:  Convergent Synthesis of a Decasaccharide Representative of a Dimer of the Branched Repeating Unit<sup>1</sup>

The <b>D</b>‘<b>A</b>‘<b>B</b>‘<b>(E</b>‘<b>)C</b>‘<b>DAB(E)C</b> decasaccharide representative of a dimer of a frame-shifted pentasaccharide repeating unit of the O-specific polysaccharide of <i>Shigella flexneri</i> 2a was synthesized as its methyl glycoside by condensing a pentasaccharide donor (<b>D</b>‘<b>A</b>‘<b>B</b>‘<b>(E</b>‘<b>)C</b>‘) and a pentasaccharide acceptor (<b>DAB(E)C</b>-OMe). Several convergent routes to these two building blocks, involving either the <b>AB</b> linkage or the <b>BC</b> linkage as the disconnection site, were evaluated in comparison to the linear strategy. The latter was preferred. It is based on the use of the trichloroacetimidate chemistry. The target branched oligosaccharide was designed to probe the recognition at the molecular level of the natural polysaccharide by protective monoclonal antibodies.