Tetrahedron 2013; 69:10337-50
Shigella flexneri serotypes 6 and 6a are closely related bacteria causing shigellosis in humans. Their Oantigens are {/4)-b-D-GalpA-(1/3)-b-D-GalpNAc-(1/2)-[3Ac/4Ac]-a-L-Rhap-(1/2)-a-L-Rhap-(1/}n acidic polysaccharides ({ABAcCD}n), which only differ in the degree of O-acetylation. A concise synthesis of two disaccharides (BC, BAcC) and four trisaccharides, representing portions and/or analogs of the Oantigens, is described. A protected intermediate compatible with late stage 3C-O-acetylation, and/or galactosyl (A) to galacturonic acid (A) conversion, was designed and assembled from trichloroacetimidate and thioglycoside donors tuned for high yielding glycosylation and excellent stereocontrol. The galacturonic moiety was efficiently introduced from galactose using a TEMPO/NaOCl/NaClO2-based oxidation protocol optimized for full compatibility with sensitive moieties, such as allyl ethers and acetates. Final Pd/C-mediated deprotection provided the targets, including the propyl glycoside ABAcC, its non O-acetylated counterpart ABC, and the non acidic analogs ABAcC and ABC. The BC and ABC oligosaccharides are also portions of the O-antigen from Escherichia coli O147, which causes diarrhea in pigs