Synthesis of Two Hexasaccharides Related to the Repeating Unit of the O‐Antigen from Escherichia coli TD2158

Abstract

Efficient strategies for the synthesis of two hexasaccharides related to the O‐antigen polysaccharide of E. coli TD2158 are reported. Both of the oligosaccharides were prepared by the sequential glycosylation of rationally functionalized monosaccharide synthons derived from commercially available sugars. Azido and phthalimido groups were utilized as the precursors for the natural acetamido functionality depending on the stereochemical requirement of the glycosidic bond. The target oligosaccharides were prepared in the form of their 4‐methoxyphenyl (OPMP) glycoside to leave the scope for further glycoconjugate formation. All glycosylation reactions were performed through the activation of thioglycosides using NIS and H2SO4‐silica resulting in desired stereoselectivity and good to excellent yields.

Total synthesis of the O-antigen repeating unit of Providencia stuartii O49 serotype through linear and one-pot assemblies

“…With the monosaccharide building blocks in hand, the galactosamine donor 6 was coupled with galactose acceptor 7 by activation of the thioglycoside using N -iodosuccinimide (NIS) in the presence of TMSOTf to afford the desired disaccharide β-ᴅ-Gal p NHTroc-(1→4)-α-ᴅ-Gal p ( 4 ) in 85% yield, as a single isomer ( Scheme 3 ). Then, the chloroacetyl group was selectively removed using thiourea and 2,4,6-collidine [ 47 ] to afford the 3’-OH acceptor 5 in 87% yield. Finally, NIS/TMSOTf-promoted coupling of donor 3 with disaccharide acceptor 5 provided the desired β-linked trisaccharide 2 in 89% yield ( Scheme 3 ).…”