Oligosaccharides corresponding to biological repeating units of Shigella flexneri variant Y polysaccharide: part 3. Synthesis and 2D-nuclear magnetic resonance analysis of a heptasaccharide hapten

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Abstract
The block synthesis of a heptasaccharide portion of the biological repeating unit, [2)-α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap-(1→3)-β-D-GlcpNAc-(1], of the Shigella flexneri variant Y polysaccharide is described. The synthetic strategy relies on the use of the key trisaccharide intermediate α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap, both as a glycosyl acceptor and as a donor. Thus, the trisaccharide bromide in conjunction with the β-D-GlcpNPhth-(1→2)-α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap unit under Helferich conditions yielded the blocked heptasaccharide in 86% yield. The latter unit was obtained, in turn, from the key trisaccharide intermediate functioning as an acceptor molecule. Attempts at coupling the tetrasaccharide donor, α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap-(1→3)-β-D-GlcpNPhth, with the trisaccharide acceptor α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap, to give the heptasaccharide under a variety of conditions were unsuccessful. The blocked derivatives were synthesized as their allyl glycosides. Removal of the blocking groups, hydrogenation of the allyl group, and N-acetylation yielded the heptasaccharide hapten, α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap-(1→3)-β-D-GlcpNAc-(1→2)-α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap, as its propyl glycoside, for use in inhibition studies with complementary monoclonal antibodies, and in NMR and X-ray studies. The detailed NMR analysis of the protected and deprotected heptasaccharides by use of two-dimensional NMR techniques is also described.