Synthesis and conformational studies of the tyvelose capped, Lewis-x like tetrasaccharide epitope of Trichinella spiralis

Chemical synthesis and high resolution NMR studies are reported for the tetrasaccharide epitope and constituent structures that occur as terminal elements of Trichinella spiralis cell surface glycans. The 2-(trimethylsilyl)ethyl and methyl glycosides of Lewis-x type trisaccharides, in which βGalNAc replaces βGal were synthesized from monosaccharide synthons utilizing thioglycoside and halide-ion glycosylation methods. The unique 3,6-dideoxy-β- d-arabinohexopyranosyl residue that caps the structure was introduced to selectively protected 2-(trimethylsilyl)ethyl and methyl trisaccharide glycosides by utilizing an insoluble silver zeolite catalyst and a glycosyl halide. In separate reactions not only were the principal targets obtained but also the corresponding α-linked tetrasaccharides. A comparison of the NMR spectra of the methyl tetrasaccharide glycosides showed that at the site of the α-linked tyvelose structure, specific GalNAc resonances ( C-1, C-2, C-3) possess uncharacteristically wide 13C (8–21 Hz) and 1H lines. The β-linked tetrasaccharide glycoside, that represents the native parasite epitope, exhibited only narrow line widths (3 Hz, 13C). Since NOE derived distance constraints for the α-linked tyvelose structure were not consistent with the existence of unusual glycosidic conformers, the origin of the limited number of wide lines was attributed to local rigidity in the GalNAc residue, at the site of tyvelose glycosylation. Tetrasaccharide epitopes present in glycoprotein glycans of Trichinella spiralis have been synthesized as Me and TMSEt glycosides with terminal β- ( 5 and 6) and α-linked tyvelose residues, using monosaccharide synthons such as 12 and 14. NMR data suggest that the chair conformation of the GalNAc residue in the α-linked tetrasaccharide is unusually rigid.