Sialyl Lewis tetrasaccharide

Scheme 8.1 Synthesis of a sialyl-Lewis tetrasaccharide employing C-enriched protecting groups for the quantitative reaction monitoring using gated decoupling NMR spectroscopy.

Nicolaou s synthesis [7] of the Sialyl-Lewis tetrasaccharide is discussed in detail as an example of the stepwise route (Scheme 5.4). 

A number of non-natural fucosyl donors have also been probed with this enzyme [89]. As can be seen from Fig. 12, the C-6-atom of fucose is open for modifications. L-Gal and o-Ara are good substrates, whereas replacement of the 2-OH group of the fucose donor apparently is not tolerated by FucT VI. Indeed, Hindsgaul and coworkers succeeded in attaching the blood group A trisaccharide to the 6-position of the fucose donor and proved the biocatalytic transfer of this strange sugar [90]. The combination of non-natural donors with non-natural acceptors by enzymatic fucosyltransfer has also been probed which proved to be instrumental in the assembly of a library of sialyl-Lewis tetrasaccharides (Fig. 12) [91]. 

The FucT III transfers an L-fucose unit from GDP-fucose onto the 4-OH-group of a galactosylated N-acetylglucosamine in an a-niode to give the Lewis trisaccharide or the sialyl-Lewis tetrasaccharide, respectively (Fig. 13). 

The replacement of the N-acetyl group of the N-acetylglucosaniine unit is widely tolerated (Table 9). Especially striking is again the selective fucosylation of the glycuronamide derivatives (entries 11-15). Heterocyclic substituents on the acceptor apparently do not affect the enzyme either (entries 8-10). Nonnatural fucose donors are also recognized by FucT III and transferred in the expected way to form Lewis trisaccharides or sialyl-Lewis tetrasaccharides, respectively (see Fig. 14). 

Approaches to pharmaceutically apphcable inhibitors of selectin-hgand interactions began with target structures modeled on the entire sialyl-Lewis tetrasaccharide. However, the scope of these studies was soon focused when structure-activity relationships emerged from consideration of the structures of naturally occurring ligands of the selectins (Section V). 

Selectins are C-type lectins that form calcium-dependent bonds with their physiological ligands. The leukocyte L-selectin and the E- and P-selectins presented on the endothelium recognize all the sialyl Lewis tetrasaccharide ligands (sLe Fig. 6.23) that are presented by membrane-bound proteins or lipids on both interacting cellular surfaces (Fig. 6.22). 

Oligosaccharides are mediators of biospecific information by virtue of their structural complexity. As an example, the sialyl-Lewis tetrasaccharide is one of the crucial molecules involved in cell adhesion and trafficking [4, 15, 16]. The mechanisms of their synthesis are therefore crucial and the glycosyltransferases are at the centre stage of the synthesis of glycoside bonds [17]. 

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