The recognition of cell‐surface l‐selectin by its carbohydrate ligands causes lymphocytes to roll on capillary endothelium at sites of inflammation. As this primary contact is a prerequisite for extravasation of the leukocytes to the tissue, its inhibition by free oligosaccharides capable of competing with the natural l‐selectin ligands is an attractive therapeutic possibility.

The exact structures of the biological ligands of l‐selectin are not yet known, but the principal carbohydrate epitopes share some structural features: they are O‐glycosidically linked mucin‐type oligosaccharides with N‐acetyllactosamine backbone, which is 3′‐sialylated or 3′‐sulfated, 3‐fucosylated and sometimes 6‐ or 6′‐sulfated at the distal N‐acetyllactosamine termini. Multivalency of the ligand, which is believed to enhance the binding, is achieved by a branched polylactosamine backbone or by a clustered array of O‐glycans.

We report here enzymic synthesis of a large oligosaccharide fulfilling several of the features characteristic to the l‐selectin ligands: it is a dodecameric O‐glycosidic core‐2‐type oligosaccharide alditol with a branched polylactosamine backbone carrying two distal α‐2,3′‐sialylated and α‐1,3‐fucosylated N‐acetyllactosamine groups (sialyl Lewis x, sialyl Le^x). The structure of each saccharide on the synthesis route from disaccharide Galβ1–3GalNAc to the dodecasaccharide alditol was established by several methods including one‐ and two‐dimensional ¹H‐NMR spectroscopy. The last step of the synthesis, the α‐1,3‐fucosylation of the 6‐linked arm proceeded sluggishly, and was associated with a noticeable shift in H1 resonance of the GlcNAc residue of the branch‐bearing N‐acetyllactosamine unit.

The final synthesis product and its analogs lacking one or both of the fucose residues were tested as inhibitors of l‐selectin‐mediated lymphocyte‐endothelium interaction in vitro in rejecting rat kidney transplant. While the non‐fucosylated O‐glycosidic oligosaccharide alditol did not possess any inhibitory activity, the mono‐fucosylated one (i.e. monovalent sialyl Le^x) prevented the binding significantly and the difucosylated dodecasaccharide alditol (i.e. divalent sialyl Le^x) was a very potent inhibitor (IC₅₀, inhibitory concentration preventing 50% of binding = 0.15 μM). Besides the multivalency, also the Galβ1–3GalNAc‐ol sequence of the O‐glycosidic core appeared to increase the affinity of the glycan to l‐selectin. This was indicated by parallel inhibition experiments, where a disialylated and difucosylated branched polylactosamine decasaccharide, similar to the divalent dodecasaccharide alditol, but lacking the reduced O‐glycosidic core, was a less effective inhibitor (IC₅₀= 0.5 μM) than the O‐glycosidic dodecasaccharide alditol.