SYNTHESIS AND STRUCTURAL STUDIES OF BRANCHED 2-LINKED TRISACCHARIDES RELATED TO BLOOD-GROUP DETERMINANTS

A series of trisaccharide glycosides, Fuc-(1 --> 2)-beta-Gal-(1 --> 3) -beta-X-OMe (X = GlcNAc, Glc, 2-deoxy-Glc) related to the blood group determinant Led have been synthesised both as their alpha- and beta-Fu c anomers together with the component disaccharide starting compounds. The conformational properties of the six trisaccharides together with their parent disaccharides have been investigated by NMR spectroscopy (proton and carbon chemical shifts and proton NOEs) in combination wi th computer modeling using the Monte Carlo approach and the HSEA force field using the GEGOP programme. The interaction between the terminal fucose unit and the reducing unit was probed by substitution of the b ulky NAc group with hydroxyl and deoxy substituents, respectively. Com pounds with severe steric interactions were identified by the non-addi tivity of their carbon chemical shifts. This was subsequently confirme d by the detailed conformational assessment by NOE spectroscopy and co mputer modeling. The most severe contacts arose in the alpha-L-Fuc der ivatives, whereas the beta-linked L-Fuc derivatives only in one case e xhibit severe steric interaction as probed by the NMR parameters. (C) 1996 Elsevier Science Ltd.