Synthesis and structural studies of "branched" 2-linked trisaccharides related to H-type 2 blood group determinants

A series of fucosylated trisaccharides L-Fuc-(1 -->2)-beta -D-Gal-(1 -->4)-beta -X-OMe (1-6, X = D-GlcNAc, D-Qui (6-deoxy-Glc), D-Xyl) related to H type 2 blood group determinant have been synthesized both as their alpha- and beta- L-Fuc anomers together with the component disaccharide starting compounds (7-11). The conformational properties of the trisaccharides together with their parent disaccharides have been investigated by NMR spectroscopy (proton and carbon chemical shifts and proton NOEs) in combination with computer modeling using the Monte Carlo approach and the HSEA force field using the GEGOP program with the main focus on the alpha -linked fucose series. The series of compounds allow for the investigation of interaction between the sugar units in the-in principle-linear structures, which in practice behave as branched trisaccharides. The interaction between the terminal fucose unit and the unit at the reducing end has been probed by substitution of the bulky CH2OH group with CH3 and H substituents, respectively. Compounds with severe steric interactions can be identified by the non-additivity of their carbon chemical shifts and subsequently confirmed by the detailed conformational assessment by NOEs and computer modeling. The most severe contacts arise in the GlcNAc and Qui trisaccharide series, whereas the Xyl-containing trisaccharide derivatives only exhibit weak steric interaction as probed by the NMR parameters.