Rhizobial Saccharides Part 3. Unexpected α-Selective Glycosylation During Synthesis of Pyruvated Tetrasaccharide Fragments Related to Exopolysaccharides of Rhizobium Leguminosarum

Abstract

The two diastereomeric tetrasaccharide allyl glycosides 3,4-Py-β-D-Galp-(1→6)-β-D-Galp-(1→6)-β-D-Glcp-(1→6)-α-D-Galp-(1→O-allyl) related to the exopolysaccharide of Rhizobium leguminosarum biovar phaseoli 127 K 87 having an (R)- and (S)-configurated pyruvic acid acetal, respectively at positions 3 and 4 of the galactopyranosyl residue of the non-reducing end, were prepared from two disaccharide blocks. The fully benzoylated diastereomeric disaccharide donors having the structure 3,4-Py-β-D-Galp-(1→6)-α-D-Galp-(1→X) with leaving groups X = bromide and trichloroacetimidate were synthesised by a stepwise approach from benzoylated 3,4-pyruvated galactosyl donors (chlorides, bromides, trichloroace-timidates) and benzyl and phenyl 1-thio- 2,3,4-tri-O-benzoyl-β-D-galactopyranosides, respectively. The disaccharide acceptors having the structure β-D-Glcp-(1→6)-α-D-Galp-(1→O-allyl) were obtained by coupling allyl 2,3,4-tri-O-benzoyl-α-D-galactopyranoside and its 6-O-trimethylsilyl ether with suitably protected glucosyl chlorides or fluorides. The coupling of the pyruvated disaccharide trichloroacetimidates with the disaccharide acceptors afforded the β-(1→6)-linked tetrasaccharides whereas the coupling of the pyruvated disaccharide bromide resulted in an α-(1→6)-selective glycosylation. The anomeric selectivity was attributed to a double diastereoselection caused by the configuration of the pyruvic acid acetal. Deblocking of the three tetrasaccharides provided the corresponding tetrasaccharide allyl glycosides which are suitable for preparation of glycoconjugates. Dedicated to Professor Hans Paulsen on the occasion of his 75th birthday