The glycosyl-aldonolactone approach for the synthesis of β-d-Galf-(1→3)-d-Manp and 3-deoxy-β-d-xylo-hexofuranosyl-(1→3)-d-Manp

A convenient synthesis of free β-d-Galf-(1→3)-d-Manp (8a) is reported. The disaccharide is present as external unit in the lipopeptidophosphoglycan (LPPG) of Trypanosoma cruzi and internally in the lipophosphoglycan (LPG) of Leishmania. Condensation of 2,5,6-tri-O-benzoyl-d-mannono-1,4-lactone (1) with 1,2,3,5,6-penta-O-benzoyl-d-galactofuranose, promoted by SnCl4, led to the β-glycosyl-lactone, a key intermediate for disaccharide 8a, readily obtained by successive reduction of the lactone with diisoamylborane and debenzoylation. As in the LPG of Leishmania the HO-3 group of the galactofuranose is glycosylated by α-d-Galp, we also synthesized 3-deoxy-β-d-xylo-hexofuranosyl-(1→3)-d-Manp (8b) and p-nitrophenyl 3-deoxy-β-d-xylo-hexofuranoside for studying the influence of HO-3 in the interaction with specific glycosidases. The disaccharide 8a, and its corresponding alditol, were good substrates for the β-d-galactofuranosidase from Penicillium fellutanum, whereas the 3-deoxyglycosides were not hydrolyzed by the enzyme.