Synthesis of galactofuranose disaccharides of biological significance

Abstract

Methyl β- d-galactofuranoside ( 3) was readily obtained by tin(IV) chloridecatalyzed glycosylation of penta- O-benzoyl-α,β- d-galactofuranose ( 1), followed by debenzoylation with sodium methoxide. Glycosylation of 1 with 2,3,5-tri- O-benzoyl- d-galactono-1,4-lactone ( 4) or with the 6- O-trityl-lactone derivative 5 gave the benzoylated β- d-galactofuranosyl-(1→6)- d-galactono-1,4-lactone 6 in excellent yield. The structure of disaccharide 6 was confirmed by borohydride reduction to the glycosyl-alditol 7. A byproduct of the condensation reaction of 1 with 4 or 5 was identified as the benzoylated (1→1)-β,β′- d-galactofuranosyl disaccharide 8. Compound 8 was readily prepared (88% yield) by controlled addition of water to 1, in the presence of stannic chloride. O-Debenzoylation of 8 afforded crystalline β′- d-galactofuranosyl-(1→1)-β- d-galactofuranoside ( 9). The glycosyl-lactone 6 constitutes a key intermediate for the synthesis of a disaccharide derivative having both units in the furanoid form. Thus, diisoamylborane reduction of the lactone function of 6 led to the disaccharide derivative 10, from which the methyl glycoside 12 was prepared. O-Debenzoylation of 12 gave the corresponding methyl β- d-galactofuranosyl-(1→6)-β- d-galactofuranoside ( 13). The free disaccharide β- d-Gal f-(1→6)- d-Gal p ( 14) and its acetylated derivative ( 15) were also synthesized from 10.