Synthesis of 4-nitrophenyl beta-D-fucofuranoside and beta-D-fucofuranosyl-(1-->3)-D-mannopyranose: modified substrates for studies on catalytic requirements of beta-D-galactofuranosidase.

Syntheses of 4-nitrophenyl beta-D-fucofuranoside (6) and beta-D-fucofuranosyl-(1-->3)-D-mannopyranose (10) are reported. These compounds, as analogues of galactofuranosides, were used for studying the influence of the hydroxyl group at C-6 in the interaction of the substrate with beta-D-galactofuranosidase. For the synthesis of the fucofuranosides, 2,3,5-tri-O-benzoyl-6-bromo-6-deoxy-D-galactono-1,4-lactone (1) was the key intermediate, which upon reduction of the lactone group with diisoamylborane, acetylation of the anomeric hydroxyl group, and catalytic hydrogenolysis of the bromine at C-6, led to 1-O-acetyl-2,3,5-tri-O-benzoyl-alpha,beta-D-fucofuranose (4), a convenient derivative for the preparation of fucofuranosides. Compound 4 was glycosylated in the presence of SnCl4, either with 4-nitrophenol for the preparation of 6, or with 2,5,6-tri-O-benzoyl-D-mannono-1,4-lactone (7), for the synthesis of disaccharide 10, via the glycosyl-aldonolactone approach. The synthetic route developed for the beta-D-fucofuranosides is simple and efficient. Compound 6 was not hydrolyzed by incubation with the exo beta-D-galactofuranosidase from Penicillium fellutanum, showing that HO-6 is essential for interaction of the substrate with the enzyme.