Synthesis of alpha-D-Galf-(1-2)-D-Gal and alpha-D-Galf-(1-2)[beta-D- Galf-(1-3)]-D-Gal, constituents of Bacteroides cellulosolvens glycoproteins

GANDOLFI-DONADIO, L; GOLA, G; MUCHNIK DE LEDERKREMER, R; GALLO-RODRIGUEZ, C

Glasgow, Escocia

Simposio; 22nd International Carbohydrate Symposium; 2004

Galactose in the b-furanosic configuration is commonly found in fungi, bacteria and protozoa.1 On the contrary, the a-configuration is only described in few microorganisms as Bacteroides and Clostridium and the structures have never been confirmed by synthesis. In this work, we present the syntesis of a-D-Galf-(1-2)-D-Galol (1), and the progress made in the synthesis of a-D-Galf-(1-2)[b-D-Galf-(1-3)]-D-Galol (2), that have been isolated by reductive b-elimination from glycoproteins of Bacteroides cellulosolvens.2 Compound 1 was also obtained from Clostridium thermocellum.2 We have employed the trichloroacetimidate method to perform the a-D-Galf glycosylation, using the tetra-O-benzyl-b-D-galactofuranosyl derivative 3. As precursor of the reducing galactose, the galactono-1,4-lactone derivative 4 was used taking into account the more reactive OH in C-2. Thus, glycosidation of 4 with 1 equivalent of 3 afforded the a(1-2) disaccharide 5 in 51 % yield and the b(1-2) disaccharide in 8 % yield. Another fraction (16 %) gave a mixture of a(1-3) and b(1-3) disaccharides. The glycosidation sites were confirmed by acetylation of the products. Thus, acetylation of 5 followed by diisoamylborane reduction gave the derivative of a-D-Galf-(1-2)-D-Galf. Further deprotection and borohydride reduction of the anomeric center gave the alditol 1 that showed the same 1H-NMR spectrum as described for the natural compound.