CONICET | Buscador de Institutos y Recursos Humanos

D-Arabinofuranose (Araf) and D-galactofuranose (Galf) are found in the arabinogalactan of the cell wall of mycobacteria.1,2 D-Galf is also present in glycoconjugates of infectious protozoa like Trypanosoma cruzi and Leishmania,3,4 and of fungi.5,6 The chemistry and biology of arabinofuranosyl and galactofuranosyl containing polysaccharides has been reviewed.7 The furanose form of arabinose and galactose is absent in mammals, hence the interest of several groups in the synthesis of the oligosaccharides containing these sugars for metabolic and immunological studies. Per-O-acylated monosaccharides are important synthons for the synthesis of glycosides, in particular oligosaccharides.8 We have used galactofuranose per-O-benzoate for the first synthesis of a-D-Galf phosphate9 and 1-thio-b-D-galactofuranosides.10 Also, Araf and Galf were introduced as the terminal quasi non-reducing (upstream) unit in oligosaccharides using the tin(IV) chloride promoted condensation.11,12 The perbenzoate 1 was also used as precursor of the trichloroacetimidate derivative employed in oligosaccharide syntheses.13,14 These per-O-benzoates are obtained as a crystalline, anomeric mixture of the a,b-furanoses, and it can be used, without separation, for the glycosylation reactions. The method is based on the tautomerization of the monosaccharide in hot pyridine that yields a considerable proportion of the furanosyl forms, which are then benzoylated. We have optimized the original method for the preparation of 1,2,3,5,6-penta-O-benzoyl-a,b-D-Galf,15 and extended the reaction to D-arabinose. Peracetylated galactofuranose and arabinofuranose were previously most often synthesized from glycosides obtained by Fisher glycosidation of free sugars. The mixture of pyranose and furanose glycosides thus obtained, were then acetylated and the peracetates were acetolysed to obtain furanose peracetates. In this way, crystalline penta-O-acetyl-b-D-galactofuranose was obtained in 37-41 % yield (in three steps).16,17 Difficulties to separate the mixture of glycosides have been reported.18 When octyl galactofuranosides were used for the acetolysis reaction,19 the yield of penta-O-acetyl-b-D-galactofuranose was ~40 % but the protocol, in addition to the three synthetic steps, involved repeated chromatography. The tetra-O-acetyl-a,b-D-arabinofuranose was obtained by the Fisher glycosidation as a syrup, after column chromatography.20 Earlier, tetra-O-benzoyl-a-D-arabinofuranose was prepared in three steps in a low yield,21 starting from the methyl glycoside. A different approach comprised acetolysis of D-galactose diethyl dithioacetal, obtained from D-galactose in 47% yield.22 It afforded 1,2,3,5,6-penta-O-acetyl-b-D-galactofuranose in a yield of 30% (from galactose).23 This method has the disadvantage of employing the odorous ethanethiol for preparation of the starting material. By the present procedure, the yields of the crystalline furanose perbenzoates are also moderate (26-33%) but the easy one step synthesis from inexpensive reagents makes it an attractive approach.