ChemInform Abstract: Synthesis, Conformation, and Glycosidic Coupling Reactions of Highly Active Substituted 2,7-Dioxabicyclo(4.1.0)heptanes: 1,2-Anhydro-3,4-di- O-benzyl-α-D-xylopyranose.

Abstract

Abstract The title 1,2-anhydro sugar ( 8 ) was synthesized from d -xylose. The key intermediate for the synthesis was 2- O -acetyl-3,4-di- O -benzyl-β- d -xylopyranosyl fluoride, which was transformed into crystalline 8 by ring closure with potassium tert -butoxide. Comparison of the observed vicinal coupling constants for 8 with the results obtained from calculations by molecular mechanics suggested that the conformation of the pyranose ring is close to a 4 H 5 half chair. The anhydro sugar was highly reactive, its condensation with 1,2:3,4-di- O -isopropylidene-α- d -galactopyranose proceeding in reasonable yield without any Lewis acid catalyst, and quantitatively in the presence of molecular sieves to give the β-linked disaccharide as the major product. The stereochemical outcome of the condensation was little affected by changes in solvent or reaction temperature. The presence of some Lewis acid catalysts caused a decrement in the β:α ratio, while predominantly α-linked disaccharide was obtained when trityl perchlorate was the catalyst.