Combination of 3-O-Levulinoyl and 6-O-Trifluorobenzoyl Groups Ensures & alpha;-Selectivity in Glucosylations: Synthesis of the Oligosaccharides Related to Aspergillus fumigatus & alpha;-(1 & RARR; 3)-d-Glucan

Stereospecific & alpha;-glucosylation of primary and secondaryOH-groupat carbohydrate acceptors is achieved using glucosyl N-phenyl-trifluoroacetimidate (PTFAI) donor protected with an electron-withdrawing2,4,5-trifluorobenzoyl (TFB) group at O-6 and the participating levulinoyl(Lev) group at O-3. New factors have been revealed that might explain & alpha;-stereoselectivity in the case of TFB and pentafluorobenzoyl(PFB) groups at O-6. They are of conformational nature and confirmedby DFT calculations. The potential of this donor, as well as the orthogonalityof TFB and Lev protecting groups, is showcased by the synthesis of & alpha;-(1 & RARR; 3)-linked pentaglucoside corresponding to Aspergillus fumigatus & alpha;-(1 & RARR; 3)-d-glucan and of its hexasaccharide derivative, bearing & beta;-glucosamineresidue at the non-reducing end.