Direct Synthesis of 2,6-Dideoxy-beta-glycosides and beta-Rhamnosides with a Stereodirecting 2-(Diphenylphosphinoyl)acetyl Group

Anomeric stereocontrol is usually one of the major issues in the synthesis of complex carbohydrates, particularly those involving 3-configured 2,6-dideoxyglycoside and D/L-rhamnoside moieties. Herein, we report that 2-(diphenylphosphinoyl)acetyl is highly effective as a remote stereodirecting group in the direct synthesis of these challenging beta-glycosides under mild conditions. A deoxy-trisaccharide as a mimic of the sugar chain of landomycin E was prepared stereospecifically in high yield. The synthetic potential was also highlighted in the synthesis of Citrobacter freundii O-antigens composed of a [-> 4)-alpha-D-Manp-(1 -> 3)beta-D-Rhap(1 -> 4)-beta-D-Rhap-(1 ->] repeating unit, wherein the convergent assembly up to a nonasaccharide was realized with a strongly beta-directing trisaccharide donor. Variable-temperature NMR studies indicate the presence of intermolecular H-bonding between the donor and the bulky acceptor as direct spectral evidence in support of the concept of hydrogen-bond-mediated aglycone delivery.