Direct Synthesis of 2,6‐Dideoxy‐β‐glycosides and β‐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 β‐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 β‐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)‐α‐d‐Manp‐(1→3)‐β‐d‐Rhap(1→4)‐β‐d‐Rhap‐(1→] repeating unit, wherein the convergent assembly up to a nonasaccharide was realized with a strongly β‐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. The 2‐(diphenylphosphinoyl)acetyl group is highly stereodirecting for the construction of challenging β‐configured 2,6‐dideoxyglycosides and d/l‐rhamnosides. Its synthetic potential was demonstrated in linear or convergent approaches to relevant complex carbohydrates. Variable‐temperature NMR studies provided evidence that the reaction proceeds by hydrogen‐bond‐mediated aglycone delivery.