Stereoselective Synthesis of O -Glycosides with Borate Acceptors.

Borate esters have been applied widely as coupling partners in organic synthesis. However, the direct utilization of borate acceptors in O -glycosylation with glycal donors remains underexplored. Herein, we describe a novel O -glycosylation resulting in the formation of 2,3-unsaturated O -glycosides and 2-deoxy O -glycosides mediated by palladium and copper catalysis, respectively. This O -glycosylation method tolerated a broad scope of trialkyl/triaryl borates and various glycals with exclusive stereoselectivities in high yields. All the desired aliphatic/aromatic O -glycosides and 2-deoxy O -glycosides were generated successfully, without the hemiacetal byproducts and O→C rearrangement because of the nature of borate esters. The utility of this strategy was demonstrated by functionalizing the 2,3-unsaturated glycoside products to form saturated β- O -glycosides, 2,3-deoxy O -glycosides, and 2,3-epoxy O -glycosides.