Stereoselective Synthesis of -Glycosides with Borate Acceptors.

Borate esters have been applied widely as coupling partners in organic synthesis. However, the direct utilization of borate acceptors in -glycosylation with glycal donors remains underexplored. Herein, we describe a novel -glycosylation resulting in the formation of 2,3-unsaturated -glycosides and 2-deoxy -glycosides mediated by palladium and copper catalysis, respectively. This -glycosylation method tolerated a broad scope of trialkyl/triaryl borates and various glycals with exclusive stereoselectivities in high yields. All the desired aliphatic/aromatic -glycosides and 2-deoxy -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--glycosides, 2,3-deoxy -glycosides, and 2,3-epoxy -glycosides.