Stereoselective Synthesis of <i>O</i>-Glycosides with Borate Acceptors

Abstract

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