On the relevance of glycosyl oxonium ions to 1,2- cis -selective O -glycosylation in ether solvents

Since no later than the 1970s, organic chemists have speculated on the role of glycosyl oxonium ions in chemical O-glycosylation. Such species result from the attack of ethers on glycosyl oxocarbenium ions and are invoked to explain 1,2-cis-selectivity in ether solvents. However, a systematic study to probe this phenomenon appears to be lacking in the chemical literature. Herein, we study the effects of solvent, counteranion, protecting group electron-withdrawing effects, and acceptor on O-glycosylation stereoselectivity with D-glucosyl trichloroacetimidate donors. While many of these transformations proceed with 1,2-cis-selectivity, our results suggest that glycosyl oxonium ions play minimal, if any, role in O-glycosylation.