Reversing Reactivity: Stereoselective Desulfurative 1,2-trans-O-Glycosylation of Anomeric Thiosugars with Carboxylic Acids under Copper or Cobalt Catalysis.

We have discovered a new mode of reactivity of 1-thiosugars in the presence of Cu(II) or Co(II) for a stereoselective O-glycosylation reaction. The process involves the use of a catalytic amount of Cu(acac)2 or Co(acac)2 and Ag2CO3 as an oxidant in α,α,α-trifluorotoluene. Moreover, this protocol turned out to have a broad scope, allowing the preparation of a wide range of complex substituted O-glycoside esters in good to excellent yields with an exclusive 1,2-trans-selectivity. The late-stage modification of pharmaceuticals by this method was also demonstrated. To obtain a closer insight into the reaction mechanism, cyclic voltammetry was performed.