α- and β-Glycosyl sulfonium ions: Generation and reactivity

A study was conducted to observe the generation and reactivity of α- and β-glycosyl sulfonium ions. Preparation of a glycosyl triflate of 2-deoxy-2-amino sugar was done from a thioglycoside using glycosyl triflate pool method. The triflate oxygen was bound covalently to the anomeric carbon due to chemical shift of the anomeric carbon. It was found that α- and β-glycosyl sulfonium ions were produced from an electrochemically generated glycosyl triflate. The NMR spectroscopy and mass spectrometry were used to characterized these ions. The reaction with MeOH showed that the α-glycosyl sulfonium ion had more reactivity compared to β-glycosyl sulfonium ion. The stereochemical results revealed that the reaction failed to proceed by a simple S_N2 mechanism and proceeded through glycosyl cation intermediate. The study opened new opportunities for mechanistic studies on glycosylation reactions.