Catching elusive glycosyl cations in a condensed phase with HF/SbF₅ superacid

Glycosyl cations are universally accepted key ionic intermediates in the mechanism of glycosylation, the reaction that covalently links carbohydrates to other molecules. These ions have remained hypothetical species so far because of their extremely short life in organic media as a consequence of their very high reactivity. Here, we report the use of liquid hydrofluoric acid-antimony pentafluoride (HF/SbF5) superacid to generate and stabilize the glycosyl cations derived from peracetylated 2-deoxy and 2-bromoglucopyranose in a condensed phase. Their persistence in this superacid medium allows their three-dimensional structure to be studied by NMR, aided by complementary computations. Their deuteration further confirms the impact of the structure of the glycosyl cation on the stereochemical outcome of its trapping.