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Abstract

Glycosyl cations are the key intermediates during the glycosylation reaction that covalently
links building blocks during the synthetic assembly of carbohydrates. The exact structure of
these ions remained elusive due to their transient and short-lived nature. Structural insights
into the intermediate would improve our understanding of the reaction mechanism of glycosidic
bond formation. Here, we report an in-depth structural analysis of glycosyl cations
using a combination of cold-ion infrared spectroscopy and first-principles theory. Participating
C2 protective groups form indeed a covalent bond with the anomeric carbon that leads
to C1-bridged acetoxonium-type structures. The resulting bicyclic structure strongly distorts
the ring, which leads to a unique conformation for each individual monosaccharide. This gain
in mechanistic understanding fundamentally impacts glycosynthesis and will allow to tailor
building blocks and reaction conditions in the future.