Direct Experimental Characterization of a Sialyl Cation

Area of Science:

• Carbohydrate Chemistry
• Organic Reaction Mechanisms
• Spectroscopy

Background:

• Sialic acids are crucial monosaccharides in biological processes.
• Controlling stereoselectivity in sialylation reactions remains a significant challenge.
• Mechanistic studies on the sialyl cation intermediate are limited.

Purpose of the Study:

• To generate and structurally characterize the elusive sialyl cation.
• To elucidate the factors governing stereoselectivity in sialylation reactions.
• To provide mechanistic insights into sialyl cation formation and reactivity.

Main Methods:

• Generation and isolation of a sialyl cation from an ionized sialic acid precursor.
• Structural characterization using cryogenic infrared spectroscopy.
• Computational analysis with quantum chemical calculations.

Main Results:

• The sialyl cation was successfully generated and isolated.
• A novel bridged structure of the sialyl cation was identified for the first time.
• The C5-NHAc group stabilizes the positive charge via remote participation, forming the bridged structure.
• The bridged structure shields the β-side, explaining observed α-selectivity in S_N1 sialylation.

Conclusions:

• The study provides the first structural characterization of a sialyl cation.
• The findings explain the α-selectivity in sialylation reactions through a bridged intermediate.
• This work offers critical mechanistic insights into carbohydrate chemistry and sialylation processes.