Evaluating Participation Modes in Peracetylated Glycosyl Cations

Neighboring group participation and remote participation are fundamental concepts in carbohydrate chemistry and are commonly applied to achieve stereocontrol in glycosylation reactions. The corresponding intermediates can be glycosyl cations, which are challenging to characterize due to their short-lived nature. Using gas-phase infrared spectroscopy supported by density functional theory calculations, we directly assess and rank the participation modes of acetyl protecting groups in peracetylated glucosyl, galactosyl, and mannosyl cations, showing that neighboring group participation and hence the formation of C2-dioxolenium ions is found experimentally in all three cases. Energetic ranking of theoretical structures <i>in vacuo</i> revealed differences in the remote participation preferences depending on the hexose, showing that C3-dioxolenium ions are preferred for glucose and particularly mannose, whereas C4-dioxolenium ions are most stable in the case of galactose. These findings contribute to our fundamental understanding of protecting group participation and could facilitate glycosylation strategies in the future.