Mechanism of C-3 Acyl Neighboring Group Participation in Mannuronic Acid Glycosyl Donors.

One of the main challenges in oligosaccharide synthesis is the stereoselective introduction of the glycosidic bond. In order to understand and control glycosylation reactions, thorough mechanistic studies are required. Reaction intermediates found by NMR spectroscopy often cannot explain the glycosylation's stereochemical outcome. Hence, reactions may proceed through low-abundance reaction intermediates that are difficult to detect, according to a Curtin-Hammett scenario. We have previously observed that manno-type sugars can engage in C-3 acyl neighboring group participation. Herein, we report the detection of glycosyl dioxanium ions that result from C-3 neighboring group participation in mannuronic acid donors. Using a suite of exchange NMR techniques, we were able to dissect the kinetics of the conformational ring-flip that precedes C-3 acyl participation and the participation event itself in various manno-type sugars. Hence, this study provides a complete picture of mannosyl dioxanium ion formation.