Cation Clock Permits Distinction Between the Mechanisms of α- and β‑O- and β‑C-Glycosylation in the Mannopyranose Series: Evidence for the Existence of a Mannopyranosyl Oxocarbenium Ion

The use of a cationic cyclization reaction as a probe of the glycosylation mechanism has been developed and applied to the 4,6-O-benzylidene-protected mannopyranoside system. Cyclization results in the formation of both cis- and trans-fused tricyclic systems, invoking an intermediate glycosyl oxocarbenium ion reacting through a boat conformation. Competition reactions with isopropanol and trimethyl­(methallyl)­silane are interpreted as indicating that β-O-mannosylation proceeds via an associative SN2-like mechanism, whereas α-O-mannosylation and β-C-mannosylation are dissociative and SN1-like. Relative rate constants for reactions going via a common intermediate can be estimated.