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 beta-O-mannosylation proceeds via an associative S(N)2-like mechanism, whereas alpha-O-mannosylation and beta-C-mannosylation are dissociative and S(N)l-like. Relative rate constants for reactions going via a common intermediate can be estimated.