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.

Min Huang , Pascal Retailleau (1) , Luis Bohé (1) , David Crich (1)

1 ICSN - Institut de Chimie des Substances Naturelles

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 S(N)2-like mechanism, whereas α-O-mannosylation and β-C-mannosylation are dissociative and S(N)1-like. Relative rate constants for reactions going via a common intermediate can be estimated.

Domaines: Chimie organique

Min Huang, Pascal Retailleau, Luis Bohé, David Crich. 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.. Journal of the American Chemical Society, 2012, 134 (36), pp.14746-9. ⟨10.1021/ja307266n⟩. ⟨hal-00745057⟩