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. 2012 Sep 12;134(36):14746-9.
doi: 10.1021/ja307266n. Epub 2012 Aug 31.

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  1 , Pascal Retailleau, Luis Bohé, David Crich
Affiliations

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 et al. J Am Chem Soc. .

Abstract

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.

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Figures

Figure 1
Figure 1
X-Ray crystallographic structures of tricycles 5 and 6
Figure 2
Figure 2
O- and C-Glycoside to cyclized products ratio as a function of nucleophile concentration
Scheme 1
Scheme 1
Preparation and activation of sulfoxide 4; formation of tricyclic compounds 5 and 6.
Scheme 2
Scheme 2
Mechanism of formation of the tricycles 5 and 6
See this image and copyright information in PMC

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