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Oxocarbenium ion
- Author: Stephen Withers
- Responsible Editor: Spencer Williams
An oxocarbenium ion is the species that is formed upon formal ‘ionization’ of a glycosidic bond. Also called an oxacarbenium ion, or glycosylium ion, they are a positively charged carbocation that is stabilized by resonance. Lifetimes of oxocarbenium ions are estimated at around 10-12 sec: right at the limit of existence as free species. Further, in the presence of anionic nucleophiles their lifetime was shown to be considerably shorter [1]. Therefore it seems unlikely that oxocarbenium ion intermediates exist in the active sites of glycoside hydrolases, which contain nucleophilic carboxylate residues. Indeed there is no experimental evidence for their existence. However, kinetic isotope effect studies have shown that transition states for glycosyl transfer have considerable oxocarbenium ion character.
Because the oxocarbenium ion has double-bond character between the anomeric carbon and the ring oxygen, this part of the structure must be planar. Thus for a 6-membered ring, C5, O5, C1 and C2 must lie in a plane. This arrangement can be accommodated in a 6-membered ring only in the 4H3 or 3H4 half chair conformations or 2,5B or B2,5 boat conformations.
References
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Banait, N. S., and Jencks, W. P. (1991) Journal of the American Chemical Society 113, 7951-7958.