Direct observation of fructosyl oxocarbenium ions: key intermediates in biology and biofuels
Geoffrey Akien and Bala Subramaniam
Oxocarbenium ions are key intermediates for glycosylation in nearly all living things as well as in the conversion of biomass into chemicals and fuels. Their detection has been a long-standing goal, but so far this has not been achieved (C. R. Chim., 2011, 14, 3-16.). The tertiary ions formed from fructose were expected to be more feasible initial targets due to their increased stability, but this is a significant increase in complexity from previously detected oxocarbenium ions (Russ. Chem. Rev., 1984, 53, 888-899.).
We report the first direct observation of the fructopyranosyl oxocarbenium ion (2) by the addition of methylated fructopyranoses (1) to fluorosulfonic acid at low temperature, with detection by NMR. Attempts to apply the same methodology to the five-membered furanosyl oxocarbenium ion (4) lead to rearrangement via a 1,2-hydride shift to give the alkoxycarbenium ion 5.