Boric acid promotes the dehydration of glucose to 5-(hydroxy)methylfurfural in ionic liquids. Computational analyses by DFT calculations show a significant decrease in energy for the isomerization of glucose to fructose when the sugars are bound to boric acid and isotopic labeling NMR studies confirm that the mechanism proceeds via an ene–diol intermediate as described by A. Riisager, P. Fristrup et al. in their Full Paper on page 1456 ff.