Fructofuranosyl cation

The ratio of the different isomeric products was found to vary with time, temperature, and initial concentration. This suggested that some kind of equilibration was occurring between isomers. I3C NMR spectroscopy of a reaction mixture showed, upon cooling, the reversible formation of a pair of signals in the anomeric region. These signals were ascribed to the anomeric carbon atoms of fructofuranosyl fluorides (10), which were presumed to be in equilibrium with the reactive fructofuranosyl cation, 11. 

Fischer projection, monosaccharides acyclic form, 56-57 cyclic forms, 59-60 modified, 60-61 Fluorine chemistry, 18-19 D-Fructofuranosides, degradation, 445 Fructofuranosyl cation, 217 Fructofuranosyl fluorides, 217 Fructose... 

Considering that the acid-catalysed hydrolysis ( inversion ) of sucrose has been studied perfunctorily for the last century and a half, surprisingly little is known about the hydrolysis of ketosides. The positive entropies of activation of methyl -D-fructofuranoside (-1-13 eu) and sucrose (-f8 eu) argue in favour of an A-1 mechanism [1] involving a fructofuranosyl cation. 

The formation of 5-hydroxymethylfuran-2-carbaldehyde from saccharose proceeds via hydrolysis to fructofuranosyl cation and glucose (Figure 4.26). Ehmination of a proton and two molecules of water from the fructofuranosyl cation yields 5-hydroxymethylfuran-2-carbaldehyde. Under dry pyrolytic conditions and at temperatures above 250 °C, 90% of 5-hydroxymethylfuran-2-carbaldehyde originates from the fructose moiety and only 10% originates from glucose. Pentoses and L-ascorbic acid dehydrate in the same way as hexoses under addic conditions yielding furan-2-carbaldehyde (via reactive 3-deoxy-L-t/jreo-pentos-2-ulose and 3,4-dideoxypentosulos-3-ene) as the main product (see Section 5.14.6.1.5). 6-Deoxyhexoses, such as L-rhamnose, yield, analogously, 5-methylfuran-2-carbaldehyde (4-195). 

Linear Homotrisaccharides.— Pyrolysis of sucrose at 100 C in the presence of citric acid caused the formation of the fructofuranosyl cation and thence the six non-reducing trisaccharides comprising sucrose with fructofuranose a- and - bonded to C-6, C-1 and C-6 4-0-a-D-Glucopyranosyl-c(,a-trehalose was made by use of perbenzylated glucosyl fluoride. ... 

On the other hand. Binder et al. [1] proposed two variations on the mechanism previously suggested by Zhao et al (Figure 19). It was concluded that the fructofuranosyl cation undergoes attacks by chloride, bromide, or iodide. As bromide and iodide are better leaving groups than chloride, they were deemed as effective ionic additives. Qiromium salts play an important role in the yield of 5-HMF. Research has shown that the yield of 5-HMF using chromium correlates with metal coordination. It was also proposed that the halide additives serve two roles (1) as ligands for the chromium cation and (2) they facilitate the selective conversion of fructose. 

In 1952, Wolfrom and Hilton demonstrated that L-sorbose was also capable of forming dimeric dianhydrides,22 and they postulated sorbofuranosyl and pyra-nosyl cationic intermediates. In 1955, Boggs and Smith23 postulated a fructofu-ranosyl cationic intermediate in the formation of per-O-acetyl ot-D-Fru/-1,2 2,l -p-D-Fru/[di-D-fructose anhydride I (5)] from inulin triacetate. They indicated that three adjacent P-2,l -linked fructofuranosyl units would be required for formation of the dianhydride. 

It might be pertinent to consider the basis of the extremely facile isomerization in anhydrous HF or pyridinium poly(hydrogen fluoride). HF is an extremely strong proton donor, but it is also a potent fluorinating agent. It is highly probable that the postulated cationic intermediates in these isomerizations are fluori-nated and serve as reactive intermediates in the same way as the fructofuranosyl... 

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