Hexos-5-ulose solution

There are at least ten forms in an aqueous solution of D-n7w-hexos-3-ulose,37 of which eight have been identified. Most forms have a free or a hydrated keto group at 23 °, there is 44% of the a-furanose and 1.5% of its hydrate 22% of the -pyranose and 12% of its hydrate 5% of the a-pyranose and 2% of its hydrate, 1% of a bicyclic y -furanose, and 8% of a dimer. The... 

The major components in the equilibrium of 6-deoxy-D-xp/ohexos-5-ulose43 are the a-aldofuranose (36%), the / -aldofuranose (28%), and the pyranose involving both the aldehyde and the ketone group (36%). The ketone group in the furanoses is not hydrated the configuration of the pyranose is / -1, a-5 (1 R,5R). The same type of pyranose is the main component (67%) in an aqueous solution of D-xy/o-hexos-5-ulose.44... 

High-held H and 13C NMR spectra of aqueous solution of D-xylo-hexos-5-ulose (242) have provided evidence for the presence of at least six isomeric forms and one anhydro form. The dominant isomeric form was the /f-pyranose 242a (67%) with the next most abundant form being the anhydro structure 242c (18%). The a- and /My4-furanoses (242d,e) and 1-aldehydrol /1-5,2-furanose structure (242f) were also observed.459... 

P. E. Morris, Jr., K. D. Hope and D. E. Kiely, The isomeric composition of D-ribo-hexos-3-ulose (3-keto-D-glucose) in aqueous solution, J. Carbohydr. Chem., 8 (1989) 515-530. 

D. E. Kiely, J. W. Talhouk, J. M. Riordan, and K. Gray, The isomeric composition of 6-deoxy-D-xyfo-hexos-5-ulose in aqueous solution as determined by H and 13C NMR, J. Carbohydr. Chem., 2 (1983) 427-438. 

At low temperatures, D-glucose and D-fructose in the presence of ferrous sulfate are converted into D-uru/u770-hexos-2-ulose (36), which can be degraded by further oxidation to glycolic acid, glyoxylic acid, and D-erythronic acid. The nature of the products formed under various conditions and the mechanism of the reaction have been described (see Ref. 1, p. 1133). In dilute solution, in the presence of ferrous sulfate at low temperature, compound 36 gave D-ura mo-2-hexulosonic acid (37) and D-ery//zro-hexo-2,3-diulosonic acid (38). In concentrated solutions, formaldehyde was also found. The formation of these products at low temperature was ascribed to the series of reactions in Scheme 19. 

Deoxy-D-x2/Zo-hexos-5-ulose reduced cold Fehling solution and readily formed a bis[(p-nitrophenyl)hydrazone] at room temperature and was rather alkali-labile. It showed mutarotation. The structure was deduced from elemental analysis of the compound and its derivative, and from the mode of preparation. 

Acetoxylation of the double bond is caused by lead tetraacetate thus, compound (123, R = Ac, Ri = OMe, Rj = H), gives crystalline methyl 5-acetoxy-tetra-0-acetyl-/8-D-glucopyranoside (126), which is hydrolyzed, on standing at room temperature in aqueous solution, to tetra-O-acetyl-n-a 2/lo-hexos-5-ulose (124, R = Ac, Ri = OAc). By means of this reaction, D-glucose has been converted, by way of 2-acetoxy-tri-O-acetyl-D-glucal and its main hydrogenation-deacetylation product, namely, styracitol, into 1,5-anhydro-2,3,4-tri-0-benzoyl-6-deoxy-D-lya o-hex-5-enitol (127), and thence, by acetoxylation and de-esterification, into n-fructose (128) ... 

Deoxy-D-eryt/jro-hexos-2-ulose occurs mainly as a- or P-pyranose in aqueous solutions. Owing to the presence of a keto group at C-2, other cyclic forms may also be present, such as furanoses (shown in Figure 4.30). In weakly acidic and neutral solutions, 3-deoxy-D-eryt/iro-hexos-2-ulose eliminates water and yields y-lactone of metasaccharinic acid and other produds... 

Deoxy-D-glycero-hexo-2,3-diulose is a typical product of degradation and 4-0-substituted derivatives of fructose (e.g. lactulose, which arises by isomerisation of lactose). 1,2-Enolisation of 4-deoxy-D-g lycero-hexo-2,3-diulose yields, via the appropriate l-ene-l,2-diol, unstable 4-deoxyhexos-3-ulose, which splits into formic acid and 1,2-enediol of 3-deoxy-D-glycero-pentulose in neutral or alkaline solutions by a reverse Claisen condensation mechanism. This sugar can be oxidised to 3-deoxy-D-glycero-pentos-2-ulose or may dehydrate to form 3,4-dideoxypentosulose (Figure 4.32). Other reaction products are isosaccharinic acids. 

Optimum conditions for the electro-oxidation of l,2 5,6-di-Q-isopropylidene-a-D-glucofuranose to the corresponding 3-uIose have been developed In a study of the hydration of methyl 4,6-0-benzylidene-2-bromo- and 2-chloro-2-deo) -a- and -D-xvlo-hexopvranosid-3-ulose in acetone solution, only the fl-D-xylo-derivatives were observed to hydrate and in another investigation the isomeric composition of freshly prepared D-ribo-hexos-3-ulose in DjO has been examined by H-n.m.r. spectroscopy. Full details have been published on the regioselective monooxidation of unprotected glycosides by reaction of Q-stannyl derivatives with bromine. ... 

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