Fructose in aqueous solution

Figure 25.5 Pyranose and furanose forms of fructose in aqueous solution. The two pyra-nose anomers result from addition of the C6 -OH group to the C2 carbonyl the two furanose anomers result from addition of the C5 -OH group to the C2 carbonyl.

Evans, Nicoll, Strause and Waring46 oxidized D-glucose and D-fructose in aqueous solution with excess cupric acetate at 50° for the purpose of ascertaining whether the general principles underlying the mechanism of carbohydrate oxidation in alkaline solutions are sufficient to explain the course of such oxidations in acid solutions. D-Glucosone was claimed to be one of the first products of oxidation the osone was not isolated, and,... 

Following a report60 that D-fructose, but not D-glucose, is oxidized by selenious acid, Dixon and Harrison61 used this reagent to prepare D-glucosone from D-fructose in aqueous solution isolation and purification were carried out after the manner of Fischer,4 but no yield was given. By this... 

Bands Observed" in the Laser-Raman Spectrum of o-Fructose in Aqueous Solution... 

Fig. 9.7. Stereochemistry of the hemiketal formation from D-fructose in aqueous solution at 25°C (in the formulas for the six-membered ring hemiketal the carbon skeleton of 5-valerolactol is black, which emphasizes the relationship to the reaction from Figure 9.5, whereas the extra substituents as well as the bonds leading to them are red).

Figure 4-4 Tautomeric Forms of Fructose in Aqueous Solution at Room Temperature...

FIGURE 11.2 A molecule of fructose In aqueous solution. Note the attractions between the hydroxyl (O —H) groups of the fructose and molecules of water. The fructose molecule Is equated the exact number and arrangement of the attached water molecules fluctuate. Also shown Is one hydrogen bond between a water molecule and an oxygen atom In the fructose ring. 

D-mannitol is widely used as sweetening agent and finds also different application in the food industry and related areas [1]. D-mannitol can be directly prepared from mannose or by stereoselective hydrogenation of D-fructose. However, the hydrogenation of D-fructose in aqueous solution over different heterogeneous catalysts leads to the formation of two isomers, i.e. D-mannitol and D-sorbitol, near to a ratio of one to one [1]. 

Studies of carbohydrates in solution have included an investigation of the intramolecular hydrogen bonding of methyl 4,6-O-benzylidene-a-D-hexopyrano-side derivatives. Diols, monomethyl ethers, and monodeoxy-compounds with various configurations were studied in the 3600 cm" region. A similar investigation was carried out on methyl ethers and benzylidene derivatives of D-aldo-pyranoses. A laser Raman study of D-fructose in aqueous solution indicated that furanose forms could be distinguished from pyranoses and that at equilibrium the ratio of the two forms was 41 59. ... 

Laser-Raman spectroscopy of D-fructose in aqueous solution has given results for the proportions of furanose and pyranose similar to those from other tech-... 

Under mild acid conditions (viz., pH 5-6 at 0-60 C) reducing sugars ionize and mu-tarotate, at lower pH (v/z., down to pH 3 or 4) and at higher temperatures viz., up to ca. 100°C) enolization and isomerization occurs. In acid solution enolization is initiated by direct protonation of the carbonyl group (see Figure 2). In fact, acids are far less effective enolization catalysts than alkalies and as a consequence D-glucose and D-fructose in aqueous solution show maximum stability between pH 3 and 4 e.g., McDonald, 1950). 

Results from measurements of the spin-lattice relaxation times of fructose in aqueous solutions containing alkali metal halides have been interpreted in terms of structure-making and -breaking properties of the alkali metal ions. ... 

Alkaline Degradation. At high pH, sucrose is relatively stable however, prolonged exposure to strong alkaU and heat converts sucrose to a mixture of organic acids (mainly lactate), ketones, and cycHc condensation products. The mechanism of alkaline degradation is uncertain however, initial formation of glucose and fructose apparendy does not occur (31). In aqueous solutions, sucrose is most stable at —pH 9.0. 

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

A long-wavelength probe 29 signaling carbohydrates in aqueous solutions by increasing of fluorescence was developed by Akkaya and Kukre on the basis of a symmetrical squaraine dye containing two phenylboronic acid functions [89]. The emission maximum of this probe is at 645 nm. A maximal response of about 25% was found for fructose. 

Thermolysis of D-fructose in acid solution provides 11 and 2-(2-hydrox-yacetyl)furan (44) as major products. Earlier work had established the presence of 44 in the product mixtures obtained after acid-catalyzed dehydrations of D-glucose and sucrose. Eleven other products were identified in the D-fructose reaction-mixture, including formic acid, acetic acid, 2-furaldehyde, levulinic acid, 2-acetyl-3-hydroxyfuran (isomaltol), and 4-hydroxy-2-(hydroxymethyl)-5-methyl-3(2//)-furanone (59). Acetic acid and formic acid can be formed by an acid-catalyzed decomposition of 2-acetyl-3-hydroxyfuran, whereas levulinic acid is a degradation prod-uct of 11. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4//-pyran-4-one has also been isolated after acid treatment of D-fructose.The pyranone is a dehydration product of the pyranose form of l-deoxy-D-eo f o-2,3-hexodiulose. In aqueous acid seems to be the major reaction product of the pyranone. 

This enzyme [EC 2.4.1.7], also known as sucrose gluco-syltransferase, catalyzes the reaction of sucrose with orthophosphate to produce D-fructose and a-D-glucose 1-phosphate. In the forward reaction, arsenate may replace phosphate as the substrate. However, the resulting product is unstable in aqueous solutions. In the reverse reaction, various ketoses and L-arabinose may replace D-fructose. See Arsenolysis... 

Figure 1. Solubility of fructose in aqueous alcoholic solutions, 25-60 C.

Pyrones are also formed from D-fhreo-2,5-hexodiulose ( 5-keto-D-fructose ) (67) when it is heated in aqueous solution.89 Compounds 68 and 69 are formed in high yield, with 68 preponderating. Kojic acid (68) results from /3-eliminations in which the ring remains intact, but the formation of 5-hydroxymaltol (69) requires ring opening, because of the absence of a proton on C-2. 

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