Aqueous Solution Ketoses

Composition in Aqueous Solution Ketoses 1. Hexuloses and Pentuloses 

The composition of aqueous solutions of all of the 2-hexuloses, of several 3-hexuloses, 2-pentuloses, and their deoxy derivatives, and some of the hexulosonic acids is shown in Table IV. 

D-fhreo-2,5-Hexodiulose ( 5-keto-D-fructose ) is mainly ( 95%) in the / -pyranose form (7) in aqueous solution98 furanose forms involving 

D-fhreo-2,5-Hexodiulosonic acid, having a keto group at both C-2 and C-5 is, like the corresponding diulose, preponderantly in the yS-pyranose form in solution, with the 5-keto group fully hydrated.100 

The composition of solutions of the 2-heptuloses has been determined, and discussed, by Angyal and Tran.92 These ketoses are different from other reducing sugars inasmuch as there are two hydroxymethyl side chains attached to the pyranose ring. In the a-pyranose form, they are cis to each other and will therefore both be equatorial in the preponderant chair form. In the / -pyranose, however, one or other of the hydroxymethyl groups has to be axial and, in consequence, the / anomers are disfavored in only one solution (that of the altro isomer) was the yS-pyra-nose detected in the 13C-n.m.r. spectrum.92 

If a sugar has two carbonyl groups, each can form pyranose or furanose rings bicyclic forms sometimes result. An example is D-t/irao-3,4-hexodiu-lose42 because this compound is symmetrical, and pyranoses are not possible, only two forms are present at equilibrium. In aqueous solution, and also in dimethyl sulfoxide, 72% of the a,a- and 28% of the / ,/ -difuranose were found at 27°. The a,/ form, which would have two transfused five-mem-bered rings, was not observed. In dimethyl sulfoxide, there is also a small proportion of the monocyclic furanose form present. 

If a sugar contains both an aldehyde and a ketone function, the aldehyde will form rings mainly the composition can be quite complex. Eight monomeric forms (out of a possible 18) were identified in the spectra of each of the pentos-2-uloses.36 The erythro isomer was found to consist, at 23° in D20, of 4.5% of the a-aldopyranose and 20.5% of its hydrate, 4.5% of the / -aldopyr-anose and 38.8% of its hydrate. 10.9% of the a-aldofuranose hydrate, 14.2% of the / -aldofuranose hydrate, 5.2% of the a-ketofuranose hydrate, and 1.1% of the / -ketofuranose hydrate. For the threo isomer, the composition, in the same order, wasfound to beO.9,59.8,1.0,29.1,1.5,1.3,1.0, and 5.4%. These compositions were also determined at 80°. 

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 

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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%). 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

The problems of anomeric equilibrium may be avoided by investigating 2-ketoses. Both a hydroxyl group and a hydroxymethyl group are attached to the anomeric carbon atom in such sugars, and the bulky hydroxymethyl group favors the equatorial position. These authors measured c.d. spectra for three ketoses, the 2-(hydroxymethyl) derivatives of a-L-xylose, a-D-xylose, and a-D-mannose, in aqueous solution. 

In aqueous solution the rate law for oxidation of simple a-ketols, such as acetoin and benzoin and various aldoses and ketoses o. sog-sioj markedly different from that of Wiberg and Nigh (vide supra), viz. 

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... 

For simplicity, we have thus far represented the structures of aldoses and ketoses as straight-chain molecules (Figs 7-3, 7-4). In fact, in aqueous solution, aldotet-roses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl... 

By contrast, the keto forms of the ketoses seem to be hydrated to only a slight extent, if at all. When the proportion of the keto form is 5% or more, as it is for the 1-deoxyhexuloses, the hydrated form should be readily detectable ip the -n.m.r. spectrum, but it has not been found. A model compound, l-deoxy-3,4,5,6-tetra-0-methyl-D-fructose, shows only the signals of the keto form in aqueous solution.16 Ketones are, on the whole, hydrated to a lesser extent than aldehydes77 in the case of ketoses having more than four carbon atoms in the chain, there would also be a 1,3-parallel interaction of one of the geminal hydroxyl groups with another hydroxyl group. 

The Composition (%) of Aqueous Solutions of Ketoses and Amino Sugars... 

The irradiation of starch in aqueous gels, sols, and solutions leads to somewhat different results because of the operation of different mechanisms. Although all reactions in the solid state involve free-radical processes, those in aqueous solutions may involve hydrated electrons, which with oxygen give O2 ions. Kochetkov et al. 90 have assumed that deoxy sugars result from the involvement of Oj ions alone, whereas the formation of deoxy ketoses is assisted by hydroxyl radicals. Stockhausen et a/.191 have proposed the following sequences of reactions. Ionization of water leads to solvated electrons and further radicals. 

The aldehyde group of aldoses can either be oxidized or reduced and ketoses can be reduced. The best laboratory reagent for reduction is sodium boro-hydride which acts rapidly in neutral aqueous solutions (see Eq. 4-2). Since both NaB H4 and NaB H4 are available, radioactive or heavy isotope labels can be introduced in this way. The aldehyde groups can be oxidized by a variety of agents to the corresponding aldonic acids, a fact that accounts for the reducing properties of these sugars. In alkaline solution aldoses reduce Cu + ions to cuprous oxide (Eq. 4-3), silver ions... 

Unprotected aldoses and ketoses can be reduced to afford alditols while aldonolactones can be reduced to give either aldoses or alditols. The reagent of choice for reduction to alditols is sodium borohydride since it is both cheap and convenient to use. The reduction is carried out under mild conditions at room temperature in an aqueous solution. Sodium borohydride is stable in water at pH 14 while it reacts with the solvent at neutral or slightly acidic pH, but at a slower rate than the rate of carbonyl reduction. In some cases, the product will form esters with the generated boric acid. These borate complexes can be decomposed by treatment with hydrochloric acid or a strongly acidic ion-exchange resin and the boric acid can be removed in the work-up as the low boiling trimethyl borate by repeated co-evaporation with methanol at acidic pH [155]. 

Aldoses can be distinguished from ketoses by observing what happens to the color of an aqueous solution of bromine when it is added to the sugar. Br2 is a mild oxidizing agent and easily oxidizes the aldehyde group, but it cannot oxidize ketones or alcohols. Consequently, if a small amount of an aqueous solution of Br2 is added to an unknown monosaccharide, the reddish-brown color of Br2 will disappear if the monosaccharide is an aldose, but will persist if the monosaccharide is a ketose. The product of the oxidation reaction is an aldonic acid. 

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