Ketose equilibrium

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%). 

Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol called an alditol. The reduction occurs by reaction of the open-chain form present in the aldehyde/ketone hemiacetal equilibrium. Although only a small amount of the open-chain form is present at any given time, that small amount is reduced, more is produced by opening of the pyranose form, that additional amount is reduced, and so on, until the entire sample has undergone reaction. 

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. 

Xylose isomerases with higher thermostability were found in the strains of Streptomyces and relaxed Actinoplanaceae (which includes the generdLAmpullariella and Actinopianes). High thermo-tolerance is desirable for production of HFCS because at equilibrium, as the temperature of the enzyme reaction is increased, the ketose/aldose ratio increases proportionately 30). In addition, reactors running at higher temperatures have less risk of microbial contamination, allowing for less frequent and less costly enzyme replacement. 

However, the applicability of this strategy is limited by the substrate specificity of the isomerases so that only a fraction of the ketoses that can be obtained from the aldose-catalyzed reaction can be enzymatically isomerized to the corresponding aldose. Moreover, the isomerization reaction is reversible and, as a ketone is generally more stable than an isomeric aldehyde, the equilibrium produces substantial aldose isomer only if the aldose sugar can exist in a very stable aldopyra-nose form [38b]. 

In the production of the sweetener high fructose com symp (HFCS), glucose isomerase is used to convert glucose into fmctose (equilibrium mixture appr. 50 50) since fmctose is nearly three times as sweet as glucose. The enzyme belongs to the subclass intramolecular oxidoreductases interconverting aldoses and ketoses (EC 5.3.1). About 8 million tonnes pr. year of glucose is treated in this way. For further details, see Chapter 4. 

A similar series of reactions was performed on a 6-azido-6-deoxy-2-ketose.246,252 In most of the foregoing syntheses, the piperidine formed is accompanied by the tautomeric furanose. The tautomers can, however, be separated without occurrence of equilibration (which can be induced by changing the pH of the medium). For the ketose derivative 183, the equilibrium mixture includes246 compound 184 and a dehydration product (182). The conformation and anomeric effect in these piperidines have been studied by n.m.r. spectroscopy,253 and their fragmentation pathways have been determined by mass spectrometry.254... 

Problem 22.7 (a) Explain how in basic solution an equilibrium is established between an aldose, its C epimer (a diastereomer with a different configuration at one chiral C) and a 2-ketose. (b) Will fructose give a positive Fehling s test which is done in a basic solution <4... 

The proportion of the acyclic form also increases with increasing temperature this is true for aldoses and ketoses,16,31 as well as for simple hydroxyketones.74 This would be expected from considerations of entropy, as the acyclic form has a greater degree of freedom, but studies on y- and d-hydroxyketones show that change in enthalpy contributes even more to the changing position of the equilibrium with increasing temperature. Evidently, cyclization of hydroxyketones is exothermic, and is favored by lower temperatures.74... 

Most, if not all, of the stable forms of crystalline aldose and ketose monosaccharides exist in the pyranose structure. Each in solution, as with D-glucose, exists as an equilibrium mixture of open chain and of a- and / -anomers of the cyclic forms. The cyclic five- and six-membered structures formulated below are an illustrative selection of monosaccharides. 

A test for reducing sugars, employing the same silver-ammonia complex used as a test for aldehydes. A positive test gives a silver precipitate, often in the form of a silver mirror. Tollens reagent is basic, and it promotes enediol rearrangements that interconvert ketoses and aldoses. Therefore, both aldoses and ketoses give positive Tollens tests if they are in their hemiacetal forms, in equilibrium with open-chain carbonyl structures, (p. 1118)... 

When an aldose or ketose is dissolved in water, a complex equilibrium may be established. The present article deals with the reactions involved and the rates of interconversion of the molecular species... 

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