Ketoses, detection

Reducing sugars can be detected by reaction with phenylhydrazine to yield a hydrazone product, except the result of the reaction is not what one might imagine giving the structure of aldoses and ketoses. Glucose, for example, can react with phenylhydrazine to yield the anticipated... 

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

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 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... 

Brands and van Boekel124 noticed that there were significant differences between aldoses and ketoses, the latter seeming more reactive in sugar degradation reactions. Whereas Amadori compounds were detected in aldose-casein systems, no such intermediates were found for ketose-casein systems. 

More drastic conditions were used with iodic acid, high temperatures and strongly acid solutions being employed. The oxidation products w ere not isolated. Williams and Woodsused 35% sulfuric acid containing 1% potassium iodate, the sugar being heated for ninety minutes at 100° with this oxidant. Ketoses, sucrose and pentoses were attacked, but L-rhamnose was oxidized more slowly. It was remarkable that none of the aldohexoses or lactose vere detectably oxidized. The reactions of cellobiose and trehalose were ascribed to impurities. 

All carbohydrates (aldoses and ketoses) and polyalcohols produce a large anodic peak response at ca. +0.15 V. The peak for glucose corresponds to a reaction with n approaching 10 equiv/mol for fluid velocities typical of flow-through detection cells. This n value is consistent with an oxidative cleavage for the C1-C2 and Cs-C bonds to form two moles of formate and one mole of dicarboxylate dianion [24]. 

The easy oxidation of the aldehyde group using a mild oxidizing agent such as copper (II) and silver (I) can detect the presence of carbohydrate. The carbohydrates are referred to as reducing sugars. This type of test cannot distinguish between aldoses and ketoses,... 

It has been emphasized that glycosuloses are absent from aldose and ketose solutions. Isolation of a D-am6mo-hexosulose- C derivative, together with independent chromatographic resolution, must finally establish the presence of glycosulose, at least from D-glucose, as previously proposed. The fact that glycosuloses have not been detected in some instances ... 

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