Ketoses, reaction with

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... 

The reaction with ethyl acetoacetate has been extended to glycolaldehyde, and to carbohydrates other than n-glucose, by employing different experimental conditions it is probably applicable to aldoses in general. With d-fructose, yields are lower, but two molar proportions of water are liberated and a crystalline product results. This has a constitution similar to that of II but with the D-omhfno-tetrahydroxybutyl chain at the /3-position on the furan ring. The reaction has been applied successfully to other ketoses and... 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

Aromatic amines that have been used include o-toluidine, p-aminosali-cylic acid, p-aminobenzoic acid, diphenylamine and p-aminophenol. Their ability to react preferentially with a particular carbohydrate or class of carbohydrate is often useful, e.g. p-aminophenol, which shows some specificity for ketoses compared with aldoses and is useful for measuring fructose. These reagents have proved particularly useful for the visualization and identification of carbohydrates after separation of mixtures by paper or thin-layer chromatography, when colour variations and the presence or absence of a reaction aid the interpretation of the chromatogram. 

The reaction with aldoses and ketoses is different and the procedure may be used to distinguish between them. Free monosaccharides and glycosides do not react in an identical manner with periodic acid and this reveals information about the structure, sequence and linkage of monosaccharides or their derivatives in a polysaccharide. 

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

CA 53, 17513 (1959). Materials useful as rocket fuels, semisolid or gelled fuels for bursting and tail-ejection-type bombs, and incendiary fuels for flame throwers and hand grenades are described. They are made by mixing 0.1-25% by wt of satd, unsatd, or aromatic nitrohydrocarbons or their mixts, such as nitro- or dinitromethane, -ethane, -propane, or -butane with divinylated ketoses or diaryl deoxyketitols prepd by reaction of C3 g ketose sugars with C6 M aromatic hydrocarbons. The latter include CfiH, toluene, naphthalene, anthracene and their alkylated derivs... 

Reaction with Fehling s (and Benedict s) reagent, aldehydes and ketones (i.e. with sugars - aldoses and ketoses) can reduce Fehling s (and Benedict s) reagents, and they themselves are oxidized. 

On the other hand, the acyl esters of 2-ketoses follow a completely different pathway, as heterocyclic, nitrogenated compounds and melanoidins are the principal products of their reaction with ammonia. The free 2-ketoses show similar behavior, but they present a significant difference in the extent of formation of these compounds, the presence of the esterifying acyl groups in the molecule being decisive in increasing their yields. 

Amadori compounds (N-substituted-l-amino-l-deoxy-2-ketoses) are potential precursors to the formation of many of these heterocyclic volatile products. The secondary nitrogen in most Amadori compounds is weakly basic and is therefore a likely site for rapid nitrosation reactions via normal reactions with nitrous acid, under mildly acidic conditions. However, purified Amadori compounds are usually obtained only after tedious isolation procedures are invoked to separate them from the complex mixtures of typical Maillard browning systems. Takeoka et al. ( 5) reported high performance liquid chromatographic (HPLC) procedures to separate Amadori compounds in highly purified form on a wide variety of columns, both of hydrophilic and hydrophobic nature. They were able to thus demonstrate that reaction products could be followed for kinetic measurements as well as to ensure purity of isolated products. 

A similar reaction with ketoses is more complex. Thus fructose is converted into furfuryl alcohol (79% yield). 

Zhdanov and Polenov have reported58 a synthesis of the first sugar phosphorane (70), which was shown to react with p-nitro-and o-hy-droxy-benzaldehyde, forming unsaturated ketoses 71 and 72, respectively. Attempted reactions with p-dimethylamino-, p-hydroxy-, and 2,4-dihydroxy-benzaldehyde were unsuccessful. The formation of the dienic ketose 72 is undoubtedly caused by /3-elimination of a methoxyl group during treatment of the reaction mixture with aqueous sodium hydroxide during isolation. 

Anhydro-C-(p-methoxyphenyl)aldoses and anhydro- 1,3-dideoxy-ketoses were found to be compounds usually produced on heating free and partially protected aldoses with p-methoxybenzoyl- and acetyl-methylenetriphenylphosphoranes, respectively, in N,N-di-methylformamide. Many of them were isolated by thin-layer chromatography on alumina, although, on several occasions, these compounds were available only as mixtures of the five- and six-membered anhydro derivatives, because of their similar chromatographic mobilities. Five-membered anhydro derivatives are favored, but exclusive formation of six-membered anhydro derivatives was observed when five-membered ring-closure was impossible. Only L-rhamnose on reaction with acetylmethylenephosphorane formed a six-membered derivative, 196b. 

Some ketoses and aldoses undergo rearrangement upon reaction with diethylaminosulfur trifluoridc. In 2-uloses, the axial orientation of an a-methoxy group can give rise to its 1,2-migration, cf. formation of 9 and 10. Thus, l,2-difluoro-2-alkoxy sugars arc obtained as anomcric mixtures. 

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