Aldonic acids, sugar oxidation

Oxidation of Aldonic Acids. Preferential oxidation of the secondary alcoholic group adjacent to the carboxyl group in sugar acids or aldonic acids such as L-gulonic acid (XXV) can be carried out with chromic acid12 or with chlorates in the presence of a vanadium catalyst.13... 

The field of halogen oxidation has developed primarily in two directions. The first is the preparation of aldonic acids by oxidation with bromine in acid solution. The second is the use of alkaline hypoiodite as a quantitative oxidant. Little work has been done on the mechanism of these reactions. The conditions under which the reactions are carried out have usually been established empirically without extensive study. The further use of solutions of known concentration and the comparison of various oxidants under similar conditions are to be desired. Critical oxidation potentials of various sugars used in relation to possible oxidants and ranges of pH might yield promising results. 

The oxidation of the primary hydroxyl function at C1 of a sugar alcohol to a carboxyl function leads to the corresponding aldonic acid. Further oxidation of the remaining terminal hydroxyl function of an aldonic acid to a carboxyl function leads to an aldaric acid. Both, aldonic and aldaric acids easily form lactones through intramolecular condensation reactions. As an exam-... 

Sugars with 0-benzyl substituents can be oxidized to the corresponding aldonic acid. The oxidation of 2-0-benzyl-n-fucose to 2-0-benzyl-n-fuconic... 

Sugars Unusual sugars (amino, deoxy and methyl sugars, and sugars with branched chains). Reduction products (sugar alcohols, cyclitols, streptidine). Oxidation products (uronic acids, aldonic acids, sugar dicarboxylic acids). 

Oxidation to Sugar Acids and Lactones. When the aldehyde group of an aldose is oxidized, the resulting compound is an aldonic acid (salt form = aldonate) (11)4. Some aldonic acids are products of carbohydrate metaboHsm. 

Although the Tollens reaction is a useful test for reducing sugars, it doesn t give good yields of aldonic acid products because the alkaline conditions cause decomposition of the carbohydrate. For preparative purposes, a buffered solution of aqueous Br2 is a better oxidant. The reaction is specific for aldoses ketoses are not oxidized by aqueous Br2. 

In a similar fashion it should be a relatively simple task to relate the well-known D-gala-L-manno-heptose (XX) to a-rhamnohexose (XXII)81 through the known 1-desoxy-D-gala-L-mcmno-heptitol (syn., 7-desoxy-L-manno-L- aZa-heptitol) (XXI).81 In. this particular case, however, there is already ample proof of the configuration of the latter sugar (XXII) because nitric acid has been found to oxidize the corresponding aldonic acid to mucic acid.82... 

There are three possible classes of sugar acids which may be produced by the oxidation of monosaccharides (Figure 9.11). The aldonic acids are produced from aldoses when the aldehyde group at carbon 1 is oxidised to a carboxylic acid. If, however, the aldehyde group remains intact and only a primary alcohol group (usually at carbon 6 in the case of hexoses) is oxidised then a uronic acid is formed. Both aldonic and uronic acids occur in nature as intermediates in... 

Pfannemtiller et al. showed that it is possible to obtain carbohydrate-containing amphiphiles with various alkyl chains via amide bond formation. For this, mal-tooligosaccharides were oxidized to the corresponding aldonic acid lactones, which could subsequently be coupled to alkylamines [128-136]. Such sugar-based surfactants are important industrial products with applications in cosmetics, medical applications etc. [137-139]. The authors were also able to extend the attached mal-tooligosaccharides by enzymatic polymerization using potato phosphorylase, which resulted in products with very interesting solution properties [140, 141]. 

Natural derivatives of sugars. The aldehyde group of an aldose can be oxidized readily to a carboxyl group to form an aldonic acid. Among the several aldonic acids that occur naturally is 6-phosphogluconic acid, which is pictured here as the 6-phosphogluconate ion ... 

Two free-radical chain reactions, in addition to the ionic enolate mechanism, seem reasonable for the oxidation of the sugars by oxygen. With an aldose-2-f one of the free-radical mechanisms would yield non-labeled formic acid and the next lower aldonic acid the other would yield labeled formic acid and the same aldonic acid. 

Three kinds of sugar acids can be formally obtained from the corresponding aldoses. They are aldonic acids, produced by oxidation at C-l of the aldose uronic acids, formed by oxidation of the primary alcohol group of the aldose and aldaric acids, formed by oxidation of both the aldehyde and the primary alcohol group. 

L-erythronic acids, respectively, and D-xylose to D-threonic acid.44 Isbell et al. elucidated the mechanism of this process45 (see the following Chapter). Degradation to a lower aldonic acid can be achieved by oxidation of an unsaturated sugar derivative. The method of Reichstein et al.46 for the preparation of L-threono-1,4-lactone by permanganate oxidation of 5,6-0-isopropylidene-L-ascorbic acid was improved by Perel and Dayton to afford the crystalline lactone in 65% yield.47... 

Two useful preparations of lower sugars by degradation of aldonic acids are discussed in the following Chapter. These are the traditional oxidation of salts of aldonic acids with hydrogen peroxide in the presence of ferric acetate (Ruff degradation) and the related oxidation of aldonic acids by hypochlorite, which was developed by Whistler and Schweiger.79... 

Fully acetylated aldonic acids can be prepared by oxidation of aldehydo-sugar peracetates. Direct acetylation of certain aldonic acid salts is possible, and addition of cadmium salts, in particular, affords high yields of acetates. The synthesis can also be accomplished by deamination of the readily prepared, acetylated amides with nitrous acid or nitrosyl chloride. Examples of the various methods are given in Ref. 86. 

This section deals with acids, that are formally modified aldonic acids, such as keto, deoxy, and branched-chain acids (including the so-called saccharinic acids). The aminoaldonic acids, which are oxidation products of amino sugars, and, in particular, the important nonulosaminic acids (neuraminic acids) and muramic acid, are not discussed here. The formation of saccharinic acids by the treatment of sugars with alkali, and the mechanisms involved, are likewise outside the scope of this chapter. 

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