Monosaccharide aldonic acids from

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... 

In the second place, oxidation by Fehling s or Tollens reagent cannot be used for the preparation of aldonic acids (monocarboxylic acids) from aldoses. Both Fehling s and Tollens reagents are alkaline reagents, and the treatment of sugars with alkali can cause extensive isomerization and even decomposition of the chain. Alkali exerts this effect, in part at least, by establishing an equilibrium between the monosaccharide and an enediol structure. 

Complexes of Cr111 with ascorbate, which are likely to form during the reactions of Crvl with this major intracellular reductant, have been studied (mainly by Cieslak-Golonka and co-workers),1062-1064 but no definitive structural information has been obtained to date. Partially characterized Crm complexes with monosaccharides or their oxidized derivatives (e.g., aldonic acid) have been isolated from the reactions with either Cr1111092 or CrVI.1093,1094... 

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. 

Although monosaccharides undergo complex isomerizations in base (see Section 22.5A), aldonic acids are epimerized specifically at C2 when they are heated with pyridine. Show how you could make use of this reaction in a synthesis of D-mannose from D-glucose. 

The aldehydic or hemiacetal group is the most easily oxidised portion of the monosaccharide molecule. All aldopyranoses (CLVIII) are oxidised with aqueous buffered bromine solution (pH 5.6). The initial product is the 1,5-lactone (CLIX) which then hydrolyses to the aldonic acid (CLX). Evaporation of the aqueous solution yields the y-lactone (CLXI), but from certain solvents such as p uidine which inhibit lactone formation the... 

Aldoses and reducing oligosaccharides are relatively easily oxidised without loss of carbon to acids, and four types of acidic monosaccharides, namely aldonic, oxoaldoiiic, hexuronic, and ascorbic acids, have been isolated from natural environments. 

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