Uronic acid from aldoses

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. 

Uronic acid (Section 25.6) The monocarboxylic acid resulting from enzymatic oxidation of the -CH2OH group of an aldose. 

Uridine, biosynthesis of, 1124 Uronic acid, 994 from aldoses, 994 Urushiols, structure of, 600 UV, see Ultraviolet... 

Monocarboxylic acids formally derived from aldoses by replacement of the CH2OH group with a carboxy group are termed uronic acids (see 2-Carb-22). 

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... 

Aldaric acids may be prepared from aldoses or aldonic acids by oxidation in aqueous solution with oxygen over platinum-charcoal255 or platinum-on-alumina.256 The effect of such promoters as bismuth or gold has also been studied.257 Hydrogen peroxide in the presence of iron salts has been used for the oxidation of uronic acids to aldaric acids.258... 

Methylated uronic acids are usually separated from methylated aldoses by virtue of the insolubility of their barium salts in chloroform. Recently, 4-0-methyl-D-glucuronic acid has been separated from a mixture of non-acidic reducing sugars by absorption on a weakly basic... 

As depicted in Scheme 21.1 for the oxidation of glucose, three general types of acids can be produced by the oxidation of a sugar aldonic acids in which only the aldehyde of the parent aldose has been oxidized to the acid aldaric acids that are dicarboxylic acids with one carboxy group coming from the oxidation of the aldehyde and the other from the primary alcohol at the other end of the carbon chain or uronic acids derived from the selective oxidation of the primary alcohol without the oxidation of the aldehyde. 

The oxidation of aldoses produces onic acids. Thus, the oxidation of glucose and mannose produces gluconic and mannonic acids, respectively. These should not be confused with the uronic acids (glucuronic and mannuronic) that result from the oxidation of the primary alcohol function. 

Bromine and hypoiodite oxidations are particularly suitable for the preparation of aldonic acids from aldoses. Similarly, uronic acids are converted to saccharic acids. Of less value is the oxidation of primary alcoholic to aldehydic groups. In this manner, glycosides can be converted to uronides and polyols to aldoses and aldonic acids. 

Uronic acids are derived from the aldoses by oxidation of the primary alcohol group to a carboxyl group. 

Uronic acids (XXIII) are derived from the aldoses (XXII) by oxidation of the terminal CHjOH group to COOH. These acids have not so far been... 

Low yields of uronic acid usually result from the action of selective oxidants on glucosides and non-reducing acetals of the aldoses in which all the hydroxyl groups are free. However, a 56% yield of D-galacturonoside has been claimed for the oxidation of methyl a-D-galactopyranoside with nitrogen dioxide . 

---