Chemical and Functional Properties of Food Saccharides

crystalline salts, esters, amides, and hydrazides. These derivatives have been helpful
to identify aldonic acids and sugars.
D-Gluconic acid is commercially important. A number of its salts exhibit biological
activity. Ferrous D-gluconate is used to treat iron-deficiency anemia. Calcium
deficiency is treated with calcium D-gluconate. Because of the sequestering ability
in alkaline solutions, D-gluconic acid is added to various cleaning compositions. In
the dairy industry, it prevents milk scale; similarly, in breweries, it prevents deposition
of beer scale.
Oxidation of the primary hydroxyl group of aldoses to the carboxyl group results
in the formation of uronic acids (aldehydocarboxylic acids), which, similar to
aldoses, take two possible pyranose forms as shown for D-galacturonic (2.18) and
D-mannuronic (2.19) acids. Depending on their configuration, uronic acids can also
COOH COOH
OH O
O
OH
H, OH OH
OH
OH H, OH
OH
2.18 2.19
form lactone rings. Prior to oxidation of the primary hydroxyl groups, the secondary
hydroxyl groups should be protected from oxidation to the carbonyl group. The
isopropylidene group appeared to be the most suitable. It is stable under neutral and
alkaline conditions and after oxidation can be removed by acid hydrolysis. Thus,
D-glucose (2.20) is reacted with acetone under proton catalysis. Protection involves
the hydroxyl groups at C-1 and C-2 and transformation of pyranose to furanose. In
consequence, 1,2-O-isopropylidene-α-D-glucofuranose (2.21) is formed, which is
subsequently oxidized over platinum at C-6 with a high yield of acid (2.22), which
is then hydrolyzed to D-glucuronic acid (2.23).
OH
© 2004 by CRC Press LLC
Pt, O2
OH
O
OH
OH
2.20
HO
H, OH
+
CH3COCH3
+
H
- H2O
HO
COOH
COOH
O
OH
+
H2O, H
O OHH
O
O
OH
2.22
O CH3
CH3
OH
2.23
CH2OH
O
OH
O
2.21
O CH3
CH3
H, OH