Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
C
O
H
(CH2)n
H
C
H
43
+ ROH
OH
+
H
anomerize to give an equilibrium mixture with α/β = 0.2. Several mechanisms for
the formation and anomerization of furanosides have been proposed. The furanosides
undergo a ring expansion to α- and β-pyranosides formed in the α/β ratio of 0.6.
Pyranosides (2.45) anomerize into (2.50) and (2.51). An intermediary cyclic
carboxonium ion (2.49) is postulated. The α/β ratio can be predicted taking into
account the interaction between substituents in the tetrahydropyranose ring (Table
2.1). Alcohols as solvents and the OCH3 group cooperate with the anomeric effect.
Thus, the α-pyranoside structure becomes favorable.
Glycosides and acetals readily hydrolyze in acid media. The rate of hydrolysis
strongly depends on the structure of the aglycone and sugar. Examination of the
hydrolysis of methyl glucosides, mannosides, galactosides, and xylosides reveals
that furanosides hydrolyze more readily than pyranosides. The rate of hydrolysis of
β-D-glycosides with the equatorial methoxy group is higher than that of α-anomers
with the axial methoxy group. The hydrolysis of methyl aldofuranosides with the
1,2-cis configuration of the hydroxyl group at C-2 and aglycone proceeds at higher
rate, and hydrolysis of glucopyranosides and glucofuranosides resembles that of
simple acetals. It requires protonation of the acetal oxygen atom followed by substitution
at the anomeric carbon atom. At room temperature, in contrast to aryl
glycosides, alkyl glycosides are stable on hydrolysis in dilute alkali. The hydrolysis
of aryl glycosides (2.52) involves a 1,2-epoxide intermediate (2.53), which is
attacked by the C-6 hydroxyl group to give a 1,6-anhydride (2.54).
Glycoside formation and degradation are fundamental biochemical processes.
Several natural biologically active compounds are complex glycosides (flavanoids,
cardiac glycosides, antibiotics). Such glucoconjugates play essential roles in several
molecular processes, such as hormone activity, induction of protective antibody
response, control the development and defense mechanism of plants, and cell proliferation.
2.3.6 BIOLOGICAL SIGNIFICANCE OF THE EFFICIENCY OF
O-GLYCOSYLATION
There are no general methods for glycoside synthesis. An interglycosidic bond is
formed in a reaction of suitably O-protected glucosyl donor bearing a leaving group,
for instance the bromine atom in (2.55), on its anomeric carbon atom with a suitable
nucleophile. For example, phenolate anion resulting from the dissociation of potassium
phenolate (2.56) provides phenylglycoside (2.57).
H
H
(CH 2)n
44
O
H, OR + H 2O