2009_Book_FoodChemistry

290 4 Carbohydrates
This then reacts, probably through its glycosyl
cation form. Due to the steric influence of the
acetylated group on C-2, the 1,2-trans-glycoside
is preferentially obtained, e. g., in the case of
D-glucose, β-glucoside results.
Acetylglycosyl halogenides are also used for
a highly stereoselective synthesis of α-glycosides.
The compound is first dehalogenated
into a glycal. Then, addition of nitrosylchloride
follows, giving rise to 2-deoxy-2-nitrosoglycosylchloride.
The latter, in the presence of
alcohol, eliminates HCl and provides a 2-deoxy-2-oximino-α-glycoside.
Reaction with ethanal
yields the 2-oxo compound, which is then
reduced to α-glycoside:
Table 4.17. Relative rate of hydrolysis of glycosides (a:
2mol/l HC1,60 ◦ C; b: 0.5mol/lHCl,75 ◦ C)
Hydrolysis
Compound condition K rel
Methyl-α-D-glucopyranoside a 1.0
Metyhl-β-D-glucopyranoside a 1.8
Phenyl-α-D-glucopyranoside a 53.7
Phenyl-β-D-glucopyranoside a 13.2
Methyl-α-D-glucopyranoside b 1.0
Methyl-β-D-glucopyranoside b 1.9
Methyl-α-D-mannopyranoside b 2.4
Methyl-β-D-mannopyranoside b 5.7
Methyl-α-D-galactopyranoside b 5.2
Methyl-β-D-galactopyranoside b 9.2
(4.104)
O-Glycosides are widely distributed in nature
and are the constituents, such as glycolipids,
glycoproteins, flavanoid glycosides or saponins,
of many foods.
O-Glycosides are readily hydrolyzed by acids.
Hydrolysis by alkalies is achieved only under
drastic conditions which simultaneously decompose
monosaccharides.
The acid hydrolysis is initiated by glycoside protonation.
Alcohol elimination is followed by addition
of water:
The hydrolysis rate is dependent on the aglycone
and the monosaccharide itself. The most favored
form of alkylglycoside, α-pyranoside, usually is
the isomer most resistant to hydrolysis. This is
also true for arylglycosides, however, due to steric
effects, the β-pyranoside isomer is synthesized
preferentially and so the β-isomer better resists
hydrolysis.
The influence of the sugar moiety on the rate of
hydrolysis is related to the conformational stability.
Glucosides with high conformational stability
are hydrolyzed more slowly (cf. data compiled in
Table 4.17).
4.2.4.6 Esters
Esterification of monosaccharides is achieved by
reaction of the sugar with an acyl halide or an acid
anhydride. Acetylation, for instance with acetic
anhydride, is carried out in pyridine solution:
(4.106)
(4.105)
Acyl groups have a protective role in some
synthetic reactions. Gluconic acid nitrile acetates
(aldonitrile acetates) are analytically suitable
sugar derivatives for gas chromatographic separation
and identification of sugars. An advantage