Food Chemistry

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-gly-
cosides. The compound is first dehalogenated
into a glycal. Then, addition of nitrosylchloride
follows, giving rise to 2-deoxy-2-nitroso-
glycosylchloride. The latter, in the presence of
alcohol, eliminates HCl and provides a 2-de-
oxy-2-oximino-α-glycoside. Reaction with etha-
nal yields the 2-oxo compound, which is then
reduced toα-glycoside:

(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 decom-
pose monosaccharides.
The acid hydrolysis is initiated by glycoside pro-
tonation. Alcohol elimination is followed by ad-
dition of water:

(4.105)

Table 4.17.Relative rate of hydrolysis of glycosides (a:

2mol/lHC1,60◦C; b: 0.5mol/lHCl,75◦C)

Hydrolysis

Compound conditionKrel

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

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, theβ-pyranoside isomer is synthesized

preferentially and so theβ-isomer better resists

hydrolysis.

The influence of the sugar moiety on the rate of

hydrolysis is related to the conformational stabil-

ity. 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)

Acyl groups have a protective role in some

synthetic reactions. Gluconic acid nitrile acetates

(aldonitrile acetates) are analytically suitable

sugar derivatives for gas chromatographic sepa-

ration and identification of sugars. An advantage