25.13
Glycosides
Glycosides
Glycosides have a substituent other than OH at
the anomeric carbon.
Usually the atom connected to the anomeric
carbon is oxygen.
Example
HOCH2
O
HO
HO
OH
OH
D-Glucose
Linamarin is
an O-glycoside
derived from
D-glucose.
HOCH2
HO
HO
O
CH3
OCC
OH
CH3
N
Glycosides
Glycosides have a substituent other than OH at
the anomeric carbon.
Usually the atom connected to the anomeric
carbon is oxygen.
Examples of glycosides in which the atom
connected to the anomeric carbon is something
other than oxygen include S-glycosides and Nglycosides.
Example
Adenosine is an Nglycoside derived from
D-ribose
NH2
N
HOCH2
H O
H
OH
OH
H
H
OH
D-Ribose
N
N
HOCH2 N
H O H
H
OH
H
OH
Adenosine
Example
HOCH2
O
HO
HO
OH
OH
D-Glucose
Sinigrin is an
HOCH2
S-glycoside HO
derived from
HO
D-glucose.
O
NOSO3K
SCCH2CH
OH
CH2
Glycosides
O-Glycosides are mixed acetals.
O-Glycosides are mixed acetals
CH
O
O
OH
H
CH2OH
hemiacetal
O-Glycosides are mixed acetals
CH
O
O
OH
hemiacetal
H
CH2OH
ROH
O
OR
H
acetal
Preparation of Glycosides
Glycosides of simple alcohols (such as
methanol) are prepared by adding an acid
catalyst (usually gaseous HCl) to a solution of a
carbohydrate in the appropriate alcohol.
Only the anomeric OH group is replaced.
An equilibrium is established between the a and
b-glycosides (thermodynamic control). The
more stable stereoisomer predominates.
Preparation of Glycosides
CH
O
H
OH
HO
H
H
OH
H
OH
CH2OH
D-Glucose
HOCH2
HO
HO
CH3OH
O
OCH3
OH
HCl
+
HOCH2
HO
HO
O
OH
OCH3
Preparation of Glycosides
HOCH2
Methyl
b-D-glucopyranoside
HO
HO
O
OCH3
OH
+
Methyl
a-D-glucopyranoside
(major product)
HOCH2
HO
HO
O
OH
OCH3
Mechanism of Glycoside Formation
HOCH2
HO
HO
•• •
O•
OH
OH
HCl
HOCH2
HO
HO
••
O ••
carbocation is stabilized
by lone-pair donation from
+
oxygen of the ring
H
OH
Mechanism of Glycoside Formation
HOCH2
•• •
O•
HO
HO
OH
HOCH2
HO
HO
CH3
HOCH2
O
HO
O •• + HO
+
H
OH +
O
H3C •• H
CH3
••
•• O ••
O ••
H
+
OH
H
Mechanism of Glycoside Formation
HOCH2
HO
HO
•• •
O•
OH
HOCH2
HO
HO
CH3
HOCH2
O
HO
O •• + HO
+
H
+
–H
OH +
O
H3C •• H
HOCH2
••
O ••
OH
HO
HO
••
OCH3 +
••
••
O ••
OH
•• OCH
3
••
25.14
Disaccharides
Disaccharides
Disaccharides are glycosides.
The glycosidic linkage connects two
monosaccharides.
Two structurally related disaccharides are
cellobiose and maltose. Both are derived from
glucose.
Maltose and Cellobiose
HOCH2
HOCH2
O
HO
1
HO
a
OH
O
O
OH
4
HO
Maltose
OH
Maltose is composed of two glucose units linked
together by a glycosidic bond between C-1 of
one glucose and C-4 of the other.
The stereochemistry at the anomeric carbon of
the glycosidic linkage is a.
The glycosidic linkage is described as a(1,4)
Maltose and Cellobiose
HOCH2
HOCH2
O
HO
1
HO
b
OH
O
O
OH
4
HO
Cellobiose
OH
Cellobiose is a stereoisomer of maltose.
The only difference between the two is that
cellobiose has a b(1,4) glycosidic bond while
that of maltose is a(1,4).
Maltose and Cellobiose
Maltose
Cellobios
e
Cellobiose and Lactose
HOCH2
HOCH2
O
HO
1
HO
b
OH
O
O
OH
4
HO
Cellobiose
OH
Cellobiose and lactose are stereoisomeric
disaccharides.
Both have b(1,4) glycosidic bonds.
The glycosidic bond unites two glucose units in
cellobiose. It unites galactose and glucose in
lactose.
Cellobiose and Lactose
HOCH2
HOCH2
O
HO
1
HO
b
OH
O
O
OH
4
HO
Lactose
OH
Cellobiose and lactose are stereoisomeric
disaccharides.
Both have b(1,4) glycosidic bonds.
The glycosidic bond unites two glucose units in
cellobiose. It unites galactose and glucose in
lactose.
25.15
Polysaccharides
Cellulose
Cellulose is a polysaccharide composed of
several thousand D-glucose units joined by
b(1,4)-glycosidic linkages. Thus, it can also be
viewed as a repeating collection of cellobiose
units.
Cellulose
Four glucose units of a cellulose chain.
Starch
Starch is a mixture of amylose and amylopectin.
Amylose is a polysaccharide composed of 100
to several thousand D-glucose units joined by
a(1,4)-glycosidic linkages.
25.16
Cell-Surface Glycoproteins
Glycoproteins
Glycoproteins have a protein backbone to which
carbohydrates are attached. One example is
the cell-surface glycoprotein that is recognized
by the influenza virus.
O
OH
HO
CO2H
CH3CNH
O
O
O
PROTEIN
OH
CH2O
O
NHCCH3
HO
OH
CH2OH
Glycoproteins
A second example is the group of glycoproteins
that define the various human blood groups.
Human-Blood-Group Glycoproteins
HO
R
CH2OH
O
O
O
O
H3C
O
OH
HO
HO
N-Acetylgalactosamine
polymer
PROTEIN
Type O:
R
=
H
HO
Type A:
R
=
CH2OH
O
HO
CH3CNH
O
HO
Type B:
R
=
CH2OH
O
HO
OH