Download Glycosides - Chemistry With BT

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.
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
••