O
O
H
O
H
OH
HOOC
O
O
H
O
H
O
O
HOOC
H
H
O
COOH
H
Glycyrrhizinic acid, a glycoside of liquorice
O
O
H
O
H
OH
O
H
O
O
H
O
H
OH
O
O
O
O
H
O
H
OH
O
O
H
O
H
O
O
H
O
H
O
H
H
OH
H
H
Ginseonoside R b1 , a glycoside of ginseng
Cardiac glycosides
Glycosides that exert a prominent effect on heart muscle are called cardiac
glycosides, e.g. digitoxin from Digitalis purpurea. Their effect is specifically on myocardial contraction and atrioventricular conduction. The
aglycones of cardiac glycosides are steroids with a side-chain containing
an unsaturated lactone ring, either five membered g-lactone (called cardenolides) or six membered d-lactone (called bufadienolides). The sugars
present in these glycosides are mainly digitoxose, cymarose, digitalose,
rhamnose and sarmentose. Digitoxose, cymarose and sarmentose are
2-deoxysugars.
Cardiac glycosides are found only in a few plant families, e.g. Liliaceae,
Ranunculaceae, Apocynaceae and Scrophulariaceae are the major sources of
these glycosides. Among the cardiac glycosides isolated to date, digitoxin
and digoxin, isolated from Digitalis purpurea and Digitalis lanata, respectively, are the two most important cardiotonics. Digitoxin and digoxin are
also found in in Strophanthus seeds and squill. Both these cardiac glycosides
are cardenolides, and the sugar present is the 2-deoxysugar digitoxose.
O
O
O
O
Cardenolide
Bufadienolide
6.4 GLYCOSIDES
327