Isoprenoid glycosides

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The aglycone of this type of glycoside is biosynthetically derived from isoprene units. There are two major classes of isoprenoid glycosides: saponins and cardiac glycosides.

Saponin glycosides possess 'soaplike' behaviour in water, i.e. they produce foam. On hydrolysis, an aglycone is produced, which is called sapogenin. There are two types of sapogenin: steroidal and triterpenoidal. Usually, the sugar is attached at C-3 in saponins, because in most sapogenins there is a hydroxyl group at C-3.

The two major types of steroidal sapogenin are diosgenin and hecogenin. Steroidal saponins are used in the commercial production of sex hormones for clinical use. For example, progesterone is derived from diosgenin.

Saponins

A steroid nucleus

A triterpenoid nucleus

A steroid nucleus

A triterpenoid nucleus

The most abundant starting material for the synthesis of progesterone is diosgenin isolated from Dioscorea species, formerly supplied from Mexico, and now from China. The spiroketal group attached to the D ring of diosgenin can easily be removed. Other steroidal hormones, e.g. cortisone and hydrocortisone, can be prepared from the starting material hecogenin, which can be isolated from Sisal leaves found extensively in East Africa.

In triterpenoidal saponins, the aglycone is a triterpene. Most aglycones of triterpenoidal saponins are pentacyclic compounds derived from one of the three basic structural classes represented by a-amyrin, b-amyrin and lupeol. However, tetracyclic triterpenoidal aglycones are also found, e.g. ginseno-sides. These glycosides occur abundantly in many plants, e.g. liquorice and ginseng roots contain glycyrrhizinic acid derivatives and ginsenosides, respectively. Most crude drugs containing triterpenoid saponins are usually used as expectorants. Three major sources of triterpenoidal glycosides along with their uses are summarized below.

Plants Botanical names (Family)

Main constituents

Uses

Liquorice root Glycyrrhiza

Glycyrrhizinic acid

In addition to expectorant

glabra (Fabaceae)

derivatives

action, it is also used as a

flavouring agent.

Quillaia bark Quillaja

Several complex

Tincture of this plant is

saponaria (Rosaceae)

triterpenoidal saponins,

used as an emulsifying

e.g. senegin II

agent.

Ginseng Panax ginseng

Ginsenosides

As a tonic, and to

(Araliaceae)

promote the feeling of

.COOH

HO OH Ginocunuoiuc Rb1, a yiycooide oi ynioeiiy

HO OH Ginocunuoiuc Rb1, a yiycooide oi ynioeiiy

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