Introduction

Glycosides are naturally occurring organic compounds in which a sugar molecule (glycone) is linked to a non-sugar molecule (aglycone or genin) through a glycosidic bond, formed by condensation between the hemiacetal –OH of the sugar and a functional group of the aglycone, with elimination of a water molecule.

Glycoside = Glycone + Aglycone (joined by glycosidic linkage)

On hydrolysis (by acids, enzymes, or alkalies), a glycoside splits back into its sugar and non-sugar components.

Glycosides can be classified in two main ways chemically:

1. Based on the nature of the glycosidic linkage
2. Based on the chemical nature of the aglycone (the major pharmacognostic classification)

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I. Classification Based on Type of Glycosidic Linkage

1. O-Glycosides

The sugar is linked to the aglycone through an oxygen atom (most common type).

• Example: Salicin (willow bark), Digoxin (Digitalis)

2. S-Glycosides (Thioglycosides)

The sugar is linked to the aglycone through a sulphur atom.

• Example: Sinigrin (from Brassica nigra – black mustard)

3. N-Glycosides

The sugar is linked through a nitrogen atom; common in nucleosides.

• Example: Adenosine, Cytidine, Nucleic acid glycosides

4. C-Glycosides

The sugar is linked directly to the aglycone by a carbon–carbon bond (resistant to acid hydrolysis).

• Example: Aloin (from Aloe), Cascaroside (from Cascara sagrada)

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II. Classification Based on Chemical Nature of Aglycone

This is the principal chemical classification used in pharmacognosy:

1. Anthraquinone Glycosides (Anthracene glycosides)

Aglycone is an anthraquinone/anthracene derivative; act as purgatives/laxatives.

• Examples: Senna (Sennosides A & B), Aloe (Barbaloin), Rhubarb (Rhein), Cascara sagrada

2. Cardiac Glycosides

Aglycone is a steroidal nucleus with an unsaturated lactone ring; act on cardiac muscle.

• Types:
• Cardenolides – 5-membered lactone ring (e.g., Digoxin, Digitoxin from Digitalis)
• Bufadienolides – 6-membered lactone ring (e.g., Squill, Bufalin from toad venom)

3. Saponin Glycosides

Aglycone (sapogenin) produces foam/froth on shaking with water; hemolytic.

• Types:
• Steroidal saponins (spirostane nucleus) – e.g., Diosgenin (Dioscorea)
• Triterpenoid saponins (pentacyclic nucleus) – e.g., Glycyrrhizin (Liquorice), Ginseng saponins

4. Cyanogenetic (Cyanophore) Glycosides

Yield hydrocyanic acid (HCN) on hydrolysis; toxic in nature.

• Examples: Amygdalin (bitter almond), Prunasin (cherry laurel), Linamarin (linseed)

5. Bitter Glycosides (Bitters)

Possess intense bitter taste; used as appetite stimulants/tonics.

• Examples: Picrocrocin (Saffron), Amarogentin (Gentian), Quassin (Quassia)

6. Flavonoid Glycosides

Aglycone is a flavonoid nucleus (C6-C3-C6 skeleton).

• Examples: Rutin, Hesperidin, Quercitrin (Citrus, Buckwheat)

7. Coumarin Glycosides

Aglycone is a coumarin (benzo-α-pyrone) derivative.

• Examples: Aesculin (Horse chestnut), Umbelliferone

8. Isothiocyanate (Mustard oil) Glycosides

On hydrolysis, yield pungent isothiocyanates.

• Examples: Sinigrin (black mustard), Sinalbin (white mustard)

9. Aldehyde Glycosides

Aglycone contains an aldehyde group.

• Example: Vanillin glycoside (Vanilla)

10. Alcoholic Glycosides

Aglycone is a simple alcohol.

• Example: Salicin (from Salix – willow bark), which yields saligenin (salicyl alcohol)

11. Phenolic Glycosides

Aglycone contains a phenolic group.

• Example: Arbutin (from Uva-ursi / bearberry) – used as a urinary antiseptic

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Summary Table

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