Glycosides
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Direct answer
Glycosides couple an active aglycone (or genin) to a sugar through an oxygen, nitrogen, sulphur or carbon bond, and the sugar is what makes the molecule soluble, stable and absorbable — remove it and the plant's chemistry becomes either inactive or unusable. Cardiac glycosides from digitalis, anthraquinone glycosides from senna, saponins from Dioscorea and cyanogenetic glycosides from bitter almond are the four groups every examination returns to, each carried by a signature chemical test: Keller-Kiliani, Borntrager, the foam test and the sodium picrate paper. Hydrolysis by dilute acid or by specific enzymes — emulsin from bitter almond, myrosin from black mustard — is the reaction on which the whole classification rests.
What you must remember
- Bond types: O-glycosides (most common), C-glycosides (barbaloin of aloe — resist acid hydrolysis), S-glycosides (sinigrin), N-glycosides.
- Cardiac glycosides: digitoxin and the purpurea glycosides from Digitalis purpurea, digoxin from D. lanata, ouabain from Strophanthus; act on the heart; Keller-Kiliani test detects their deoxy sugars.
- Anthraquinone glycosides: senna (sennosides A and B), cascara, rhubarb, aloe — stimulant purgatives; Borntrager's test gives a pink-red colour in ammoniacal layer.
- Saponin glycosides: diosgenin from Dioscorea (steroid precursor), glycyrrhizin from licorice; foam test positive, haemolytic; fame as the starting material for corticosteroids and oral contraceptives.
- Cyanogenetic glycosides: amygdalin from bitter almond; sodium picrate paper turns brick red from liberated hydrogen cyanide.
- Isothiocyanate glycosides: sinigrin in black mustard, hydrolysed by myrosin to allyl isothiocyanate.
- Other classes: flavonoid (rutin), alcohol (salicin from willow), aldehyde (glucovanillin), phenolic (arbutin from bearberry).
- Emulsin is a mixture including beta-glucosidase that hydrolyses O-glycosides; myrosin is the specific enzyme for thioglycosides.
- C-glycosides like barbaloin need oxidising hydrolysis — the modified Borntrager test — the classic "why modify Borntrager?" answer.
Two tests the examiner loves, walked through
The Borntrager test for anthraquinone glycosides is asked as a procedure, so learn it as a sequence. Hydrolyse the drug with dilute sulphuric acid — the glycoside splits to its anthraquinone aglycone, which is now free but still in the acidic aqueous phase. Shake this with benzene or chloroform; the aglycone partitions into the organic layer, taking the colour with it. Add dilute ammonia and shake: a pink to red ammoniacal layer confirms anthraquinone. Senna presents the twist — its sennosides are dianthrone C-glycosides, so plain hydrolysis is inadequate; oxidise first with ferric chloride and hydrochloric acid, then proceed, which is the modified Borntrager test.
The Keller-Kiliani test identifies cardiac glycosides by their deoxy sugars. Dissolve the extract in glacial acetic acid, add a trace of ferric chloride, then layer concentrated sulphuric acid beneath: a reddish-brown ring at the junction, with the upper acetic layer showing greenish-blue, indicates the deoxy sugars of digitalis. Run it mentally alongside the story of the drug — digitalis leaves collected in the second year, standardised by bioassay historically and by HPLC today.
The confusion that decides grades
The recurring error is treating glycosides as "sugars with something attached" and nothing more. The pharmacology lives in the aglycone; the sugar is the handle for solubility and absorption. That single sentence answers three exam questions: why the plant stores active chemistry as inactive glycosides, why digoxin works though glucose does not, and why hydrolysis is the key reaction of this chapter.
The second confusion is O- versus C-glycosides. Ordinary acid hydrolysis cleaves the O-link but cannot break a carbon-carbon bond, which is why aloe's barbaloin survives acid and demands the modified test. Third, students quote the foam test without its interpretation: persistent honeycomb foam after shaking an aqueous extract indicates saponins, and haemolysis distinguishes them further — but saponins given intravenously would be dangerous, a fact that explains why Dioscorea's diosgenin is a precursor for steroids rather than a drug in its own right.
Frequently asked questions
Define a glycoside and name the bond types.
A glycoside is a compound in which a sugar is linked through oxygen, nitrogen, sulphur or carbon to a non-sugar aglycone that usually carries the pharmacological activity; examples include O-glycosides like digitoxin and C-glycosides like barbaloin.
Describe the Borntrager test and its modification.
Hydrolyse with dilute acid, extract the free anthraquinone into benzene or chloroform, shake with ammonia — pink-red ammoniacal layer is positive; for C-glycosides like senna sennosides, oxidise with ferric chloride-hydrochloric acid before extraction, which is the modified test.
What does the Keller-Kiliani test detect?
The deoxy sugars of cardiac glycosides; a reddish-brown ring forms where sulphuric acid meets the glacial acetic acid layer containing ferric chloride, confirming digitalis-type drugs.
Which enzyme hydrolyses sinigrin and what is the product?
Myrosin (thioglucosidase), released when black mustard is crushed with water, converts sinigrin to allyl isothiocyanate, the pungent oil.
Why is diosgenin industrially important?
The sapogenin from Dioscorea saponins is the classic starting material for partial synthesis of corticosteroids and steroid hormones.