It is a semi quantitative method to find out the presence of sugars that is polysaccharides. We have to hydrolyze the drug and then chemical tests are carried out. Different tests are done because of the different chemical entities of the aglycone moiety.
The procedure for the isolation of glycosides also varies. The general method is Stass Otto method.
• Powdered drug is extracted continuously in soxhlet apparatus with alcohol.
• Extract is treated with lead acetate to precipitate tannins filtered to remove nonglycosidal impurities.
• Excess of lead acetate is precipated as lead sulphide by passing hydrogen sulphide through the solution.
• The extract is filtered and concentrated to get crude extract.
They are classified according to chemical nature of aglycone moiety as
1. Anthracene glycosides
2. Cardiac glycosides
3. Saponin glycosides
4. Flavonoidal glycosides
5. Cyanogenetic glycoside
6. Isothiocyanate glycoside
7. Phenol glycoside
8. Alcoholic glycoside
9. Lactone glycoside
10. Bitter glycoside
11. Aldehydic glycoside.
A. Classification according to the linkage between aglycone and glycone.
Linkage takes place between -OH group of glycone and hydrogen from SH, NH,CH and OH to form:
1. G- glycoside :
Glycone -OH+CH-aglyconeâ†'glycone-C-aglycone +water.
e.g. anthracene glycoside-aloe,Flavonoids.
2. O-glycoside: e.g. senna, rhubarb.
3. S-glycoside: Isothiocyanate glycoside that is sinigrin from black mustard
4. N- glycoside: they are present in nucleosides where amino group of base reacts
with -OH group of ribose or deoxy ribose to form n-glycoside.
B. They are also classified according to the type of sugars present:
e.g. Glucoside which contain glucose
Rhamnoside which contain Rhamnose
Pentoside which contain pentose sugars etc.
CARDIAC GLYCOSIDES or STEROL GLYCOSIDES:
• These are the glycosides which consists aglycone which is steroidal in nature with Î±-Î˛ unsaturated lactone ring at C-17 position and shows slowing and strengthening effect on failing heart.
• They are found in higher plants.
• They are abundant in family apocyanaceae,asclepedaceae,Ranunculaceae, sterculiaceae, leguminosae etc.
• They are also found in monocots like liliaceae family. E.g. squill.
• They are also found in frogs.
• These two glycosides are distinguished according to the presence of 5 or 6 membered lactone ring.
• Those with five membered lactone ring are cardenolides e.g. digitoxigenin, digoxigenin, gitaloxigenin, gitoxigenin, strophanthidin.
• Those with six membered lactone ring are bufadicaoleates e.g. scillaren A
• Maximum plants contains cardenolides.
STRUCTURAL ACTIVITY RELATIONSHIP:
• To have activity at C-3 position beta orientation is required.
• The ring A, B, C and D should have configuration except one compound called Uzarin which have trans configuration and inactive.
• At C-14 OH group present should be of beta orientation, but recently it is confirmed that there is no absolute requirement of this to enhance the activity.
• At C-17 position unsaturated lactone ring is required but recent study shows that even open chain with alpha and beta unsaturation is also enough.
• The sugar moiety attached at C 3 position of genin are composed of 4 sugar units. Which includes glucose or rhamnose along with deoxy sugar like digitoxose.
• Many of them are soluble in chloroform and also in alcohols.
• They are identified by the color tests:-
KELLER KELLANI TEST:-
For deoxy sugars, all cardiac glycosides except quobain and squill glycoside that is scillaridin A which do not give e.g. strophanthus and digitalis contains are cardiac glycosides which contain deoxy sugars
The sugar is dissolved in acetic acid containing a trace of ferric chloride and transferred to the surface of conc. sulphuric acid. At the junction of the liquids reddish-brown colour is produced which gradually becomes blue.
Baljet tests, Legal tests, Raymond tests for the presence of lactone ring.
Substance + sod. Picrate +alkaline â†'orange colour.
Substance + sod nitropruside + pyridine+ alkaliâ†'red colour.
Substance + dinitro benzene in ethanol +alkaliâ†'voiletâ†'blue colour.
Substance+dinitrobenzoic acid + hot alcoholic KOH â†' deep blue color.
FOR DEOXY SUGARS:
Substance +xanthydrol reagent in acetic acid + 1% Hcl â†' Red.
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