1. GLYCOSIDES
Introduction

2. Overview Non-reducing organic compounds
On hydrolysis with acids, alkalis or enzymes 
A sugar moiety (= glycone, formed of one or more sugar units).
A non-sugar moiety (= aglycone = genin).
Widely distributed in nature
Usually accompanied with enzymes used for their synthesis or hydrolysis.

3. Chemistry Acetals or sugar ethers
–OH of anomeric carbon (e.g. at C-1 of aldose sugar) is replaced by a moiety having a nucleophilic atom e.g.:
O, S, N or C  O-, S-, N- or C-glycosides

4. Most common is the -O- linkage connecting the reducing group of the sugar part & an OH (alcoholic or phenolic) of the aglycone  O-glycosides.

5. Chemistry Sugars exist in - or - forms & can  a - or -glycosides.
Most naturally occurring glycosides are of -type.

6. Sugar moiety (= Glycone)
Type of sugar
Monosaccharide (e.g. glucose, rhamnose)
Oligosaccharide (e.g. di-, tri- or tetrasaccharide)
Most common is -D-glucose
Others : pentoses & deoxy-hexoses (2-deoxy sugars)
Sugar derivatives e.g. glucuronic acid
Position of Attachment
Mostly at only 1 position, sometimes at 2 different positions.
Role of sugar moiety
Helps in stabilization & solubilization of glycoside.
Carries aglycone to site of action.

7. Non-sugar moiety (= Aglycone = Genin)
Contains free -OH (in O-glycosides), free -NH (in N-glycosides) etc….  acetal link with sugar
Type : belongs to different chemical groups e.g. phenols, alcohols, flavonoids, steroids etc…….
Role: pharmacologically active part of glycosides.

8. Pharmacological activity
Mainly associated with & due to aglycone part
Anticancer, anti-inflammatory, antihepatotoxic, immunomodulating, antiviral, analgesic, diuretic antipyretic, hypoglycemic & cardiotonic activities.
Examples:
Arbutin: urinary antiseptic
Salicin: anti-rheumatic & analgesic
Rhein: anti-inflammatory
Sennosides & barbaloin: laxatives
Digoxin & lanatosides: cardiotonics

9. Function in the plant Energy producers Detoxifying agents
Stabilizers for labile substances.
Decorative or attractive (pigments).
Protective

10. Stability & hydrolytic cleavage
Glycosides may be subjected to acid, alkaline
or enzymatic hydrolysis.
Effect of acid hydrolysis
Acetal linkage is cleaved before linkage between different sugars
Glycosides containing normal sugars are more resistant to acid hydrolysis, than those containing 2-deoxy sugars.
Drastic conditions may cause chemical changes in aglycone.

11. Stability & hydrolytic cleavage
Effect of alkaline hydrolysis
Being acetals, glycosides are relatively stable
to alkaline hydrolysis. Alkalis may modify:
Aglycone part e.g. strong alkalis open lactone ring of cardiac glycosides  loss of cardiotonic activity.
Sugar part e.g. mild alkalis remove acetyl group of acetyldigitoxose in lanatosides.

12. Stability & hydrolytic cleavage
Effect of enzymatic hydrolysis
Generally, a specific enzyme is required to hydrolyze a particular glycoside.
Certain enzymes hydrolyze a large number of glycosides e.g.
Emulsin (all b-linked glycosides).
Myrosin (all S-containing glycosides).
Enzymes are usually named after the type of sugar :
e.g. glycosides, which contain b-glucose, are hydrolyzed
by b-glucosidase, & those containing rhamnose by
rhamnosidase

13. Classification & Nomenclature
According to:
Chemical nature of aglycones : alcoholic, ester, phenolic, flavone, anthraquinone, steroid, lactone, monoterpenoid, diterpenoid & triterpenoid glycosides
Presence of a special functional group: aldehydic (aldehyde group), cyanophore or cyanogenic (cyanide group) & thio- glycosides (thiol group, -SH)
Therapeutic activity: cardiac, urinary antiseptic & laxative glycosides
Natural form: primary & secondary glycosides
Nomenclature
Names are usually derived from botanical origin e.g.salicin from
Salix, sennosides from Senna etc…...

14. Properties
Colorless solids, mostly bitter in taste (except populin, stevioside & neoastilbin 
sweet).
Generally soluble in water & hydroalcoholic solutions.
Non-reducing (do not reduce Fehling’s solution) except if aglycone part contains a reducing group (e.g. K-strophanthoside).

15. Extraction & Isolation
Due to difference in physical & chemical properties, no common general method is used for isolation of glycosides
Common solvents used for extraction: water & aqueous alcohols
Precautions before extraction
Deactivation of enzymes
Maintenance of neutral conditions
Removal of fats (defatting)

16. Deactivation of enzymes
Specific hydrolytic enzymes present with glycosides
should be deactivated before or during extraction,
especially when using fresh plant material.
Methods of enzyme deactivation:
Drying plant material for min at 100 oC then slow drying at a low temperature.
Freeze-drying (lyophilization) of plant material before extraction.
Dipping fresh plant material in boiling water or boiling alcohol for min.
Boiling fresh plant material with acetone.
Carrying out extraction at very low temperature.
Carrying out extraction in presence of (NH4)2SO4.

17. Maintenance of neutral conditions & Defatting
Maintenance of neutral conditions before & during extraction since:
Acidity may cause hydrolysis.
Mild alkalinity may produce racemization.
Defatting of fat-rich organs (e.g. seeds) before extraction using petroleum ether.

18. Purification of extracted glycosides
Methods of purification:
+ Lead acetate solution  precipitate non-glycosidic constituents (tannins, proteins & coloring substances). Excess lead removed from filtrate by H2S or Na phosphate. Not used in case of flavonoid glycosides which may be precipitated.
+ Acetone or ether  precipitate or crystallize out glycosides from alcohol or aqueous alcohol solutions.
Chromatographic techniques e.g. CC or TLC

19. Quantitative Determination
Spectroscopic techniques:
Colorimetric methods e.g. cardiac glycosides using Balget's or Kedde's reagents.
Spectrometric methods e.g. measuring of UV absorption.
Chromatographic methods:
Applied on glycosides or after hydrolysis:
Aglycone extracted with organic solvent (e.g. ethyl acetate) & determined by HPLC or GC.
Sugar part (in aqueous layer) determined by GC after derivatization (silylation) or HPLC.

20. Phenolic glycosides

21. Classification Grouped according to chemical structure into:
Simple phenolic glycosides e.g. Arbutin.
Phenolic & ester glycosides e.g. Gaultherin & Monotropin.
Phenolic & alcoholic glycosides e.g. Salicin & Populin.
Anthracene glycosides e.g. Sennosides.
Coumarin glycosides e.g. Aesculin & Daphnetin.
Flavonoid glycosides e.g. Rutin & Hesperidin.

22. 1-Simple phenolic glycosides - Arbutin
Source
Dried leaves of Uva ursi (Ericaceae).
Hydrolysis
Acid or enzymatic (emulsin) 
hydroquinone + b-D- glucose.
Chemical tests
Arbutin+ FeCl3  Blue color (phenolic)
Arbutin + HCl / heat  crystals of hydroquinone.
Uses
Diuretic & urinary antiseptic &
Both hydroquinone and arbutin have
skin bleaching action.

23. 2- Phenolic & ester glycosides- Gaultherin & Monotropin
Source
Gaultheria procumbens (Wintergreen), & Monotropa & Betula spp.
Gaultherin is a 1ry glycoside &
monotropin a 2ry glycoside.

24. Gaultherin & Monotropin
Hydrolysis
Hydrolysis by acid or enzyme:
Both  Volatile aglycone = methyl salycilate (oil of wintergreen)
Sugar part:
Gaultherin disaccharide primeverose = xylose + glucose
Monotropin  glucose
Uses
Antipyretic, analgesic & anti-rheumatic

25. 3-Phenolic & alcoholic glycosides- i-Salicin
Source
Salix fragilis (4-10%) & Populus spp.
Hydrolysis
Enzyme or mild acids  saligenin + glucose.

26. Salicin-Hydrolysis + HCl +Heat for a long time : 2 molecules
of saligenin  (dehydration)  saliretin
(water insoluble).

27. i-Salicin Isolation Extract from Salix bark with hot H2O & filter.
+ lead acetate solution (tannins ) & filter.
Pass H2S gas (to remove excess lead) & filter.
Neutralize, concentrate & cool  salicin crystals.
Chemical tests
+ conc. H2SO4  red color + H2O  no color.
+ Froehd’s  violet color.
+ K2Cr4O7 + dilute H2SO4  benzaldehyde odor.
Uses: analgesic, antipyretic & anti-inflammatory.

28. ii-Populin (6'-Benzoyl salicin)
Source:
Sweet glycoside isolated from Populus spp
Synthesis: From salicin.
Chemical tests:
Populin + Froehd’s reagent  violet color.
Hydrolysis:
Uses: analgesic, antipyretic & anti-inflammatory.

29. iii-Coniferin Source Hydrolysis:
Cambium tissues of pine trees & lignified tissues of beet root.
Hydrolysis:
Coniferin  coniferyl alcohol + glucose

30. iii-Coniferin Chemical tests
Coniferin + concentrated HCl  blue color.
Coniferin + concentrated H2SO4  red color.
Uses
Coniferin & its aglycone coniferyl alcohol are used as starting material for semisynthesis of vanillin.