Pharmacognosy of Alkaliods, Glycosides and Terpenoids: Notes for GPAT

From the point of view of GPAT, Pharmacognosy is considered as the most scoring subject. If one can prepare thoroughly for pharmacognosy then there are fair chances of qualifying the GPAT with flying colors. In the preparation one should pay due attention to the general pharmacognosy of the alkaloids, glycosides and terpenoids as these are one of the most asked sections in the GPAT. Here I am discussing this whole topic in detail covering the areas asked in the GPAT from these sections.

General Pharmacognosy of Alkaloids:

Alkaloids are organic products of the natural or synthetic origin which are basic in nature and contain one or more nitrogen atom. This is the most simple and widely accepted definition of alkaloids. Normally, nitrogen is heterocyclic in nature in alkaloids. Alkaloids posses specific physiological actions.

Protoalkaloids- Protoalkaloids are the compounds which do not contain nitrogen in the heterocyclic but are derived from the amino acids as precursor [e.g. Muscaline, colchicine, ephedrine and tryptamine].

Pseudoalkaloids- Pseudoalkaloids contain nitrogen in the heterocyclic ring but they are not derived from the amino acids [Steroidal alkaloids, terpenoidal alkaloids and purines]. Specific examples of pseudoalkaloids include connesine and caffeine.

Properties of alkaloids:

a) Free bases of alkaloids are soluble in organic bases and insoluble in water (physical property).

b) Salts of alkaloids are soluble in water and insoluble in organic solvents (physical property).

c) Alkaloids are basic in nature due to presence of lone pair of electrons on nitrogen (chemical property).

Classification of alkaloids:

Basic rings of all major classes of alkaloids

Alkaloids are classified in the following categories-

a) Pyrrole/Pyrrolidine alkaloids – Hygine and Coca

b) Pyridine/Piperidine alkaloids- Arecoline, Lobeline, Anabasine & Conine

c) Pyrrolidine alkaloids- Symphitine & Echnimidine

d)  Tropane alkaloids (Piperidine/N-methyl pyrrolidine) - Atropine, Cocaine & Hyoscine

e) Quinoline alkaloids- Quinine, Quinidine, Cinchonine & cinchonidine

f) Isoquinoline alkaloids- Morphine, Codeine, Emetine, Narcotine & Papaverine

g) Aporphine alkaloids- Boline

h) Indole alkaloids- Ergot, Vincristine, Strychinine, Reserpine &  Physostigmine

i) Imidazole alkaloids- Pilocarpine

j) Norlupinane alkaloids- Cytisine & Sparteine

k) Purine (Pyridine/Imidazole) alkaloids- Caffeine & Theophylline

l) Steroidal alkaloids-Protoveratine, Solanidine & Connessine.

m) Diterpene alkaloids- Aconitine

n) Alkylamine alkaloids- Ephedrine

General pharmacognosy of glycosides:

Glycosides in general may be defined as the organic compounds from plant or animal origin which on enzymatic or acid hydrolysis gives one or more sugar moiety along with a non-sugar moiety (Aglycone moiety).

Alpha & Mehtyl glycosides

Classification of glycosides on the basis of aglycone moiety

On the basis of aglycone moiety glycosides are classified in the following categories-

a) Anthraquinone glycosides- Senna, aloe, cascara & Rhubarb

b) Sterol or cardiac glycosides- Digitoxin & Digoxin

c) Saponin glycosides- Dioscera, Liquorice, shatavari & brahmi

d) Cyanogenetic glycosides- Bitter almond & Punarnava

e) Isothiocynate glycosides- Sinigrin

f) flavonoid glycosides- Ginkgo & buck wheat

g) Coumarin alkaloid- Visnaga & Psoralea

h) Aldehyde glycosides- Vanilla pods

i) Phenol glycosides- Bearberry

j) Steroidal Glycoalkaloids- Solanum

k) Glycosidal bitters- Gentian, pycorrhiza, chirata & henna.

Classification of glycosides on the basis of glycosidic linkage:

Glycosides are also classified on the basis of the glycosidic linkage present in them. On this basis they are classified in the following categories-

a) C-glycosides-They are represented as- Glycone-OH + HC-aglycone = Glycone-C-Aglycone + H₂O. Examples of C-glycosides are Aloe, cochineal & cascara.

b) O-glycosides- They are represented as- Glycone-OH + HO-aglycone = Glycone-O-Aglycone + H₂O. Examples of C-glycosides are Senna, rhubarb & cyanogenetic glycosides.

c) S-glycosides- They are represented as- Glycone-OH + HS-aglycone = Glycone-S-Aglycone + H₂O. Example of C-glycosides is Sinigrin (Isothiacyanate glycoside).

d) N-glycoside- They are represented as- Glycone-OH + HN-aglycone = Glycone-N-Aglycone + H₂O. Examples of C-glycosides are adenine, guanine, cytosine & thiamine (nucleosides).

General pharmacognosy of terpenoids:

Terpenoids are regarded as derivatives of polymers of isoprene; structurally CH₂=C(CH₃)CH=CH₂ or simply C₅H₈. Isoprene is also called hemiterpene and do not occur in the nature. Only polymers of isoprene exist which are called terpenoids.

Classification of terpene hydrocarbons:

Terpenoids are classified in the following categories-

a) Monoterpenes- They contains C10H16 units or 2 isoprene units. Further they are divided into following two groups-

i) Acyclic monoterpenes- Geraniol & Nerol

ii) Monocyclic monoterpenes- Limonene, Phellandrene, Terpinene & Menthol

b) Sesquiterpene- Sesquiterpene contains C15H24 units or 3 isoprene units. Further they are divided into following two groups-

i) Acyclic sesquiterpenes- Zingibrene

ii) Bucyclic sesquiterpenes- Santonin

c) Diterpenes- They contains C20H32 units or 4 isoprene units. Further they are divided into following four groups-

i) Acyclic diterpenes- Phytol

ii) Monocyclic diterpenes- Vitamin A

iii) Bicyclic diterpenes- Manool

iv) Tricyclic diterpenes- Abietic acid

d) Tripterpenes- They contains C30H48 units or 6 isoprene units. Further they are divided into following three groups-

i) Acyclic triterpenes- Squalene

ii) Tetracyclic triterpenes- Lanosterol

iii) Pentacyclic triterpene- Glycyrhetic acid

e) Polyterpenes- Saffron & Annatto