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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
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
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 &
b) Sterol or cardiac glycosides- Digitoxin & Digoxin
c) Saponin glycosides- Dioscera, Liquorice, shatavari & brahmi
Classification of glycosides on the basis of glycosidic
Glycosides are also classified on the basis of the
glycosidic linkage present in them. On this basis they are classified in the
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
Classification of terpene hydrocarbons:
Terpenoids are classified in the following categories-
They contains C10H16 units or 2 isoprene units. Further they are divided into
following two groups-