origin. In most alkaloids, the nitrogen atom is a part of the ring. Alkaloids
are biosynthetically derived from amino acids. The name ‘alkaloid’ derives
from the word ‘alkaline’, which means a water soluble base. A number of
natural alkaloids and their derivatives have been developed as drugs to treat
various diseases, e.g. morphine, reserpine and taxol.
6.2.1 Properties
Alkaloids are basic in nature, and form water soluble salts with mineral
acids. In fact, one or more nitrogen atoms that are present in an alkaloid,
typically as 1
, 2
or 3
amines, contribute to the basicity of the alkaloid.
The degree of basicity varies considerably, depending on the structure of the
molecule, and presence and location of the functional groups. Most
alkaloids are crystalline solids and are bitter in taste.
6.2.2 Classification of alkaloids
Alkaloids are generally classified according to the amino acid that provides
both the nitrogen atom and the fundamental alkaloidal skeleton. However,
alkaloids can also be grouped together on the basis of their generic structural
similarities. The following table shows different major types of alkaloid,
their generic skeletons and specific examples.
Class/structural types
Generic structure
Examples
Aporphine
(Tyrosine derived)
N R
Aporphine R = Me
Noraporphine R = H
Boldine
Betaines
N
+
O
O
Betaine
Choline, muscarine and neurine
Imidazole
N
N
H
Imidazole
Pilocarpine
Indole
(Tryptophan derived)
N
Indole
6.2 ALKALOIDS
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