The alkaloids, as an important and enormously large conglomerate of naturally occurring nitrogencontaining plant substances having very specific as well as most diversified pharmacological properties may be classified in a number of modes and means.
Hegnauer* (1963) conveniently classified alkaloids into six important groups, corresponding to the six amino-acids legitimately considered as the starting points for their biosynthesis, such as: anthranilic acid, histidine, lysine, ornithine phenylalanine and tryptophan. Price* (1963) further took a leading clue from the earlier observation and considered in details the alkaloids present in one of the families, (Rutaceae) and logically placed them in the following nine chemical-structural categories, namely: acridines, amides, amines, benzylisoquinolines, canthinones, imidazoles, indolquinazolines, furoquinolines, and quinazolines.
Another school of thought classifies alkaloids in the following four heads, namely:
(a) Biosynthetic Classification In this particular instance the significance solely lies to the precursor from which the alkaloids in question are produced in the plant biosynthetically. Therefore, it is quite convenient and also logical to group together all alkaloids having been derived from the same precursor but possessing different taxonomic distribution and pharmacological activities.
(i) Indole alkaloids derived from tryptophan.
(ii) Piperidine alkaloids derived from lysine.
(iii) Pyrrolidine alkaloids derived from ornithine.
(iv) Phenylethylamine alkaloids derived from tyrosine.
(v) Imidazole alkaloids derived from histidine.
(b) Chemical Classification It is probably the most widely accepted and common mode of classification of alkaloids for which the main criterion is the presence of the basic heterocyclic nucleus (i.e., the chemical entity).
(i) Pyrrolidine alkaloids e.g., Hygrine;
(ii) Piperidine alkaloids e.g., Lobeline;
(iii) Pyrrolizidine alkaloids e.g., Senecionine;
(iv) Tropane alkaloids e.g., Atropine;
(v) Quinoline alkaloids e.g., Quinine;
(vi) Isoquinoline alkaloids e.g., Morphine;
(vii) Aporphine alkaloids e.g., Boldine;
(viii) Indole alkaloids e.g., Ergometrine;
(ix) Imidazole alkaloids e.g., Pilocarpine;
(x) Diazocin alkaloids e.g., Lupanine;
(xi) Purine alkaloids e.g., Caffeine;
(xii) Steroidal alkaloids e.g., Solanidine;
(xiii) Amino alkaloids e.g., Ephedrine;
(xiv) Diterpene alkaloids e.g., Aconitine.
(c) Pharmacological Classification Interestingly, the alkaloids exhibit a broad range of very specific pharmacological characteristics. Perhaps this might also be used as a strong basis for the general classification of the wide-spectrum of alkaloids derived from the plant kingdom, such as: analgesics, cardio-vascular drugs, CNS-stimulants and depressants, dilation of pupil of eye, mydriatics, anticholinergics, sympathomimetics, antimalarials, purgatives, and the like. However, such a classification is not quite common and broadly known.
(i) Morphine as Narcotic analgesic;
(ii) Quinine as Antimalarial;
(iii) Strychnine as Reflex excitability;
(iv) Lobeline as Respiratory stimulant;
(v) Boldine as Choleretics and laxatives;
(vi) Aconitine as Neuralgia;
(vii) Pilocarpine as Antiglaucoma agent and miotic;
(viii) Ergonovine as Oxytocic;
(ix) Ephedrine as Bronchodilator;
(x) Narceine as Analgesic (narcotic) and antitussive.
(d) Taxonomic Classification This particular classification essentially deals with the ‘Taxon’ i.e., the taxonomic category. The most common taxa are the genus, subgenus, species, subspecies, and variety. Therefore, the taxonomic classification encompasses the plethora of alkaloids exclusively based on their respective distribution in a variety of Plant Families, sometimes also referred to as the ‘Natural order’. A few typical examples of plant families and the various species associated with them are stated below, namely:
(i) Cannabinaceous Alkaloids: e.g., Cannabis sativa Linn., (Hemp, Marijuana).
(ii) Rubiaceous Alkaloids: e.g., Cinchona Sp. (Quinine); Mitragyna speciosa Korth (Katum, Kratum, Kutum); Pausinystalia johimbe (K. Schum) (Yohimbe).
(iii) Solanaceous Alkaloids: e.g., Atropa belladona L., (Deadly Nightshade, Belladona); Brunfelsia uniflorus (Pohl) D. Don (Manaca, Manacan); Capsicum annuum L., (Sweet Peppers, Paprika); Datura candida (Pers.) Saff. (Borrachero, Floripondio); Duboisia myoporoides R. Br. (Corkwood Tree, Pituri); Hyoscyamus niger L. (Henbane, Henblain, Jusquaime); Mandragora officinarum L. (Mandrake, Loveapple); Nicotiana glauca R. Grah. (Tree Tobacco); Seopolia carniolica Jacq. (Scopolia); Solanum dulcamara L., (Bittersweet, Bitter Nightshade, Felonwood); Withania somniferum (L.) Dunal (Ashwagandha), etc.
Invariably, they are grouped together according to the name of the genus wherein they belong to, such as: coca, cinchona, ephedra.
Some ‘phytochemists’ have even gone a step further and classified the alkaloids based on their chemotaxonomic classification.
In the recent past, the alkaloids have been divided into two major categories based on the analogy that one containing a non-heterocyclic nucleus, while the other having the heterocyclic nucleus. These two classes of alkaloids shall be discussed briefly as under.
(a) Non-heterocyclic Alkaloids A few typical alkaloids having non-heterocyclic nucleus are erumerated below:

Non-heterocyclic Alkaloids
(b) Heterocyclic Alkaloids A large number of specific alkaloids possessing heterocyclic nucleus are stated below:

Heterocyclic Alkaloids
Heterocyclic Alkaloids
Heterocyclic Alkaloids

It is, however, pertinent to mention at this juncture that the enormous volume of authentic information accumulated so far with regard to the isolation of alkaloids from a variety of plant species and their subsequent characterization by the help of latest analytical techniques they may be classified as follows:
A. Alkaloids derived from Amination Reactions
(i) Acetate-derived Alkaloids
(ii) Phenylalanine-derived Alkaloids
(iii) Terpenoid Alkaloids
(iv) Steroidal Alkaloids
B. Alkaloids derived from Anthranilic Acid
(i) Quinazoline Alkaloids
(ii) Quinoline Alkaloids
(iii) Acridine Alkaloids
C. Alkaloids derived from Histidine
Imidazole Alkaloids
D. Alkaloids derived from Lysine
(i) Piperidine Alkaloids
(ii) Quinolizidine Alkaloids
(iii) Indolizidine Alkaloids
E. Alkaloids derived from Nicotinic Acid
Pyridine Alkaloids
F. Alkaloids derived from Ornithine
(i) Pyrrolidine Alkaloids
(ii) Tropane Alkaloids
(iii) Pyrrolizidine Alkaloids
G. Alkaloids derived from Tyrosine
(i) Phenylethylamine Alkaloids
(ii) Simple Tetrahydro iso-quinoline Alkaloids
(iii) Modified Benzyl Tetrahydro iso-quinoline Alkaloids
H. Alkaloids derived from Tryptophan
(i) Simple Indole Alkaloids
(ii) Simple b-Carboline Alkaloids
(iii) Terpenoid Indole Alkaloids
(iv) Quinoline Alkaloids
(v) Pyrroloindole Alkaloids
(vi) Ergot Alkaloids
* Swain, T. (ed), ‘Chemical Plant Taxonomy’, Academic Press, London, (1963).

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