Introduction:

Evaluated by most of the other classes of natural compounds, alkaloids are characterized via a great structural diversity and there is no uniform categorization of alkaloids. In history, the first classification processes combined alkaloids via the common natural source, example, a certain kind of plants. This categorization was justified via the lack of knowledge regarding the chemical structure of alkaloids and is now considered obsolete.

Main classes of Alkaloids:

Alkaloids are frequently categorized into the given major groups: 

1) True alkaloids that encompass nitrogen in the heterocycle and originate from the amino acids. Their characteristic illustrations are atropine, nicotine and morphine. This group as well comprises a few alkaloids that beside nitrogen heterocycle have terpene (example: evonine) or peptide fragments (example: ergotamine). This group as well comprises piperidine alkaloids coniine and coniceine however they don't originate from amino acids. 

2) Protoalkaloids that encompass nitrogen and as well originate from amino acids. Illustrations comprise mescaline, adrenaline and ephedrine.

3) Polyamine alkaloids - derivatives of spermidine, putrescine and spermine.

4) Peptide and cyclopeptide alkaloids. 

5) Pseudalkaloids - alkaloid-like compounds which don't originate from the amino acids. This group comprises terpene-like and steroid-like alkaloids, and also purine-like alkaloids like caffeine, theobromine and theophylline. Many categorize as pseudo alkaloids such compounds like ephedrine and cathinone. Those originate from the amino acid phenylalanine; however obtain their nitrogen atom not from the amino acid however via transamination. 

Some of the alkaloids don't have the carbon skeleton feature of their group. Therefore, galantamine and homoaporphines don't have isoquinoline fragment, however are usually attributed to the isoquinoline alkaloids. 

Main categorization of Alkaloids:

The alkaloids can be classified in the given four heads, namely:

1) Biosynthetic Classification:

In this particular case the importance solely lies to the precursor from which the alkaloids in question are formed in the plant biosynthetically. Thus, it is quite convenient and as well logical to group altogether all alkaloids having been derived from the similar precursor however possessing various taxonomic distribution and pharmacological activities.

Illustrations:

a) Indole alkaloids derived from the tryptophan.

b) Piperidine alkaloids derived from lysine.

c) Phenylethylamine alkaloids derived from tyrosine.

d) Pyrrolidine alkaloids derived from ornithine.

e) Imidazole alkaloids derived from histidine.

2) Chemical Classification:

This is probably the most broadly accepted and common mode of categorization of alkaloids for which the major criterion is the presence of the fundamental heterocyclic nucleus (that is, the chemical entity).

Illustrations:

a) Pyrrolidine alkaloids example: Hygrine.

b) Pyrrolizidine alkaloids example: Senecionine.

c) Piperidine alkaloids example: Lobeline.

d) Quinoline alkaloids example: Quinine.

e) Tropane alkaloids example: Atropine.

f) Purine alkaloids example: Caffeine.

g) Imidazole alkaloids example: Pilocarpine.

h) Amino alkaloids example: Ephedrine.

i) Diazocin alkaloids example: Lupanine.

j) Aporphine alkaloids example: Boldine.

k) Diterpene alkaloids example: Aconitine.

l) Isoquinoline alkaloids example: Morphine.

m) Indole alkaloids example: Ergometrine.

n) Steroidal alkaloids example: Solanidine.

3) Pharmacological Classification:

Amusingly, the alkaloids show a wide range of extremely specific pharmacological features. Possibly this might as well be employed as a strong basis for the general categorization of the wide-spectrum of alkaloids derived from the plant kingdom, like: analgesics, cardio-vascular drugs, dilation of pupil of eye, CNS-stimulants and depressants, mydriatics, anticholinergics, sympathomimetics, antimalarial, purgatives and the similar. Though, such a categorization is not quite general and generally known.

Illustrations:

a) Aconitine as Neuralgia

b) Lobeline as Respiratory stimulant

c) Quinine as Antimalarial

d) Morphine as Narcotic analgesic

e) Ephedrine as Bronchodilator

f) Strychnine as Reflex excitability

g) Narceine as Analgesic (that is, narcotic) and antitussive.

h) Boldine as Choleretics and laxatives

i) Pilocarpine as Antiglaucoma agent and miotic

j) Ergonovine as Oxytocic

4) Taxonomic Classification:

This specific categorization basically deals by the 'Taxon' that is, the taxonomic category. The most general taxa are the genus, subgenus, species, subspecies and diversity. Thus, the taxonomic categorization encompasses the plethora of alkaloids completely based on their corresponding distribution in a diversity of Plant families, at times as well termed to as the 'Natural order'. Some typical illustrations of plant families and the different species related with them are illustrated below, namely:

a) Cannabinaceous Alkaloids: Example - Cannabis sativa Linn., (Hemp, Marijuana).

b) Rubiaceous Alkaloids: Example - Cinchona Sp. (Quinine); Mitragyna speciosa Korth (Katum, Kratum, Kutum); Pausinystalia johimbe (K. Schum) (Yohimbe).

c) Solanaceous Alkaloids: Example - 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); and so forth.

=> In the recent past, the alkaloids have been categorized into two main classes based on the analogy that one having a non-heterocyclic nucleus, whereas the other having the heterocyclic nucleus. Such two classes of alkaloids shall be illustrated in brief as under.

I) Non-heterocyclic Alkaloids: Some typical alkaloids having non-heterocyclic nucleus are itemized below:

Fig: Non-heterocyclic Alkaloids

II) Heterocyclic Alkaloids: A huge number of specific alkaloids possessing heterocyclic nucleus are described below:

Fig: Heterocyclic Alkaloids

It is, though, pertinent to illustrate at this juncture which the huge volume of authentic information accumulated so far with regard to the isolation of alkaloids from a diversity of plant species and their subsequent characterization via the assistance of latest analytical methods they might be categorized as follows:

A) Alkaloids derived from Amination reactions:

• Acetate-derived Alkaloids
• Phenylalanine-derived Alkaloids
• Steroidal Alkaloids
• Terpenoid Alkaloids

B) Alkaloids derived from Anthranilic Acid:

• Quinazoline Alkaloids
• Quinoline Alkaloids
• Acridine Alkaloids

C) Alkaloids derived from the Histidine:

• Imidazole Alkaloids

D) Alkaloids derived from Lysine

• Piperidine Alkaloids
• Indolizidine Alkaloids
• Quinolizidine Alkaloids

E) Alkaloids derived from the Nicotinic Acid:

• Pyridine Alkaloids

F) Alkaloids derived from the Ornithine:

• Pyrrolidine Alkaloids
• Tropane Alkaloids
• Pyrrolizidine Alkaloids

G) Alkaloids derived from the Tyrosine:

• Phenylethylamine Alkaloids
• Simple Tetrahydro iso-quinoline Alkaloids
• Modified Benzyl Tetrahydro iso-quinoline Alkaloids

H) Alkaloids derived from the Tryptophan:

• Simple Indole Alkaloids
• Simple b-Carboline Alkaloids
• Terpenoid Indole Alkaloids
• Quinoline Alkaloids
• Pyrroloindole Alkaloids
• Ergot Alkaloids

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