Alkaloids are generally classified by their common molecular precursors, based on the biological pathway used to construct the molecule. From a structural point of view, alkaloids are divided according to their shapes and origins. There are three main types of alkaloids: (1) true alkaloids, (2) protoalkaloids and (3) pseudoalkaloids. True alkaloids and protoalkaloids are derived from amino acids, whereas pseudoalkaloids are not derived from these compounds.
1. True alkaloids
True alkaloids derive from amino acid and they share a heterocyclic ring with nitrogen. These alkaloids are highly reactive substances with biological activity even in low doses. All true alkaloids have a bitter taste and appear as a white solid, with the exception of nicotine which has a brown liquid appearance. True alkaloids form water-soluble salts. Moreover, most of them are well-defined crystalline substances which unite with acids to form salts. True alkaloids may occur in plants (1) in the free state, (2) as salts and (3) as N-oxides. These alkaloids occur in a limited number of species and families, and are those compounds in which decarboxylated amino acids are condensed with a non- nitrogenous structural moiety. The primary precursors of true alkaloids are such amino acids as l-ornithine, l-lysine, l-phenylalanine/l-tyrosine, l-tryptophan and l-histidine. Examples of true alkaloids include such biologically active alkaloids as cocaine, quinine, dopamine, morphine and usambarensin.
Protoalkaloids are compounds, in which the N atom derived from an amino acid is not a part of the heterocyclic bond. Such kinds of alkaloid include compounds derived from l-tyrosine and l-tryptophan. Protoalkaloids are those with a closed ring, being perfect but structurally simple alkaloids. They form a minority of all alkaloids. Hordenine, mescaline and yohimbine are good examples of these kinds of alkaloid. Chini et al. have found new alkaloids, stachydrine and 4-hydroxystachydrine, derived from Boscia angustifolia, a plant belonging to the Capparidacea family. These alkaloids have a pyrroline nucleus and are basic alkaloids in the genus Boscia. The species from this genus have been used in folk medicine in East and South Africa. Boscia angustifolia is used for the treatment of mental illness, and occasionally to combat pain and neuralgia.
Pseudoalkaloids are compounds, the basic carbon skeletons of which are not derived from amino acids. In reality, pseudoalkaloids are connected with amino acid pathways. They are derived from the precursors or postcursors (derivatives the indegradation process) of amino acids. They can also result from the amination and transamination reactions of the different pathways connected with precursors or postcursors of amino acids.
These alkaloids can also be derived from non-aminoacid precursors. The N atom is inserted into the molecule at a relatively late stage, for example, in the case of steroidal or terpenoid skeletons. Certainly, the N atom can also be donated by an amino acid source across a transamination reaction, if there is a suitable aldehyde or ketone. Pseudoalkaloids can be acetate and phenylalanine- derived or terpenoid, as well as steroidal alkaloids. Examples of pseudoalkaloids include such compounds as coniine, capsaicin, ephedrine, solanidine, caffeine, theobromine and pinidine.
Aniszewski, T. (2015). Definition, typology, and occurrence of alkaloids. Alkaloids.