Classification
Compared with most other classes of natural compounds, alkaloids are characterized by a great structural diversity and there is no uniform classification of alkaloids. First classification methods have historically combined alkaloids by the common natural source, e.g., a certain type of plants. This classification was justified by the lack of knowledge about the chemical structure of alkaloids and is now considered obsolete.
More recent classifications are based on similarity of the carbon skeleton (e.g., indole-, isoquinoline-, and pyridine-like) or biogenetic precursor (ornithine, lysine, tyrosine, tryptophan, etc.). However, they require compromises in borderline cases; for example, nicotine contains a pyridine fragment from nicotinamide and pyrrolidine part from ornithine and therefore can be assigned to both classes.
Alkaloids are often divided into the following major groups:
- "True alkaloids", which contain nitrogen in the heterocycle and originate from amino acids. Their characteristic examples are atropine, nicotine, and morphine. This group also includes some alkaloids that besides nitrogen heterocycle contain terpene (e.g., evonine) or peptide fragments (e.g. ergotamine). This group also includes piperidine alkaloids coniine and coniceine although they do not originate from amino acids.
- "Protoalkaloids", which contain nitrogen and also originate from amino acids. Examples include mescaline, adrenaline and ephedrine.
- Polyamine alkaloids – derivatives of putrescine, spermidine, and spermine.
- Peptide and cyclopeptide alkaloids.
- Pseudalkaloids – alkaloid-like compounds that do not originate from amino acids. This group includes, terpene-like and steroid-like alkaloids, as well as purine-like alkaloids such as caffeine, theobromine and theophylline. Some authors classify as pseudoalkaloids such compounds such as ephedrine and cathinone. Those originate from the amino acid phenylalanine, but acquire their nitrogen atom not from the amino acid but through transamination.
Some alkaloids do not have the carbon skeleton characteristic of their group. So, galantamine and homoaporphines do not contain isoquinoline fragment, but are, in general, attributed to isoquinoline alkaloids.
Main classes of monomeric alkaloids are listed in the table below:
| Class | Major groups | Main synthesis steps | Examples |
|---|---|---|---|
| Alkaloids with nitrogen heterocycles (true alkaloids) | |||
| Pyrrolidine derivatives | Ornithine or arginine → putrescine → N-methylputrescine → N-methyl-Δ1-pyrroline | Cuscohygrine, hygrine, hygroline, stachydrine | |
| Tropane derivatives | Atropine group |
Ornithine or arginine → putrescine → N-methylputrescine → N-methyl-Δ1-pyrroline | Atropine, scopolamine, hyoscyamine |
| Cocaine group |
Cocaine, ecgonine | ||
| Pyrrolizidine derivatives | Non-esters | In plants: ornithine or arginine → putrescine → homospermidine → retronecine | Retronecine, heliotridine, laburnine |
| Complex esters of monocarboxylic acids | Indicine, lindelophin, sarracine | ||
| Macrocyclic diesters | Platyphylline, trichodesmine | ||
| 1-aminopyrrolizidines (lolines) | In fungi: -proline + -homoserine → N-(3-amino-3-carboxypropyl)proline → norloline | Loline, N-formylloline, N-acetylloline | |
| Piperidine derivatives | Lysine → cadaverine → Δ1-piperideine | Sedamine, lobeline, anaferine, piperine | |
| Octanoic acid → coniceine → coniine | Coniine, coniceine | ||
| Quinolizidine derivatives | Lupinine group | Lysine → cadaverine → Δ1-piperideine | Lupinine, nupharidin |
| Cytisine group | Cytisine | ||
| Sparteine group | Sparteine, lupanine, anahygrine | ||
| Matrine group | Matrine, oxymatrine, allomatridine | ||
| Ormosanine group | Ormosanine, piptantine | ||
| Indolizidine derivatives | Lysine → δ-semialdehyde of α-aminoadipic acid → pipecolic acid → 1 indolizidinone | Swainsonine, castanospermine | |
| Pyridine derivatives | Simple derivatives of pyridine | Nicotinic acid → dihydronicotinic acid → 1,2-dihydropyridine | Trigonelline, ricinine, arecoline |
| Polycyclic noncondensing pyridine derivatives | Nicotine, nornicotine, anabasine, anatabine | ||
| Polycyclic condensed pyridine derivatives | Actinidine, gentianine, pediculinine | ||
| Sesquiterpene pyridine derivatives | Nicotinic acid, isoleucine | Evonine, hippocrateine, triptonine | |
| Isoquinoline derivatives and related alkaloids | Simple derivatives of isoquinoline | Tyrosine or phenylalanine → dopamine or tyramine (for alkaloids Amarillis) | Salsoline, lophocerine |
| Derivatives of 1- and 3-isoquinolines | N-methylcoridaldine, noroxyhydrastinine | ||
| Derivatives of 1- and 4-phenyltetrahydroisoquinolines | Cryptostilin | ||
| Derivatives of 5-naftil-isoquinoline | Ancistrocladine | ||
| Derivatives of 1- and 2-benzyl-izoquinolines | Papaverine, laudanosine, sendaverine | ||
| Cularine group | Cularine, yagonine | ||
| Pavines and isopavines | Argemonine, amurensin | ||
| Benzopyrrocolines | Cryptaustoline | ||
| Protoberberines | Berberine, canadine, ophiocarpine, mecambridine, corydaline | ||
| Phthalidisoquinolines | Hydrastine, narcotine (Noscapine) | ||
| Spirobenzylisoquinolines | Fumaricine | ||
| Ipecacuanha alkaloids | Emetine, protoemetine, ipecoside | ||
| Benzophenanthridines | Sanguinarine, oxynitidine, corynoloxine | ||
| Aporphines | Glaucine, coridine, liriodenine | ||
| Proaporphines | Pronuciferine, glaziovine | ||
| Homoaporphines | Kreysiginine, multifloramine | ||
| Homoproaporphines | Bulbocodine | ||
| Morphines | Morphine, codeine, thebaine, sinomenine | ||
| Homomorphines | Kreysiginine, androcymbine | ||
| Tropoloisoquinolines | Imerubrine | ||
| Azofluoranthenes | Rufescine, imeluteine | ||
| Amaryllis alkaloids | Lycorine, ambelline, tazettine, galantamine, montanine | ||
| Erythrina alkaloids | Erysodine, erythroidine | ||
| Phenanthrene derivatives | Atherosperminine | ||
| Protopins | Protopine, oxomuramine, corycavidine | ||
| Aristolactam | Doriflavin | ||
| Oxazole derivatives | Tyrosine → tyramine | Annuloline, halfordinol, texaline, texamine | |
| Isoxazole derivatives | Ibotenic acid → Muscimol | Ibotenic acid, Muscimol | |
| Thiazole derivatives | 1-Deoxy-D-xylulose 5-phosphate (DOXP), tyrosine, cysteine | Nostocyclamide, thiostreptone | |
| Quinazoline derivatives | 3,4-Dihydro-4-quinazolone derivatives | Anthranilic acid or phenylalanine or ornithine | Febrifugine |
| 1,4-Dihydro-4-quinazolone derivatives | Glycorine, arborine, glycosminine | ||
| Pyrrolidine and piperidine quinazoline derivatives | Vazicine (peganine) | ||
| Acridine derivatives | Anthranilic acid | Rutacridone, acronicine | |
| Quinoline derivatives | Simple derivatives of quinoline derivatives of 2 – quinolones and 4-quinolone | Anthranilic acid → 3-carboxyquinoline | Cusparine, echinopsine, evocarpine |
| Tricyclic terpenoids | Flindersine | ||
| Furanoquinoline derivatives | Dictamnine, fagarine, skimmianine | ||
| Quinines | Tryptophan → tryptamine → strictosidine (with secologanin) → korinanteal → cinhoninon | Quinine quinidine cinchonine, cinhonidine | |
| Indole derivatives See also: indole alkaloids | Non-isoprene indole alkaloids | ||
| Simple indole derivatives | Tryptophan → tryptamine or 5-hydroxitriptofan | Serotonin, psilocybin, dimethyltryptamine (DMT), bufotenin | |
| Simple derivatives of β-carboline | Harman, harmine, harmaline, eleagnine | ||
| Pyrroloindole alkaloids | Physostigmine (eserine), etheramine, physovenine, eptastigmine | ||
| Semiterpenoid indole alkaloids | |||
| Ergot alkaloids | Tryptophan → chanoclavine → agroclavine → elimoclavine → paspalic acid → lysergic acid | Ergotamine, ergobasine, ergosine | |
| Monoterpenoid indole alkaloids | |||
| Corynanthe type alkaloids | Tryptophan → tryptamine → strictosidine (with secologanin) | Ajmalicine, sarpagine, vobasine, ajmaline, yohimbine, reserpine, mitragynine, group strychnine and (Strychnine brucine, aquamicine, vomicine ) | |
| Iboga-type alkaloids | Ibogamine, ibogaine, voacangine | ||
| Aspidosperma-type alkaloids | Vincamine, vinca alkaloids, vincotine, aspidospermine | ||
| Imidazole derivatives | Directly from histidine | Histamine, pilocarpine, pilosine, stevensine | |
| Purine derivatives | Xanthosine (formed in purine biosynthesis) → 7 methylxantosine → 7-methyl xanthine → theobromine → caffeine | Caffeine theobromine theophylline saxitoxin | |
| Alkaloids with nitrogen in the side chain (protoalkaloids) | |||
| β-Phenylethylamine derivatives | Tyrosine or phenylalanine → dioxyphenilalanine → dopamine → adrenaline and mescaline tyrosine → tyramine phenylalanine → 1-phenylpropane-1,2-dione → cathinone → ephedrine and pseudoephedrine | Tyramine, ephedrine, pseudoephedrine, mescaline, cathinone, catecholamines (adrenaline, noradrenaline, dopamine) | |
| Colchicine alkaloids | Tyrosine or phenylalanine → dopamine → autumnaline → colchicine | Colchicine, colchamine | |
| Muscarine | Glutamic acid → 3-ketoglutamic acid → muscarine (with pyruvic acid) | Muscarine, allomuscarine, epimuscarine, epiallomuscarine | |
| Benzylamine | Phenylalanine with valine, leucine or isoleucine | Capsaicin, dihydrocapsaicin, nordihydrocapsaicin | |
| Polyamines alkaloids | |||
| Putrescine derivatives | ornithine → putrescine → spermidine → spermine | Paucine | |
| Spermidine derivatives | Lunarine, codonocarpine | ||
| Spermine derivatives | Verbascenine, aphelandrine | ||
| Peptide (cyclopeptide) alkaloids | |||
| Peptide alkaloids with a 13-membered cycle | Nummularine C type | From different amino acids | Nummularine C, Nummularine S |
| Ziziphine type | Ziziphine A, sativanine H | ||
| Peptide alkaloids with a 14-membered cycle | Frangulanine type | Frangulanine, scutianine J | |
| Scutianine A type | Scutianine A | ||
| Integerrine type | Integerrine, discarine D | ||
| Amphibine F type | Amphibine F, spinanine A | ||
| Amfibine B type | Amphibine B, lotusine C | ||
| Peptide alkaloids with a 15-membered cycle | Mucronine A type | Mucronine A | |
| Pseudoalkaloids (terpenes and steroids) | |||
| Diterpenes | Lycoctonine type | Mevalonic acid → izopentenilpyrophosfate → geranyl pyrophosphate | Aconitine, delphinine |
| Steroids | Cholesterol, arginine | Solasodine, solanidine, veralkamine, batrachotoxin | |
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