Monoterpene, Sesquiterpene and Diterpene alkaloids

Monoterpene alkaloids

Iridoid monoterpene-derived alkaloids have already been discussed as indole alkaloids. However, there are a number of other monoterpene alkaloids, most are derived from iridoid monoterpenes. These compounds are essentially iridoid monoterpenes with a nitrogen, usually of obscure origin, inserted into the structure. Examples are actinidine, b- skytanthine, and boschniakine. Gentianine may or may not be naturally occuring in any particular case. This substance has often been reported, but often arises from basicification of extracted mixtures with ammonium hydroxide.

Neither loganin or actinidine are precursors of ß-skytanthine.

Some monoterpene alkaloids are derived from non-iridoid precursors.

Species of Penstemon and Castilleja (Scrophulariaceae) contain rhexifoline, but many of them also contain alkaloids derived from the hosts of these parasitic plants.

Gentianine is a sedative. Iridoid derived alkaloids occur in the sedative plant Valeriana officinalis (Valerianaceae).

Sesquiterpene alkaloids

Many of the species of Dendrobium (the largest genus of the family Orchidaceae) contain sesquiterpene alkaloids as well as sesquiterpene picrotoxins. Most of the alkaloids are related to picrotoxins. One of these, dendrobine, has long been used in Chinese medicines. Dendrobine lowers blood pressure and has weak antipyretic and analgesic action.

Nupharidine and castoramine are "quinolizidine alkaloids" (there are also lots of erroneous structures for these in the literature, but are derived from sesquiterpene precursors.

They are found in Nuphar and Nymphaea species, but also in beaver Castor fiber scent glands. The beavers probably acquire the alkaloids from plants that they eat.

Alkaloids from the Celastraceae and Hippocreataceae

These are related families. Examples are alkaloids like evonine, maytoline, maytine, and hippocrateine. One plant, cancerina, Hippocratea excelsa contains alkaloids of this series that are active as antitumor compounds.

Similar sesquiterpene alkaloids are found in khat, Catha edulis, of the Celastraceae. However, the active compounds are related to ephedrine in structure.

Other sesquiterpene alkaloids

A number of other sesquiterpenoid alkaloids have structures that appear to have had a nitrogen inserted, but otherwise resemble sesquiterpenes.

Diterpene alkaloids derived from kaurene

Diterpene alkaloids derived from kaurene are found in several families. They are best known from the Ranunculaceae (Aconitum and Delphinium) and Garryaceae. These compounds are similar in structure to kaurene and other diterpenes of this series, but contain a nitrogen atome, usually of obscure origin.

Two main structural types are found. One is a series of comparatively simple and relatively non- toxic amino alcohols and esters modelled on a C20 skeleton. The second group is based on a C19 skeleton. These compounds are more heavily substituted and mich more toxic. The best known compound, aconitine, is exceedinly toxic.

Most of these diterpene alkaloids are neurotoxins. They hold sodium channels open. This action is similar to that of grayanotoxins (of Rhododendron and related plants, Ericaceae).

These alkaloids are exceedinly toxic. As little as 2-5 mg of some is enough to kill an adult human.

Many of these plants have been used as traditional and other medicine, but their toxicity limits their usefulness. They are responsible for numerous cases of livestock poisoning.

A number of relatively simple types are found in the genus Garrya, the only genus of the family Garryaceae.

Other similar diterpene alkaloids are found in Inula royaleana (Asteraceae) and in Spiraea japonica (Rosaceae).

Alkaloids from Aconitum and Delphinium have been used to prepare arrow poisons in Alaska, Siberia, and Hokkaido.

Other diterpene alkaloids

Diterpene alkaloids from Erythrophleum species (Fabaceae: Caesalpinoideae) are highly toxic and are often responsible for livestock poisoning. As few as one or two leaves can kill animals. In Africa, Erythrophleum guineense has been used as a trial-by-ordeal drug. These alkaloids have cardiac activity similar to digitalis.

Taxol or paclitaxel, from Taxus species inhibits cell division at late G1/M phase, promotes microtubule assembly, but microtubules don't depolymerize, and is used as an antitumor drug. Most alkaloids from Taxus species (Taxaceae, a gymnospermous family), are exceedingly toxic. Yew cuttings are responsible for a number of livestock deaths each year.

Related Images

Monoterpene Alkaloids

Sesquiterpene Alkaloid Labelling

Sesquiterpene Alkaloids

Celastracenae Alkaloids

Assorted Sesquiterpene Alkaloids

Diterpene Alkaloids

More Diterpene Alkaloids

Additional Diterpene Alkaloids

Lecture Slides

Plants with Monoterpene, Sesquiterpene, and Diterpene Alkaloids

© David S. Seigler, Plant Biology 363, Plant Secondary Metabolism, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577.