Diterpenes and Sesterterpenes

Diterpenes and Sesterterpenes

There are about 2500 known diterpenes that belong to 20 major structural types.

Diterpenes occur in all plant families.

Gibberellins are plant hormones and phytol occurs as a sidechain on chlorophyll.


Resins in plants


Impure mixtures of diterpenes from gymnospermous trees are called naval stores. These substances were formerly of great economic importance for caulking ships.


Diterpenes and Sesterterpenes


The biosynthesis occurs in plastids as is true for monoterpenes. Interestingly mixtures of monoterpenes and diterpenes are the major constituents of plant resins.

In a similar manner to monoterpenes, diterpenes arise from the DOXP pathway, not from mevalonate.


Acyclic diterpenes


Acyclic diterpenes, other than phytol, are not commonly accumulated, although small amounts do occur in a number of plants. Some of these compounds are kairomones and pheromonal substances for insects.


Monocyclic diterpenes


Macrocyclic diterpenes are commonly isolated from gymnosperms. Examples are cembrene and its relatives. Among plants, the taxanes occur only in the gymnospermous family Taxaceae. They may also be synthesized by some fungi.

Complex derivatives of macrocyclic precursors, the cocarcinogenic diterpenes, occur in two plant families, the Euphorbiaceae and the Thymelaeaceae. Many of these compounds are extremely caustic. They are not carcinogenic, but when an animal is exposed to cocarcinogens and later to carcinogens, the activity of the latter is enhanced. The caustic properties and cocarcinogenicity are not directly linked.


Bicyclic series


Two major series, those with 5α,10β- and 5β,10α- are common. The former series is common in gymnosperms. The two types were once thought to be quite distinct biosynthetically, but now both are even known to occur in the same species.

Manool, sclareol, and labdane derivatives are common in resins of the Pinaceae. Highly oxygenated derivatives occur in members of the Asteraceae, Lamiaceae, Verbenaceae, and the genus Croton of the Euphorbiaceae.

Another oxygenated type, the clerodanes, are common in these families. Many of these compounds have insect antifeedant activity.


Tricyclic series


There are four main types of tricyclic diterpenes, those with pimarane, abietane, rosane and cassaine skeleta. Labda-8(17),13-dien-15-yl pyrophosphate or copalyl pyrophosphate is the precursor of (-)- or ent-kaurene.

Diterpenes derived from ent-kaurene


ent-Kaurene is the precursor, in turn, of many other structural types of diterpenes. These include enmeins from Lamiaceae (Labiatae), gibberellic acids (found in all plants and in some fungi), and grayanotoxins, extremely poisonous compounds from the family Ericaceae. These last compounds are common in Rhododendron species. This precursor is also the source of stevioside, a sweeteners from the asteraceous plant Stevia rebaudiana.

Phytoalexins


Two series of phytoalexins are found in rice, Oryza sativa. These are the momilactones and oryzalexins. The oryzalexins appear to be derivatives of sandaracopimaridiene. Momilactones arise from 9-βH-pimari-7,15-diene.

Casbene, a macrocyclic diterpene is a phytoalexin in castor beans, Ricinus communis.

Phytotoxins


Several well known diterpene phytotoxins are produced by fungi. Among these are the colletotrichins and the cotylenins. Fusicoccin is a similar C-25 compound.

Medicinals


Ginkgolides are one set of active ingredients of Ginkgo biloba, an important herbal medicine. These compounds are involved in increasing capillary blood flow in the brain and improve brain function.

Forskolin, from Coleus forskohlii is an active inhibitor of adenylate cyclase.

Allomones


Many types of diterpenes are allomonal in action. Resins often flow over and capture small insects. Many diterpenes, expecially highly oxygenated ones, are antifeedant to insects. Others, such as the enmeins possess insect feeding-deterrent activity.

Trachyloban and other diterpenes from Helianthus species also protect the plants from insect attack.

Sesterterpenoids

C25-compounds

. They mostly appear to be diterpenes with an extra 5-carbon unit attached. Many of them are fungal products and some are phytotoxins.

Related Images

Assorted bicyclic and tricyclic diterpenes

GGPP biosynthesis

Acyclic diterpenes

Monocyclic and derived diterpenes

Phorbol esters

Formation of 5β, 10α- and 5 α,10β-Diterpene Series

ent-Kaurene biosynthesis

Compounds derived from ent-kaurene


Lecture Slides

Plants with Diterpenes and Sesterterpenes



© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577. seigler@life.uiuc.edu.