IRREGULAR MONOTERPENES

Idrissa Chirombo

A number of natural monoterpene structures contain carbon skeletons which, although obviously derived from isoprene C5 units, do not seem to fit the regular head-to-tail coupling mechanism as shown below

These structures are termed irregular monoterpenes and seem to be limited almost exclusively to members of the plant family the Compositae/Asteraceae.

Two isoprene units appear to have coupled in another manner and this is borne out by information available on their biosynthesis, though this is far from fully understood.

DMAPP and IPP are utilized in their biosynthesis, geranyl PP and neryl PP do not appear to be involved

Pre-eminent amongst these structures are chrysanthemic acid and pyrethric acid found in ester form as the pyrethrins (pyrethrins, cinerins, ∗and jasmolins, , which are valuable insecticidal components in pyrethrum flowers, the flower heads of Chrysanthemum cinerariaefolium (Compositae/ Asteraceae).

1. IRIDOIDS (C10) The iridane skeleton found in iridoids is

monoterpenoid in origin and contains a cyclopentane ring which is usually fused to a six-membered oxygen heterocycle, e.g. nepetalactone from catmint Nepeta cataria (Labiatae/Lamiaceae), a powerful attractant and stimulant for cats.

The iridoid system arises from geraniol by a type of folding which is different from that already encountered with monoterpenoids, and also different is the lack of phosphorylated intermediates and subsequent carbocation mechanism in its formation. The fundamental cyclization to iridodial is formulated as attack of hydride on the dialdehyde, produced by a series of hydroxylation and oxidation reactions on geraniol.

Further oxidation gives iridotrial, in which hemiacetal formation then leads to production of the heterocyclic ring.

2. Pyrethrins The pyrethrins are valuable insecticidal

components of pyrethrum flowers, Chrysanthemum cinerariaefolium (= Tanacetum cinerariifolium) (Compositae/Asteraceae).

Pyrethrum cultivation is conducted in East Africa, especially Kenya, and more recently in Ecuador and Australia.

The natural pyrethrins are used as a constituent of insect sprays for household use and as post-harvest insecticides, having a rapid action on the nervous system of insects, whilst being biodegradable and non-toxic tomammals, though they are toxic to fish and amphibians.

inseides, having a rapid action on the nervous system of insects, whilstg

The flowers may contain 0.7–2% of pyrethrins, representing about 25–50% of the extract.

A typical pyrethrin extract contains pyrethrin I (35%), pyrethrin II (32%), cinerin I (10%), cinerin II (14%), jasmolin I (5%), and jasmolin II (4%), which structures represent esters of chrysanthemic acid or pyrethric acid with the alcohols pyrethrolone, cinerolone, and jasmolone.

Pyrethrin I is the most insecticidal component, with pyrethrin II providing much of the rapid knock-down (paralysing) effect.

3. Valerian Valerian root consists of the dried underground

parts of Valeriana officinalis (Valerianaceae), a perennial herb found throughout Europe. Drug material comes from wild and cultivated plants, and is carefully dried at low temperature (less than 40 °C) to minimize decomposition of constituents.

Valerian preparations are widely used as herbal tranquillizers to relieve nervous tension, anxiety, and insomnia.

Valerian was especially popular during the First World War, when it was used to treat shell-shock.

The drug does possess mild sedative and tranquillizing properties, but for maximum activity the roots need to be freshly harvested and carefully dried. The major active principles are generally held to be a number of epoxyiridoid esters called valepotriates (0.5–1.6%), the principal component of which is valtrate (about 80%)

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