Description
Pyrethrins described originally are natural products
obtained from chrysanthemum flowers. Pyrethrins are
chiral and composed of several optically active isomers.
They have been extracted and purified to be used as insectcontrolling
agents. Later, many synthetic compounds of
simpler structures based on modifications of the natural
pyrethrins have been examined for their insecticidal
activities, and numerous “synthetic pyrethroids” have
been developed and marketed nowadays. Many of them
are optically active and enantiopure because, since the
initiation of such efforts, strategy of the synthetic chemists
for constructing active pyrethroid structures involved
extensive utilization of naturally occurring enantiopure
pyrethrin molecules. For example, chrysanthemic acid
(acid portion of the natural pyrethrin I) was esterified
with a simpler alcohol of a structure similar to natural
pyrethrolone, etc. (S)-bioallethrin (6) (2), the ester of (1R)-
trans-chrysanthemic acid with (S)-allethrolone, is one of
the earliest examples. In this study, the large difference
in biological activity between stereoisomers was clearly
indicated.
Uses
The pyrethroids formed chronologically the fourth major group of insecticides
after the organochlorines, the organophosphates and the carbamates.
They are synthetic analogues of the pyrethrins, the naturally
occurring insecticidal constituents of the flowers of Tanaceturn cinerariaefdiurn.
The pyrethroids were first commercialised in 1978 (fenvalerate) and
the class now consists of 42 active ingredients, some of which differ only
in relative stereoisomer content. This figure (PM) includes natural
pyrethrum and four non-ester pyrethroids. The members of the class control
a wide range of insects, especially Lepidoptera (and also Coleoptera,
Diptera, Hemiptera and others) in a wide range of crops, ornamentals and
trees. They are also important in public health and veterinary medicine
where they are used as ectoparasiticides. The more volatile compounds
and those prone to photodecomposition have uses in domestic insect
control.
The pyrethroids are non-systemic (in plants), and are contact and stomach
poisons; they also have an anti-feeding action. They possess good
residual activity on treated plants. The compounds act on the central and
peripheral nervous systems of target insects at very low doses. Their
action occurs on sodium channels within the lipophilic environment of
the membranes at, or close to, the Na
+
gate proteins and they act by
modulating the opening and closing of the channels, leading to synaptic
discharge, repetitive discharge, depolarisation and ultimately death.
The major components of the natural insecticide mixture are: the pyrethrins
(I and II), the cinerins (I and II) and the jasmolins (I and II) in the
percentage ratio 71 : 21 : 7. The designation I refers to the chrysanthemate
esters and II refers to the ’pyrethrate’ esters. Their structures are shown in
the pyrethrum entry. Pyrethrin I (1) is the most important component for
insecticidal action but pyrethrin II is very effective at rapid ‘knock-down’.
This is not synonymous with death but it is very effective in the rapid
removal of a nuisance factor (important in domestic use). Pyrethrin I is
an ester of 2,2-dimethyl-3-dimethylvinylcyclopropanecarboxylica cid and
2-pentadienyl-3-methylcyclopentenolone. The potential for complex
stereochemistry is immediately apparent.
Uses
Pyrethroids are neurotoxie synthetic compounds used
as insecticides. Cypermethrin and fenvalerate have
been reported as causing positive allergie patch tests,
but only fenvalerate was relevant in an agricultural
worker.
