Structure and Chemistry 321 Pyrethrins

Pyrethrins are prepared from dried Chrysanthemum cinerariaefolium and/or C. cineum flower heads and are composed of six insecticidally active esters. Pyrethrin extracts are highly viscous liquids with high boiling points, sensitivity to oxidation, and are difficult to store for long periods.12 Annual world

High Yield Permethrin

Etofenprox

Figure 3.2 The chemical structure of type I (resmethrin and permethrin), type II (fenvalerate and cypermethrin) and pseudopyrethroids.

Etofenprox

Figure 3.2 The chemical structure of type I (resmethrin and permethrin), type II (fenvalerate and cypermethrin) and pseudopyrethroids.

production of dried flowers has rarely exceeded 20 000 tonnes and, with an average pyrethrins content of 1.5%, the potential yield is 30 kg of 50% extract per tonne. With losses at various processing stages, however, the actual yield is only about 25 kg, giving a potential annual world production of 500 tonnes. Since availability is highly variable, demand often far exceeds supply.1

In 1924, Staudinger and Ruzicka13 elucidated that the active constituents, pyrethrin I and II, are esters of 2,2-dimethyl-3-(2-methyl-l-propenyl)-l-cyclo-propanecarboxylic acid (chrysanthemic) and of 3-(2-methoxycarbonyl-l-prope-nyl)-2,2-dimethyl-l-cyclopropanecarboxylic acid (pyrethric acid), respectively. The six constituents of pyrethrins are pyrethrin I and II, cinerin I and II, and jasmolin I and II. They are collectively known as pyrethrins, which are the esters of two carboxylic acids, chrysanthemic and pyrethric acid (see Figure 3.1). Naming of the six esters of pyrethrins is derived from the alcohol component distinguished by name and number, which are designated by the Roman numeral I and II that represent the esters of the chrysanthemic and pyrethric acid, respectively (see Figure 3.1). There is considerable variation in the proportions of the different constituents of pyrethrins, with the average extract containing 73% pyrethrin I and II, 19% cinerin I and II, and 8% jasmolin I and II.14 Pyrethrin I and II differ in their insecticidal properties, with pyrethrin I showing greater lethality and pyrethrin II showing greater knockdown.15

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