Natural pyrethrins

Pyrethrins Naturally occurring lipophilic esters that are toxic to many insects. 

Crowley MP, Inglis HS, Snarey M et al (1961) Biosynthesis of the pyrethrins. Nature... 

Chrysanthemic acid (TM355) is an important constituent of pyrethrins - naturally occurring insecticides which are virtually harmless to mammals. What feature of this molecule will dominate our strategic thinking ... 

Pyrethroid insecticides are synthetic analogs of natural pyrethrins. Natural pyrethrins were widely used in Europe during the 19th century, when few effective insecticides were available. Natural pyrethrins, which contain six insecticidally active components extracted from the dried heads of the pyrethrum flower (Chrysanthemum cinariaefolium), have high insecticidal properties and low manunalian... 

Synonyms Chrysanthemum monocarboxylic acid pyrethrolone ester Cyclopropanecarboxylic acid, 2,2-dimethyl-3-(2-methylpropenyl)-, ester with 4-hydroxy-3-methyl-2-(2,4-pentadienyl)-2-cyclopenten-1-one Pyrethrolone, chrysanthemum monocarboxylic acid ester (+)-Pyrethronyl (+)-trans-chrysanthemate Classification Pyrethrins (natural)... 

Figure 7. Synthetic Pyrethroid modeled on pyrethrin natural insecticide found in Chrysanthemum cinerariaefli...

Pyrethrins Natural insecticide originally extracted from chrysanthemums and used for insect control in households and storage facilities. Among insecticides, pyrethrins are known to have the least toxic effects for humans. There are synthetic pyrethrins with a similar structure to natural pyrethrins. 

Piperonyl butoxide [51-03-6] is 5-[2-(2-butoxyethoxy)ethoxy]methyl-6-propyl-l,3-benzodioxole (11) d 1.04—1.07, vp 0.13 kPa at 25°C). The rat oral LD qS are 7500, 6150 mg/kg. Piperonyl butoxide is the synergist commonly used with natural pyrethrins in aerosol sprays. 

Pyrethroids from Chiysanthemic Acid. The unsaturated side chains of the aHethrolone alcohol moieties of the natural pyrethrins are readily epoxidized by microsomal oxidases and converted to diols, thus detoxifying the insecticides. Esterification of chrysanthemic acid (9), R = CH3, with substituted ben2yl alcohols produces usehil insecticides barthrin [70-43-9J, 2-chloro-3,4-methylenedioxyben2yl (+)-i7j ,/n7 j -chrysanthemate, and dimethrin [70-38-2] 2,4-dimethylben2yl (+)-i7j ,/n7 j -chrysanthemate. These have alimited spectmm of insecticidal activity but are of very low mammalian toxicity, ie, rat oralLD s >20,000 mg/kg. 

Pyrethrins are a group of naturally occurring insecticidal substances found in the flowers of various plants of the chrysanthemum family. The following is the structure of a typical pyrethrin, cinerin I (exclusive of stereochemistry) ... 

Pyrethrolone and cinerolone make up the keto alcohol moiety of the pyrethrins. Both of these keto alcohols have one asymmetric carbon at the 4-position and a double bond in the side chain which is capable of cis-trans isomerism in the 2-position. It is possible, therefore, to have four stereoisomers for each keto alcohol. Katsuda et al. (22) show that only the ( + ) form occurs in the natural esters. Elliott (8) has shown recently, by a new procedure developed to obtain pure ( + ) pyrethrolone, that the hitherto unidentified prye-throlone C is in reality pyrethrolone contaminated with thermally isomerized material. (+) Pyrethrolone forms a crystalline monohydrate from which the pure alcohol is obtained. The natural configurations of the keto alcohols in the esters are insecticidally more active, as is the case with the acid moiety. 

More detailed information on the chemistry of pyrethrins is given by Zechmeister (35). The chapter on the Chemistry of the Natural Pyrethrins by Crombie and Elliott is an excellent and complete review of this field. 

Page et al. (28) studied the activation threshold of pyrethrins for certain insects. This threshold point of toxicant in the insect occurred when the natural activity was replaced by forced activity caused by the action of the pyrethrins on the peripheral nervous system. 

Thin-layer plates were made with silica gel-calcium sulfate and each contained a mixture of zinc silicate and zinc cadmium sulfide as phosphors. Separated components are generally visible under ultraviolet light by fluorescence quenching. This was true, in part, for the pyrethrins, except that some of the separated components possessed a natural fluorescence under the ultraviolet lamps. 

STAHL, for instance, was able to demonstrate that on irradiation with long-wavelength UV light the naturally occurring contact insecticides pyrethrin I and II, cinerin I and II and jasmolin I and II present in Chrysanthemum cinerariifolium are converted to inactive pyrethrin oxides by the incorporation of oxygen [7]. 

Many pesticides are not as novel as they may seem. Some, such as the pyre-throid and neonicotinoid insecticides, are modeled on natural insecticides. Synthetic pyrethroids are related to the natural pyrethrins (see Chapter 12), whereas the neo-nicotinoids share structural features with nicotine. In both cases, the synthetic compounds have the same mode of action as the natural products they resemble. Also, the synthetic pyrethroids are subject to similar mechanisms of metabolic detoxication as natural pyrethrins (Chapter 12). More widely, many detoxication mechanisms are relatively nonspecific, operating against a wide range of compounds that... 

The compounds featured in Table 1.1 are considered briefly here. Pyrethrins are lipophilic esters that occur in Chrysanthemum spp. Extracts of flower heads of Chrysanthemum spp. contain six different pyrethrins and have been used for insect control (Chapter 12). Pyrethrins act upon sodium channels in a manner similar to p,p -DDT. The highly successful synthetic pyrethroid insecticides were modeled on natural pyrethrins. 

The structures of some pyrethroid insecticides are shown in Figure 12.1. They are all lipophilic esters showing some structural resemblance to the natural pyrethrins. They can all exist in a number of different enantiomeric forms. Permethrin, cypermethrin, and deltamethrin, for example, all have three asymmetric carbon atoms... 

These synthetic pyrethroids mimic natural counterparts, of which the most important is pyrethrin 1 (10.265). Unfortunately, the natural products lack the photochemical and hydrolytic stability necessary for use as wool insect-resist agents. The synthetic products have the required stability, yet retain the low mammalian toxicity and low environmental retention of the natural products. Permethrin, however, is toxic to aquatic life and is therefore subject to increasingly severe discharge limits. There is some evidence that permethrin is less effective against larvae of a certain beetle. This can be compensated for by using a combination of permethrin with the hexahydropyrimidine derivative 10.264. Some possible alternative pyrethroids have been mentioned [517] as development products (10.266-10.269). 

Keywords Cross resistance Natural pyrethrins Safety Synthetic pyrethroid... 

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