Pyrethrins structure

A more marked departure from the pyrethrin structure was represented by esters of chrysanthemic acid formed with substituted benzyl alcohols, such as... 

Most pyrethrins and pyrethroids are toxic to fish, but this is not the case with two hybrids of DDT and pyrethrin structures, namely cyclophenthrin and phencyelate. The former is a-cyano-3-phenoxybenzyl l-(4-ethoxyphenyl)-2,2-dimethylcyclopropanecarboxylate, and phencyclate is the 2,2-dichloro analogue (Holan et aL, 1978,1983 Wakita etal., 1983). These are racemates, the inactive isomer of which prevents the fish receptor from taking up the active isomer whereas no such protection occurs at the insect receptor (G. Holan, personal communication). 

The insecticidal properties of pyrethrum result from six esters that are collectively called pyrethrins. After the isolation and identification of these compounds, a number of derivatives have been synthesized that are more effective than the natural pyrethrins. For example, dimethrin is effective against mosquito larvae and is safe to use (oral LD q for rats is greater than 10,000 mg/kg). The basic pyrethrin structure is shown in black, and the groups that vary in different pyrethrins are shown in yellow. 

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) ... 

The great advance in the field of instrumentation, coupled with the discovery of the heterogeneity of the pyrethrolone radical, has advanced the knowledge of pyrethrum chemistry considerably beyond that known in 1945. LaForge and Barthel (24,25) have shown the structure of the active ingredients of pyrethrum, known collectively as pyrethrins, to be esters as represented by the structure shown in Table I. 

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 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... 

As described in the section on Cross-resistance in this chapter, it was found that some insect species showed extremely low cross-resistance to three ingredients, pyrethrins as well as d-allethrin and prallethrin, although they developed resistance to photostable synthetic pyrethroids. The latter two compounds of d-allethrin and prallethrin have quite similar chemical structures and the same configuration as cinerin I (an ingredient of pyrethrins). It is considered preferable to develop pyrethroids retaining the characteristics of natural pyrethrins and household insecticides containing them in the perspectives of safety and low cross-resistance. 

Fujitani [6] separated the insecticidally active syrupy ester from pyrethrum flowers in 1909 and named the ester pyrethron. Yamamoto [7, 8] subjected the hydrolysis product of this pyrethron to ozone oxidation, and isolated Iram-caronic acid and aldehyde (1 and 2, respectively, Fig. 3). Although Yamamoto did not determine the structure of this acid, he presumed it to be pyrethron acid (Fig. 3). Eventually, the presence of a cyclopropane ring in the molecule of natural pyrethrins became clear for the first time in 1923. 

The main application fields of pyrethrins are limited to indoor use because of their instability to heat, light, and oxygen. Since the absolute configuration of the six insecticidal components of pyrethrins were elucidated in 1958, various researches on structural modifications have been carried out actively in many countries for more than half a century, leading to the development of a variety of photostable pyrethroids. As a result, they have been widely put into outdoor use for agriculture, forestry, animal health, termite control, and so on. 

As shown in Fig. 6, the chemical structure of natural pyrethrins consists of six chemical components pyrethrin I and II, cinerin I and II, and jasmolin I and II. 

Except for the side chain structure of the alcohol moiety, there is a great difference between natural pyrethrins and dxT/ram-allethrin in that the former consists of the mixture of pyrethrins I and II (5-10, Fig. 5), whereas the latter does not contain pyrethrin II homologs. 

Cross-resistance to pyrethroids for outdoor use has developed markedly in M. domestica, mosquitoes, cockroaches, and so on however, it has also been found that natural pyrethrins as well as d-allethrin and prallethrin (ETOC ), which have very similar chemical structures and the same configuration as natural pyrethrins, show an extremely low degree of cross-resistance development by these highly-resis-tant sanitary pests compared to photostable pyrethroids. Many novel synthetic pyrethroids recently developed as household insecticides have tended to pursue efficacy improvements in terms of rapid knock-down effects, residual efficacy or volatility. 

Natural pyrethrins are a neurotoxin and repel, knock down, and kill by contact with insects at a low concentration. On the other hand, they have ideal features for household insecticides because of their quite low dermal and oral toxicides to warm-blooded animals. Neither plants other than pyrethrum nor synthetic insecticides have been reported to have such properties. Numerous synthetic pyrethroids have been developed by chemists since the complicated chemical structure of natural pyrethrins was elucidated in the middle of the twentieth century. Allethrin was the first synthetic pyrethroid put into practical use. 

Some of them were only slightly insecticidal. But their foresight is notable regarding natural pyrethrins as lead compounds without knowledge of the real structures of alcohol moieties at that time. 

Through extensive studies during the past 60 years, natural pyrethrins proved to possess ample possibilities for structural modifications. Namely, just after elucidation of the structures of pyrethrin I (1) and pyrethrin II (2) of natural pyrethrins in 1947 [2], extensive efforts began to modify mainly the alcohol moieties. In Fig. 2 commercialized household use pyrethroids from Sumitomo Chemical Co.Ltd are listed. 

Although structurally-diverse as evidenced above, the insecticidal pyrethroids still conform to a unique, operationally-defined, structure-activity relationship based on the physical characteristics and three-dimensional shape of the entire molecule conforming to those originally evidenced in the natural pyrethrins [13]. From this relationship, it becomes apparent that there is no single molecular aspect or reactive moiety that serves as a true toxophore for the pyrethroids and that their actions at target sites are dependent upon the entire stereospecific structure of these insecticides [1]. 

Begley MJ, Crombie L, Simmonds DJ, et al. (1972) Absolute configuration of pyrethrins. Configuration and structure of (+)-allethronyl (+)-tra s-chrysanthemate 6-bromo-2,4-dinitrophenylhydrazone by X-ray methods. J Chem Soc Chem Commun 1276-1277... 

Pyrethroids are a class of synthetic insecticides designed and optimized based on the structure of the pyrethrins found in natural pyrethrum extracted from chrysanthemum flowers [1, 2], Pyrethroids are widely used to control insect pests in agriculture and public health because of their relative safety for humans and high insecticidal potency [3]. 

Pyrethrum has been used as an insecticide for around 150 years, and there has been no other insecticide which has so successfully contributed to the control of sanitary pests. Numerous analogs have been developed by chemists worldwide since the elucidation of the chemical structure of pyrethrins, which are the insecticidal ingredients of pyrethrum. As a result, their application has expanded extensively to various fields. To date, many eminent books have been published by scientists in this field and have contributed to advancing pyrethroid science. 

Pyrethroids are a collective term for compounds that are obtained by modifying the structure of natural insecticidal ingredients, pyrethrins, contained in pyrethrum while maintaining safety, to improve efficacy and provide different characteristics from pyrethrins that show high selective toxicity comparable to pyrethrins. 

Pyrethroids are a class of synthetic chemicals that are similar in structure to natural pyrethrins. They have been used in field crops and urban pest management for nearly 30 years, and within the last 5 to 10 years new products have been registered for specific use against stored-product insects. Resmethrin is labeled for use as an aerosol in food plants, mills, and warehouse facilities, but could have potential side effects such as discoloration of surfaces and odor contamination and may be more appropriate for use in empty facilities. Labels generally state to cover any food prior to application. The pyrethroids esfenvalerate (Conquer) and prallethrin (Etoc) are also labeled for use in some situations as an aerosol space treatment in... 

Alicyclic hydrocarbons are saturated carbon chains that form ring structures. Naturally occurring alicyclic hydrocarbons are common (Chap. 1). For example, alicyclic hydrocarbons are a major component of crude oil, comprising 20-67 vol.%. Other examples of complex, naturally occurring alicyclic hydrocarbons include camphor (a plant terpene) and cyclohexyl fatty acids (components of microbial lipids). Anthropogenic sources of alicyclic hydrocarbons to the environment include fossil-fuel processing and oil spills, as well as the use of such agrochemicals as the pyrethrin insecticides (Chap. 1, and references therein). 

Mention was made of the natural product pyrethrins and the structure of pyrethrin I was given in this chapter, Section 3.1. Because of the unique structures of these cyclopropane-containing natural products and their high insecticidal properties, syntheses of analogs have been studied. The isobutenyldimethylcyclopropanecarboxylic acid moiety, called chrysan-themic acid, has been modified by using different ester groups. As a result a number of synthetic pyrethroids are available for certain specific uses. 

Pyrethroids are based on mimicking the structure of the natural insecticide pyrethrin. Pyrethins are found in the flowers of chrysanthemums. Ground flowers were traditionally used to obtain pyrethin insecticides and used to kill lice in the early 1800s. Synthetic pyrethins were first produced in the early 1970s. The exact nature of how pyrethroids work is unknown, but because they paralyze insects it is speculated that they affect the nervous or muscular system. Pyrethroids are effective in low dosages and are nonpersistent. 

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