Pyrethrin I

They indicated that this method might be advantageous for the partial separation of pyrethrins I and II on an industrial scale, thus tailoring pyrethrin extracts for specific uses depending upon the insect to be controlled. 

Sawicki et al. (33) prepared by reconstitution pure samples of pyrethrins I and II and cinerins I and II and compared the toxicity of these esters with the regular pyrethrum extract on houseflies 5 to 6 days old. 

Pyrethrins I and II are, respectively, more toxic per unit weight ... 

Sawicki and Elliott (31) re-examined the insecticidal activity of pyrethrin extracts and its four insecticidal constituents against four strains of houseflies, and checked the relative toxicity of pyrethrins I and II. This work confirmed the earlier results, showing that pyrethrin II was 1.3 to 1.6 times more toxic than pyrethrin I, but that the relative toxicities of pyrethrins I and II against the four strains of flies differed little. Resistance to knockdown but not to killing was associated in these strains with resistance to organophos-phorus and chlorinated insecticides. 

The development and adaptation of procedures for the separation, isolation, purification, identification, and analysis of the components of the pyrethrum mixture have been studied and evaluated. Results of studies to determine the molar extinction coefficient of pyrethrin I as well as a gas chromatographic procedure for the determination of pyrethrins are reported. The use of chromatographic separation procedures (including partition, adsorption, gas, and thin-layer chromatography), colorimetry, and infrared spectrophotometry are discussed. 

Figure 7. Infrared spectrum of pyrethrin I Isolated after dual partition chromatography First partition column. Celite-acetonitrile-hexane Second partition column. Silicic acid-nitromethane-hexane (with 5% acetone). Corresponds to peak 3 of gas chromatographic separation of pyrethrum mixture...

The separations allowed by the partition column provided a rather pure sample of pyrethrin I, demonstrated by the gas chromatograph and by comparison with known infrared spectra. The purified pyrethrin I was weighed quantitatively and a color test performed to determine the extinction coefficient. The figure obtained from ten runs is 1120, calculated from the formula ... 

Figure 9. Infrared spectrum of pyrethrin I decomposition product isolated from thin-layer chromatographic plate...

The columns labeled PI reflect the total of pyrethrin I and cinerin I just as in the AO AC procedure. The gas chromatographic results are in terms of the total amount of the mixture but were analyzed as the methyl ester of chrysanthemic acid. The present state of the determination of PII (pyrethrin II plus cinerin II) is not complete because of the erratic extractability of the dicarboxylic acids from the hydrolysis mixture. The gas chromatographic pattern is distinct and straightforward. As the extraction procedure for PII is improved, the gas chromatographic method will be more applicable. The present recovery of PII is in the range of 80 to 90%. The average of the values shown in Table II for PI is 98.0%. 

Figure 1 presents the gas chromatographic responses obtained from a pyrethrum concentrate. Peak I has been identified as cinerin I, peak II as a cinerin-type compound, peak III as pyrethrin I, peak... 

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]. 

Determination of the contents of pyrethrin I and pyrethrin II was then made, for about 2 years (Fig. 2). Having detected pyrethrin I throughout the whole growing process of pyrethrum leaves, they reported that the pyrethrin I content, which had a close relationship with flowering, reached a peak of 0.27-0.40 wt% during flowering and was slightly lower than that in dried flowers. 

Pyrethrin II was also detected in young leaves 2 months after seeding, similarly to pyrethrin I, but the content remained at about 0.05 wt% without seasonal change for 2 years. The insecticidal potency of pyrethrins obtained from pyrethrum leaves was confirmed with Musca domestica. 

While it is conceivable that a part of pyrethrin I is biosynthesized in pyrethrum leaves and moves to flowers sequentially, biosynthesis of pyrethrin II is quite an interesting theme. 

Fig. 2 Seasonal changes in pyrethrins contents in pyrethrum leaves. Filled circles pyrethrin I, open circles pyrethrin II, filled triangles pyrethrin I + pyrethrin II. There were significant differences between changes in pyrethrin I contents and those in pyrethrin II contents (F test, P < 0.05)...

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. 

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. 

Simplification of the diene moiety of pyrethrin I resulted in finding allethrin (4) by Schechter [3], which was the first pyrethroid commercialized for household use. Matsui et al. made an extensive effort on the process of allethrin, which resulted in the launch of the first pyrethroid in Japan in 1954. As the propynyl analog of pyrethrin I, prallethrin (11) was commercialized by Sumitomo Chemical in 1988 in the most insecticidally active form [4]. Allethrin and prallethrin have been used widely for control of mosquitoes. 

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