Development of Pyrethroids for Household Use

Natural pyrethrins have long been used as most favored household insecticides. Pyrethrum flowers are still cultivated in certain areas including Africa, Australia, and China. On the other hand, commercial use of pyrethroids is one of the most remarkable success stories in insecticide development originated from natural products as a lead. It should be noted that in 1924 Staudinger and Ruzicka reported [1] several pyrethrin analogs including piperonyl chrysanthemate (3) as shown in Fig. 1. 

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. 

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. 

The most important breakthrough in terms of modifications of the alcohol moiety of pyrethrins was the aforementioned piperonyl ester (3) followed by the inventions of resmethrin (7) by Elliott in 1965 [5] and phenothrin (5) by Itaya in 1968 [6]. Further exploratory work in Sumitomo Chemical on modification of 

---