Pyrethroids

Abstract Pyrethroids are derived from the natural pyrethrins, the in secticidal ingredients of pyrethrum flowers (Chrysanthe mum spp.) (82). Early synthetic pyrethroid insecticides, [e.g., allethrin (112)], resemble pyrethrin I, a major constituent of the natural pyrethrins. Pyrethrins and most pyrethroids are esters with an acid moiety derived from or sterically resembling chrysanthemic acid, and an aromatic alcohol moiety. However, the structure of some synthetic pyrethroids is only remotely related to that of the natural pyrethrins. Chemical structures of some of the current pyrethroid insecticides are shown in Figure 29. Activity and stability of pyrethroids have been greatly enhanced by the halogenation of the isobutenyl side chain of chrysanthemic acid, and by the introduction of novel alcohol moieties, in particular the 3-phenoxy-benzyl alcohol. Pyrethroids containing an a-cyano-substituted 3-phenoxybenzyl alcohol moiety are commonly referred to as a-cyano or type II pyrethroids. Virtually all pyrethroids are chiral molecules; up to 16 isomers of a single compound may be distinguished. With few exceptions, technical pyrethroids are mixtures of several active and several inactive isomers. Some products are enriched in specific isomer content, and this is generally indicated by a prefix to the common name [e.g., bioallethrin (lR,trans,aS-allethrin); /3-cyfluthrin, A-cyhalothrin, a methrin (1:1 mixtures of active lR,cis,aS and inactive 1S,cis,aR enantiomers of cyfluthrin, cyhalothrin, and cypermethrin, respectively); and esfenvalerate (2S,aS fenvalerate)].