SESSION VIII SYNTHETIC PYRETHROIDS Bull. Org. mond. Sante 1970, 44, 315-324 Bull. Wld Hlth Org. The Relationship between the Structure and the Activity of Pyrethroids M. ELLIOTT1 There is considerable scope for developing new non-persistent insecticides with little hazard Jor man and mammals by modifying the structures of the natural pyrethrins. New compounds already synthesized are more effective against some insect species than are the natural compounds, are even less hazardous to mammals, and do not need synergists to supplement their insecticidal action. Other examples show considerable insect species specificity. These compounds may help to control insect vectors when other insecticides are no longer effective because resistance has developed or because their residues can no longer be tolerated. When the detailed chemical structures of the The insecticidal compounds (Fig. 1) in pyrethrum natural pyrethrins were established, the foundation extract (Crombie & Elliott, 1961) are esters of cyclo- for rational structure-activity studies was laid and propanecarboxylic acids with alkenylmethyl cyclo- it might then have been predicted that related com- pentenolones. Investigations of the metabolism of pounds, as toxic or more toxic to insects, would be these esters in insects (Yamamoto et al., 1969) and discovered by persistent investigations. However, in mammals (Casida et al., 1971) have detected no it was unforeseen and welcome to find that some of hydrolysis products of the cyclopropane ester link. the new compounds with greatest insecticidal activ- This and other evidence (Elliott, 1969) indicates ity are also considerably less toxic to mammals than that these compounds act against insects as intact are the natural pyrethrins. Table 1 shows that one esters. An essential feature of this action seems to be that the ester bond is difficult to cleave, and it Table 1 probably provides a position of appropriate polar- Toxicity of parathion and bioresmethrin to the housefly ity at the centre of the molecule. and the rat LDso (mg/kg) to Compoundc Fig. 1 housefly a rat b Structures of the natural pyrethrins parathion 1 5 RR\ ,H C=C CH3 bioresmethrin 0.25 8 000 C/H \CH1-CH-CO O-CH-C CHCHCX3CH, \C/ CH,CHLCH-R. a Topical application. II b Oral administration. 0 R R synthetic compound is more toxic than parathion Pyrethrin CH3- -CH=CH2 to houseflies, but much less toxic to rats. Such ,. II CH,O.CO- -CH=CH2 results justify continued work with these non- Cinerin CH,- -CH, persistent compounds. 11 CH,O.CO- -CH, Jasmolin CH3- -CH,CH, 1 Organic chemist, Department of Insecticides and Fun- CHA0.CO- -CH,CH, gicides, Rothamsted Experimental Station, Harpenden, Herts., England. 2643 -315 316 M. ELLIOTT Fig. 2 shows the structural features at present con- 1969). In practice this means that the carbon atom sidered essential for powerful pyrethrin-like activity. bearing the acyloxy group AB must be tetrahedral. All compounds as active as or more active than the Thus, in all pyrethroids based on cyclopentenolones natural pyrethrins contain the unit B substituted in (the natural esters, allethrin, furethrin, etc.), the various ways at A; thus they are derived from gem- unit C is the asymmetric methine group at C-4 of dimethyl-substituted cyclopropane acids. Natural the 5-membered ring, but in acetylenic, benzylic, and chrysanthemic acid has a trans-isobutenyl group at heterocyclic chrysanthemates, and in tetramethrin, C-3 on the cyclopropane ring, but other substituents 0-acyl is attached to the CH2 group and there is no there produce greater or more rapid action. Even centre of asymmetry in the alcohol. with no substituents at C-3 on the cyclopropane When C is CH, D is the rest of the cyclopen- ring, appropriate alcohols give esters with con- tenolone ring; when C is CH2, D is a unit such siderable insecticidal and knock-down action (Barlow as C6H4, furan, or C - C, so that, in both cases, et al., 1971). the carbon atoms in C, D, and E are co-planar. The exact nature of the substituent A at C-3 on The unit E is -CH2-, -0-, or -CO-, or a sterically the cyclopropane ring is, therefore, not critical for equivalent link, such that an unsaturated centre F toxicity to insects, so this position is probably not (an olefinic or acetylenic bond, a conjugated system directly involved in the poisoning process, in contrast of double bonds, or an aromatic ring) can adopt a to the gem-dimethyl group on C-2. Substituents at position skew to the direction defined by C, D, and E. C-3 on the cyclopropane ring influence the reactivity If no unsaturated centre F is present, as in the methyl- and physical properties of the molecules and partly benzyl chrysanthemates (Barthel, 1961, 1964; Elliott, determine the ease with which the compounds are Ford & Janes, 1970; Elliott, Janes & Jeffs, 1970), detoxified in insect and mammalian systems. dimethrin (Barthel, 1964), trimethrin (Elliott et al., A second structural feature common to all potent 1965a), barthrin (Barthel, 1964), and tetramethrin pyrethroids examined so far is the ability of the (Kato et al., 1964) insect killing power is limited 0-acyl bond in the ester to adopt a position approxi- to a lower level. mately parallel to a line or plane through the units C, Such requirements for insecticidal potency can be D, and E (for a more detailed discussion, see Elliott, satisfied in many structures, and those compounds most active against insects have the appropriate physical properties to penetrate and remain active Fig. 2 inside the organism and the fewest positions at which they are attacked by detoxifying systems. Active but Structural requirements for pyrethrin-like activity easily detoxified compounds are frequently recog- nized by a great increase in toxicity in the presence A CHCOO Cl D EIF| of a synergist to inhibit the mixed-function oxidase system (Casida, 1970). With houseflies, this inhibi- C| CH, tion is conveniently achieved by pretreatment of each insect with a constant large dose (2 of :H,OCO ,tg) .C=CH sesamex, irrespective of the amount of insecticide A=H CH, A = ; CH NC (CH.,),C=CH4,C CHI C,; "'H) CH( (Sawicki & Famham, 1967, 1968). Numerous results obtained in this way are discussed in the present CH,,H, the best example of an outstanding insecticide R S Cl CH,-CH2 paper; ,C etc. that is easily deactivated in houseflies is pyrethrin I, H Cl CH,-CH2 for which a synergistic factor (LD50 without syner- gist/LD50 with synergist) of 350 was obtained C = CH (cyclopentenofnyl derivatives) (A. W. Farnham, personal communication). CH2 (all other derivatives) A specific application of the generalized structure D = C ; -_CC-; . (Fig. 2) is the relationship seen (Elliott, 1967; Elliott et al., 1965a; Elliott, Janes & Pearson, 1967; Elliott CH,CI et al., 1967b) between allethrin (Fig. 3) (Schechter 0 et al., 1949) and substituted benzyl chrysanthemates 2 orr+3- such as piperonyl chrysanthemate (Staudinger & Ruzicka, 1924; Synerholm, 1949); barthrin, 6-chloro- RELATIONSHIP BETWEEN STRUCTURE AND ACTIVITY OF PYRETHROIDS 317 Fig. 3 1970; Elliott, Janes & Jeffs, 1970). This led from Structures of synthetic pyrethroids 4-allylbenzyl chrysanthemate (Fig. 3) and 2,6-dime- thyl-4-allylbenzyl chrysanthemate (Elliott et al., As-CH- CH, 1965a, 1965b; Elliott, 1967) to 4-benzylbenzyl chry- 11 santhemate and 5-benzyl-3-furylmethyl chrysanthe- E"C/ CH,CH=CH, 11 mate (Elliott et al., 1967a, 1967b, 1971). A parallel 0 alletriin line of progress evolved N-hydroxymethyl chrysan- themates such as tetramethrin (Kato et al., 1964), which has rapid knock-down action. Table 2 shows AB-CH, F\1CH2,F§ the relative toxicities of these compounds and of a AB-CH,K R natural pyrethroid to houseflies and mustard beetles. e.g., barthrin dimethrin 4- BBC Since 5-benzyl-3-furylmethyl (+)-trans-chrysan- themate (bioresmethrin) is more active against insects AB-CH, CH,frj_\ than other synthetic pyrethroids, it and related 11CH2CHCH2 compounds were investigated further, as follows. AB-CH, The xanthenylmethyl chrysanthemate (Fig. 4) was ABC bores.me'h in 0 CH, 11 Fig. 4 AB-CH2 CH,CH=CH, A B-CH, N CI Structures of different chrysanthemates CH, 0 Ih DMABC tet-3methrin A8-CH 0°j AB-CH,I 5 - f CHb 3-xanthenylmethyl- 5-benzyl-3-fturylmethyl - piperonyl chrysanthemate (Barthel & Alexander, 1958); dimethrin, 2,4-dimethylbenzyl chrysanthemate AB-CH,bn (Barthel, 1958; Barthel et al., 1959; Piquett & C' Gersdorff, 1958); and other methylbenzyl chrysanthe- 4-benzylbenzyl - H H mates (Elliott et el., 1965a; Elliott, Ford & Janes, Table 2 Relative toxicities of pyrethroids to the housefly and the mustard beetle Compound Relative toxicity a to (+)-trans-chrysanthemate of: other designationstdesignations housefly mustard (+)-pyrethrolone pyrethrin 12 1 600 2,4-dimethylbenzyl alcohol dimethrin 1 9 7 6-chloropiperonyl alcohol barthrin 20 5 4-benzylbenzyl alcohol 4-BBC 15 20 N-hydroxymethyltetrahydrophthalimide tetramethrin 60 58 (+)-allethrolone (+)-(+)-allethrin 100 42 4-allylbenzyl alcohol ABC 190 3 2,6-dimethyl-4-allylbenzyl alcohol DMABC 190 50 5-benzyl-3-furylmethyl alcohol bioresmethrin 1 000 1 000 a Toxicity by topical application, relative to that of bioresmethrin taken as 1 000. 318 M. ELLIOTT synthesized because it resembled the furan super- Table 3 ficially and was also related to 4-benzylbenzyl chry- Relative toxicities of allylbenzyl and benzylbenzyl santhemate, but it had no insecticidal activity. This chrysanthemates to the housefly and the mustard beetle suggested that the toxicity for insects of the 4-benzyl- benzyl compound and, by implication, of the 5-ben- Relative toxicity b to Structure of compound a zyl-3-furylmethyl chrysanthemate, is associated with housefly Imustard beetle the ability to act in a conformation in which the two aromatic rings are non-planar. In substituted AB-CH //3 CH2CH=CH2 85 0.9 diphenylmethanes the benzene rings are rotated by 520 about the axes containing the p, p' positions in opposite senses from a hypothetical planar con- -CH, 13 2.4 formation (data obtained for 3,3'-dichloro-4,4'- dihydroxydiphenylmethane: Sutton, 1958), and H2CH=CH, the lack of steric hindrance in phenylfuryl meth- anes, such as 5-benzyl-3-furylmethyl esters, would make a similar The potency disposition possible.