The pyrethrins and related compounds; Part XXIV: synthesis, ¹³C‐nuclear magnetic resonance spectra and insecticidal activity of cycloalkyl analogues of fenvalerate

Close isosteres of fenvalerate [( RS )‐α‐cyano‐3‐phenoxybenzyl ( RS )‐2‐(4‐chlorophenyl)‐3‐methylbutyrate], in which cyclopropyl groups replace isopropyl have insecticidal activity close to or greater than the parent compounds, and diminished intravenous toxicity to rats. A direct toxicological relationship of these compounds to fenvalerate itself and to chrysanthemate esters is indicated by the consistently greater activity of esters from one of an enantiomeric pair of acids. Other esters with larger alkyl or cycloalkyl groups, or spiropentane analogues of chrysanthemates are less active insecticides. 13 C‐Nuclear magnetic resonance spectra suggest that in the α‐cyanobenzyl esters there is an intramolecular through‐space interaction in solution. The relationships between the chemical structures of the compounds synthesised and their relative activities to different insect species and toxicity to rats are discussed.