AbstractBACKGROUND: The novel natural product cinnacidin was isolated from a fungal fermentation extract of Nectria sp. DA060097. The compound was found to contain a cyclopentalenone ring system with an isoleucine subunit linked through an amide bond. Initial biological characterization of cinnacidin suggested promising herbicidal activity.RESULTS: Two synthetic analogs, (2S,3S)‐2‐[(3RS,3aSR,6aRS)‐3‐methoxy‐4‐oxo‐3,3a,4,5,6,6a‐hexahydropentalen‐1‐ylcarbamoyl]‐3‐methylvaleric acid and benzyl (2S,3S)‐2‐[(3RS,3aSR,6aRS)‐3‐methoxy‐4‐oxo‐3,3a,4, 5,6,6a‐hexahydropentalen‐1‐ylcarbamoyl]‐3‐methylvalerate, were prepared for further characterization, and their herbicidal activities were compared with that of cinnacidin.CONCLUSIONS: The synthetic compounds were highly phytotoxic on a range of weeds. Based on the symptoms in treated plants, the mode of action of these compounds is suggested to be similar to that of coronatine and jasmonic acid. Coronatine was more active against warm‐season grasses, while the cinnacidin benzyl ester analog was more effective on cool‐season grasses. In a seedling growth bioassay conducted on bentgrass, the cinnacidin analog was equivalent in activity to coronatine. Copyright © 2008 Society of Chemical Industry