Chemical Behaviour and Herbicidal Activity of Cyclohexanedione Oxime Herbicides

Benzoximate (I; Fig. 1) is an acaricide developed by Nippon Soda in 1971 (Iwataki, 1992). However, scientifics of the company observed that some benzohydroxamates showed weak herbicidal activity. After much synthetic developments, a new lead compound, an ethoxyimino dehydroacetic acid derivative (II; Fig. 1), showed a strong pre-emergence herbicidal activity against annual grass weed without any effects towards broadleaf plants. Further developmental research was performed on the cyclohexanedione skeleton to develop a new post-emergence herbicide. It was observed that the ethoximine group between the two keto groups was essential for the herbicidal activity (III; Fig. 1). Besides, when hetero atoms were introduced in the ring, the compounds showed high preemergence activity. When the ring was formed by carbons, so-called cyclohexane derivatives, the compounds showed high preand post-emergence activities. Therefore, the synthetic research was focused towards the substituents on the cyclohexanodione skeleton. The activity was higher when side chain substituents R1 and R2 (Fig. 1) were alkyl groups. As for the ring substituents, mono i-Pr and germinal dimethyl at the R3 and R3 position and ciano and methoxycarbonyl groups at the R4 position provided the maximum activity. This way, alloxydim–sodium was discovered and introduced in the market in 1978.