Herbicides cinmethylin

GraysonBT, Webb JD, Factors offecting the performance and crop phytotoxicity of a new rice herbicide, cinmethylin. 1. Effects of water depth and soil type on the distribution and uptake of anmethylin by transplanted and direct-seeded rice,... 

The rice herbicides cinmethylin (Figure 2.23), mefenacet (Figure 2.24) and daimuron and methyldymron (Figure 2.25) also interfere with meristematic activity in susceptible species. 

Figure 2. Effect of 1,4-cineole (circles), cis-2-hydroxy-1,4-cineole (triangles) and the commercial herbicide cinmethylin (squares) on the activity of asparagine synthetase from lupin. The dotted line represents 50% inhibition of enzyme activity.

The herbicide cinmethylin has been reported to be derived Irom the monoterpene backbone of the cineole class of natural products. Indeed, cinmethylin is identical to 1,4-cineole, with the addition of a benzyl ether moiety that was added to lower the volatility of the natural product. Cinmethylin was in fact developed by Shell Chemical during a biorational synthetic program to discover new insecticides that targeted the glycerol-3-phos-phate shuttle. A phytotoxic dioxalane intermediate was identified. Optimization of the monocyclic structures led to rigid bicyclic structures with improved herbicidal activity. Some of the bicyclic structures possessed the basic backbone of the monoterpene cineoles found in nature. 

Endothall is a structural analogue of cantharidin (Fig. 20). Its mode of action is apparently inhibition of protein phosphatase(s). A struc-ture/activity relationship study demonstrated that the presence of the oxygen bridge and the location of the two carboxylic groups play important roles in the activity of the molecules. The unusual 7-oxabicyclo[2.2.1]heptane ring of endothal is similar to that of cinmethylin, another natural product-like herbicide for which the mode of action has also eluded scientists for many years. 

Vaughn, S.F. and Spencer, G.F. (1996) Synthesis and herbicidal activity of modified monoterpenes structurally similar to cinmethylin. Weed Sci. 44, 7-11... 

Cineole, 13, is a natural herbicide [73]. Its hydroxy derivative 14 (2-hydroxy-1,4-cineole 1,4-epoxy-p-menthane-2-ol) is a constituent of oil from rhizomes of Ferula jaeschkeana [74]. Its 2-methylbenzyl ether 15 (cinmethylin) is a preemergence grass herbicide [75, 76]. Alcohol 14 can be prepared by microbial hydroxy-lation of 13 [77]. This also produces ketone 16 and its enantiomer [78]. The fragrance of ketone 16 and isomeric l-isopropyl-4-methyl- 7-oxabicyclo[2.2.1] heptan-2-one is very similar to that of 14 and menthone [79]. Mullilam diol 17, a dihydroxy derivative of 13, has been isolated from Zanthoxylum rhetsa, a plant that exhibits antibiotic activity which is prescribed in dyspepsia and diarrhea. The eight-carbon system rengyoxide has been found in Forsythia suspensa fruits [80] (Fig. 4). 

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