Mesotrione is a new herbicide being developed for the selective pre- and post-emergence control of a wide range of broad-leaved and grass weeds in maize (Zea mays). It is a member of the benzoylcyclohexane-1,3-dione family of herbicides, which are chemically derived from a natural phytotoxin obtained from the Californian bottlebrush plant, Callistemon citrinus. The compound acts by competitive inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD), a component of the biochemical pathway that converts tyrosine to plastoquinone and alpha-tocopherol. Mesotrione is an extremely potent inhibitor of HPPD from Arabidopsis thaliana, with a Ki value of c 6-18 pM. It is rapidly taken up by weed species following foliar application, and is distributed within the plants by both acropetal and basipetal movement. Maize is tolerant to mesotrione as a consequence of selective metabolism by the crop plant. Slower uptake of mesotrione, relative to susceptible weed species, may also contribute to its utility as a selective herbicide for use in maize.

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A series of 1,3,4(2H)-isoquinolinetriones have been found to be fastacting post-emergence herbicides, producing symptoms of desiccation. These redox-active compounds are very potent stimulators of the light-dependent consumption of oxygen at photosystem I in isolated chloroplasts. Pulse radiolysis studies on 2-ethyl-l,3,4(2H)-isoquinolinetrione have shown it to have free-radical properties which could enhance the generation of superoxide radicals in plants.Electrochemical studies further support a redox mediator mode of action for the series. The compounds were found to be unstable towards hydrolysis, and this was considered to be a major factor limiting the overall herbicidal effects. Other parameters, related to uptake and/or translocation, which may limit the full expression of the herbicidal activity of certain compounds, are discussed.which gives rise to the herbicidal activity of the isoquinolinetriones, and describe the results of a preliminary study designed to probe the structureactivity-property relationships within a series of compounds. EXPERIMENTAL METHODS Chemical synthesisThe three general methods used for the preparation of 1,3,4(2H)-isoquinolinetriones are outlined in Fig. 1.Method A involves the oxidation of a homophthalimide derivative to the corresponding isoquinolinetrione. This transformation has been conveniently carried out using a two-step one-pot procedure, in which the homophthalimide is first condensed with N,N-dimethyl-p-nitrosoaniline to give an intermediate imine which is then hydrolysed in situ by the addition of aqueous acid." Method B requires the 49 Pestic. Sci. 0031-613X/95/$09.00 0 1995 SCI. Printed in Great Britain 1,3,4(2H)-isoquinolinetrione herbicides~~~~~~ ~ ~ Average score across weed species, on a scale of 0-9 where 0 indicates no damage five days after treatment and 9 signifies Estimated from a half life of 3.8 min at 40°C. Estimated. Estimated from a measurement made at pH 6. Estimated from a measurement made at pH 5. complete kill (see Section 2.2).' Not measured; this compound could not be obtained >90% pure.

: Examples of pesticides containing an arylsulfur pentafluoride group were made and their biological activities compared to the corresponding trifluoromethyl analogues. A phenylsulfur pentafluoride analogue of the insecticide fipronil, screened against a resistant strain of Musca domestica, showed higher activity than the corresponding trifluoromethyl analogue.

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