History of the discovery of the triketone class of HPPD inhibitors 4-hydroxyphenylpyruvate dioxygenase inhibitor

i

to investigate effect, took soil underneath these plants , extracted , fractionated it. resulting extracts applied soil flats containing watergrass (echinochloa crus-galli) indicator species @ high application rate of 100 lb./acre. there herbicidal effect test, extracts developed on preparative thin layer chromatographic sheet. same echinochloa crus-galli seeds placed on sheet , germinated. active ingredient identified bleaching symptoms on test species.

the area herbicidal activity seen extracted , submitted isolated active ingredient ken cheng @ western research center who, using proton nmr, ir , mass spectrometry identified structure being of leptospermone (iv) known natural product had been derived steam-volatile oils of australian plants, had never been cited having biological activity

he approached chemist, ron rusay, @ western research center independently synthesized compound , submitted further greenhouse testing. these tests showed had modest herbicidal activity against grass weeds @ high application rate of 100 lbs./acre. prepared series of analogs in alkanoyl group modified , patent obtained on series of compounds. these compounds had similar, weak herbicidal activity similar found lead compound. because of weak herbicidal activity, work on other analogs not pursued.

shortly after this, wrc chemist, bill michaely, synthesized aroyl triketone (v) unexpected byproduct when attempting synthesize sethoxydim (vi) analog.

while compound did not have herbicidal activity, did show activity in screen designed show antidotal activity toward other herbicides. when attempting optimize antidote activity, several aryl substituted analogs prepared. discovered compounds ortho substituent had herbicidal activity no antidote activity. observation coupled knowledge of herbicidal activity of earlier leptospermone analogs pivotal in formulating idea of potential toxophore class of herbicides.

a small task force consisting of david lee, bill michaely , don james prepared number of substituted triketones chloro-, bromo- , methyl-substitutents in ortho position. biological activity remained modest , task force disbanded after short while.

one working hypothesis active ingredient in these triketones cyclized tetrahydroxanthenones(vii). bill michaely prepared several of these, herbicidal activity remained modest , work in area terminated. not until david lee able show these compounds in equilibrium 2-hydroxy triketones(viii) trapping intermediate methyl iodide reason biological activity understood.

david lee had strong background in quantitative structure–activity relationships (qsar) after post-doctoral year professors manfred wolff , peter kollman used prophet system. looking on qsar (quantitative structure–activity relationships) of triketones, david lee saw potential discrepancy in existing structure-activity analysis. other 2-chloro-4-nitro substitution pattern, no other triketones electron-withdrawing substituents in 4-position had ever been prepared. hypothesized activity of triketones correlated electron withdrawing ability of substituents. activity of 2-chloro-4-nitro analog outlier, , theorized perhaps nitro group being reduced in vivo. 4-methylsulfonyl group prepared test hypothesis, , become commercial herbicide mikado (ii) prepared. prospect of improving biological activity new aromatic substitution patterns totally rejuvenated work on triketones.

a key discovery in preparation of these compounds finding jim heather in wrc process development group acetone cyanohydrin catalyst preparation of o-chloro analogs. use of catalyst allowed first time production of o-nitro triketones.

at point large effort on synthesis of analogs commenced david lee coordinating effort. key chemists participating in charles carter, bill michaely, hsiao-ling chin, nhan nguyen , chris knudsen, although @ 1 time every synthesis chemist @ wrc worked on project. within relatively short time, major progress made in optimizing substitution combinations. sc-0051 (sulcotrione) synthesized , tested in sept. – oct. 1983, , sc-1296 (mesotrione) , sc-0735 (nitisinone) both synthesized , tested in 1984. triketones in widespread university field trials in 1985. first of triketone patents published in 1986.

with long history of working bleaching herbicides inhibit phytoene desaturase including commercial herbicide flurochloridone(ix), there considerable interest find these compounds not inhibit phytoene desaturase in vitro. fact phytoene desaturase inhibitors typically have high log p, whereas triketones not, further suggested different mode of action.

there had been toxicological concern corneal , paw lesions observed rats had repeatedly dosed triketone. several chemists , toxicologists came upon paper describing similar ocular, not skin, lesions inhibitors of tyrosine hydroxylase. linda mutter in wrc toxicology section used spot test tyrosine on urine of treated rats , had positive results. plasma tyrosine analysis further confirmed buildup of tyrosine in treated rats.

work on mode of action , toxicology of triketones took on broader range of interactions when stauffer chemical purchased ici in june 1987. ici split off pharmaceutical , agrochemical businesses zeneca , syngenta formed in 2000 merger of novartis agribusiness , zeneca agrochemicals.

as part of toxicology studies, martin ellis @ ici central toxicology laboratory identified triketone inhibition of tyrosine catabolism in rat liver , found tyrosine hydroxylase not inhibited triketones. furthermore found urine of rats treated iii showed elevated levels of both p-hydroxyphenylpyruvate , p-hydoxyphenyllactic acids. these results suggested p-hydroxyphenylpyruvate dioxygenase (hppd) enzyme inhibited, fact confirmed s. lindstedt. further tests established hppd enzyme inhibited in plants mammals.