| Abstract
| - Four series of new pyrazoles, namely, 5 4-carboxypyrazolo-3-tert-butylcarboxamide and 6 4-carboxypyrazolo-3-cyclopropylcarboxamide derivatives and 10 pyrazolo[3,4-d][1,3]thiazine-4-one and9 pyrazolo[3,4-d][1,3]thiazine-4-thione derivatives, were synthesized and screened as potentialinhibitors of photosynthetic electron transport. The structures were confirmed by 1H NMR, elemental,and IR analyses. Their biological activity was evaluated in vitro as the ability to interfere with thelight-driven reduction of ferricyanide by isolated spinach chloroplasts. Only a few compounds exhibitedexcellent inhibitory properties in the micromolar range, comparable to those of commercial herbicidessharing the same target, such as diuron, lenacil, and hexazinone. Nevertheless, most of the remainingmolecules exerted a remarkable inhibition in the millimolar range. Combined with previous results on6 pyrazolo[1,5-a][1,3,5]triazine-2,4-dione and 4 pyrazolo[1,5-c][1,3,5]thiadiazine-2-one derivatives,these data allowed a comprehensive analysis of structure−activity relationship. Molecular modelingstudies were undertaken to rationalize the structural determinants of activity in terms of shape, size,and molecular fields. Results suggested that the inhibitory potential of these compounds is associatedmainly with their electrostatic properties. Keywords: Herbicides; photosynthetic electron transport inhibitors; pyrazole derivatives; structure−activity relationship
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