4-Hydroxyphenylpyruvate
dioxygenase (HPPD, EC 1.13.11.27) is one
of the most promising herbicide targets for the development of agricultural
chemicals owing to its unique mechanism of action in plants. We previously
reported on the co-crystal structure of Arabidopsis
thaliana (At) HPPD complexed with
methylbenquitrione (MBQ), an inhibitor of HPPD that we
previously discovered. Based on this crystal structure, and in an
attempt to discover even more effective HPPD-inhibiting herbicides,
we designed a family of triketone-quinazoline-2,4-dione derivatives
featuring a phenylalkyl group through increasing the interaction between
the substituent at the R1 position and the amino acid residues
at the active site entrance of AtHPPD. Among the
derivatives, 6-(2-hydroxy-6-oxocyclohex-1-ene-1-carbonyl)-1,5-dimethyl-3-(1-phenylethyl)quinazoline-2,4(1H,3H)-dione
(23) was identified as a promising compound. The co-crystal
structure of compound 23 with AtHPPD
revealed that hydrophobic interactions with Phe392 and Met335, and
effective blocking of the conformational deflection of Gln293, as
compared with that of the lead compound MBQ, afforded
a molecular basis for structural modification. 3-(1-(3-Fluorophenyl)ethyl)-6-(2-hydroxy-6-oxocyclohex-1-ene-1-carbonyl)-1,5-dimethylquinazoline-2,4(1H,3H)-dione
(31) was confirmed to be the best subnanomolar-range AtHPPD inhibitor (IC50 = 39 nM), making it approximately
seven times more potent than MBQ. In addition, the greenhouse
experiment showed favorable herbicidal potency for compound 23 with a broad spectrum and acceptable crop selectivity against
cotton at the dosage of 30–120 g ai/ha. Thus, compound 23 possessed a promising prospect as a novel HPPD-inhibiting
herbicide candidate for cotton fields.