Pyrazole-Isoindoline-1,3-dione Hybrid: A Promising Scaffold for 4-Hydroxyphenylpyruvate Dioxygenase Inhibitors

The discovery of 4-hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27) inhibitors has been an active area of research due to their great potential as herbicides for weed control. Starting from the binding mode of known inhibitors of HPPD, a series of HPPD inhibitors with new molecular scaffolds were designed and synthesized by hybridizing 2-benzoylethen-1-ol and isoindoline-1,3-dione fragments. The results of the in vitro tests indicated that the newly synthesized compounds showed good HPPD inhibitory activity with IC50 values against the recombinant Arabidopsis thaliana HPPD (AtHPPD) ranging from 0.0039 mu M to over 1 mu M. Most promisingly, compound 4ae, 2-benzyl-5-(5-hydroxy-1,3-dimethyl-1H-pyrazole-4-carbonyl)isoindoline-1,3-dione, showed the highest AtHPPD inhibitory activity with a K-i value of 3.92 nM, making it approximately 10 times more potent than pyrasulfotole (K-i = 44 nM) and slightly more potent than mesotrione (K-i = 4.56 nM). In addition, the cocrystal structure of the AtHPPD-4ae complex was successfully resolved at a resolution of 1.8 angstrom. The X-ray diffraction analysis indicated that the two carbonyl groups of 2-benzoylethen-l-ol formed a bidentate chelating interaction with the metal ion, while the isoindoline-1,3-dione moiety formed pronounced pi-pi stacking interactions with Phe381 and Phe424. Moreover, water-mediated hydrogen bonding interactions were observed between Asn282 and the nitrogen atoms of the pyrazole ring of 4ae. The above results showed that the pyrazole-isoindoline-1,3-dione hybrid is a promising scaffold for developing HPPD inhibitors.