Design, Synthesis, and Bioactivity of Novel Quinazolinone Scaffolds Containing Pyrazole Carbamide Derivatives as Antifungal Agents

In this study, 32 novel quinazolinone-scaffold-containing pyrazole carbamide derivatives were designed and synthesized in a search for a novel fungicide against Rhizoctonia solani. Single-crystal X-ray diffraction of 3-(difluoromethyl)-N-(2-((6,7-difluoro-4-oxoquinazolin-3(4H)-yl)methyl)phenyl)-1-methyl-1H-pyrazole-4-carboxamide (6a11) confirmed the structure of the target compounds. The in vitro antifungal activity of the target compounds against R. solani was evaluated at 100 µg/mL. The structure–activity relationship analysis results revealed that antifungal activity was highest when the substitution activity was at position 6. Moreover, the position and number of chlorine atoms directly affected the antifungal activity. Further in vitro bioassays revealed that 6a16 (EC50 = 9.06 mg/L) had excellent antifungal activity against R. solani that was higher than that of the commercial fungicide fluconazole (EC50 = 12.29 mg/L) but lower than that of bixafen (EC50 = 0.34 mg/L). Scanning electron microscopy), 7.33 (SEM) revealed that N-(2-((6,8-dichloro-4-oxoquinazolin-3(4H)-yl)methyl)phenyl)-3-(difluoromethyl)-1-methyl-1H-pyrazole-4-carboxamide (6a16) also affected the mycelial morphology. The findings revealed that molecular hybridization was an effective tool for designing antifungal candidates. Meanwhile, pyrazolecarbamide derivatives bearing a quinazolinone fragment exhibited potential antifungal activity against R. solani.