In order to obtain antibacterial candidate compounds, eighteen isoxazoleamide derivatives containing phosphonate fragment were designed and synthesized by combining methylisoxazole and phosphonate via amide bonds based on molecular hybridization strategy. Their structures were confirmed by ¹H NMR, ¹³C NMR and elemental analysis. Antibacterial activity tests revealed that these derivatives exhibited varying degrees of inhibition against the tested bacteria, with particularly significant activity against Gram-negative bacteria. Diethyl ((3-(2-chloro-6-fluorophenyl)-5-methylisoxazole-4-carboxamido) (4-hydroxyphenyl)methyl)phosphonate (Ⅲl) stood out as the most potent, showing minimum inhibitory concentrations (MIC) of 2 μg/mL against both E. coli and fluoroquinolone-resistant E. coli (FREC), outperforming the control drug gentamicin. Additionally, compound Ⅲl demonstrated a low tendency to induce resistance. Time-kill curve analysis indicated that at concentrations≥1 MIC, compound Ⅲl exhibited bactericidal effects throughout the growth cycle of E. coli, with the most pronounced effect observed at 4 MIC. Intracellular bactericidal assays confirmed that compound Ⅲl possesses intracellular antibacterial activity against E. coli-infected macrophages. Furthermore, compound Ⅲl showed no toxicity toward murine mononuclear macrophages, suggesting its potential as a promising lead compound for further antibacterial development.

Key words: phosphonateptide, isoxazole, amide derivatives, synthesis, antibacterial activities