# Synthesis and Biological Studies of New 2-Benzoxazolinone Derivatives as Antibacterial Agents
## 3 Results and Discussion
### Biology
#### 3.2.1 Evaluation of the Antibacterial Activity by the Determination of MIC and MBC Values
Hydra zones **9** and **11-13** with halophenyl substitutions inhibited the growth of the bacillus at a concentration of 62.5 \(\mathrm{\SIUnitSymbolMicro m}\)L. The incorporation of the 4-bromophenyl (**14**), the bulkier 2-chloro-5-nitrophenyl (**17**), and the very bulky 3,4,5-trimethoxyphenyl (**19**) moieties reduced the growth inhibition properties of these compounds, resulting in an MIC of 250 \(\mathrm{\SIUnitSymbolMicro m}\)L.  
The MBC values of hydrazide **3** and its first-series hydrazones for _S. aureus, E. coli_, and _S. enteritidis_ were identical and equal to 500 \(\mathrm{\SIUnitSymbolMicro m}\)L. The higher range (125-500 \(\mathrm{\SIUnitSymbolMicro m}\)L) of MBC values was when treating _B. subtilis_. Hydrazones **9**, **11-13**, and **15**, bearing an electron-withdrawing halo and nitro substituents on the phenyl rings in the molecules, killed \(\geq\)99.9% of the bacillus at a concentration of 125 \(\mathrm{\SIUnitSymbolMicro m}\)L, and in this case, it was found to be the best bactericidal result in this part of the bactericidal evaluation. Hydrazones **8**, **10**, **16**, and **18**, with unsubstituted phenyl, 2,4-difluoro, 4-dimethylamino, and 2,3,4-trimethoxy substitutions on the additional phenyl rings, respectively, retained the bactericidal effect of the initial hydrazide **3** and killed \(\geq\)99.9% of the _B. subtilis_ strain at a two-fold higher concentration with an MIC of 250 \(\mathrm{\SIUnitSymbolMicro m}\)L.  
The comparative evaluation of the antibacterial activity of hydrazide **3** and its second series of hydrazones **20-23** showed a higher range of results. Identical or close (the difference \(\pm\)1 log\({}_{2}\) dilution) MIC and MBC values were determined for hydrazones **20-22** for all tested bacteria strains. The MIC values of compound **3** and its hydrazones **20-22** for _S. enteritidis_ and _S. aureus_ were identical and equal to 125 \(\mathrm{\SIUnitSymbolMicro m}\)L, while 2-furyl substituted hydrazone **22** was effective against _B. subtilis_ and _E. coli_ strains after the treatment with the 1-2 log\({}_{2}\) lower concentration of 62.5 \(\mathrm{\SIUnitSymbolMicro m}\)L. The hydrazone **23** with the incorporated 5-nitro-2-thienyl fragment was more effective when compared with hydrazide **3** against all of the tested bacteria strains, and the MIC values were 62.5 \(\mathrm{\SIUnitSymbolMicro m}\)L for _S. aureus_, _B. subtilis_, and _S. enteritidis_, while 15.6 \(\mathrm{\SIUnitSymbolMicro m}\)L was found for _E. coli_. Compared to structure **22**, the attached electron-withdrawing nitro group in molecule **23** highly increased the inhibition properties of the compound and led to improved action against _S. aureus_, _E. coli_, and _S. enteritidis_. The MBC values of hydrazide **3** and its second-series hydrazones for _S. aureus_, _E. coli_, and _S. enteritidis_ were identical and equal to 500 \(\mathrm{\SIUnitSymbolMicro m}\)L. The higher range (125-500 \(\mathrm{\SIUnitSymbolMicro m}\)L) of MBC values was found when treating the _B. subtilis_ strain. The 2-furyl- and 5-nitro-2-furylhydrazones **22** and **23** killed \(\geq\)99.9% of the bacillus at a two-fold lower concentration of 125 \(\mathrm{\SIUnitSymbolMicro m}\)L, whereas 5-nitro-2-thienylhydrazone **21** required 500 \(\mathrm{\SIUnitSymbolMicro m}\)L to produce the same bactericidal effect in comparison with the MBC of hydrazide **3**. The comparative evaluation of the antibacterial activity of compound 3 and its azoles **24-26** shows that _B. subtilis_ and _S. enteritidis_ were the most resistant to the action of these compounds. Identical 125 \(\mathrm{\SIUnitSymbolMicro m}\)L MIC values were determined for compound **3** and its azoles **24**, **25**, and **26** for the tested bacteria strains of _B. subtilis_ and _S. enteritidis_.