# Synthesis and Biological Studies of New 2-Benzoxazolinone Derivatives as Antibacterial Agents
## 3 Results and Discussion  
### Chemistry  
Since the \(\beta\)-amino acid core is a valuable building block for many biologically active compounds [50] and can be used for the synthesis of pharmacologically important hydrazones and heterocycles, it was thus decided to incorporate a carboxylly1 moiety at the 3-position of the fused benzoxazolin-2-one structure. For this purpose, sodium 3-(2-oxobenzo[d]oxazol-3(2\(H\))-yl)propanoate was applied, which has shown good properties as a plant growth promoter in vitro and in vivo [46; 51]. The initial acid **1** was obtained by the procedure described in [52]. To reach the target acid hydrazide **3** (Scheme 1), the standard synthesis pathway \"acid \(\rightarrow\) ester \(\rightarrow\) hydrazide\" was followed, i.e., after the esterification of the carboxylic acid **1** with methanol in the presence of sulfuric acid as a catalyst, the obtained ester **2** was subjected to hydrazinolysis to afford acid hydrazide **3**. The molecular structures of the obtained compounds **2** and **3** were confirmed by the \({}^{1}\)H, \({}^{13}\)C NMR, IR spectroscopy, and microanalysis data. The \({}^{1}\)H NMR spectrum of **3** exhibited two characteristic proton singlets at 9.11 and 4.16 ppm due to the presence of the CONHNH\({}_{2}\) fragment. Four aromatic protons resonated in the range of 7.12-7.32 ppm. Two triplets of the alkyl chain NCH\({}_{2}\)CH\({}_{2}\) were observed at 2.50 and 4.02 ppm, with a coupling constant of \(J\) = 6.8 Hz. After evaluating the peak values of the \({}^{13}\)C NMR spectrum of molecule **3**, it can be stated that the resonance lines were in good agreement with the target molecular structure. The sulfanilamide derivative **4** was synthesized by reacting acid **1** with sulfanilamide in DMSO at room temperature for 20 h in the presence of triethylamine as a strong base and HBTU, a standard coupling agent for carboxylic acid activation. The additional spectral lines of aromatic (7.67 and 7.73 ppm (2d, \(J\) = 8.5 Hz)), NH (10.39 ppm), and NH\({}_{2}\) (7.24 ppm) protons of the attached sulfanilamide moiety in the \({}^{1}\)H NMR spectrum confirmed the formed structure **4**. In the \({}^{13}\)C NMR spectrum, six new resonances in the aromatic region of the spectrum indicated the presence of an additional phenyl ring, and the presence of a resonance line at 169.20 ppm confirmed the C=O of the amide moiety. Knowing that the incorporation of the benzimidazole structure in a benzoxazole-based molecule influences an increase in its biological activity [53; 54], we decided to explore it in our study. The Phillip's reaction [55] of compound **1** with a corresponding benzene-1,2-diamine in 17.5% hydrochloric acid afforded the scheduled structures **5-7**. The molecular structures of the benzimidazole-containing compounds **5-7** were easily confirmed by the data of the NMR spectra, where the presence of signals between 6.97 and 7.61 ppm integrated for 8 (compound **5**) and 7 (compounds **6** and **7**) protons reflected the presence of two aromatic rings in the synthesized derivatives. The singlets at the characteristic 12.30 (**5**), 12.46 (**6**), and 12.54 (**7**) ppm confirmed the existing NH of benzimidazole heterocycle in the molecules. In the \({}^{13}\)C NMR spectra, the resonances at 151.52 (**5**), 152.82 (**6**), and 152.94 (**7**) ppm approved the existence of the N=C-NH fragment of the benzimidazole core. To incorporate the hydrazzone moiety into the molecule, hydrazide **3** was condensed with a number of aromatic or carbaldehydes in propan-2-ol at reflux temperature. The time required for the completion of the reaction varied from 2 to 4 h, and the yields obtained were from 70.5 to 91.7%. The chemical structure of the synthesized hydrazones **8-23** was established based on the analysis of the \({}^{1}\)H NMR and \({}^{13}\)C NMR spectra. Hydrazones can form four isomers owing to the presence of amide and azomethine groups in their structure. The geometrical isomers originate from the azomethine N=CH group.