2.4.1. Antioxidant Activity Assay by DPPH Method

The results of the study concerning the antioxidant effect evaluation of the new 4-[(4 bromophenyl)sulfonyl]benzoic acid derivatives **5**–**8** by the DPPH method are presented in Table 6. In terms of experimental results obtained by this test, of all the newly synthesized compounds, 4*H*-1,3-oxazol-5-one **6** had the best antioxidant activity with a DPPH inhibition rate of 16.75 ± 1.18%, its effect being better than that of key starting materials (*L*-valine and carboxylic acid **3**), but lower than the standard antioxidants used. This compound was followed by *N*-acylated α-amino acid **5** with a DPPH inhibition percentage of 4.70 ± 1.88%. In contrast, compounds **7a**,**b**, and **8a**,**b** had the lowest scavenging effect values in the range of 1.35–1.82%.


**Table 6.** Results of the assessment of the antioxidant effect of the compounds tested by the DPPH method.

#### 2.4.2. Antioxidant Activity Assay by ABTS Method

The results of the assessment of the antioxidant potential of the compounds tested by the ABTS method are shown in Table 7. The obtained values of the percentage scavenging effect of new compounds **5**–**8** are small in the interval of 0.40–7.66%. Among the new compounds, 1,3-oxazole **8b** showed the best antioxidant activity (7.66 ± 0.71%), followed by α-acylamino ketone **7a** (7.14 ± 1.51%), while compound **7b** had the lowest (0.40 ± 0.27%). By this method, *L*-valine had a better effect (44.71 ± 0.66%) than the new products **5**–**8**, but key raw material **3** showed a lower activity (0.17 ± 0.85%) than these.


**Table 7.** Results of the evaluation of the scavenging effect of the compounds analyzed by the ABTS method.
