*2.4. Evaluation of Antioxidant Activity*

The antioxidant effect of newly synthesized derivatives **5**–**8** was studied using three methods based on electron transfer reactions, namely DPPH, ABTS, and ferric reducing power tests. The results were compared with those of the following standard antioxidant agents: ascorbic acid (AA), butylated hydroxyanisole (BHA, a mixture of 2-(*tert*-butyl)- 4-methoxyphenol and 3-(*tert*-butyl)-4-methoxyphenol) and butylated hydroxytoluene (BHT, 2,6-di-*tert*-butyl-4-methylphenol), used as positive controls. The antioxidant capacity of the key starting materials *L*-valine and 4-[(4-bromophenyl)sulfonyl]benzoic acid **3** was also determined.
