## 3 Results and Discussion
### Antioxidant Activity  
For synthesized compounds, antioxidant activity assessment was performed via the ABTS radical removal method. In the assay, ABTS radical cations are generated from the ammonium ABTS salt using a strong oxidizing agent. The radical cation has an intense absorption band at 734 nm; when an antioxidant is added, the concentration of radical cations decreases, which is determined photometrically [46, 47, 48]. The results are given in Figure 5 in terms of inhibition percentages calculated against concentration using BHT as the standard. Most of the compounds showed better results compared to the standard. The exceptions were samples **2c**, **3b**, **3c**, **4a**, **4d**, and **5d**. Compounds **1c**, **4c**, and **5a-c** exhibited the best results. In the series of obtained thiosemicarbazones **1a-d**, the best results were shown by the 2,6-di-tert-butylphenol derivative **1c**, which can be explained by the formation of a stable di-_tert_-butylphenoxyyl radical. Vanillin's thiosemicarbazone **1b** showed the poorest result. The antioxidant activity of diatomic phenol derivatives **1a** and **1d** turned out to be intermediate. This can be explained by the interaction of the phenolic hydroxyl group and the CH=N fragment at the _ortho_-position. Formylresorcinol's thiosemicarbazone **1a** showed slightly higher activity compared to the butylated pyrocatechol derivative **1d** due to the coplanarity of the **1a** molecule. Among thiazoles with phenyl and coumaryl substitutes, the same pattern was observed as for thiosemicarbazones: compound **2b**, based on 2,6-di-_tert_-butylphenol exhibited the highest activity, and the effectiveness of thiazole **2a** based on phenylresorcinol was higher than for butylated pyrocatechol derivative. In general, the activity of derivatives of this series was lower than that of the corresponding thiosemicarbazones. In the case of thiazoles with a coumarin fragment, the resorcinol derivative **2d** also turned out to be more active than the pyrocatechol derivative **2e**. The introduction of the coumarin fragment increased the antioxidant activity compared to thiazoles containing a phenyl substituent. For thiazoles prepared from bromodimedone, completely different results were observed: compounds based on diatomic phenols **3a** and **3d** showed the highest efficiency, but just as in previous cases, the resorcinol derivative **3a** turned out to be more active. In case of thiazolidinones **4** and **5**, compounds with 2,6-di-_tert_-butylphenol showed the best results, while vanillin derivatives **4b** and **5b** exhibited intermediate activity, formyl-resorcinol derivatives **4a** and **5a** demonstrated comparable activity, and the pyrocatechol derivative **4d** and **5d** again showed the lowest results. In general, compounds with thiazolidinone moieties and carboxyl group **5** have higher antioxidant activity compared to other derivatives. To assess the electron-donating capacity of prepared compounds, PFRAP assay was used. In this assay, ferricyanide reduces to ferrocyanide, and a green-blue color of the solution appears because of the Perl's Prussian blue formation [49,50]. The resulted compound has an absorption peak at \(\lambda\) = 700 nm. The results are presented in Figure 6.  
The results of the PFRAP assay displayed a stronger electron-donating capacity for thiosemicarbazone **1b**, thiazoles **2d**, **2e**, and **3c**, and thiazolidinones **4d** and **5d** compared to  
Figure 5: ABTS\({}^{+}\) (2,2\({}^{\prime}\)-azino-bis(3-ethylbenzothazoline-6-sulfonic acid cation radical) scavenging activity of the compounds prepared. Figure 6: Ferric ion-reducing capacity of the compounds prepared. BHT. An intermediate activity was found for thiazoles **2a** and thiazoltionnes **4b**, **4c**, **5a**, and **5b**. The rest of the synthesized compounds displayed moderate or low antioxidant capacity compared to that of BHT. Among thiazoles **2a-c** with a phenyl fragment, the best antioxidant activity was observed for **2a**, which contained a resorcinol fragment.