## 1 Introduction  
The formation of reactive oxygen species (ROS), including free radicals and peroxides of both inorganic and organic origin, is a result of normal cells functions. An overproduction of ROS causes redox imbalance of the cell's homeostasis and triggers oxidative stress. This state is very harmful for the cell, since ROS are responsible for the oxidation of cellular macromolecules such as carbohydrates, lipids, proteins, and DNA [1]. Oxidative stress is the cause or an important component of many severe diseases, including hypertension [2], atherosclerosis [3], Alzheimer's disease [4], diabetes [5], and infertility [6], and is also one of the components of chronic fatigue syndrome [7]. The endogenous antioxidant defense system of the cell controls the balance between produced and eliminated ROS through the use of different enzymes [8, 9]. However, to complement endogenous antioxidant defense systems' exogenous antioxidants, both natural and synthetic antioxidants are used. Natural antioxidants are represented mainly by vitamin C, vitamin E, carotenoids, and polyphenols [10]. Many classes of compounds have established themselves as synthetic antioxidants: arylamines [11], alkylbenzotrizoles [12], alkylaminothiadiazole [13], flavones and flavonoids [14, 15], hydroxycumarins [16, 17], and phenols [18, 19, 20, 21]. The mechanism of antioxidants' action in different systems has been studied for several decades, but no universal approach for the development of new antioxidant molecules has been identified yet. The most up-to-date rational design approach for effective antioxidants is to combine in one molecule fragments that interfere with the action of various oxidizing agents. For example, phenols can be used for their ability to interrupt radical reactions. Likewise, S- and N-containing heterocycles can be useful to decompose hydroperoxides and chelate metals [22]. Thus, hydrazones [23,24] and thiocarbohydrazones [25,26] with phenol fragments exhibit strong antioxidant activity. Of note is that antioxidants containing heterocyclic fragments in their chemical structure are also of considerable interest. 1,2,4-Triazole-5-thione **I** (Figure 1) showed a higher inhibition effect of 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radicals compared to standard antioxidant butylated hydroxytoluene (BHT), and demonstrated the in vitro inhibition of lipid peroxidation, decreasing the Fe\({}^{2+}\) ion-induced lipid peroxidation of essential oils [27]. Pyrazole **II** showed activity in inhibiting the oxidation of DNA induced by 2,2\({}^{\prime}\)-azobis(2-amidinopropane) dihydrochloride (AAPH) [28]. Compound **III** was found to have high radical scavenging activity, reduced the ROS levels induced by H\({}_{2}\)O\({}_{2}\), and was also found to exhibit strong activity against lung carcinoma and human promyelocytic leukemia [29]. Very few systematic studies have been conducted on structure-antioxidant activity correlations in thiazoles with phenolic fragments [30,31], but the available data show the promising antioxidant activity of this class of compound. The key intermediate for the preparation of thiazoles and many other classes of heterocycle are thiosemicarbazones, which also exhibit antioxidant properties. The relationship between the ability of thiosemicarbazone to inhibit oxidation and the coplanarity of several conjugated fragments of the molecule is known; in addition, in the case of thiosemicarbazones with a phenolic substituent, the ability to intercept radicals is influenced by the possibility of forming a hydrogen bond between the phenolic OH and the azomethine group. To investigate the influence of these factors on the antioxidant properties of the substances, a conformational analysis can be performed [32,33,34]. The aim of this study was to synthesize new antioxidants containing both phenols and a thiosemicarbazone group or thiazole and a thiazolinone ring.