## 1 Introduction  
Reactive Oxygen Species (ROS) have a role in cell signaling, including apoptosis, gene expression, and activation of cell signaling cascades and they also have the potential to cause a number of deleterious events in the cells [1]. In normal aerobic cells, enzymatic (glutathione peroxidase, catalase, and superoxide dismutase) and non-enzymatic (antioxidants a-tocopherol, ubiquitinone, beta-carotene, ascorbate, and glutathione) antioxidant systems are those that maintain a balance between oxidative damage and/or antioxidant protection of the cells. Insufficient antioxidant protection and/or excessive production of ROS generate oxidative stress, which has an important role in the development of cancers, aging, and many other diseases [2, 3]. Oxidative damage and mitochondrial dysfunction contribute to the cascade of events leading to degeneration, for example, of the dopaminergic neurons (supported by postmortem brain analyses) which show increased levels of by-products of lipid peroxidation and DNA and RNA oxidation products [4]. Oxidative stress can be effectively neutralized by enhancing cellular defenses by applying higher levels of antioxidants [5]. Thiazoles are an important class of chemical compounds based on a simple five-membered heterocyclic ring. Due to its already known pharmacological activity, a lot of research studies have been done with different thiazole analogues to find possible agents with potential therapeutic use. The main and the most evaluated effects are antioxidant, anti-inflammatory, antimicrobial, antifungal, antiviral, diuretic, neuroprotective, and antitumor and cytotoxic potential [6; 7; 8; 9; 10; 11; 12]. This multiple effects of thiazole derivatives are due to a broad spectrum of pharmacological activities with priority to the following: (1) modulators of nicotinic, glutamate, cannabinoid and/or G protein-coupled receptors; (2) ligands of dopamine and histamine-3 receptors; (3) agonists of the \(\gamma\)-aminobutyric acid receptor and sphingosine-1-phosphate receptor [13]. Because thiazoles are core in the structure of a number of drug formulations, and because of the high potential for pharmacological activity, where, for our work, presumed antioxidant properties evoke specific interest, the present study is dedicated to the assessed antioxidant effect of synthesized bicyclic thiazolopyrimidine derivatives. These compounds are of great importance as potential therapeutic agents alone or in combination with reported drugs in the attenuation of neurological disorders. There are already studies that declared them as potent A\({}_{\text{2A}}\) receptor antagonists [14; 15; 16; 17; 18] particularly, the 2-thioso-thiazolo-pyrimidine derivatives, which not only showed a remarkable potential as A\({}_{\text{2A}}\) receptor antagonists [16] but also exhibited a neuroprotective effect [19; 20]. The C-2 of the 2-thiosothiazole ring of thiazolopyrimidine sandwiched between the heteroatoms sulfur and nitrogen and the exocyclic sulfur at C-2 may act as electron sinks. The 2-thiosothiazole ring of thiazolopyrimidines seemed to resemble to the 2-oxo-furon ring of ascorbic acid. This property gives thiazolopyrimidines potential for antioxidant activity. In a number of neurological diseases an imbalance in the body of oxidants/antioxidants is detected as one of the mechanisms with pathophysiological importance for disease development, therefore, the main goal of our study was to find if some of investigated compounds could be good antioxidant candidates.