## 1 Introduction  
Tetrazines are aromatic heterocyclic compounds consisting of a six-membered ring with four nitrogen atoms in their structure, accounting for approximately 68% of the mass in the unsubstituted moiety. These compounds exist as three isomers that differ in the positions of the nitrogen atoms (Figure 1): 1,2,3,4-tetrazine (\(v\)-tetrazine), 1,2,3,5-tetrazine (\(as\)-tetrazine), and 1,2,4,5-tetrazine (\(s\)-tetrazine) [1,2]. The first of the isomers presented in Figure 1, 1,2,3,4-tetrazine (**A**), is highly unstable and is usually not isolated on its own but rather as a structural scaffold in fused systems. Stable monocyclic derivatives of tetrazine **A**, in the form of the corresponding 1,3-dioxides, were obtained for the first time by [PERSON] et al. in 2006 [3]. The literature reveals that cyclic 1,2,3,4-tetrazines exist in equilibrium with the corresponding diazonium salts, formed by the opening of the six-membered heterocyclic ring [4, 5, 6]. This equilibrium state between the respective forms depends strongly on the structure of the fused molecule and the nature of the substituents. Derivatives of 1,2,3,4-tetrazine are being studied, among other purposes,  
Figure 1: Isomeric structures of tetrazine: \(v\)-tetrazine (**A**), \(as\)-tetrazine (**B**), and \(s\)-tetrazine (**C**). for their potential applications in medicine as platelet aggregation inhibitors (**D**, Figure 2), guanylate cyclase activators [7,8], or as high-energy density materials for industrial use (**H**, **I**, Figure 2) [9,10]. The second isomeric form, 1,2,3,5-tetrazine (**B**, Figure 1), and its derivatives are relatively poorly described in the literature. The first example of a monocyclic derivative, 2,6-diphenyl-1,2,3,5-tetrazine, with complete spectroscopic characterization and reactivity in Diels-Alder cycloaddition reactions, was reported in 2019 by [PERSON] and [PERSON] [11]. For this class of compounds, studies predominantly focus on fused polycyclic systems and  
Figure 2: Selected isomeric tetrazine derivatives for different purposes: compounds exhibiting biological activity and optoelectronic properties, high energy density materials, and compounds for use in biorthogonal chemistry. their biological activity, particularly anticancer properties. Mitozolomide (**E**, Figure 2) and temozolomide (**F**, Figure 2) are examples of fused arrangements that have found practical applications in the treatment of malignant melanoma [12,13]. Due to the high nitrogen content in the 1,2,3,5-tetrazine ring, candidates for use in high-energy materials are also being investigated. Polycyclic compounds are designed in which the six-membered unit is fused with other heterocyclic compounds, such as 1,2,4-triazole or pyrazole, or converted into corresponding \(N\)-oxides (**J**, Figure 2) [14,15]. The last of the possible isomers of tetrazine is 1,2,4,5-tetrazine (**C**, Figure 1), also referred to as \(s\)-tetrazine. X-ray structural analysis reveals that this aromatic molecule has a planar structure, with carbon-nitrogen and nitrogen-nitrogen bond lengths of 1.334 A and 1.321 A, respectively [16]. Most 1,2,4,5-tetrazine compounds are characterized by intense colors, often in shades of violet and red. This phenomenon is a direct consequence of weak, forbidden n\(\rightarrow\)\(\pi^{\ast}\) electronic transitions, which are also associated with the fluorescent properties observed in some derivatives [17,18,19]. The 1,2,4,5-tetrazine derivatives are notable for their stability compared to all other possible isomers. Consequently, compounds of this type have attracted significant interest from scientists and are widely studied for their diverse applications. Over the last decade, approximately 300 articles have been published annually, covering various aspects of 1,2,4,5-tetrazine chemistry (Figure 3).