## 1 Introduction  
Oxadazoles belong to the group of heterocyclic compounds which contain two nitrogen atoms and one oxygen atom. Due to the position of heteroatoms in the ring, they comprise few isomers [1]. 1,2,4-Oxadazoles and 1,3,4-oxadiazoles belong to the group of the most frequently studied compounds due to their high biological activity and good stability [2]. These derivatives have anti-inflammatory, analgesic [3], anticancer [4], antibacterial [5], antifungal [6], antiviral [7], or blood-pressure-lowering effects [8]. Oxadazoles are also successfully used in agriculture as herbicides, insecticides, and plant protection agents against bacteria, viruses, and fungi (Figure 1) [9; 10; 11; 12]. Conjugated macro-cyclic arrangements containing 1,3,4-oxadiazol core exhibit interesting electron-transfer or luminescent properties and are applied in the production of different types of conducting systems including laser dyes, scintillators, optical brighteners, or organic light-emitting diodes [13; 14; 15; 16]. Furthermore, in materials science, compounds of this type are used in the production of blowing agents, heat-resistant polymers, and anti-corrosion agents [17; 18; 19; 20]. 1,3,4-Oxadazoles can be obtained by a number of cyclization methods. The most popular procedures include the cyclodehydration reaction of diacylhydrazines with sulfuric acid [21], though chloride [22], polyphosphoric acid [23], phosphoryl chloride [24], trifluoromethane anhydride [25], phosphorus pentoxide [26], or triphenylphosphine derivatives [27]. Other methods of effective synthesis of 1,3,4-oxadiazolzoles also include the oxidative cyclization of \(N\)-acylhydrazones under the action of bromine [28], potassium permanganate [29], 2,3-dichloro-5,6-dicyano-1,4-benzoquinone [30], or cerium ammonium nitrate [31]. Micronutrient chelates belong to complex compounds in which the organic ligand is connected to the metal ion using more than one coordination center. In modern agriculture, they are successfully used as effective fertilizers. One of the main limitations in plant cultivation is the various types of micronutrient deficiencies, which require the application of substances containing a given element in their structure. The most commonly used micronutrients include copper (Cu\({}^{2+}\)), manganese (Mn\({}^{2+}\)), iron (Fe\({}^{3+}\)), and zinc (Zn\({}^{2+}\)). On the other hand, organic compounds that may act as potential ligands usually contain electron-donating groups, including aminopolycarboxylic functionality. The currently used compounds with metal cation complexing properties comprise ethylenediaminetetraacetic acid (EDTA), ethylenediamine-di-(\(o\)-hydroxyphenylacetic acid) (EDDHA), ethylenediamine-di-(5-carboxy-2-hydroxyphenyl)acetic acid (EDDCHA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), glucoheptanoic acid, and citric acid [33]. An interesting group of new organic ligands with complexing properties could constitute heterocyclic arrangements such as 1,3,4-oxadiazoles, substituted with the bis(carboxymethyl)amino group. It seems to be quite probable that these hybrids, unknown in the literature to date, could exhibit even stronger properties toward complexation due to the presence of ring oxygen and nitrogen heteroatoms with lone electron pairs. Previously, we elaborated the methodology for the construction of symmetrical derivatives of 2,5-dialkyl-1,3,4-oxadiazoles substituted with bis(carboxymethyl)amino groups at the terminal alkyl positions [34]. This study initiated a broad research program devoted to the synthesis and properties of new five-membered heterocyclic ligands, which may be of interest to researchers in agriculture who are still looking for new, effective micronutrient chelates. The work presented here describes a convenient and practical methodology for preparation of 5-phenyl-1,3,4-oxadiazoles connected to the bis(carboxymethyl)amino group via benzylene and alkylene linkers.