## 2 Results  
The target 1,3,4-oxadiazoles bearing bis(carboxymethyl)amino groups were synthesized according to a few-step procedure. The first step was the preparation of benzhydrazide (**2**) by means of methyl benzoate (**1**), hydrazine hydrate and ethanol, used in this case as a solvent. The reaction proceeded at reflux for about 10 h (Scheme 1) [35]. The main product was obtained in a high yield of 90% without the need of purification. In the next step, the benzhydrazide (**2**) formed was reacted with commercially available 4-bromomethylbenzoyl chloride (a) or alkanecarboxylic acid chlorides (b-e), which differed in the length of the alkyl chain and contained a bromine substituent in the terminal position (Scheme 1). We applied similar reaction conditions which were elaborated previously while working on the preparation of symmetrical \(N\),\(N^{\prime}\)-diacylhydrazine derivatives [34]. The reaction was conducted at room temperature using diethyl ether as a solvent in which the benzhydrazide substrate was suspended. Due to the high reactivity of the acid chlorides, they were carefully added to the reaction mixture. Sodium carbonate dissolved in a small amount of water was used as the base to neutralize the evolving hydrogen chloride. The crude products were recrystallized by means of methanol or ethanol. The yields of the resulted hydrazine derivatives (**3a-e**) varied from 46% to 78% (Table 1, Entries 1-5). Figure 2: Key intermediates in the synthesis of new 5-phenyl-1,3,4-oxadiazoles containing bis(carboxymethyl)amino group: unsymmetrical \(N\),\(N^{\prime}\)-diacylhydrazines and iminodiacetic acid. The formation of 1,3,4-oxadiazole derivatives (**4a-e**) consisted in the reaction of \(N\),\(N^{\prime}\)-diacylhydrazines **3a-e** with a range of cyclodehydrating agents such as phosphorus pentachloride (PCIs), thionyl chloride (SOCI2), perchloric acid (HClO4), phosphorus pentoxide (P2O5), and phosphoryl trichloride (POCl3). Among them, phosphoryl trichloride POCI3, acting both as a reagent and a solvent, proved to be an effective agent for most of the products (**4a-b**, **4d-e**). Only in the case of 2-(2-bromoethyl)-5-phenyl-1,3,4-oxadiazole (**4c**) was a better yield obtained with the use of phosphorus pentoxide P2O5 in toluene. Regardless of the method used, the reaction mixture was heated to 70 \({}^{\circ}\)C, and the progress of the reaction was controlled by TLC (Scheme 2). Heating to a higher temperature most likely caused decomposition of the formed 1,3,4-oxadiazole product. The duration of the reaction depended on the type of substrate used. The best result was achieved for the cyclization reaction of the \(N\),\(N^{\prime}\)-diacylhydrazine derivative containing the 4-(bromomethyl)phenyl group (**3a**), where the time needed for complete reaction of the substrate was about 3 h (Table 2, Entry 1). In the case of transformations making use of \(N\),\(N^{\prime}\)-diacylhydrazines-bearing bromoalkyl substituents (**3b-e**), reaction times were considerably longer, and even reaching 8 h (Table 2, Entries 2-5). Thus, we decided to carry out the same reactions under microwave irradiation (Table 2, Entries 6-10). Conducting the reaction using a microwave reactor was based on preliminary optimization of the process parameters. For this purpose, two different modes offered by the CEM device software (Synergy 1.58, Version: 201A19) were used: SPS and Dynamic. During the process optimization, the impact of the microwave heating power was also tested. It was found that setting the power higher than 100 W (130, 150, 200 W) resulted in too intense heating of the mixture and the release of POCI3 from the reaction vessel. The SPS method, which is based on intensive heating of the reaction mixture to the indicated temperature, was ineffective due to the temporary activation of microwave radiation and too large temperature changes.