## 3 Results and Discussion  
The target compounds **1-5** were prepared as it is shown on Schemes 1 and 2. Initially, the number of thiosemicarbazones was synthesized by the condensation of thiosemicarbazide and series of aldehydes: formylresorcinol--**1a**; vanillin--**1b**; 2,3-dihydroxy-4,6-di-_tert_-butylbenzaldehyde--**1c**; and 4-hydroxy-3,5-di-_tert_-butylbenzaldehyde--**1d**. The IR spectra of compounds **1a-d** contain absorption at 1587-1625 cm\({}^{-1}\), corresponding to the C=N stretching frequency, and signals near 3450-3200 cm\({}^{-1}\), which correspond to N-H symmetric and asymmetric stretching vibrations, which correlates with calculated and literature data [43]. In the \({}^{1}\)H NMR spectrum of compounds **1a-d**, the protons of the formyl group of the starting compounds were not presented, and in the region 7.94-8.91 ppm protons of CH=N moiety were observed. To ascertain the prevalence of one particular conformer, a potential energy surface (PES) scan was applied using the B3 LYP/6-311+G (d,p) approximation. The configurations of synthetic compounds, corresponding to energy minimum, were explored through the consideration of the torsion angle H5-C5-C6-C7 and torsion angle H3-N3-N4-C5. The torsion angles were rotated between 0\({}^{\circ}\) and 360\({}^{\circ}\), and the step was 10\({}^{\circ}\). The structures corresponding to the energy minimum isomers are shown in Figure 2, and the results of the PES scan are shown in Figure 3.  
Figure 2: Structures of the low-energy conformers of thiosemicarbazones **1a**,**b**,**d**. It was observed that for \(\varphi\)1, the E configurations reduce the stereochemical repulsions in relation to the \(Z\), and \(E\)-isomers are more stable for **1a** and **1b**. For **1d**, a strong spatial interaction of thiosemicarbazide fragment is observed with both OH and _tert_-butyl groups, forming the most stable rotamer, with a value of \(\varphi\)1 = 106.5. For \(\varphi\)2, \(Z\)-isomers were found to be more stable because of the mutual repulsion of sulfur and oxygen atoms of the phenolic group. In order to consider the predicted energy values with the structure of the compound **1d**, the 2D-NOESY spectrum was recorded (Figure 4). In the spectrum, a correlation between the CH proton (8.94) and the protons of the _tert_-butyl group (1.34) is observed, as well as the absence of correlation between CH and OH protons, which reveals the structure of **1d** conformer, as shown in Figure 2.  
The reaction of thiosemicarbazones with bromoacetophenone and bromoacetyloumarin by reflux in ethanol led to the formation of the corresponding thiazoles **2a-e** (Scheme 2). The IR spectra of compounds **2a-e** show C=N stretching vibrations in the thiazole ring at  
Figure 4: 2D-NOESY spectrum of **1d**. Figure 3: PES analysis of compounds **1a**, **1b**, and **1d**. 1504-1577 cm\({}^{-1}\) (according to potential energy distribution (PED)), and there is no absorption band corresponding to the vibrations of N-H bond at between 3150 and 3200 cm\({}^{-1}\). The \({}^{1}\)H NMR spectra of compounds **2a-c** contain signals of the phenyl ring in the region of 7.5 ppm, as well as a singlet in the region of 8 ppm corresponding to the proton of the thiazole ring. In the \({}^{1}\)H NMR spectra of substances **2d,e**, the singlet near 8.5 ppm corresponding to the CH proton of the thiazole ring group appears, and a series of peaks from 7.73 to 6.33 ppm which correspond to the protons of the coumarin scaffold are observed. Additionally, there are absorption bands of C=O vibrations in coumarin fragments between 1710 and 1716 cm\({}^{-1}\) in the IR spectra of these compounds. The best yields of thiazoles **3a-d** were achieved by refluxing the starting thiosemicarbazones and bromodimedone in acetic acid in the presence of sodium acetate. The IR spectra of the obtained compounds show strong absorption bands of C=O stretching vibrations near 1630 cm\({}^{-1}\) and signals of C-O stretching vibrations near 1050 cm\({}^{-1}\).