# Synthesis, Characterization, and Antimicrobial Activity of New 1,2,3-Thiadiazole and 1,2,3-Selenadiazole Derivatives

Authors: Mousa L. Al-Smadi; Fatima Esmadi; Mohammad Al-Smadi; Karem H. Alzoubi; Osama Alzoubi; Yousef S. Khader
Journal: Journal of Chemistry
 (2022)

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## Abstract

1,2,3-Thiadiazole, 1,2,3-selenadiazole, and semicarbazones that are prepared from ketones are promising moieties for lead compound development. New 1,2,3-thiadiazole (2c-4c) and 1,2,3-selenadiazole derivatives (2d–4d) were prepared from the corresponding semicarbazones (2b-4b). The semicarbazones (2b-4b) were prepared from the corresponding ketones (2a-4a). Characterization of the synthesized compounds was performed using infrared spectra (IR), proton nuclear magnetic resonance (1H-NMR) spectra, carbon nuclear magnetic resonance (13C-NMR), ultraviolet spectra, mass spectrometry, and elemental analysis. The antimicrobial activity of the prepared compounds was explored in vitro against various pathogenic microbes. All heterocyclic compounds had positive antimicrobial activity, but these activities varied in the extent of antimicrobial coverage. Compounds (2c) and (2d) had positive activity against Staphylococcus aureus and Escherichia coli but without any antipseudomonal activity. Compound (3c) had the most activity against Candida albicans with potential as a novel antifungal agent along with activity against some Gram-positive and Gram-negative bacteria. Compounds (4c) and (4d) exhibited broad-spectrum coverage in which both compounds demonstrated antimicrobial activity against all microorganisms explored. Interestingly, they both had substantial antipseudomonal activity against local resistant Pseudomonas aeruginosa and reference P. aeruginosa (ATCC 27853). This may suggest the potential for compounds (4c) and (4d) as novel broad-spectrum antibacterial agents with promising antipseudomonal activity. In conclusion, new 1,2,3-thiadiazole (2c-4c) and 1,2,3-selenadiazole (2d-4d) derivatives were identified as potential lead compounds for novel antibacterial agents.


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## Body

## 1. Introduction

1,2,3-thiadiazole or 1,2,3-selenadiazole, are five-membered aromatic ring compounds containing three contiguous heteroatoms: one sulfur or selenium and two nitrogen atoms. Further exploration of the therapeutic potential of new 1,2,3-thiadiazole or 1,2,3-selenadiazole is an active area of research. Previous studies have demonstrated that some of the new 1,-2,3-thiadiazole or 1,2,3-selenadiazole derivatives exhibit antifungal [1, 2] antiviral [3, 4], and anticancer activities [5]. The concept of isosteric exchange has long been used as a tool for modifying the activity of biologically important molecules. The incorporation of the isosteric pair (sulfur and selenium) as heteroatoms in the heterocyclic system is an important example [6].Hurd and Mori have previously prepared 1,2,3-thiadiazoles through cyclization reaction of hydrazones upon reaction with thionyl chloride [7–9]. This method is widely employed for the synthesis of 1,2,3-thiadiazoles with high yields [7, 9, 10]. The synthesis of 1,2,3-selenadiazoles by analogy with the 1,2,3-thiadiazole system by Hurd and Mori has been previously reported [7–9]. In the present study, we report the synthesis of new 1,2, 3-thiadiazole and 1 2 3-selenadiazole derivatives using the methods of Hurd and Mori and Lalezari et al. [7, 8] with subsequent characterization of the synthesized compounds by infrared spectra (IR), proton nuclear magnetic resonance (1H-NMR) spectra, carbon nuclear magnetic resonance (13C-NMR), ultraviolet spectra (UV), mass spectrometry, and elemental analysis. Furthermore, we explored the antimicrobial activity of these newly synthesized heterocyclic compounds against common human pathogenic organisms by the hole diffusion method. Spectrum of antimicrobial action against Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria Escherichia coli, local resistant Pseudomonas aeruginosa and reference Pseudomonas aeruginosa (ATCC 27853), and fungus (Candida albicans) were examined and reported. The extent of antimicrobial activity was compared between different compounds by reporting the diameter of the inhibition zone at a constant compound concentration. This study gives insight into compounds with potential as novel antibacterial and antifungal agents. This comes in the current situation where antibacterial, as well as antifungal, resistance among Candida species is a major problem [11–14].


## 2. Method

Acetone was dried and distilled prior to use from phosphorus pentoxide (P2O5). Chloroform was dried and distilled over anhydrous calcium chloride, collected over magnesium sulfate then filtered, and stored over molecular sieves (3 A°). Absolute ethanol and glacial acetic acid were used without further drying. All solvents were obtained from Scharalab. 4-hydroxy acetophenone, selenium dioxide, and thionyl chloride were obtained from ACROS. These chemicals were used without further purification. Semicarbazide hydrochloride was obtained from Schuchardt (Merck). Sodium acetate anhydrous extra pure, potassium carbonate, Aliquat 336, and sodium bicarbonate were obtained from Scharalau. 4-chloro-3-methylphenol, 4-nitrobenzyl bromide, 2-bromo-4-chlorophenol, and 1,2-dibromoethane were obtained from Aldrich.Melting points (mp) were determined on electrothermal digital melting point apparatus and were uncorrected. Infrared (IR) spectra were recorded using IR Prestige-21 Fourier transform spectrophotometers version 1.50 as KBr pellets. Proton nuclear magnetic resonance (1H-NMR) spectra were recorded on 400 MHz (JEOL, JNM = ECP400, FT-NMR system), (200 MHz with AC200 Instrument from Bruker company), tetramethylsilane (TMS) was used as an internal reference. Carbon nuclear magnetic resonance (13C-NMR) spectra were recorded on 100 MHz (JEOL, JNM-ECP400, FT-NMR System), (50 MHz with AC200 Instrument from Bruker company). Ultraviolet spectra were recorded using UV-1700 Pharma. Spec. Shimadzu corporation spectrometer. The mass spectra were carried out using instruments MAT CH7A of Varian Company (EI: 70Ev ionization energy, electron ionization) and using MAT95 of Finnigan Company (FD: 5 KV ionizing energy, field desorption). The signals were given as m/z with the relative intensity between brackets. The analytical thin layer chromatography (TLC) was carried out using TLC-silica plates (0.2 mm) from Merck Company. The elemental analysis was carried out by using an elemental analyzer from NCHS Euro 3000Italy.


### 2.1. General Preparative Procedures

#### 2.1.1. Synthesis of 1-(4-(2-Bromoethoxy) Phenyl) Ethanone (1a)

The synthesis was carried out according to the method described in the literature [7–9]. A mixture of 4-hydroxyacetophenone (3.4 g, 25 mmol), with an excess amount of 1,2-dibromoethane (46.96 g, 250 mmol), potassium carbonate (34.6 g, 250 mmol), and a few drops of Aliquat 336 in 150 mL dry acetone was refluxed for 40 hours. The reaction was followed with TLC in chloroform until completion. The reaction mixture was cooled, and the precipitated salt was removed by filtration. The solvent and excess of 1,2-dibromoethane were removed under vacuum and the product was purified using silica gel column chromatography (40 cm). Traces of 1,2-dibromoethane was eluted with petroleum ether (40–70°C), then the product at the top of the column was eluted by chloroform. The solvent was evaporated under a vacuum to obtain a white powder (2.5 g, 74%), which melts at 62–63°C. Spectroscopic and analytical results of compound (1a) are shown in supplementary Table 1. Synthesis of compounds (1a-4a) is illustrated in Scheme 1.Scheme 1
Synthesis of ketones (1a–4a).


#### 2.1.2. Synthesis of 1-(4-(2-(2-Bromo-4-Chlorophenoxy) Ethoxy) Phenyl) Ethanone (2a) and 1-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl) Ethanone (3a)

A mixture of 1-(4-(2-bromoethoxy) phenyl) ethanone(1a) (2.45 g, 10 mmol) and either 2-bromo-4-chlorophenol (2.07 g, 10 mmol) or 4-chloro-3-methylphenol (1.43 g, 10 mmol), potassium carbonate (1.38 g, 10 mmol), and few drops of Aliquat 336 in 100 mL dry acetone was refluxed for 40 hours. The reaction was followed with TLC in chloroform until completion. The reaction mixture was then cooled, the precipitated salt was removed by simple filtration, and the solution was evaporated to dryness under vacuum. The remaining white solid was washed using a small amount of cold ethanol and dried to give 3.6 g for both compounds (97% for (2a) and 92% for (3a). Spectroscopic and analytical results of compounds (2a) and (3a) are shown in supplementary Tables 2 and 3. Synthesis of compounds (2a) and (3a) is illustrated in Scheme 1.


#### 2.1.3. Synthesis of 1-(4-((4-Nitrobenzyl) Oxy) Phenyl) Ethanone (4a)

A similar procedure to that used for the preparation of compounds(2a) and (3a) was followed for the preparation of compound (4a) except that 3.4 g, 10 mmol of 4-hyroxyacetophenone, and 5.4 g, 25 mmol 4-nitrobenzyl bromide were used. After the drying step, a pale-yellow oil was obtained, which quickly transformed into solid at room temperature to give 7.47 g, 97% of the product. Spectroscopic and analytical results of compound (4a) are shown in supplementary Table 4. Synthesis of compound (4a) is illustrated in Scheme 1.


#### 2.1.4. Synthesis of (E)-2-(1-(4-(2-(2-Bromo-4-Chlorophenoxy)ethoxy) Phenyl) Ethylidene) Hydrazinecarboxamide (2b) and (E)-2-(1-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl) Ethylidene) Hydrazinecarboxamide (3b)

A mixture of semicarbazide hydrochloride (1.67 g, 15 mmol) and sodium acetate (1.23 g, 15 mmol) was dissolved in 50 mL absolute ethanol and heated to reflux for 30 minutes. The mixture was filtered while hot, to remove precipitated sodium chloride salt. Then, an equivalent amount (15 mmol) of ketones(2a, 5.54 g) or (2a), 4.58 g was added to the hot filtrate. The mixture was refluxed for 1 hour and water generated was continuously removed using succulent with magnesium sulfate. After 30 minutes, the reaction started producing precipitate. After precipitation completes, the reaction mixture was cooled to room temperature. Then, the precipitated semicarbazone was filtered off, washed with cold ethanol, and dried to give a white solid (5.2 g, 81.3%) of (2b) and 4.62 g, 85% of (3b). Spectroscopic and analytical results of compounds (2b) and (3b) are shown in supplementary Tables 5 and 6. Synthesis of compounds (2b) and (3b) is illustrated in Scheme 2.Scheme 2
Synthesis of semicarbazone derivatives (2b–4b).


#### 2.1.5. Synthesis of (E)-2-(1-(4-((4-Nitrobenzyl) Oxy) Phenyl) Ethylidene) Hydrazinecarboxamide (4b)

A similar procedure to that used for the preparation of semicarbazones(2b) and (3b) was used for the preparation of semicarbazone (4b) from ketone (4a). The product obtained was a yellow solid (3.94 g, 80% yield) of semicarbazone (4b). Spectroscopic and analytical results of compound (4b) are shown in supplementary Table 7. Synthesis of compound (4b) is illustrated in Scheme 2.


#### 2.1.6. Synthesis of 4-(4-(2-(2-Bromo-4-Chlorophenoxy)Ethoxy)Phenyl)-1,2,3-Thiadiazole (2c) and 4-(4-(2-(4-Chloro-3-Methylphenoxy)Ethoxy)Phenyl)-1,2,3-Thiadiazole (3c)

10 mmol of semicarbazones(2b, 4.27 g) or (3b, 3.62 g) were slowly added to thionyl chloride (20 g, 16 mmol) in several portions with vigorous stirring. The mixture was stirred overnight at room temperature until no more hydrogen chloride is being produced. The excess thionyl chloride was removed under vacuum. The remaining residues were washed with several portions of petroleum ether (60–70°C). The products have pale-yellow color and included 2.9 g, 70% yield of (2c) and 2.1 g, 60% yield of (3c). Spectroscopic and analytical results of compounds (2c-3c) are shown in supplementary Tables 8 and 10. Synthesis of compounds (2c-3c) is illustrated in Scheme 3.Scheme 3
Synthesis of 1,2,3-thiadiazole derivatives (2c–4c).


#### 2.1.7. Synthesis of 4-(4-(2-(2-Bromo-4-Chlorophenoxy) Ethoxy) Phenyl)-1,2,3-Selenadiazole (2d) and 4-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl)-1,2,3-Selenadiazole (3d)

Compound(2b) (1.7 g, 4 mmol) or (3b) (1.45 g, 4 mmol) was dissolved in 50 mL glacial acetic acid with vigorous stirring and gentle heating (35–40°C). The solution was treated with selenium dioxide powder (0.6 g, 5.41 mmol), and the mixture color became red after 2 minutes of treatment. The mixture was kept under gentle heating and vigorous stirring overnight followed by TLC until the reaction was complete. The precipitated solid was removed by simple filtration and the filtrate was poured over ice water and extracted with chloroform (3 × 50 mL). The combined organic layers were washed with saturated sodium hydrogen carbonate solution and dried using magnesium sulfate. The solvent was removed under vacuum and the remaining product was recrystallized from acetone to give the brown solid (1.1 g, 60% yield) of compound (2d) and 0.94 g, 60% yield of compound (3d). Spectroscopic and analytical results of compounds (2d-3 d) are shown in supplementary Tables 9 and 11. Synthesis of compounds (2d) and (3d) is illustrated in Scheme 4.Scheme 4
Synthesis of 1,2,3-selenadiazole derivatives (2d–4d).


#### 2.1.8. Synthesis of 4-(4-((4-Nitrobenzyl) Oxy) Phenyl)-1,2,3-Thiadiazole (4c)

Similar procedure to that used for the preparation of compounds(2c-3c) was used for the preparation of compound (4c) from semicarbazone (4b) (2.10 g, 6.00 mmol). The product obtained is a yellow solid with a 70% yield (0.66 g). Spectroscopic and analytical results of compound (4c) are shown in supplementary Table 12. Synthesis of compound (4c) is illustrated in Scheme 3.


#### 2.1.9. Synthesis of 4-(4-((4-Nitrobenzyl) Oxy) Phenyl)-1,2,3-Selenadiazole (4d)

Similar procedure to that used for the preparation of compounds(2d-3d) was followed for the preparation of compound (4d) from semicarbazone (4b) (1.2 g, 3.49 mmol). The product obtained was a light brown solid (1.86 g, 84% yield). Spectroscopic and analytical results of compound (4d) are shown in supplementary Table 13. Synthesis of compound (4d) is illustrated in Scheme 4.


### 2.2. Antimicrobial Activity

The antimicrobial activity of the heterocyclic compounds(2c), (2d), (3c), and (3d) was tested against certain human pathogenic microbes including Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli), local resistant Pseudomonas aeruginosa and a reference Pseudomonas aeruginosa ATCC 27853), and the fungus, Candida albicans, by the hole diffusion method, grown on nutrient agar as previously described [9,15]. The solutions of compounds (2c), (2d), (3c), (3d), (4c), and (4d) were prepared by dissolving them in 0.01 g/mL dimethylsulfoxide (DMSO). Standard antimicrobials were used for each of the above-tested microorganisms. The diameter of the inhibition zone was measured at the constant low concentration of 0.01 g/mL.


## 2.1. General Preparative Procedures

### 2.1.1. Synthesis of 1-(4-(2-Bromoethoxy) Phenyl) Ethanone (1a)

The synthesis was carried out according to the method described in the literature [7–9]. A mixture of 4-hydroxyacetophenone (3.4 g, 25 mmol), with an excess amount of 1,2-dibromoethane (46.96 g, 250 mmol), potassium carbonate (34.6 g, 250 mmol), and a few drops of Aliquat 336 in 150 mL dry acetone was refluxed for 40 hours. The reaction was followed with TLC in chloroform until completion. The reaction mixture was cooled, and the precipitated salt was removed by filtration. The solvent and excess of 1,2-dibromoethane were removed under vacuum and the product was purified using silica gel column chromatography (40 cm). Traces of 1,2-dibromoethane was eluted with petroleum ether (40–70°C), then the product at the top of the column was eluted by chloroform. The solvent was evaporated under a vacuum to obtain a white powder (2.5 g, 74%), which melts at 62–63°C. Spectroscopic and analytical results of compound (1a) are shown in supplementary Table 1. Synthesis of compounds (1a-4a) is illustrated in Scheme 1.Scheme 1
Synthesis of ketones (1a–4a).


### 2.1.2. Synthesis of 1-(4-(2-(2-Bromo-4-Chlorophenoxy) Ethoxy) Phenyl) Ethanone (2a) and 1-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl) Ethanone (3a)

A mixture of 1-(4-(2-bromoethoxy) phenyl) ethanone(1a) (2.45 g, 10 mmol) and either 2-bromo-4-chlorophenol (2.07 g, 10 mmol) or 4-chloro-3-methylphenol (1.43 g, 10 mmol), potassium carbonate (1.38 g, 10 mmol), and few drops of Aliquat 336 in 100 mL dry acetone was refluxed for 40 hours. The reaction was followed with TLC in chloroform until completion. The reaction mixture was then cooled, the precipitated salt was removed by simple filtration, and the solution was evaporated to dryness under vacuum. The remaining white solid was washed using a small amount of cold ethanol and dried to give 3.6 g for both compounds (97% for (2a) and 92% for (3a). Spectroscopic and analytical results of compounds (2a) and (3a) are shown in supplementary Tables 2 and 3. Synthesis of compounds (2a) and (3a) is illustrated in Scheme 1.


### 2.1.3. Synthesis of 1-(4-((4-Nitrobenzyl) Oxy) Phenyl) Ethanone (4a)

A similar procedure to that used for the preparation of compounds(2a) and (3a) was followed for the preparation of compound (4a) except that 3.4 g, 10 mmol of 4-hyroxyacetophenone, and 5.4 g, 25 mmol 4-nitrobenzyl bromide were used. After the drying step, a pale-yellow oil was obtained, which quickly transformed into solid at room temperature to give 7.47 g, 97% of the product. Spectroscopic and analytical results of compound (4a) are shown in supplementary Table 4. Synthesis of compound (4a) is illustrated in Scheme 1.


### 2.1.4. Synthesis of (E)-2-(1-(4-(2-(2-Bromo-4-Chlorophenoxy)ethoxy) Phenyl) Ethylidene) Hydrazinecarboxamide (2b) and (E)-2-(1-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl) Ethylidene) Hydrazinecarboxamide (3b)

A mixture of semicarbazide hydrochloride (1.67 g, 15 mmol) and sodium acetate (1.23 g, 15 mmol) was dissolved in 50 mL absolute ethanol and heated to reflux for 30 minutes. The mixture was filtered while hot, to remove precipitated sodium chloride salt. Then, an equivalent amount (15 mmol) of ketones(2a, 5.54 g) or (2a), 4.58 g was added to the hot filtrate. The mixture was refluxed for 1 hour and water generated was continuously removed using succulent with magnesium sulfate. After 30 minutes, the reaction started producing precipitate. After precipitation completes, the reaction mixture was cooled to room temperature. Then, the precipitated semicarbazone was filtered off, washed with cold ethanol, and dried to give a white solid (5.2 g, 81.3%) of (2b) and 4.62 g, 85% of (3b). Spectroscopic and analytical results of compounds (2b) and (3b) are shown in supplementary Tables 5 and 6. Synthesis of compounds (2b) and (3b) is illustrated in Scheme 2.Scheme 2
Synthesis of semicarbazone derivatives (2b–4b).


### 2.1.5. Synthesis of (E)-2-(1-(4-((4-Nitrobenzyl) Oxy) Phenyl) Ethylidene) Hydrazinecarboxamide (4b)

A similar procedure to that used for the preparation of semicarbazones(2b) and (3b) was used for the preparation of semicarbazone (4b) from ketone (4a). The product obtained was a yellow solid (3.94 g, 80% yield) of semicarbazone (4b). Spectroscopic and analytical results of compound (4b) are shown in supplementary Table 7. Synthesis of compound (4b) is illustrated in Scheme 2.


### 2.1.6. Synthesis of 4-(4-(2-(2-Bromo-4-Chlorophenoxy)Ethoxy)Phenyl)-1,2,3-Thiadiazole (2c) and 4-(4-(2-(4-Chloro-3-Methylphenoxy)Ethoxy)Phenyl)-1,2,3-Thiadiazole (3c)

10 mmol of semicarbazones(2b, 4.27 g) or (3b, 3.62 g) were slowly added to thionyl chloride (20 g, 16 mmol) in several portions with vigorous stirring. The mixture was stirred overnight at room temperature until no more hydrogen chloride is being produced. The excess thionyl chloride was removed under vacuum. The remaining residues were washed with several portions of petroleum ether (60–70°C). The products have pale-yellow color and included 2.9 g, 70% yield of (2c) and 2.1 g, 60% yield of (3c). Spectroscopic and analytical results of compounds (2c-3c) are shown in supplementary Tables 8 and 10. Synthesis of compounds (2c-3c) is illustrated in Scheme 3.Scheme 3
Synthesis of 1,2,3-thiadiazole derivatives (2c–4c).


### 2.1.7. Synthesis of 4-(4-(2-(2-Bromo-4-Chlorophenoxy) Ethoxy) Phenyl)-1,2,3-Selenadiazole (2d) and 4-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl)-1,2,3-Selenadiazole (3d)

Compound(2b) (1.7 g, 4 mmol) or (3b) (1.45 g, 4 mmol) was dissolved in 50 mL glacial acetic acid with vigorous stirring and gentle heating (35–40°C). The solution was treated with selenium dioxide powder (0.6 g, 5.41 mmol), and the mixture color became red after 2 minutes of treatment. The mixture was kept under gentle heating and vigorous stirring overnight followed by TLC until the reaction was complete. The precipitated solid was removed by simple filtration and the filtrate was poured over ice water and extracted with chloroform (3 × 50 mL). The combined organic layers were washed with saturated sodium hydrogen carbonate solution and dried using magnesium sulfate. The solvent was removed under vacuum and the remaining product was recrystallized from acetone to give the brown solid (1.1 g, 60% yield) of compound (2d) and 0.94 g, 60% yield of compound (3d). Spectroscopic and analytical results of compounds (2d-3 d) are shown in supplementary Tables 9 and 11. Synthesis of compounds (2d) and (3d) is illustrated in Scheme 4.Scheme 4
Synthesis of 1,2,3-selenadiazole derivatives (2d–4d).


### 2.1.8. Synthesis of 4-(4-((4-Nitrobenzyl) Oxy) Phenyl)-1,2,3-Thiadiazole (4c)

Similar procedure to that used for the preparation of compounds(2c-3c) was used for the preparation of compound (4c) from semicarbazone (4b) (2.10 g, 6.00 mmol). The product obtained is a yellow solid with a 70% yield (0.66 g). Spectroscopic and analytical results of compound (4c) are shown in supplementary Table 12. Synthesis of compound (4c) is illustrated in Scheme 3.


### 2.1.9. Synthesis of 4-(4-((4-Nitrobenzyl) Oxy) Phenyl)-1,2,3-Selenadiazole (4d)

Similar procedure to that used for the preparation of compounds(2d-3d) was followed for the preparation of compound (4d) from semicarbazone (4b) (1.2 g, 3.49 mmol). The product obtained was a light brown solid (1.86 g, 84% yield). Spectroscopic and analytical results of compound (4d) are shown in supplementary Table 13. Synthesis of compound (4d) is illustrated in Scheme 4.


## 2.1.1. Synthesis of 1-(4-(2-Bromoethoxy) Phenyl) Ethanone (1a)

The synthesis was carried out according to the method described in the literature [7–9]. A mixture of 4-hydroxyacetophenone (3.4 g, 25 mmol), with an excess amount of 1,2-dibromoethane (46.96 g, 250 mmol), potassium carbonate (34.6 g, 250 mmol), and a few drops of Aliquat 336 in 150 mL dry acetone was refluxed for 40 hours. The reaction was followed with TLC in chloroform until completion. The reaction mixture was cooled, and the precipitated salt was removed by filtration. The solvent and excess of 1,2-dibromoethane were removed under vacuum and the product was purified using silica gel column chromatography (40 cm). Traces of 1,2-dibromoethane was eluted with petroleum ether (40–70°C), then the product at the top of the column was eluted by chloroform. The solvent was evaporated under a vacuum to obtain a white powder (2.5 g, 74%), which melts at 62–63°C. Spectroscopic and analytical results of compound (1a) are shown in supplementary Table 1. Synthesis of compounds (1a-4a) is illustrated in Scheme 1.Scheme 1
Synthesis of ketones (1a–4a).


## 2.1.2. Synthesis of 1-(4-(2-(2-Bromo-4-Chlorophenoxy) Ethoxy) Phenyl) Ethanone (2a) and 1-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl) Ethanone (3a)

A mixture of 1-(4-(2-bromoethoxy) phenyl) ethanone(1a) (2.45 g, 10 mmol) and either 2-bromo-4-chlorophenol (2.07 g, 10 mmol) or 4-chloro-3-methylphenol (1.43 g, 10 mmol), potassium carbonate (1.38 g, 10 mmol), and few drops of Aliquat 336 in 100 mL dry acetone was refluxed for 40 hours. The reaction was followed with TLC in chloroform until completion. The reaction mixture was then cooled, the precipitated salt was removed by simple filtration, and the solution was evaporated to dryness under vacuum. The remaining white solid was washed using a small amount of cold ethanol and dried to give 3.6 g for both compounds (97% for (2a) and 92% for (3a). Spectroscopic and analytical results of compounds (2a) and (3a) are shown in supplementary Tables 2 and 3. Synthesis of compounds (2a) and (3a) is illustrated in Scheme 1.


## 2.1.3. Synthesis of 1-(4-((4-Nitrobenzyl) Oxy) Phenyl) Ethanone (4a)

A similar procedure to that used for the preparation of compounds(2a) and (3a) was followed for the preparation of compound (4a) except that 3.4 g, 10 mmol of 4-hyroxyacetophenone, and 5.4 g, 25 mmol 4-nitrobenzyl bromide were used. After the drying step, a pale-yellow oil was obtained, which quickly transformed into solid at room temperature to give 7.47 g, 97% of the product. Spectroscopic and analytical results of compound (4a) are shown in supplementary Table 4. Synthesis of compound (4a) is illustrated in Scheme 1.


## 2.1.4. Synthesis of (E)-2-(1-(4-(2-(2-Bromo-4-Chlorophenoxy)ethoxy) Phenyl) Ethylidene) Hydrazinecarboxamide (2b) and (E)-2-(1-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl) Ethylidene) Hydrazinecarboxamide (3b)

A mixture of semicarbazide hydrochloride (1.67 g, 15 mmol) and sodium acetate (1.23 g, 15 mmol) was dissolved in 50 mL absolute ethanol and heated to reflux for 30 minutes. The mixture was filtered while hot, to remove precipitated sodium chloride salt. Then, an equivalent amount (15 mmol) of ketones(2a, 5.54 g) or (2a), 4.58 g was added to the hot filtrate. The mixture was refluxed for 1 hour and water generated was continuously removed using succulent with magnesium sulfate. After 30 minutes, the reaction started producing precipitate. After precipitation completes, the reaction mixture was cooled to room temperature. Then, the precipitated semicarbazone was filtered off, washed with cold ethanol, and dried to give a white solid (5.2 g, 81.3%) of (2b) and 4.62 g, 85% of (3b). Spectroscopic and analytical results of compounds (2b) and (3b) are shown in supplementary Tables 5 and 6. Synthesis of compounds (2b) and (3b) is illustrated in Scheme 2.Scheme 2
Synthesis of semicarbazone derivatives (2b–4b).


## 2.1.5. Synthesis of (E)-2-(1-(4-((4-Nitrobenzyl) Oxy) Phenyl) Ethylidene) Hydrazinecarboxamide (4b)

A similar procedure to that used for the preparation of semicarbazones(2b) and (3b) was used for the preparation of semicarbazone (4b) from ketone (4a). The product obtained was a yellow solid (3.94 g, 80% yield) of semicarbazone (4b). Spectroscopic and analytical results of compound (4b) are shown in supplementary Table 7. Synthesis of compound (4b) is illustrated in Scheme 2.


## 2.1.6. Synthesis of 4-(4-(2-(2-Bromo-4-Chlorophenoxy)Ethoxy)Phenyl)-1,2,3-Thiadiazole (2c) and 4-(4-(2-(4-Chloro-3-Methylphenoxy)Ethoxy)Phenyl)-1,2,3-Thiadiazole (3c)

10 mmol of semicarbazones(2b, 4.27 g) or (3b, 3.62 g) were slowly added to thionyl chloride (20 g, 16 mmol) in several portions with vigorous stirring. The mixture was stirred overnight at room temperature until no more hydrogen chloride is being produced. The excess thionyl chloride was removed under vacuum. The remaining residues were washed with several portions of petroleum ether (60–70°C). The products have pale-yellow color and included 2.9 g, 70% yield of (2c) and 2.1 g, 60% yield of (3c). Spectroscopic and analytical results of compounds (2c-3c) are shown in supplementary Tables 8 and 10. Synthesis of compounds (2c-3c) is illustrated in Scheme 3.Scheme 3
Synthesis of 1,2,3-thiadiazole derivatives (2c–4c).


## 2.1.7. Synthesis of 4-(4-(2-(2-Bromo-4-Chlorophenoxy) Ethoxy) Phenyl)-1,2,3-Selenadiazole (2d) and 4-(4-(2-(4-Chloro-3-Methylphenoxy) Ethoxy) Phenyl)-1,2,3-Selenadiazole (3d)

Compound(2b) (1.7 g, 4 mmol) or (3b) (1.45 g, 4 mmol) was dissolved in 50 mL glacial acetic acid with vigorous stirring and gentle heating (35–40°C). The solution was treated with selenium dioxide powder (0.6 g, 5.41 mmol), and the mixture color became red after 2 minutes of treatment. The mixture was kept under gentle heating and vigorous stirring overnight followed by TLC until the reaction was complete. The precipitated solid was removed by simple filtration and the filtrate was poured over ice water and extracted with chloroform (3 × 50 mL). The combined organic layers were washed with saturated sodium hydrogen carbonate solution and dried using magnesium sulfate. The solvent was removed under vacuum and the remaining product was recrystallized from acetone to give the brown solid (1.1 g, 60% yield) of compound (2d) and 0.94 g, 60% yield of compound (3d). Spectroscopic and analytical results of compounds (2d-3 d) are shown in supplementary Tables 9 and 11. Synthesis of compounds (2d) and (3d) is illustrated in Scheme 4.Scheme 4
Synthesis of 1,2,3-selenadiazole derivatives (2d–4d).


## 2.1.8. Synthesis of 4-(4-((4-Nitrobenzyl) Oxy) Phenyl)-1,2,3-Thiadiazole (4c)

Similar procedure to that used for the preparation of compounds(2c-3c) was used for the preparation of compound (4c) from semicarbazone (4b) (2.10 g, 6.00 mmol). The product obtained is a yellow solid with a 70% yield (0.66 g). Spectroscopic and analytical results of compound (4c) are shown in supplementary Table 12. Synthesis of compound (4c) is illustrated in Scheme 3.


## 2.1.9. Synthesis of 4-(4-((4-Nitrobenzyl) Oxy) Phenyl)-1,2,3-Selenadiazole (4d)

Similar procedure to that used for the preparation of compounds(2d-3d) was followed for the preparation of compound (4d) from semicarbazone (4b) (1.2 g, 3.49 mmol). The product obtained was a light brown solid (1.86 g, 84% yield). Spectroscopic and analytical results of compound (4d) are shown in supplementary Table 13. Synthesis of compound (4d) is illustrated in Scheme 4.


## 2.2. Antimicrobial Activity

The antimicrobial activity of the heterocyclic compounds(2c), (2d), (3c), and (3d) was tested against certain human pathogenic microbes including Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli), local resistant Pseudomonas aeruginosa and a reference Pseudomonas aeruginosa ATCC 27853), and the fungus, Candida albicans, by the hole diffusion method, grown on nutrient agar as previously described [9,15]. The solutions of compounds (2c), (2d), (3c), (3d), (4c), and (4d) were prepared by dissolving them in 0.01 g/mL dimethylsulfoxide (DMSO). Standard antimicrobials were used for each of the above-tested microorganisms. The diameter of the inhibition zone was measured at the constant low concentration of 0.01 g/mL.


## 3. Results and Discussion

New 1,2,3-thiadiazole, and 1,2,3-selenadiazole derivatives were prepared via methods that were first reported by Hurd and Mori for 1,2,3-thiadiazole compounds and by Lalezariet al for 1,2,3-selenadizole compounds [7–9]. In both methods, the semicarbazones derived from the corresponding ketones were applied for the preparation of 1,2,3-thiadiazole and 1,2,3-selenadiazole derivatives. To obtain the 1,2,3-thiadiazoles and 1,2,3-selenadiazoles, the starting ketones and semicarbazones have been prepared and characterized as illustrated in Schemes 1–4. The IR, MS, elemental analysis, UV, 1H-NMR, 13C-NMR data, and physical properties of compounds (1a–4a), (2b–4b), (2c–4c), and (2d–4d) are shown in supplementary Tables 1–13. The IR, 1H-NMR, and 13C-NMR spectra of compounds (1a–4a), (2b–4b), (2c–4c), and (2d–4d) are shown in supplementary Figures 1–39.


### 3.1. Ketones

Scheme1 shows the preparation of (1a) by the reaction of the commercially available 4-hydroxyacetophenone with 1,2-dibromoethane in the presence of potassium carbonate and 2–3 drops of Aliquat 336 (catalyst) in refluxing acetone. The potassium carbonate was added to the phenol derivative (acid) to undergo an acid-base reaction to convert the weak nucleophile (the phenol derivative) into a stronger nucleophile (the potassium phenoxide derivative) that can accomplish the SN2 substitution reaction with the primary alkyl halide faster. The reaction goes to completion in about 40 hours to give the expected ketone (1a).The reaction is a nucleophilic substitution one. Compound(1a) is the starting material for the preparation of ketones (2a) and (3a).The structure of ketone(1a) was confirmed by spectroscopic measurements. The IR-spectrum of ketone (1a) exhibited a band at 1668 cm−1 for the carbonyl group (supplementary Table 1). The 1H-NMR spectrum showed a single peak at 2.51 ppm for CO(CH3) and two triplets at 4.5 and 3.84 ppm for –CH2CH2–O, whereas the aromatic protons appeared as two doublets in the aromatic region from 7.10 to 8.01 ppm. The 13C-NMR spectrum showed one peak for CH3-carbon at 26.5 ppm and the carbonyl carbon at 196.13 ppm. The mp, mass spectroscopy, elemental analysis, and UV-spectra are shown in supplementary Table 1. Scheme 2 shows the preparation of compounds (2a) and (3a). The reaction involves nucleophilic substitution reaction of ketone (1a) with 2-bromo-4-chlorophenol or 4-chloro-3-methylphenol in presence of potassium carbonate and 2–3 drops of Aliquat 336 (catalyst) in refluxing acetone. The reaction undergoes completion in about 40 hours to give expected ketones (2a), (3a).The structures of ketones(2a), (3a) were confirmed by spectroscopic measurements. The IR spectra of ketones (2a) and (3a) exhibited bands at 1677 or 1665 cm−1 for the carbonyl group, supplementary Tables 2–3, respectively. The 1H-NMR spectra showed single peaks at 2.51 or 2.58 ppm for CO(CH3) and two triplets at 4.4 and 4.38, 4.43 ppm for –CH2 CH2–O, the aromatic protons appeared as four doublets in the aromatic region from 6.8 to 7.97 ppm and for both ketones (2a) and (3a), one singlet at 7.9 for ketone (2a) and at 6.99 ppm for ketone (3a) and one singlet at 2.36 ppm for the methyl group of ketone (3a). The 13C-NMR spectra of ketones (2a), (3a) showed peaks for carbon CH3 at 26.3, 26.37 ppm and for carbonyl carbon at 196.7, 196.78 ppm, respectively. The mp, mass spectroscopy, elemental analysis, and UV-spectra are shown in supplementary Tables 2–3.Ketone(4a) was prepared by nucleophilic substitution reaction of the commercially available 4-hydroxyacetophenone with 4-nitrobenzyl bromide in presence of potassium carbonate and 3 drops of Aliquat 336 (catalyst) in refluxing acetone. Again, this reaction undergoes completion in about 40 hours to give the expected ketone (4a) as shown in Scheme 3. The structure of ketone (4a) was confirmed by spectroscopic measurements. The IR-spectrum of ketone (4a) exhibited a band at 1675 cm−1 for the carbonyl group. The 1H-NMR spectrum showed two single peaks at 2.49 ppm for CO(CH3) and 5.36 ppm for –CH2-O, whereas the aromatic protons appeared as four doublets in the aromatic region from 6.84 to 7.92 ppm. The 13C-NMR spectrum showed a peak for carbon atom of CH3 at 25.58 ppm and carbonyl carbon at 195.57 ppm. The mp, mass spectroscopy, elemental analysis, IR, 1H-NMR, 13C-NMR, and UV-spectra results are shown in supplementary Table 4.


### 3.2. Semicarbazones

Semicarbazones(2b–4b) were prepared by condensation reaction between ketones (2a–4a) and the semicarbazide in refluxing ethanol as shown in Schemes 1 and 2. The semicarbazones (2b–4b) were obtained as white powders with 81.3, 82.8, 80% yields and mp 203–205, 196, 238–240 C, respectively.The structure of compounds(2b–4b) was confirmed by IR, 1H-NMR, 13C-NMR, and mass spectroscopic measurements. The IR spectra of the semicarbazones (2b–4b) showed characteristic bands at 3480, 3334, 3427 cm−1 for–NH, (3233, 3194), (3100, 3066), (3203, 3175) cm−1 for–NH2, and 1678, 1675, 1689 cm−1 for carbonyl group, respectively. 1H-NMR spectra showed the disappearance of the characteristic peak of CH3–C=O and the formation of new peak for CH3–C=N at 2.14, 1.31, 2.13 ppm (s, 3H). 13C-NMR showed peaks at 20.24, 20.24, 13.26 ppm for CH3 and at 147.7, 137.03, 147.07 ppm for C=N, respectively. In addition, the appearance of two singlet peaks, one of them for –NH at 9.18, 8.36, 6.99 ppm and the second for –NH2 at 6.42, 5.59, 6.44 ppm, respectively, confirms the formation of the semicarbazones. The 1H-NMR, 13C-NMR, elemental analysis, UV-spectra, and mass spectroscopy are shown in supplementary Tables 5–7.In previous studies, the preparation of semicarbazones R1R2C = NNHC(O)NH2, showed that the configuration of the C=N bond exists mostly in the E form, which was found to be the major product according to the nuclear over house effect experiments of the 1H-NMR spectra due to less steric factor [7]. Therefore, the structures of compounds (2b-4b) are drawn in the E form depending on 1H-NMR spectra.


### 3.3. 1,2,3-Thiadiazoles

The 1,2,3-thiadiazoles were obtained by treating semicarbazones(2b-4b) with thionyl chloride at room temperature as shown in Scheme 3. 1,2,3-Thiadiazoles (2c-4c) were obtained in 70, 60, and 68.4% yields and found to decompose at 140, 135, and 180 C, respectively. IR spectra for compounds (2c-4c) from supplementary Tables 8, 10, and 12 showed the absence of the bands at 3480, 3334, and 3427 cm−1 for–NH and formation of new bands at (1469, 1442), (1477, 1450), (1514, 1449) cm−1 for the 1,2,3-thiadiazole rings, respectively.1H-NMR spectra showed a singlet peak at 8.53, 8.56, 8.51 ppm, respectively, which is attributed to the proton at carbon number five of the 1,2,3-thiadiazole ring. 13C-NMR spectra showed that the peaks due to C4 and C5 appeared at 162.6, 161.07, 162.7 ppm and 129.93, 130.10, 158.77 ppm, respectively. From these data, the active α-methyl group of the semicarbazones (2b-4b) had disappeared. The 1H-NMR, 13C-NMR, IR, mp, mass spectroscopy, and elemental analysis data of compounds (2c-4c) are shown in supplementary Tables 8, 10, and 12.


### 3.4. 1,2,3-Selenadiazoles

The 1,2,3-selenadiazole compounds(2d-4d) were obtained by treating semicarbazones (2b-4b) with selenium dioxide in glacial acetic acid as shown in Scheme 1. Compounds (2d) and (3d) were obtained in 60% yield whereas (4d) was obtained in 68.4% yield. The reaction of semicarbazones (2b-4b) is very slow due to low solubility in acetic acid.IR spectra of the 1,2,3-selenadiazoles(2d–4d) showed appearance of characteristic bands in 1421–1480 cm−1 region for 1,2,3-selenadiazole heterocyclic ring and the disappearance of the peaks at 3480, 3334, 3427 cm−1 that is characteristic of semicarbazones–NH group. The 1H-NMR spectra showed a single peak that appears downfield at 9.26, 9.19, 9.31 ppm which is due to the proton at carbon number five of the heterocyclic ring. The 13C-NMR spectra showed a chemical shift due to C4 and C5 at 162.60, 162.79, 162.70 ppm and 129.93, 131.94, 156.79 ppm for 1,2,3-selenadiazoles (2d–4d), respectively. The 1H-NMR, 13C-NMR, IR, mp, mass spectroscopy, and elemental analysis data of compounds (2c–4c) are shown in supplementary Tables 9, 11, and 13.


### 3.5. Antimicrobial Activity

The activity of the heterocyclic compounds(2c), (2d), (3c), (3d), (4c), and (4d) was tested against some human pathogenic microbes including Gram-positive S. aureus, Gram-negative E. coli, local resistant P. aeruginosa and reference P. aeruginosa (ATCC 27853), and C. albicans by the hole diffusion method [9,15]. As shown in Table 1, all six heterocyclic compounds had positive antimicrobial activity at a concentration of 0.01 g/mL, but they varied in their antimicrobial coverage. The extent of antimicrobial action also differed among compounds as noted by examining the diameter of the inhibition zone at a constant low concentration when using the hole diffusion technique. Heterocyclic compounds (2c) and (2d) had positive activity against S. aureus and E. coli, along with some activity of compound (2c) against C. albicans. However, they did not show any antipseudomonal activity. Compound (3c), like compound (2c), showed positive activity against S. aureus and even better activity against E. coli and C. albicans. Furthermore, compounds (3c) and (3d) showed positive antimicrobial activity against local resistant P. aeruginosa.Table 1
Sensitivity of human pathogenic microbes to the new synthetic heterocyclic compounds, and some complexes using the hole method.
CompoundCompound concentration (g/mL)Staphylococcus aureusEscherichia coliCandida albicansLocal resistantPseudomonas aeruginosaReferencePseudomonas aeruginosa ATCC 278532c0.01+++++−−2d0.01++−−−3c0.01++++++++−3d0.01−+++++−4c0.01+++++++++++++4d0.01++++++++++++Results were expressed as ranges of diameters of zones of inhibition of antimicrobial activity. An inhibition zone diameter (ID) < 5 mm was denoted as “+”, ID of 10 mm-5 mm was denoted as “++”, and ID of >10 mm was denoted as “+++”. The “−” indicates no inhibition zone. Standard antimicrobials were used for each of the above-tested microorganisms, where the zone of inhibition for each standard compound was >10 mm “+++” for its corresponding microorganism.Another remarkable finding was the broad-spectrum coverage of heterocyclic compounds (4c) and (4d) in which both compounds exhibited antimicrobial activity against all microorganisms explored. After examining the diameter of inhibition zone, interestingly, they both had substantial antipseudomonal activity against local resistantP. aeruginosa and reference P. aeruginosa (ATCC 27853). The solvent showed no activity against any of the tested pathogens.The results of the present study demonstrated that heterocyclic compounds (4c) and (4d) have high potential as novel broad-spectrum antibacterial agents with promising antipseudomonal activity. Additionally, by observing the diameter of the inhibition zone, heterocyclic compound (3c) had the most activity againstC. albicans with activity against some Gram-positive and Gram-negative bacteria. Thus, compound (3c) has potential as a novel antifungal and antibacterial agent.


## 3.1. Ketones

Scheme1 shows the preparation of (1a) by the reaction of the commercially available 4-hydroxyacetophenone with 1,2-dibromoethane in the presence of potassium carbonate and 2–3 drops of Aliquat 336 (catalyst) in refluxing acetone. The potassium carbonate was added to the phenol derivative (acid) to undergo an acid-base reaction to convert the weak nucleophile (the phenol derivative) into a stronger nucleophile (the potassium phenoxide derivative) that can accomplish the SN2 substitution reaction with the primary alkyl halide faster. The reaction goes to completion in about 40 hours to give the expected ketone (1a).The reaction is a nucleophilic substitution one. Compound(1a) is the starting material for the preparation of ketones (2a) and (3a).The structure of ketone(1a) was confirmed by spectroscopic measurements. The IR-spectrum of ketone (1a) exhibited a band at 1668 cm−1 for the carbonyl group (supplementary Table 1). The 1H-NMR spectrum showed a single peak at 2.51 ppm for CO(CH3) and two triplets at 4.5 and 3.84 ppm for –CH2CH2–O, whereas the aromatic protons appeared as two doublets in the aromatic region from 7.10 to 8.01 ppm. The 13C-NMR spectrum showed one peak for CH3-carbon at 26.5 ppm and the carbonyl carbon at 196.13 ppm. The mp, mass spectroscopy, elemental analysis, and UV-spectra are shown in supplementary Table 1. Scheme 2 shows the preparation of compounds (2a) and (3a). The reaction involves nucleophilic substitution reaction of ketone (1a) with 2-bromo-4-chlorophenol or 4-chloro-3-methylphenol in presence of potassium carbonate and 2–3 drops of Aliquat 336 (catalyst) in refluxing acetone. The reaction undergoes completion in about 40 hours to give expected ketones (2a), (3a).The structures of ketones(2a), (3a) were confirmed by spectroscopic measurements. The IR spectra of ketones (2a) and (3a) exhibited bands at 1677 or 1665 cm−1 for the carbonyl group, supplementary Tables 2–3, respectively. The 1H-NMR spectra showed single peaks at 2.51 or 2.58 ppm for CO(CH3) and two triplets at 4.4 and 4.38, 4.43 ppm for –CH2 CH2–O, the aromatic protons appeared as four doublets in the aromatic region from 6.8 to 7.97 ppm and for both ketones (2a) and (3a), one singlet at 7.9 for ketone (2a) and at 6.99 ppm for ketone (3a) and one singlet at 2.36 ppm for the methyl group of ketone (3a). The 13C-NMR spectra of ketones (2a), (3a) showed peaks for carbon CH3 at 26.3, 26.37 ppm and for carbonyl carbon at 196.7, 196.78 ppm, respectively. The mp, mass spectroscopy, elemental analysis, and UV-spectra are shown in supplementary Tables 2–3.Ketone(4a) was prepared by nucleophilic substitution reaction of the commercially available 4-hydroxyacetophenone with 4-nitrobenzyl bromide in presence of potassium carbonate and 3 drops of Aliquat 336 (catalyst) in refluxing acetone. Again, this reaction undergoes completion in about 40 hours to give the expected ketone (4a) as shown in Scheme 3. The structure of ketone (4a) was confirmed by spectroscopic measurements. The IR-spectrum of ketone (4a) exhibited a band at 1675 cm−1 for the carbonyl group. The 1H-NMR spectrum showed two single peaks at 2.49 ppm for CO(CH3) and 5.36 ppm for –CH2-O, whereas the aromatic protons appeared as four doublets in the aromatic region from 6.84 to 7.92 ppm. The 13C-NMR spectrum showed a peak for carbon atom of CH3 at 25.58 ppm and carbonyl carbon at 195.57 ppm. The mp, mass spectroscopy, elemental analysis, IR, 1H-NMR, 13C-NMR, and UV-spectra results are shown in supplementary Table 4.


## 3.2. Semicarbazones

Semicarbazones(2b–4b) were prepared by condensation reaction between ketones (2a–4a) and the semicarbazide in refluxing ethanol as shown in Schemes 1 and 2. The semicarbazones (2b–4b) were obtained as white powders with 81.3, 82.8, 80% yields and mp 203–205, 196, 238–240 C, respectively.The structure of compounds(2b–4b) was confirmed by IR, 1H-NMR, 13C-NMR, and mass spectroscopic measurements. The IR spectra of the semicarbazones (2b–4b) showed characteristic bands at 3480, 3334, 3427 cm−1 for–NH, (3233, 3194), (3100, 3066), (3203, 3175) cm−1 for–NH2, and 1678, 1675, 1689 cm−1 for carbonyl group, respectively. 1H-NMR spectra showed the disappearance of the characteristic peak of CH3–C=O and the formation of new peak for CH3–C=N at 2.14, 1.31, 2.13 ppm (s, 3H). 13C-NMR showed peaks at 20.24, 20.24, 13.26 ppm for CH3 and at 147.7, 137.03, 147.07 ppm for C=N, respectively. In addition, the appearance of two singlet peaks, one of them for –NH at 9.18, 8.36, 6.99 ppm and the second for –NH2 at 6.42, 5.59, 6.44 ppm, respectively, confirms the formation of the semicarbazones. The 1H-NMR, 13C-NMR, elemental analysis, UV-spectra, and mass spectroscopy are shown in supplementary Tables 5–7.In previous studies, the preparation of semicarbazones R1R2C = NNHC(O)NH2, showed that the configuration of the C=N bond exists mostly in the E form, which was found to be the major product according to the nuclear over house effect experiments of the 1H-NMR spectra due to less steric factor [7]. Therefore, the structures of compounds (2b-4b) are drawn in the E form depending on 1H-NMR spectra.


## 3.3. 1,2,3-Thiadiazoles

The 1,2,3-thiadiazoles were obtained by treating semicarbazones(2b-4b) with thionyl chloride at room temperature as shown in Scheme 3. 1,2,3-Thiadiazoles (2c-4c) were obtained in 70, 60, and 68.4% yields and found to decompose at 140, 135, and 180 C, respectively. IR spectra for compounds (2c-4c) from supplementary Tables 8, 10, and 12 showed the absence of the bands at 3480, 3334, and 3427 cm−1 for–NH and formation of new bands at (1469, 1442), (1477, 1450), (1514, 1449) cm−1 for the 1,2,3-thiadiazole rings, respectively.1H-NMR spectra showed a singlet peak at 8.53, 8.56, 8.51 ppm, respectively, which is attributed to the proton at carbon number five of the 1,2,3-thiadiazole ring. 13C-NMR spectra showed that the peaks due to C4 and C5 appeared at 162.6, 161.07, 162.7 ppm and 129.93, 130.10, 158.77 ppm, respectively. From these data, the active α-methyl group of the semicarbazones (2b-4b) had disappeared. The 1H-NMR, 13C-NMR, IR, mp, mass spectroscopy, and elemental analysis data of compounds (2c-4c) are shown in supplementary Tables 8, 10, and 12.


## 3.4. 1,2,3-Selenadiazoles

The 1,2,3-selenadiazole compounds(2d-4d) were obtained by treating semicarbazones (2b-4b) with selenium dioxide in glacial acetic acid as shown in Scheme 1. Compounds (2d) and (3d) were obtained in 60% yield whereas (4d) was obtained in 68.4% yield. The reaction of semicarbazones (2b-4b) is very slow due to low solubility in acetic acid.IR spectra of the 1,2,3-selenadiazoles(2d–4d) showed appearance of characteristic bands in 1421–1480 cm−1 region for 1,2,3-selenadiazole heterocyclic ring and the disappearance of the peaks at 3480, 3334, 3427 cm−1 that is characteristic of semicarbazones–NH group. The 1H-NMR spectra showed a single peak that appears downfield at 9.26, 9.19, 9.31 ppm which is due to the proton at carbon number five of the heterocyclic ring. The 13C-NMR spectra showed a chemical shift due to C4 and C5 at 162.60, 162.79, 162.70 ppm and 129.93, 131.94, 156.79 ppm for 1,2,3-selenadiazoles (2d–4d), respectively. The 1H-NMR, 13C-NMR, IR, mp, mass spectroscopy, and elemental analysis data of compounds (2c–4c) are shown in supplementary Tables 9, 11, and 13.


## 3.5. Antimicrobial Activity

The activity of the heterocyclic compounds(2c), (2d), (3c), (3d), (4c), and (4d) was tested against some human pathogenic microbes including Gram-positive S. aureus, Gram-negative E. coli, local resistant P. aeruginosa and reference P. aeruginosa (ATCC 27853), and C. albicans by the hole diffusion method [9,15]. As shown in Table 1, all six heterocyclic compounds had positive antimicrobial activity at a concentration of 0.01 g/mL, but they varied in their antimicrobial coverage. The extent of antimicrobial action also differed among compounds as noted by examining the diameter of the inhibition zone at a constant low concentration when using the hole diffusion technique. Heterocyclic compounds (2c) and (2d) had positive activity against S. aureus and E. coli, along with some activity of compound (2c) against C. albicans. However, they did not show any antipseudomonal activity. Compound (3c), like compound (2c), showed positive activity against S. aureus and even better activity against E. coli and C. albicans. Furthermore, compounds (3c) and (3d) showed positive antimicrobial activity against local resistant P. aeruginosa.Table 1
Sensitivity of human pathogenic microbes to the new synthetic heterocyclic compounds, and some complexes using the hole method.
CompoundCompound concentration (g/mL)Staphylococcus aureusEscherichia coliCandida albicansLocal resistantPseudomonas aeruginosaReferencePseudomonas aeruginosa ATCC 278532c0.01+++++−−2d0.01++−−−3c0.01++++++++−3d0.01−+++++−4c0.01+++++++++++++4d0.01++++++++++++Results were expressed as ranges of diameters of zones of inhibition of antimicrobial activity. An inhibition zone diameter (ID) < 5 mm was denoted as “+”, ID of 10 mm-5 mm was denoted as “++”, and ID of >10 mm was denoted as “+++”. The “−” indicates no inhibition zone. Standard antimicrobials were used for each of the above-tested microorganisms, where the zone of inhibition for each standard compound was >10 mm “+++” for its corresponding microorganism.Another remarkable finding was the broad-spectrum coverage of heterocyclic compounds (4c) and (4d) in which both compounds exhibited antimicrobial activity against all microorganisms explored. After examining the diameter of inhibition zone, interestingly, they both had substantial antipseudomonal activity against local resistantP. aeruginosa and reference P. aeruginosa (ATCC 27853). The solvent showed no activity against any of the tested pathogens.The results of the present study demonstrated that heterocyclic compounds (4c) and (4d) have high potential as novel broad-spectrum antibacterial agents with promising antipseudomonal activity. Additionally, by observing the diameter of the inhibition zone, heterocyclic compound (3c) had the most activity againstC. albicans with activity against some Gram-positive and Gram-negative bacteria. Thus, compound (3c) has potential as a novel antifungal and antibacterial agent.


## 4. Conclusions

New compounds of ketones, semicarbazones, 1,2,3-thiadiazoles, and 1,2,3-selenadizoles were prepared. All prepared compounds were fully characterized by spectroscopic techniques. The antimicrobial activity of the prepared heterocyclic 1,2,3-thiadiazoles and 1,2,3-selenadizoles was explored. All heterocyclic compounds had positive antimicrobial activity but differed in their spectrum of antimicrobial coverage and extent of action. Results were promising of several compounds with broad-spectrum antimicrobial and antipseudomonal action, in addition to other compounds with antifungal activity. These results come at the time of urgent need to develop novel antimicrobial agents to overcome the emerging resistance worldwide. Future work based on the current study should focus on molecular docking and X-ray crystallographic analysis. In the present study, the compounds were obtained as fine amorphous powder. In a future study, we recommend obtaining fine crystals to carry out the X-ray crystallographic study for other compounds. We also recommend preparing their metal complexes and testing their antimicrobial activity against certain standard or reported drug.



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