Ultrasound-assisted green synthesis and antimicrobial assessment of 1,3-thiazoles and 1,3,4-thiadiazines

ABSTRACT In this article, we applied ultrasonic irradiation as an ecofriendly, green, efficient approach for the synthesis of a new 1,3-thiazoles and 1,3,4-thiadiazines under solvent-free conditions. Thus, thiocarbohydrazide condenses with N-(thiazol-2-yl)acetamide (2) in different molar ratios under ultrasonic irradiation afforded mono and bis thiocarbohydrazons 3a,b, respectively. Thiocarbohydrazones were reacted with α-haloketone, α-haloester as well as hydrazonoyl halides under ultrasonic conditions afforded 1,3-thiazole 4, 1,3-thiazolinone 5 and 1,3,4-thiadiazines 6 and 11a–c, respectively. The reaction of monohydrazone 3a with phthalic anhydride derivatives afforded phthalazines 7a,b in a good yield. Ultrasonic irradiation has benefits such as greater purity, lower cost, high yields, and simple workups compared to the conventional methods. All the new compounds were screened for antimicrobial activity against different tested microbes. Most of the newly synthesized compounds exhibited potent to moderate activities toward the tested microbes. Compounds that exhibited potent antimicrobial activities were selected to measure their MIC and MBC values. The results revealed that compounds 3a, 3b, 8, 10, and 11a showed lower MIC against Staphylococcus aureus with the values of 39.06, 78.13, 9.77, 39.06 and 39.06 µg/ml, respectively. Meanwhile, the MBC values were found to be acceptable for 3a and 10 of 156 and 312, respectively. GRAPHICAL ABSTRACT


Chemistry
The synthetic pathways for the target compounds are illustrated in Schemes 1-3. Ultrasonic irradiation is a simple method for preparing organic compounds with high yield, pure products, low costs, less energy consumption for chemical transformations and less release of hazardous chemicals to air, as it is solvent-free reactions. Firstly before we move on to the ultrasoundassisted green synthesis of 1,3-thiazoles and 1,3,4-thiadiazines, we would like to refer that, in 2015 Abdel-Aziem reported the synthesis of new derivatives of 1,3thiazoles and 1,3,4-thiadiazines linked to the coumarin moiety under thermal conditions (traditional method) (32). By applying the ultrasound technique, the reaction is completed in a short time in addition, high yields and pure products are obtained compared to the traditional method. Thus, acetylation of 2-aminothiazole (1) with acetic anhydride by irradiation for 20 min at 70°C under ultrasound irradiation afforded N-(thiazol-2-yl)acetamide (2). The formation of acetyl derivative 2 was confirmed by the 1 H NMR spectrum which displayed new signals at 2.14 and 12.06 ppm for CH 3 and NH function, besides two doublet signals at 7.17 and 7.44 ppm assigned for H2 and H3 of thiazole, respectively. The reaction of 2 with thiocarbohydrazide (40) in 1:1 and 2:1 ratio under ultrasonic irradiation for 20 min at 70°C furnished the desired mono and bishydrazones 3a,b, respectively, in a good yield (Scheme 1). The amide nitrogen is poor nucleophile (they do not undergo nucleophilic addition on carbonyl carbon) due to the mesomeric effect (Scheme 1). An iminol nitrogen is much more nucleophilic than the corresponding amide nitrogen (41). Since amides are stable to nucleophilic substitution, they must be activated before the reaction. The addition of acid protonates the amide oxygen converting it to the iminol form. As a result, it is reasonable to suppose that compounds 3a and 3b are produced by protonation of the carbonyl oxygen followed by nucleophilic addition of the thiocarbohydrazide molecule to the positively charged carbonyl carbon. The final step is the formation of the final product via the elimination of water molecules.
Based on spectroscopic data, the structures of the synthesized compounds were verified. The IR spectrum of hydrazones 3a,b demonstrated peaks in the region 3400, 3350, 3270 cm −1 assignable to NH 2 , and N-H function, additionally to 1260-1282 cm −1 for C═S function. Furthermore, 1  Moreover, the treatment of bishydrazones 3b with phenacyl chloride and/or ethyl bromoacetate in anhydrous sodium acetate under ultrasonic irradiation for 60 min at 60°C afforded 1,3-thiazole 4 and 1,3-thiazolinone 5, respectively (Scheme 2). The spectroscopic data of compounds 4 and 5 supported their structures. In the infrared spectrum of 5 absorption peaks at 3250, 3190, 1710 and 1605 cm −1 were observed, which correspond to 2NH, C=O and C=N, respectively. The 1 H NMR of 5 demonstrated a singlet signal at δ 2.11 and 4.33 ppm for two methyl and methylene protons, 7. 13 and 7.40 ppm, two doublet signals for H2 and H3 thiazole and singlet signals at 12.02 for the 2NH function.
Similarly, monohydrazones 3a was reacted with phenacyl chloride under the previous ultrasonic conditions afforded 1,3,4-thiadiazines 6 based on spectral data. The 1 H NMR spectrum of 6 recoded singlet signals at δ 2.14 and 4.99 ppm corresponds to methyl and methylene groups. In addition, two doublet signals at 7.14 and 7.44 ppm are attributed to H2 and H3 of the thiazole moiety besides multiplet at δ 7.56-8.04 ppm for the phenyl group and NH protons, as well as a singlet signal at 12.06 ppm for NH. Also, the reaction of 3a with tetrachloro or tetrabromophthalic anhydrides gave the corresponding phthalazines 7a,b, respectively (Scheme 3). The infrared spectrum of 7a,b displayed characteristic absorption peaks at 1750 and 1760 cm −1 for two carbonyl groups, while the 1 H NMR spectrum of 7b recorded a singlet signal at δ 2.11 ppm corresponding to the CH 3 group. In addition, two doublet signals at δ 7.14 and 7.44 ppm are attributed to H2 and H3 of thiazole, besides singlet signals at δ 9.60 and 12.02 ppm for 2NH and NH protons. The hydrazinyl hydrazine 8 was prepared from the reaction of monohydrazone 3a with hydrazine hydrate via loss of hydrogen sulphide.

Antimicrobial activity and (SAR)
All the newly prepared compounds have been screened for the antimicrobial activity against different tested microbes including Staphylococcus aureus ATCC 6538 (G+ve), Escherichia coli ATCC 25922 (G−ve), Candida albicans ATCC 10231 (yeast), and Aspergillus niger NRRL A-326 (fungus), using the cup diffusion method (45) at a single dose of 250 µg. The antibiotic neomycin and the antifungal cyclohexamide were used as positive controls. The results are shown in Table 1 and Figure 1 as a growth inhibition zone (mm).
The following are the findings of the SAR investigation: It has been noticed that the antimicrobial activity is significantly influenced by different groups attached to thiazolyl moiety. The most active compounds were mono and bishydrazons 3a,b, thiazol 4, hydrazinyl 8, and 1,3,4-thiadiazine bearing p-nitrophenyl group 11c.   were evaluated from the cup diffusion method (Table 2, Figure 2

Experimental section
All reagents and solvents were of analytical grade. All melting points of the synthesized compounds were measured on a digital Gallen Kamp MFB-595 instrument and maybe uncorrected. A Shimadzu 440 spectrophotometer was utilized for recording the infrared spectra. 1 H and 13 C NMR spectra of the new derivatives were measured on a Bruker spectrometer at 400 and 100 MHz, respectively, and TMS is the internal standard. The symbols (s) singlet, (d) doublet, (t) triplet, (q) quartet, and (m) multiplet were expressed for hydrogen coupling patterns and chemical shifts (δ) were defined as parts per million (ppm) relative to the solvent peak.

Synthesis of thiocarbohydrazones 3a,b
A mixture of N-(thiazol-2-yl)acetamide (3) and thiocarbohydrazide (in molar ratio 1:1 or 2:1) were mixed together with a few drops of acetic acid and then transferred to Pyrex tubes. The reaction mass was irradiated in a water bath under ultrasonic irradiation at 60°C for 20 min. The reaction progress was monitored through this period using TLC. The produced solid recrystallized from ethanol furnished 3a, b, respectively.

General method for synthesis of 1,3-thiazole 4 and 1,3-thiazolinone 5
An equimolar ratio of 3b, phencyl bromide and/or ethyl bromoacetate (0.01 mol each), and (0.004 mol) anhydrous sodium acetate were mixed together then transferred to Pyrex tubes. After the addition of 3-4 drops of ethanol, the reaction mass was irradiated in a water bath under ultrasonic irradiation at 60°C for 60 min. The reaction progress was monitored through this period using TLC until it is completed. The obtained products 4 and 5 were recrystallized using ethanol. An equimolar ratio of 3a, phencyl bromide (0.01 mol), and (0.004 mol) of anhydrous sodium acetate were mixed together then transferred to Pyrex tubes. After the addition of 3-4 drops of ethanol, the reaction mass was irradiated in a water bath under ultrasonic irradiation at 60°C for 60 min. The reaction progress was monitored through this period using TLC. The orange solid was recrystallized using ethanol afforded 6.

General method for synthesis of phthalazines 7a,b
An equimolar ratio of 3a (0.01 mol), tetrachlorophthalic anhydride, or tetrabromophthalic anhydride (0.01 mol) and a catalytic amount of triethylamine were mixed together, then transferred to Pyrex tubes. The reaction mass was irradiated in a water bath under ultrasonic irradiation at 60°C for 60 min. The reaction progress was monitored through this period using TLC. The solid produced was recrystallized from ethanol to give pure products of 7a,b, respectively.

Synthesis of hydrazine derivative 8
A mixture of 3a (0.01 mol) and (0. 1 mol) of hydrazine hydrate was mixed together then transferred to Pyrex tubes. After the addition of 3-4 drops of ethanol, the reaction mass was irradiated in a water bath under ultrasonic irradiation at 60°C for 60 min. The reaction progress was monitored through this period using TLC. The formed solid was recrystallized from ethanol to give 8. An equimolar ratio of 3a (0.01 mol), the appropriate hydrazonoyl halides 9a-d (0,01 mol) and three drops of triethylamine were mixed together and then transferred to Pyrex tubes. The reaction mass was irradiated in a water bath under ultrasonic irradiation at 60°C for 60 min. The reaction progress was monitored through this period using TLC. The product was recrystallized from ethanol to give pure products of 10, 11 (a-c), respectively.

Conclusions
Considering the ultrasound irridiation as a green, environmentally friendly and as an efficient organic chemical methodology, we reported herein the synthesis of some new 1,3-thiazole and 1,3,4-thiadiazine derivatives by utilizing the ultrasound irradiation technique. All the new compounds have been screened for antimicrobial activity against different tested microbes. Most of the newly synthesized compounds exhibited potent to moderate activities toward the tested microbes. The MIC and MBC values were found to be acceptable for the most synthesized compounds.

Disclosure statement
No potential conflict of interest was reported by the author(s).