Introduction

Thiazolidin-4-one derivatives are a bunch of heterocyclic organic compounds with a 5-membered saturated ring. There has been considerable interest in the chemistry of thiazolidin-4-one ring systems 1, which is the core structure in a variety of synthetic pharmaceuticals with a wide range of biological activities [1] such as antifungal [2], antiproliferative [3], anti-inflammatory [4], antimalarial [5], herbicidal [6] and antiviral properties [7].

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In recent years, great advances have been carried out in the treatment of diseases, but there are still challenges such as antibiotic resistance. So, there is a serious need to design and synthesis of new antibiotics [8]. Heterocyclic compounds such as thiazolidin-4-one to have sulfur and nitrogen atoms in their molecular structure exhibit antimicrobial properties. Environmental and health issues caused by chemicals have developed the use of natural compounds in the synthesis of compounds. Essential oils are composed of complex volatile secondary plant metabolites that can be physically separated from other plant components or membranous tissue [9]. The compositions of essential oils, a widespread range of antimicrobial potentials and low level of toxicity [10] have enhanced their use in perfumes, pharmaceutical and cosmetic industries as well as food preservatives and additives [11]. Anethum graveolens L. (dill) is an aromatic and annual herb of apiaceae family and it has been used in folk medicines such as carminative, antispasmodic, sedative, [12]. Studies on the chemical compositions of the essential oil of A. graveolens have revealed carvone and limonene as the main components [13]. Semi-synthesis is a good way to use of natural compound as raw material for synthesis of chemical compounds. Production of biologically active compound from plants (especially essential oil of plants) without isolation and purification is a new method and cost-effective. Essential oils having hydroxy and carbonyl functional groups are eco-friendly materials for chemical reactions. In this report, some novel thiazolidin-4-one derivatives were synthesized in the reaction of essential oils containing carbonyl groups with thiosemicarbazide 2.

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Experimental

General

Ethanol and thiosemicarbazide were obtained from Merck (Darmstadt, Germany). Magnesium sulfate, dimethyl acetylenedicarboxylate (DMAD), diethyl acetylenedicarboxylate (DEAD) and di-tert-butyl acetylenedicarboxylate (DTAD) were purchased from Sigma-Aldrich Chemical Company, UK. All other chemicals and solvents in analytical grade were available commercially. Melting points were recorded on an Electrothermal-9100 apparatus. UV–Vis spectra were obtained with Optizen 2120UV plus. FT-IR spectra were recorded on a JASCO FT-IR 460plus spectrometer. 1H NMR and 13C NMR spectra were recorded on a BRUKE DRX-400 & 300 Avance spectrometer using CDCl3 as solvent. Mass spectra were recorded on an Agilent Technologies MS-5973 (70 eV) mass spectrometer.

Extraction of essential oils

Anethum graveolens was cultivated at the greenhouse of Sistan and Baluchestan University (latitude/longitude 6051′32″, 2927′35″) in Zahedan, Iran, and was collected in November 2016 and then was identified by Herbarium of Shiraz University (Voucher number: 55030). Seeds of A. graveolens were used as the crude source, dried in the shade and powdered by the grinder. 100 gr of dried powder was exposed to hydro-distillation for 3 h using a Clevenger type apparatus. The obtained essential oils were collected and anhydrous magnesium sulfate was used to absorb the small amount of water containing essential oils and then stored at 4 °C until use.

GC/MS analysis

The hydrodistillate oil was subjected to GC/MS analysis using a GC/MS Agilent Technologies (GC: 7890B, MS: 5977AMSD) with a HP-5MS capillary column (30 m × 0.25 mm, 0.25 µm). Helium (99.99%) was used as the carrier gas, at a flow rate of 1 mL/min. Injection port temperature was set at 270 °C, column temperature was initially kept at 60 °C for 1 min, then gradually increased to 240 °C at a rate of 5°C/min. Retention indices were calculated for all components using a homologous series of n-alkanes injected in conditions equal to samples ones. Identification of components of essential oil was based on retention indices (RI) relative to n-alkanes and computer matching with the NIST libraries, as well as comparisons of the fragmentation pattern of the mass spectra with data published in the literature [14].

Synthesis of carvonethiosemicarbazone

Ethanol (7 mL) was added to the essential oils containing carbonyl group specially carvone 3, and then add enough distilled water until the solution was turbid. A solution of thiosemicarbazide 2 (1 mM) in ethanol (5 mL) was added slowly to this solution. After a little heating, if necessary, by adding distilled water a homogeneous solution was obtained and then stored until white precipitate of carvonethiosemicarbazone (CTC) 4 was formed and then recrystallized in ethanol 95% to give desired product.

Synthesis of thiazolidin-4-one

A mixture of carvonethiosemicarbazone (CTC) 4 (1 mM), acetylenic ester 5 (1 mM) were dissolved and stirred in 4 mL EtOH. Next, again acetylene ester and EtOH were added drop by drop. After few minutes, precipitate was formed.

In vitro antibacterial and antifungal assays

Antibiotics and antifungal agents were purchased from Sigma-Aldrich. The concentration of bacterial and fungal suspensions was determined using Jenway 6405 UV–Vis spectrophotometer. Gram-negative bacterial strains including Pseudomonas aeruginosa (PTCC 1310), Klebsiella pneumoniae (PTCC 1290), Escherichia coli (PTCC 1399), Shigella flexneri (PTCC 1234), Shigella dysenteriae (PTCC 1188), Proteus mirabilis (PTCC 1776) and Salmonella enterica subsp. enterica (PTCC 1709), and Gram-positive bacterial strains including Enterococcus faecalis (PTCC 1778), Streptococcus agalactiae (PTCC 1768), Streptococcus pneumoniae (PTCC 1240), Listeria monocytogenes (PTCC 1297), Staphylococcus aureus (PTCC 1189), Staphylococcus epidermidis (PTCC 1435), Bacillus cereus (PTCC 1665), Bacillus thuringiensis subsp. kurstaki (PTCC 1494), Rhodococcus equi (PTCC 1633), and fungi including Aspergillus fumigatus (PTCC 5009), Candida albicans (PTCC 5027) and Fusarium oxysporum (PTCC 5115) were prepared from the Persian Type Culture Collection (PTCC), Tehran, Iran. Broth microdilution and disk diffusion susceptibility tests were performed according to CLSI (Clinical and Laboratory Standards Institute) guidelines M07-A9, M26-A, M02-A11, M44-A and M27-A2 [15, 16]. Solution of all synthetic compounds were prepared in 10% DMSO at initial concentrations of 10,240 µg/mL, antibiotics Cefazolin and Gentamicin, and antifungal drugs Canazole and Nystatin were dissolved in double-distilled water at concentrations of 17.6 and 10,240 µg/mL, respectively. The IZD (inhibition zone diameter) values were measured at initial concentrations. All tests were repeated three times and the results were expressed as the average of three independent experiments.

Result and discussion

There are various methods for the synthesis of thiazolidin-4-one using thiosemicarbazone. Ahmadi et al. [17] synthesized thiazolidin-4-one derivatives from thiosemicarbazone in the presence of acetylenedicarboxylate derivatives. They used water as a green solvent and PPh3 as catalyst and also tetrabutylammonium bromide to dissolve raw materials in the water. Additionally, Hassan et al. reported that 2-substituted thiosemicarbazones [18] and aryl thiosemicarbazones [19] react quickly with dimethyl acetylenedicarboxylate (DMAD) to give 1,3-thiazolidin-4-one in high yields. In this work a number of thiazolidin-4-one derivatives were synthesized without catalyst by natural compounds. Since the natural compounds are not harmful for the environment, therefore, interest in the use of natural materials has increased in recent years. Starting material of this reaction were essential oils of A. graveolens seeds commonly known as dill. It is an annual medicinal plant with tiny yellow flowers belonging to the family apiaceae. The concentration of the essential oils is different in various parts of the plant. For example, the aerial parts and leaves contain a high concentration of α-Phellandrene (56–62%) [20, 21] and seeds contain a high concentration of carvone (36–75%) [22,23,24,25]. Since the desired starting material was carvone, therefore, the essential oils of dill seeds were extracted. Yield of the essential oils extracted from 100 gr of seeds was 1.24%. Results from GC–MS of the essential oils are presented in Table 1. Carvone (37.7%) had the highest amound and after d-limonene (33.8%) and Apiol (17.3%) were the abundant compounds. Due to the lack of carbonyl functional group they do not participate in the reaction. It should be noted is that (S)-(+)-carvone isomer is the only isomer presents in dill seeds [26]; therefore, racemic mixture is not formed. The reaction of essential oil containing carvone with thiosemicarbazide in ethanol produces carvonethiosemicarbazone (CTC) precipitate (Scheme 1). The ratio of thiosemicarbazide to carvone of essential oil 1:1 was chosen. The synthesized carvonethiosemicarbazone (CTC) was used for synthesis of thiazolidin-4-one derivatives. The mechanism of synthesis of thiazolidin-4-one derivatives is shown in Scheme 2. Structures and purity of CTC and 6a–c were confirmed by FT-IR, 1H-NMR, 13C-NMR and MS spectral data. The results are shown in Table 2. The reaction of CTC with DMAD was carried out fast and without any catalyst and 6a (95%) was obtained. Additionally, in the reaction with DEAD and DTAD, 6b (92%) and 6c (98%) were synthesized. The reaction was carried out with DTAD at a slower rate, because of spatial prevention of DTAD (Table 2).

Table 1 Chemical compositions of essential oil of Dill (Anethum graveolens)
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Scheme 1
scheme 1

Total reaction of synthesis of thiazolidin-4-one derivatives with essential oil (containing carvone), thiosemicarbazide and acetylenic esters

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Scheme 2
scheme 2

The general mechanism of synthesis of thiazolidin-4-one derivatives from carvonethiosemicarbazone and acetylenic esters

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Table 2 Synthesis of thiazolidin-4-one derivatives from carvonthiosemicarbazone and acetylenic esters
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Inhibitory property of carvonethiosemicarbazone (CTC) 4 and the newly synthesized compounds 6a–c were studied against 9 Gram-positive and 7 Gram-negative pathogenic bacteria as well as 3 fungal strains. The activities as inhibition zone diameter (IZD), minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and minimum fungicidal concentration (MFC) values were presented in Tables 3 and 4. The results were compared with those obtained by antibiotics Cefazolin and Gentamicin, and antifungal drugs Canazole and Nystatin.

Table 3 Antibacterial activity of compounds 4 and 6a–c
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Table 4 Antifungal activity of compounds 4 and 6a-c
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No antibacterial and antifungal activities were observed by thiazolidines 6a and 6b. Derivative 6c was only effective against S. dysenteriae, B. cereus and all the three fungi mentioned above. It seems that increasing the volume of alkoxy ester groups on C-5 thiazolidine were improved antimicrobial effects. Relative membrane permeability of microorganisms allows the exchange of ions and small molecules between the extracellular media and the cell interior. Antimicrobial agents can inhibit their growth by blocking the permeability of the outer membrane [27]. Possibly, the permeability is reduced by increasing the size of the molecule. Inhibitory activity of compound 4 was determined against all the tested bacteria and fungi. This broad-spectrum antimicrobial agent like antibiotic closthioamide contains a thiourea moiety [28]. In addition, action mechanism of thioamide drugs was determined against bacteria tuberculosis and leprosy [29].

Conclusions

In summary, we reported green synthesis of thiazolidin-4-one derivatives using essential oil mixture as starting material. Functional groups in the essential oil compounds are good starting material for synthesis of various compounds. The other advantages of this method compared to other methods are low cost, eco-friendly and easy work-up. In addition, this method represents a simple and efficient procedure, uses mild reaction conditions. Furthermore, it gives nearly quantitative yields without any by-products.

S-(+)-Carvone thiosemicarbazone

Yield: 65% white crystals. m.p: 116–118 °C. Rf (CH2Cl2/AcOEt, v/v 2/1): 0.9. Molecular formula: C11H17N3S. IR (KBr, cm−1), 3158 (NH); 3412, 3280 (NH2). 1H NMR δ (CDCl3, ppm): 1.76 (s, 3H, CH3); 1.90 (s, 3H, CH3); 2.15 (m, 4H, 2CH2); 2.70 (q, 1H, H2C–CH–CH2); 4.75–4.79 (d, 2H, C=CH2); 6.25 (s, 1H, CH=C); 6.40 (s, 1H, NH2); 7.25 (s, 1H, NH2); 8.71 (s, 1H, C=NNH–). 13C NMR δ (CDCl3, ppm): 17.73; 20.60; 29.07; 30.10; 40.62; 110.86; 132.01; 135.36; 149.97 (C-carvone); 146.82 (C=N); 178.98 (C=S). MS m/z: [MH+] found: 224.31; [M+]: 223.34.

{2-[(5-Isopropenyl-2-methyl-cyclohex-2-enylidene)-hydrazono]-4-oxothiazolidin-5-ylidene}-acetic acid methyl ester (6a):

Yield: 95% yellow crystals. m.p: 209–213 °C. Rf (CH2Cl2/AcOEt, v/v 2/1): 0.9. Molecular formula: C16H19O3N3S. UV/Vis 50 ppm λmax: 340 nm. IR (KBr, cm−1): 1181 (OCH3); 1605, 1622 (2 double bond of carvone); 1693, 1716 (2C=O); 2920, 2954 (2 CH3); 3615 (NH). 1H NMR δ (CDCl.3, ppm): 1.68 (s, 3H, CH3); 1.87 (s, 3H, CH3); 2.08–2.35 (m, 4H,, 2CH2); 3.18–3.25 (broad dd, 1H, H2C–CH–CH2); 3.79 (s, 3H, –OCH3); 4.67–4.70 (d, 2H, C=CH2); 6.20 (s, 1H, CH=C); 6.80 (s, 1H, C=CH–CO); 9.46 (s, 1H, NH). 13C NMR δ (CDCl3, ppm): 17.88; 20.91; 30.70; 31.13; 39.13–40.92; 52.83; 110.52; 114.42; 132.64; 137.29; 143.78; 148.10; 159.08; 164.56; 166.14; 166.38. MS m/z (%): 292(100), 253 (61), 227 (23), 187 (71), 132 (90), 106 (85), 85 (83), 53 (75). [M+]: 333 (87).

{2-[(5-Isopropenyl-2-methyl-cyclohex-2-enylidene)-hydrazono]-4-oxo-thiazolidin-5ylidene}-acetic acid ethyl ester (6b):

Yield: 92% yellow crystals. m.p: 164–166 °C. Rf: (AcOEt 10%) 0.1. Molecular formula: C17H21O3N3S. UV/Vis 50 ppm λmax: 320 nm. IR (KBr, cm−1): 1196 (OCH2CH3); 1615, 1635 (2 double bond of carvone); 1698, 1709 (2C=O); 2919, 2969 (2CH3), 3132 (NH). 1H NMR δ (CDCl.3, ppm): 1.25–1.30 (t, 3H, –O–CH2–CH3); 1.65 (s, 3H, CH3); 1.87 (s, 3H, CH3); 2.04–2.34 (m, 4H, 2CH2); 3.17–3.23 (broad dd, 1H, H2C–CH–CH2); 4.20–4.27 (q, 2H, –O–CH2–CH3); 4.66–4.69 (d, 2H, C=CH2); 6.19 (s, 1H, CH=C); 6.79 (s, 1H, C=CH–CO); 9.65 (s, 1H, NH). 13C NMR δ (CDCl3, ppm): 14.20; 17.83; 20.58; 31.03–31.10; 41.26; 61.68; 110.17; 116–66; 133.34; 136.97; 142.10; 147.76; 157.92; 165.64–165.96. MS m/z (%): 106 (100), 306 (73), 267 (45), 201 (57), 132 (94), 79 (74), 53 (99). [M+]: 347 (71).

{2-[(5-Isopropenyl-2-methyl-cyclohex-2-enylidene)-hydrazono]-4-oxo-thiazolidin-5ylidene}-acetic acid tert-butyl ester (6c)

Yield: 98% yellow crystals. m.p: 153–156 °C. Rf: (AcOEt 10%) 0.4. Molecular formula: C19H25O3N3S. UV/Vis 50 ppm λmax: 335 nm. IR (KBr, cm−1): 1156 (t-BuO); 1607, 1627 (2 double bond of carvone); 1691, 1711 (2C=O), 2923, 2979 (2CH3), 3144 (NH). 1H NMR δ (CDCl.3, ppm): 1.47 (s, 9H, –O–C(CH3)3); 1.64 (s, 3H, CH3); 1.87 (s, 3H, CH3); 2.02–2.38 (m, 4H, 2CH2); 3.16–3.22 (broad dd, 1H, H2C–CH–CH2); 4.65–4.68 (d, 2H, C=CH2); 6.19 (s, 1H, CH=C); 6.71 (s, 1H, C=CH–CO); 10.11 (s, 1H, NH). 13C NMR δ (CDCl3, ppm): 17.88; 20.56; 27.92–28.10; 31.03–31.06; 41.28; 82.60; 110.18; 118.95; 133.32; 136.80; 140.58; 147.76; 158.42; 164.42–166.22. MS m/z (%) 57 (100), 319 (18), 278 (56), 239 (37), 213 (15), 173 (40), 132 (66), 107 (60), 81 (34). [M+]: 375 (32).