All chemicals were of analytical grade and commercially available obtained from Sigma-Aldrich, Germany. All reagents and solvents were used without further purification and drying. ¹H NMR spectra of compounds in DMSO-d6 solution were registered on a spectrometer Varian Mercury VX-400, USA (400 MHz), internal reference TMS. The elemental analysis experimental data on contents of Carbon, Hydrogen and Nitrogen correspond to those calculated (±0.3%) and were registered on a Elementar Vario L cube, Germany. Chemical shifts are reported in ppm units with use of δ scale.

Ascorbic acid was purchased from a medical store.

General procedure for the synthesis 3-aryl-4-imino-thiazolidine-2-ones (1a-d). The mixture of the 25% ammonia solution (5 mmol) and the appropriate 3-aryl-4-thioxo-thiazolidine-2-one (5 mmol) was refluxed for 15 min. On cooling, the crystalline precipitate was filtered off, washed with acetone and dried. The obtained compounds were re-crystallised from acetic acid.

4-Imino-3-phenyl-thiazolidin-2-one (1a). Yield 58%, m.p.= 245 °C. ¹H NMR (400 MHz, DMSO-d6) d 5.83 (s, 2H, CH₂), 6.90 (t, 1H, J = 7.3 Hz, C₆H₅), 7.22 (t, 2H, J = 8.0 Hz, C₆H₅), 7.39 (d, 2H, J = 7.7 Hz, C₆H₅), 8.50 (s, 1H, NH). Calcd for C₉H₈N₂OS%: C, 56.23; H, 4.19; N, 14.57. Found %: C, 56.13; H, 4.07; N, 14.36.

4-Imino-3-(4-nitro-phenyl)-thiazolidin-2-one (1b). Yield 54%, m.p.= 230 °C. ¹H NMR (400 MHz, DMSO-d6) d 5.81 (s, 2H, CH₂), 7.25 (d, 2H, J = 8.1 Hz, C₆H₄), 7.48 (d, 2H, J = 7.3 Hz, C₆H₄), 8.53 (s, 1H, NH). Calcd for C₉H₇N₃O₃S %: C, 45.57; H, 2.97; N, 17.71. Found %: C, 45.59; H, 2.88; N, 17.66.

4-Imino-3-(4-chloro-phenyl)-thiazolidin-2-one (1c). Yield 40%, m.p.= 197 °C. ¹H NMR (400 MHz, DMSO-d6) d 5.64 (s, 2H, CH₂), 7.12 (d, 2H, J = 7.9 Hz, C₆H₄), 7.38 (d, 2H, J = 8.1 Hz, C₆H₄), 8.51 (s, 1H, NH). Calcd for C₉H₇ClN₂OS %: C, 47.69; H, 3.11; N, 12.36. Found %: C, 47.78; H, 3.19; N, 12.47.

4-Imino-3-(4-fluoro-phenyl)-thiazolidin-2-one (1d). Yield 45%, m.p.= 224 °C. ¹H NMR (400 MHz, DMSO-d6) d 5.75 (s, 2H, CH₂), 7.21 (d, 2H, J = 7.7 Hz, C₆H₄), 7.46 (d, 2H, J = 8.4 Hz, C₆H₄), 8.48 (s, 1H, NH). Calcd for C₉H₇FN₂OS %: C, 51.42; H, 3.36; N, 13.33. Found %: C, 52.07; H, 3.28; N, 13.15.

General procedure of synthesis of 5-arylidene derivatives 3-phenyl-4-iminothiazolidine-2-one (2a-2i). To 15 ml of acetate acid, 0.005 mol of compound 1a, 0.005 mol of the corresponding aromatic aldehyde and a few drops of monoaminoethanol were added. The mixture is boiled for 30 minutes. The crystalline precipitate, which was obtained after cooling, is filtered off, washed with water and dried. The resulting compounds are recrystallised from acetate acid.

5-(4-Fluoro-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2a). Yield 70%, m.p.= 260–262 °C. ¹H NMR (400 MHz, DMSO-d6) d 6.93 (t, 1H, J = 7.3 Hz, C₆H₅), 7.28 (t, 2H, J = 8.0 Hz, C₆H₅), 7.44 (d, 2H, J = 7.7 Hz, C₆H₅), 7.42 (d, 2H, J = 8.5 Hz, C₆H₄), 7.59–7.64 (m, 2H, C₆H₄), 7.84 (s, 1H, CH), 8.54 (s, 1H, NH). Calcd for C₁₆H₁₁FN₂OS %: C, 64.42; H, 3.72; N, 9.39. Found %: C, 64.25; H, 3.74; N, 9.33.

5-(4-Chloro-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2b). Yield 61%, m.p.= 253–254 °C. ¹H NMR (400 MHz, DMSO-d6) d 6.89 (t, 1H, J = 7.3 Hz, C₆H₅), 7.22 (t, 2H, J = 8.0 Hz, C₆H₅), 7.36 (d, 2H, J = 7.7 Hz, C₆H₅), 7.45 (d, 2H, J = 8.5 Hz, C₆H₄), 7.54 (d, 2H, J = 8.5 Hz, C₆H₄), 7.83 (s, 1H, CH), 8.57 (s, 1H, NH). Calcd for C₁₆H₁₁ClN₂OS %: C, 61.05; H, 3.52; N, 8.90. Found %: C, 61.27; H, 3.44; N, 8.85.

5-(4-Methoxy-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2с). Yield 65%, m.p.= 248 °C. ¹H NMR (400 MHz, DMSO-d6) d 3.79 (s, 1H, O-CH₃), 6.91 (t, 1H, J = 7.3 Hz, C₆H₅), 7.15 (d, 2H, J = 8.8 Hz, C₆H₅), 7.31 (t, 2H, J = 8.0 Hz, C₆H₅), 7.47 (d, 2H, J = 8.7 Hz, C₆H₄), 7.55 (d, 2H, J = 8.8 Hz, C₆H₄), 7.79 (s, 1H, CH), 8.55 (s, 1H, NH). Calcd for C₁₇H₁₄N₂O₂S %: C, 65.79; H, 4.55; N, 9.03. Found %: C, 65.07; H, 4.59; N, 9.15.

5-(3,4-Dimethoxy-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2d). Yield 55%, m.p.= 240–242 °C. ¹H NMR (400 MHz, DMSO-d6) d 3.77 (s, 1H, O-CH₃), 3.81 (s, 1H, O-CH₃), 6.95 (t, 1H, J = 7.3 Hz, C₆H₅), 7.04 (s, 1H, J = 8.5Hz, C₆H₃), 7.15 (d, 2H, J = 8.8 Hz, C₆H₅), 7.26 (t, 2H, J = 8.0 Hz, C₆H₅), 7.42 (d, 2H, J = 7.7 Hz, C₆H₃), 7.77 (s, 1H, CH), 8.51 (s, 1H, NH). Calcd for C₁₇H₁₄N₂O₂S %: C, 63.51; H, 4.74; N, 8.23. Found %: C, 63.11; H, 4.78; N, 8.16.

5-(4-Hydroxy-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2e). Yield 52%, m.p.= 220 °C. ¹H NMR (400 MHz, DMSO-d6) d 6.89 (d, 2H, J = 8.0 Hz, C₆H₄), 6.95 (t, 1H, J = 7.3 Hz, C₆H₅), 7.22 (t, 2H, J = 8.1 Hz, C₆H₅), 7.37 (d, 2H, J = 7.6 Hz, C₆H₅), 7.44 (d, 2H, J = 8.0 Hz, C₆H₄), 7.77 (s, 1H, CH), 8.54 (s, 1H, NH), 10.25 (s, 1H, OH). Calcd for C₁₆H₁₂N₂O₂S %: C, 64.95; H, 4.08; N, 9.45. Found %: C, 65.05; H, 4.14; N, 9.33.

5-(4-Hydroxy-3-methoxy-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2f). Yield 57%, m.p.= 235–236 °C. ¹H NMR (400 MHz, DMSO-d6) d 3.81 (s, 1H, O-CH₃), 6.93 (t, 1H, J = 7.3 Hz, C₆H₅) , 7.05 (s, 1H, J = 8.0Hz, C₆H₃), 7.20 (d, 2H, J = 8.1 Hz, C₆H₅), 7.35 (t, 2H, J = 7.6 Hz, C₆H₅), 7.46 (d, 2H, J = 8.0 Hz, C₆H₃), 7.79 (s, 1H, CH), 8.52 (s, 1H, NH), 9.91 (s, 1H, OH). Calcd for C₁₇H₁₄N₂O₃S %: C, 62.56; H, 4.32; N, 8.58. Found %: C, 62.09; H, 4.33; N, 8.49.

5-(4-Methyl-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2g). Yield 57%, m.p.= 212 °C. ¹H NMR (400 MHz, DMSO-d6) d 2.12 (s, 1H, CH₃), 6.88 (t, 1H, J = 7.3 Hz, C₆H₅), 7.11 (d, 2H, J = 8.8 Hz, C₆H₅), 7.29 (t, 2H, J = 8.0 Hz, C₆H₅), 7.38 (d, 2H, J = 7.9 Hz, C₆H₄), 7.55 (d, 2H, J = 7.9 Hz, C₆H₄), 7.87 (s, 1H, CH), 8.60 (s, 1H, NH). Calcd for C₁₇H₁₄N₂O₂S %: C, 69.36; H, 4.79; N, 9.52. Found %: C, 69.25; H, 4.77; N, 9.49.

5-(4-Bromo-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2h). Yield 68%, m.p.= 253 °C. ¹H NMR (400 MHz, DMSO-d6) d 6.77 (t, 1H, J = 7.3 Hz, C₆H₅), 7.18 (t, 2H, J = 8.0 Hz, C₆H₅), 7.33 (d, 2H, J = 7.7 Hz, C₆H₅), 7.54 (d, 2H, J = 8.0 Hz, C₆H₄), 7.65 (d, 2H, J = 8.0 Hz, C₆H₄), 7.77 (s, 1H, CH), 8.51 (s, 1H, NH). Calcd for C₁₆H₁₁BrN₂OS %: C, 53.49; H, 3.09; N, 7.80. Found %: C, 53.37; H, 3.12; N, 7.82.

5-(4-Nitro-benzylidene)-4-imino-3-phenyl-thiazolidin-2-one (2i). Yield 49%, m.p.= 269–270 °C. ¹H NMR (400 MHz, DMSO-d6) d 6.87 (t, 1H, J = 7.3 Hz, C₆H₅), 6.96 (d, 2H, J = 8.0 Hz, C₆H₄), 7.24 (t, 2H, J = 8.0 Hz, C₆H₅), 7.41 (d, 2H, J = 7.7 Hz, C₆H₅), 7.52 (d, 2H, J = 8.0 Hz, C₆H₄), 7.77 (s, 1H, CH), 8.51 (s, 1H, NH). Calcd for C₁₆H₁₁N₃O₃S %: C, 59.07; H, 3.41; N, 12.92. Found %: C, 59.79; H, 3.44; N, 12.87.

4-Imino-3-phenyl-thiazolidine-2,5-dione 5-oxime (3). 0.05 mol of compound 1a are added in 50 ml of 10% HCl, cooled to 0 °C and a solution of 10.5 g sodium nitrite in 25 ml of water is added to the suspension by dropping during stirring and cooling for 1 h. The mixture is left at room temperature for 12 hours. The precipitate is filtered off, washed with water, acetone and dried at 60 °C. Yield 65%, m.p.= 182 °C. ¹H NMR (400 MHz, DMSO-d6) d 6.89 (t, 1H, J = 7.3 Hz, C₆H₅), 7.19 (t, 2H, J = 8.0 Hz, C₆H₅), 7.39 (d, 2H, J = 7.7 Hz, C₆H₅), 8.50 (s, 1H, NH), 10.05 (s, 1H, OH). Calcd for C₉H₇N₃O₂S %: C, 48.86; H, 3.19; N, 18.99. Found %: C, 49.03; H, 3.21; N, 18.90.

The antioxidant activity was determined on the basis of the free radical scavenging activity of stable 2,2-Diphenyl-1-picrylhydrazyl (DPPH). The effect of the studied compounds on DPPH radicals was estimated according to the method of Blois (Blois 1958; Molyneux 2004) with minor modifications. The solution of DPPH in ethanol with a concentration of 150 μmoles/l (4 ml) was mixed with the compound or control solution in ethanol, its concentration being 250 μmoles/l (0.2 ml). The reaction mixture was thoroughly vortex-mixed and incubated at room temperature in the dark for 60 min. Simultaneously, a control was prepared as ascorbic acid solution in ethanol (0.2 ml) mixed with DPPH solution in ethanol (4 ml) without sample fraction. Reduction in the absorbance of the mixture was measured at 540 nm using ethanol as blank. Ascorbic acid was used as a standard. The absorbance of DPPH solution was also measured. The percentage of free-radical-scavenging activity was expressed as percent inhibition and it was calculated using the following formula:

;

where A_DPPH is the absorbance of DPPH free radicals solution, A_c is the absorbance of a sample.

Each experiment was performed in triplicate and average values were recorded. Results are expressed as the means ± S.D.

A classical approach to the formation of the 4-thiazolidone ring is the reaction of [2+3]-cyclocondensation. The corresponding 3-aryl-thiazolidine-2,4-diones obtained by the known method (Komaritsa 1970) was introduced into the thionation reaction with phosphorus pentasulphide, which led to yielding the corresponding 3-aryl-4-thioxo-thiazolidine-2-ones. These substances are obtained under conditions of similar synthesis of the known 4-thioxo-thiazolidin-2-one. The synthetic potential of the resulting compounds allowed them to react with a 25% aqueous ammonia solution with the generation 3-aryl-4-imino-thiazolidine-2-ones (1a-d). The reaction mixture reflux for 15 min in 25% aqueous ammonia solution was the optimal condition for compounds 1a-d formation proceeding in good yields (Scheme 1).

Scheme 1. 

Synthesis of 3-aryl-4-imino-thiazolidine-2-ones (1a-d).

This transformation was made possible by the fact that 3-aryl-4-thioxo-thiazolidine-2-ones are cyclic thioamides and, as a consequence, have a significant activity of the thiol group resulting from the greater electrophilicity of the carbon atom in thiocarbonyl group of the mentioned compounds as compared to the carbonyl position of C⁴ 4-aryl-thiazolidine-2,4-diones. One of the proofs of the obtained compounds structure is their acid hydrolysis to the known 3-aryl-thiazolidine-2,4-diones (Scheme 2):

Scheme 2. 

Hydrolysis to the known 3-aryl-thiazolidine-2,4-diones.

The methylene group presence in C⁵ position of compound 1a provides an entry for 5-aryliden derivatives 3-phenyl-4-iminothiazolidine-2-one (2a-i). Acetic acid was found to be the most suitable medium for the Knoevenagel condensation of compound 1a with aromatic aldehydes. The synthetic strategy developed showed the high yielding compounds 2a-i may be achieved by using monoaminoethanol as a catalyst (Scheme 3).

Scheme 3. 

Synthesis of 5-aryliden derivatives 3-phenyl-4-iminothiazolidine-2-one (2a-i).

The next stage of our work was the further functionalisation of compound 1a in position C⁵, in particular, the reaction of nitrosation of the mentioned heterocycle with nitric acid was investigated. It was found that compound 1a reacts with nitric acid formed by the interaction of sodium nitrite with hydrochloric acid. As a result of the reaction, the corresponding 4-imino-3-phenyl-thiazolidine-2,5-dione 5-oxime (3) is formed (Scheme 4).

Scheme 4. 

Synthesis of 4-imino-3-phenyl-thiazolidine-2,5-dione 5-oxime (3) under the nitrosation reaction.

The structures of the obtained compounds were confirmed by ¹H NMR spectroscopy and elemental analysis. All these new compounds gave spectroscopic data in accordance with the proposed structures. The ¹H NMR spectra of all compounds show the protons signals of the imino group in the thiazolidine ring as a singlet in the 8.48–8.57 ppm. The spectra of compounds 1a-d show the signal of methylene group at the C⁵ position as a singlet at 5.64–5.83 ppm. The spectrum of compound 1a shows the signal of the phenyl radical as a duplet and triplet at 7.22 and 7.39 ppm. The lack of the methylene group signals for compounds 2a-i proves the Knoevenagel condensation of the basic scaffold.

The antioxidant activity was determined on the basis of the free radical scavenging activity of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical. The DPPH radical has found many applications due to its high stability in a methanolic solution and its having an intense purple colour. In its oxidised form, the DPPH radical has an absorbance maximum centred at a wavelength about 540 nm. The absorbance decreases when the radical is reduced by antioxidants. Its reduction affords 2,2-diphenyl-1-picrylhydrazine (DPPH-H) or the corresponding anion (DPPH–) in basic medium. The DPPH radical acts as a scavenger for other odd-electron species which provide(?) para-substitution products at phenyl rings.

The DPPH method is described as a simple, rapid and convenient method for screening of many samples for radical scavenging activity. These advantages make the DPPH method interesting for testing newly synthesised compounds to scavenge radicals and to identify promising antioxidant drug candidates.

In the present paper, we demonstrate that the modified spectrophotometric method uses the DPPH radical and its specific absorbance properties. The free-radical-scavenging activities of each compound were assayed using a stable DPPH and were quantified by decolourisation the solution being mixed with DHHP at a wavelength of 540 nm. The absorbance of DPPH solution in ethanol (150 μ moles/l) was measured as 0.770. The absorbances and free-radical-scavenging activities percentage inhibitions of standard (ascorbic acid) and each compound are listed in Table 1.

Table 1.Download as CSV 

Values of absorbance and % inhibition of some novel 4-iminothiazolidine-2-ones.

The compound or standard	Absorbance of a sample, As	% Inhibition	The compound or standard	Absorbance of a sample, As	% Inhibition
Control	0.770±0.025	–	2d	0.731±0.020	5.05
1a	0.689 ±0.015	10.50	2e	0.738 ±0.025	4.11
1b	0.713±0.020	7.41	2f	0.707±0.025	8.17
1c	0.731±0.020	5.03	2g	0.720±0.020	6.52
1d	0.730±0.020	5.22	2h	0.679±0.020	11.83
2a	0.747±0.025	3.05	2i	0.572±0.015	25.65
2b	0.708±0.020	8.08	3a	0.561±0.015	27.15
2c	0.711±0.020	7.63	Ascorbic acid	0.580±0.015	24.68

Antioxidant activity evaluation results showed that most of the researched compounds have a small effect on the release of free radicals in the range 3.05% - 11.83%. c (25.65%, 27.15%) exceeding that for ascorbic acid. Thus, the structure of the substituents in the 5 position of 3-phenyl-4-iminothiazolidin-2-one showed that 4-bromo-benzylidene and 4-nitro-benzylidene increases the antioxidant activity compared with the basic scaffold.