Synthesis, Modeling Study and Antioxidants Activity of New Heterocycles Derived from 4-Antipyrinyl-2-Chloroacetamidothiazoles

Abstract

The present work reports the preparation of twelve new heterocyclic scaffolds containing an antipyrinyl-thiazole hybrid through the reaction of 4-antipyrinyl-2-chloroacetamido-thiazoles 1 and 6 with various types of nucleophiles, namely; ethyl thioglycolate, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, ammonium thiocyanate, malononitrile, and salicylaldehyde. The constructed compounds were characterized by conventional spectroscopic techniques (IR, ¹H NMR, ¹³C NMR, and mass analysis). A DFT method (material studio package) was used to predict the geometry, bond lengths, bond angles, and dipole moments as well as other global chemical reactivities of the constructed antipyrinyl-thiazole compounds. Also, their semi-core pseudopods calculations (dspp) were carried out with DNP (double numerical basis sets plus polarization functional) to predict the properties of materials. In addition, the antioxidant activity of these antipyrinyl-thiazole scaffolds has been screened by the ABTS method. The results indicated that 2-(4-antipyrinylthiazolylamino)-5-substituitedbenzylidene-thiazol-4(5H)-ones 10b and 10c exhibited the best antioxidant activity with a percentage inhibition of 85.74% and 83.51%, respectively.

1. Introduction

The antipyrine nucleus has been utilized as a useful precursor for the construction of many biologically important heterocycles [1,2]. Antipyrine derivatives are of great interest in medicine as a result of their broad range of pharmacological activity and clinical applications, including antibacterial [3], analgesic [4], and anti-inflammatory [4,5] as well as antitumor activity [6,7]. Antipyrine scaffolds are strong inhibitors of cyclooxygenase isoenzymes and platelet thromboxane and prostanoids synthesis [8,9]. In addition, thiazoles are valuable basic units in the field of medicinal science and are found in a wide assortment of bioactive scaffolds [10,11]. It has been well established that they possess antibacterial [12,13], anti-inflammatory [14,15], antitubercular [16,17], anticonvulsant [18,19], and anticancer [20,21,22,23] activities. Aminothiazole-containing medicines have been applied in clinical use for over thirty years, e.g., Famotidine is used to treat and prevent gastroesophageal reflux disease [24,25], Abafungin as an antifungal agent is used in the treatment of dermatomycoses [26] and Cefdinir is a well-known FDA-approved antibiotic and a third generation broad-spectrum cephalosporin [27]. Recently, some new thiazoles have been synthesized and proved to possess antioxidant power and DNA damage inhibition ability [28,29]. In light of these previous reports, we envisaged that integrating antipyrine and thiazole nuclei in one molecule could potentially produce new molecular hybrids with significant synergistic antioxidant activities.

2. Results and Discussion

2.1. Synthesis

The focus of this research paper, 4-antipyrinyl-2-chloroacetamidothiazole (1), has been obtained by stirring 2-amino-4-antipyrinylthiazole with chloroacetyl chloride in DMF and triethylamine for 4 h [30]. Heating of chloroacetamide derivative 1 with ethyl thioglycolate in absolute ethyl alcohol and sodium acetate furnished one product (TLC), which was identified as ethyl 2-[(2-((4-antipyrinylthiazol-2-yl)amino)-2-oxoethyl)thio]acetate (2) based on its spectral and elemental analyses, see Scheme 1.

Chloroacetamide derivative 1 reacted with 3-cyano-4,6-dimethyl-2-mercaptopyridine (3) in acetone and sodium carbonate to afford N-(4-antipyrinylthiazol-2-yl)-2-((3-cyano-4,6-dimethylpyridin-2-yl)thio)-acetamide (4), which underwent intramolecular cyclization in hot ethanolic sodium ethoxide solution furnishing the conforming 3-aminothieno[2,3-b]pyridine derivative 5, as shown in Scheme 1. The formation of 5 may be interpreted through the nucleophilic attack of active methylene on the cyano group according to the mechanistic consideration. Structural proof of 4 and 5 was verified from their compatible spectral analyses including IR, ¹H NMR, and ¹³C NMR. The infrared spectrum of 4 revealed the absorption of nitrile function at 2219 cm⁻¹ that disappeared from the infrared spectrum of compound 5, indicating the cyclization process had occurred. The ¹H NMR spectrum of 4 displayed the two protons of the methylene group as a singlet signal at 4.05 ppm, this signal was no longer present in the spectrum of the product 5, providing additional evidence for the successful cyclization reaction.

The reactivity of C-5 of thiazole ring in the key compound, 4-antipyrinyl-2-chloroacetamidothiazole (1) towards electrophilic diazo-coupling reaction was investigated. Thus, the coupling reaction of compound 1 with diazotized sulphanilamide was carried out in ethanol containing sodium acetate at 0–5 °C to furnish 4-antipyrinyl-2-chloroacetamido-5-(4-sulfamoylphenylazo)thiazole (6), see Scheme 2. The infrared spectrum of 6 showed three characteristic absorptions at 3351, 3264, and 1697 cm⁻¹ to indicate the presence of -NH₂, N-H, and C=O functional groups, respectively. The ¹H NMR spectrum displayed four singlet signals at δ 2.72 ppm for three protons of CH₃-C, at δ 3.28 ppm for three protons of CH₃-N, at δ 4.44 ppm for two protons of -CH₂- group, and at δ 12.92 ppm for one proton of N-H. In addition, a multiplet signal for the aromatic protons and NH₂ (δ 7.28–7.61 ppm), and a doublet signal for the two protons at δ 7.94–8.00 ppm (AA’-BB’ system of the 1,4-disubstituted phenyl ring).

The reaction of 2-chloroacetamido-thiazole derivative 6 with 2-mercaptobenzothiazole and/or 2-mercaptobenzoxazole was achieved by refluxing in ethyl alcohol and sodium acetate for 2 h to furnish the corresponding sulfide derivatives 7a and 7b, respectively. The chemical structures of 7a and 7b were deduced from their compatible spectral and elemental analyses.

Treatment of the key compound 1 with ammonium thiocyanate in refluxing ethanol has been described to furnish 2-((4-antipyrinylthiazol-2-yl)imino)thiazolidin-4-one (9). A plausible mechanism for the reaction is indicated in scheme 3. The reaction proceeds via intramolecular cyclization of the thiocyanate intermediate 8 and the Dimroth-like rearrangements [31]. The product of this reaction was designed as the lactam derivative 9 and finds support from the literature of Vicini et al. [32]. The constructed thiazolidin-4-one scaffold 9 underwent condensation with three para-substituted benzaldehyde derivatives in glacial CH₃COOH and anhydrous CH₃COONa (Knoevenagel condensation reaction) furnishing the corresponding 2-(4-antipyrinylthiazol-2-ylamino)-5-(substituted-benzylidene)-thiazol-4(5H)-one derivatives 10a–c in good yields, see Scheme 3. The chemical structures of 10a–c were determined based on their compatible spectral analyses. Thus, the IR spectrum of 10a showed the characteristic absorptions at 3176 and 1698 cm⁻¹ referring to the functional groups -NH- and carbonyl (C=O), respectively. The ¹H NMR spectrum of the same scaffold exhibited a singlet for three protons at δ 2.61 ppm (CH₃-C), a singlet for three protons at δ 2.28 ppm (CH₃-N), a singlet for three protons at δ 3.84 ppm (OCH₃), and a doublet for two protons at δ 7.14 ppm (aromatic protons). The aromatic and thiazole C-5 protons resonated as a multiplet (δ 7.28–7.62 ppm), the olefinic proton resonated as a singlet at δ 7.73 ppm, while the proton on N-H function resonated as a singlet at δ 12.62 ppm. In compounds 10a–c, the Z conformation of the exocyclic C=C double bond was assigned on the basis of ¹H NMR spectroscopy and on the basis of literature data for analogous 4-thiazolidinones [33]. The ¹H NMR spectra of compounds 10a–c showed only one kind of methine proton that, deshielded by the adjacent C=O, was detected at 7.73–7.78 ppm, which are higher chemical shift values than those expected for E isomers.

Condensation reaction of equimolar amounts of chloroacetamido derivative 1 and malononitrile in absolute ethyl alcohol containing a few drops of triethylamine afforded the corresponding condensation product 2-amino-1-(4-antipyrinylthiazol-2-yl)-3-cyano-4,5-dihydro-5-oxo-1H-pyrrole (12) which was identified based on its compatible spectral analyses, see Scheme 4. Thus, the infrared spectrum of compound 12 exhibited the characteristic absorptions at 3293, 3191, 2216, and 1705 cm⁻¹ related to the functional groups amino (NH₂), nitrile (C≡N), and carbonyl (C=O), respectively. In the ¹H NMR spectrum, the protons of two methyl, methylene, thiazole-H5, and amino functions were secured by the presence of their characteristic signals at δ 2.65 ppm (singlet), δ 3.32 ppm (singlet), δ 4.36 ppm (singlet), δ 7.29 ppm (singlet), and δ 12.47 ppm (singlet), respectively.

Treatment of chloroacetamide derivative 1 with salicylaldehyde in DMSO containing potassium carbonate was achieved by stirring the reaction mixture for 8 h followed by neutralization with dilute HCl to furnish N-(4-antipyrinylthiazol-2-yl) benzofuran-2-carboxamide (13). It was reasoned that the chlorine atom in acetamido derivative 1 could be substituted by nucleophiles, so the plausible pathway for the formation of benzofuran derivative 14 could be via formation of intermediate 13, which subsequently underwent a condensation reaction between the formyl and active methylene group, see Scheme 5. The structural proof of compound 14 was determined based on its correct spectral analyses. Thus, the infrared spectrum of compound 14 showed absorptions at 3369 and 1689 cm⁻¹ to secure the functional groups (N-H) and (C=O), respectively. The ¹H NMR spectrum clearly indicated the presence of a singlet for three protons at 2.62 (CH₃-C), a singlet for three protons at 3.33 (CH₃-N), a singlet for one proton at 7.28 ppm (thiazole-H5), a multiplet for ten protons in the region of δ 7.36–7.71 ppm (aromatic-H and furan-H3), and a singlet for one proton at δ 12.31 ppm (NH).

2.2. Computational Studies

DMOL3 module calculations were used to examine the cluster estimations [34] and DNP, the double numerical basis sets plus polarization functional (DNP) implemented in the Materials Studio bundle [35]. It is constructed to realize the large-scale density functional theory (DFT) calculations [36,37,38,39]. The geometric optimization is performed with no symmetry confinement.

2.2.1. Geometry Optimization

The molecular structures along with atomic numbering of the title 4-antipyrinylthiazole scaffolds (2, 4, 5, 6, 7, 9, 10, 12, and 14) are represented in Figure 1 and Figure 2. The bond lengths and bond angles are included in the supplementary material, Tables S1–S24. The data obtained in these tables reveal that the bond lengths or bond angles are altered to some extent upon the formation of a new thiazole derivative, which is in turn dependent (or influenced) by the nature of the attacking electrophile or the experimental conditions.

2.2.2. Global Reactivity Descriptors

Density functional theory (DFT) was utilized to understand the chemical reactivity and site selectivity of the molecular systems. As highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are the orbitals that are the most likely to be involved in chemical reactivity, where the HOMO (π donor) is the highest energy orbital that is still occupied; therefore, energetically, it is the orbital which is easiest to remove electrons from, thus acting as a Lewis base. On the other hand, the LUMO (π acceptor) is the lowest lying orbital that is empty; therefore, energetically, this orbital is the easiest to add more electrons to (acts as Lewis acid). The energy gap = EHOMO − ELUMO and clarifies the inevitable charge exchange interaction inside the molecule. The global electrophilicity index (ω), chemical potential (μ), electronegativity (χ), global softness (S), and global hardness (η), are determined by the well-known equations [40] and the data are listed in

Table 1. Calculated global reactivity descriptors for the antipyrinyl-thiazole derivatives.

Cpd. No.	HOMO	LUMO	EHOMO − ELUMO	χ	μ	η	S	ω	σ	Binding Energy *
2	−3.090	−1.705	−1.385	2.397	−2.397	0.692	0.722	4.150	1.444	−5683.573
4	−3.389	−2.232	−1.157	2.810	−2.810	0.578	0.864	6.827	1.728	−6312.004
5	−3.274	−2.243	−1.031	2.758	−2.758	0.515	0.969	7.380	1.939	−6356.229
6	−3.839	−3.017	−0.822	3.428	−3.428	0.411	1.216	14.295	2.433	−6204.837
7a	−3.759	−2.934	−0.825	3.346	−3.346	0.412	1.212	13.574	2.424	−7817.104
7b	−3.852	−3.019	−0.833	3.435	−3.435	0.416	1.200	14.168	2.400	−7859.676
9	−3.329	−2.158	−1.171	2.743	−2.743	0.585	0.853	6.427	1.707	−4635.508
10a	−3.533	−2.612	−0.921	3.072	−3.072	0.460	1.085	10.250	2.171	−6444.896
10b	−4.098	−3.616	−0.482	3.857	−3.857	0.241	2.074	30.864	4.149	−6271.616
10c	−3.768	−2.966	−0.802	3.367	−3.367	0.401	1.246	14.135	2.493	−6013.436
12	−5.005	−2.237	−2.768	3.621	−3.621	1.384	0.361	4.736	0.722	−5973.095
14	−3.506	−2.456	−1.050	2.981	−2.981	0.525	0.952	8.463	1.904	−5768.652

* = KJ·mol⁻¹.

.

χ = −1/2 (E_LUMO + E_HOMO)

(1)

µ = −χ = 1/2 (E_LUMO + E_HOMO)

(2)

η = 1/2 (E_LUMO − E_HOMO)

(3)

S = 1/2 η

(4)

ω = µ²/2 η

(5)

The softness σ = 1/η

(6)

From the obtained data we can deduced that:

• The values of Frontier molecular orbitals energies (FMOs) namely E_HOMO and E_LUMO as well as their neighboring orbitals are negative, indicating the stability of the synthesized antipyrinyl-thiazole derivatives [41].
• Based on FMOs theory, the reaction occurs with maximum overlap between the HOMO on one molecule and the LUMO on the other and this is a controlling factor in many reactions. Therefore, orbitals of the derivative with the largest value of molecular orbital coefficients may be considered as the sites of electron donation. Thus, the HOMO level is mostly localized on N(2), N(3), S(17), C(16), C(18), C(19), C(4), and O(14) atoms, see Figure 1 and Figure 2, indicating the most preferable sites for attack of the incoming nucleophile.
• It is well documented that the smaller the energy gap (E_HOMO − E_LUMO) of a molecule the greater the reactivity, polarizability, and readiness to offer electrons to an acceptor and the molecule is considered to be “soft”, which in turn affects its biological activity. Thus, the title compounds follow the order: 10b > 10c > 6 > 7a > 7b > 10a > 5 > 14 > 4 > 9 > 2 > 12. This means that compound 10b possesses the smallest energy gap and the highest electrophilicty index (ω = 30.864) among all newly synthesized thiazoles and, therefore, has the highest softness, polarizability, and reactivity [13].
• Furthermore, it is obvious that the values of the binding energy increase for the new 4-antipyrinylthiazole derivatives (except thiazole compound 9) when compared to that of the starting compound (i.e., 4-antipyrinyl-2-chloroacetamidothiazole), revealing higher stability of the newly-formed thiazoles. The newly synthesized thiazoles can be arranged according to stability as: 7b > 7a > 10a > 5 > 4 > 10b > 6 > 10c > 12 > 14 > 2 > 9.

2.3. Antioxidant Activity

The antioxidant activity assay was estimated using an ABTS free radical scavenging activity assay (2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) [42]. Inhibition free radical ABTS in percent (I %) was calculated as per the equation:

I% = (A_blank − A_sample)/(A_blank) × 100

where A_blank is the absorbance of the control reaction (containing all reagents except the test compound), and A_sample is the absorbance of the test sample.

The antioxidant activities of the newly synthesized 4-antipyrinylthiazole scaffolds 2, 4, 5, 6, 7, 9, 10, 12, and 14 were evaluated using the ABTS Radical Cation Decolorization Assay [43]. The results, shown in

Table 2. ABTS scavenging activity of the synthesized 4-antipyrinylthiazole scaffolds.

Compound Number	Absorbance	ABTS Inhibition (%)
Control of ABTS	0.540	0
l-Ascorbic acid	0.060	88.88
2	0.306	43.33
4	0.274	49.25
5	0.219	59.44
6	0.144	73.33
7a	0.118	78.14
7b	0.223	58.70
9	0.211	60.92
10a	0.182	66.29
10b	0.077	85.74
10c	0.089	83.51
12	0.347	35.74
14	0.212	60.74

, indicated that compounds 10b, 10c, and 7a have excellent antioxidant properties compared to the reference of the test (l-Ascorbic acid, 88.88%). The derivatives of 2-(4-antipyrinylthiazolylamino)-5-substituitedbenzylidene-thiazol-4(5H)-one substituted with electron-withdrawing groups at the benzylidene moiety exhibited the best antioxidant activity. The percentage inhibition of scaffolds 10b and 10c substituted with nitro and bromide groups at the benzylidene moiety are 85.74% and 83.51%, respectively.

The replacement of the chlorine atom of the 2-chloroacetamido-thiazole compound 6 (percentage inhibition of 73.33%) by the benzothiazolyl moiety to afford the corresponding antipyrinyl-thiazole scaffold 7a enhanced the antioxidant activity with percentage inhibition 78.14%.

3. Materials and Methods

3.1. General Methods

Melting points have been determined on Gallenkamp electric apparatus (capillary method, Gallenkamp Co., London, UK). IR spectra (KBr discs) have been obtained on a Mattson 5000 FT-IR spectrometer (Shimadzu Co., Kyoto, Japan). The nuclear magnetic resonance spectra have been recorded using a WP 300 spectrometer (Bruker Co., Billerica, MA, USA) at 300 MHz for ¹H-NMR or 75.5 MHz for ¹³C-NMR. The mass analysis (EI technique, 70 eV) was acquired by a Qp-2010 mass spectrometer (Shimadzu, Tokyo, Japan).

3.2. Preparation of Ethyl 2-[(2-((4-antipyrinylthiazol-2-yl)amino)-2-oxoethyl)-thio]acetate (2)

A suspension of chloroacetamido derivative 1 (2 mmol, 0.72 g) and ethyl thioglycolate (2 mmol, 0.24 mL) in 15 mL ethyl alcohol containing CH₃COONa (0.2 g) was heated under reflux for 3 h. The resulting precipitate upon cooling was filtered, dried, and recrystallized from EtOH to furnish the sulfide compound 2. White solid, yield = 84%, m.p. = 161–163 °C. IR ($\overline{\nu }$/cm⁻¹): 3195 (N-H), 1723 (C=O), 1682 (C=O). ¹H-NMR (CDCl₃): δ 1.22 (t, J = 7.15 Hz, 3H, CH₃), 2.66 (s, 3H, CH₃), 3.16 (s, 3H, CH₃), 3.32 (s, 2H, CH₂), 3.64 (s, 2H, CH₂), 4.12 (q, J = 7.15 Hz, 2H, CH₂), 7.08 (s, 1H, thiazole-H5), 7.26–7.52 (m, 5H, Ar-H), 11.24 (s, 1H, NH). ¹³C-NMR (CDCl₃): δ 12.48, 14.02, 34.28, 35.16, 36.37, 61.68, 104.23, 109.18, 124.44 (2C), 126.87, 129.31 (2C), 135.08, 142.67, 151.92, 156.41, 163.85, 164.91, 170.67. Analysis for C₂₀H₂₂N₄O₄S₂ (446): Calcd.: C, 53.80; H, 4.97; N, 12.55%. Found: C, 53.89; H, 4.94; N, 12.50%.

3.3. Preparation of N-(4-antipyrinylthiazol-2-yl)-2-((3-cyano-4,6-dimethylpyridin-2-yl)thio)-acetamide (4)

To a stirred suspension of 3-cyano-4,6-dimethylpyridine-2-thiol (0.82 g, 0.005 mol) and anhydrous K₂CO₃ (0.70 g, 0.005 mol) in 40 mL acetone, 4-antipyrinyl-2-chloroacetamidothiazole (1.81 g, 0.005 mol) was added. The reaction suspension was heated under reflux for two h and then cooled to 25 °C. The solid, obtained after dilution by 20 mL cold water, was filtered and dried. Recrystallization was achieved by heating in ethanol. Yellow crystals, yield = 80%, m.p. 178–180 °C. IR ($\overline{\nu }$/cm⁻¹): 3371 (N-H), 2219 (C≡N), 1688 (C=O). ¹H-NMR (CDCl₃): δ 2.41 (s, 6H, 2CH₃), 2.64 (s, 3H, CH₃), 3.33 (s, 3H, CH₃), 4.05 (s, 2H, CH₂), 7.05 (s, 1H, pyridine-H₅), 7.21 (s, 1H, thiazole-H₅), 7.31–7.55 (m, 5H, Ar-H), 12.14 (s, 1H, CONH). ¹³C-NMR (CDCl₃): δ 12.44, 20.00, 24.53, 34.49, 40.26, 104.70, 105.81, 112.31, 119.78, 124.23, 126.16 (2C), 127.74, 129.51 (2C), 133.81, 142.42, 151.68, 155.62, 157.74, 158.26, 160.37, 163.38, 164.91. Analysis for C₂₄H₂₂N₆O₂S₂ (490): Calcd.: C, 58.76; H, 4.52; N, 17.13%. Found: C, 58.82; H, 4.50; N, 17.06%.

3.4. Synthesis of 3-amino-N-(4-antipyrinylthiazol-2-yl)-4,6-dimethylthieno[2,3-b]pyridine-2-carboxamide (5)

The sulfide derivative 4 (0.98 g, 0.002 mol) was heated under reflux in sodium ethoxide solution (prepared by stirring small parts of 0.08 g sodium metal in 20 mL dry ethyl alcohol) for 2 h. The reaction mixture was cooled, then poured into ice-cold water and neutralized with 0.1 N HCl. The obtained solid was filtered and recrystallized from ethyl alcohol. Yellow crystals, yield = 54%, m.p. = 271–273 °C. IR ($\overline{\nu }$/cm⁻¹): 3412, 3353, 3184 (N-H and NH₂), 1658 (C=O). ¹H-NMR (DMSO-d₆): δ 2.43 (s, 6H, 2CH₃), 2.68 (s, 3H, CH₃), 3.34 (s, 3H, CH₃), 6.92 (s, 2H, NH₂), 7.04 (s, 1H, pyridine-H₅), 7.26 (s, 1H, thiazole-H₅), 7.34–7.58 (m, 5H, Ar-H), 12.38 ppm (s, 1H, CONH). ¹³C-NMR (DMSO-d₆): δ 12.58, 20.33, 23.68, 34.16, 98.74, 107.86, 109.47, 118.63, 122.03, 126.13 (2C), 128.81, 129.49 (2C), 133.14, 143.56, 145.42, 154.50, 159.27, 160.49, 162.41, 163.83, 164.43, 166.32. Analysis for C₂₄H₂₂N₆O₂S₂ (490): Calcd.: C, 58.76; H, 4.52; N, 17.13%. Found: C, 58.62; H, 4.58; N, 17.24%.

3.5. Synthesis of 4-antipyrinyl-2-chloroacetamido-5-(4-sulfamoylphenylazo)thiazole (6)

A cold suspension of sulphanilamide (10 mmol, 1.70 g) in concentrated HCl (3 mL) was diazotized at 0–5 °C with a solution of NaNO₂ (0.70 g in 10 mL H₂O) with stirring for 30 min. The diazonium solution was added dropwise to a mixture of chloroacetamide derivative 1 (10 mmol, 3.62 g) and sodium acetate (3 g) in 35 mL ethanol with stirring in an ice-bath for 1 h. The precipitate was filtered and recrystallized from EtOH/DMF mixture (5:1) to give the sulfamoylphenylazo-thiazole derivative 6. Reddish-brown solid, yield = 74%, m.p. = 241–242 °C. IR ($\overline{\nu }$/cm⁻¹): 3351, 3264 (NH₂ and N-H), 1697 cm⁻¹ (C=O). ¹H-NMR (DMSO-d₆): δ 2.72 (s, 3H, CH₃), 3.28 (s, 3H, CH₃), 4.44 (s, 2H, CH₂), 7.28–7.61 (m, 7H, Ar-H and NH₂), 7.94–8.00 (dd, 4H, Ar-H), 12.92 ppm (s, 1H, NH). ¹³C-NMR (DMSO-d₆): δ 12.57, 35.16, 42.11, 108.81, 121.58, 124.41 (2C), 127.08 (2C), 129.21 (2C), 129.79 (2C), 132.25, 135.81, 140.27, 142.72, 147.47, 155.53, 161.21, 162.78, 165.31. Analysis for C₂₂H₂₀ClN₇O₄S₂ (545): Calcd.: C, 48.39; H, 3.69; N, 17.96%. Found: C, 48.27; H, 3.63; N, 17.85%.

3.6. Preparation of 2-substituted-N-(5-((4-sulfamoylphenyl)azo)thiazol-2-yl)acetamide Derivatives 7

A suspension of compound 6 (1 mmol, 0.54 g), 2-mercaptobenzothiazole or 2-mercaptobenzoxazole (1 mmol) and CH₃COONa (0.16 g) was heated under reflux in 15 mL ethyl alcohol for 2 h; after which, the formed precipitate upon cooling was filtered, washed by cold ethanol, and dried to give the anticipated compounds 7a and 7b, respectively.

2-(Benzothiazol-2-ylthio)-N-(5-((4-sulfamoylphenyl)azo)-4-antipyrinylthiazol-2-yl)acetamide (7a). Brown solid, yield = 72%, m.p. = 285–287 °C. IR ($\overline{\nu }$/cm⁻¹): 3341, 3257 (NH₂ and N-H), 1705 cm⁻¹ (C=O). ¹H-NMR (DMSO-d₆): δ 2.75 (s, 3H, CH₃), 3.36 (s, 3H, CH₃), 4.42 (s, 2H, CH₂), 7.32–7.94 (m, 15H, Ar-H and NH₂), 12.76 ppm (s, 1H, NH). MS (EI): m/z (%) = 676 (molecular ion, 13.8%). Analysis for C₂₉H₂₄N₈O₄S₄ (676): Calcd.: C, 51.47; H, 3.57; N, 16.56%. Found: C, 51.68; H, 3.66; N, 16.68%.

2-(Benzoxazol-2-ylthio)-N-(5-((4-sulfamoylphenyl)azo)-4-antipyrinylthiazol-2-yl)acetamide (7b). Brown solid, yield = 64%, m.p. = 274–275 °C. IR ($\overline{\nu }$/cm⁻¹): 3336, 3248 (NH₂ and N-H), 1699 cm⁻¹ (C=O). ¹H-NMR (DMSO-d₆): δ 2.68 (s, 3H, CH₃), 3.33 (s, 3H, CH₃), 4.41 (s, 2H, CH₂), 7.40-7.92 (m, 15H, Ar-H and NH₂), 12.86 (s, 1H, NH). MS (EI): m/z (%) = 660 (molecular ion, 21.2%). Analysis for C₂₉H₂₄N₈O₅S₃ (660): Calcd.: C, 52.72; H, 3.66; N, 16.96%. Found: C, 52.90; H, 3.58; N, 17.07%.

3.7. Preparation of 2-((4-antipyrinylthiazol-2-yl)imino)thiazolidin-4-one (9)

A solution of chloroacetamido derivative 1 (10 mmol, 3.62 g) and ammonium thiocyanate (15 mmol, 1.14 g) in 20 mL ethanol was heated under reflux for 5 h. The precipitate that formed upon cooling was isolated by filtration and recrystallized from EtOH to furnish thiazolidin-4-one derivative 9. Yellow solid, yield = 80%, m.p. = 201–202 °C. IR ($\overline{\nu }$/cm⁻¹): 3128 (N-H), 1714 (C=O). ¹H-NMR (DMSO-d₆): δ 2.65 (s, 3H, CH₃), 3.31 (s, 3H, CH₃), 4.05 (s, 2H, CH₂), 7.31–7.60 (m, 6H, Ar-H and thiazole-H5), 12.06 (s, 1H, NH). ¹³C-NMR (DMSO-d₆): δ 11.80, 33.76, 35.48, 104.51, 108.71, 122.66 (2C), 124.71, 128.31 (2C), 133.64, 141.46, 151.48, 159.24, 162.66, 165.29, 174.39. Analysis for C₁₇H₁₅N₅O₂S₂ (385): Calcd.: C, 52.97; H, 3.92; N, 18.17%. Found: C, 52.86; H, 3.95; N, 18.21%.

3.8. Synthesis of 2-((4-antipyrinylthiazol-2-yl)imino)-5-arylidenethiazolidin-4-one Derivatives 10a–c

A suspension of thiazolidin-4-one derivative 9 (2 mmol, 0.77 g) and the appropriate para-substituted benzaldehyde derivative (2 mmol) in 15 mL glacial CH₃COOH containing 0.5 g fused CH₃COONa was heated under reflux for 4 h. The reaction mixture was cooled and then poured into ice-water and the precipitate was filtered and recrystallized from ethanol.

2-((4-Antipyrinylthiazol-2-yl)imino)-5-(4-methoxybenzylidene)thiazolidin-4-one (10a). Yellow powder, yield = 74%, m.p. = 241–243 °C. IR ($\overline{\nu }$/cm⁻¹): 3176 (N-H), 1698 (C=O). ¹H-NMR (DMSO-d₆): δ 2.61 (s, 3H, CH₃), 3.28 (s, 3H, CH₃), 3.84 (s, 3H, OCH₃), 7.14 (d, J = 8.80 Hz, 2H, Ar-H), 7.28–7.62 (m, 8H, Ar-H and thiazole-H5), 7.73 (s, 1H, olefinic CH=C), 12.62 (s, 1H, NH). ¹³C-NMR (DMSO-d₆): δ 12.45, 34.36, 55.18, 106.26, 113.87 (2C), 117.37, 122.41, 123.32 (2C), 127.35, 129.49 (2C), 130.31 (2C), 133.89, 142.26, 145.60, 151.05, 155.83, 159.07, 161.71, 162.66, 164.09, 167.63. Analysis for C₂₅H₂₁N₅O₃S₂ (503): Calcd.: C, 59.63; H, 4.20; N, 13.91%. Found: C, 59.46; H, 4.26; N, 13.98%.

2-((4-Antipyrinylthiazol-2-yl)imino)-5-(4-nitrobenzylidene)thiazolidin-4-one (10b). Orange solid, yield = 69%, m.p. = 294–295 °C. IR ($\overline{\nu }$/cm⁻¹): 3192 (N-H), 1718 (C=O). ¹H-NMR (DMSO-d₆): δ 2.64 (s, 3H, CH₃), 3.31 (s, 3H, CH₃), 7.31–7.60 (m, 6H, Ar-H and thiazole-H5), 7.78 (s, 1H, olefinic CH=C), 7.82 (d, J = 8.80 Hz, 2H, Ar-H), 8.30 (d, J = 8.40 Hz, 2H, Ar-H), 12.56 (s, 1H, NH). MS m/z (%): 518 (M⁺, 47.4%). ¹³C-NMR (DMSO-d₆): δ 12.58, 34.19, 108.62, 116.45, 121.09, 123.27 (2C), 124.81 (2C), 125.48, 129.16 (2C), 130.34 (2C), 135.27, 139.96, 142.18, 147.29, 151.06, 156.41, 160.20, 161.38, 165.29, 168.40. MS (EI): m/z (%) = 518 (molecular ion, 65.7%). Analysis for C₂₄H₁₈N₆O₄S₂ (518): Calcd.: C, 55.59; H, 3.50; N, 16.21%. Found: C, 55.77; H, 3.42; N, 16.09%.

2-((4-Antipyrinylthiazol-2-yl)imino)-5-(4-bromobenzylidene)thiazolidin-4-one (10c). Yellow solid, yield = 83%, m.p. = 275–276 °C. IR ($\overline{\nu }$/cm⁻¹): 3132 (N-H), 1702 (C=O). ¹H-NMR (DMSO-d₆): δ 2.62 (s, 3H, CH₃), 3.32 (s, 3H, CH₃), 7.28 (s, 1H, thiazole-H5), 7.36–7.78 (m, 10H, Ar-H and olefinic CH=C), 12.48 (s, 1H, NH). ¹³C-NMR (DMSO-d₆): δ 12.67, 34.49, 108.57, 114.72, 120.30, 122.75, 124.92 (2C), 125.16, 128.38 (2C), 129.64 (2C), 131.06 (2C), 133.29, 143.04, 148.38, 151.36, 155.91, 160.47, 161.84, 164.85, 167.76. MS (EI): m/z (%) = 551 (molecular ion, Br-79, 73.4%) and 553 (M⁺, Br-81, 75.6%). Analysis for C₂₄H₁₈BrN₅O₂S₂ (552): Calcd.: C, 52.18; H, 3.28; N, 12.68%. Found: C, 52.39; H, 3.21; N, 12.81%.

3.9. Synthesis of 2-amino-1-(4-antipyrinylthiazol-2-yl)-3-cyano-4,5-dihydro-5-oxo-1H-pyrrole (12)

To a suspension of chloroacetamido derivative 1 (2 mmol, 0.72 g) and malononitrile (2 mmol, 0.13 g) in 20 mL ethanol, five drops of Et₃N were added, then the reaction mixture was heated under reflux for 2 h; after which, the reaction mixture was left to cool at room temperature and neutralized with dilute HCl. The precipitate was filtered and recrystallized from EtOH to afford thiazolyl-pyrrole derivative 12. Orange solid, yield = 52%, m.p. = 284–285 °C. IR ($\overline{\nu }$/cm⁻¹): 3293, 3191 (NH₂), 2216 (C≡N), 1705 (C=O). ¹H-NMR (DMSO-d₆): δ 2.65 (s, 3H, CH₃), 3.32 (s, 3H, CH₃), 4.36 (s, 2H, CH₂), 7.26 (s, 1H, thiazole-H5), 7.34–7.58 (m, 5H, Ar-H), 12.47 ppm (s, 2H, NH₂). ¹³C-NMR (DMSO-d₆): δ 11.69, 29.26, 34.41, 64.21, 102.79, 107.82, 113.76, 123.48 (2C), 126.14, 128.25 (2C), 134.23, 141.61, 150.97, 152.19, 155.47, 159.36, 164.73. Analysis for C₁₉H₁₆N₆O₂S (392): Calcd.: C, 58.15; H, 4.11; N, 21.42%. Found: C, 58.10; H, 4.15; N, 21.33%.

3.10. Synthesis of N-(4-antipyrinylthiazol-2-yl)benzofuran-2-carboxamide (14)

A solution of chloroacetamido derivative 1 (2 mmol, 0.72 g) and salicylaldehyde (2 mmol, 0.21 mL) in 15 mL DMSO containing anhydrous potassium carbonate (2 mmol, 0.28 g) was stirred at room temperature for 8 h; after which, the reaction mixture was poured into ice-cooled water and neutralized by diluted HCl. The precipitate was filtered off and recrystallized from methanol to afford thiazolyl-benzofuran derivative 14. Grey solid, yield = 64%, m.p. = 187–188 °C. IR ($\overline{\nu }$/cm⁻¹): 3369 (N-H), 1689 (C=O). ¹H-NMR (DMSO-d₆): δ 2.62 (s, 3H, CH₃), 3.33 (s, 3H, CH₃), 7.28 (s, 1H, thiazole-H5), 7.36–7.71 (m, 10H, Ar-H and furan-H3), 12.31 (s, 1H, NH). ¹³C-NMR (DMSO-d₆): δ 12.24, 34.35, 106.75, 108.84, 112.36, 115.27, 120.04, 122.17, 122.58 (2C), 123.80, 124.91, 127.38, 129.11 (2C), 133.39, 143.64, 151.48, 155.08, 157.42, 159.88, 161.40, 163.71. Analysis for C₂₃H₁₈N₄O₃S (430): Calcd.: C, 64.17; H, 4.21; N, 13.02%. Found: C, 64.29; H, 4.16; N, 13.11%.

3.11. Anti-Oxidant Activity Screening Assay—ABTS Method

The 4-antipyrinylthiazole scaffolds 2, 4, 5, 6, 7, 9, 10, 12, and 14 were tested in vitro for their antioxidant activity by ABTS method [43].

4. Conclusions

Twelve 4-antipyrinyl-thiazole derivatives were synthesized from the readily available 4-antipyrinyl-2-chloroacetamido-thiazole by its reaction with various types of nucleophiles such as ethyl thioglycolate, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, ammonium thiocyanate, malononitrile, and salicylaldehyde. The semi-core pseudopods calculations (dspp) were carried out with the double numerical basis sets plus polarization functional (DNP) to predict properties of materials using the hybrid FT/B3LYP method. Antioxidant properties of the synthesized 4-antipyrinyl-thiazole scaffolds were screened by the ABTS radical cation decolorization assay. Among the analogous compounds, 10b and 10c showed excellent antioxidant activity (percentage inhibition of 85.74% and 83.51%, respectively) very close to the reference inhibitor.