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Current Chemistry Letters 8 (2019) 211–224 Contents lists available at GrowingScience Current Chemistry Letters homepage: www.GrowingScience.com Green synthesis of 4-methoxybenzylidene thiazole derivatives using potassium carbonate as base under ultrasound irradiation Dattatraya N. Pansarea*, Rohini N. Shelkea, Chandraknat D. Pawarb, Aniket P. Sarkateb, Pravin N. Chavanc, Shankar R. Thopated and Devanand B. Shindee a Department of Chemistry, Deogiri college, Station road, Aurangabad 431 005, MS, India Department of Chemical Technology, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad-431 004, MS, India c Department of Chemistry, Doshi Vakil College, Goregaon, District-Raigad, (MS), India d Department of Chemistry, S.S.G.M. College, Kopargaon, Ahmednagar, (MS), India e Shivaji University, Vidyanagar, Kolhapur, 416 004, MS, India b CHRONICLE Article history: Received February 26, 2019 Received in revised form June 2, 2019 Accepted June 2, 2019 Available online June 2, 2019 Keywords: 4-Methoxybenzylidene Water Ultrasonic Irradiation Potassium Carbonate ABSTRACT An environmentally benign aqueous protocol for the synthesis of novel 2-((5-(4methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)substituted acid by using potassium carbonate as a base has been achieved. These ultrasound irradiation and conventional technique reaction proceed efficiently in water in the absence of organic solvent. In comparison with conventional methods, our protocol is convenient and offers several advantages, such as shorter reaction time, higher yields, milder conditions and environmental friendliness. © 2019 by the authors; licensee Growing Science, Canada. 1. Introduction The Nitrogen-containing five and six-member heterocyclic compounds and their derivatives, which can be easily synthesized in laboratories, are particularly important and often found in natural sources. The 2-thioxothiazolidin-4-one (Rhodanine) based molecules and thiazole have been reported to exhibit a broad spectrum of biological activities, such as anti-inflammatory,1,2 antipyretic,3,4 antidiabetic,5 anticancer,6 antitubercular,7,8 anti-HIV,9-11 antiparasitic,12 hypnotic13 and antiproliferative agents.14,15 Rhodanine was discovered in 1877, so there have been several attempts to design antimicrobial agents based on this heterocycles. There are various reports available on rhodanine derivatives as antimicrobial agents.16-21 These reports suggested that a chain containing free carboxyl group at rhodanine nuclei was important to the observed levels of biological activity22 and synthesized structures of rhodanine containing moiety is shown (Fig. 1). * Corresponding author. Fax: +91 0240-2400413, Mobile no. +91 9850108474 E-mail address: dattatraya.pansare7@gmail.com (D. N. Pansare) © 2019 by the authors; licensee Growing Science, Canada doi: 10.5267/j.ccl.2019.006.001 212 Fig. 1. Previously reported antimicrobial agents and synthesized compounds. The most common protocol for the synthesis of thioxothiazolidinone involves active methylene group followed by intermolecular condensation with aromatic substituted aldehyde. However, these reactions required long reactions times, high temperatures, produce by-products, expensive reagents and, in general, have difficult purifications.23-25 Ultrasound irradiation, an efficient and innocuous technique for reagent activation in the synthesis of organic compounds, and in particular heterocyclic compounds, has been applied with success, and generates products in good to excellent yields.26-28 Ultrasound-promoted synthesis has attracted much attention during the past few decades. One advantage of using cavitation as an energy source to promote organic reactions includes shorter reaction times. Compared with conventional synthetic methods, the ultrasound- assisted method is reported as a fast, simple, convenient, time saving, economical, and environmentally benign method for the synthesis of novel materials.29-31 It was known that the ultrasound agitation generate notable effects of chemical and physical effects due to the acoustic cavitation.32,33 Ultrasonic irradiation has been acknowledged as an innocuous, green technique and its application today has been a boon in serving a new pathway for several chemical processes like reagent activation in the synthesis of organic and inorganic compounds.34 In view of the above considerations and in continuation of our previous work on thiazoles, thiazolidinones and sulphonamide derivatives of pharmaceutical interest,35-47 we wish to report a simple, mild, competent and environmentally benign method for the synthesis and characterization of novel rhodanine derivatives 3, 4 and 6a-l by ultrasound irradiation and conventional technique via potassium carbonate catalyzed in water media. 2. Results and discussion The synthetic protocols employed for the synthesis of rhodanine derivatives 3 and 4 presented in scheme 1, scheme 2 and 6a-l are presented in scheme 3. The compound (Z)-5-(4methoxybenzylidene)-2-thioxothiazolidin-4-one 3 was prepared via a Knoevenagel condensation between and 4-methoxybenzaldehyde (1) with rhodanine (2). The compound (Z)-5-(4methoxybenzylidene)-2-(methylthio)thiazol-4(5H)-one 4 was obtained via reaction of the compound (3) with iodomethane in water using triethylamine as base. D. N. Pansare et al. / Current Chemistry Letters 8 (2019) 213 Table 1. Ultrasound irradiation: Screening of base, solvents, reaction time and yield for the synthesis (6a)a Entry Base Solvent Time (min) Yieldb (%) 1 Diethylamine Water 12 62 2 Diethylamine Methanol 18 32 3 Diethylamine Acetic acid 15 32 4 Diethylamine DCM 16 43 5 Diethylamine Toluene 14 34 6 Triethylamine Water 8 72 7 Triethylamine Methanol 13 42 8 Triethylamine Acetic acid 14 33 9 Triethylamine DCM 12 44 10 Triethylamine Toluene 18 36 11 Potassium carbonate Water 1 99 12 Potassium carbonate Methanol 12 72 13 Potassium carbonate Acetic acid 10 73 14 Potassium carbonate DCM 8 72 15 Potassium carbonate Toluene 10 66 a All the reaction was carried out in equimolar amounts of each compound in 1 mL of solvent b Isolated yield. 2.1. Effect of base and solvents A variety of bases were screened under ultrasound irradiation in order to validate the right choice and the results are shown in Table 1. To justify the influence of the base, the reaction was carried out in the presence of base potassium carbonate wherein a maximum yield of 99% could be obtained (Table 1, Entry 11). It was further observed that the yield of the reaction hardly improved in the presence of other like diethylamine and triethylamine bases (Table 1, Entries 1 and 6), whereas the use of potassium carbonate as base significantly improved the yield to 99% (Table 1, Entry 11). Hence potassium carbonate under ultrasonic irradiation was selected for our further studies. a Reaction condition: (i) Method A: Ultrasound irradiation: Sodium acetate, Acetic acid, 25 ºC, 25 min. (i) Method B: Conventional method: Sodium acetate, Acetic acid, reflux, 2 h. (ii) Method A: Ultrasound irradiation: Triethylamine, Iodomethane, Water, rt, 3 min. (ii) Method B: Conventional method: Triethylamine, Iodomethane, Water, rt, 1 h. Scheme 1. Synthesis of (Z)-5-(4-methoxybenzylidene)-2-(methylthio)thiazol-4(5H)-one (4)a 214 a Reaction condition: Method A: Ultrasound irradiation: Compound 4 (1 mmol), L-Alanine (5a) (1.2 mmol), base (1 mmol), solvent 1 mL, rt. 1-18 min. Method B: Conventional method: Compound 4 (1 mmol), L-Alanine (5a) (1.2 mmol), base (1 mmol), solvent 1 mL, rt. 10-98 min. Scheme 2. Screening of model reaction (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol2-yl)amino)propanoic acid (6a)a We synthesized and screening of model reaction under ultrasound irradiation and conventional method of the compound (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino) propanoic acid 6a (Scheme 2, Table 1, Table 2). The reaction in which the compound 4 (1 mmol) and the compound 5a (1.2 mmol), various base and various solvents were selected as a model reaction to optimize the reaction conditions. In terms of the effect of solvents and base on the condensation reaction, potassium carbonate was found to be the better base and water was found to be the best solvent for the reaction (Table 1, entry 11); other solvents, including methanol, acetic acid, dichloromethane (DCM) and toluene were less efficient (Table 1, entries 2–5, 7–10 and 12–15). Table 2. Conventional method: Screening of base, solvents, reaction time and yield for the synthesis (6a)a Entry Base Solvent Time (min) Yieldb (%) 1 Diethylamine Water 80 58 2 Diethylamine Methanol 90 30 3 Diethylamine Acetic acid 95 35 4 Diethylamine DCM 98 40 5 Diethylamine Toluene 92 30 6 Triethylamine Water 82 68 7 Triethylamine Methanol 84 40 8 Triethylamine Acetic acid 86 30 9 Triethylamine DCM 84 45 10 Triethylamine Toluene 88 35 11 Potassium carbonate Water 10 88 12 Potassium carbonate Methanol 40 79 13 Potassium carbonate Acetic acid 44 76 14 Potassium carbonate DCM 46 78 15 Potassium carbonate Toluene 48 68 a b All the reaction was carried out in equimolar amounts of each compound in 1 mL of solvent Isolated yield. Water gave the corresponding product in 62–99% yield, which was the best among these solvents (Table 1, entries 1, 6 and 11). To increase the efficiency of the condensation reaction, the effects of different base were investigated (Table 1, entries 1–15). Potassium carbonate exhibited the best performance with used solvents and gave better yield, (Table 1, entries 11–15). Sodium acetate and triethylamine gave lower yields with other solvents, but gave better yield in water as a solvent (Table 1, entries 1 and 6). All the reactions were carried out in equimolar amounts of each compound in 1 mL D. N. Pansare et al. / Current Chemistry Letters 8 (2019) 215 of solvent. Among these reactions same amounts of the solvent, namely 1 mL of water turned out to be the best choice with yields of 62%, 72% and 99% (Table 1, entries 1, 6 and 11). a Reaction condition: Method A: Ultrasound irradiation: potassium carbonate, water, rt, 1-4 min. Method B: Conventional: potassium carbonate, water, rt, 10-30 min. Scheme 3. Synthesis of (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino) substituted acid (6a-l).a We also synthesized and screening of model reaction under conventional method and the results of these findings are presented in Table 2. The reaction in which the compound 4 (1 mmol) and the compound 5a (1.2 mmol), various base and various solvents were selected as a model reaction to optimize the reaction conditions. In terms of the effect of solvents and base on the condensation reaction, potassium carbonate was found to be the better base and water was found to be the best solvent for the reaction (Table 2, entry 11); other solvents, including methanol, acetic acid, DCM and toluene were less efficient (Table 2, entries 2–5, 7–10 and 12–15). Nevertheless, all of these yields were generally low before further optimizations. Water gave the corresponding product in 58–88% yield, which was the best among these solvents (Table 2, entries 1, 6 and 11). Table 3. Physical data for synthesized rhodanine derivatives 6(a-l)a Sr. No. a 6a 6b 6c 6d 6e 6f 6g 6h 6i 6j -CH3 -CH(CH3)2 -CH(CH3)CH2CH3 -CH2C6H5 -CH2CH2SCH3 -CH2CH(CH3)2 -CH2OH -CH2SH -CH2COOH 6k 6l -CH2C6H4OH -CHOHCH3 Yieldb ( %) Time (min) Substituent (R) Melting point (ºC) Ultrasound irradiation 1 1 2 2 3 3 4 4 4 2 Conventional method 10 30 25 25 25 30 25 28 25 30 Ultrasound irradiation 99 98 98 97 98 98 97 96 97 98 Conventional method 88 85 88 88 92 90 88 90 90 90 220-222 212-214 148-150 178-180 176-178 238-240 241-243 256-258 173-175 198-200 2 3 25 22 98 98 88 82 157-159 181-183 Reaction condition (6a-l). Compound (4) (1 mmol), amino acids (5a-l) (1.2 mmol), Method A: Ultrasound irradiation: potassium carbonate, Water, rt, 1-4 min. Method B: Conventional method: potassium carbonate, Water, rt, 10-30 min. b Isolated yields 216 To increase the efficiency of the condensation reaction, the effects of different base were investigated (Table 2, entries 1–15). Potassium carbonate exhibited the best performance with used solvents and gave better yield, (Table 2, entries 11–15). Sodium acetate and triethylamine gave lower yields with other solvents, but gave better yield in ethanol as a solvent (Table 2, entries 1 and 6). All the reactions were carried out in equimolar amounts of each compound in 1 mL of solvent. Among these reactions same amounts of the solvent, namely 1 mL of ethanol turned out to be the best choice with yields of 58%, 68% and 88% (Table 2, entries 1, 6 and 11). We would like to mention here that water as a solvent with potassium carbonate as base was the best choice with a yield of 99% and less time required for the completion of the reaction (Table 1, entry 11). Thus we decided to carry out the further reactions in water as a solvent with potassium carbonate as a base. As a result the reaction time was shortened; thermal decomposition was also minimized, at room temperature stirring, resulting in higher isolated yields. But in this synthesis, we compared to the reaction between ultrasound irradiation and conventional method, the ultrasound irradiation is the best method. Because the studies indicated that the use of ultrasound irradiation made the reactions very fast, very less time required to complete the reaction, and recorded high product yields 62%, 72% and 99% (Table 1, entries 1, 6 and 11) and surprisingly, in the conventional method, the reactions very sluggish and recorded low yields 58%, 68% and 88% (Table 2, entries 1, 6 and 11). The physical data of the synthesized compounds are presented in Table 3. All the reactions proceeded well in 1-4 min to give products in very good yields (96–99%) by ultrasound irradiation and in conventional method, the reactions proceeded in 10-30 min to give products in yields (82–90%). The purity of the synthesized compounds was checked by TLC on silica gel precoated F254 Merck plates and melting points were recorded on SRS Optimelt, melting point apparatus and are uncorrected. The structure of the synthesized compounds was confirmed by IR, 1H NMR, 13C NMR and Mass spectral analysis. 3. Conclusions With the pervasive applicability and pharmacoactivity of these derivatives, we have herein devised an energy efficient, general, cost effective and eco sustainable method for the synthesis of a series of rhodanine derivatives 3, 4 and 6a-l. The promising salient features of this strategy are absence of toxic organic solvents, minimization of waste, ease of product isolation, rapid, avoids laborious column purification steps, economically viable, easy to operate, rate and yield enhancements. The present method will permit a further increase of the diversity within rhodanine derivatives. It is envisaged that, the utility of sonication in combination with water as solvent and potassium carbonate as a base will make further development and good prospects for industrial application, synthetic chemistry and chemical science. Acknowledgement The authors are thankful to the Head, Department of Chemical Technology, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431004 (MS) India, for providing the laboratory facility. The authors would like to also thank the anonymous referees for constructive comments on earlier version of this paper. 4. Experimental section 4.1. Material and methods Rhodanine, 4-methoxybenzaldehyde, anhydrous sodium acetate, triethylamine, dichloromethane, iodomethane and various solvents were commercially available. The major chemicals were purchased D. N. Pansare et al. / Current Chemistry Letters 8 (2019) 217 from Sigma Aldrich and Avra labs. Reaction courses were monitored by TLC on silica gel precoated F254 Merck plates. Developed plates were examined with UV lamps (254 nm). IR spectra were recorded on a FT-IR (Bruker). Melting points were recorded on SRS Optimelt, melting point apparatus and are uncorrected. The Ultrasonic Bath, sonicator of PCI Analytics having ultrasound cleaner with a frequency of 35 kHz and constant frequency 100 W maintained at 25ºC by circulating water. 1H NMR spectra were recorded on a 400 MHz Varian NMR spectrometer and DMSO solvent is used. The following abbreviations are used; singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and broad (br). Mass spectra were taken with Micromass-QUATTRO-II of WATER mass spectrometer. 4.2. General procedure for the synthesis of compounds (3) 4.2.1. Method A: Ultrasound irradiation A 50 mL flask was charged with 4-methoxybenzaldehyde 1 (1 mmol), 2-thioxothiazolidin-4-one 2 (1 mmol), anhydrous sodium acetate (1 mmol), acetic acid (1 mL). The mixture was sonicated (35 kHz, constant frequency) at 25 ºC for 25 min. The progress of the reaction was monitored by TLC (20% ethyl acetate: n-hexane). After completion of the reaction, the reaction mixture was poured into the icecold water. The precipitate was filtered off and washed with water (3×10 mL), dried and purified by recrystallized in ethanol as solvent to give 98 % yield. 4.2.2. Method B: Conventional method A 50 mL round bottom flask, an equimolar amount of 4-methoxybenzaldehyde 1 (1 mmol), 2thioxothiazolidin-4-one 2 (1 mmol), anhydrous sodium acetate (1 mmol) and acetic acid (1 mL) were added. The mixture was stirred under reflux condition for 2 h. The progress of the reaction was monitored by TLC (20% ethyl acetate: n-hexane). After completion of the reaction, the reaction mixture was poured into the ice-cold water. The precipitate was filtered off and washed with water (3×10 mL), dried and purified by recrystallized in ethanol as solvent to give 82 % yield. 4.2.2.1. (Z)-5-(4-methoxybenzylidene)-2-thioxothiazolidin-4-one (3) Yellow solid, Yield: 95%. mp 247–249 ºC; ES-MS m/z: 251.32. IR νmax/cm–1: 3082 (NH), 1729 (C=O), 1575 (C=C), 1442 (C=N), 1277 (C=S), 1194(C–N). 1H NMR (400 MHz, DMSO-d6, ppm) = 3.90 (s, 3H, OCH3), 6.60–6.62 (d, J = 7.2 Hz, 2H, Ar–CH), 7.30–7.32 (d, J = 7.2 Hz, 2H, Ar–CH), 7.70 (s, 1H, =CH), 13.70 (s, 1H, NH). 13C NMR: δppm = 55.8, 114.3, 116.0, 130.7, 142.9, 143.4, 160.5, 168.4, 193.7. 4.2.3. General procedure for the synthesis of compounds (4) 4.2.3.1. Method A: Ultrasound irradiation A 50 mL flask was charged with, the compound (Z)-5-(4-methoxybenzylidene)-2-thioxothiazolidin4-one 3 (1 mmol), triethylamine (1.2 mmol), iodomethane (1.2 mmol) and water (1 mL). The mixture was sonicated (35 kHz, constant frequency) at 25 ºC for 3 min. The progress of the reaction was monitored by TLC (10% methanol: chloroform). After completion of the reaction, the reaction mixture was concentrated in-vacuo. The residue was washed with water (3×15 mL) to afford the crude product. The crude product was recrystallized using ethanol as solvent to give yield in the range 95%. 4.2.3.2. Method B: Conventional method In a 50 ml round bottom flask, the compound (Z)-5-(4-methoxybenzylidene)-2-thioxothiazolidin4-one 3 (1 mmol), triethylamine (1.2 mmol), iodomethane (1.2 mmol), water (1 mL) stirred at room 218 temperature up to 2 h. The progress of the reaction was monitored by TLC (10% methanol: chloroform). After completion of the reaction, the reaction mixture was concentrated in-vacuo. The residue was washed with water (3×15 mL) to afford the crude product. The crude product was recrystallized using ethanol as solvent to give yield in the range 85 %. 4.2.3.2.1. (Z)-5-(4-methoxybenzylidene)-2-(methylthio)thiazol-4(5H)-one (4) Yellow solid, Yield: 95%. mp 162–164 ºC; ES-MS m/z: 265.35. IR νmax/cm–1: 1681 (C=O), 1572 (C=C), 1503 (C=N), 1149 (C-S), 911 (C–N). 1H NMR (400 MHz, DMSO-d6, ppm)= 2.80 (s, 3H, SCH3), 3.80 (s, 3H, OCH3), 6.70–6.72 (d, 2H, Ar–CH), 7.30–7.32 (d, 2H, Ar–CH), 7.90 (s, 1H, =CH). 13 C NMR: δppm = 14.4, 55.8, 114.2, 127.5, 130.5, 132.6, 152.3, 160.1, 162.7, 167.2. 4.3. General procedure for the synthesis of (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5dihydrothiazol-2-yl)amino)substituted acid (6a-l) 4.3.1. Method A: Ultrasound irradiation: A 50 mL flask was charged with, the compound (Z)-5-(4-methoxybenzylidene)-2(methylthio)thiazol-4(5H)-one 4 (1 mmol), amino acids 5a-l (1.2 mmol), potassium carbonate (1 mmol) and water (1 mL). The mixture was sonicated (35 kHz, constant frequency) at 25 ºC for 1-4 min. The progress of the reaction was monitored by TLC (10% methanol: chloroform). After completion of the reaction, the reaction mixture was concentrated in-vacuo. The residue was washed with water (3×15 mL) to afford the crude product. The compounds (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5dihydrothiazol-2-yl)amino) substituted acid 6a-l were recrystallized from ethanol and isolated as yellowish solids. 4.3.2. Method B: Conventional method: In a 50 ml round bottom flask, the compound (Z)-5-(4-methoxybenzylidene)-2-(methylthio)thiazol4(5H)-one 4 (1 mmol), amino acids 5a-l (1.2 mmol), potassium carbonate (1 mmol) and water (1 mL) were added to the reaction mixter and stirred for 10-30 min at room temperature. The progress of the reaction was monitored by TLC (10% methanol: chloroform). After completion of the reaction, the reaction mixture was concentrated in-vacuo. The residue was washed with water (3×15 mL) to afford the crude product. The compounds (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2yl)amino) substituted acid 6a-l were recrystallized from ethanol and isolated as yellowish solids. 4.3.2.1. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)propanoic acid (6a) Yellow solid, Yield: 99%, mp 220–222 ºC; ES-MS m/z: 306.34. IR νmax /cm–1: 3384 (OH), 2657 (CH– Ar), 1737 (HO–C=O), 1690 (C=O), 1599(C=C), 1553 (C=N), 1006 (C-S), 761 (C–N). 1H NMR (400 MHz, DMSO-d6, ppm) = 1.30–1.32 (d, 3H, CH3), 3.80 (s, 3H, OCH3), 4.50–4.52 (q, 1H, CH), 7.50– 7.70 (m, 4H, Ar–CH), 7.80 (s, 1H, =CH), 8.40 (s, 1H, NH), 10.15 (s, 1H, COOH). 13C NMR: δppm = 16.8, 53.5, 55.5, 56.2, 114.1, 130.4, 132.5, 143.4, 152.3, 158.6, 160.7, 167.7, 174.2. 4.3.2.2. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-methyl butanoic acid (6b) Yellow solid, Yield: 98%, mp 212–214 ºC; ES-MS m/z: 334.39. IR νmax /cm–1: 3744 (OH), 3011 (NH), 1737 (HO–C=O), 1689 (C=O), 1553 (C=C), 1509 (C=N), 1232 (C-S), 1010 (C–N). 1H NMR (400 MHz, DMSO-d6, ppm) = 0.90–0.92 (d, 6H, CH3), 2.20–2.22 (m, 1H, CH), 3.80 (s, 3H, OCH3), 4.50–4.52 (d, 1H, CH), 7.40–7.42 (d, J = 7.2 Hz, 2H, Ar–CH), 7.60–7.62 (d, J = 7.2 Hz, 2H, Ar–CH), D. N. Pansare et al. / Current Chemistry Letters 8 (2019) 219 7.80 (s, 1H, =CH), 10.02 (s, 1H, NH), 13.15 (s, 1H, COOH). 13C NMR: δppm = 18.2, 30.1, 55.6, 61.3, 114.3, 127.6, 130.7, 132.4, 153.3, 157.6, 161.7, 168.7, 174.4. 4.3.2.3. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-methyl pentanoic acid (6c) Yellow solid, Yield: 98%, mp 148–150 ºC; ES-MS m/z: 348.42. IR νmax/cm–1: 3624 (OH), 3563 (NH), 2969 (Ar-CH), 1730 (HO–C=O), 1641 (C=O), 1609 (C=C), 1584 (C=N), 1183 (C-S), 1093 (C–N). 1H NMR (400 MHz, DMSO-d6, ppm) = 1.10–1.12 (m, 8H, CH2CH3), 1.30–1.32 (m, 1H, CH), 3.20–3.22 (d, 1H, CH), 3.70 (s, 3H, OCH3), 7.40–7.42 (d, J = 7.6 Hz, 2H, Ar–CH), 7.90 (s, 1H, =CH), 8.20 (d, J = 7.6 Hz, 2H, Ar–CH), 8.80 (s, 1H, NH), 10.30 (s, 1H, COOH). 13C NMR: δppm = 11.2, 15.2, 25.2, 37.5, 55.3, 55.6, 56.2, 55.8, 130.6, 132.7, 143.4, 152.3, 161.7, 167.7, 174.6. 4.3.2.4. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-phenyl propanoic acid (6d) Yellow solid, Yield: 97%, mp 178–180 ºC; ES-MS m/z: 382.43. IR νmax /cm–1: 3392 (OH), 3210 (NH), 2976 (CH–Ar), 1730 (HO–C=O), 1699 (C=O), 1563 (C=C), 1544 (C=N), 1012 (C-S), 1068 (C– N). 1H NMR (400 MHz, DMSO-d6, ppm) = 2.50–2.52 (d, 2H, CH2), 3.80 (s, 3H, OCH3), 4.40–4.42 (q, 1H, CH), 7.20–7.70 (m, 9H, Ar–CH), 7.90 (s, 1H, =CH), 9.14 (s, 1H, NH), 11.02 (s, 1H, COOH). 13 C NMR: δppm = 55.4, 56.2, 36.4, 58.4, 114.5, 125.9, 127.7, 128.6, 128.9, 135.3, 136.9, 143.4, 152.2, 158.5, 167.2, 174.2. 4.3.2.5. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-4-(methylthio) butanoic acid (6e) Yellow solid, Yield: 98%, mp 176–178 ºC; ES-MS m/z: 366.46. IR νmax /cm–1: 3475 (OH), 3213 (NH), 2922 (CH–Ar), 1719 (HO–C=O), 1699 (C=O), 1582 (C=C), 1452 (C=N), 1215 (C-S), 1029 (C– N). 1H NMR (400 MHz, DMSO-d6, ppm) = 2.10 (s, 3H, CH3), 2.30–2.32 (q, 2H, CH2), 2.60–2.62 (t, 2H, CH2), 3.30–3.32 (q, 1H, CH), 3.80 (s, 3H, OCH3), 7.10–7.12 (d, J = 7.2 Hz, 2H, Ar–CH), 7.50– 7.52 (d, J = 7.2 Hz, 2H, Ar–CH), 7.80 (s, 1H, =CH), 9.30 (s, 1H, NH), 10.20 (s, 1H, COOH). 13C NMR: δppm = 15.2, 29.2, 30.5, 55.4, 56.6, 56.8, 114.6, 130.4, 132.3, 143.5, 152.3, 161.7, 167.7, 174.6. 4.3.2.6. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-4-methyl pentanoic acid (6f) Yellow solid, Yield: 98%, mp 238–240 ºC; ES-MS m/z: 348.44. IR νmax /cm–1: 3382 (OH), 3212 (NH), 3020 (CH–Ar), 1721 (HO–C=O), 1699 (C=O), 1515 (C=C), 1574 (C=N), 1023 (C-S), 1051 (C– N). 1H NMR (400 MHz, DMSO-d6, ppm) = 0.92–0.94 (d, 6H, CH–(CH3)2), 1.42–1.44 (m, 1H, CH), 1.70–1.72 (t, 2H, CH2), 3.82 (s, 3H, OCH3), 4.40–4.42 (q, 1H, CH), 7.20–7.22 (d, J = 7.2 Hz, 2H, Ar– CH), 7.50–7.52 (d, J = 7.2 Hz, 2H, Ar–CH), 7.80 (s, 1H, =CH), 9.20 (s, 1H, NH), 11.84 (s, 1H, COOH). 13 C NMR: δppm = 22.8, 24.5, 40.1, 55.2, 55.8, 114.2, 127.2, 130.1, 132.4, 152.2, 158.1, 160.1, 167.1, 174.2. 4.3.2.7. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-hydroxy propanoic acid (6g) Yellow solid, Yield: 97%, mp 241–243 ºC; ES-MS m/z: 332.34. IR νmax /cm–1: 3420 (OH), 3211 (NH), 3017 (CH–Ar), 1730 (HO–C=O), 1698 (C=O), 1543 (C=C), 1521 (C=N), 1029 (C-S), 1097 (C– N). 1H NMR (400 MHz, DMSO-d6, ppm) = 3.60 (t, 1H, CH), 3.85 (s, 3H, OCH3), 4.01–4.03 (d, 2H, CH2), 5.30 (s, 1H, OH), 7.10–7.12 (d, 2H, Ar–CH), 7.30 (d, 2H, Ar–CH), 7.78 (s, 1H, =CH), 9.12 (s, 220 1H, NH), 10.86 (s, 1H, COOH). 13C NMR: δppm = 55.5, 59.2, 62.3, 114.8, 127.1, 132.9, 135.1, 151.9, 158.1, 159.6, 167.9, 171.2. 4.3.2.8. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-mercapto propanoic acid (6h) Yellow solid, Yield: 96%, mp 256–258 ºC; ES-MS m/z: 338.40. IR νmax /cm–1: 3415 (OH), 3211 (NH), 3011 (CH–Ar), 2510 (SH), 1735 (HO–C=O), 1698 (C=O), 1559 (C=C), 1501 (C=N), 1011 (CS), 1099 (C–N). 1H NMR (400 MHz, DMSO-d6, ppm) = 1.40 (s, 1H, SH), 3.10–3.12 (d, 2H, CH2), 3.82 (s, 3H, OCH3), 4.15–4.17 (t, 1H, CH), 7.02–7.04 (d, J = 7.6 Hz, 2H, Ar–CH), 7.42–7.44 (d, J = 7.6 Hz, 2H, Ar–CH), 7.78 (s, 1H, =CH), 9.88 (s, 1H, NH), 11.54 (s, 1H, COOH). 13C NMR: δppm = 26.9, 55.2, 60.5, 114.7, 128.8, 130.1, 132.5, 143.6, 152.9, 158.3, 167.2, 178.2. 4.3.2.9. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)succinic acid (6i) Yellow solid, Yield: 97%, mp 173–175 ºC; ES-MS m/z: 350.37. IR νmax /cm–1: 3426 (OH), 3210 (NH), 3016 (CH–Ar), 1732 (HO–C=O), 1705 (C=O), 1532 (C=C), 1511 (C=N), 1014 (C-S), 1040 (C– N). 1H NMR (400 MHz, DMSO-d6, ppm) = 2.61–2.63 (d, 2H, CH2), 3.71–3.73 (t, 1H, CH), 3.90 (s, 3H, OCH3), 7.12–7.14 (d, J = 7.2 Hz, 2H, Ar–CH), 7.52–7.54 (d, J = 7.2 Hz, 2H, Ar–CH), 7.78 (s, 1H, =CH), 9.70 (s, 1H, NH), 11.74 (s, 2H, COOH). 13C NMR: δppm = 35.9, 53.2, 55.3, 114.7, 127.5, 128.8, 132.5, 135.6, 152.9, 158.3, 167.2, 172.2, 178.2. 4.3.2.10. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-(1H-imidazol4-yl)propanoic acid (6j) Yellow solid, Yield: 98%, mp 198–200 ºC; ES-MS m/z: 372.40. IR νmax /cm–1: 3442 (OH), 3296 (NH), 2921 (CH–Ar), 1693 (C=O), 1500 (C=C), 1455 (C=N), 1015 (C-S), 824 (C–N). 1H NMR (400 MHz, DMSO-d6, ppm) = 3.10 (s, 2H, CH2), 3.60 (s, 1H, CH), 3.82 (s, 3H, OCH3), ), 7.10–7.12 (d, J = 7.2 Hz, 2H, Ar–CH), 7.30–7.32 (d, J = 7.2 Hz, 2H, Ar–CH), 7.64 (s, 1H, =CH imidazole ring), 7.80 (s, 1H, =CH), 8.64 (s, 1H, =CH imidazole ring), 8.70 (s, 2H, NH), 10.90 (s, 1H, COOH). 13C NMR: δppm = 28.9, 58.3, 117.9, 55.2, 114.5, 124.7, 127.9, 128.4, 129.2, 132.1, 135.2, 152.3, 159.2, 168.5, 175.2. 4.3.2.11. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-(4hydroxyphenyl) propanoic acid (6k) Yellow solid, Yield: 98%, mp 157–159 ºC; ES-MS m/z: 398.43. IR νmax /cm–1: 3445 (O=C-OH), 3393 (OH), 3214 (NH), 2974 (CH–Ar), 1731 (HO–C=O), 1699 (C=O), 1543 (C=C), 1591 (C=N), 1011 (CS), 1082 (C–N). 1H NMR (400 MHz, DMSO-d6, ppm) = 2.80–2.82 (d, 2H, CH2), 3.80 (s, 3H, OCH3), 4.40–4.42 (t, 1H, CH), 5.32 (s, 1H, OH), 7.10–7.12 (d, J = 7.6 Hz, 4H, Ar–CH), 7.50–7.52 (d, J = 7.6 Hz, 4H, Ar–CH), 7.72 (s, 1H, =CH), 9.32 (s, 1H, NH), 11.16 (s, 1H, COOH). 13C NMR: δppm = 36.4, 55.6, 56.6, 115.9, 127.7, 128.6, 128.9, 129.2, 130.2, 135.3, 152.2, 155.7, 158.5, 167.7, 174.2. 4.3.2.12. (Z)-2-((5-(4-methoxybenzylidene)-4-oxo-4,5-dihydrothiazol-2-yl)amino)-3-hydroxy butanoic acid (6l) Yellow solid, Yield: 98%, mp 181–183 ºC; ES-MS m/z: 336.36. IR νmax /cm–1: 3454 (OH), 3212 (NH), 3012 (CH–Ar), 1733 (HO–C=O), 1691 (C=O), 1555 (C=C), 1597 (C=N), 1046 (C-S), 1112 (C– N). 1H NMR (400 MHz, DMSO-d6, ppm) = 1.15–1.17 (d, 3H, CH3), 3.50–3.52 (d, 1H, CH), 3.60 (s, 1H, OH), 3.80 (s, 3H, OCH3), 3.93–3.95 (m, 1H, CH), 7.20–7.22 (d, J = 7.2 Hz, 2H, Ar–CH), 7.50–