In the present study a novel series of (2-substituted-4-methylthiazol-5-yl)(4-substituted piperazin-1-yl)methanone derivatives were synthesized by reaction of 2-substituted-4-methylthiazole-5-carboxylic acid with N-substituted benzyl piperazine by using 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDCl) and 1-hydroxybenzotriazole (HOBt) in DMF. All the newly synthesized compounds were characterized by spectral methods. The title compounds were screened for in vitro antibacterial activity. Most of the compounds show moderate to good antimicrobial activity. The in vitro antibacterial activity was performed against Gram-positive bacteria including S. aureus, B. subtilis and Gram-negative bacteria including E. coli and K. pneumonia . The antifungal activity was screened against fungi including F. solani , C. lunata and A. niger . To evaluate the activity of the synthesized compounds, the zone of inhibition were determined. The in vitro antimicrobial screening results of tested compounds are listed in 2 and 3 . Most of the synthesized compounds (except compounds 5d, 5i and 5l ) exhibited moderate to excellent antimicrobial activities. It is noteworthy that compounds 5e and 5k (R=2,4-(Cl) ₂ C ₆ H ₃ CH ₂ −, R ¹ =2,4-(Cl) ₂ C ₆ H ₃ − and 4-ClC ₆ H ₄ CH ₂ , respectively), compound 5g (R = CH ₃ −, R ¹ =4-ClC ₆ H ₄ CH ₂ −) and 5h (R = C ₆ H ₅ −, R ¹ = 4-ClC ₆ H ₄ CH ₂ −) showed excellent activity against all bacterial strain as compared to standard drugs. Even compounds 5a, 5b, 5c and 5j showed good antibacterial activities against most of the strains. The result of antifungal activity revealed that all the tested compounds show moderate to good antifungal activity as compared to the standard drug ketoconazole. Compound 5c (R = 4-ClC ₆ H ₄ CH ₂ −, R ¹ = 2,4-(Cl) ₂ C ₆ H ₃ −), 5d (R = 4- FC ₆ H ₄ CH ₂ −, R ¹ = 2,4-(Cl) ₂ C ₆ H ₃ −) and 5l (R = 3,4-(MeO) ₂ C ₆ H ₃ CH ₂ −, R ¹ = 4-ClC ₆ H ₄ CH ₂ −) exhibited good activities against all the fungal species. However compounds 5d , 5i and 5l were inactive against bacterial strain but showed good antifungal activity. Thus, it is concluded that compounds with R = CH ₃ −, C ₆ H ₅ − and 2,4-diClC ₆ H ₃ CH ₂ − show good to excellent antibacterial activity. Melting points were determined in an open capillary on Veego melting point apparatus and are uncorrected. The purity of the compounds was checked on silica gel-G plates. The compounds 5a−l was purified on silica gel (100−200) column chromatography using ethyl acetate: hexane (2:8) as eluent. Infrared spectra (cm ⁻¹ ) were recorded in KBr on a Shimadzu Model FTIR-435 spectrophotometer. ¹ H NMR and ¹³ C NMR spectra were recorded in CDCl ₃ or DMSO- d ₆ solution on a Varian Mercury YH-300 spectrometer operating at 300 MHz for ¹ H and 75 MHz for ¹³ C. Chemical shifts are expressed relative to tetramethylsilane (TMS) and were reported as δ (ppm). Mass spectral (MS) measurements were made on a Jeol-JMS-DX 303 mass spectrometer. The isotopic peak at M+2 was observed in the mass spectrum of all the compounds due to S, Br and/or Cl. 2-Methyl, phenyl or benzyl substituted-4-methylthiazole-5-carboxylic acid (3a−3f) Mixture of 2-chloro-3-oxo ethyl butanoate (0.05 mol) and substituted thioamides (0.055 mol) in methanol (50 mL) was refluxed for 5−6 hours. After completion of the reaction, as monitored on TLC, 2N NaOH (20 mL) was added and refluxed further for 4 hours. Methanol was distilled off and the mixture was acidified with 4N HCl to pH 2. The precipitated product was filtered, washed with water and recrystallized from aqueous ethanol. (2-methyl, phenyl or benzyl-substituted-4-methylthiazol-5-yl)(4-phenyl or benzyl substituted piperazin-1-yl)methanone (5a−5l) A mixture of 2-substituted-4-methylthiazole-5-carboxylic acid ( 3a−f ) (1 mmol), DIPEA (0.35 mL, 2 mmol), HOBt (0.14 g, 1 mmol) in DMF (10 mL) was cooled to 0 ℃. To this N-aryl or benzyl piperazine ( 4a−b ) (1 mmol) was added followed by EDC.HCl (0.19 g, 1 mmol) at 0 ℃ and stirred overnight at room temperature. The reaction was quenched with water and the product was filtered, washed with water and purified by column chromatography using ethyl acetate: hexane (2:8). (4-(2,4-Dichlorophenyl)piperazin-1-yl)(2,4-dimethylthiazol-5-yl)methanone (5a) IR (KBr): 3056, 2971, 2911, 2802, 1631, 1542, 1480, 1417, 1251, 1130, 1006, 934, 830, 822 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.42 (s, 3H), 2.68 (s, 3H), 3.18 (s, 4H), 3.76 (s, 4H), 6.72 (m, 1H), 6.95 (s, 1H), 7.24−7.45 (m, 1H); ¹³ C NMR (CDCl ₃ , 75 MHz): 15.0, 20.1, 48.1, 49.3, 115.4, 121.6, 126.8, 127.0, 128.9, 132.5, 149.3, 151.7, 164.2, 168.5; LCMS: 370.0 (M+H) ⁺ . Anal. calcd. for C ₁₆ H ₁₇ Cl ₂ N ₃ OS: C, 51.90; H, 4.63; N, 11.35. Found: C, 51.79; H, 4.51; N, 11.22. (4-(2,4-Dichlorophenyl)piperazin-1-yl)(4-methyl-2-phenylthiazol-5-yl)methanone (5b) IR (KBr): 2991, 2891, 2850, 1616, 1541, 1438, 1365, 1242, 1151, 1010, 947, 850, 813 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.44 (s, 3H), 3.11 (s, 4H), 3.71 (s, 4H), 6.66 (dd, J = 3 and 9 Hz, 1H), 6.88, (d, J = 3 Hz, 1H), 7.21 (d, J = 9 Hz, 1H), 7.35−7.37 (m, 3H), 7.81−7.85 (m, 2H); ¹³ C NMR (CDCl ₃ , 75 MHz): 16.6, 49.1, 115.9, 117.9, 123.3, 123.7, 126.5, 129.0, 130.5, 130.6, 132.7, 132.9, 150.0, 153.1, 162.6, 167.9; LCMS: 432.1 (M+H) ⁺ . Anal. calcd. for C ₂₁ H ₁₉ Cl ₂ N ₃ OS: C, 58.34; H, 4.43; N, 9.72. Found: C, 58.26; H, 4.37; N, 9.59. (2-(4-Chlorobenzyl)-4-methylthiazol-5-yl)(4-(2,4-dichlorophenyl)piperazin-1-yl)methanone (5c) IR (KBr): 3059, 2930, 2879, 2823, 1631, 1475, 1427, 1365, 1271, 1240, 1157, 1089, 1016, 956, 873 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300MHz): 2.41 (s, 3H), 3.16 (s, 4H), 3.61 (s, 4H), 3.84 (s, 2H), 6.75 (dd, J = 2.4 and 8.4 Hz, 1 H), 7.00 (d, J = 2.4 Hz, 1H), 7.30 (d, J = 8.4 Hz, 1H), 7.37−7.42 (m, 4H); ¹³ C NMR (CDCl ₃ , 75 MHz): 14.9, 41.0, 47.9, 51.0, 115.2, 117.8, 125.0, 125.5, 128.1, 128.8, 130.6, 131.4, 131.6, 134.3, 148.9, 152.0, 164.5, 169.1; LCMS: 480.0 (M+H) ⁺ . Anal. calcd. for C ₂₂ H ₂₀ Cl ₃ N ₃ OS: C, 54.95; H, 4.19; N, 8.74. Found: C, 54.88; H, 4.11; N, 8.63. (2-(4-Fluorobenzyl)-4-methylthiazol-5-yl)(4-(2,4-dichlorophenyl)piperazin-1-yl)methanone (5d) IR (KBr): 3076, 2908, 2831, 1947, 1506, 1435, 1348, 1246, 1138, 1006, 852, 763 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.44 (s, 3H), 3.13 (s, 4H), 3.72 (s, 4H), 4.24 (s, 2H), 6.71 (dd, J = 2.75 and 9 Hz, 1H), 6.83, (d, J = 2.70 Hz, 1H), 7.01 (t, J = 8.7 Hz, 2H), 7.24−7.30 (m, 3H); ¹³ C NMR (CDCl ₃ , 75 MHz): 16.2, 38.4, 48.6, 48.8, 115.4, 117.6, 122.8, 124.0, 130.3, 130.5, 132.5, 132.6, 132.7, 149.9, 151.7, 161.5, 162.4, 170.3; LCMS: 464.1 (M+H) ⁺ . Anal. calcd. for C ₂₂ H ₂₀ Cl ₂ FN ₃ OS: C, 56.90; H, 4.34; N, 9.05. Found: C, 56.80; H, 4.23; N, 8.94. (2-(2,4-Dichlorobenzyl)-4-methylthiazol-5-yl)(4-(2,4-dichlorophenyl)piperazin-1-yl)methanone (5e) IR (KBr): 3084, 2928, 2833, 1631, 1558, 1487, 1242, 997, 954, 856, 788 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.36 (s, 3H), 3.06 (s, 4H), 3.64 (s, 4H), 4.29 (s, 2H), 6.64 (dd, J = 3 and 9 Hz, 1H), 6.86, (d, J = 3 Hz, 1H), 7.13−7.23 (m, 3H), 7.33 (d, J = 2.1 Hz, 1H); ¹³ C NMR (CDCl ₃ , 75 MHz): 16.4, 29.6, 36.4, 49.0, 115.4, 117.9, 123.2, 124.2, 127.5, 129.5, 130.5, 131.9, 132.8, 133.4, 134.0, 134.7, 150.0, 151.9, 162.5, 168.1; LCMS: 514.0 (M+H) ⁺ . Anal. calcd. for C ₂₂ H ₁₉ Cl ₄ N ₃ OS: C, 51.28; H, 3.72; N, 8.15. Found: C, 51.20; H, 3.60; N, 8.04. (2-(3,4-Dimethoxybenzyl)-4-methylthiazol-5-yl)(4-(2,4-dichlorophenyl)piperazin-1-yl)methanone (5f) IR (KBr): 3031, 2965, 2910, 2824, 1631, 1524, 1418, 1315, 1251, 1140, 1089, 1006, 750, 819 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.43 (s, 3H), 3.14 (s, 4H), 3.73 (s, 4H), 3.86 (s, 3H), 3.87 (s, 3H), 4.19 (s, 2H), 6.72 (dd, J = 3 and 9 Hz, 1H), 6.80−6.87, (m, 3H), 6.94 (d, J = 3 Hz, 1H), 7.28 (d, J = 9 Hz 1H); ¹³ C NMR (CDCl ₃ , 75 MHz): 15.4, 39.2, 46.4, 51.1, 56.2, 56.3, 115.2, 117.0, 117.6, 119.1, 123.4, 125.2, 126.0, 128.1, 129.5, 131.5, 145.8, 148.4, 149.7, 152.2, 163.1, 168.2; LCMS: 506.1 (M+H) ⁺ . Anal. calcd. for C ₂₄ H ₂₅ Cl ₂ N ₃ O ₃ S: C, 56.92; H, 4.98; N, 8.30. Found: C, 56.78; H, 4.86; N, 8.19. (4-(4-Chlorobenzyl)piperazin-1-yl)(2,4-dimethylthiazol-5-yl)methanone (5g) IR (KBr) 3060, 2976, 2908, 2808, 1639, 1546, 1487, 1429, 1265, 1138, 1006, 935, 831, 756 cm ⁻¹ ; ¹ H NMR: 2.37 (s, 7H), 2.60 (s, 3H), 3.59 (s, 4H), 4.23 (s, 2H), 7.15−7.34 (m, 4H); ¹³ C NMR: 15.1, 20.2, 48.9, 59.1, 65.3, 116.0, 128.9, 130.5, 132.9, 135.1, 152.0, 164.5, 168.3; LCMS: 350.1 (M+H) ⁺ . Anal. calcd. for C ₁₇ H ₂₀ ClN ₃ OS: C, 58.36; H, 5.76; N, 12.01. Found: C, 58.23; H, 5.67; N, 11.89. (4-(4-Chlorobenzyl)piperazin-1-yl)(4-methyl-2-phenylthiazol-5-yl)methanone (5h) IR (KBr): 3065, 2956, 2900, 2815, 1630, 1484, 1440, 1321, 1250, 1005, 865, 815 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.26 (s, 4H), 2.35 (s, 3H), 3.49 (s, 4H), 4.09 (s, 2H), 7.01− 7.24 (m, 7H), 7.74−7.77 (m, 2H); ¹³ C NMR (CDCl ₃ , 75 MHz): 18.1, 51.5, 74.7, 77.2, 119.4, 126.2, 127.4, 128.4, 128.7, 130.2, 133.6, 140.1, 140.6, 152.2, 162.2, 167.6; LCMS: 412.1 (M+H) ⁺ . Anal. calcd. for C ₂₂ H ₂₂ ClN ₃ OS: C, 64.14; H, 5.38; N, 10.20. Found: C, 64.08; H, 5.31; N, 10.09. (2-(4-Chlorobenzyl)-4-methylthiazol-5-yl)(4-(4-chlorobenzyl)piperazin-1-yl)methanone (5i) IR (KBr): 3068, 2949, 2912, 2809, 1628, 1504, 1478, 1324, 1248, 1006, 860, 809 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.32 (s, 3H), 2.60 (s, 4H), 3.32 (s, 4H), 4.21 (s, 2H), 4.23 (s, 2H), 7.15−7.36 (m, 8H); ¹³ C NMR (CDCl ₃ , 75 MHz): 14.2, 47.7, 51.2, 61.2, 127.2, 127.5, 128.3, 128.5, 128.9, 129.7, 131.2, 132.6, 133.8, 135.5, 140.4, 153.2, 168.9; LCMS: 460.1 (M+H) ⁺ . Anal. calcd. for C ₂₃ H ₂₃ Cl ₂ N ₃ OS: C, 60.00; H, 5.04; N, 9.13. Found: C, 59.88; H, 5.00; N, 9.02. (4-(4-Chlorobenzyl)piperazin-1-yl)(2-(4-fluorobenzyl)-4-methylthiazol-5-yl)methanone (5j) IR (KBr): 3051, 2908, 2823, 1629, 1487, 1305, 1240, 1132, 1006, 846, 746 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.41 (s, 4H), 2.44 (s, 3H), 3.62 (s, 4H), 4.24 (s, 2H), 4.26 (s, 2H), 7.03 (t, J = 8.4 Hz, 2H), 7.20−7.40 (m, 6H); ¹³ C NMR (CDCl ₃ , 75 MHz): 16.2, 38.5, 51.6, 60.1, 75.0, 115.5, 124.5, 127.5, 128.6, 128.9, 130.4, 132.8, 141.0, 151.3, 160.2, 163.5, 170.0; LCMS: 444.1 (M+H) ⁺ . Anal. calcd. for C ₂₃ H ₂₃ ClFN ₃ OS: C, 62.22; H, 5.22; N, 9.46. Found: C, 62.09; H, 5.17; N, 9.33. (2-(2,4-Dichlorobenzyl)-4-methylthiazol-5-yl)(4-(4-chlorobenzyl)piperazin-1-yl)methanone (5k) IR (KBr): 3070, 2970, 2910, 2808, 1627, 1545, 1477, 1440, 1311, 1257, 1099, 1006, 864, 813 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.30 (s, 7H), 3.49 (s, 4H), 4.12 (s, 2H), 4.23 (s, 2H), 7.06−7.29 (m, 7H); ¹³ C NMR (CDCl ₃ , 75 MHz): 16.2, 36.3, 51.6, 74.9, 124.5, 127.3, 127.5, 128.6, 128.9, 129.4, 131.8, 133.5, 133.8, 134.6, 140.4, 141.2, 151.3, 162.2, 167.7; LCMS: 494.1 (M+H) ⁺ . Anal. calcd. for C ₂₃ H ₂₂ Cl ₃ N ₃ OS: C, 55.82; H, 4.48; N, 8.49. Found: C, 55.69; H, 4.36; N, 8.35. (2-(3,4-Dimethoxybenzyl)-4-methylthiazol-5-yl)(4-(4-chlorobenzyl)piperazin-1-yl)methanone (5l) IR (KBr): 3024, 2974, 2918, 2810, 1626, 1533, 1429, 1317, 1249, 1141, 1095, 1004, 754, 709 cm ⁻¹ ; ¹ H NMR (CDCl ₃ , 300 MHz): 2.37 (s, 4H), 2.62 (s, 3H), 3.58 (s, 4H), 3.83 (s, 6H), 4.17 (s, 2H), 4.21 (s, 2H), 6.81 (t, J = 8.4 Hz, 2H), 7.15−7.35 (m, 5H); ¹³ C NMR (CDCl ₃ , 75 MHz): 15.2, 39.8, 48.0, 55.3, 56.2, 56.3, 64.3, 115.1, 116.8, 117.5, 123.3, 128.8, 129.7, 130.5, 132.9, 133.6, 146.0, 148.6, 152.1, 163.1, 168.2; LCMS: 485.2 (M+H) ⁺ . Anal. calcd. for C ₂₅ H ₂₈ ClN ₃ O ₃ S: C, 61.78; H, 5.81; N, 8.65. Found: C, 61.64; H, 5.70; N, 8.49. The synthesized compounds 5a−5l were screened for their in vitro antimicrobial activity against the standard strains B. subtilis , S. aureus (Gram-positive) and E. coli , K. pneumonia (Gram-negative) by the disk diffusion method. 17 18 Disks measuring 6 mm in diameter were punched from whatman no.1 filter paper. Batches of 100 disks were dispensed to each screw-caped bottle and sterilized by dry heat at 145 ℃ for one hour. The test compounds were prepared with 100 μg/mL and 200 μg/mL concentration in dimethyl sulfoxide (DMSO). Disks of each concentration were placed in nutrient agar medium inoculated with fresh bacteria strains separately. Ciprofloxacin and Chloramphenicol was used as positive controls and DMSO was used as negative control. The incubation was carried out at 37 ℃ for 24 h. The diameter of the zone of growth inhibition around each well was measured after incubation using vernier caliper. The compounds were screened for their antifungal activity against F. solani , C. lunata and A. niger in DMSO by disc diffusion method under standard conditions using Sabourad Dextrose Agar medium as described by NCCLS. 19 Sterile filter paper discs (6 mm diameter) containing specific amount of anti fungal agent (100 μg for the synthesized compounds) were placed on the surface of an agar plate inoculated with the standardized suspension of microorganisms tested. The plates were incubated at 37 ℃ for 2 days for evaluating antifungal activity. The diameters of inhibition zones (in mm) were measured. Ketoconazole was used as positive control. In summary, we have synthesized a series of novel (2-methyl, phenyl or benzyl-substituted-4-methylthiazol-5-yl)(4-phenyl or benzyl substituted piperazin-1-yl)methanone ( 5a−5l ) and screened for their antimicrobial activity. The in vitro antimicrobial results revealed that the compounds 5e, 5g, 5h and 5k show significant antibacterial activity while compounds 5a, 5b, 5c and 5j showed good antibacterial activities. All the tested compounds showed moderate to good antifungal activity. The antimicrobial activity results make them interesting lead molecules for further synthetic and biological evaluation. Further studies are in progress to acquire more information regarding structure activity relationship.