Introduction

Over the past few years, there has been an increased use of antimicrobial drugs and this has resulted in the development of resistance to these drugs. Different mechanisms in microbes contribute to the development of resistance to antibiotics. This leads to search for new heterocyclics as new antimicrobials with modified action [1]. Scientific evidence suggests that free metals and radicals become more harmful, which play an important role in the pathogenesis of many diseases [2]. Generally, compounds having antioxidant activity have been proven to exhibit anticancer [3], anti-hypertensive [4], anti-Alzheimer’s, anti-inflammatory, and many other activities [5]. This fact attracted the special attention of researchers in the identification and development of novel antioxidants that can prevent radical-induced damages.

Pyrimidines with a vast literature and long history have been regarded as important constituents of nucleic acids and many biological processes. Pyrimidine moiety was also found to be embedded in a large number of alkaloids, drugs, antibiotics, agrochemicals, and antimicrobial agents as parent component [6]. 1, 2, 3, 4-tetrahydro pyrimidine derivatives synthesis has recently came out of a growing area of research to determine the most promising target molecules because of their important medicinal and pharmacological properties such as antiviral [7], anti-malarial [8], anticarcinogenic [9], hypoglycemic [10], and calcium channel modulators [11]. Hence, keeping in view of the facts mentioned above and continuing our research work on synthesis and biological evaluation of heterocyclic compounds [12–14] in our present work, we report the synthesis of two series of thiazolo and imidazo N-(4-nitrophenyl)-7-methyl-5-aryl-pyrimidine-6 carboxamide derivatives, which can be synthesized by taking N-(4-nitrophenyl)-3-oxobutamide (Biginelli reaction; a three-component cyclocondensation reaction is one of the methods described to synthesize dihydropyrimidine derivatives) with different aromatic aldehydes and thiourea, guanidine in the presence of concentrated HCl [15].

Materials and methods

Melting points were determined in open glass capillaries on a Sonar melting point apparatus and are uncorrected. The percentage yield was based upon the products obtained after recrystallization. Reaction progress was monitored by thin-layer chromatography (TLC) performed on silica gel G-coated plate in the solvent system toluene: ethyl acetate (7.5:2.5), and the spots were located by iodine. Infrared (IR) analysis was performed on a SHIMAZDU FT-IR instrument using KBr pellets and was recorded in cm−1. 1H NMR spectra were recorded on Bruker DRX-300 FTNMR spectrometer using DMSO-d6 solvent and are expressed in parts per million (d, ppm) downfield from the internal standard TMS. Elemental analysis of all the synthesized compounds was carried out on a Thermo Scientific Flash 2000 CHN analyzer. Mass spectra were taken on a Bruker Compass Data Analysis 4.0 mass spectrometer.

Procedure for synthesis of N-(4-nitrophenyl)-3-oxobutamide (3) [16]

A mixture of ethylacetoacetate (0.01 mol) and 4-nitro aniline (0.01 mol) in toluene containing catalytic amount (0.05 ml, 40 %) of NaOH was refluxed on oil bath for 8 h. After completion of the reaction (TLC monitoring), the reaction mixture was kept overnight; separated solid was filtered, washed with petroleum ether and recrystallized from ethanol to obtain N-(4-nitrophenyl)-3-oxobutamide.

General procedure for synthesis of 5-(2-hydroxynaphthalen-1-yl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo/imidazo[3,2-a]pyrimidine-6-carboxamide (7f, 9h) [16]

A mixture of N-(4-nitrophenyl)-3-oxobutamide (0.01 mol), 2-hydroxynaphthaldehyde (0.01 mol), guanidine/thiourea (0.01 mol), and ethanol (8 ml), containing 0.4 ml of concentrated HCl was heated under reflux for 6–8 h. The solution was deep frozen overnight and precipitates were obtained. 1,2-dichloroethane (0.01 mol) was added to a boiling solution of intermediate obtained above (0.01 mol) in dimethyl formamide and then refluxed for 1 h. After completion of the reaction, the contents were poured on crushed ice. The precipitates obtained were filtered, dried, and recrystallized from ethanol.

General procedure for synthesis of substituted N-(4-nitrophenyl)-6-methyl-4-phenyl-2-thioxo/imino-1,2,3,4-tetrahydropyrimidine-5-carboxamide derivatives (6a–g)(8a–g) [16]

A mixture of N-(4-nitrophenyl)-3-oxobutamide (0.01 mol), different aromatic aldehyde (0.01 mol), guanidine/thiourea (0.01 mol), and ethanol (8 ml), containing 0.4 ml of concentrated HCl was heated under reflux for 6–8 h. The solution was deep frozen overnight and precipitates were obtained.

General procedure for synthesis of 5-substituted N-(4-nitrophenyl)-7-methyl-3,5-dihydro-2H-imidazo/thiazolo [3,2-a]pyrimidine-6-carboxamide derivatives (7a–g)(9a–g) [16]

1,2-dichloroethane (0.01 mol) was added to a boiling solution of substituted N-(4-nitrophenyl)-6-methyl-4-phenyl-2-thioxo/imino-1,2,3,4-tetrahydropyrimidine-5-carboxamide derivatives (0.01 mol) in dimethyl formamide and then refluxed for 1 h. After completion of the reaction, the contents were poured on crushed ice. The precipitates obtained were filtered, dried, and recrystallized from ethanol.

Spectral data

Intermediate 4-(4-chlorophenyl)-6-methyl-N-(4-nitrophenyl)-2-thioxo-1,2,3,4 tetrahydropyrimidine-5-carboxamide 8d solid yellow; Anal. Calcd for C18H15ClN4O3S C, 53.67; H, 3.75; N, 13.91 Found C, 53.58; H, 3.60; N, 13.92 IR (KBr pellets, cm−1): 1,690 (C=O str., 20 amide), 3,343(N–H str., sec. amide), 1,581 (C=C str., skeletal vibration of phenyl nucleus), 860 (C–Cl), 1,211(C=S).

5-(2-hydroxyphenyl)-7-methyl-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide 7a solid brown (yield: 82 %), solid yellow; m.p.: 264–266 °C Rf 0.42; Anal. Calcd for C20H19N5O4 C, 61.06; H, 4.87; N, 17.80 Found C, 60.68; H, 4.56; N, 16.83; IR (KBr pellets, cm−1): 1,690 (C=O str., 20 amide), 3,046 (C=C–H asymmetrical str., aromatic), 3,156 (N–H str., sec. amide), 1,173 (C–N str., 30 aryl amine), 3,208 (OH str., phenyl), 1,583 (C=C str., skeletal vibration of phenyl nucleus), 1,397 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6) 7.25–7.76(m, 4H, Ar–H), 7.95–8.34(m, 3H, Ar–H), 6.91 (s, 1H, CH, pyrimidine ring), 8.37(s, 1H, NH), 2.53–3.39(s, 2H, CH2), 5.47(1H, OH) m/e: 394.15 (M+1).

5-(4-(dimethylamino)phenyl)-7-methyl-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide 7b solid red; Anal. Calcd for C22H24N6O3 C, 62.84; H, 5.75; N, 19.99 Found C, 61.74; H, 5.95; N, 18.90; IR (KBr pellets, cm−1): 1,687 (C=O str., 20 amide), 3,075 (C=C–H Asymmetrical str., aromatic), 3,170 (N–H str., sec, amide), 1,645 (C=N str., pyrimidine ring), 1,279 (C–N str., 30 aryl amine), 1,317 (C–N str., aryl 30 amine), 1,017 (C–O str., OCH3), 1,573 (C=C str., skeletal vibration of phenyl nucleus), 1,514 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6)7.47–7.56 (m, 4H, Ar–H), 7.53–7.65(m, 3H, Ar–H), 6.91 (s, 1H, CH, pyrimidine ring), 8.05(s, 1H, NH), 2.67–3.91 (s, 2H, CH2) m/e: 422.20 (M+2).

5-(4-hydroxyphenyl)-7-methyl-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide (7c) solid yellow; Anal. Calcd for C20H19N5O4 C, 61.06; H, 4.87; N, 17.80 Found C, 60.98; H, 4.77; N, 17.84; IR (KBr pellets, cm−1): 1,688 (C=O str., 20 amide), 3,077 (C=C–H Asymmetrical str., aromatic), 3,108 (N–H str., sec. amide), 1,073 (C–N str., 30 aryl amine), 1,290 (C–O str., OCH3), 3,216 (OH str., phenyl), 1,584 (C=C str., skeletal vibration of phenyl nucleus), 1,502 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6) 7.14–7.74(m, 4H, Ar–H), 7.81–8.83(m, 3H, Ar–H), 6.91 (s, 1H, CH, pyrimidine ring), 8.76 (s, 1H, NH), 2.91–3.86 (s, 2H, CH2), 5.03(1H, OH) m/e: 395.15 (M+2).

5-(4-formylphenyl)-7-methyl-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide (7d) solid dark brown; Anal. Calcd for C21H19N5O4 C, 62.22; H, 4.72; N, 17.27 Found C, 61.98; H, 4.78; N, 17.29; IR (KBr pellets, cm−1): 1,683 (C=O str., 20 amide), 3,072 (C=C–H asymmetrical str., aromatic), 3,212 (N–H str., sec. amide), 1,646 (C=N str., pyrimidine ring), 1,124 (C–N str., 30 aryl amine), 1,301 (C–O str., OCH3), 1,725 (C=O str., aldehyde), 1,581 (C=C str., skeletal vibration of phenyl nucleus), 1,480 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6) 7.28–7.79(m, 4H, Ar–H), 7.95–8.35(m, 3H, Ar–H), 6.81 (s, 1H, CH, pyrimidine ring), 8.39(s, 1H, NH), 2.45–3.49(s, 2H, CH2), 5.49(1H, OH) m/e: 406.15 (M+1).

7-methyl-5-(2-nitrophenyl)-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide (7e) solid white; Anal. Calcd for C20H18N6O5 C, 56.87; H, 4.30; N, 19.90 Found C, 55.78; H, 4.36; N, 19.99; IR (KBr pellets, cm−1): 1,684 (C=O str., 20 amide), 3,076 (C=C–H Asymmetrical str., aromatic), 3,171 (N–H str., sec. amide), 1,645 (C=N str., pyrimidine ring), 1,106 (C–N str., 30 aryl amine), 1,247 (C–O str., OCH3), 1,572 (C=C str., skeletal vibration of phenyl nucleus), 1,440 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6) 7.45–7.77 (m, 4H, Ar–H), 7.93–8.35 (m, 3H, Ar–H), 6.97 (s, 1H, CH, pyrimidine ring), 8.38 (s, 1H, NH), 2.57–3.35(s, 2H, CH2), 5.87(1H, OH) m/e: 423.14 (M+1).

5-(2-hydroxynaphthalen-1-yl)-7-methyl-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide (7f) solid red; Anal. Calcd for C24H21N5O4 C, 65.00; H, 4.77; N, 15.79 Found C, 65.98; H, 4.79; N, 14.91; IR (KBr pellets, cm−1): 1,685 (C=O str., 20 amide), 3,075 (C=C–H Asymmetrical str., aromatic), 3,168 (N–H str., sec. amide), 1,645 (C=N str., pyrimidine ring), 1,106 (C–N str., 30 aryl amine), 1,217 (C–O str., OCH3), 1,571 (C=C str., skeletal vibration of phenyl nucleus), 1,440 (C–NO2 str., C6H4NO2) 1H NMR δppm (DMSO-d6) 7.55–7.73 (m, 4H, Ar–H), 7.94–8.34 (m, 3H, Ar–H), 6.96 (s, 1H, CH, pyrimidine ring), 8.38 (s, 1H, NH), 2.57–3.33(s, 2H, CH2), 5.82(1H, OH) m/e: 444.16 (M+1).

5-(3,4-dimethoxyphenyl)-7-methyl-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide (7g) solid brown; Anal. Calcd for C22H23N5O5 C, 60.40; H, 5.30; N, 16.01Found C, 60.87; H, 5.74; N, 16.56; IR (KBr pellets, cm−1): 1,688 (C=O str., 20 amide), 3,025 (C=C–H Asymmetric str., aromatic), 3,168 (N–H str., sec. amide), 1,645 (C=N str., pyrimidine ring), 1,166 (C–N str., 30 aryl amine), 1,278 (C–O str., OCH3), 1,575 (C=C str., skeletal vibration of phenyl nucleus), 1,440 (C–NO2 str., C6H4NO2) 1H NMR δppm (DMSO-d6) 7.55–7.78 (m, 4H, Ar–H), 7.83–8.34 (m, 3H, Ar–H), 6.98 (s, 1H, CH, pyrimidine ring), 8.35 (s, 1H, NH), 2.67–3.36 (s, 2H, CH2), 5.86 (1H, OH) m/e: 438.17 (M+1).

5-(4-hydroxy-3-methoxyphenyl)-7-methyl-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide (7h °) solid yellow; Anal. Calcd for C21H21N5O5 C, 59.57; H, 5.00; N, 16.54 Found C, 57.92; H, 5.67; N, 16.76; IR (KBr pellets, cm−1): 1,687 (C=O str., 20 amide), 3,077 (C=C–H Asymmetric str., aromatic), 3,108 (N–H str., sec. amide), 1,109 (C–N str., 30 aryl amine), 1,290 (C–O str., OCH3), 1,584 (C=C str., skeletal vibration of phenyl nucleus), 3,216 (OH str., phenyl), 1,467 (C–NO2 str., C6H4NO2) 1H NMR δppm (DMSO-d6) 7.55–7.78 (m, 4H, Ar–H), 7.94–8.37 (m, 3H, Ar–H), 6.98 (s, 1H, CH, pyrimidine ring), 8.37 (s, 1H, NH), 2.67–3.37 (s, 2H, CH2), 5.77 (1H, OH) m/e: 424.16 (M+1).

5-(4-hydroxyphenyl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9a) solid green; Anal. Calcd for C20H18N4O4S C, 58.53; H, 4.42; N, 13.65 Found C, 57.94; H, 4.49; N, 13.87; IR (KBr pellets, cm−1): 1,690 (C=O str., 20 amide), 2,981 (C=C–H asymmetrical str., aromatic), 3o72 (N–H str., sec. amide), 1,082 (C–N str., 30 aryl amine), 3,209 (OH str., phenyl), 678 (C–S–C str., Thiazole ring), 1,577 (C=C str., skeletal vibration of phenyl nucleus), 1,465 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6) 7.05–7.45 (m, 4H, Ar–H), 7.70–7.86 (m, 3H, Ar–H), 6.89(s, 1H, CH, pyrimidine ring), 8.01 (s, 1H, NH), 2.66–3.92(s, 2H, CH2), 5.50 (1H, OH) m/e: 411.11(M+1).

5-(2-formylphenyl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9b) solid yellow; Anal. Calcd for C21H18N4O4S C, 59.70; H, 4.29; N, 13.26 Found C, 59.76; H, 4.34; N, 13.67; IR (KBr pellets, cm−1): 1,690 (C=O str., 20 amide), 3,076 (C=C–H asymmetrical str., aromatic), 3,120 (N–H str., sec. amide), 1,138 (C–N str., 30 aryl amine), 1,736 (C=O str., aldehyde), 1,232 (C–O str., OCH3), 667 (C–S–C str., Thiazole ring), 1,537 (C=C str., skeletal vibration of phenyl nucleus), 1,414 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6)7.27–7.75 (m, 4H, Ar–H), 7.79–8.17(m, 3H, Ar–H), 6.92 (s, 1H, CH, pyrimidine ring), 8.78 (s, 1H, NH), 2.64–3.64 (s, 2H, CH2), 9.89 (m, 1H, CHO) m/e: 423.11 (M+1).

5-(4-formylphenyl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9c) solid brown; Anal. Calcd for C21H18N4O4S C, 59.70; H, 4.29; N, 13.26 Found C, 59.87; H, 4.33; N, 13.35; IR (KBr pellets, cm−1): 1,689 (C=O str., 20 amide), 3,068 (C=C–H asymmetrical str., aromatic), 3,280 (N–H str., sec. amide), 1,092 (C–N str., 30 aryl amine), 1,601 (C=N str., pyrimidine ring), 1,727 (C=O str., aldehyde), 1,260 (C–O str., OCH3), 640 (C–S–C str., Thiazole ring), 1,506 (C=C str., skeletal vibration of phenyl nucleus), 1,384 (C–NO2 str., C6H4NO2) 1H NMR, δppm (DMSO-d6) 7.42–7.87(m, 4H, Ar–H), 7.87–7.98 (m, 3H, Ar–H), 6.91 (s, 1H, CH, pyrimidine ring), 8.21 (s, 1H, NH), 2.49–3.84(s, 2H, CH2), 10.48 (s, 1H, CHO) m/e: 423.11 (M+1).

5-(4-chlorophenyl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9d) solid red; Anal. Calcd for C20H17ClN4O3S C, 56.01; H, 4.00; N, 13.06 Found C, 55.57; H, 4.67; N, 13.12; IR (KBr pellets, cm−1): 1,690 (C=O str., 20 amide), 3,024 (C=C–H Asymmetric str., aromatic), 3,165 (N–H str., sec. amide), 1,645 (C=N str., pyrimidine ring), 1,166 (C–N str., 30 aryl amine), 1,217 (C–O str., OCH3), 1,570 (C=C str., skeletal vibration of phenyl nucleus), 760 (C–CI str., phenyl), 1,439 (C–NO2 str., C6H4NO2) 636 (C–S–C str., Thiazole ring), 1H NMR δppm (DMSO-d6) 7.45–7.79 (m, 4H, Ar–H), 7.73–8.55 (m, 3H, Ar–H), 6.98 (s, 1H, CH, pyrimidine ring), 8.37 (s, 1H, NH), 2.67–3.55(s, 2H, CH2), 5.97(1H, OH) m/e: 430.07 (M+2).

5-(4-(dimethylamino)phenyl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9e) solid yellow; Anal. Calcd for C22H23N5O3S C, 60.39; H, 5.30; N, 16.01 Found C, 60.40; H, 5.36; N, 16.23; IR (KBr pellets, cm−1): 1,688 (C=O str., 20 amide), 3,024 (C=C–H asymmetric str., aromatic), 3,168 (N–H str., sec. amide), 1,645 (C=N str., pyrimidine ring), 1,167 (C–N str., 30 aryl amine), 1,279 (C–O str., OCH3), 1,572 (C=C str., skeletal vibration of phenyl nucleus), 1,341 (C–N str., 30 aryl amine), 637 (C–S–C str., Thiazole ring), 1,440 (C–NO2 str., C6H4NO 12 H NMR δppm (DMSO-d6) 7.45–7.77 (m, 4H, Ar–H), 7.93–8.35 (m, 3H, Ar–H), 6.97 (s, 1H, CH, pyrimidine ring), 8.38 (s, 1H, NH), 2.57–3.35 (s, 2H, CH2), 5.89 (1H, OH) m/e: 438.16 (M+1).

5-(4-methoxyphenyl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9f) solid red; Anal. Calcd for C21H20N4O4S C, 59.42; H, 4.75; N, 13.20 Found C, 60.55; H, 4.79; N, 13.26; IR (KBr pellets, cm−1): 1,688 (C=O str., 20 amide), 3,026 (C=C–H asymmetric str., aromatic), 3,171 (N–H str., sec. amide), 1,640 (C=N str., pyrimidine ring), 1,161 (C–N str., 30 aryl amine), 1,218 (C–O str., OCH3), 1,556 (C=C str., skeletal vibration of phenyl nucleus), 1,466 (C–NO2 str., C6H4NO2) 1H NMR δppm (DMSO-d6) 7.45–7.87 (m, 4H, Ar–H), 7.83–8.45 (m, 3H, Ar–H), 6.98 (s, 1H, CH, pyrimidine ring), 8.48 (s, 1H, NH), 2.57–3.37 (s, 2H, CH2), 5.88(1H, OH) m/e: 426.12 (M+2).

5-(3,4-dimethoxyphenyl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9g) solid white; Anal. Calcd for C22H22N4O5S C, 58.14; H, 4.88; N, 12.33 Found C, 59.19; H, 4.85; N, 12.34; IR (KBr pellets, cm−1): 1,687 (C=O str., 20 amide), 3,015 (C=C–H asymmetric str., aromatic), 3,155 (N–H str., sec. amide), 1,160 (C–N str., 30 aryl amine), 1,245 (C–O str., OCH3), 1,597 (C=C str., skeletal vibration of phenyl nucleus), 686 (C–S–C str., Thiazole ring), 1,459 (C–NO2 str., C6H4NO2) 1H NMR δppm (DMSO-d6) 7.35–7.78 (m, 4H, Ar–H), 7.94–8.36 (m, 3H, Ar–H), 6.87 (s, 1H, CH, pyrimidine ring), 8.37 (s, 1H, NH), 2.47–3.38 (s, 2H, CH2), 5.88 (1H, OH) m/e: 455.13 (M+1).

5-(2-hydroxynaphthalen-1-yl)-7-methyl-N-(4-nitrophenyl)-3,5-dihydro-2H-thiazolo[3,2-a]pyrimidine-6-carboxamide (9h) solid red; Anal. Calcd for C24H20N4O4S C, 62.60; H, 4.38; N, 12.17 Found C, 62.76; H, 4.42; N, 12.76; IR (KBr pellets, cm−1): 1,688 (C=O str., 20 amide), 3,047 (C=C–H asymmetric str., aromatic), 3,154 (N–H str., sec. amide), 1,169 (C–N str., 30 aryl amine), 1,208 (C–O str., OCH3), 1,589 (C=C str., skeletal vibration of phenyl nucleus), 627 (C–S–C str., Thiazole ring), 3,254 (OH str., phenyl), 1,484 (C–NO2 str., C6H4NO2) 1H NMR δppm (DMSO-d6) 7.43–7.78 (m, 4H, Ar–H), 7.97–8.45 (m, 3H, Ar–H), 6.96 (s, 1H, CH, pyrimidine ring), 8.37 (s, 1H, NH), 2.53–3.37 (s, 2H, CH2), 5.87 (1H, OH) m/e: 461.12 (M+1) (Tables 1, 2).

Table 1 Arrangement of substituents used for targeted compounds
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Table 2 Physicochemical properties of synthesized pyrimidine derivatives
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Biological screening of synthesized pyrimidine derivatives

Evaluation of antimicrobial activity

Determination of minimum inhibitory concentration (MIC) in-vitro antimicrobial activity of synthesized compounds was performed against Gram-positive bacteria: Staphylococcus aureus MTCC 3,160, Bacillus subtilis MTCC 441, Gram-negative bacterium: Escherichia coli MTCC 443 and fungal strains: Candida albicans MTCC 227 and Aspergillus niger MTCC 281 using tube dilution method using ciprofloxacin and fluconazole as standard drugs for antibacterial and antifungal activity, respectively [17]. Dilutions of test and standard compounds were prepared in double-strength nutrient broth—I.P. (bacteria) or Sabouraud dextrose broth I.P. (fungi) [18]. The inoculated tubes were incubated at 37 ± 1 °C for 24 h (bacteria), at 25 ± 1 °C for 7 d (A. niger) and at 37 ± 1 °C for 48 h (C. albicans), respectively, and the results were recorded in terms of MIC (the lowest concentration of test substance which inhibited the growth of microorganisms) given in Table 3.

Table 3 Minimum inhibitory concentration for the antimicrobial activity of newly synthesized pyrimidine derivatives
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Free radical scavenging activity

Free radical scavenging activity of synthesized compounds against stable free radical 2,2-diphenyl-2-picrylhydrazyl hydrate (DPPH) was determined spectrophotometrically. When DPPH reacts with an antioxidant compound, which can donate hydrogen, it is reduced. Following the reduction, its deep violet color in methanol bleaches to yellow, and shows a significant absorption decrease at 517 nm. Fifty milliliters of various concentrations (25, 50, 75, and 100) µg/ml of the compounds dissolved in methanol was added to 5 ml of a 0.004 % methanol solution of DPPH. After a 30-min incubation period at room temperature, the absorbance was read against a blank at 517 nm. Tests were carried out in triplicate, and ascorbic acid was used as a positive control. IC50 values were calculated (Table 4). Scavenging of DPPH free radical was calculated as: DPPH scavenging activity (%) = [(Ac–At)/Ac] × 100 where, Ac is the absorbance of the control reaction At is the absorbance of the test compound.

Table 4 Percentage inhibition and IC50 value for free radical scavenging activity of the synthesized pyrimidine derivatives
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Results and discussion

Chemistry

The synthesis of two series of pyrimidine (7a–h) and (9a–h) derivatives was carried out as outlined in the Scheme 1. All the compounds were obtained in appreciable yield and their physicochemical characteristics are presented in Table 2. The structures of the synthesized compounds were established on the basis of their consistent IR, NMR, and mass spectral characteristics in addition to elemental analysis (C, H, N), which were in full agreement with the assigned molecular structures and the data. IR spectrum showed a definite absorption at 1,690 cm−1 due to carboxamide group. The formation of compound 9d was further supported by disappearance of the IR band at 1,211 cm−1 (8d) due to the C=S group and appearance of a new band at 636 cm−1 due to the C–S–C bond. The strong absorption bands appeared between 3,141 and 3,189 cm−1 in the IR spectra of compounds (7a–h) corresponding to secondary amine (N–H) group present in pyrimidine ring. The 1H NMR data of compounds revealed signals between 6.84 and 6.97 δppm for CH of pyrimidine ring. The 1H NMR data of compounds revealed signals between 8.07 and 8.19 δppm NH of pyrimidine ring.

Scheme 1
scheme 1

Synthesis of methyl 5-substituted N-(4-nitrophenyl)-7-methyl-3, 5-dihydro-2H-thiazolo/imidazo[3,2-a]pyrimidine-6-carboxamide derivatives

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Biological evaluation

Antimicrobial activity

All the synthesized compounds were screened for their antimicrobial activity and the results showed that p-chloro substituted pyrimidine derivative 7-methyl-5-(2-nitrophenyl)-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide 7e was found to be the most active antimicrobial comparable to standards taken. Compounds 7a, 7c, 9a and 9d also showed significant antibacterial and antifungal activity. This may be due to the presence of electron-withdrawing group present on the aromatic ring attached to pyrimidine moiety. The role of the electron-withdrawing group in increasing the antimicrobial activity is supported by the results of Mostafa et al. [19].

Free radical scavenging activity

Compounds concentration providing 50 % inhibition (IC50) was calculated as per the method described by Bondet et al. [20]. Tests were carried out in triplicate, and ascorbic acid was used as a positive control. Standard curve was plotted for different concentrations of ascorbic acid. The results of antioxidant activity showed that few synthesized compounds exhibit considerable antioxidant activity. Chloro substitution 9d at the para position of phenyl ring increases the antioxidant activity, which is also supported by Kumar et al. [21]. Substitution of bulky group like naphthol (9h and 7f) also increases the activity, which is also favored by Bhalghat et al. [22].

Structure–activity relationship (SAR)

From the results of the antimicrobial and antioxidant activities of the synthesized substituted pyrimidine 6-carboxamide derivatives, the following structure–activity relationships can be derived (Fig. 1):

Fig. 1
figure 1

Structure–activity relationship of the synthesized derivatives

Full size image
  1. 1.

    The results of antibacterial activity indicated that compound 7-methyl-5-(2-nitrophenyl)-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide 7e was found to be most potent among all the synthesized compounds. This may be due to the presence of the electron-withdrawing nitro group.

  2. 2.

    p-nitro group at N-substituted phenyl ring of all synthesized pyrimidine derivatives increases the overall antimicrobial activity.

  3. 3.

    Substitution of chloro group in compound 9d at phenyl ring attached to pyrimidine ring increases the antioxidant activity.

  4. 4.

    Presence of bulky group in compound (9h and 7f) also found to increase the antioxidant activity.

Conclusions

Two series of new thiazolo and imidazo pyrimidine derivatives were synthesized and their antimicrobial and antioxidant activities have been evaluated. Compound 7-methyl-5-(2-nitrophenyl)-N-(4-nitrophenyl)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide 7e was found to be the most active antimicrobial comparable to standards taken. Compounds 7a, 7c, 9a, and 9d also have shown significant antibacterial and antifungal activity. Compounds 7f, 9d, and 9h have shown promising antioxidant activity with IC50 comparable with standard compound. Hence, this research represents a class of wide-spectrum antimicrobial agents and potential antioxidant agents, which can further be explored for future research.