Synthesis and Antimicrobial Activity of Fluorine Containing Pyrazole-clubbed Dihydropyrimidinones

*Corresponding Author:

N. C. Desai
Division of Medicinal Chemistry, Department of Chemistry (DST-FIST Sponsored & UGC NON-SAP), Mahatma Gandhi Campus, Maharaja Krishnakumarsinhji Bhavnagar University, Bhavnagar-364 002, India
E-mail: dnisheeth@rediffmail.com

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Abstract

In the present communication synthesis of pyrazole-containing dihydropyrimidinone motifs (4a-o) and their antimicrobial activity and cytotoxicity were reported. The newly synthesized compounds were characterized using infrared, proton nuclear magnetic resonance, carbon-13 nuclear magnetic resonance and mass spectral techniques. Compounds 4b, 4c, 4f, 4g, 4i and 4j were the most active derivatives identified during antimicrobial activity screening. On the basis of antibacterial activities, it was observed that compounds 4b and 4c exhibited activity against methicillin resistant Staphylococcus aureus with minimum inhibitory concentrations of 12.5 and 6.25 µg/ml, respectively. From structure activity relationship studies, it could be concluded that electron withdrawing groups played a crucial role in enhancing antimicrobial and cytotoxic effects of title compounds. In addition, the results of the cytotoxicity studies indicated that compounds 4b, 4c, 4g and 4j possessed lower levels of cytotoxicity.

Keywords

Pyrazole, dihydropyrimidinones, Biginelli adduct, antimicrobial screening, cytotoxicity

Continuing progress in the treatment of many infections is now threatened by the increasing number of pathogens resistant to antimicrobial drugs. There is a need to both stewards the use of existing drugs better and to develop new therapeutic antimicrobials. Presence of fluorine atom leads to modification of some physicochemical properties such as basicity or lipophilicity, bioavailability and increase in the binding affinity of drug molecules to the target protein [1]. Hybrid heterocycles containing fluorine atoms have many applications in pharmaceutical industry [2-7].

Recently dihydropyrimidones (DHPMs)-based compound like monastrol [8,9] has led to the devotion for efficient pharmacophore variation of Biginelli DHPMs. DHPMs are also used as orally active antihypertensive agents [10,11], as α1a adrenoceptor selective antagonists [12] and cyclooxygenase-2 inhibitors [13]. The batzelladine alkaloids A and B are used for the treatment of the epidemic, acquired immune deficiency syndrome (AIDS), which inhibited the binding of human immunodeficiency virus envelope protein gp-120 to human CD4 cells to become potential new leads for AIDS therapy [14]. In the past, a broad range of biological effects, including antibacterial [15], antitubercular [16,17], antitumor [18], antiinflammatory [19], antioxidant [20] and antiamoebic [21] activities have been ascribed to these partly reduced pyrimidine derivatives. Apart from synthetic DHPM derivatives, several marine natural products with fascinating biological activities containing the dihydropyrimidine-5-carboxylate core have been isolated [22].

The chemistry of pyrazole ring system has occupied prime position in medicinal chemistry for diverse biological activities such as antibacterial [23], antifungal [24], antioxidant [25], anticancer [26], BRAF(V600E) inhibitors [27] idiopathic or immune thrombocytopenic agents [28], antitubercular [29] and antiinflammatory. The pyrazole ring is present as a core in a variety of prominent drugs such as celecoxib, sildenafil, difenamizole, ionazolac and pyrazofurin.

In the present investigation the cytotoxicity of the title compounds has been evaluated. For the development of novel bioactive antimicrobial therapeutics, cytotoxicity test is very essential. Cytotoxicity is the quality of being toxic to cells; indicating the dose at which the cells are killed. Cytotoxicity assays are commonly used to screen chemical libraries. A compound to be tested for any biological activity, first of all should not be cytotoxic, so that cell can easily survive. Previously, our research group has synthesized various heterocyclic derivatives as potential antimicrobial agents [30-33]. Looking to the role of dihyropyrimidinones and pyrazole in the current drugs discovery; we have incorporated these two moieties in the core structure of title compounds.

Materials and Methods

The required chemicals were purchased from E. Merck KG, Darmstadt, Germany. Melting points were recorded on Gallenkamp apparatus and were left uncorrected. Completion of reaction and purity of all compounds were checked on aluminium coated TLC plates 60, F245 (E. Merck KG) using n-hexane:ethyl acetate (7:3, v/v) as mobile phase and visualized under ultraviolet (UV) light, or iodine vapour. The compositions of elements (% C, H, N) for the synthesized compounds were determined by using Perkin-Elmer 2400 CHN analyser. Infrared (IR) spectra were also recorded on Perkin Elmer FT-IR spectrophotometer. Proton nuclear magnetic resonance (¹H NMR) spectra were recorded on a Bruker Avance II 400 MHz and carbon-13 NMR spectra on a Varian Mercury-400 (100 MHz) in DMSO-d₆ as a solvent and tetramethylsilane (TMS) as an internal standard. Chemical shifts were reported in parts per million (ppm). Mass spectra carried out using Shimadzu LCMS 2010 spectrophotometer.

Synthesis of ethyl 6-methyl-2-oxo-4-phenyl-1,2,3,4- tetrahydropyrimidine-5-carboxylate (1)

Synthesis of ethyl 6-methyl-2-oxo-4-phenyl-1,2,3,4- tetrahydropyrimidine-5-carboxylate (Biginelli adduct) was achieved using previously published methods [34].

Synthesis of 4-(2-fluorophenyl)-6-methyl-2-oxo-1,2, 3,4-tetrahydropyrimidine-5-carbohydrazide (2)

Compound 1 (0.01 mol) was dissolved in 1,4-dioxane (20 ml) and to this hydrazine hydrate (99 %, 0.01 mol) was added followed by the addition of a catalytic amount of con. H₂SO₄ and allowed to stir for 3 h at 100°. After completion of reaction, the crude mass was allowed to cool and poured on crushed ice. Product obtained as yellowish precipitate, was filtered and dried. Purification was done by crystallization using ethanol (99 %) to give compound 2.

Percent yield: 69; melting point (MP): 198-200°, IR (KBr, ν_max, cm^-1): 3424 (–N–H, 1º amine), 3310 (N–H, 2º amine), 3080 (Ar–H), 2922 (C–H, –CH₃), 1717 (C=O, amide), 1676 (C=O, urea), 1574, 1520 (C=C), 1124 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.02 (s, 2H, –N–NH₂), 2.26 (s, 3H, Ar–CH₃), 5.50 (s, 1H, –CH of pyrimidine ring), 5.86 (s, 1H, –NH-N), 6.12 (s, 1H, –NH–C–Ph), 7.10–7.46 (m, 4H, Ar–H), 7.56 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 18.1 (–CH₃), 47.3 (–CH of pyrimidine ring), 150.3 (NH₂–CO– NH₂), 165.2 (–CONH), 159.4 (–C–F); LCMS (ESI); m/z: 264.10 (M⁺). Ana. calcd. (%) for C₁₂H₁₃FN₄O₂: C, 54.54; H, 4.96; N, 21.20. Found: C, 54.44; H, 4.89; N, 21.15.

Synthesis of 4-(2-fluorophenyl)-6-methyl-5-(3-methyl-5-oxo-4,5-dihydro-1H-pyrazole-1-carbonyl)-3,4-dihydropyrimidin-2(1H)-one (3)

To a mixture of compound 2 (0.01 mol) and ethyl acetoacetate (0.01 mol) in absolute ethanol (99 %, 20 ml), catalytic amount of triethylamine (1 ml) was added to a round bottom flask. The reaction mixture was refluxed for 12 h at 78° using reflux condenser equipped with magnetic stirrer. After completion of reaction, the resultant heavy reddish syrup was allowed to cool at room temperature. It was washed thoroughly with ether to remove impurities. The solid thus separated was filtered off under vacuum and recrystallized from ethanol (99 %) to give product 3.

Percent yield: 62; MP: 220-222°, IR (KBr, ν_max, cm^-1): 3082 (Ar–H), 2922, 2918 (C–H, pyrimidine and pyrazole –CH₃), 2882 (C–H, –CH2), 1680 (C=O, urea), 1653 (C=O, 3° amide), 1574, 1520 (C=C), 1124 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.10 (s, 3H, pyrazole ring –CH₃), 2.20 (s, 2H, pyrazole ring –CH2), 2.36 (s, 3H, pyrimidine ring –CH₃), 5.43 (s, 1H, –CH of pyrimidine ring), 6.20 (s, 1H, –NH–C–Ph), 7.10–7.50 (m, 4H, Ar–H), 7.55 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 16.4 (–CH₃ of pyrazole ring), 18.1 (–CH₃ of pyrimidine ring), 42.4 (–CH2 of pyrazole ring), 47.3 (–CH of pyrimidine ring), 150.3 (NH₂–CO–NH₂), 165.6 (–C=O–N–), 159.6 (–C–F), 163.1 (–C=O of pyrazole ring); LCMS (ESI); m/z: 330.11 (M⁺). Ana. calcd. (%) for C₁₆H₁₅FN₄O₃: C, 58.18; H, 4.58; N, 16.96. Found: C, 58.29; H, 4.55; N, 16.92.

General synthesis of 5-(4-arylidene-3-methyl-5- oxo-4,5-dihydro-1H-pyrazole-1-carbonyl)-4-(2-fluorophenyl)-6-methyl-3,4-dihydropyrimidin-2(1 H)-ones (4a-o)

A mixture of a solution of 4-(2-fluorophenyl)-6- methyl-5-(3-methyl-5-oxo-4,5-dihydro-1H-pyrazole- 1-carbonyl)-3,4-dihydropyrimidin-2(1H)-one 3 (0.01 mol) and different derivatives of aldehyde (0.01 mol) suspended in dry toluene were taken in a flask equipped with a Dean-Stark apparatus fitted with calcium guard tube. Catalytic amount of piperidine (0.5 ml) was added and the mixture was refluxed with stirring for 8 h. On cooling, the product was precipitated, filtered under vacuum and washed with cold methanol to give series of compounds 4a-o. Product was crystallized from ethanol/chloroform (1:1).

5-(4-benzylidene-3-methyl-5-oxo-4,5-dihydro-1Hpyrazole-1-carbonyl)-4-(2-fluorophenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-one (4a)

Light yellowish crystals; % yield: 72; MP: 231-233°, IR (KBr, ν_max, cm^-1): 3080, 3025 (Ar–H), 2936 (C–H, –CH₃), 1713 (C=O, 3° amide), 1697 (C=O, urea), 1578 (C=N), 1545 (C=C), 1125 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.13 (s, 3H, pyrimidine ring –CH₃), 2.51 (s, 3H, pyrazole ring –CH₃), 5.50 (s, 1H, –CH of pyrimidine ring), 6.10 (s, 1H, –NH–C– Ph), 6.95 (s, 1H, ethylene >C=CH), 7.02–7.42 (m, 9H, Ar–H), 7.54 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 13.5 (–CH₃ of pyrazole ring), 16.3 (–CH₃ of pyrimidine ring), 53.5 (–CH of pyrimidine ring), 128.5, 145.1 (ethylene >C=CH), 155.5 (NH₂– CO–NH₂), 157.6 (–C=O–N–), 158.6 (–C–F), 165.1 (–C=O of pyrazole ring); LCMS (ESI); m/z: 418.14 (M⁺). Ana. calcd. (%) for C₂₃H₁₉FN₄O₃: C, 66.02; H, 4.58; N, 13.39. Found: C, 66.07; H, 4.63; N, 13.35.

5-(4-(2-fluorobenzylidene)-3-methyl-5-oxo-4,5- dihydro-1H-pyrazole-1-carbonyl)-4-(2-fluoro phenyl)-6-methyl-3,4-dihydropyrimidin-2(1 H)-one (4b)

Light yellowish crystals; % yield: 76; MP: 262-264°, IR (KBr, ν_max, cm^-1): 3082, 3030 (Ar–H), 2929 (C–H, –CH₃), 1710 (C=O, 3° amide), 1690 (C=O, urea), 1579 (C=N), 1548 (C=C), 1124 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.16 (s, 3H, pyrazole ring –CH₃), 2.48 (s, 3H, pyrimidine ring –CH₃), 5.53 (s, 1H, –CH of pyrimidine ring), 6.18 (s, 1H, –NH–C–Ph), 6.96 (s, 1H, ethylene >C=CH), 7.10–7.51 (m, 8H, Ar–H), 7.56 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 13.7 (–CH₃ of pyrazole ring), 16.1 (–CH₃ of pyrimidine ring), 52.5 (–CH of pyrimidine ring), 128.2, 144.4 (ethylene >C=CH), 155.2 (NH₂–CO–NH₂), 157.7 (–C=O–N–), 158.8 (–C–F), 166.0 (–C=O of pyrazole ring); LCMS (ESI); m/z: 436.13 (M⁺). Ana. calcd. (%) for C₂₃H₁₈F₂N₄O₃: C, 63.30; H, 4.16; N, 12.84. Found: C, 63.37; H, 4.13; N, 12.80.

5-(4-(4-fluorobenzylidene)-3-methyl-5-oxo-4,5- dihydro-1H-pyrazole-1-carbonyl)-4-(2-fluoro phenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-one (4c)

Yellowish crystals; % yield: 73; MP: 275-277°, IR (KBr, ν_max, cm^-1): 3075, 3055 (Ar–H), 2925, 2920 (C–H, –CH₃), 1714 (C=O, 3° amide), 1693 (C=O, urea), 1577 (C=N), 1536 (C=C), 1102 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.24 (s, 3H, pyrazole ring –CH₃), 2.60 (s, 3H, pyrimidine ring –CH₃), 5.56 (s, 1H, –CH of pyrimidine ring), 6.15 (s, 1H, –NH–C–Ph), 6.98 (s, 1H, ethylene >C=CH), 6.99–7.45 (m, 8H, Ar–H), 7.53 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 13.8 (–CH₃ of pyrazole ring), 16.2 (–CH₃ of pyrimidine ring), 51.4 (–CH of pyrimidine ring), 126.2, 143.5 (ethylene >C=CH), 155.3 (NH₂–CO–NH₂), 158.5 (–C=O–N–), 159.3 (–C–F), 167.1 (–C=O of pyrazole ring); LCMS (ESI); m/z: 436.13 (M⁺). Ana. calcd. (%) for C₂₃H₁₈F₂N₄O₃: C, 63.30; H, 4.16; N, 12.84. Found: C, 63.34; H, 4.09; N, 12.77.

4-(2-fluorophenyl)-5-(4-(2-hydroxybenzylidene)-3- methyl-5-oxo-4,5-dihydro-1H-pyrazole-1- carbonyl)-6-methyl-3,4-dihydropyrimidin-2(1H)- one (4d)

Light brown crystals; % yield: 57; MP: 221-223°, IR (KBr, ν_max, cm^-1): 3313 (O–H, Ar–OH), 3077, 3060 (Ar–H), 2926, 2919 (C–H, –CH₃), 1715 (C=O, 3° amide), 1686 (C=O, urea), 1575 (C=N), 1530 (C=C), 1125 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO– d₆): 2.25 (s, 3H, pyrazole ring –CH₃), 2.45 (s, 3H, pyrimidine ring –CH₃), 5.53 (s, 1H, –CH of pyrimidine ring), 6.74 (s, 1H, Ar–OH), 6.14 (s, 1H, –NH–C–Ph), 6.81 (s, 1H, ethylene >C=CH), 6.89–7.37 (m, 8H, Ar–H), 7.53 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.0 (–CH₃ of pyrazole ring), 16.0 (–CH₃ of pyrimidine ring), 49.5 (–CH of pyrimidine ring), 126.3, 143.4 (ethylene >C=CH), 154.1 (NH₂– CO–NH₂), 159.6 (–C=O–N–), 159.0 (–C–F), 167.0 (– C=O of pyrazole ring); LCMS (ESI); m/z: 434.14 (M⁺); Ana. calcd. (%) for C₂₃H₁₉FN₄O₄: C, 63.59; H, 4.41; N, 12.90. Found: C, 63.62; H, 4.49; N, 12.94.

4-(2-fluorophenyl)-5-(4-(4-hydroxybenzylidene)-3- methyl-5-oxo-4,5-dihydro-1H-pyrazole-1-carbonyl)-6-methyl-3,4-dihydropyrimidin-2(1H)- one (4e)

Light brown crystals; % yield: 65; MP: 252-254°, IR (KBr, ν_max, cm^-1): 3329 (O–H, Ar–OH), 3052, 3061 (Ar–H), 2924, 2920 (C–H, –CH₃), 1716 (C=O, 3° amide), 1682 (C=O, urea), 1568 (C=N), 1503 (C=C), 1133 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.24 (s, 3H, pyrazole ring –CH₃), 2.56 (s, 3H, pyrimidine ring –CH₃), 5.61 (s, 1H, –CH of pyrimidine ring), 6.04 (s, 1H, –NH–C–Ph), 6.75 (s, 1H, Ar–OH), 6.98 (s, 1H, ethylene >C=CH), 7.08–7.55 (m, 8H, Ar–H), 7.55 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.2 (–CH₃ of pyrazole ring), 16.5 (–CH₃ of pyrimidine ring), 49.4 (–CH of pyrimidine ring), 126.3, 143.3 (ethylene >C=CH), 154.3 (NH₂–CO–NH₂), 159.5 (–C=O–N–), 159.2 (–C–F), 167.9 (–C=O of pyrazole ring); LCMS (ESI); m/z: 434.14 (M⁺). Ana. calcd. (%) for C₂₃H₁₉FN₄O₄: C, 63.59; H, 4.41; N, 12.90. Found: C, 63.61; H, 4.42; N, 12.80.

5-(4-(2-chlorobenzylidene)-3-methyl-5-oxo-4,5- dihydro-1H-pyrazole-1-carbonyl)-4-(2-fluoro phenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-one (4f)

Yellow crystals; % yield: 61; MP: 262-264°, IR (KBr, ν_max, cm^-1): 3071, 3063 (Ar–H), 2929, 2922 (C–H, –CH₃), 1719 (C=O, 3° amide), 1683 (C=O, urea), 1579 (C=N), 1518 (C=C), 1134 (C–F), 783 (C–Cl); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.26 (s, 3H, pyrazole ring –CH₃), 2.46 (s, 3H, pyrimidine ring –CH₃), 5.53 (s, 1H, –CH of pyrimidine ring), 6.05 (s, 1H, –NH–C–Ph), 6.80 (s, 1H, ethylene >C=CH), 7.10–7.66 (m, 8H, Ar–H), 7.52 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.6 (–CH₃ of pyrazole ring), 16.3 (–CH₃ of pyrimidine ring), 48.5 (–CH of pyrimidine ring), 126.2, 143.5 (ethylene >C=CH), 134.2 (–C–Cl), 153.5 (NH₂–CO–NH₂), 159.6 (–C–F), 160.5 (–C=O–N–), 167.2 (–C=O of pyrazole ring); LCMS (ESI); m/z: 452.11 (M⁺). Ana. calcd. (%) for C₂₃H₁₈ClFN₄O₃: C, 61.00; H, 4.01; N, 12.37. Found: C, 61.04; H, 4.06; N, 12.29.

5-(4-(4-chlorobenzylidene)-3-methyl-5-oxo-4,5- dihydro-1H-pyrazole-1-carbonyl)-4-(2-fluoro phenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-one (4g)

Yellow crystals; % yield: 72; MP: 277-279°, IR (KBr, ν_max, cm^-1): 3076, 3065 (Ar–H), 2927, 2919 (C–H, –CH₃), 1720 (C=O, 3° amide), 1685 (C=O, urea), 1576 (C=N), 1519 (C=C), 1138 (C–F), 781 (C–Cl); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.24 (s, 3H, pyrazole ring –CH₃), 2.45 (s, 3H, pyrimidine ring –CH₃), 5.56 (s, 1H, –CH of pyrimidine ring), 6.07 (s, 1H, –NH–C– Ph), 6.84 (s, 1H, ethylene >C=CH), 7.06–7.72 (m, 8H, Ar–H), 7.56 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.8 (–CH₃ of pyrazole ring), 16.1 (–CH₃ of pyrimidine ring), 47.9 (–CH of pyrimidine ring), 126.4, 143.4 (ethylene >C=CH), 133.6 (–C–Cl), 153.2 (NH₂–CO–NH₂), 156.7 (–C–F), 160.4 (–C=O–N–), 167.3 (–C=O of pyrazole ring); LCMS (ESI); m/z: 452.11 (M⁺). Ana. calcd. (%) for C₂₃H₁₈ClFN₄O₃: C, 61.00; H, 4.01; N, 12.37. Found: C, 60.59; H, 4.07; N, 12.28.

4-(2-fluorophenyl)-6-methyl-5-(3-methyl-4-(2-nitro benzylidene)-5-oxo-4,5-dihydro-1H-pyrazole-1- carbonyl)-3,4-dihydropyrimidin-2(1H)-one (4h)

Dark brown crystals; % yield: 68; MP: 204-206°, IR (KBr, ν_max, cm^-1): 3057 (Ar–H), 2918 (C–H, –CH₃), 1720 (C=O, 3° amide), 1680 (C=O, urea), 1549 (C=N), 1511 (C=C), 1384 (NO₂), 1107 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.34 (s, 3H, pyrazole ring –CH₃), 2.40 (s, 3H, pyrimidine ring –CH₃), 5.38 (s, 1H, –CH of pyrimidine ring), 6.04 (s, 1H, –NH–C– Ph), 6.19 (s, 1H, ethylene >C=CH), 7.21–7.47 (m, 8H, Ar–H), 7.48 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.9 (–CH₃ of pyrazole ring), 16.3 (–CH₃ of pyrimidine ring), 47.4 (–CH of pyrimidine ring), 126.2, 143.6 (ethylene >C=CH), 147.5 (–C–NO₂), 153.4 (NH₂–CO–NH₂), 156.5 (–C–F), 160.7 (–C=O–N–), 167.4 (–C=O of pyrazole ring); LCMS (ESI); m/z: 463.13 (M⁺). Ana. calcd. (%) for C₂₃H₁₈FN₅O₅: C, 59.61; H, 3.92; N, 15.11. Found: C, 59.52; H, 3.97; N, 15.08.

4-(2-fluorophenyl)-6-methyl-5-(3-methyl-4-(3-nitro benzylidene)-5-oxo-4,5-dihydro-1H-pyrazole-1- carbonyl)-3,4-dihydropyrimidin-2(1H)-one (4i)

Dark brown crystals; % yield: 66, MP: 272-274°, IR (KBr, ν_max, cm^-1): 3061 (Ar–H), 2916 (C–H, –CH₃), 1725 (C=O, 3° amide), 1677 (C=O, urea), 1550 (C=N), 1522 (C=C), 1460 (NO₂), 1121 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.32 (s, 3H, pyrazole ring –CH₃), 2.41 (s, 3H, pyrimidine ring –CH₃), 5.35 (s, 1H, –CH of pyrimidine ring), 6.06 (s, 1H, –NH–C–Ph), 7.16–7.50 (m, 8H, Ar–H), 6.15 (s, 1H, ethylene >C=CH), 7.47 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.8 (–CH₃ of pyrazole ring), 16.2 (–CH₃ of pyrimidine ring), 47.5 (–CH of pyrimidine ring), 126.2, 143.5 (ethylene >C=CH), 147.9 (–C–NO₂), 152.3 (NH₂–CO–NH₂), 157.6 (–C–F), 162.8 (–C=O–N–), 167.3 (–C=O of pyrazole ring); LCMS (ESI); m/z: 463.13 (M⁺). Ana. calcd. (%) for C₂₃H₁₈FN₅O₅: C, 59.61; H, 3.92; N, 15.11. Found: C, 59.52; H, 3.84; N, 15.05.

4-(2-fluorophenyl)-6-methyl-5-(3-methyl-4-(4-nitro benzylidene)-5-oxo-4,5-dihydro-1H-pyrazole-1- carbonyl)-3,4-dihydropyrimidin-2(1H)-one (4j)

Dark brown crystals; % yield: 58; MP: 275-277°, IR (KBr, ν_max, cm^-1): 3087, 3058 (Ar–H), 2925 (C–H, –CH₃), 1723 (C=O, 3° amide), 1691 (C=O, urea), 1591 (C=N), 1527 (C=C), 1481 (NO₂), 1097 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.27 (s, 3H, pyrazole ring –CH₃), 2.48 (s, 3H, pyrimidine ring –CH₃), 5.41 (s, 1H, –CH of pyrimidine ring), 6.01 (s, 1H, –NH– C–Ph), 7.20–7.77 (m, 8H, Ar–H), 7.71 (s, 1H, ethylene >C=CH), 7.81 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.7 (–CH₃ of pyrazole ring), 17.1 (–CH₃ of pyrimidine ring), 47.3 (–CH of pyrimidine ring), 126.2, 143.8 (ethylene >C=CH), 147.1 (–C–NO₂), 151.2 (NH₂–CO–NH₂), 157.5 (–C–F), 163.7 (–C=O–N–), 167.2 (–C=O of pyrazole ring); LCMS (ESI); m/z: 463.13 (M⁺). Ana. calcd. (%) for C₂₃H₁₈FN₅O₅: C, 59.61; H, 3.92; N, 15.11. Found: C, 59.70; H, 3.90; N, 15.03.

5-(4-(2,6-dichlorobenzylidene)-3-methyl-5-oxo-4,5-dihydro-1H-pyrazole-1-carbonyl)-4-(2-fluoro phenyl)-6-methyl-3,4-dihydropyrimidin-2(1H)-one (4k)

Light brown crystals; % yield: 55; MP: 215-217°, IR (KBr, ν_max, cm^-1): 3074, 3064 (Ar–H), 2930 (C–H, –CH₃), 1720 (C=O, 3° amide), 1688 (C=O, urea), 1574 (C=N), 1535 (C=C), 1135 (C–F), 779 (C–Cl); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.34 (s, 3H, pyrazole ring –CH₃), 2.43 (s, 3H, pyrimidine ring –CH₃), 5.42 (s, 1H, –CH of pyrimidine ring), 6.12 (s, 1H, –NH–C–Ph), 6.88 (s, 1H, ethylene >C=CH), 7.10– 7.58 (m, 7H, Ar–H), 7.52 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.8 (–CH₃ of pyrazole ring), 16.2 (–CH₃ of pyrimidine ring), 53.4 (–CH of pyrimidine ring), 126.2, 143.5 (ethylene >C=CH), 132.6 (–C–Cl), 154.5 (NH₂–CO–NH₂), 156.8 (–C=O–N–), 159.5 (–C–F), 165.1 (–C=O of pyrazole ring); LCMS (ESI); m/z: 486.07 (M⁺). Ana. calcd. (%) for C₂₃H₁₇C_l2FN₄O₃: C, 56.69; H, 3.52; N, 11.50. Found: C, 56.71; H, 3.44; N, 11.45.

4-(2-fluorophenyl)-5-(4-(2-methoxybenzylidene)-3- methyl-5-oxo-4,5-dihydro-1H-pyrazole-1- carbonyl)-6-methyl-3,4-dihydropyrimidin-2(1H)- one (4l)

Dark yellowish crystals; % yield: 77; MP: 268-270°, IR (KBr, ν_max, cm^-1): 3074, 3036 (Ar–H), 2925 (C–H, –CH₃), 1715 (C=O, 3° amide), 1688 (C=O, urea), 1574 (C=N), 1535 (C=C), 1246 (C-O-C, Ar-O-CH₃), 1133 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.36 (s, 3H, pyrazole ring –CH₃), 2.45 (s, 3H, pyrimidine ring –CH₃), 3.83 (s, 3H, –OCH₃), 5.45 (s, 1H, –CH of pyrimidine ring), 6.13 (s, 1H, –NH–C–Ph), 6.89 (s, 1H, ethylene >C=CH), 7.10–7.67 (m, 8H, Ar–H), 7.50 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.7 (–CH₃ of pyrazole ring), 16.2 (–CH₃ of pyrimidine ring), 52.4 (–CH of pyrimidine ring), 56.5 (–OCH₃), 126.2, 143.9 (ethylene >C=CH), 153.2 (NH₂–CO–NH₂), 157.8 (–C=O–N–), 159.5 (–C–F), 166.4 (–C=O of pyrazole ring); LCMS (ESI); m/z: 448.15 (M⁺). Ana. calcd. (%) for C₂₄H₂₁FN₄O₄: C, 64.28; H, 4.72; N, 12.49. Found: C, 64.16; H, 4.67; N, 12.40.

4-(2-fluorophenyl)-5-(4-(4-methoxybenzylidene)-3- methyl-5-oxo-4,5-dihydro-1H-pyrazole-1- carbonyl)-6-methyl-3,4-dihydropyrimidin-2(1H)- one (4m)

Dark yellowish crystals; % yield: 59; MP: 256-258°, IR (KBr, ν_max, cm^-1): 3075, 3030 (Ar–H), 2927 (C–H, –CH₃), 1716 (C=O, 3° amide), 1684 (C=O, urea), 1572 (C=N), 1540 (C=C), 1240 (C-O-C, Ar-O-CH₃), 1135 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.35 (s, 3H, pyrazole ring –CH₃), 2.47 (s, 3H, pyrimidine ring –CH₃), 3.80 (s, 3H,–OCH₃), 5.50 (s, 1H, –CH of pyrimidine ring), 6.15 (s, 1H, –NH–C–Ph), 6.84 (s, 1H, ethylene >C=CH), 7.13–7.46 (m, 8H, Ar–H), 7.55 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.6 (–CH₃ of pyrazole ring), 16.5 (–CH₃ of pyrimidine ring), 49.6 (–CH of pyrimidine ring), 55.7 (–OCH₃), 126.0, 143.6 (ethylene >C=CH), 154.4 (NH₂–CO–NH₂), 157.5 (–C=O–N–), 159.3 (–C–F), 169.2 (–C=O of pyrazole ring); LCMS (ESI); m/z: 448.15 (M⁺). Ana. calcd (%) for C₂₄H₂₁FN₄O₄: C, 64.28; H, 4.72; N, 12.49. Found: C, 64.15; H, 4.63; N, 12.42.

4-(2-fluorophenyl)-6-methyl-5-(3-methyl-4-(2-methylbenzylidene)-5-oxo-4,5-dihydro-1Hpyrazole-1-carbonyl)-3,4-dihydropyrimidin-2(1H)- one (4n):

Dark yellow crystals; % yield: 75; MP: 244-246°, IR (KBr, ν_max, cm^-1): 3075, 3034 (Ar–H), 2977 (C–H, Ar-CH₃), 2927 (C–H, –CH₃), 1751 (C=O, 3° amide), 1684 (C=O, urea), 1572 (C=N), 1534 (C=C), 1135 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.36 (s, 3H, pyrazole ring –CH₃), 2.40 (s, 3H, pyrimidine ring –CH₃), 2.50 (s, 3H, Ar–CH₃), 5.34 (s, 1H, –CH of pyrimidine ring), 6.20 (s, 1H, –NH–C–Ph), 6.89 (s, 1H, ethylene >C=CH), 7.01–7.56 (m, 8H, Ar–H), 7.56 (s, 1H, –NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.9 (–CH₃ of pyrazole ring), 17.1 (–CH₃ of pyrimidine ring), 19.2 (Ar–CH₃), 49.3 (–CH of pyrimidine ring), 126.3, 143.8 (ethylene >C=CH), 153.1 (NH₂–CO–NH₂), 156.6 (–C=O–N–), 159.5 (–C–F), 165.2 (–C=O of pyrazole ring); LCMS (ESI); m/z: 432.16 (M⁺). Ana. calcd. (%) for C₂₄H₂₁FN₄O₃: C, 66.66; H, 4.89; N, 12.96. Found: C, 66.72; H, 4.79; N, 12.90.

4-(2-fluorophenyl)-6-methyl-5-(3-methyl-4-(4-methylbenzylidene)-5-oxo-4,5-dihydro-1Hpyrazole-1-carbonyl)-3,4-dihydropyrimidin-2(1H)-one (4o)

Dark yellow crystals; % yield: 78, MP: 234-236°, IR (KBr, ν_max, cm^-1): 3079, 3028 (Ar–H), 2980 (C–H, Ar– CH₃), 2925 (C–H, –CH₃), 1775 (C=O, 3º amide), 1689 (C=O, urea), 1575 (C=N), 1546 (C=C), 1131 (C–F); ¹H NMR (400 MHz, δ ppm, DMSO–d₆): 2.35 (s, 3H, Ar–CH₃), 2.38 (s, 3H, pyrazole ring –CH₃), 2.46 (s, 3H, pyrimidine ring –CH₃), 5.43 (s, 1H, –CH of pyrimidine ring), 6.18 (s, 1H, –NH–C–Ph), 6.85 (s, 1H, ethylene >C=CH), 7.10–7.55 (m, 8H, Ar–H), 7.56 (s, 1H, – NH–C–CH₃); ¹³C NMR (100 MHz, δ ppm, DMSO–d₆): 14.8 (–CH₃ of pyrazole ring), 17.3 (–CH₃ of pyrimidine ring), 21.3 (Ar–CH₃), 50.5 (–CH of pyrimidine ring), 126.5, 143.5 (ethylene >C=CH), 153.0 (NH₂–CO– NH₂), 157.5 (–C=O–N–), 159.7 (–C–F), 165.0 (–C=O of pyrazole ring); LCMS (ESI); m/z: 432.16 (M⁺). Ana. calcd (%) for C₂₄H₂₁FN₄O₃: C, 66.66; H, 4.89; N, 12.96. Found: C, 66.69; H, 4.79; N, 12.85.

In vitro antimicrobial screening:

Antibacterial activity of newly synthesized compounds (4a-o) was carried out against the representative panel of bacteria such as Escherichia coli MTCC- 443, Pseudomonas aeruginosa MTCC-1688, Staphylococcus aureus MTCC-96 and Streptococcus pyogenes MTCC-442 using ciprofloxacin as the standard antibacterial drug. Antifungal activity was screened against three fungal species, Candida albicans MTCC-227, Aspergillus niger MTCC-282, Aspergillus clavatus MTCC-1323, and griseofulvin was used as the standard antifungal drug. The minimal inhibitory concentration (MIC) of all the synthesized compounds was determined by the broth microdilution method according to National Committee for Clinical Laboratory Standards (NCCLS) [35]. All the synthesized compounds (1, 2, 3 and 4a-o) were screened for antibacterial and antifungal activities in six sets (n=6) against bacteria and fungi used in the present protocol.

In vitro cytotoxicity studies

After identification of active antimicrobial agents, the next step was to determine the toxicity of drug contenders. In vitro cytotoxic activity of newly synthesized compounds (4a-o) were evaluated against 3T3 mouse embryonic fibroblast cell line and human cervical cancer cell line (HeLa) using the tetrazolium dye, 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide (MTT) colorimetric assay. The IC₅₀ determination was achieved according to the NCCLS recommendations [36].

Results and Discussion

We have synthesized new analogues in which pyrazole scaffold was linked to the DHPMs systems. The synthetic pathway for final compounds is illustrated in Figure 1. Compound 1 on reaction with hydrazine hydrate furnished 4-(2-fluorophenyl)-6-methyl-2-oxo-1,2,3,4- tetrahydro-pyrimidine-5-carbohydrazide 2. Cyclic condensation of compound 2 with ethyl acetoacetate resulted the compound 4-(2-fluorophenyl)-6-methyl- 5-(3-methyl-5-oxo-4,5-dihydro-1H-pyrazole-1- carbonyl)-3,4-dihydropyrimidin-2(1H)-one 3. This was further converted into 5-(4-(arylmethylene)-3- methyl-5-oxo-4,5-dihydro-1H-pyrazole-1-carbonyl)- 4-(2-fluorophenyl)-6-methyl-3,4-dihydropyrimidin- 2(1H)-ones (4a-o) by Knoevenagel condensation with different aryl aldehydes in the presence of piperidine catalyst.

Figure 1: Synthetic pathway of novel compounds 4a-o
Where, 4a. Ar=-C₆H₅; 4b. Ar=-2-F-C₆H₄; 4c. Ar=-4-F-C₆H₄; 4d. Ar=-2-OH-C₆H₄; 4e. Ar=-4-OH-C₆H₄; 4f. Ar=-2-Cl-C₆H₄; 4g. Ar=-4-Cl-C₆H₄; 4h. Ar=-2-NO₂-C₆H₄; 4i. Ar=-3-NO₂-C₆H₄; 4j. Ar=-4-NO₂-C₆H₄; 4k. Ar=-2,6(Cl)₂-C₆H₃; 4l. Ar=-2-OCH₃-C₆H₄; 4m. Ar=-4-OCH₃-C₆H₄; 4n. Ar=-2-CH₃-C₆H₄; 4o Ar=-4-CH₃-C₆H₄

IR spectrum of targeted compound 4c gave stretching vibrations at 3075 and 3055 cm^-1 showed strong intensity absorption peaks corresponding to Ar-H groups. The absorption peaks at 2925 and 2920 cm^-1 were assigned to stretching vibration corresponding to -CH₃ groups of pyrimidine and pyrazole ring system, respectively. The strong absorption at 1693 cm^-1 observed due to the stretching vibration of >C=O group present in pyrimidine ring system. Apart from that >C=O group present in amide linkage, showed strong stretching absorption at 1714 cm^-1. Absorption bands at 1577 and 1536 cm^-1 were observed due to the C=N and C=C stretching in aromatic rings. A strong intensity absorption band at 1102 cm^-1 was shown due to -C-F stretching vibration (Figure 2).

Figure 2: Carbon enumerations of compound 4c

¹H NMR spectra of final compound 4c showed that protons of methyl groups present on pyrazole and pyrimidine ring systems were observed as a singlet, and it gave a shift at δ= 2.24 and δ= 2.60 ppm, respectively. Proton of the -CH-C- linkage, which was attached to C-6 appeared as a singlet at δ= 5.56 ppm. In pyrimidine ring proton group’s linkage at -NH-CPh proton and -NH-C-CH₃ proton furnished a singlet values at δ= 6.15 ppm and δ= 7.53 ppm, respectively. Proton at ethylene group linkage at C-23 shown δ= 6.98 ppm as singlet. Looking to the ¹³C NMR spectra, values of the chemical shift of final compound 4c was observed in between the range of δ= 167.1-13.8 ppm. In the carbon of methyl groups of at C-8 and C-22 appeared at δ= 16.2 and 13.8 ppm, respectively. The C-6 carbon of pyrimidine ring appeared at δ= 51.4 ppm. The C-18 and C-23 carbon of ethylene groups appeared at δ= 126.2 and 143.5 ppm, respectively. The carbonyl carbon of urea in pyrimidine ring, carbonyl in pyrazole ring and carbonyl in amide linkage appeared at δ= 155.3, 167.1 and 158.5 ppm, respectively. The C-15 and C-27 carbon of fluoro phenyl rings appeared at δ= 159.3 ppm due to the influence of fluoro groups. Carbon enumeration of compound 4c is described in Figure 2. Additionally, the mass spectrum of compound 4c showed a molecular ion peak at m/z= 436.13 (M⁺) was also in harmony to the molecular formula C₂₃H₁₈F₂N₄O₃. The spectral values for all the compounds and C, H, and N analysis are listed in experimental section.

The outcome of antimicrobial activity evaluation of the synthesized compounds revealed that these compounds possessed antibacterial and antifungal activities. From antimicrobial activity data (Table 1), key precursor, fluoro hydrazide of tetrahydropyrimidine (2) showed poor antimicrobial activity at MIC= 1000 μg/ml. Compound (3) displayed moderate antibacterial activity at MIC= 25 and 50 μg/ml and good antifungal activity at MIC= 50 μg/ml against tested bacteria and fungi, respectively. Subsequently, when compound (3) was converted into final compounds (4a-o), these showed significant broad-spectrum antimicrobial activity.

Table 1: Results of Antibacterial Screening of Compounds 4a-o

Primary microbiological screening results showed that compounds 4a (-C₆H₅) and 4k (-2,6-(Cl)₂-C₆H₃) possessed moderate activity against E. coli. The antibacterial activity against E. coli improved when substitutions pattern was changed by the installation of fluoro and nitro groups in compounds 4b, 4c, 4i and 4j. Compound 4g was found to be most active against E. coli (MIC= 12.5 μg/ml). Compound 4f (-2-Cl-C₆H₄) possessed moderate activity against P. aeruginosa, while compounds 4a, 4b, 4c and 4g showed very good activity against P. aeruginosa. Moreover, when we introduced nitro group as a substituent at meta position in compound 4a, the activity was enhanced and showed excellent activity against P. aeruginosa. In case of S. aureus, electro withdrawing group at 2nd position like in 4b (-2-F-C₆H₄) obsessed moderate activity at MIC= 100 μg/ml, while electro withdrawing groups at 4th position like in 4g (-4-Cl-C₆H₄) and 4j (-4-NO₂-C₆H₄) displayed very good activity and highest inhibition (MIC= 12.5 μg/ml) observed in compound possessing fluoro group at para position i.e. compound 4c. It was observed from Table 1, compounds 4g (-4-Cl-C₆H₄) and 4i (-3-NO₂-C₆H₄) possessed moderate activity against S. pyogenes, while compounds 4a (-C₆H₄) and 4c (-4-F-C₆H₄) showed very good activity against S. pyogenes at 50 μg/ml MIC. The highest MIC inhibition at 12.5 μg/ml flaunted for S. pyogenes in compound 4b having fluoro group at ortho position.

MIC values of antifungal activity were observed against C. albicans, A. niger and A. clavatus by conventional broth micro dilution method using various concentrations for screening. On the basis of antifungal activity results, we concluded that compounds 4f (-2-Cl- C₆H₄) and 4j (-4-NO₂-C₆H₄) possessed moderate to very good activity against C. albicans. Again antifungal data revealed that the introduction of fluoro group as a substituent in targeted compound enhanced the activity against C. albicans. Furthermore, compounds 4c, 4h and 4j having -4-F-C₆H₄, -2-NO₂-C₆H₄, -4-NO₂-C₆H₄ functional groups, exhibited moderate activity against A. niger, while very good activity was shown against A. niger by -2-Cl-C₆H₄ (4f). Ortho fluoro substitution showed excellent results against A. niger. We have employed A. clavatus to check the activity of newly synthesized compounds. Compound 4h (-2-NO₂- C₆H₄) showed moderate activity against A. clavatus, on the other hand compound 4b (-2-F-C₆H₄) displayed very good activity against A. clavatus. The MIC= 25 μg/ml was demonstrated in compounds having electron withdrawing group, i.e. compounds 4f and 4j. The remaining compounds of the series showed feeble antifungal activity (Table 1).

The enhancement in activity of these compounds was endorsed to the presence of electron withdrawing groups like nitro and halogen in reported compounds. The most active compounds, 4b, 4c, 4g, and 4j against S. aureus and S. pyogenes (MIC= 12.5–25 μg/ml) were also tested against methicillin-resistant S. aureus (MRSA isolate ATCC 43300) and the results were given in Table 2. Compounds 4b and 4c exhibited more potent activity than the standard drugs against MRSA. Compound 4c, with MIC value of 6.25 μg/ml against MRSA, showed fourfold more potency than ciprofloxacin (MIC= 25 μg/ml) and eightfold more activity than chloramphenicol (MIC= 50 μg/ml). In addition, compound 4b endowed with fluoro group showed twofold more activity at MIC value of 12.5 μg/ml than ciprofloxacin and fourfold higher potency than chloramphenicol against MRSA.

Table 2: Inhibitory Activity (MIC, µG/ML) of Compounds 4B, 4C, 4G, AND 4J against Methicillin Resistant S. aureus

The IC₅₀ values achieved for these compounds were shown in Table 3. Cytotoxicity results displayed that the derivatives 4b, 4c, 4f, 4g, 4i and 4j accounted no toxicity at concentration of 100 μM (IC₅₀>100 μM), while other derivatives exhibited moderate toxicity against HeLa cell lines. It was established that none of the tested compounds revealed any significant cytotoxic effects on HeLa cell line, signifying that compounds were potential for their in vivo use as antimicrobial agents. Furthermore, none of the derivatives displayed cytotoxicity against 3T3 cell lines (IC₅₀>100 μM).

Table 3: Levels of Cytotoxicity Prompted by Compounds 4a-o on HeLa Cells and 3T3 Cell Lines

In the present investigation we have used the pyrazole bearing dihydropyrimidinone motif for the development of potential antimicrobial agents. Both the pharmacophores are therapeutically very important as dihydropyrimidines are potential inhibitors of dihydrofolate reductase, which is a promising drug target for treatment of mycobacterial infections. Similarly pyrazole moiety is also very promising as it possesses varieties of biological activities. This prompted us to use the hydride of both the pharmacophores for the present investigation [37,38]. SAR study helped in enlightening the use of different substitution and their electronic effect on microbial strains. Substitution pattern of pyrazole and dihydropyrimidine derivatives were chosen carefully for deliberating different electronic environment of the new molecules [37,38]. Electron donating groups on aromatic ring, such as methyl, methoxy and hydroxy, and electron withdrawing groups from aromatic ring, such as fluoro, chloro and nitro, were chosen as substituents in the molecular diversities of the targeted compounds. Results of antimicrobial activity of final compounds showed that the presence of hydrophobic substituent at ortho and para (-F, 4b and 4c) position of phenyl ring provided a positive impact on antimicrobial activity. The amplified activity was due to the hydrophobic nature of fluorine, which was responsible for the influence of substituent group’s physicochemical properties. In addition, compounds 4b and 4c have two fluoro groups in structure which may also be responsible for the influence in antimicrobial activity. The antibacterial activity data (Table 1) revealed that the presence of electron withdrawing functional groups, specifically -F, -Cl and -NO₂ exhibited excellent activity against all type of bacterial strains, which is better than the electron donating groups like -OH, -CH₃ and -OCH₃. In nut shell, we can conclude that the incorporation of electron donating groups such as hydroxy, methyl, and methoxy diminished the antibacterial property. Effect of halogen group’s as substitution was clearly visible in activity enhancement (Table 1). It was noted that the formation of pyrazole ring (compound 3) from hydrazide precursor slightly improved the activity than its precursor but the drastic change was observed in antimicrobial activity of final Knoevenagel adduct (Table 1). The four compounds 4b, 4c, 4g, and 4j were screened against MRSA. Among these, compounds 4b and 4c showed superior activity than two representatives’ ciprofloxacin and chloramphenicol. This may be due to presence of fluoro group in molecular framework. On the basis of SAR studies of compounds 4a-o, it was presumed that the presence of electron donating groups increased the cytotoxic activity.

The newly targeted compounds (4a-o) presented here clearly vary in their corresponding antimicrobial activity depending on the type of substituents. The antimicrobial activity data of the synthesized compounds indicated that electron withdrawing groups such as fluoro and chloro at ortho and para position in targeted molecule increased the antibacterial, antifungal and MRSA activities as well as negative cytotoxic effect. SAR study also support the positive evidence of fluoro substituent in hybrid molecule, showing the prime attention of fluorine atom in synthesis. Outcome of cytotoxicity study favours the role of electron withdrawing functional groups, specifically -F, -Cl and -NO₂ in antimicrobial and effect of cytotoxicity enhancement.

Acknowledgements

The authors thank the University Grants Commission, New Delhi for NON-SAP and UGC-BSR one-time grant and Department of Science and Technology, New Delhi for DST-FIST programs financial support. Authors also are grateful to Prof. Bharti P. Dave, Head Department of Life Sciences, M. K. Bhavnagar University, Bhavnagar, India for her valuable inputs to the microbial activity study and for the critical comments during the preparation of manuscript. Dr. Bonny Y Patel wishes to thank the CSIR, New Delhi for (CSIR EMR-II 02(0188)14/EMR-II) a Research Associateship -I.

Conflicts of interest

There are no conflicts of interest.

Financial support and sponsorship

Nil.

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