A series of 25 new chalcones were synthesized by Claisen-Schmidt condensation, well characterized by spectroscopic data, and evaluated for their antibacterial and antifungal activities by serial tube dilution method. Among the compounds tested,
The use of antimicrobial agents is critical for the successful treatment of infectious diseases. The existing batteries of antimicrobial agents we have in hand for the treatment of infectious diseases are insufficient to protect us over the long term [
Chalcones are a group of natural products containing two aryl rings (rings A and B) connected through a three-carbon spacer in the form of ketovinyl group (Figure
Structure of chalcone.
All the chemicals used were of analytical grade and purchased from SD Fine and Himedia. 4-Isobutylacetophenone was obtained from Aldrich Chemical Co. Silica gel-G for TLC (Merck) was used as stationary phase and ethyl acetate : hexane (2 : 8) as mobile phase to check the purity of the compounds. UV light (254 nm) and iodine vapours were used to visualize the spots. Melting points were determined in open capillaries, using Boitus melting point apparatus, expressed in °C, and are uncorrected. The IR spectra were recorded using Bruker Vertex 80v spectrometer. 1H and 13C NMR spectra were recorded on Bruker AMX 400 MHz and chemical shifts are given in units as per million, downfield from tetramethylsilane (TMS) as the internal standard. MS spectra were recorded on Agilent LC-MS spectrometer and elemental analyses were carried out using a Carlo Erba 1108 elemental analyzer for C, H, and N.
Equimolar quantities of 4-isobutylacetophenone (0.001 moles) and the appropriate aldehyde (0.001 moles) were dissolved in ethanol (7.5 mL). To this mixture 7.5 mL of 50% aqueous KOH was added dropwise and the reaction mixture was left for 24 h at room temperature. Later, it was acidified with a mixture of hydrochloric acid and water (1 : 1), which resulted in the precipitation of target compounds
Synthesis of chalcones
Yield 92%; m.p. 136–138°C; IR (KBr, cm−1): 1659 (C=O), 1585 (C=C of Ar), 1505 (CH=CH), 835 (C-Cl), 3050 (Ar C-H), 2833 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 87%; m.p. 128–13°C; IR (KBr, cm−1): 1655 (C=O), 1602 (C=C of Ar), 1505 (CH=CH), 3010 (Ar C-H), 2921 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 85%; m.p. 149–151°C; IR (KBr, cm−1): 1655 (C=O), 1581 (C=C of Ar), 1510 (CH=CH), 833 (C-Cl), 3057 (Ar C-H), 2877 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 65%; m.p. 140–142°C; IR (KBr, cm−1): 1652 (C=O), 1583 (C=C of Ar), 1502 (CH=CH), 833 (C-Cl), 3120 (Ar C-H), 2920 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 85%; m.p. 142–144°C; IR (KBr, cm−1): 1664 (C=O), 1580 (C=C of Ar), 1524 (CH=CH), 928 (C-F), 3127 (Ar C-H), 2954 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 79%; m.p. 163–165°C; IR (KBr, cm−1): 1655 (C=O), 1581 (C=C of Ar), 1510 (CH=CH), 925 (C-F), 926 (C-F), 3040 (Ar C-H), 2933 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 82%; m.p. 138–140°C; IR (KBr, cm−1): 1650 (C=O), 1586 (C=C of Ar), 1505 (CH=CH), 1178 (-N(CH3)2), 3198 (Ar C-H), 2940 (Alkyl C-H); 1H NMR (100 MHz, CDCl3, ppm):
Yield 80%; m.p. 107–109°C; IR (KBr, cm−1): 1650 (C=O), 1605 (C=C of Ar), 1502 (CH=CH), 969 (C-Br), 3155 (Ar C-H), 2836 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 73%; m.p. 156–158°C; IR (KBr, cm−1): 3460 (O-H), 1648 (C=O), 1606 (C=C of Ar), 1505 (CH=CH), 3060 (Ar C-H), 2852 (Alkyl C-H); 1H NMR (100 MHz, CDCl3, ppm):
Yield 65%; m.p. 152–154°C; IR (KBr, cm−1): 3520 (O-H), 1648 (C=O), 1612 (C=C of Ar), 1505 (CH=CH), 3111 (Ar C-H), 2928 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 95%; m.p. 190–192°C; IR (KBr, cm−1): 1652 (C=O), 1610 (C=C of Ar), 1502 (CH=CH), 1541 (N=O, asymmetric), 1346 (N=O, symmetric), 3092 (Ar C-H), 2951 (Alkyl C-H). 1H NMR (400 MHz, CDCl3,
Yield 79%; m.p. 149–151°C; IR (KBr, cm−1): 1655 (C=O), 1605 (C=C of Ar), 1508 (CH=CH), 1125 (-OCH3), 3054 (Ar C-H), 2956 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 66%; m.p. 146–148°C; IR (KBr, cm−1): 1655 (C=O), 1605 (C=C of Ar), 1500 (CH=CH), 1130 (-OCH3), 3066 (Ar C-H), 2839 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 70%; m.p. 180–182°C; IR (KBr, cm−1): 1652 (C=O), 1585 (C=C of Ar), 1462 (CH=CH), 1127 (-OCH3), 3110 (Ar C-H), 2853 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 76%; m.p. 132–134°C; IR (KBr, cm−1): 1651 (C=O), 1581 (C=N), 1604 (C=C of Ar), 1505 (CH=CH), 1368 (C-N), 3006 (Ar C-H), 2799 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 86%; m.p. 143–145°C; IR (KBr, cm−1): 1645 (C=O), 1590 (C=N), 1603 (C=C of Ar), 1502 (CH=CH), 1370 (C-N), 3098 (Ar C-H), 2937 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 89%; m.p. 165–167°C; IR (KBr, cm−1): 1650 (C=O), 1581 (C=N), 1605 (C=C of Ar), 1505 (CH=CH), 1373 (C-N), 3101 (Ar C-H), 2811 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 82%; m.p. 189–191°C; IR (KBr, cm−1): 1652 (C=O), 1588 (C=N), 1605 (C=C of Ar), 1506 (CH=CH), 1375 (C-N), 3121 (Ar C-H), 2935 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 86%; m.p. 179–181°C; IR (KBr, cm−1): 1655 (C=O), 1610 (C=C of Ar), 1505 (CH=CH), 624 (C-S), 3119 (Ar C-H), 2954 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 85%; m.p. 149–151°C; IR (KBr, cm−1): 1652 (C=O), 1585 (C=C of Ar), 1503 (CH=CH), 2959 (Ar C-H), 2713 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 78%; m.p. 102–104°C; IR (KBr, cm−1): 1643 (C=O), 1574 (C=C of Ar), 1500 (CH=CH), 1240 (O-CH2-O), 3122 (Ar C-H), 2963 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 66%; m.p. 146–148°C; IR (KBr, cm−1): 1658 (C=O), 1605 (C=C of Ar), 1503 (CH=CH), 3011 (Ar C-H), 2883 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 60%; m.p. 139–141°C; IR (KBr, cm−1): 1663 (C=O), 1610 (C=N), 1588 (C=C of Ar), 1510 (CH=CH), 1391 (C-N), 3105 (Ar C-H), 2732 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 71%; m.p. 126–128°C; IR (KBr, cm−1): 3450 (O-H), 1648 (C=O), 1606 (C=C of Ar), 1510 (CH=CH), 1225 (-OCH3), 3001 (Ar C-H), 2833 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
Yield 84%; m.p. 131–133°C; IR (KBr, cm−1): 1658 (C=O), 1603 (C=C of Ar), 1515 (CH=CH), 824 (C-Cl), 1525 (N=O, asymmetric), 1348 (N=O, symmetric), 3012 (Ar C-H), 2823 (Alkyl C-H); 1H NMR (400 MHz, CDCl3, ppm):
The antimicrobial activity was performed against four bacterial and two fungal strains. The organisms selected were the Gram-positive
The employed methodology deals with development of atom based 3D-QSAR models to predict the antibacterial and antifungal activity for the synthesized chalcones. By these studies, it is possible to understand how the compounds interact with the target. The results emerging out of these studies can be used to identify new active ligands. For this reason, PHASE v 3.1 (Schrödinger LLC, Portland, Oregon, USA;
The data set consists of structurally diverse compounds having antibacterial and antifungal activities against
Antibacterial and antifungal activities of chalcones (MIC in
Compound |
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4′′-Chlorophenyl | 32 | 32 | 64 | 64 | 32 | 64 |
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4′′-Methylphenyl | 256 | 128 | 128 | 128 | 256 | 256 |
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2′′-Chlorophenyl | 64 | 64 | 128 | 128 | 64 | 64 |
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4′′-Fluorophenyl | 32 | 32 | 64 | 64 | 16 | 32 |
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4′′-Dimethylaminophenyl | 256 | 128 | 256 | 256 | 128 | 64 |
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3′′-Bromophenyl | 128 | 128 | 128 | 128 | 64 | 128 |
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4′′-Hydroxyphenyl | 64 | 64 | 64 | 64 | 128 | 128 |
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3′′-Hydroxyphenyl | 256 | 256 | 128 | 256 | 128 | 64 |
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4′′-Nitrophenyl | 128 | 64 | 64 | 128 | 64 | 64 |
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4′′-Methoxyphenyl | 128 | 128 | 256 | 128 | 32 |
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3′′,4′′-Dimethoxyphenyl | 256 | 128 | 256 | 128 | 64 | 128 |
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3′′,4′′,5′′-Trimethoxyphenyl | 128 | 128 | 128 | 256 | 32 | 32 |
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2′′-Pyridinyl | 64 | 128 | 64 | 128 | 32 | 64 |
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3′′-Pyridinyl | 64 | 128 | 128 | 64 | 64 | 128 |
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4′′-Pyridinyl | 64 | 128 | 128 | 64 | 32 |
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2′′-Pyrrolyl | 128 | 256 | 128 | 256 | 64 | 32 |
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2′′-Thienyl | 128 | 256 | 256 | 256 | 32 | 64 |
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5′′-Bromofuran-2′′-yl | 64 | 128 | 64 | 128 | 64 | 32 |
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3′′,4′′-Methylenedioxyphenyl | 256 | 256 | 64 | 64 | 64 | 128 |
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9′′-Anthracenyl | 128 | 256 | 128 | 128 | 32 | 32 |
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1′′-Phenyl-3′′-methylpyrazole-4′′-yl | 256 | 128 | 128 | 256 | 128 | 64 |
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3′′-Methoxy-4′′-hydroxyphenyl | 264 | 128 | 64 | 64 | 64 | 32 |
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2′′-Chloro-5-nitrophenyl | 64 | 64 | 32 | 32 | 64 | 32 |
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<1 | <1 | <1 | <1 | -- | -- | |
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-- | -- | -- | -- | <2 | <2 |
The structures of all the 25 synthesized chalcones were modeled using Chemdraw Ultra 10.0 (Cambridge software), and then the modeled structure was copied to Chem3D Ultra 10.0 to create a 3D model which was subjected to energy minimization using molecular mechanics (MM2). The minimization was executed until the root-mean-square gradient value reached a value smaller than 0.001 kcal/mol. Such type of energy minimized structures was considered for molecular docking and 3D-QSAR studies. However, the corresponding MDL MOL files were prepared using Chem3D Ultra 10.0 integral options (save as /MDL MOL).
In order to understand the structural differences between the active and inactive compounds, it was thought that it was proper to do a visual inspection of the aligned ligands and quantify these differences by building a 3D-QSAR model that identifies which functional groups contribute, either positively or negatively, to activity. PHASE generated individual atom based 3D-QSAR models by using a set of 25 chalcones, which all have been aligned in a three-dimensional space with reference of their antibacterial and antifungal activity data. From this set of 25 compounds, 20 compounds were selected to generate the 3D-QSAR model (i.e., the training set; see supplementary data for tables (available
PHASE can use two alternative ways to generate the structural components that constitute the 3D-QSAR models, that is, the pharmacophore model and atom based 3D-QSAR. The atom based model is more useful for elucidating the structure activity relationships. In atom based 3D-QSAR models, a regression is performed by using the partial least-squares (PLS) method [where the independent variables are the binary-valued occupancies (i.e., the values are either 0 or 1) of the cubes by the different structural components]. Thus, in this model, the number of independent variables used is the 6
PHASE supports only the use of a true external test set (i.e., compounds which have not been used to build the model) to validate the 3D- QSAR model. For this reason, it is necessary to analyze the statistics obtained from the training and test sets. The main statistical properties that describe the 3D-QSAR model when the training set data is used are as follows: (a) the
Once the 3D-QSAR models have been generated, we have to visualize and analyze them. Thus, to understand how the structures of the ligands contribute either positively or negatively to the computed activity, the three-dimensional aspects of the 3D-QSAR model were examined. The visualization allows viewing the cubic volume elements occupied by one specific ligand or all the cubes in the 3D-QSAR model which shows the volume occlusion maps (i.e., the union of the cubes occupied by all the compounds from the set). In this visualization, the maps represent the regions of favourable and unfavourable interactions. The volume occlusion maps describe the spatial arrangement of favourable interactions to accept groups of the target protein. In Figures
Atom based 3D-QSAR model for antibacterial activity of chalcones against
Atom based 3D-QSAR model for antibacterial activity of chalcones against
Atom based 3D-QSAR model for antibacterial activity of chalcones against
Atom based 3D-QSAR model for antibacterial activity of chalcones against
Atom based 3D-QSAR model for the antifungal activity of chalcones against
Atom based 3D-QSAR model for the antifungal activity of chalcones against
The conventional base-catalyzed Claisen-Schmidt condensation led to the synthesis of target compounds. Purification of the compounds was done by recrystallization employing ethanol as solvent. Structural elucidation of the compounds was done with the help of spectroscopic studies including IR, 1H NMR, 13C NMR, and MS. The spectral data of the compounds was in accordance with the anticipated structures. The IR spectra of these compounds give away characteristic -C=O stretching at 1645–1660 cm−1 and -C=C- stretching at 1450–1520 cm−1, respectively. Additional -C=C- and -C-H stretching at 1580–1610 cm−1 and 3010–3150 cm−1 had established the occurrence of aromatic rings. The 1H NMR spectra of the compounds were done by dissolving the compounds in CDCl3 and two diagnostic doublets in the spectrum around
The antimicrobial activity of the prepared chalcones was performed by serial tube method and their MIC values are summarized in Table
The set of 25 chalcones
Summary of the atom based 3D-QSAR statistical data results.
Organism | PLS factors | SD |
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RMSE |
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Pearson- |
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5 | 0.1405 | 0.9296 | 17.9 |
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0.22 | 0.5219 | 0.6622 |
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4 | 0.1345 | 0.9211 | 35 |
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0.14 | 0.4623 | 0.7484 |
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5 | 0.0963 | 0.9623 | 56.2 |
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0.15 | 0.4529 | 0.7285 |
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5 | 0.1081 | 0.7262 | 51.5 |
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0.28 | 0.5607 | 0.788 |
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5 | 0.041 | 0.9215 | 56.1 |
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0.327 | 0.4529 | 0.7285 |
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5 | 0.1068 | 0.9312 | 38.2 |
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0.27 | 0.566 | 0.7109 |
The results clearly indicated the potential antibacterial activity of the chalcone
We report the synthesis, structural elucidation, and antimicrobial activities of a series of 4-isobutylacetophenone chalcones. These compounds can be synthesized in good yields. The compounds
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors would like to acknowledge the Ministry of Minority Affairs, Indian Government, for providing UGC-Maulana Azad National Fellowship for Afzal Basha Shaik to carry out the proposed work (F1-17.1/2012-13/MANF-2012-13-MUS-AND-15992/(SA-III/Website)).
Table 3: experimental and predicted MIC (