Synthesis and antimicrobial evaluation of some 1-( 4-arylthiazol -2-yl )-1 ′-( aryl / heteroaryl )-3 , 3 ′-dimethyl-[ 4 , 5 ′-bi-1 H-pyrazol ]-5-ols

A series of sixteen 1-(4-arylthiazol-2-yl)-1′-(aryl/heteroaryl)-3,3′-dimethyl-[4,5′-bi-1H-pyrazol]-5-ols (7a–p) was synthesized starting from dehydroacetic acid (DHA, 1) via the stepwise formation of thiosemicarbazone (2), 3-(1-(2-(4-arylthiazol-2-yl)hydrazono)ethyl)-4-hydroxy-6-methyl-2H-pyran-2-ones (4a–d) and 1-(1-(4-arylthiazol-2-yl)-5-hydroxy-3-methyl-1H-pyrazol-4-yl)butane-1,3-diones (5a–d) in high yields. The in vitro antibacterial and antifungal activities of the synthesized bipyrazoles 7a–p were investigated against two Gram-positive bacterial strains, viz. Bacillus subtilis (MTCC 441) and Staphylococcus aureus (MTCC 7443), one Gram-negative bacterial strain, viz. Escherichia coli (MTCC 42), and two fungal strains, viz. Candida albicans (MTCC 183) and Aspergillus niger (MTCC 282). The compounds 7a and 7e were found to exhibit better inhibitory activity against A. niger than the reference fluconazole. Moreover, the antifungal activities of the title compounds were more prolific than their antibacterial activities. Furthermore, in order to study binding interactions, docking simulations of compounds 7a, 7m and 7o were performed into the active site of S. aureus 1,4-dihydroxy-2-naphthoyl-CoA synthase.


INTRODUCTION
Infectious diseases caused by various microbes, such as bacteria, fungi, viruses and parasites, are still a leading cause and major threat to public health, despite much important progress having been made in recent years. 1 -3 The discovery, development and synthesizing a new efficient, active and less toxic molecules for systemic activities remains the aim and subject of many molecular architectures and drug researchers.The growing clinical importance of drug resistant bacterial and fungal pathogens has lent an additional urgency in the field 128 of microbiological research and development of new antimicrobial agents.][6][7][8][9] Their usefulness is reported in the synthesis of heat resistant polymers. 10Bipyrazolyl derivatives have also been reported to act as active components in the capture of active oxygen and free radicals in vivo and act as useful agents for preventing and treating various diseases induced by active oxygen, while some of them have found application as agents for detecting singlet oxygen. 11On the other hand, thiazole derivatives also constitute an attractive class of organic heterocycles as they have been reported to exhibit a wide range of pharmacological properties, including anticonvulsant, 12 analgesic, 13 antimicrobial, 14 anti-inflammatory, 15 antitumor, 16 antipyretic, 17 antitubercular, 18 anti-HIV, 19 antioxidant, 20 diuretic 21 etc.Likewise, benzothiazole derivatives have attracted continuing interest because of their widespread biological activities, viz.][24][25][26][27] The pervasiveness of the bipyrazole nucleus in bioactive molecules has stimulated the need for elegant and different ways for their synthesis.Recently, synthetic methodologies to bipyrazoles along with their applications have been reviewed as a new class of supramolecular complexes, organometallics, cage-like structures and self-assembling, metallomacrocycles with bipyrazole ligands as promising catalysts, molecular mimics, molecular magnetic devices and sensors. 28[32] Protein and small molecule docking is a potent strategy that has grown into an essential component of computer-aided drug design. 33It gives an appropriate picture of protein−ligand interactions and facilitates the design of potential active leads.Docking tools normally explore ligand conformations to generate a list of docked poses along with scores and detect a single highest scoring pose as the best solution. 34wing to the importance of bipyrazoles, thiazoles and bezothiazoles that qualify them as excellent scaffolds in therapeutic and medicinal research and in continuation of an ongoing research program on the synthesis of pyrazoles, 35 -37 herein, the synthesis, antimicrobial evaluation and molecular docking studies of several 1-(4-arylthiazol-2-yl)-1′-(aryl/heteroaryl)-3,3′-dimethyl-[4,5′-bi-1H-pyrazol]-5-ols (7a-p) are reported.
The structures of all the bipyrazoles 7a-p were well characterized by satisfactory spectroscopic (IR, 1 H-NMR, 13 C-NMR and mass) and analytical data.The IR spectra of the bipyrazoles 7a-d exhibited characteristic broad absorption bands of medium intensity in the region 3099-3107 cm -1 due to the C5-OH of the pyrazole moiety.The 1 H-NMR spectra of the bipyrazoles 7a-d, in each case, displayed two sharp singlets in the δ regions 1.87-1.88and 2.39-2.43ppm, each integrating for three protons due to methyl groups situated at the 3-and 3′-positions, respectively.In the aromatic region, at the highest field, a singlet integrating for one proton in the δ region 6.29-6.30ppm was displayed, which was assigned to C4′-H of the pyrazole moiety.All these assignment were in obeisance with the chemical shifts of similar protons, as reported in literature. 45 -47he broad signal exhibited in the δ region 13.05-13.16ppm (exchangeable with D 2 O) was safely assigned to C5-OH.The other aliphatic and aromatic protons were observed in the expected regions.The structures of the bipyrazoles 7e-p, tethered with a benzothiazole/6-substituted benzothiazole moiety were corroborated by employing spectral as well as analytical results such as those for 7a-d.The compounds 7e-p exhibited the characteristic broad absorption bands of medium intensity in the region 3055-3107 cm -1 due to C5-OH stretching in their IR spectra.The 1 H-NMR spectra of 7e-p displayed two sharp singlets in the δ regions 2.24-2.26and 2.33-2.39ppm, integrating for three protons each due to methyl groups situated at the 3-and 3′-positions, respectively.In the aromatic _________________________________________________________________________________________________________________________ (CC) 2017 SCS.
Available on line at www.shd.org.rs/JSCS/ region, the key proton of the pyrazole ring C4′-H appeared as a singlet in the δ region 6.52-6.57ppm.In the most downfield region, the broad signal exhibited in the δ region 13.02-13.17ppm (exchangeable with D 2 O) was easily assigned to C5-OH.The other aliphatic and aromatic protons were observed in the expected regions.Furthermore, the 13 C-NMR and mass spectral results of the bipyrazoles 7a-p were also found satisfactory (vide experimental).Scheme 1. Synthesis of bipyrazoles 7a-p.
Available on line at www.shd.org.rs/JSCS/SYNTHESIS AND ANTIMICROBIAL EVALUATION OF BIPYRAZOLES 131 (MTCC 42), and two fungal strains, viz.Candida albicans (MTCC 183) and Aspergillus niger (MTCC 282).Ciprofloxacin and fluconazole were used as standards against the bacteria and fungi, respectively.The serial tube dilution technique 48 was used to determine the minimum inhibitory concentrations (MIC) of the test compounds.The MIC values of 7a-p against the tested microorganisms are depicted in Table I.The data presented in Table I revealed that bipyrazoles 7g and 7p exhibited the least inhibitory activity, 7a, 7b, 7e, 7f, 7h and 7k-o exhibited low activity, 7c, 7i and 7j were found to be moderately active whereas 7d (MIC, 0.0075 µmol mL -1 ) was found to exhibit noticeable inhibitory activity against B. subtilis; against S. aureus, 7e was found the least active, 7b-d, 7f-l, 7n and 7p exhibited low activity whereas 7a, 7m and 7o were found moderately active; 7a, 7c-e, 7g-j, 7l and 7n-p were found less effective, and 7b, 7k and 7m were found moderately active.Compound 7f (MIC, 0.0060 µmol mL -1 ) exhibited considerable inhibitory activity against E. coli as compared to the reference, i.e., ciprofloxacin (MIC, 0.0047 µmol mL -1 ).The compounds 7e and 7j were found least effective, 7d was found less effective, 7a and 7m were found moderately active, and 7b, 7c, 7f-i, 7k, 7l and 7n-p exhibit appreciable inhibitory activity against C. albicans, while 7j and 7n were found least effective, and 7b-d, 7f-i, 7k-m, 7o and 7p were found to exhibit considerable inhibitory activity against A. niger as compared to the reference, i.e., fluconazole.It is interesting to mention here that 7a (MIC, 0.0035 µmol mL -1 ) and 7e (MIC, 0.0029 µmol mL -1 ) were found to exhibit greater inhibitory activity than fluconazole against A. niger.
From the antimicrobial activity data, the following structure-activity relationships were established: a) Substitution of hydrogen by larger groups, such as methyl, methoxy and chloro, resulted in increased activity against E. coli.
b) Against B. subtilis, compounds containing a methoxy group were generally more active.c) Derivatives containing chloro groups were found to exhibit greater inhibitory activity against C. albicans.d) In case of S. aureus and A. niger, compounds with chloro or methoxy groups were more active.

Docking simulations
In order to determine the probable mechanism of action responsible for the antimicrobial activity of the synthesized compounds, the common structural skeleton was screened using the BAITOC web server (www.scfbio-iitd.res.in//software/drugdesign/baitocnew.jsp(March, 2017)), which provides probable biological targets for organic compounds.The structural framework under study was found to be active against S. aureus 1,4-dihydroxy-2-naphthoyl-CoA synthase.Therefore, the most active compounds against S. aureus, namely 7o, 7m and 7a, were docked into the active site of 1,4-dihydroxy-2-naphthoyl-CoA synthase (Fig. 1).The X-ray crystal structure of the enzyme was obtained from the RCSB Protein Data Bank (PDB ID: 2UZF) and the Autodock Vina docking program 49 was used for performing the docking simulations.
The most preferred solutions of most active compound 7o displayed various types of interactions with residues of the active site (Fig. 2).The hydroxyl group of this compound displayed a hydrogen bond with Ser73 and the nitrogen of the thiazole ring created a hydrogen bond with Thr143.The hydroxypyrazole ring and the phenyl ring exhibited π-donor type hydrogen bonding with Thr143 and Ser149, respectively.The benzothiazole and thiazole rings were involved in T-shaped π-π interactions with Phe258 while the hydroxyl on the pyrazole ring showed π-π stacking interactions with the same residue.The pyrazole ring and thiazole ring made π-σ bonding with Val119 and Val147, respectively.The same conformation of compound 7m exhibited most of the interactions (Fig. 3) as displayed by compound 7o.However, one hydrogen bond with Ser73 and the T-shaped π-π interactions with Phe258 were missing in case of 7m.However, the benzothiazole ring presented three π-alkyl type interactions with Val33, Arg34 and Ala36.Similarly, com-_________________________________________________________________________________________________________________________ (CC) 2017 SCS.
Available on line at www.shd.org.rs/JSCS/pound 7a exhibited interactions with the same residues as that of 7o except one hydrogen bond with Ser73, one T-shaped π-π interaction with Phe258 (Fig. 4).
In addition, one π-sulphur interaction existed between the sulphur atom of the benzothiazole ring and the π electrons of Phe258.Thus, the latter features may be the cause of the higher activity of compound 7o.The greater activity of compound 7m than 7a may be due to the shorter length of the hydrogen bond with Thr143 (H … N = 2.92 for 7m; H … N = 2.97 for 7a), which causes tighter binding of the compound with the enzyme, resulting in greater inhibition.Available on line at www.shd.org.rs/JSCS/SYNTHESIS AND ANTIMICROBIAL EVALUATION OF BIPYRAZOLES 135 spectra were recorded on a Bruker Avance II 400 spectrometer at 400 MHz, using dimethyl sulfoxide-d 6 (DMSO-d 6 ) as solvent.The chemical shifts (δ) are reported in ppm using tetramethylsilane (TMS) as an internal standard.Coupling constants (J) are valued in Hz.The mass spectra were recorded on a Waters Micromass Q-Tof Micro (ESI) spectrometer.The figure given in parentheses represents relative intensities corresponding to the base peak taken as 100.Elemental analysis was carried out using Vario Micro Cube Elementar CHNS analyser.Analytical results for C, H and N were within ±0.4 % of the theoretical values.The purity of the compounds was checked by thin layer chromatography (TLC) using readymade silica gel (SIL G/UV254, ALUGRAM) plates.The spots were visualized under a ultraviolet (UV) lamp.Solvents were dried using standard literature procedures.
Physical and spectral data for the synthesised bipyrazoles are given in the Supplementary material to this paper.

Antimicrobial activity
The in vitro antibacterial and antifungal activities of all sixteen synthesized bipyrazoles 7a-p were tested against five microorganisms, i.e., two Gram-positive bacteria, viz.(MTCC 441) and S. aureus (MTCC 7443), one Gram-negative bacterium, viz.E. coli (MTCC 42), and two fungi, viz.C. albicans (MTCC 183) and A. niger (MTCC 282), by the serial tube dilution technique 48 using two solid media, double strength nutrient broth and Sabouraud dextrose broth for bacterial and fungal growth, respectively.The stock solutions (100 μg mL -1 ) of all the test compounds were prepared by dissolving 1 mg of the test compound in 10 mL of dimethyl sulphoxide (DMSO).Ciprofloxacin and fluconazole were used as references against the bacterial and fungal strains, respectively.Fresh cultures were obtained by inoculation of the respective microorganism in a suitable media (double strength nutrient broth in case of the bacteria and Sabouraud dextrose broth in the case of the fungi) followed by incubation at 37±1 °C.The stock solutions of the test compounds were serially diluted in test tubes containing 1 mL of sterile medium to obtain concentrations of 50-3.12 μg mL -1 and then inoculated with 100 μL of a suspension of the respective microorganism in sterile saline.The inoculated test tubes were incubated at 37±1 °C for 24 h in case of B. subtilis, S. aureus and E. coli, at 37±1 °C for 48 h in case of C. albicans and at 37±1 °C for 120 h in case of A. niger and their minimum inhibitory concentration (MIC) values were determined.In microbiology, MIC is the lowest concentration of an antimicrobial agent that will inhibit the visible growth of a microorganism after incubation.The reference compounds ciprofloxacin and fluconazole were also tested under similar conditions to compare with the results of tested compounds.

Docking studies
The course of action for the docking studies was taken in accordance with the modus operandi given by Kumar et al. 51 The structures of the molecules were drawn with Mavin-Sketch 5.10. 52The 3D X-ray crystallographic arrangement of Staphylococcus aureus enzyme, 1,4-dihydroxy-2-naphthoyl-CoA synthase, together with co-crystallized ligand acetoacetyl--CoA (CAA, PDB ID: 2UZF) was taken from the Brookhaven Protein Databank (http:// //www.rcsb.org/pdb).The necessary changes in enzyme were realised with UCSF Chimera 1.10. 53The Dunbrack rotamer library 54 and Antechamber, 55 and were applied to correct imperfect side chains and to compute Gasteiger charges.AutoDock tools 56 were exercised to make pdbqt files.
Co-crystallized ligand CAA was redocked into the active site of the enzyme for validation of the docking protocols.The accurate docked conformation confirmed that the opted protocols were correct for the docking simulations against this enzyme.Visualization of results was realised with the PyMOL 57 and Discovery Studio 4.0. 58
Available on line at www.shd.org.rs/JSCS/SYNTHESIS AND ANTIMICROBIAL EVALUATION OF BIPYRAZOLES 137 containing chloro or methoxy groups exhibited better antimicrobial activity.Docking studies of compounds 7a, 7m and 7o against S. aureus 1,4-dihydroxy--2-naphthoyl-CoA synthase showed that hydrogen bonding and T-shaped π-π interactions were responsible for the antibacterial activity.Thus, some of these studied compounds may emerge as potential molecules for further development as antimicrobial drug candidates.