Synthesis, antimicrobial activity and advances in structure-activity relationships (SARs) of novel tri-substituted thiazole derivatives

Accepted Manuscript Synthesis, antimicrobial activity and advances in structure-activity relationships (SARs) of novel tri-substituted thiazole derivatives Guda Mallikarjuna Reddy, Jarem Raul Garcia, Vemulapati Hanuman Reddy, Ageo Meier de Andrade, Alexandre Camilo, Junior, Renan Augusto Pontes Ribeiro, Sergio Ricardo de Lazaro PII:

S0223-5234(16)30625-0

DOI:

10.1016/j.ejmech.2016.07.062

Reference:

EJMECH 8780

To appear in:

European Journal of Medicinal Chemistry

Received Date: 21 June 2016 Revised Date:

18 July 2016

Accepted Date: 25 July 2016

Please cite this article as: G.M. Reddy, J.R. Garcia, V.H. Reddy, A.M. de Andrade, A. Camilo Junior., R.A. Pontes Ribeiro, S.R. de Lazaro, Synthesis, antimicrobial activity and advances in structureactivity relationships (SARs) of novel tri-substituted thiazole derivatives, European Journal of Medicinal Chemistry (2016), doi: 10.1016/j.ejmech.2016.07.062. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis, antimicrobial activity and advances in the structure-activity relationships (SARs) of novel tri-substituted thiazole derivatives Guda Mallikarjuna Reddy, Jarem Raul Garcia*, Vemulapati Hanuman Reddy, Ageo Meier de Andrade, Alexandre Camilo Junior, Renan Augusto Pontes Ribeiro and Sergio Ricardo de Lazaro

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Trisubstituted thiazoles were synthesized and studied for their antimicrobial activity and supported by theoretical calculations. In addition, MIC, MBC and MFC were also tested. SAR analysis was analyzed to scrutinize comprehensive structure-activity relationships. In fact, LUMO orbital energy and its orbital orientation was reliable to explain their antibacterial and

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antifungal assay. Amongst the tested compounds, tri-methyl-substituted thiazole compound

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showed higher antimicrobial activity and low MIC value due to highest LUMO energy.

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Synthesis, antimicrobial activity and advances in structure-activity relationships (SARs) of novel tri-substituted thiazole derivatives

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Guda Mallikarjuna Reddya, Jarem Raul Garciaa*, Vemulapati Hanuman Reddyb, Ageo Meier de Andradec, Alexandre Camilo Juniorc, Renan Augusto Pontes Ribeiroc, Sergio Ricardo de Lazaroc, a

Department of Chemistry, State University of Ponta Grossa, Ponta Grossa, Parana, Brazil., Department of Chemistry, Indian Institute of Chemical Sciences, Hyderabad, Telangana, India., c Group of Chemical Simulation, State University of Ponta Grossa, Ponta Grossa, Parana, Brazil., b

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Abstract: Trisubstituted thiazoles were synthesized and studied for their antimicrobial activity and supported by theoretical calculations. In addition, MIC, MBC and MFC were also tested. SAR

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analysis was analyzed to scrutinize comprehensive structure-activity relationships. In fact, LUMO orbital energy and orbital orientation was reliable to explain their antibacterial and antifungal assay. Amongst the tested compounds, tri-methyl-substituted thiazole compound showed higher antimicrobial activity and low MIC value due to highest LUMO energy.

1. Introduction

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Keywards: Thiazoles; Antimicrobial; Simulation studies; MIC; MBC/MFC.

In recent research field, electronic structure of chemical compounds displays a fundamental role in mechanism associated with pharmacological activities [1]. Therefore,

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theoretical predictions are useful tools for identification of efficient biologically active compounds because of produced results like frontier orbitals (HOMO and LUMO), band-gap (Egap), molecular electrostatic potential (MEP) and other electronic parameters which are good

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agreement with experimental results [2]. In fact, several years onwards, the second major cause of death in the world, particularly in developed countries is due to infectious diseases and is the third leading cause for death [3-5]. Withal, we are observing today a dramatic world-wide increasing of serious infections by only cause of microbes and also microorganism resistance to multiple antibacterial and antifungal agents have become a serious problem [6,7]. Based on the above facts, need and considerable interest in the discovery of new lead structures and novel chemical entities which will act as antimicrobials. Further, thiazole moiety is a key pharmacophore for the synthesis of several biological molecules because of its low toxicity. For

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example, thiazole contained vitamin B1 (thiamine) helps in the normal functioning of the nervous system by its role in the synthesis of acetylcholine [8]. Some synthesized thiazolidinone derivatives possess antimicrobial [9-12], antioxidant [13], anti-candida & cytotoxicity [14] and anti-inflammatory [15] activities. Recently, 1,3-thiazole derivatives combined good in vitro

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activity against P. falciparum with oral efficacy in a Plasmodium berghei mouse model has been identified [16]. In addition, the cross-coupling palladium catalyzed reactions between aryl halides and alkynes are most widely used methodologies in organic synthesis [17-19]. As part of our ongoing program [20], we aimed towards the development of new heterocycles as

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therapeutic agents, herein we reported synthesis of unknown tri substituted thiazole derivatives and to screened their antimicrobial assay. In addition, in this work, we calculated some

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molecular parameters of compounds in order to correlate them with their antimicrobial activity. 2. Results and discussion 2.1. Chemistry

Initially, targeted compounds 3(a–l) were synthesized from synthetic intermediate 4-(4bromophenyl)-2,5-dimethylthiazole (1). Reaction of 2-bromo-1-(4-bromophenyl)propan-1-one

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with thioacetamide produced compound 1 (Scheme 1) with good yield. Further, treatment of compound 1 with arylboronic acids 2(a–l) in the presence of palladium chloride (PdCl2), triphenylphosphine (PPh3) and potassium carbonate in dimethyl formamide (DMF) gave the corresponding thiazole derivatives 3(a–l). Moreover, reaction of trifluro substituted boronic acid

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with bromophenylthiazole moiety gave higher yield 90% than the other compounds. However, most of the compounds formed with comparatively good yields. All the synthesized compounds 13

C NMR and HRMS and all the spectral data were in agreement

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were characterized by 1H &

with the proposed structures. 2.2.

Biological assay

All the synthesized compounds 3(a–l) were screened for their antibacterial activity at 200

µg/mL concentration and the results were presented in Table 1, which revealed that, compound 3l displayed higher antibacterial activity than other compounds against all the Gram-positive and Gram-negative bacteria. This may be due to the presence of three methyl moieties on benzene ring forced to higher activity. On the other hand, compounds 3d, 3e, 3h, 3i and 3j exhibited good

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antibacterial activity and 3e was showed good activity than 3j except on P. vulgaris, while, the other compounds possessed moderate to low activity. Furthermore, entire compounds showed excessive antibacterial activity towards Gram-negative Escherichia coli bacteria. In addition, all the tested compounds inhibited the spore germination against the fungi

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Aspergillus niger and Aspergillus flavus. In fact, when compared with standard drug all the compounds were showed slightly higher activity towards A. niger. Among all the synthesized compounds, compound 3l exhibited efficient antifungal activity particularly toward A. flavus at 200 µg/mL and its activity nearly equivalent to the standard drug Ketoconazole (Table 1).

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Compounds 3d and 3h displayed excellent antifungal activity. Moreover, compounds 3c, 3k, 3e, 3f, 3i and 3j gave moderate to least activity. Interestingly, compound 3g pushed antifungal

2.2.1. MIC, MBC/MFC studies

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activity towards A. flavus bacteria only and the compounds 3a and 3b were inactive.

Compounds 3d, 3e, 3h, 3i, 3j and 3l were screened for minimum inhibitory concentration (MIC) and minimum bactericidal/fungicidal concentration (MBC/MFC) against Gram-positive [Bacillus subtilis (ATCC 6633), Staphylococcus aureus (ATCC 19433)], Gram-negative

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[Escherichia coli (ATCC 8739), Proteus vulgaris (ATCC 29213)] bacterial stains, Aspergillus niger (MTCC 1881) and Aspergillus flavus (MTCC 1884) fungicidal stains. In fact, the lowest concentration required to arrest the growth of bacteria was regarded as MIC. In this results (Table 2), between these compounds, compound 2,5-dimethyl-4-(2',4',6'-trimethylbiphenyl-4-

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yl)thiazole (3l) exhibited low MIC values than other compounds. Once the minimum inhibitory

concentration (MIC) is determined, there is an extra set of steps performed for MBC/MFC. On

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the other hand, the lowest concentrations of prepared drugs required to kill a particular bacterial/fungicidal is called as MBC/MFC. The antimicrobials are usually regarded as bactericidal/fungicidal if the MBC/MFC is not greater than four times the MIC [21]. The compounds 3d & 3l have the 2 x MIC = MBC value in case of Escherichia coli bacteria and MFC value is 2 x MIC in case of A. flavus. Moreover, compound 3h has 2 x MIC of MBC value in case of S. aureus bacteria and 2 x MIC = MBC value in case of P. vulgaris was belongs to compound 3i. However, the other compounds (3e and 3j) showed bactericidal and fungicidal effects greater than 2 x MIC. The results obtained in this work clearly indicated that, among the described compounds, compound 3l displayed much stronger antibacterial activity against

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Escherichia coli bacteria with an inhibition zone of 27.98 mm at 200 µg/mL and also MIC & MBC of 6.25 and 12.5 µg, as well as it exhibited antifungal activity against A. flavus with an inhibition zone of 27.81 mm at 200 µg/mL and also MIC & MFC of 12.5 and 25 µg, respectively. The results summarized in Table 1 and Table 2, clearly reveal that, electron

2.3.

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donating substituent derivatives displayed comparatively higher antimicrobial activity. Electrostatic results

From Koopmans’s theorem, the ionization potential (I) and electron affinity (A) can be

I = -EHOMO

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A = -ELUMO

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expressed via HOMO and LUMO orbital potential as [22]

the LUMO energy presents the ability of a molecule receiving an electron or implies reactivity towards active groups. A Higher HOMO energy implies that the molecules is a good electron donor. In other way, lower HOMO energy values indicates molecule donating ability is poor [23, 24]. Table 3 summarized the theoretical electronic parameters for investigating the thiazole derivatives. In all the compounds, it was observed that, there was no much difference in HOMO

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energy of all the compounds, even though compounds have different substituents. In addition, the same situation happened in case of I and A results. However, the LUMO orbital energies were affected by the presence of electron donating or withdrawing or both groups on the structure; consequently, there was a change an energy according to attached groups. According

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to Mabkhot et. al., [25] HOMO-LUMO energy gap (Egap) is a measure of the intramolecular charge transfer and was used in pharmaceutical studies. In this study, the compounds exhibited

2.4.

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different antimicrobial activity, this may be due to the difference in LUMO energy levels. Correlation of biological assay with Electrostatic results. The LUMO results presented in Table 2 were plotted against the experimental biological

activity of synthesized compounds (Figure 1). Among the investigated parameters, the relation between biological activity and LUMO orbital energy displayed a clear evidence for the identification of excellent to low biological active compounds. The compound 3l showed illustrious LUMO energy -0.95 eV and also

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exhibited higher antibacterial & antifungal activity than the other compounds. It was observed that compounds with magnificent biological activity has the excessive LUMO energy (Figure 1) also revealed that, LUMO energy has a direct effect on biological activity of the compounds. This may be due to the presence of electron donating methyl groups in compound 3l force to

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higher LUMO energy and activity. This was evidenced by the comparison of compounds 3b and 3g. In this case, compounds 3b and 3g both have different results in theoretical and bioassay studies even though they have electron with drawing fluorine as a common substitution. But, additionally methyl substituted compound 3g has highest LUMO energy -1.16 eV than the

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cyanide substituted compound 3b which has low LUMO energy -1.79 eV and also 3g has good antimicrobial activity than compound 3b (Table 1 & 3). The LUMO energy of compound 3d has

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-1.02 eV and displayed excellent antimicrobial activity. Even though the compound 3h has two methoxy groups, it showed less biological activity than compound 3d which has only one methoxy group, this may be due to the influence of low LUMO energy -1.03 eV than compound 3d. Moreover, the compound 3f exhibited low antimicrobial activity and has less LUMO energy -1.06 eV. This may be due to the presence of methoxy group at meta position. In addition, the compounds 3a, 3b and 3c were showed least biological activity and also have low LUMO energy

activity.

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value. While, the other compounds observed moderate results in LUMO energy and in biological

Once LUMO energy related to biological activity, need to investigate the other aspect like location of these orbitals on the molecule (Figure 2). Such investigation of electronic

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densities revealed the location of particular part in a compound influenced the biological activity. In fact, LUMO orbitals of lowest biological active compounds were located on the aryl moiety.

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This was clearly observed an image of compounds 3b, 3f & 3l and their correlation of LUMO energy orbital in figure 2. In compound 3b the LUMO energy orbitals located on aryl group (Electronic structure I), LUMO energy orbitals of compound 3f were in between aryl group and thiazole group (Electronic structure II), whereas, the same energy orbitals of compound 3l were located on thiazole moiety (Electronic structure I). These electronic distributions demonstrated that, electron donating groups were responsible to active the thiazole ring consequently increased biological activity. Such electron donating groups also changed the charge distribution on the molecular structure from aryl group to thiazole ring (Figure 2.) through intramolecular charge transfer. Besides, electron withdrawing groups that localized charge density on both thiazole and

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aryl rings. Therefore, different biological activity of thiazole derivatives were theoretically

Electronic structure II

Electronic structure III

3. Conclusion

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Electronic structure I

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suggested based on localization of LUMO orbital.

A series of biologically active trisubstituted thiazole derivatives were synthesized and tested for their antimicrobial activity. In addition, electronic parameters were also calculated. Theoretical calculations evidenced for the identification of excellent to low biological compounds amongst the tested compounds. The compound 3l has excessive LUMO energy and

4. Experimental

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enhanced biological activity in MIC also.

4.1. Chemistry and Computational Methodology

Melting points were determined in open capillaries on a Mel-Temp apparatus and were

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uncorrected. The 1H NMR spectra were recorded in CDCl3 or DMSO–d6 or CDCl3+DMSO–d6 on a Jeol JNM l –300 MHz. The

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C NMR spectra were recorded in CDCl3 or DMSO–d6 or

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CDCl3+DMSO–d6 on a Jeol JNM spectrometer operating at 75 MHz. All chemical shifts were reported in δ (ppm) using TMS as an internal standard. HRMS data were obtained using Electro spray ionization (ESI). The microanalyses were performed on a Perkin Elmer 24 0C elemental analyzer. Theoretical investigation of the thiazole derivatives (3(a-l)) in the framework of Density Functional Theory (DFT) calculations applying B3LYP hybrid functional and 631+G(p) basis set for C, Cl, F, H, N, O, S atoms and Sapporo Double Zeta Potential (SPK-DZP) [26] basis set for Iodine atom, implemented on GAMESS-US [27] program package. The structure of compounds was full-relaxed in relation to the total energy of each system; while, the electronic description was analyzed by means of HOMO (Highest Occupied Molecular Orbital)

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LUMO (Lowest Unoccupied Molecular Orbitals), Electron affinity energy (A, where A = E-- E0), Ionization potential energy (I, where I = E+- E0) and Molecular Electrostatic Potential (MEP). Such electronic properties were calculated from total electronic energies of the neutral (E0),

4.2.

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cationic (E+) and anionic (E-) models.

General procedure for the synthesis of 4-(4-bromophenyl)-2,5-dimethylthiazole (1): 2-Bromo-1-(4-bromophenyl)propan-1-one (1 mmol) in 10 mL of ethanol. To this,1 mmol

of thioacetamide was added. The mixture was refluxed at 80 0C temperature for 12 h. Then, the

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reaction solution was cooled to room temperature, diluted with ethyl acetate (30 mL) and then washed with sodium bicarbonate solution followed by brine. The organic layer was dried over

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anhydrous magnesium sulfate and the solvent was removed under vacuum. The resultant residue was recrystallized from methanol to obtain compound 4-(4-bromophenyl)-2,5-dimethylthiazole (1) with good yield.

4.2.1. 4-(4-Bromophenyl)-2,5-dimethylthiazole (1)

Off white solid in(0.26g, 82%); m.p. 143–145 °C; 1H NMR (300 MHz, CDCl3): δ 2.60 (s, 3H, (C5 -CH3)), 3.25 (s, 3H, (C2-CH3)), 7.60-7.80 (m, 4H, Ar-H); 13C NMR (75 MHz, CDCl3)

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δ 11.2 (C5-CH3), 16.5 (C2 -CH3), 124.1, 126.5, 128.0, 129.5, 141.7 (C-5), 147.0 (C-4),

159.1(C-2); HRMS: m/z calcd for C11H10BrNS (M+H)+ 269.1802; found 269.1806. 4.3. General procedure for the synthesis of tri substituted thiazole derivatives 3(a–l):

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The compounds 4-(4-bromophenyl)-2,5-dimethylthiazole (1) (1.5 mmol), arylboranic acid 2(a–l) (1 mmol), PdCl2 (0.3mmol), PPh3 (0.5mmol) and K2CO3 (2 mmol) were kept in DMF

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(20 mL) solvent for 4–5 h at 90 0C. After, reaction mixture was cooled and filtered. The filtrate was diluted, extracted with dichloromethane and the organic layer was removed under vacuum. The resultant residue was purified by column chromatography (silica gel, 60–120 mesh) using hexane/EtOAc (4:1) as eluent. 4.3.1. 2,5-Dimethyl-4-(4'-(trifluoromethyl)biphenyl-4-yl)thiazole (3a). Off white solid in (0.49g, 90%); m.p. 181–183 °C; 1H NMR (300 MHz, DMSO): δ 2.16 (s, 3H, (C5-CH3)), 2.28 (s, 3H, (C2-CH3)), 7.12–7.37 (m, 8H, Ar–H);

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C NMR (75 MHz,

DMSO) δ 12.2 (C5-CH3), 18.5 (C2-CH3), 125.6, 126.8, 127.2, 129.0, 128.0, 128.5, 134.7, 137.0, 140.0, 143.4 (C-5), 148.4 (C-4), 161.1(C-2); HRMS: m/z calcd for C18H15F3 NS

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(M+H)+ 334.0825; found 334.0836. 4.3.2. 4'-(2,5-Dimethylthiazol-4-yl)-5-fluorobiphenyl-3-carbonitrile (3b) Off white solid in (0.46g, 83%), mp 195–197 °C; 1H NMR (300 MHz, DMSO): δ 2.05 (s, 3H, (C5-CH3)), 2.16 (s, 3H, (C2-CH3)), 6.86–6.88 (d, 1H, Ar-H), 7.09–7.15 (dd, 4H, Ar-H), 13

C NMR (75 MHz, DMSO): δ 12.2 (C5-CH3), 18.5 (C2-CH3),

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7.24–7.25 (d, 2H, Ar-H);

113.3, 117.5, 117.9, 118.3, 118.6, 126.6, 126.9, 128.5, 130.0, 132.1, 135.2, 143.2 (C-5), 148.3 (C-4), 161.2 (C-2); HRMS: m/z calcd for C18H14N2FS (M+H)+ 309.0856; found 309.0852.

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4.3.3. 4'-(2,5-Dimethylthiazol-4-yl)biphenyl-3-carbaldehyde (3c)

Off white solid in (0.43g, 85%), mp 173–175 °C; 1H NMR (300 MHz, DMSO): δ 2.28 (s,

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3H, (C5-CH3)), 2.40 (s, 3H, (C2-CH3)), 7.16–7.18 (d, 1H, Ar-H), 7.38–7.49 (m, 4H, Ar-H), 7.54–7.73 (dd, 2H, Ar-H), 7.81–7.89 (s, 1H, Ar-H), 9.81(s, 1H, CHO);

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C NMR (75 MHz,

DMSO): δ 12.3 (C5-CH3), 18.6 (C2-CH3), 126.6, 127.9, 128.6, 129.8, 130.1 132.3, 134.4, 136.7, 137.4, 138.60, 140.3 (C-5), 148.5 (C-4), 161.1 (C-2), 193.1 (CHO); HRMS: m/z calcd for C18H16NOS (M+H)+ 294.0886; found 294.0879.

4.3.4. 4-(4'-Methoxybiphenyl-4-yl)-2,5-dimethylthiazole (3d)

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Off white solid in (0.44g, 87%), mp 188–190 °C; 1H NMR (300 MHz, DMSO): δ 2.27 (s, 3H, (C5-CH3)), 2.40 (s, 3H, (C2-CH3)), 3.57 (s, 3H (OCH3)), 6.35–7.97 (m, 8H);

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C NMR

(75 MHz, DMSO): δ 12.2 (C5-CH3), 18.6 (C2-CH3), 53.1 (OCH3), 110.5, 126.0, 127.6, 128.5, 133.7, 135.6, 137.3, 144.5 (C-5), 148.6 (C-4), 161.1 (C-2), 163.0; HRMS: m/z calcd for

4.3.5.

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C18H18NOS (M+H)+ 296.1054; found 296.1052. (4'-(2,5-Dimethylthiazol-4-yl)biphenyl-4-yl)methanol (3e)

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Off white solid in (0.44g, 81%), mp 169–171 °C; 1H NMR (300 MHz, DMSO): δ 2.28 (s, 3H, (C5-CH3)), 2.41 (s, 3H, (C2-CH3)), 4.453–4.45 (d, 2H, CH2), 7.07–7.44 (m, 8H, Ar-H); 13

C NMR (75 MHz, CDCl3+DMSO): δ 12.35 (C5-CH3), 18.8 (C2-CH3), 62.5 (CH2), 102.6,

106.0, 121.9, 123.4, 124.7, 126.1, 128.4, 131.2, 131.4 (C-5), 139.7 (C-4), 159.9 (C-2); HRMS: m/z calcd for C18H18NOS (M+H)+ 296.1102; found 296.1103. 4.3.6.

4-(3'-Methoxybiphenyl-4-yl)-2,5-dimethylthiazole (3f)

Off white solid in (0.44g, 86%), mp 152–154 °C; 1H NMR (300 MHz, CDCl3+DMSO): δ 2.09 (s, 3H, (C5-CH3)), 2.21 (s, 3H, (C2-CH3)), 3.50 (s, 3H, OCH3), 6.36-–7.25 (m, 8H, ArH);

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C NMR (75 MHz, CDCl3+DMSO): δ 11.9 (C5-CH3 ), 18.3 (C2-CH3), 54.6 (OCH3),

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113.6, 115.3, 125.6, 126.7, 127.2, 128.1, 132.3, 132.8, 138.7, 142.3 (C-5), 148.1 (C-4), 158.3, 160.8 (C-2); HRMS: m/z calcd for C18H18NOS (M+H)+ 296.1048; found 296.1052. 4.3.7.

4-(4'-Fluoro-3'-methylbiphenyl-4-yl)-2,5-dimethylthiazole (3g)

Off white solid in (0.44g, 83%), mp 177–179 °C; 1H NMR (300 MHz, CDCl3+DMSO): δ

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2.16 (s, 3H, (C5-CH3 )), 2.20 (s, 3H, (C2-CH3)), 2.28 (s, 3H, CH3), 7.12–7.66 (m, 7H, Ar-H); 13

C NMR (75 MHz, CDCl3+DMSO): δ 12.2 (C5-CH3), 18.5 (C2-CH3), 35.5 (CH3), 113.2,

115.5, 121.9, 125.6, 126.8, 127.2, 128.0, 128.5, 134.7, 137.0, 143.4 (C-5), 148.4 (C-4), 161.1 (C-2); HRMS: m/z calcd for C18 H17 FNS (M+H)+ 298.0998; found 298.0985. 4-(3',4'-Dimethoxybiphenyl-4-yl)-2,5-dimethylthiazole (3h)

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4.3.8.

Off white solid in (0.48g, 82%), mp 177–179 °C; 1H NMR (300 MHz, CDCl3 δ 2.18 (s, 3H, (C5-CH3)), 2.22(s, 3H, (C2-CH3 )), 3.89 (s, 3H, OCH3), 3.97 (s, 3H, OCH3), 6.94 (d, 2H, Ar13

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H), 7.28 (s, 1H, Ar-H), 7.640–7.74 (dd, 4H, Ar-H);

C NMR (75 MHz, CDCl3) δ 11.3 (C5-

CH3), 18.1 (C2-CH3), 53.1 (OCH3), 68.9 (OCH3), 113.2, 116.6, 124.1, 126.7, 127.8, 128.6, 130.0, 132.4, 133.5, 134.0, 139.2 (C-5), 146.6 (C-4), 160.2 (C-2); HRMS: m/z calcd for C19H20NO2S(M+H)+

4.3.9. 2,5-Dimethyl-4-(4'-methylbiphenyl-4-yl)thiazole (3i)

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Off white solid in (0.41g, 80%), mp 163–165 °C; 1H NMR (300 MHz, CDCl3+DMSO): δ 2.09 (s, 3H, (C5-CH3)), 2.18 (s, 3H, Ar-CH3); 2.21 (s, 3H, (C2-CH3)), 7.06–7.75 (m, 8H, ArH);

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C NMR (75 MHz, CDCl3+DMSO): δ 12.1 (C5-CH3), 18.9 (C2-CH3), 36.4 (CH3), 124.2,

125.9, 127.0, 128.1, 128.6, 131.5, 133.5, 138.0, 141.5 (C-5), 147.4 (C-4), 161.5 (C-2);

4.3.10.

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HRMS: m/z calcd for C18H18NS (M+H)+ 280.1126; found 280.1138. 4'-(2,5-Dimethylthiazol-4-yl)biphenyl-4-ol (3j)

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Off white solid in (0.41g, 85%), mp 170–172 °C; 1H NMR (300 MHz, CDCl3+DMSO): δ 2.14 (s, 3H, (C5-CH3)), 2.18 (s, 3H, (C2-CH3)), 7.12–7.37(m, 8H Ar-H);

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C NMR (75 MHz,

CDCl3+DMSO): δ 12.5 (C5-CH3), 17.5 (C2-CH3), 124.1, 125.7, 127.0, 128.5, 128.9, 131.2, 134.2, 137.6, 143.9 (C-5), 147.1(C-4), 160.5 (C-2); HRMS: m/z calcd for C17H16NOS (M+H)+ 280.0898; found 280.0879. 4.3.11.

4-(4'-Iodobiphenyl-4-yl)-2,5-dimethylthiazole (3k)

Off white solid in (0.58g, 83%), mp 159–161 °C; 1H NMR (300 MHz, CDCl3+DMSO): δ 2.16 (s, 3H, (C5-CH3)), 2.28 (s, 3H, (C2-CH3)), 7.20–7.80 (m, 8H, Ar-H);

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C NMR (75

MHz, CDCl3+DMSO): δ 12.1 (C5-CH3), 18.1 (C2-CH3), 123.6, 125.8, 126.5, 128.1, 128.5,

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131.2, 134.5, 137.0, 140.1 (C-5), 148.8 (C-4), 159.3 (C-2); HRMS: m/z calcd for C17H15INS (M+H)+ 392.0112; found 392.0124. 4.3.12.

2,5-Dimethyl-4-(2',4',6'-trimethylbiphenyl-4-yl)thiazole (3l)

Off white solid in (0.46g, 85%), mp 210–212 °C;

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H NMR (300 MHz, CDCl3+DMSO): δ

7.25 (m, 6H, Ar-H);

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2.05 (s, 3H, (C5-CH3)), 2.16 (s, 3H, (C2-CH3)), 2.35 (s, 3H, CH3), 2.40 (s, 6H, CH3), 6.88– C NMR (75 MHz, CDCl3+DMSO): δ 11.5 (C5-CH3), 19.0 (C2-CH3),

31.5 (CH3), 34.0 (CH3), 113.3, 117.5, 117.9, 118.6, 128.5, 126.6, 126.9, 135.2, 143.2 (C-5), 148.3 (C-4), 161.2 (C-2); HRMS: m/z calcd for C20H22OS (M+H)+ 308.1436; found

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308.1448.

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Acknowledgements

The author Guda Mallikarjuna Reddy is grateful to Brazilian Higher Education Personnel Training Coordination (CAPES: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for the fellowship under the PNPD program. References

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P.A. Villanova 1992, National committee for clinical laboratory standards, reference broth dilution antifungal susceptibility testing of yeasts, proposed

standard NCCLS document M27–P.

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Figure captions Synthesis of tri substituted thiazole derivatives.

Table 1.

In vitro antimicrobial activity of compounds 3(a–l).

Table 2.

MIC, MBC, and MFC of compounds 3d, 3e, 3h, 3i, 3j and 3l.

Table 3.

Theoretical results of compounds 3(a–l).

Figure 1.

Graph between LUMO energy and biological activity of the thiazole derivatives.

Figure 2.

Molecular Electrostatic Potential (MEP) and LUMO orbital of lowest, moderate and

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Scheme 1.

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excessive biological activity compounds 3b, 3f and 3l by DFT/B3LYP calculations.

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Figure 1. Graph between LUMO energy and biological activity of the thiazole derivatives.

Figure 2. Molecular Electrostatic Potential (MEP) and LUMO orbital of lowest, moderate and

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excessive biological activity compounds 3b, 3f and 3l by DFT/B3LYP calculations

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Table 1. In vitro antimicrobial activity of compounds 3(a–l). Compound No.

Zone of inhibition (in mm) at conc. 200 µg/mL after 24 h In vitro antifungal activity

S. aureus (ATCC19433)

E. coli (ATCC8739)

P. vulgaris (ATCC29213)

3a

05.37

7.86

08.70

6.91

3b

04.40

06.00

08.29

06.31

3c

06.65

08.02

09.56

07.39

3d

22.13

25.54

3e

20.03

21.56

3f

15.23

17.48

3g

14.64

16.44

3h

22.01

24.11

3i

21.11

3j

17.71

3k

12.08

3l

A. niger (MTCC-1881)

A. flavus (MTCC-1884)

-

-

-

-

03.92

06.29

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B. subtilis (ATCC6633)

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In vitro antibacterial activity

21.58

23.42

24.91

23.78

17.64

14.52

17.09

19.20

18.84

06.32

09.11

18.05

15.22

-

05.63

25.64

20.95

22.54

24.16

23.05

24.17

19.52

19.71

21.34

19.20

21.11

20.03

13.63

15.32

14.812

16.25

15.09

08.00

09.57

24.65

26.07

27.98

22.15

25.17

27.81

Ciprofloxacin

25.52

27.04

29.75

23.19

-

-

Ketoconazole

-

-

-

-

25.42

28.35

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Table 2.

MIC, MBC and MFC of compounds 3d, 3e, 3h, 3i, 3j and 3l

Compound No.

MIC MIC (MBC/MFC)

B. subtilis

S. aureus

E. coli

P. vulgaris

A. niger

A. flavus

6.25 (100)

50 (>200)

12.5 (25)

25 (100)

12.5 (100)

25 (50)

3e

25 (100)

12.5 (>200)

25 (100)

100 (>200)

25 (200)

25 (100)

3h

12.5 (100)

25 (50)

50 (200)

25 (100)

25 (100)

50 (200)

3i

25 (100)

100 (>200)

12.5 (100)

50 (100)

25(>200)

25 (100)

3j

50 (200)

25 (100)

25 (>200)

50 (200)

50 (200)

25 (100)

3l

6.25 (50)

25 (100)

6.25 (12.5)

12.5 (50)

6.25 (100)

12.5 (25)

Ciprofloxacin

6.25

6.25

6.25

12.5

-

-

Ketoconazole

-

-

-

-

6.25

12.5

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3d

Table 3. Theoretical results of compounds 3(a–l). Compound HOMO No. (eV)

LUMO (eV)

Egap (eV)

A (eV)

I (eV)

-1.42

-4.57

-0.23

7.42

-1.79

-4.30

-0.41

7.54

-1.87

-4.04

-0.34

7.33

-1.02

-4.41

0.33

6.80

-1.05

-4.52

0.21

6.96

-5.99

3b

-6.10

3c

-5.91

3d

-5.43

3e

-5.58

3f

-5.62

-1.06

-4.56

0.28

7.00

3g

-5.72

-1.16

-4.56

0.19

7.12

3h

-5.42

-1.03

-4.39

0.28

6.76

3i

-5.58

-1.05

-4.53

0.28

6.97

3j

-5.49

-1.07

-4.45

0.31

6.90

3k

-5.79

-1.35

-4.44

-0.08

7.10

3l

-5.83

-0.95

-4.88

0.48

7.22

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1. Synthesis and simulation studies. 2. HOMO-LUMO orbital orientation and calculated electronic parameters. 3. Correlation between biological assay with electronic parameters

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4. LUMO orbitals influenced the biological activity.