Design, synthesis and biological evaluation of some novel N'-(1,3-benzothiazol-2-yl)-arylamide derivatives as antibacterial agents

Chemical Papers

https://doi.org/10.1007/s11696-021-01730-8

ORIGINAL PAPER

Design, synthesis and biological evaluation of some novel

N’‑(1,3‑benzothiazol‑2‑yl)‑arylamide derivatives as antibacterial

agents

Swarupa Rani Gurram¹· Mohammed Afzal Azam¹

Received: 13 February 2021 / Accepted: 5 June 2021

Abstract

In the present work, we carried out hydroxybenzotriazole (HOBT) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide

hydrochloride (EDCl)-mediated synthesis of new N’-(1,3-benzothiazol-2-yl)-arylamides C_1-18in high yields under relatively

milder reaction conditions using dimethyl formamide as solvent. Synthesized compounds were characterized by FTIR, ¹H-

NMR, ¹³C-NMR and HRMS spectral data. The MIC values of synthesized compounds C_1-18were determined by the broth

microdilution method using Mueller Hinton medium. Tested compounds showed variable activity against the tested Gram-

positive and Gram-negative bacterial strains. Compounds C₃, C₅, C₉, C_13-15and C₁₇exhibited promising activity against

Staphylococcus aureus NCIM 5021 with MIC values in the range of 19.7–24.2 μM. Among all tested compounds, C₁₃pos-

sessing thiophene ring attached to the benzothiazole moiety via amide linkage exhibited maximum activity against S. aureus

NCIM 5022 with MIC of 13.0 μM. Compound C₁₃showed maximum activity against S. aureus ATCC 43300 with MIC of

15.0 μM and exhibited bactericidal activity against this strain in minimum bactericidal concentration determination. This

compound also eliminated S. aureus ATCC 43300 strain after 24-h exposure indicating its bactericidal activity. ADMET

calculation showed favourable pharmacokinetic profle of synthesized compounds C_1-18

.

Graphic abstract

Keywords N’-(1,3-benzothiazol-2-yl)-arylamides · HOBt/EDCl · MIC · MBC · Time-kill study

Introduction

* Mohammed Afzal Azam

Benzothiazole (BT) is a privileged bicyclic heterocyclic

afzal9azam@hotmail.com; afzal@jssuni.edu.in

moiety present in a wide variety of synthetic and natural

products. BT derivatives are manufactured worldwide for a

wide variety of applications (Azam and Suresh 2012). BT

derivatives act as important scafold and played an important

1

Department of Pharmaceutical Chemistry, JSS College

of Pharmacy, Tamil Nadu (JSS Academy of Higher

Education and Research), Ooty, Nilgiris 643001, India

Vol.:(0123456789)

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Chemical Papers

role in the feld of medicinal chemistry. BTs have been inves-

tigated extensively and associated with diverse biological

activities such as antifungal (Catalano et al. 2013), antipro-

tozoal (Delmas et al. 2002), antimicrobia l (Amnerkar and

Bhusari 2011), anticancer (Cai et al. 2013; Cindric et al.

2018 ), anticon vulsant (Liu et al. 2016 ), antihypertensive

(Meltzer-Mats et al. 2013 ), antidiabetic (Mar iappan et al.

2012), anti-infammatory (Ugwu et al. 2018) activities.

2-Aminobenzothiazoles are also investigated for their lar-

vicidal and adulticidal activities against Aedes aeaegypti

(Sever et al. 2019).

high lipophilicity and potential for nonspecifc binding.

Replacement of the adamantyl group with methyl-substi-

tuted cyclohexyl and substitution of trifuoromethyl group

at position fve of benzothiazole ring resulted in compound

2 with potent activity against M. tuberculosis H37Rv

(MIC,≤0.12 μg/mL). Further, replacement of the adamantyl

group with 1-methylbicyclo[3.3.1]nonane ring and substitu-

tion of 5-Br-/5,7-dichloro-/4,5,6-trifuoro-2-aminobenzothi-

azole amide groups (compounds 3, 4 and 5, respectively)

in place of 2-aminobenzothiazole amide resulted in potent

and specifc activity against M. tuberculosis H37Rv (MIC

of ≤ 0.12 μg/mL in all three cases). However, compounds

2–6 exhibited lower activity against Mycobacterium avium

101 and Mycobacteroides abscessus 19,977 (MICs, 1.0

to>64 μg/mL). Further, replacement of 5-trifuoromethyl-

2-aminobenzothiazole ring in compound 2 by 4,6-difuoro-

2-amino benzothiazol resulted in compound 6 with potent

activity against M. tuberculosis H37Rv (MIC of≤0.12 μg/

mL). Replacement of 5-Br-/5,7-dichloro-/4,5,6-trifuoro-

2-aminobenzothiazoles in compounds 3–5 by 5,7-dichloro

and 7-trifuoromethoxy-2-aminobenzothiazoles resulted,

respectively, in compounds 7 and 8 (De Groote et al. 2018).

These two compounds were observed to be signifcantly

active against the tested rapid growing non-tuberculous

mycobacteria (MICs, 0.03 to ≤ 0.06 μg/mL). However,

these two compounds showed poor or less activity (MICs, 8

to>64 μg/mL) against tested strains of both Gram-positive

and Gram-negative bacteria Staphylococcus aureus ATCC

29213, Enterococcus faecalis ATCC 29212, Streptococcus

pyogenes ATCC 19615, Streptococcus pneumoniae ATCC

49619, Escherichia coli tolC CGSC 5633 and Pseudomonas

aeruginosa ATCC 35151. It is evident from the above result

that the substitution of halogen atoms or halo-containing

groups imparted specifc activity either against M. tuber-

culosis or non-tuberculous mycobacteria. Benzothiazole-

bearing amide 9 (Fig. 1) also displayed promising activity

against S. aureus ATCC 25323 (MIC, 15.6 μg/mL), E. coli

ATCC 35218 (MIC, 7.81 μg/mL) and K. pneumonia ATCC

31488 (MIC, 3.91 μg/mL) (Bhat et al. 2017). In another

attempt, Patel et al. (2012) identifed benzothiazole aceta-

mide derivative (compound 10 Fig. 1), with significant

antibacterial activity against Staphylococcus aureus (MIC,

3.12 μg/mL), Bacillus cereus (MIC, 6.25 μg/mL) and Shi-

gella fexneri MIC, 6.25 μg/mL). Benzothiazole derivatives

11 possessing acetamide function have been synthesized and

evaluated for their antimicrobial activity (Srivastava and Sen

2008). Some of the compounds also exhibited promising

activity against S. aureus, E. coli, K. pneumoniae and Bacil-

lus subtilis.

Several drugs are also available in the market contain-

ing benzothiazole nucleus, including glutamate transporter

inducer riluzole (Zoccolella et al. 2007) intended for the

treatment of amyotrophic lateral sclerosis, carbonic anh y-

drase 1–4 and 7 inhibitor ethoxzolamide (Heck et al. 1994)

indicated for the treatment of glaucoma and as a diuretic,

mast cell stabilizer Tiaramide (Berkin and Kerr 1982) used

as an antiasthmatic drug, Dopamine D2–D4 receptor recep-

tor agonist Pramipexole (Mierau et al. 1995) used to treat

Parkinson’s disease, dual D2 dopamine receptor and β2-

adrenoceptor agonist Sibenadet (Laursen et al. 2003) for

alleviating the symptoms of chronic obstructive pulmonary

disease, Frentizole an inhibitor of the Aβ-ABAD interac-

tion used as an immunosuppressive agent (Xie et al., 2006).

In addition, Phortress (Bradshaw and Westwell 2004 ) is in

clinical trials for the treatment of solid tumour (formation

of extensive DNA adducts). Many patents have been pub-

lished on benzothiazole derivatives highlighting its impor-

tance, and few derivatives are in diferent phases of clinical

trials (Svendsen et al. 2021 ; Waynne et al., 2009; Klunk

et al. 2014; Dahl et al. 2015; Sutton et al. 2009). Further,

N-benzothiazol-2-y l-amides have been shown to be associ-

ated with wide range of biological activities such as inhibi-

tion of ubiquitin ligase (Parlati et al. 2005), selective cyto-

toxicity against tumorigenic cell lines (Yoshida et al. 2005),

prophylaxis and treatment of rotavirus infections (Bailey

and Pevear 2004 ), the adenosine A_2Areceptor modulators

(Alanine et al. 2001) and therapeutic agents for disorders

associated wit h nuclear hormone receptors (Kerwin et al.

1997). In particular, some benzothiazoles substituted at the

second position with a substituted benzoylamino moiety

showed activity against Brugia malayi thymidylate kinase

(Sashidhara et al. 2015) and Mycobacterium tuberculosis

H37Rv strain (Hazra et al. 2012). The amide functionality is

known to be important functional group due to its presence

in nature and chemical industry, biomolecules and medicinal

agents (Greenberg 2000).

High-throughput screening campaign by Franzblau et al.

(2012) identif ed unsubstituted benzothiazole adamantly

amide 1 (Fig. 1) with promising activity against Mycobac-

teroides abscessus (MIC, 1 μg/mL). This compound was

further optimized by Graham et al. (2018) to overcome its

Molecular hybridization is known medicinal chemistry

strategy used to combine two diferent scafolds into a single

chemical entity. It is a well-known approach employed for

the design of ligands to increase the afnity towards targets

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Chemical Papers

Fig. 1 Design concept of title

compounds C_1-18

of interest (Makhaeva et al. 2020; Gontijo et al. 2020). Based

on these observations, we sought to combine unsubstituted

benzothiazole ring with alkyl/aryl hydrazide moieties in a

single molecular scafold in an attempt to develop broad-

spectrum antibacterial agents.

by coupling of 2-amin-4-nitrobenzothiazole and acid chlo-

ride using Eaton’s reagent under microwave. Several other

approaches (Shaik et al. 2019; Wang et al. 2017; Castanheiro

et al. 2016; Kim et al. 2013 ) have also been used for the syn-

thesis of N-benzothiazol-2-yl-amide. In the present work, we

designed eighteen molecules C_1-18by linking benzothiazole

nucleus with substituted aryl/aralkyl pharmacophores via

amide linkage. All these molecules were synthesized in high

yields by direct coupling of 2-aminobenzothiazole (A) with

carboxylic acids (B_1-18) under the catalysis of 1-hydroxyben-

zotriazole (HOBt) and 1-ethyl-3-(3-dimethylaminopropyl)

carbodiimide hydrochloride (EDCl) and subsequently evalu-

ated for their antibacterial activity against selected strains

of bacteria.

Amidation reaction is among the most used transforma-

tions in medicinal and organic chemistry. Although N‐acyl

2‐aminobenzothiazoles play an important role in medicinal

chemistry, the available synthetic strategies that lead to these

compounds are limited (Santos et al. 2020). These molecules

have been synthesized by the N‐heterocyclic carbene (NHC)

organocatalysed direct oxidative amidation of araldehydes

with 2‐aminobenzothiazoles in dichloromethane using tria-

zolium salt as carbine precursor and Cs₂CO₃as base (Pre-

maletha et al. 2017). These compounds showed diverse

biological activities like anti-infective, inhibitors of pro-

tein–protein interaction, inhibitor of nuclear hormone recep-

tors, anticonvulsant, antitubercular and anticancer. Direct

coupling of 2-aminobenzothiazole with acid chlorides in

glacial acetic acid has yielded N-benzothiazol-2-yl-amides

in 55–80% yield (Saraswat et al. 2018). Coupling of 2-amin-

obenzothiazole with diferent acid chlorides using dimethyl-

formamide and triethylamine has also been applied (Cindric

et al. 2018). N-benzothiazol-2-yl-amide was also prepared

Experimental

Material and methods

All glass wares were oven-dried prior to use. The purifca-

tion and drying of solvents were carried wherever required.

All reactions were examined by thin-layer chromatog-

raphy (TLC) using pre-coated aluminium back thin layer

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Chemical Papers

chromatography (TLC) silica gel F₂₅₄plates (Merck Ltd.,

Germany). Chemicals and solvents were purchased from

Sigma-Aldrich, Merck, Alfa Aesar, Finar Pvt. Ltd., India,

and LOBA Chemie Pvt., Ltd. India. The melting point of

the synthesized compounds was determined using Veego

VMP-1 melting point apparatus expressed in °C and is

uncorrected. Fourier transform infrared (FT-IR) spectra were

recorded either with Shimadzu 8400 s or with PerkinElmer-

corresponding aromatic acids (B_1-18) (0.2 g, 0.0014 mol) dis-

solved in N,N-dimethylformamide (DMF) (15 mL). The mix-

ture was cooled to 0 °C in an ice bath with stirring, and then,

EDCl (0.563 g, 0.00294 mol) was added. The reaction mixture

was then slowly allowed to reach the room temperature over

1 h, and then, stirring was further continued at this temperature

till completion of reaction. Progress of reaction was monitored

with TLC using n-hexane:ethylacetate (1:2 to 1:6) as eluent.

The reaction was quenched by adding saturated NaHCO₃

solution and then extracted with ethylacetate (20 mL×3).

The organic layer was dried with anhydrous Na₂SO₄, fltered

and concentrated under reduced pressure. The residue thus

obtained was purifed by column chromatography on silica gel

Merck (200 mesh) in glass columns (2 or 3 cm diameter) using

25–30 g of silica gel per one gram of the residue. The elution

of column was started with n-hexane, and then, eluent polarity

was gradually increased with ethyl acetate. Compounds C_1-18

thus obtained in the yield of 80–95%.

1

Spectrum Two spectrometers. H and ¹³C-NMR spectra

were recorded either in deuterated chloroform (CDCl₃) or in

dimethyl sulphoxide (DMSO-d₆) at 300–500 and 100 MHz,

respectively, using Bruker AV-III 400 spectrometer (Ger-

many). Chemical shifts were recorded in ppm using the

solvent as internal standard. The peak patterns are given as

s, singlet; d, doublet; t, triplet; td, triple doublet; dd, dou-

blet of doublets; m, multiplet. High-resolution mass spectra

(HRMS) were measured using Xevo G2-XS QT of Quadru-

pole Time-of-Flight Mass Spectrometer (USA) with positive

electrospray ionization (ESI) mode at 70 eV, as shown in

Table 1.

Synthetic route and chemical structures of com pounds C_1-18

are presented, respectively, in Scheme 1 and Table 2.

General procedure for the synthesis of N’‑(1,3‑benzothia‑

N‑(1,3‑benzothiazol‑2‑yl)‑2‑fuorobenzamide (C₁)

zol‑2‑yl)‑4‑substituted benzamides (C_1‑18

)

White solid; yield: 85.6%. M.P: 170–172 °C. Rf = 0.66

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3260

(NH), 3021 (Ar C–H), 1656 (>C=O), 1623 (>C=N), 1595,

1566 (Ar –C=C–), 1518 (amide II), 767 (ortho-substituted

1,3-Benzothiazole-2-amine (A) (0.242 g, 0.0014 mol) and

HOBt (0.450 g, 0.0029 mol) were successively added to the

1

benzene), 671 (C–S–C). H-NMR (300 MHz, DMSO-d₆)

Table 1 Optimization of reaction conditions in the model reaction

δ_ppm12.81 (s, 1H, NH), 7.99 (d, J=7.7 Hz, 1H), 7.76 (dd,

JFH3ʹH4ʹ=91.45 Hz, 2H), 7.61 (td, JFH5ʹ=73.17, Hz, 1H),

7.39–7.46 (m, 1H), 7.25–7.38 (m, 3H). ¹³C-NMR (100 MHz,

CDCl₃) δ_ppm161.69 (> C=O), 160.96 (> C=N), 159.21,

157.36, 148.02, 134.91, 134.81, 131.94, 125.98, 125.01,

123.78, 121.04, 120.68, 118.68, 118.58, 116.31, 116.07. ¹⁹F-

NMR (377 MHz, DMSO-d₆): δ_ppm113.27. HRMS (ESI-TOF)

(m/z): [M+H]⁺Calcd. for: C₁₄H₁₀N₂OSF 273.0498; Found

273.0489.

Entry Solvent

HOBt/EDCl

Tem-

Time Yield (%)^a

perature

(°C)

1

2

3

4

5

6

7

CH₃CN

2.0 equivalent

each

25

18

24

12

24

25.3

30.2

30.1

18.6

29.2

85.6

32.1

CH₃COCH₃2.0 equivalent

each

2.0 equivalent

each

CHCl₃

CH₂Cl₂

THF

2.0 equivalent

N‑(1,3‑benzothiazol‑2‑yl)‑4‑methylbenzamide (C₂).

each

2.0 equivalent

each

White powder; yield: 80.5%. M.P: 160–162 °C. Rf=0.65

(ethylacetate/n-hexane, V/V=2:8). FT-IR (cm⁻¹): 3285 (NH),

3013 (Ar C–H), 1663 (>C=O), 1618 (>C=N), 1592, 1542

(Ar –C=C–), 1525 (amide II), 821 (para-substituted benzene),

DMF

2.0 equivalent

each

DMF

1.5 equivalent

each

1

669 (C–S–C). H-NMR (300 MHz, CDCl₃) δ_ppm8.18 (d,

^aIsolated yield

J=8.2 Hz, 2H), 8.11 (d, J=8.3 Hz, 1H), 7.62–7.52 (m, 1H),

Scheme 1 Route for the synthe-

sis of tilted compounds C_1-18

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Chemical Papers

Table 2 Chemical structures of the synthesized compounds C_1-18

Comp

Ar

Comp

Ar

Comp

Ar

C₁

C₇

C₁₃

C₂

C₈

C₁₄

C₃

C₄

C₉

C₁₅

C₁₆

C₁₀

C₅

C₆

C₁₁

C₁₂

C₁₇

C₁₈

7.47 (dd, J=8.2, 3.9 Hz, 2H), 7.41 (d, J=8.1 Hz, 2H), 2.51

(s, 3H). ¹³C-NMR (100 MHz, CDCl₃) δ_ppm162.81, 146.91,

143.62, 130.86, 129.98, 129.23, 128.89, 128.75, 124.86,

121.97, 120.60, 108.44, 22.06. HRMS (ESI-TOF) (m/z):

[M+H]⁺Calcd. for: C₁₅H₁₃N₂OS 269.0749; Found 269.0736.

(NH), 3025 (Ar C–H), 1648 (> C=O), 1622 (> C=N),

1589, 1567 (Ar –C=C–), 1512 (amide II), 1345, 1545

1

(NO₂), 665 (C–S–C). H-NMR (500 MHz, CDCl₃) δ_ppm

8.57 (d, J = 2.1 Hz, 1H), 8.17 (dd, J = 8.4, 2.1 Hz, 1H),

7.86 (d, J = 7.8 Hz, 1H), 7.71 (d, J = 8.4 Hz, 1H), 7.65 (d,

J = 8.1 Hz, 1H), 7.48–7.44 (m, 1H), 7.39–7.35 (m, 1H).

¹³C-NMR (100 MHz, DMSO-d₆) δ_ppm139.67, 125.18,

125.09, 124.19, 123.30, 122.69, 118.28, 118.00, 115.93,

113.56, 111.58. HRMS (ESI-TOF) (m/z): [M+H]⁺Calcd.

for: C₁₄H₉ClN₃O₃S 334.0047; Found 334.0042.

N‑(1,3‑benzothiazol‑2‑yl)‑4‑chloro‑3‑nitrobenzamide (C₃).

White powder; yield: 86.5%. M.P: 254–256 °C. Rf = 0.76

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3249

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Chemical Papers

N‑(1,3‑benzothiazol‑2‑yl)‑3‑nitrobenzamide (C₄).

¹³C-NMR (100 MHz, DMSO-d₆) δ_ppm¹³C-NMR (100 MHz,

DMSO-d₆) δ_ppm165.69, 159.59, 150.95, 148.11, 139.83,

131.72, 126.86, 124.44, 122.37, 120.64. HRMS (ESI-TOF)

(m/z): [M+H]⁺Calcd. for: C₁₃H₁₀N₃OS 256.0539; Found

256.0536.

Pale yellow solid; yield: 84.3%. M.P: 240–242 °C. Rf=0.68

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3288

(NH), 3011 (Ar C–H), 1659 (>C=O), 1622 (>C=N), 1590,

1577 (Ar –C=C–), 1517 (amide II), 1355, 1547 (NO₂), 672

(C–S–C). ¹H-NMR (400 MHz, CDCl₃) δ_ppm11.03 (s, 1H,

NH), 8.84 (s, 1H), 8.44 (dd, J = 8.2, 1.1 Hz, 1H), 8.36 (d,

J = 7.8 Hz, 1H), 7.93–7.85 (m, 1H), 7.71 (t, J = 8.0 Hz,

1H), 7.55 (d, J=7.5 Hz, 1H), 7.43–7.31 (m, 2H). ¹³C-NMR

(100 MHz, DMSO-d₆) δ 163.80, 158.90, 146.76, 145.63,

133.24, 130.07, 128.45, 125.41, 124.81, 122.47, 122.32,

120.09, 118.47. HRMS (ESI-TOF) (m/z): [M+H]⁺Calcd.

for: C₁₄H₁₀N₃O₃S 300.0437; Found 300.0434.

N‑(1,3‑benzothiazol‑2‑yl)‑1H‑indole‑2‑carboxamide (C₈).

White powder; yield: 82.5%. M.P: 276–278 °C. Rf = 0.68

(ethylacetate/n-hexane, V/V = 2:8). FT-IR (cm⁻¹): 3327

(NH), 3028 (Ar C–H), 1651 (>C=O), 1633 (>C=N), 1590,

1559 (Ar –C=C–), 1528 (amide II), 669 (C–S–C). ¹H-NMR

(400 MHz, DMSO-d₆) δ_ppm12.94 (s, 1H, NH), 11.98 (s,

1H, NH), 8.03 (d, J=7.6 Hz, 1H), 7.80 (d, J=8.1 Hz, 1H),

7.76–7.67 (m, 2H), 7.52–7.43 (m, 2H), 7.38–7.31 (m, 1H),

7.28 (ddd, J=8.2, 7.0, 1.0 Hz, 1H), 7.10 (t, J=7.1 Hz, 1H).

¹³C-NMR (100 MHz, CDCl₃+ DMSO-d₆) δ_ppm164.79,

163.59, 153.58, 142.45, 136.95, 134.10, 132.13, 130.69,

129.72, 128.27, 127.18, 125.96, 125.18, 117.28, 112.29,

15.08. HRMS (ESI-TOF) (m/z): [M + H]⁺Calcd. for:

C₁₆H₁₂N₃OS 294.0695; Found: 294.0690.

N‑(1,3‑benzothiazol‑2‑yl)‑2,6‑dichlorobenzamide (C₅).

White powder; yield: 81.3%. M.P: 208–210 °C. Rf = 0.56

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3297

(NH), 3032 (Ar C–H), 1661 (> C=O), 1612 (> C=N),

1593, 1561 (Ar –C=C–), 1530 (amide II), 675 (C-S-C).

¹H-NMR (500 MHz, CDCl₃) δ_ppm8.57 (d, J = 8.4 Hz,

1H), 8.04 (d, J = 8.4 Hz, 1H), 7.89 (t, J = 7.8 Hz, 1H),

7.66 (t, J = 7.8 Hz, 1H), 7.43 (d, J = 3.2 Hz, 3H). ¹³C-

NMR (100 MHz, CDCl₃+DMSO-d₆) δ_ppm160.89, 143.51,

133.19, 133.13, 129.11, 128.55, 128.52, 128.38, 125.07,

120.66, 108.59. HRMS (ESI-TOF) (m/z): [M+H]⁺Calcd.

for: C₁₄H₉Cl₂N₂OS 322.9807; Found 322.9803.

N‑(1,3‑benzothiazol‑2‑yl)‑2‑iodobenzamide (C₉).

White powder; yield: 86.9.5%. M.P: 180–182 °C. Rf=0.69

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3289

(NH), 3017 (Ar C–H), 1655 (>C=O), 1618 (>CN), 1596,

1559 (Ar –C=C–), 1509 (amide II), 670 (C–S–C). ¹H-NMR

(400 MHz, DMSO-d₆) δ_ppm12.87 (s, 1H, NH), 8.04 (d,

J=7.7 Hz, 1H), 7.98 (d, J=7.9 Hz, 1H), 7.79 (d, J=8.0 Hz,

1H), 7.59 (dd, J=7.6, 1.6 Hz, 1H), 7.54 (td, J=7.5, 0.9 Hz,

1H), 7.50–7.44 (m, 1H), 7.39–7.32 (m, 1H), 7.29 (td, J=7.7,

1.7 Hz, 1H). ¹³C-NMR (100 MHz, DMSO-d₆) δ_ppm168.64,

158.29, 149.02, 140.83, 139.71, 132.41, 132.04, 129.16,

128.61, 126.74, 124.33, 122.29, 121.23, 94.17. HRMS (ESI-

TOF) (m/z): [M+H]⁺Calcd. for: C₁₄H₁₀N₂OSI 380.9559;

Found: 380.9543.

N‑(1,3‑benzothiazol‑2‑yl)pyridine‑3‑carboxamide (C₆).

Yellow solid; yield: 87.5%. M.P: 240–244 °C. Rf = 0.57

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3277

(NH), 3012 (Ar C–H), 1649 (>C=O), 1626 (>C=N), 1589,

1561 (Ar –C=C–), 1511 (amide II), 665 (C–S–C). ¹H-NMR

(300 MHz, CDCl₃+DMSO-d₆) δ_ppm¹H-NMR (400 MHz,

DMSO-d₆) δ_ppm13.22 (s, 1H, NH), 8.83 (d, J = 5.3 Hz,

2H), 8.03 (t, J = 6.8 Hz, 3H), 7.80 (d, J = 8.0 Hz, 1H),

7.49 (t, J=7.6 Hz, 1H), 7.36 (t, J=7.5 Hz, 1H). ¹³C-NMR

(100 MHz, CDCl₃+DMSO-d₆) δ_ppm170.02, 164.17, 157.72,

154.61, 152.73, 140.85, 136.56, 133.18, 130.91, 128.61,

128.23, 126.18, 125.16. HRMS (ESI-TOF) (m/z): [M+H]⁺

Calcd. for: C₁₃H₁₀N₃OS 256.0545; Found: 256.0536.

N‑(1,3‑benzothiazol‑2‑yl)‑2,4‑dichlorobenzamide (C₁₀).

White powder; yield: 88.2%. M.P: 208–210 °C. Rf = 0.58

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3288

(NH), 3009 (Ar C–H), 1659 (>C=O), 1622 (>C=N), 1597,

1551 (Ar –C=C–), 1524 (amide II), 668 (C–S–C). ¹H-NMR

(300 MHz, DMSO-d₆) δ_ppm13.03 (s, 1H, NH), 8.05 (d,

J=7.7 Hz, 1H), 7.85–7.77 (m, 2H), 7.75 (s, 1H), 7.60 (dd,

J=8.3, 1.9 Hz, 1H), 7.52–7.44 (m, 1H), 7.36 (t, J=7.6 Hz,

1H). ¹³C-NMR (100 MHz, DMSO-d₆) δ_ppm165.40, 158.24,

148.76, 136.46, 133.54, 132.15, 131.94, 131.46, 129.96,

128.02, 126.81, 24.43, 122.35, 121.18. HRMS (ESI-TOF)

(m/z): [M + H]⁺Calcd. for: C₁₄H₉Cl₂N₂OS 322.9807;

Found: 322.9803.

N‑(1,3‑benzothiazol‑2‑yl)pyridine‑4‑carboxamide (C₇).

Yellow solid; yield: 86.4%. M.P: 242–244 °C. Rf = 0.67

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3279

(NH), 3018 (Ar C–H), 1654 (>C=O), 1632 (>C=N), 1595,

1562 (Ar –C=C–), 1519 (amide II), 665 (C–S–C). ¹H-NMR

(400 MHz, DMSO-d₆) δ_ppm13.23 (s, 1H, NH), 8.84 (d,

J=5.3 Hz, 2H), 8.04 (t, J=6.8 Hz, 3H), 7.81 (d, J=8.0 Hz,

1H), 7.50 (t, J = 7.6 Hz, 1H), 7.37 (t, J = 7.5 Hz, 1H).

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N‑(1,3‑benzothiazol‑2‑yl)‑2‑phenoxyacetamide (C₁₁).

¹H-NMR (300 MHz, DMSO-d₆) δ_ppm12.50 (s, 1H, NH),

8.70 (d, J = 8.5 Hz, 1H), 8.30 (dd, J = 8.4, 4.7 Hz, 2H),

8.16 (d, J = 8.2 Hz, 1H), 8.09 (d, J = 7.8 Hz, 1H), 7.96

(t, J = 7.7 Hz, 1H), 7.83 (dd, J = 16.8, 8.2 Hz, 2H), 7.51

(t, J = 7.6 Hz, 1H), 7.39 (t, J = 7.5 Hz, 1H). ¹³C-NMR

(100 MHz, DMSO-d₆) δ_ppm164.09, 157.96, 149.09,

148.20, 146.52, 138.96, 132.22, 131.43, 130.10, 129.81,

129.46, 128.63, 126.83, 124.46, 122.41, 121.28, 119.38.

HRMS (ESI-TOF) (m/z): [M+H]⁺Calcd. for: C₁₇H₁₂N₃OS

306.0701; Found 306.0691.

White powder; yield: 83.8%. M.P: 156–158 °C. Rf = 0.67

(ethylacetate/n-hexane, V/V = 4:6). FT-IR (cm⁻¹): 3276

(NH), 3010 (Ar C–H), 2834 (CH₂), 1650 (> C=O), 1626

(>C=N), 1589, 1564 (Ar –C=C–), 1515 (amide II), 1031,

1228 (C–O–C), 672 (C–S–C). ¹H-NMR (400 MHz, DMSO-

d₆) δ_ppm12.60 (s, 1H, NH), 8.00 (d, J=7.6 Hz, 1H), 7.78 (d,

J=8.0 Hz, 1H), 7.49–7.43 (m, 1H), 7.36–7.29 (m, 3H), 6.99

(dd, J=8.4, 7.7 Hz, 3H), 4.93 (s, 2H). ¹³C-NMR (100 MHz,

DMSO-d₆) δ_ppm168.38, 158.17, 157.87, 148.93, 131.94,

130.05, 126.70, 124.22, 122.27, 121.77, 121.13, 115.03,

66.58. HRMS (ESI-TOF) (m/z): [M + H]⁺Calcd. for:

C₁₅H₁₃N₂O₃S 285.0703; Found: 285.0709.

N‑(1,3‑benzothiazol‑2‑yl)‑2‑(1H‑indol‑2‑yl)acetamide (C₁₅).

White powder; yield: 86.5%. M.P: 180–184 °C. Rf = 0.78

(ethylacetate/n-hexane, V/V = 6:4). FT-IR (cm⁻¹): 3311

(NH), 3020 (Ar C–H), 1663 (>C=O), 1619 (>C=N), 1594,

1563 (Ar –C=C–), 1529 (amide II), 668 (C–S–C). ¹H-NMR

(400 MHz, DMSO-d₆) δ_ppm12.55 (s, 1H, NH), 10.99 (s,

1H, NH), 7.95 (d, J=7.6 Hz, 1H), 7.74 (d, J=8.0 Hz, 1H),

7.62 (d, J = 7.8 Hz, 1H), 7.43 (t, J = 7.7 Hz, 1H), 7.38 (d,

J=8.1 Hz, 1H), 7.34–7.26 (m, 2H), 7.09 (t, J=7.5 Hz, 1H),

7.01 (t, J=7.4 Hz, 1H), 3.94 (s, 2H). ¹³C-NMR (100 MHz,

DMSO-d₆) δ_ppm171.19 (>C=O), 158.58, 149.03, 136.58,

131.93, 127.60, 126.55, 124.84, 123.95, 122.15, 121.62,

120.95, 119.07, 111.95, 107.70, 67.77, 32.93 (CH₂).

HRMS (ESI-TOF) (m/z): [M+H]⁺Calcd.for:C₁₇H₁₂N₃OS

308.0858; Found: 308.0847.

N‑(1,3‑benzothiazol‑2‑yl)‑2‑methoxybenzamide (C₁₂).

White powder; yield: 90.5%. M.P: 156–158 °C. Rf = 0.71

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3283

(NH), 2928 (CH₂), 3011 (Ar C–H), 1655 (> C=O), 1621

(> C=N), 1590, 1558 (Ar–C=C–), 1519 (amide II),

1

1032, 1255 (C–O–C), 667 (C–S–C). H-NMR (400 MHz,

DMSO-d₆) δ_ppm12.05 (s, 1H, NH), 8.02 (d, J = 7.9 Hz,

1H), 7.81–7.74 (m, 2H), 7.60 (t, J = 7.8 Hz, 1H), 7.47 (t,

J=7.6 Hz, 1H), 7.34 (t, J=7.6 Hz, 1H), 7.25 (d, J=8.4 Hz,

1H), 7.12 (t, J = 7.5 Hz, 1H), 3.96 (s, 3H, OCH₃). ¹³C-

NMR (100 MHz, DMSO-d₆) δ_ppm165.27, 158.14, 157.73,

149.04, 134.21, 132.12, 130.88, 126.69, 124.17, 122.24,

121.91, 121.21, 121.07, 112.80, 56.65. HRMS (ESI-TOF)

(m/z): [M+H]⁺Calcd. for: C₁₅H₁₃N₂O₃S 285.0692; Found:

285.0687.

N‑(1,3‑benzothiazol‑2‑yl)‑4‑oxo‑3,4‑dihy‑

dro‑2H‑chromene‑3‑carboxamide (C₁₆).

Yellow powder; yield: 85.2%. M.P: 250–252 °C. Rf=0.70

(ethylacetate/n-hexane, V/V = 6:4). FT-IR (cm⁻¹): 3297

(NH), 3018 (Ar C–H), 1721 (> C=O), 1647 (> C=O),

1630 (>C=N), 1594, 1567 (Ar –C=C–), 1521 (amide II),

N‑(1,3‑benzothiazol‑2‑yl)thiophene‑2‑carboxamide (C₁₃).

White powder; yield: 84.7%. M.P: 208–210°. Rf = 0.76

(ethylacetate/n-hexane, V/V = 4:6). FT-IR (cm⁻¹): 3284

(NH), 3016 (Ar C–H), 1667 (> C=O), 1632 (> C=N),

1598, 1558 (Ar –C=C–), 1532 (amide II), 671 (C–S–C). ¹H-

NMR (300 MHz, DMSO-d₆) δ_ppm13.03 (s, 1H, NH), 8.32

(s, 1H), 8.02 (d, J=6.5 Hz, 2H), 7.78 (d, J=7.3 Hz, 1H),

7.48 (t, J=7.6 Hz, 1H), 7.36 (d, J=7.7 Hz, 1H), 7.30 (dd,

J=8.3, 4.1 Hz, 1H). ¹³C-NMR (100 MHz, DMSO-d₆) δ_ppm

159.70, 157.94, 148.02, 136.93, 136.32, 133.58, 130.86,

128.11, 125.68, 123.12, 121.22, 119.74. HRMS (ESI-TOF)

(m/z): [M+H]⁺Calcd. for: C₁₂H₉N₂O₂S 261.0156; Found:

261.0145.

1

1030, 1244 (C–O–C), 661 (C–S–C). H-NMR (400 MHz,

CDCl₃) δ_ppm9.11 (s, 1H, NH), 8.39 (dd, J = 8.0, 1.6 Hz,

1H), 7.90–7.80 (m, 3H), 7.64–7.55 (m, 2H), 7.47 (td, J=8.3,

7.3, 1.2 Hz, 1H), 7.33 (td, J=7.8, 1.1 Hz, 1H), 5.30 (s, 3H).

¹³C-NMR (100 MHz, CDCl₃) δ_ppm176.66, 163.35, 156.83,

156.15, 135.30, 132.65, 126.94, 126.64, 126.27, 124.04,

121.51, 121.32, 118.56, 114.33, 71.95, 53.46, 31.84.

HRMS (ESI-TOF) (m/z): [M-H]⁺Calcd. for: C₁₇H₁₁N₂O₃S

323.3537; Found: 323.0483.

N‑(1,3‑benzothiazol‑2‑yl)‑3‑bromobenzamide (C₁₇).

N‑(1,3‑benzothiazol‑2‑yl)quinoline‑2‑carboxamide (C₁₄).

White powder; yield: 88.6%. M.P: 160–164 °C. Rf = 0.64

(ethylacetate/n-hexane, V/V = 3:7). FT-IR (cm⁻¹): 3269

(NH), 3012 (Ar C–H), 1657 (> C=O), 1632 (> C=N),

1596, 1558 (Ar –C=C–), 1535 (amide II), 670 (C–S–C).

¹H-NMR (500 MHz, DMSO-d₆) δ_ppm13.09 (s, 1H, NH),

8.36 (s, 1H), 8.15 (d, J=7.9 Hz, 1H), 8.08–8.01 (m, 1H),

White powder; yield: 88.8%. M.P: 176–178 °C. Rf = 0.59

(ethylacetate/n-hexane, V/V = 4:6). FT-IR (cm⁻¹): 3269

(NH), 3018 (Ar C-H), 1654 (>C=O), 1633, 1621 (>C=N),

1598, 1557 (Ar –C=C–), 1514 (amide II), 669 (C-S-C).

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7.87 (d, J = 8.0 Hz, 1H), 7.80 (d, J = 8.0 Hz, 1H), 7.54 (t,

J=7.9 Hz, 1H), 7.49 (t, J=7.7 Hz, 1H), 7.36 (t, J=7.6 Hz,

1H). ¹³C-NMR (100 MHz, DMSO-d₆) δppm 166.45, 135.92,

134.80, 132.23, 131.80, 131.50, 131.28, 128.75, 127.91,

126.75, 124.28, 122.34, 122.30, 120.64. HRMS (ESI-TOF)

(m/z): [M+H]⁺Calcd. for: C₁₄H₈N₂OSBr 332.9697; Found:

332.9686.N-(1,3-benzothiazol-2-yl)-2-methylbenzamide

(C₁₈).

control. Sterile DMSO is served as a negative control. One

hundred microlitres was the fnal volume for MIC protocols,

and DMSO concentration in each assay well was below 1%.

All synthesized compounds dissolved in sterile DMSO were

screened against the selected strains in the concentration

range of 0.39–125 μg/mL. Corresponding wells were inoc-

ulated with the bacterial suspensions at 10⁵colony-form-

ing unit/mL (CFU/mL) concentrations, and then, 96-well

microtitre plates were incubated without agitation at 37 °C

for 24 h. After incubation period plates were agitated, the

absorbance was measured at 600 nm. Tests were carried

out in triplicate, and results were taken as a mean. For the

MBC determination, an aliquot of 50 μl from each well was

pipetted out and sub-cultured on sterile Mueller–Hinton agar

plates. These plates were further incubated for 24 h at 37 °C,

and the numbers of colony were counted using colony coun-

ter (Manti Lab, M316, India). The lowest concentrations of

the test compound that did not produce any bacterial growth

on agar plates were regarded as the MBC values (Pankey

and Sabath, 2004). For each strain, all experiments were

performed in triplicate. The ratio MBC/MIC≤2 is usually

regarded as the bactericidal activity, while the MBC/MIC

ratio≥4 is considered as bacteriostatic activity of test com-

pound (Levison 2004). The results of MIC and MBC deter-

minations of t he tested compounds are presented in Table 3

and Table 4, respectively.

White powder; yield: 92.5%. M.P: 160–162 °C. Rf=0.62

(ethylacetate/n-hexane, V/V = 2:8). FT-IR (cm⁻¹): 3265

(NH), 3021 (Ar C–H), 2969 (CH₃), 1653 (> C=O), 1628

(> C=N), 1591, 1558 (Ar –C=C–), 1517 (amide II), 665

1

(C–S–C). H-NMR (500 MHz, DMSO-d₆) δ_ppm12.76 (s,

1H, NH), 8.02 (d, J=7.9 Hz, 1H), 7.80 (dd, J=16.1, 7.5 Hz,

1H), 7.62 (d, J=7.4 Hz, 1H), 7.48–7.41 (m, 1H), 7.38–7.32

(m, 1H), 7.31–7.26 (m, 1H), 2.52 (s, 1H), 2.44 (s, 3H). ¹³C-

NMR (100 MHz, DMSO-d₆) δ_ppm158.68, 148.96, 139.46,

136.91, 134.25, 131.97, 131.36, 130.65, 128.64, 126.65,

126.22, 124.18, 122.21, 121.03, 20.12. HRMS (ESI-TOF)

(m/z): [M+H]⁺Calcd. for: C₁₅H₁₃N₂OS 269.0749; Found:

269.0736.

Antibacterial activity assay

Minimum inhibitory concentration (MIC)

and minimum bactericidal concentration (MBC)

The antibacterial activity of synthesized compounds C_1-18

was screened against four Gram-positive bacterial strains:

Staphylococcus aureus NCIM 5021 (ATCC 25923), Staphy-

lococcus aureus NCIM 5022 (ATCC 29213), methicillin-

resistant Staphylococcus aureus ATCC 43300 (MRSA),

Staphylococcus epidermidis NCIM 2493; Klebsiella pneu-

moniae NCIM 2706, Pseudomonas aeruginosa NCIM 2036,

Escherichia coli NCIM 2065 and Mycobacterium Sp. NCIM

2984, using the guidelines of Clinical Laboratories Standard

Institute (CLSI 2007). S. aureus NCIM 5021 strain used in

this study is sensitive to methicillin, amikacin, ampicillin,

bacitracin and benzyl penicillin, while S. aureus NCIM 5022

is sensitive to methicillin, ampicillin, carbenicillin, cepha-

lothin, chloroamphenicol, clindamycin, colistin, erythro-

mycin, gentamycin, kanamycin, nitrofurantoin, penicillin G

and tetracycline. These two strains are used as the reference

strains for susceptibility testing of diferent antibiotics. The

clinical isolate S. aureus ATCC 43300 is an oxacillin- and

methicillin-resistant strain.

Time‑kill assay

Compounds C₉and C₁₄exhibited low MIC values against

S. aureus NCIM 5021, while compound C₁₃displayed most

potent activity against S. aureus ATCC 43300. These three

compounds were selected for their time-kill kinetic assays.

Time-kill assays were performed by the broth macro-dilu-

tion method according to the guidelines of Clinical Labo-

ratories Standard Institute (CLSI 2007). S. aureus NCIM

5021 and S. aureus ATCC 43300 inoculum suspensions

were serially diluted to the concentration of 10⁵CFU/mL.

The inoculum suspensions of S. aureus NCIM 5021 were

treated with test compounds C₉(at test concentrations of

9.8 and 19.7 μM) and C₁₄(at test concentrations of 12.7

and 25.5 μM), whereas inoculum suspensions of S. aureus

ATCC 43300 were treated with compound C₁₃at test con-

centrations of 7.5 and 15 μM. Ciprofoxacin was kept as a

positive control at test concentrations of 47.9 and 95.9 μM.

The treated inoculum cultures were then incubated at 37 °C,

and at timed intervals (0, 2, 4, 6, 8, 12, and 24 h) 50 μL from

the corresponding cultures was collected and subsequently

sub-cultured on nutrient agar medium. Plates were incubated

at 37 °C for 24 h, and then, CFUs were determined (Sup-

plementary Table S1). All data were analysed by construct-

ing log₁₀CFU per millilitre versus time (h) plot (Fig. 2a–c).

All tests were carried out by the broth microdilution

method in Mueller Hinton medium (Hi-media) using

96-well microtitre plates. Compounds C_1-18dissolved in

sterile dimethyl sulphoxide (DMSO) were used to screen

their antibacterial activity, while standard drugs ciprofoxa-

cin and gentamicin in sterile DMSO were used as a positive

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Compared to the initial inoculums, a reduction of≥3 log₁₀

CFU/mL indicates the bactericidal activity, while<3 log₁₀

CFU/mL corresponds to the bacteriostatic activity.

Cytotoxicity and selective index

The cytotoxic activity of compounds C_1-18was evaluated

according to the 3-(4,5-dimethyl-2-thiazolyl)-

2,5-diphenyl-2H-tetrazolium bromide (MTT) assay pro-

tocol published by the American Type Culture Collection

(ATCC) on VERO Cells (ATCC CCL-81) using dimethyl

sulphoxide (DMSO) as a negative control under the same

dilution conditions. Experiments were repeated three times.

Activities were expressed as the concentration of drug inhib-

iting 50% cell growth (IC₅₀) and are summarized in Table 5.

Results and discussion

In the present study, we synthesized eighteen new N’-(1,3-

benzothiazol-2-yl)-substituted arylamides as outlined in

Scheme 1. HOBt- and EDCl-catalysed reaction of 1,3-ben-

zothiazole-2-amine (A) with appropriate substituted aro-

matic acids in dimethyl formamide (DMF) (Larsen et al.

2011) aforded title compounds 1–18 in 80.2–94.2% yield.

Optimization of the reaction conditions for the synthesis

of titled compounds 1–18 started by reacting 1,3-benzo-

thiazole-2-amine (A) with 2-fuorobenzoic acid as model

substrate with diferent amounts of catalyst and solvents

at varied temperatures (Table 1). The reaction was carried

out under stirring in the presence of 2.0 equivalent each of

HOBt and EDCl using acetonitrile as a solvent. The desired

product C₁(Scheme 1) was achieved only in 25.3% yield

(Table 1, entry 1). This inspired us to further examine the

optimal reaction conditions for a more satisfactory result.

We also investigated the efect of solvent on the yield of the

product. Changing the solvent from CH₃COCH₃to tetrahy-

drofuran (THF) did not improve the reaction yield (Table 1,

entry 2–5). The highest yield was obtained when DMF was

used as solvent at room temperature (Table 1, entry 6). The

amount of catalyst was also investigated, and an amount of

2.0 equivalent each of HBOt and EDCl was observed to the

best one, afording the product in 85.6% yield. Decreas-

ing the mount of catalyst, 1.5 equivalent each of HBOt and

EDCl did not improve the reaction yield (Table 1, entry 7).

The structures of newly synthesized compounds C_1-18

1

(Table 2) have been characterized by H-NMR, ¹³C-

NMR and HRMS spectral data (Supplementary Figs.

S1-18). In the Fourier transform infrared (FT-IR) spec-

tra of compounds C_1-18, characteristic absorption bands

of amide NH and > C=O appeared, respectively, in the

region 3224–3327 cm⁻¹and 1648–1667 cm⁻¹, whereas the

Fig. 2 Plots represent the time-kill profle of compounds a 9 b 14 and

c 13

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Chemical Papers

stretching absorption bands of benzothiazole ring> C=N

and C–S–C functions were observed, respectively, in the

regions of 1612–1633 cm⁻¹and 661–675 cm⁻¹. Further, the

formation of compounds C_1-18was supported by the disap-

pearance of carboxylic>C=O and OH peaks and the appear-

ance of amide>C=O peaks in the range between 1648 and

1667 cm⁻¹.

S1e). In the ¹³C-NMR spectrum of all other compounds,

the carbonyl carbon of benzamide and azomethine carbon

of benzothiazole ring appeared in the range, respectively,

at δ 162.09–176.66 and δ 134.38–161.33 ppm. Also, aro-

matic SP²-hybridized carbon signals in all other compounds

appeared in the expected region.

Formation of compounds C_1-18was supported by their

¹H-NMR spectra, which showed amide NH signals in the

region δ 8.84–13.22 ppm. However, in compounds C₂, C₃

and C₅signals due to NH protons did not appear, which

may be due to the replacement of exchangeable protons with

deuterium. In the ¹H-NMR spectra, splitting patterns for the

aromatic protons were observed to be in agreement with the

substitution pattern of respective compounds. The chemical

shift values and coupling constant (J values) of aromatic

protons were assigned accordingly with the position of aro-

matic protons. Aromatic protons on coupling with ortho and

meta protons exhibited characteristic splitting patterns such

as double doublet (dd), triplet of doublet (td), triplet (t) and

doublet (d). In the ¹H-NMR spectrum of representative com-

pound C_1,eight aromatic protons appeared in the region at δ

7.28–7.99 ppm (Supplementary Fig. S1a and S1b). A sharp

singlet signal that appeared at δ 12.86 ppm is ascribed to the

NH proton. In this compound, proton present on position

four of benzothiazole ring appeared as doublet at δ 7.99 ppm

(J = 7.3, 1.7 Hz), while a multiplet signal appeared at δ

7.25–7.38 ppm is due to three protons present on position

Minimum inhibitory concentration (MIC)

Antibacterial activity of synthesized compounds C_1-18was

evaluated by the broth microdilution method (CLSI 2007)

using Mueller Hinton medium (HiMedia). The MIC value

of tested compounds is presented in Table 3 and compared

with the standard drugs ciprofoxacin. Compounds C_1-18

exhibited variable activity against the tested Gram-posi-

tive and Gram-negative bacterial strains. Among all tested

compounds, C₃, C₅, C₉, C_13-15and C₁₇exhibited signifcant

activity against S. aureus NCIM 5021 with MIC values in

the range of 19.7–30.3 μM, while other tested compounds

showed moderate-to-low activity (MIC, 48.4 to 114 μM)

against this strain. Against S. aureus NCIM 5022 only four

compounds, i.e. C₁, C₃, C₁₃, C₁₆and C₁₇, showed promis-

ing activity (MIC, 13 to 30 μM) against S. aureus NCIM

5022 compared to the standard drugs ciprofoxacin (MIC

6.3 μM) and gentamycin (MIC, 16.9 μM). Among all tested

compounds C₁₃possessing thiophene ring attached to the

benzothiazole moiety via amide linkage exhibited maxi-

mum activity against this strain with MIC of 13 μM. It is

evident from result that two electron-withdrawing groups

at positions two and six of the phenyl ring (compound C₅

MIC, 197.3 μM) decrease activity against S. aureus NCIM

5022. In compound C₆, 3-pyridyl ring attached to the amide

function also decreased activity (MIC, 249 μM) against this

strain. This may be due to the presence of the electronegative

nitrogen in the pyridine ring making this ring system rela-

tively electron-defcient in compound C₆. Also, large aro-

matic ring systems like indole (compound C₈MIC, 106 μM)

and quinoline (compound C₁₄MIC, 102.2 μM) and also

alkaryl group (compound C₁₁MIC, 110 μM) when attached

to the benzothiazole ring via amide linkage decreased the

activity against S. aureus NCIM 5022. Against S. aureus

ATCC 43300 (MRSA), compounds C₁, C₃, C_5,C₆, C₉, C₁₀

and C_13-16showed potent activity (MIC, 15.0–26.4 μM),

while all other tested compounds exhibited moderate-to-low

activity (MIC, 44.2–114 μM). Compound C₁₃possessing

thiophene ring showed maximum activity against S. aureus

ATCC 43300 (MIC, 15.0 μM). It is evident that the substitu-

tion of electronegative halogen atom either on ortho or para

position of the phenyl ring (C₁, C₃, C_5,C₉and C₁₀) attached

to the benzothiazole moiety via amide linkage is optimum

for activity against this strain. It is also observed that larger

heterocyclic rings (compounds C₈, C₁₄and C₁₆) are also

well tolerated for activity against S. aureus ATCC 43300.

1

fve, six and seven of this ring. H spectrum of compound

C₁showed H···F interactions for H3′, H4′, H5′ and H6′ and

observed to be typical through bond coupling. A double dou-

ʹ

blet that appeared at δ 7.76 ppm is assigned to H3 and H4ʹ

coupled to ¹⁹F with J value of 91.45 Hz. A triple of doublet

that appeared at δ 7.61 ppm is assigned to H6ʹ couplet to ¹⁹

F

with J value of 73.17 Hz. The multiplet that appeared at δ

7.39–7.46 ppm is due to H5ʹ of fuorobenzene ring. In the

¹³C NMR spectrum of compound C₁, the carbonyl carbon of

amide function exhibited a singlet signal at δ 161.69 ppm,

while the azomethine carbon of the benzothiazole ring

appeared as a singlet at δ 160.69 ppm (Supplementary Fig.

S1c-d). For compound C₁¹³C-¹⁹F coupling was observed

as doublet, one bond distance ¹JCF=134.86 Hz and at two

2

bonds distance JCF=118.63 and 116.19 Hz. These were

assigned to coupling of ¹³F with to C-2ʹ at δ 134.86 ppm,

with coupling to C1ʹ at δ 118.63 ppm, and with coupling to

C3ʹ at δ 116.19 ppm, respectively (Supplementary Fig.S1e).

Remaining nine SP²-hybridized aromatic carbons appeared

at their expected region between δ 116.07–159.21 ppm. In

the ¹⁹F NMR spectrum (Supplementary Fig. S1d), fuo-

rine atom appeared as singlet signal at δ -113.27 ppm. In

the mass spectrum of compound C₁[M + H]⁺peak was

observed at m/z 273.0489 (100%), which corresponds to

its molecular formula C₁₄H₉N₂OSF (Supplementary Fig.

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Table 3 Antibacterial screening result of synthesized compounds C_1-18against selected Gram-positive and Gram-negative bacteria

Comp

Minimum inhibitory concentration (µM)*

^aS.a

^bS. a

^cS.a

^dS.e

^eM. t

^fK. p

^gP. a

^hE. c

C₁

110 1.12

114 1.2

30.1 0.97

56.7 0.49

23.3 0.49

102 1.2

197 .3 1.2

249 1.2

26.4 0.49

114 1.2

58.0 0.25

57.0 0.49

91.1 0.54

102.3 0.54

48.4 0.12

60.2 0.49

121.1 0.97

55.4 0.94

82.8 0.25

22.6 1.12

109 0.25

112.3 0.54

29.6 0.12

99.3 1.12

12.7 0.12

23.4 0.25

23.1 0.25

53.7 1.2

29.7 0.45

113 1.12

95.8 1.12

106 0.45

96.5 0.45

128 1.2

32.3 1.2

104 0.54

163.1 0.54

97.2 0.12

111 0.12

217 0.97

57.6 0.54

206 0.49

201 0.49

191 0.54

189 0.49

112 1.2

26.4 0.49

56.7 0.49

45.5 0.25

106 0.54

49.2 0.25

29.5 0.25

126.3 0.54

25.4 0.49

40.2 0.54

46.8 1.2

52.81 0.54

55.6 0.49

27.1 0.97

102 0.25

51.7 0.25

22.2 0.54

21.9 0.54

57.0 0.54

6.6 0.76

57.3 0.12

54.1 0.97

94.3 0.54

206 0.54

96.3 0.54

238 0.97

123 0.54

209 0.54

165 0.25

194 0.12

115 0.97

221 0.49

118 0.54

202 0.49

104 0.54

192 0.45

98.4 0.54

229 0.54

1.6 0.94

57.3 0.15

113 0.97

45.8 0.15

108 0.97

23.9 0.15

122 0.25

120 0.97

212 1.2

164 0.97

24.2 0.15

112 0.54

30.7 0.25

205 0.25

99.6 1.12

47.8 0.25

95.4 0.25

114 1.2

C₂

C₃

21.5 0.49

106 0.49

24.2 0.49

57.4 1.2

58.2 0.97

52.6 0.49

19.7 0.97

99.3 1.12

110 1.12

54.2 0.49

30.3 0.49

25.5 0.97

24.4 0.49

48.4 0.49

23.1 0.49

59.3 0.49

6.2 0.82

23.3 0.25

102.1 1.2

22.3 0.54

29.3 0.49

57.2 1.2

53.3 0.25

21.0 1.2

23.9 0.25

28.1 1.2

28.5 1.12

15.0 0.25

26.2 0.25

23.7 0.49

21.9 0.54

44.2 0.54

119 0.97

95.9 0.78

C₄

C₅

C₆

C₇

129 1.2

C₈

106 0.25

161.3 1.2

95.6 0.49

110 0.12

112 0.25

13.0 0.49

102.2 0.12

51.7 0.49

25.0 0.54

22.2 1.2

58.2 0.25

6.3 0.52

C₉

C₁₀

C₁₁

C₁₂

C₁₃

C₁₄

C₁₅

C₁₆

C₁₇

C₁₈

Cipro

foxacin

Genta

micin

6.3 0.52

3.6 0.52

6.9 0.52

17.1 0.99

16.9 0.92

41.2 0.78

17.1 0.98

16.9 0.62

2.3 0.88

4.6 0.83

2.0 0.52

^*Values are mean SEM (n=3)

Ciprofoxacin and Gentamicin used as positive control

b

^aS. a: Staphylococcus aureus NCIM 5021; S. a: Staphylococcus _eaureus NCIM 5022;^cS. a: Methicillin resistant Staphylococcus aureus ATCC

d

f

43,300 (MRSA); S. e: Staphylococcus epidermidis NCIM 2493; M.t. a:Mycobacterium sp. NCIM 2984; K. p: Klebsiella pneumoniae NCIM

2706; ^gP. a: Pseudomonas aeruginosa NCIM 2036; ^hE. c: Escherichia coli NCIM 2065

MIC: Minimum inhibitory concentration

On the other hand, fve compounds, i.e. C₁₀, C₁₃and C_15-17,

showed signifcant activity (MIC, 12.7–29.6 μM) against S.

epidermidis NCIM 2493, while all other tested compounds

exhibited moderate-to-low activity (MIC, 48.4–121.1 μM).

Maximum activity against this strain was observed with

compound C₁₅(MIC, 12.7 μM) possessing indole nucleus

attached to the benzothiazole moiety via actamide linkage.

Except C₁and C₇, which showed promising activity (MIC,

29.7 and 32.3 μM), all other tested compounds showed

moderate-to-low activity (MIC, 57.6–217 μM) against

Mycobacterium sp. NCIM 2984. Six tested compounds C₁,

C₆, C₈, C₁₃, C₁₆and C₁₇showed promising activity against

K. pneumoniae NCIM 2706 (MICs, 21.9–29.5 μM), while

all other tested compounds displayed moderate-to-low

activity (MIC, 45.5–126.3 μM). Moreover, no correlation

was observed between substitution pattern and antibacte-

rial activity. In addition, all tested compounds were found

to be less active against this strain when compared to the

standard drugs ciprofoxacin (MIC 6.6 μM) and gentamycin

(MIC, 2.3 μM). All tested compounds displayed less activity

against P. aeruginosa NCIM 2036 (MICs, 54.1–238 μM)

compared to the standard drug ciprofoxacin (MIC 1.6 μM)

and gentamycin (MIC, 4.6 μM). On the other hand, only

three compounds, i.e. C₅, C₁₀and C₁₃, displayed promising

activity against E. coli NCIM 2065 (MIC, 23.9–30.7 μM).

But activities of these tested compounds were observed to be

far less when compared to the standard drugs ciprofoxacin

(MIC, 6.9 μM) and gentamicin (MIC, 2 μM). It is evident

from the above results that tested compounds are more active

against Gram-positive compared to the Gram-negative bac-

teria. Low activity against Gram-bacteria may be attributed

to the poor penetration of test compounds through bacterial

cell wall.

Minimum bactericidal concentration (MBC)

MBC values were determined by the broth microdilu-

tion method (CLSI 2007) to assess the bactericidal or

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Chemical Papers

Table 4 Minimum bactericidal concentration (MBC) and MBC/MIC ratio of title molecules C_1-18against selected bacterial strains

Comp

MBC^$*& ^aS. a

^bS.a

^cMRSA

^dS.e

^eM. t

^fK. P

^gP.a

^hE. c

MBC/

MIC

C₁

MBC

MBC/MIC 4

MBC 223.8 0.54 226.8 0.89 229.8 1.21 228.2 0.1.30 405.3 0.97 203.3 0.89 193.9 0.56 415.3 0.97

MBC/MIC 2

MBC 86.3 0.88 93.4 0.25 186.9 0.74 182.2 0.54

MBC/MIC 4

MBC 212.7 0.87 416 0.97

MBC/MIC 2

MBC 96.8 1.23 395 0.97

MBC/MIC 4

MBC 230.5 0.91 498 0.95

MBC/MIC 4

459.5 0.25 120.5 0.54 105.8 0.54 232.3 0.54

119.1 0.25 108.8 0.29 458.8 0.97 229.4 0.54

4

8

4

C₂

4

2

4

C₃

383.4 0.97 182.2 0.84 188.7 0.90 92.6 0.25

4

8

2

4

2

C₄

416 0.83

4

204.6 0.99

2

212.7 0.54 414.7 0.97 414 1.13

2

436.4 0.54

4

2

C₅

90.6 0.88 192.5 0.92

193.1 0.69 389.4 0.97 386.3 0.94 100.9 0.85

2

4

2

8

4

C₆

119.2 0.79 120.0 0.67

256.4 0.83 119.2 0.90 121.4 0.86 476 0.82

2

4

2

4

2

4

C₇

MBC

MBC/MIC 4

235 0.84

258 0.97

2

58.8 1.02 230.5 0.78

253.3 0.69 123.56 0.95 495.6 0.77 489 0.97

4

2

4

C₈

MBC

MBC/MIC 4

MBC 39.2 0.65 330.5 0.70 40.81 0.95 331 0.80

MBC/MIC 2

MBC 397.5 0.97 382.6 0.97 47.8 0.25 89.4 0.25

MBC/MIC 4

MBC 440.8 0.69 219.7 0.80 121.1 0.91 219.7 0.82

MBC/MIC 4

MBC 216.9 0.54 450.7 0.97 114 0.25

MBC/MIC 4

MBC 121.5 0.81 52.3 0.95 30 0.73

MBC/MIC 4

MBC

MBC/MIC 2

MBC 97.7 0.95 207.1 0.54 95.1 0.88 101.6 0.79

MBC/MIC 4

MBC 193.8 0.54 96.9 0.88 93.8 0.79 93.8 0.86

MBC/MIC 4

MBC 95.7 0.94 86.7 0.87 177.1 0.69 94.2 0.91

MBC/MIC 4

MBC 237.3 0.81 232.8 0.88 467.5 0.90 214.9 0.79

MBC/MIC 4

Ciprofox- MBC

2.2 0.85

acin

208.8 0.74 213.6 0.92 215.6 1.22 221.1 0.85

209.5 0.69 213.3 1.21 426.6 0.84 419.7 0.97

2

4

2

8

2

C₉

326.3 0.79 82.8 0.62

2

330 1.09

2

326.3 0.97

2

4

2

C₁₀

C₁₁

C₁₂

C₁₃

C₁₄

C₁₅

C₁₆

C₁₇

C₁₈

388.8 0.97 194 0.54

338.1 0.89 96.8 0.78

4

2

4

2

4

222.5 0.68 220 0.59

231.6 0.69 442.2 0.88

2

4

2

4

2

4

224.6 0.54

435.2 0.97 110.9 0.54 885.9 1.12 442.2 0.79

4

2

4

122.3 1.25

4

230.7 0.48 240.3 0.79 475.3 0.68 123 0.89

4

2

4

8

4

49.8 0.69 204.5 0.74 105.2 0.95 198.6 0.73

412.4 0.97 204.9 0.54 811.8 0.90 410.4 0.87

2

4

2

4

2

406.5 0.97 102.6 1.15 416.9 1.21 405.2 0.97

4

8

2

4

765.4 0.73 97.2 0.55

385.1 0.97 191.3 0.98

4

2

4

756.6 0.90 94.8 0.85

375.9 0.84 190.9 0.90

4

2

225.3 0.93 466.4 0.89 466 0.99

458.2 1.21

4

2

8

2

4

2.2 0.94

<1

2.0 0.81

<1

2.0 0.78

1

2.3 0.98

<1

2.1 0.67

<1

2.1 0.80

<1

2.2 0.91

<1

MBC/MIC 1

^*Values are mean SEM (n=3)

Ciprofoxacin used as positive control

b

^aS. a Staphylococcus aureus NCIM 5021; S. a Staphylococcus aureus NCIM 5022;^cS. a Methicillin-resistant Staphylococcus aureus ATCC

d

e

f

43300 (MRSA); S. e Staphylococcus epidermidis NCIM 2493; M.t. a Mycobacterium sp NCIM 2984; K. p Klebsiella pneumoniae NCIM

2706; ^gP. a Pseudomonas aeruginosa NCIM 2036; ^hE. c Escherichia coli NCIM 2065

MIC minimum inhibitory concentration (μM)

bacteriostatic activity of synthesized compounds C_1-18

.

2) (Table 4) against S. aureus NCIM 5021, while other tested

Amongst all tested compounds, four compounds, i.e. C₂,

C₄, C₉and C₁₄, displayed bactericidal activity (MBC/MIC,

compounds displayed bacteriostatic activity (MBC/MIC,

4). Against S. aureus NCIM 5022, seven compounds (C_5-9

,

1 3

Chemical Papers

Table 5 Cytotoxic activity

(IC₅₀) and selectivity index

values of synthesized

Comp

Cytotoxicity

IC₅₀-Vero cells

(μM)*

Selectivity index (SI) (SI=IC₅₀/MIC**)

^aS.a

^bS. a

^cS.a

^dS.e

^eM. t

^fK. p

^gP. a

^hE. c

compounds C_1-18against tested

bacterial strains

C₁

250.3 0.88

189.4 0.76

349.3 1.34

104.4 0.55

198.2 1.13

124.4 0.79

152.3 0.85

276.8 0.67

234.3 1.21

253.7 0.73

277.6 0.95

176.2 0.98

349.8 1.22

198.7 0.89

351.6 0.67

324.8 0.78

152.4 1.06

147.6 0.68

2.27

8.30

3.33

14.99

1.02

1.00

1.01

1.18

2.61

1.45

2.65

2.52

1.57

26.90

1.94

6.80

12.99

6.86

2.54

9.46

4.31

8.41

1.67

3.63

0.98

2.05

1.97

4.71

2.66

1.43

2.61

2.50

0.81

6.07

0.96

1.74

1.70

0.80

1.32

9.48

3.33

7.67

0.98

4.02

8.55

1.20

10.89

5.82

5.42

5.26

3.16

12.90

1.94

6.80

14.6

6.95

2.59

4.36

3.49

3.70

0.50

2.05

1.06

1.23

1.32

1.41

1.30

2.41

0.79

2.96

0.98

3.38

1.69

1.54

0.64

4.36

1.67

7.61

0.96

8.28

2.06

1.26

1.30

1.42

10.49

2.48

1.57

11.39

0.96

3.53

6.79

1.59

1.29

C₂

1.65

16.24

0.98

8.18

4.39

2.61

5.26

11.87

2.55

2.52

3.24

30.32

7.79

14.52

6.71

6.59

2.49

1.65

14.99

1.02

8.87

8.61

2.66

5.19

11.14

10.62

9.89

6.17

23.32

7.58

14.83

3.44

3.31

3.83

1.02

4.09

4.19

1.25

4.99

2.82

11.23

2.55

1.56

11.81

2.00

27.68

13.80

6.59

2.75

C₃

C₄

C₅

C₆

C₇

C₈

C₉

C₁₀

C₁₁

C₁₂

C₁₃

C₁₄

C₁₅

C₁₆

C₁₇

C₁₈

1.24

*Values are mean SEM (n=3)

**MIC: Minimum inhibitory concentration (μM)

b

^aS. a: Staphylococcus aureus NCIM 5021; S. a: Staphylococcus aureus NCIM 5022;^cS. a: Methicillin

d

resistant Staphylococcus aureus ATCC 43,300 (MRSA); S. e: Staphylococcus epidermidis NCIM 2493;

f

g

^eM.t. a:Mycobacterium sp. NCIM 2984; K. p: Klebsiella pneumoniae NCIM 2706; P. a: Pseudomonas

aeruginosa NCIM 2036; ^hE. c: Escherichia coli NCIM 2065

C₁₁and C₁₄) displayed bactericidal activity (MBC/MIC,

2), while all other tested compounds showed bacteriostatic

activity (MBC/MIC, 4). Except four compounds, i.e. C₂,

C₉, C₁₀and C₁₃(MBC/MIC, 2), all other tested compounds

showed bacteriostatic activity (MBC/MIC, 4–8) against S.

aureus ATCC 43300. On the other hand, compounds C₃,

C₄, C₆, C₇, C₁₁C₁₂and C₁₄displayed bactericidal activity

against S. epidermidis NCIM 2493 (MBC/MIC, 2), while

in other tested compounds MIC values are much higher

than MBC against this strain. Compounds were observed

to be more active against Mycobacterium sp NCIM 2984

as evident by the bactericidal activity (MBC/MIC, 2) of

compounds C_4-9, C₁₁, C₁₂, C₁₄and C_15,Whereas only three

compounds C₉, C₁₄and C₁₅exhibited bactericidal activity

(MBC/MIC, 2) against P. aeruginosa NCIM 2036. Against

another Gram-negative bacteria E. coli NCIM 2065 fve

compounds (C₃, C₈, C₉, C₁₄and C₁₇) displayed bactericidal

activity (MBC/MIC, 2), while other tested compounds

showed bacteriostatic activity (MBC/MIC, 4).

and 19.7 μM) and C₁₄(at test concentrations of 12.7 and

25.5 μM). Results are compared with ciprofoxacin tested

at concentrations of 3.1 and 6.2 μM. The time-kill kinetics

profle of C₉against S. aureus NCIM 5021 at test concen-

tration of 19.7 μM showed 2log10 and >3log10 reduction

in number of viable cells, respectively, after 10 and 12 h of

exposure, indicating bactericidal activity against this strain.

At 9.8 μM test concentration, also this compound displayed

bactericidal activity against S. aureus NCIM 5021 with a

3log10 reduction in number of viable cells after 12-h expo-

sure; however, the rate of killing was comparatively less

compared to the test concentration 19.7 μM. C₁₄showed a

similar killing rate as C₉against S. aureus NCIM 5021 with

a 3log10 reduction in viable cell count relative to the initial

inoculums after 12-h exposure. As shown in Fig. 2a, b, com-

pounds C₉and C₁₄completely eliminated this strain after

24 h. At both test concentrations of 3.1 and 6.2 μM, cipro-

foxacin exhibited 3log10 reduction in S. aureus NCIM 5021

viable cell count after 10 h of exposure. Time-kill assay was

also performed for compound C₁₃against S. aureus ATCC

43300 (MRSA) at test concentrations of 7.5 and 15 μM. At

both concentrations, compound C₁₃displayed bactericidal

activity with a decrease of≥3log10 in viable cell count after

12-h exposure. The killing rate of ciprofoxacin was faster

Time‑kill assay

Time-kill kinetics was performed against S. aureus NCIM

5021 for compounds C₉(at test concentrations of 9.8

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Chemical Papers

at both test concentrations of 47.9 and 95.9 μM against S.

aureus ATCC 43300 with a 3log10 reduction of viable cell

count after 10 h of exposure. The results indicate that all

three tested compounds have time‐dependent bactericidal

activity.

absorption is observed to be between 88.72 and 93.82%

showing the favourable kinetic profle of these compounds.

The predicted central nervous system (CNS) permeability

of C_1-18ranges between −1.332 and −2.802, indicating

no CNS activity of these compounds (−2 inactive and+2

active). In addition, the apparent Caco-2 cell permeability

(a model for the gut-blood barrier) for C_1-18is observed to

be within the range 0.64–1.809 cm/sec (>0.90 high Caco-2

cell permeability) indicating the fast non-active transport

of compounds C₁and C_3-18. The predicted AMES toxicity

result indicates the non carcinogenic potential of compounds

C₁, C₂, C_5-11, C₁₄, C₁₇and C₁₈. Further, compounds C_1-18did

not exhibit the blockage of human ether-a-go-go (HERG)

K⁺channels indicating no cardiotoxic efect of these com-

pounds. These compounds are observed to be the inhibitors

of some of the cytochrome P450s. The predicted zero Pains

alert (PA) (Baell and Holloway, 2010) of all synthesized

compounds indicated the absence of PAN assay interference

structure and hence safety of these compounds. Compounds

C_1-18also obeyed Lipinski’s rule (Lipinski, 2000) of fve

with zero violation, indicating their drug-like character of

these compounds.

Cytotoxicity and selective index

The selectivity index (SI) of test compounds was determined

as the ratio of IC₅₀on VERO cells and MIC against bacteria.

The higher SI ratio indicates theoretically more efcacy and

would be safety of a compound during in vivo treatment for a

given bacterial infection. Result of cytotoxic activity against

VERO cell line indicated a low efect of compounds C_1-18

with IC₅₀values higher than 104 μM (Table 5). Compounds

C_1-3, C₅and C_8-16are observed to be slightly less cytotoxic

(IC₅₀~ 176.2–351.6 μM). Among the tested compounds,

C₄showed maximum cytotoxic efect with an IC₅₀value

of 104.4 µM and an SI value in the range 0.50–1.02 μM

against all tested bacterial strains. Compound C₁₄also

exhibited lower SI values of 0.96, 0.98 and 0.96 μM, respec-

tively, against Mycobacterium sp NCIM 2984, P. aeruginosa

NCIM 2036 and E. coli NCIM 2065. Compounds C₃and

C₁₃with low cytotoxic efect (IC_50s, 349.3 and 349.8 μM,

respectively) displayed high SI against all three tested strains

of S. aureus. Compound C₁₃also showed high SI against

the tested strains of S. epidermidis (SI, 11.81), K. pneumo-

niae (SI, 12.90) and E. coli (SI, 11.39). Another compound,

i.e. C_15,with less cytotoxic efect (IC_50s, 351.6 μM) showed

relatively high SI of 27.68 and 14.52, respectively, against

tested strain of S. epidermidis. However, low SI (<6.80) was

observed for this compound against all other tested bacterial

strains. On the other hand, compound C₁₆exhibited high SI

in the range of 12.99–14.83 against S. aureus NCIM 5022,

MRSA ATCC 43300, S. epidermidis NCIM 2493 and K.

pneumoniae NCIM 2706.

Drug candidates ADMET properties have been recog-

nized as an important reason for late-stage failure in drug

development. Consensus predictions of ADMET parameters

are well-known strategy to improve the putative results in

drug design feld. These parameters predicted by diferent

methods allowed for a consensus vie w of a given property

(Andrade et al. 2020; Daina et al. 2017; Moda and Andri-

copulo 2012 ). Based on t hese observations, we also used

QikProp module (Schrödinger software suite 2019–2) to pre-

dict the ADMET profles of compounds C_1-18(Supplemen-

tary Table S2). The predicted central nervous system (CNS)

activity ranges between −2–1 on a scale of −2 as inac-

tive and+2 as active, indicating no CNS activity of these

compounds. Brain/blood partition coefficient QPlogBB

(−0.03–0.39) also indicated that these compounds are CNS

negative and cannot cross the blood–brain barrier. The total

solvent-accessible surface area (SASA) (490.8–581.48 Å²)

and polar surface area (PSA) (45.48–92.87 Å²) of these

molecules are well within the recommended range. The

octanol/water partition coefcient (QPlogPo/w) (2.07–4.25)

and aqueous solubility as indicated by the QPlogS

(−5.07–−5.89) indicated the favourable absorption and dis-

tribution of compounds C_1-18. Predicted apparent Caco-2

cell permeability (QPPCaco) of these compounds is within

the range 318–3830 nm/sec, indicative of fast non-active

transport of these molecules, and is easy to absorb. Predicted

QPlogKhsa values (−0.193–0.557), indicative of binding of

these compounds to human serum albumin, are well within

the recommended range (−1.5–1.5). This further indi-

cated the favourable pharmacokinetic profle of these com-

pounds. All eighteen compounds C_1-18exhibited predicted

Prediction of absorption, distribution, metabolism,

excretion and toxicity (ADMET) properties

ADMET properties of chemical compounds play important

roles in the design and development of drug. In the present

study, ADMET properties of the synthesized compounds

C_1-18were computed by SwissADME and (http://www.

swissadme.ch ) and pkCSM (http://biosig.unimelb.edu.au/

pkcsm/prediction) online tools. ADMET properties, princi-

ple descriptors including hERG inhibition potential, AMES

toxicity, Lipinski number of violations and PAINS number

of alerts were taken into account (Table 6) to evaluate the

acceptability for rational drug design.

The predicted total polar surface area (TPSA) of C_1-18

ranges between 70.2 and 116 Å²and is well within the rec-

ommended range (7–200 Å²) (Table 5). The human intestinal

1 3

Chemical Papers

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Chemical Papers

IC₅₀values in the range −4.12–−4.88 for the blockage of

human ether-a-go-go (HERG) K⁺channels (QPlogHERG)

indicating the safety of these molecules. In addition, pre-

dicted number of primary metabolites of compounds C_1-18is

between 1 and 2, which further indicated the safety of these

compounds. All compounds obeyed Lipinski’s rule of fve

with 0 violation. Further, the drug-likeness of compounds

was also assessed according to Jorgensen’s rule of three [38],

and all compounds obeyed this rule with zero violation.

43300 in MBC determination and also eliminated this strain

after 24 h. From the above results, it is evident that chemical

structure of compound C₁₃may be utilized for further devel-

opment of potent antibacterial and antiproliferative agents.

Computed ADMET properties showed favourable pharma-

cokinetic profle of synthesized compounds C_1-18

.

Supplementary Information The online version contains supplemen-

tary material available at https://doi.org/10.1007/s11696-021-01730-8.

Acknowledgements One of the authors would like to thank All India

Council for Technical Education (AICTE) Quality Improvement Pro-

gramme for providing fellowship.

Conclusion

In the present work, we performed hydroxybenzotriazole

(HOBt) and 1-(3-dimethylaminopropyl)-3-ethylcarbodi-

imide hydrochloride (EDCl)-mediated synthesis of new

Declarations

Conflict of interest On behalf of all authors, the corresponding author

states that there is no confict of interest.

N’-(1,3-benzothiazol-2-yl)-substituted benzamides C_1-18

.

High yields (80–95%) were obtained under relatively milder

reaction conditions using dimethylformamide as solvent.

Compound C₁, C₂, C₄, C₁₂and C₁₃synthesized by the N‐het-

erocyclic carbene (NHC) organocatalysed direct oxidative

amidation (Premaletha et al. 2017) resulted in 61–93% yield.

Compound C₁₃synthesized by the coupling of 2-aminoben-

zothiazole and thiophene-2-carbonyl chloride in toluene in

the presence of triethylamine resulted in 29% yield (Sovic

et al. 2015). This compound demonstrated.

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Design, synthesis and biological evaluation of some novel N'-(1,3-benzothiazol-2-yl)-arylamide derivatives as antibacterial agents

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