Novel fluoroquinolone derivatives bearing N-thiomide linkage with 6-substituted-2-aminobenzothiazoles: Synthesis and antibacterial evaluation

1 Introduction

Since the introduction of fluoroquinolones in late 1970s, they have generated great excitement, opportunities and applications in the antibacterial chemotherapeutic world, as these agents potentially offer all the general attributes of ideal antibacterial agents. Over the last 15 years, researchers have attempted and proved these attributes as reality (Andriole, 1988; Sharma and Jain, 2008). Initially, great improvement in potency, spectrum and in vivo efficiency was achieved as a result of intensive structural modifications. Further, the systematic and well defined structure–activity relationships were established which reveal the optimal groups for each position in terms of size, shape and electronic properties. Once the multitude of new quinolones reached advanced toxicology and clinical trials, it was realized that quinolones like all other agents, do show some undesirable side effects in human and the laboratory testings. Their adverse reactions and toxicological profiles have been reviewed extensively (Lv et al., 2012). The major concern has been the growing incidences of resistance especially to Staphylococci and Enterococci. Some of the side effects of quinolone antibacterials are unacceptable, for example, grepafloxacin withdrawn from the market, due to increased cases of heart problems in clinical findings. Similarly trovafloxacin was removed from the market due to liver toxicity (Graul et al., 1999). The current phase of quinolone design is aimed at reducing or eliminating these side effects and overcome the emerging resistance through additional synthetic manipulations while further enhancing or maintaining potency.

Synthesis and evaluation of over 10,000 quinolone derivatives resulted in extensive knowledge of the structure–activity relationship for many quinolone substituents. The most intensive structural variation has been carried out on amines at the 7-position (Foroumadi et al., 2006; Foroumadi et al., 2005), partially due to the ease of their aromatic substitution reactions. Piperazine, aminopyrrolidine and their substituted derivatives have been the most successfully employed side chains, as evidenced by the compounds currently in the market (Sharma et al., 2009; Sharma et al., 2010; Domagala et al., 1988; Chu et al., 1985). Originally, the newer fluoroquinolones arose with the development of 7- piperazinyl quinolones, such as norfloxacin 1, ciprofloxacin 2 and gatifloxacin 3 (Fig. 1). The site near the C-7 substituent is regarded as drug enzyme interaction domain (Foroumadi et al., 2005).

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Figure 1

Some commercially available fluoroquinolones.

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These facts motivated our concern to prepare C-7 substituents of quinolones. The piperazine moiety of fluoroquinolones possesses enough structural flexibility to allow product optimization. Thus, we anticipated that safer and superior antibacterial compounds can be developed by attaching an appropriate moiety through N-atom of the substituents at C-7 position. In this communication, we report the synthesis, characterization and antibacterial activity of a number of N-substituent piperazinyl quinolones by introducing specific substituents in the piperazine unit of 7-piperazinyl quinolones.

Keeping in view the diverse biological activity profile of 2-amino-6-substituted benzothiazoles such as cytotoxic, antibacterial, fungicidal, analgesic, antiinflammatory, antioxidant activities and pharmacological interventions of quinolone derivatives such as antibacterial, antimycobacterial, antimalarial, antioxidant, analgesic and antiviral activities, (Chu et al., 1985; Senthilkumar et al., 2009; Jayashree et al., 2009; El-Gazzar et al., 2009; Dinakaran et al., 2008; Winter et al., 2008; German et al., 2008; Jazayeri et al., 2009) clubbing of these scaffolds was considered to be of significant relevance. Moreover, due to our ongoing endeavors (Sharma et al., 2010, 2011; Kharb et al., 2011, 2011a, b) and research program on the design and synthesis of novel antimicrobial agents, a number of potent fluoroquinolone derivatives(Lv et al., 2012; Sharma and Jain, 2008; Sharma et al., 2011) have been synthesized. Recently, we have reported the synthesis of novel fluoroquinolone derivatives annulated with benzothiazoles with promising antibacterial activity (Sharma et al., 2011a, 2015). It was clearly indicated that clubbing of fluoroquinolone with benzothiazoles is worthwhile and the synthesized compounds were of promising pharmacological significance. Thus, the present study was aimed to achieve better antimicrobial profile at lower concentrations, by preparing N-substituted piperazinyl quinolone derivatives carrying benzothiazolyl substituents.

2 Experimental

2.2

2.2 Chemistry

2.2.1

2.2.1 General procedure for the synthesis of 2-amino-6-substituted benzothiazole [26] (2a–2e)

The 2-amino-6-substituted benzothiazoles (2a–2e) were prepared by mixing p-substituted aniline (0.05 mol) (1a–1e) and potassium thiocyanate (0.2 mol) in 90 ml of 96% acetic acid. To this was added drop wise, with stirring, a solution of bromine (0.05 mol) in glacial acetic acid (37.5 ml) and temperature was maintained below 35 °C. After all the bromine solution was added, the mixture was stirred for another 10 h at room temperature and was filtered, the residue so obtained was washed with water. The combined filtrate and washings were neutralized with ammonium hydroxide solution. The precipitate thus obtained was collected and dried. Further purification was carried out by crystallization from benzene. Adopting the above procedure, five different 2-amino-6-substituted benzothiazoles (2a–2e) have been synthesized. The purity of the benzothiazoles was checked by TLC.

2.2.2

2.2.2 General procedure for the synthesis of fluoroquinolone derivatives (4a–4n)

For the synthesis of target fluoroquinolone derivatives, a mixture of 2-amino-6-substituted benzothiazole (0.05 mmol) (2a–2e), fluoroquinolone (0.05 mmol) (3a–3c) and carbon disulfide (0.05 mmol) in basic medium composed of sodium bicarbonate (NaHCO₃) (0.05 mmol) and N,N-dimethyl formamide (10 ml), was refluxed for 10 h. After complete consumption of fluoroquinolone in the reaction (monitored by thin layer chromatography), the reaction mixture was poured into ice cold water and the precipitate was filtered, washed with water to yield the crude product. The precipitate was collected and dried. Further purification was carried out by crystallization from N,N-dimethyl formamide (DMF). The purity of title compounds was ascertained by thin layer chromatography. Employing the captioned procedure, fourteen novel target compounds (4a–4n) were synthesized. Structural and physicochemical data of synthesized compounds are presented in Table 1.

Table 1 Structural and physicochemical data of target compounds (4a–4n).

Compound	R1	R2	R3	R	mp (°C)	Formula	Yield (%)
4a	–C2H5	–H	–H	–OCH3	247–250	C25H24FN5O4S2	67
4b	–C2H5	–H	–H	–NO2	278–281	C24H21FN6O5S2	60
4c	–C2H5	–H	–H	–CH3	254–257	C25H24FN5O3S2	58
4d	–C2H5	–H	–H	–F	245–248	C24H21F2N5O3S2	72
4e	–C2H5	–H	–H	–Br	256–259	C24H21BrFN5O3S2	69
4f		–H	–H	–OCH3	326–329	C26H24FN5O4S2	66
4g		–H	–H	–NO2	332–335	C25H21FN6O5S2	64
4h		–H	–H	–CH3	319–322	C26H24FN5O3S2	59
4i		–H	–H	–F	329–331	C25H21F2N5O3S2	64
4j		–H	–H	–Br	327–330	C25H21BrFN5O3S2	63
4k		–OCH3	–CH3	–OCH3	229–232	C28H28FN5O5S2	68
4l		–OCH3	–CH3	–NO2	199–202	C27H25FN6O6S2	63
4m		–OCH3	–CH3	–CH3	197–200	C28H28FN5O4S2	63
4n		–OCH3	–CH3	–F	211–214	C27H25F2N5O4S2	64

2.2.2.1

2.2.2.1 1-Ethyl-6-fluoro-7-(4-(N-(6-methoxy-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4a)

M.p. 247–250 °C (67% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.38 (3H, J = 6.7, t, CH₃ ethyl), 2.49 (2H, J = 6.7, q, CH₂ ethyl), 3.02–3.76 (8H, m, piperazine), 3.93 (3H, s, OCH₃ benzothiazole), 6.76–7.93 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.90 (1H, s, H₂-quinolone), 9.36 (1H, s, NH), 15.14 (1H, s, COOH); IR (KBr): cm⁻¹ = 1585 (C = C), 1705 (C = O), 1211 (C–O), 1157 (C–N), 3186–2854 (C–H), 3302 (COOH); MS: m/z = 541.6 (M⁺).

2.2.2.2

2.2.2.2 1-Ethyl-6-fluoro-7-(4-(N-(6-nitro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4b)

M.p. 278–281 °C (60% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.39 (3H, J = 7.2, t, CH₃ ethyl), 2.19 (2H, J = 7.2, q, CH₂ ethyl), 3.29–3.64 (8H, m, piperazine), 7.18–8.15 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.63 (1H, s, H₂-quinolone), 9.35 (1H, s, NH), 15.24 (1H, s, COOH); IR (KBr): cm⁻¹ = 1586 (C = C), 1718 (C = O), 1258 (C–O), 1052 (C–N), 2932 (C–H), 3415 (COOH); MS: m/z = 556.6 (M⁺).

2.2.2.3

2.2.2.3 1-Ethyl-6-fluoro-7-(4-(N-(6-methyl-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4c)

M.p. 254–257 °C (58% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.41 (3H, J = 6.6, t, CH₃ ethyl), 2.29–2.49 (2H, J = 6.6, q, CH₂ ethyl), 3.07–3.59 (8H, m, piperazine), 2.89 (3H, s, CH₃ benzothiazole), 6.98–7.93 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.10 (1H, s, H₂ quinolone), 8.90 (1H, s, NH), 15.19 (1H, s, COOH); IR (KBr): cm⁻¹ = 1583 (C = C), 1717 (C = O), 1262 (C-O), 1098 (C-N), 2922 (C-H), 3415 (COOH).

2.2.2.4

2.2.2.4 1-Ethyl-6-fluoro-7-(4-(N-(6-fluoro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4d)

M.p. 245–248 °C (72% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.37 (3H, J = 6.8, t, CH₃ ethyl), 1.89 (2H, J = 6.8, q, CH₂ ethyl), 3.31–4.28 (8H, m, piperazine), 6.97–8.10 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.89 (1H, s, H₂-quinolone), 9.35 (1H, s, NH), 15.02 (1H, s, COOH); IR (KBr): cm⁻¹ = 1586 (C = C), 1715 (C = O), 1255 (C–O), 1062 (C–N), 2933 (C–H), 3423 (COOH).

2.2.2.5

2.2.2.5 7-(4-(N-(6-bromo-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1-ethyl-6-fluoro-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4e)

M.p. 256–259 °C (69% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.38 (3H, J = 6.2, t, CH₃ ethyl), 1.89 (2H, J = 6.2, q, CH₂ ethyl), 2.95–3.93 (8H, m, piperazine), 7.17–8.10 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.92 (1H, s, H₂-quinolone), 9.56 (1H, s, NH), 15.26 (1H, s, COOH); IR (KBr): cm⁻¹ = 1585 (C = C), 1715 (C = O), 1227 (C–O), 1050 (C–N), 2932 (C–H), 3409 (COOH).

2.2.2.6

2.2.2.6 1-Cyclopropyl-6-fluoro-7-(4-(N-(6-methoxy-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4f)

M.p. 326–329 °C (66% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.32 (4H, d, CH₂-CH₂- Cyclopropyl), 3.25 (1H, m, CH Cyclopropyl), 3.30–3.61 (8H, m, piperazine), 3.84 (3H, s, OCH₃ benzothiazole), 7.43–8.11 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.60 (1H, s, H₂-quinolone), 9.64 (1H, s, NH), 15.19 (1H, s, COOH); IR (KBr): cm⁻¹ = 1581 (C = C), 1718 (C = O), 1257 (C-O), 1173 (C-N), 2924–2839 (C-H), 3294 (COOH).

2.2.2.7

2.2.2.7 1-Cyclopropyl-6-fluoro-7-(4-(N-(6-nitro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4g)

M.p. 332–335 °C (64% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.34 (4H, d, CH₂-CH₂- Cyclopropyl), 2.90 (1H, m, CH Cyclopropyl), 3.25–3.63 (8H, m, piperazine), 7.40–8.12 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.23 (1H, s, H₂-quinolone), 9.09 (1H, s, NH), 14.87 (1H, s, COOH); IR (KBr): cm⁻¹ = 1584 (C = C), 1715 (C = O), 1258 (C–O), 1062 (C-N), 2936 (C-H), 3448 (COOH); MS: m/z = 568.6 (M⁺).

2.2.2.8

2.2.2.8 1-Cyclopropyl-6-fluoro-7-(4-(N-(6-methyl-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4h)

M.p. 319–322 °C (59% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.18 (4H, d, CH₂–CH₂–Cyclopropyl), 2.87 (1H, m, CH Cyclopropyl), 3.33–3.80 (8H, m, piperazine), 2.49 (3H, s, CH₃ benzothiazole), 7.38–8.21 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.65 (1H, s, H₂-quinolone), 9.34 (1H, s, NH), 15.12 (1H, s, COOH); IR (KBr): cm⁻¹ = 1584 (C = C), 1718 (C = O), 1265 (C–O), 1203 (C–N), 3094–2839 (C–H), 3294 (COOH).

2.2.2.9

2.2.2.9 1-Cyclopropyl-6-fluoro-7-(4-(N-(6-fluoro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4i)

M.p. 329–331 °C (64% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.17 (4H, d, CH₂–CH₂–Cyclopropyl), 3.12 (1H, m, CH Cyclopropyl), 3.20–3.95 (8H, m, piperazine), 7.00–8.11 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.64 (1H, s, H₂-quinolone), 9.29 (1H, s, NH), 15.09 (1H, s, COOH); IR (KBr): cm⁻¹ = 1583 (C = C), 1717 (C = O), 1260 (C–O), 1057 (C–N), 2909 (C–H), 3416 (COOH).

2.2.2.10

2.2.2.10 7-(4-(N-(6-bromo-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4j)

M.p. 327–330 °C (63% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.16 (4H, d, CH₂–CH₂–Cyclopropyl), 3.02 (1H, m, CH Cyclopropyl), 3.28–3.80 (8H, m, piperazine), 7.19–8.11 {5H, m, Aromatic proton (H₅, H₈-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.60 (1H, s, H₂-quinolone), 9.64 (1H, s, NH), 15.22 (1H, s, COOH); IR (KBr): cm⁻¹ = 1585 (C = C), 1717 (C = O), 1260 (C–O), 1061 (C–N), 2917 (C–H), 3448 (COOH); MS: m/z = 602.5 (M⁺).

2.2.2.11

2.2.2.11 1-Cyclopropyl-6-fluoro-8-methoxy-7-(4-(N-(6-methoxy-1,3-benzothiazol-2-yl)amino-1-thio-methyl)-3-methylpiperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4k)

M.p. 229–232 °C (68% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.27 (4H, d, CH₂–CH₂–Cyclopropyl), 2.82 (1H, m, CH Cyclopropyl), 3.26–3.66 (7H, m, piperazine), 3.87 (3H, s, OCH₃) 2.12–2.56 (3H, m, CH₃ piperazine), 6.90–8.28 {4H, m, Aromatic proton (H₅-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.73 (1H, s, H₂-quinolone), 9.03 (1H, s, NH), 15.06 (1H, s, COOH); IR (KBr): cm⁻¹ = 1585 (C = C), 1717 (C = O), 1254 (C–O), 1073 (C–N), 2917 (C–H), 3448 (COOH).

2.2.2.12

2.2.2.12 1-Cyclopropyl-6-fluoro-8-methoxy-7-(3-methyl-4-(N-(6-nitro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4l)

M.p. 199–202 °C (63% yield). ¹H NMR (DMSO-d₆): δ/ppm = 0.93 (4H, d, CH₂–CH₂–Cyclopropyl), 3.15 (1H, m, CH Cyclopropyl), 3.28–3.85 (7H, m, piperazine), 2.18–2.65 (3H, m, CH₃ piperazine), 3.96 (3H, s, OCH₃), 7.16–8.00 {4H, m, Aromatic proton (H₅-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.44 (1H, s, H₂-quinolone), 9.76 (1H, s, NH), 15.35 (1H, s, COOH); IR (KBr): cm⁻¹ = 1586 (C = C), 1718 (C = O), 1249 (C–O), 1064 (C–N), 2947 (C–H), 3421 (COOH).

2.2.2.13

2.2.2.13 1-Cyclopropyl-6-fluoro-8-methoxy-7-(4-(N-(6-methyl-1,3-benzothiazol-2-yl)amino-1-thio-methyl)-3-methylpiperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4m)

M.p. 197–200 °C (63% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.22 (4H, d, CH₂–CH₂–Cyclopropyl), 2.92 (1H, m, CH Cyclopropyl), 3.04–3.76 (7H, m, piperazine), 2.19–2.76 (3H, m, CH₃ piperazine), 3.88 (3H, s, OCH₃), 7.02–8.20 {4H, m, Aromatic proton (H₅-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.78 (1H, s, H₂-quinolone), 9.33 (1H, s, NH), 15.18 (1H, s, COOH); IR (KBr): cm⁻¹ = 1587 (C = C), 1715 (C = O), 1268 (C–O), 1109 (C–N), 2917 (C–H), 3413 (COOH); MS: m/z = 581.7 (M⁺).

2.2.2.14

2.2.2.14 1-Cyclopropyl-6-fluoro-7-(4-(N-(6-fluoro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)-3-methylpiperazin-1-yl)-8-methoxy-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid (4n)

M.p. 211–214 °C (64% yield). ¹H NMR (DMSO-d₆): δ/ppm = 1.32 (4H, d, CH₂–CH₂–Cyclopropyl), 3.15 (1H, m, CH Cyclopropyl), 3.37–3.81 (7H, m, piperazine), 2.80–2.98 (3H, m, CH₃ piperazine), 3.94 (3H, s, OCH₃), 6.98–7.94 {4H, m, Aromatic proton (H₅-quinolone and H_4', H_5', H_7'-benzothiazole)}, 8.67 (1H, s, H₂-quinolone), 9.28 (1H, s, NH), 15.17 (1H, s, COOH); IR (KBr): cm⁻¹ = 1583 (C = C), 1705 (C = O), 1265 (C–O), 1157 (C–N), 3194–2854 (C–H), 3302 (COOH); MS: m/z = 585.6 (M⁺).

3 Results and discussion

The synthetic route of desired compounds is presented in Scheme 1. The requisite 2-amino-6-substituted benzothiazoles (2a–2e) were prepared according to the method described by Stuckwisch (1949). Reaction between 2-amino-6-substituted benzothiazole, different fluoroquinolones and carbon disulfide in the presence of basic medium composed of sodium bicarbonate and N,N-dimethyl formamide, afforded the target compounds after a refluxing time of 10 h. The fluoroquinolone derivatives (4a–4n) were found in 58–72% yields. TLC analysis was done to confirm the purity of the compound. Analytical and spectral data (IR and ¹H NMR) of all the newly synthesized compounds were found in full agreement with the proposed structures. In the ¹H NMR spectra the signals of the respective proton of the compounds were verified on the basis of their chemical shifts, multiplicities. The spectra showed a characteristic singlet of one proton in DMSO-d₆ at δ 8.8–9.4 ppm corresponding to NH group of thiomide linkage, proton at the second position of quinolone ring showed a characteristic singlet at a shift at δ 8.6–8.9 ppm and a broad singlet at δ 14.7–15.3 ppm of COOH group. The IR spectra of compounds revealed C = O, C = C, stretching at approximately 1710, 1600 cm⁻¹, whereas in general C–O, O–H and C–H bands were revealed at nearby 1300, 3400, 2900 and 3000. It was found interesting that only two compounds among all were in lesser yield than 60% whereas all other compounds are in good yields. Compound 1-ethyl-6-fluoro-7-(4-(N-(6-fluoro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid was afforded in most effective yield of 72%. Melting range of various compounds of this series is found to be higher than 200 °C. Compound 4g showed the maximum melting temperature among all synthesized compounds which is 332–335 °C.

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Scheme 1

Synthetic route of target compounds.

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All the synthesized fluoroquinolone derivatives were screened for antibacterial activities against two Gram positive bacterial strains i.e. S. auerus (NCDC 110) and Bacillus subtilis (NCDC 71) and two Gram negative strains i.e. E. coli (NCDC 134) and P. aeruginosa (NCDC 105). The antibacterial activity was determined by measuring the diameter of the zone of inhibition and minimum inhibitory concentration (MIC). The results of zone of inhibition and MIC screening are summarized in Tables 2 and 3 respectively.

These novel derivatives demonstrated varying antibacterial activity (zone of inhibition) against different strains. Compound 4j possessing bromo functionality at position-6 of benzothiazole nucleus, depicted significant zone of inhibition of 11.3, 12.8 and 15.0 mm at 10, 50 and 100 μg/ml concentrations respectively against S. auerus. Compounds bearing N-cyclopropyl functionality i.e. 4g, 4h, 4i and 4n also found to possess significantly potent antibacterial activities against S. auerus and the zone of inhibition depicted by ciprofloxacin analogs 4g, 4i and 4j was even better than the standard antibiotics ciprofloxacin, norfloxacin, and gatifloxacin when tested against S. auerus. While compound 1-cyclopropyl-6-fluoro-7-(4-(N-(6-fluoro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)-3-methylpiperazin-1-yl)-8-methoxy-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid showed moderate activity. Compounds 4g, substituted with the NO₂ group at position-6 of benzothiazolyl moiety annulated with ciprofloxacin molecule, depicted zone of inhibition of 7.3 mm at concentration 10 μg/ml against Gram positive Bacillus subtilis bacterial strain exhibit even better activity than the standard drugs norfloxacin (5.0 mm) and gatifloxacin (6.5 mm). Compounds 4d, 4h and 4j exhibited good antibacterial activities against Bacillus subtilis while 4g, 4h and 4j showed moderate activity and the activities shown by 4h and 4j are significantly potent having a zone of inhibition of 10.3 and 7.8 mm respectively at concentration 10 μg/ml against the standard antibiotics norfloxacin (4.2 mm) and gatifloxacin (6.0 mm). Analogs 4h and 4j showed even better antibacterial activities against the standard antibiotics ciprofloxacin, norfloxacin and gatifloxacin when tested against E. coli and 1-cyclopropyl-6-fluoro-7-(4-(N-(6-nitro-1,3-benzothiazol-2-yl)amino-1-thio-methyl)piperazin-1-yl)-1,4-dihydro-4-oxo-quinoline-3-carboxylic acid showed comparable activities at higher concentration (50 μg/ml and 100 μg/ml). Compounds 4b, 4e, 4f, 4g, 4h, 4j, 4l and 4m showed improved antibacterial activities against the standard antibiotics norfloxacin and gatifloxacin when tested against P. aeruginosa.

When tested against S. auerus compounds having nitro substitution at position-6 of benzothiazole clubbed with norfloxacin (4b) and gatifloxacin (4l) showed MIC 05 μg/ml, which is 10 times more potent as compared to the standard drug ciprofloxacin (MIC value 50 μg/ml). While all the norfloxacin and ciprofloxacin derivatives showed comparable activity. Compounds 4a, 4c, 4e, 4h, 4i and 4j showed significant MIC against Bacillus subtilis when compared with standards. Moreover, compounds 4d, 4f and 4l showed superior MIC (15, 10, 15 μg/ml respectively) against Bacillus subtilis than the standard antibiotics ciprofloxacin (MIC value 20 μg/ml) and gatifloxacin (MIC value 100 μg/ml). Analogs 4g, 4i, 4l and 4n showed better MIC against the standard antibiotics ciprofloxacin, norfloxacin and gatifloxacin when tested against E. coli. Compounds 4c, 4d, 4e, 4g, 4h, 4j, 4l, 4m and 4n showed comparable MIC against the standard antibiotics norfloxacin and gatifloxacin when tested against P. aeruginosa.

4 Conclusion

In conclusion, we have synthesized fourteen novel derivatives of three fluoroquinolone drugs i.e. norfloxacin, ciprofloxacin and gatifloxacin. These drugs have been linked to different 2-amino-6-substituted benzothiazoles via thiomide bridge linkage. The derivatives are characterized by physicochemical and spectral analyses such as ¹H NMR and IR. The spectral data obtained were in full agreement with the proposed structures. The in vitro evaluation of newly synthesized compounds revealed improved therapeutic effectiveness as compared to the parent drugs. Some derivatives showed more potent or equipotent antibacterial activities against different strains (S. auerus, B. subtilis, E. coli, P. aeruginosa). The experimental data of this investigation reveal that the synthesized novel derivatives of fluoroquinolones have remarkable antibacterial potential.