Novel benzothiazole containing 4H-pyrimido[2,1-b]benzothiazoles derivatives: One pot, solvent-free microwave assisted synthesis and their biological evaluation

Manoj N. Bhoi; Mayuri A. Borad; Edwin A. Pithawala; Hitesh D. Patel

doi:10.1016/j.arabjc.2016.01.012

View/Download PDF

Buy Reprints

PDF

Translate this page into:

Original article

12 (

); 3799-3813

doi:

10.1016/j.arabjc.2016.01.012

Novel benzothiazole containing 4H-pyrimido[2,1-b]benzothiazoles derivatives: One pot, solvent-free microwave assisted synthesis and their biological evaluation

Manoj N. Bhoi^{^a,⁎}, Mayuri A. Borad^{^a}, Edwin A. Pithawala^{^b}, Hitesh D. Patel^{^a,⁎}

a Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad, India

b Department of Life Sciences, School of Sciences, Gujarat University, Ahmedabad, India

⁎Corresponding authors at: Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad, Gujarat, India. Tel.: +91 079 26300969; fax: +91 079 26308545. manoj18bhoi@gmail.com (Manoj N. Bhoi), hitesh13chem@rediffmail.com (Hitesh D. Patel)

Received: 2015-11-26, Accepted: 2016-1-28, Published: 2016-12-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Abstract

Synthesis of new and desired compounds has an everlasting demand. The present work emphasizes on the one pot, three component microwave assisted synthesis of novel ethyl 2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate derivatives by the reaction of 2-aminobenzothiazole derivatives with Pyridine 2-aldehyde and Ethyl acetoacetate in the presence of PdCl₂ as an expeditious catalyst under solvent-free condition. The salient features of this approach are operational simplicity, convergence, short reaction time, high atom economy, easy workup, mild reaction conditions and environmentally benign conditions. All the newly synthesized diverse poly-functionalized tri-heterocyclic benzothiazole derivatives have been characterized by elemental analysis and various spectroscopic methods such as FT-IR, ¹H NMR, ¹³C NMR, ESI-MS and Single crystal X-ray analysis (4a). All the final scaffolds have been screened for antibacterial and antioxidant activities. Also their antitubercular activity against Mycobacterium tuberculosis H₃₇RV was screened.

Keywords

Antioxidant

2-Amino benzothiazole

Microwave irradiation

4H-pyrimido[2,1-b]benzothiazoles

Solvent-free reaction

Mycobacterium tuberculosis

Show Related Articles from PubMed

1 Introduction

Search on efficient organic synthesis approach that substantiates the rapid generation of complex organic molecules from simple and readily accessible starting materials is in vogue during recent epochs and the interests have been engrossed a current position (Chow and Shea, 2005; de Graaff et al., 2012; Jiang et al., 2010; Shiri, 2012 ). The important medicinal compounds, which are synthesized through steadfast reactions are receiving more fascinating attention. Literature shows that scientists have also eventually facilitated the rapid development of new chemical entities which are available for biological evaluation.

Non-traditional conditions and their approach to implement new synthetic organic reactions have gained popularity, primarily to circumvent growing environmental concerns (Pillai et al., 2002; Raghavendra et al., 2007). Microwave assisted reaction is an invaluable technology for synthesis of organic compounds with respect to time as it decelerates reaction time with enhanced yields and selectivity. Microwave reaction under solvent free conditions is consensus in offering reduced pollution with ease in processing and handling (Bejan et al., 2012; de la Hoz et al., 2005; Desai et al., 2006; Hayes, 2002, 2004; Kappe, 2000, 2004, 2008; Kappe et al., 2012; Khabazzadeh et al., 2008; Perreux and Loupy, 2002; Stadler et al., 2002, 2003 ).

The chemistry of fused heterocycles is known to fascinate field of investigation in therapeutic chemistry, since they have been found to display improved biological activity. Heterocyclic compounds show significant role in both medicinal chemistry and their constitution in natural products. Among them, benzothiazole derivatives have received substantial attention in synthetic organic chemistry with numerous significant and valuable applications in the pharmaceutical industry. From the current literature survey, benzothiazole derivatives have shown various biological activities (Bhoi et al., 2015a, 2015b, 2014; Boer and Gekeler, 1995; Borad et al., 2015; Bretzel et al., 1993; Hutchinson et al., 2001; Klusa, 1995; Pande et al., 1982; Singh et al., 1975; Singh and Vaid, 1986; Huang and Yang, 2006; Jiang et al., 2009, 2011; Wu et al., 2014a, 2014b; Yang et al., 2013; Zuo et al., 2012 ) used in material science and in other industrial applications (Choudhary et al., 2013; Hopper, 1993; Zhang et al., 2001 ). The important and pharmacological properties of benzothiazole derivatives along with their improved synthetic methods allow a facile access to these heterocyclic compounds. Recently, many efforts have been dedicated to improve new and highly effective synthetic protocols such as [3 + 2] cycloadditions, transition metal catalyzed cyclization and multicomponent coupling reactions for the synthesis of derivatives of benzothiazole (Bastug et al., 2012; Jaseer et al., 2010; Karle et al., 2012; Noolvi et al., 2012; Sahu et al., 2012a; Tale, 2002; Trapani et al., 1996; Xue et al., 2013 ). Typically, multi-component reactions (MCRs) are an influential tool which attract much more focus in synthetic organic reactions owing to the complex molecules with varied range of complexity where the starting materials are easily available (Benetti et al., 1995; Dömling, 2006; Kappe, 2002; Langer, 2001; Nair et al., 2003; Ramón and Yus, 2005; Simon et al., 2004 ). Thus, MCRs are influenced by microwave and solvent free condition as a powerful green alternative over the conventional synthesis (Devineni et al., 2013; Eynde et al., 2001; Kappe, 2000, 2002; Kappe and Stadler, 2004; Leadbeater et al., 2006; Oliver Kappe, 1993; Saxena and Chandra, 2011 ).

The Biginelli reaction is recognized multicomponent reaction for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles and associated polyheterocycles (Chebanov et al., 2010; Ranu et al., 2000; Sedash et al., 2012 ). Since 1893, a one pot, three-component condensation reaction between aldehyde, β-ketoesters and urea was first time reported by Biginelli (Biginelli and Gazz, 1893) to produce 3,4-dihydropyrimidin-2-(1H)one. Compound 2-amino benzothioazoles and 2-aminobenzimidazoles derivatives are used in place of urea (Alajarin et al., 1995; Shaabani et al., 2005). The Biginelli reaction can be carried out by heating as well as by acid and base catalysts. Recently, catalysts such as FeF₃ (Atar and Jeong, 2014), Kaolin (Sahu et al., 2013), N,N-dichlorobis(2,4,6-trichlorophenyl)urea (Rao et al., 2011), hydrotalcite (Sahu et al., 2012b), TBAHS (Nagarapu et al., 2013), AlCl₃ (Sahu et al., 2012c), and Zn(ClO₄)₂·6H₂O(Kaur et al., 2015; Singh et al., 2013) have shown to be effective for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles. Although these methods afford good results, there is yet a great demand for quick and eco-friendly catalytic MCRs conditions. Our aim is to find out the best environmental friendly catalytic system for a one-pot reaction through microwave assisted synthesis under solvent free condition.

Encouraged by the auspicious biological activity of the structurally diverse heterocycles with fused heterocyclic systems and our continuing research plan on the synthesis of therapeutically interesting heterocycles, we have developed new structural motifs with promising bioactivity. An efficient, one pot three component synthesis of ethyl 2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate derivatives by reaction between various derivatives of 2-amino-benzothiazole with Pyridine 2-aldehyde and Ethyl acetoacetate using PdCl₂ as an efficient catalyst under microwave assisted solvent-free conditions, with better yield and short reaction time is introduced to develop new structural motifs with promising bioactivity. We have evaluated their antibacterial activity against gram +ve and gram −ve bacterial strains and antitubercular activity against Mycobacterium tuberculosis H₃₇RV and also screened antioxidant activity.

2 Material and methods

2.1

2.1 Chemistry

All chemicals and solvents were acquired from commercial sources (Aldrich-Sigma and Merck chemical suppliers). The reactions were scrutinized by thin layer chromatography (TLC) and judged by the consumption of starting material. TLC was performed on silica gel G 60 F₂₅₄ (Merck) plate. It was visualized by UV radiation, exposure to iodine vapor and spray reagent also. Column chromatography was conducted over silica gel 60 (60–120 μm). CEM Discover microwave system (model No.: 908010; make up CEM Matthews. Inc, USA) was used for synthesis with external IR temperature control. The melting points recorded on optimelt automated melting point system were uncorrected. IR spectra were recorded on a Perkin–Elmer 377 spectrophotometer in KBr with absorption in cm⁻¹. ¹H NMR and ¹³C NMR spectra were recorded on Bruker AV 400 and 100 MHz using DMSO-d₆ as solvent and TMS as an internal standard. Mass spectra were recorded on Advion Expression CMS, USA, using Methanol:Water:Formic acid (80:20:0.1) as mobile phase. Elemental analysis was performed on vario MICRO cube, elementar CHNS analyser serial No.: 15084053. Single crystal X-ray diffraction intensity data of the compound 4a were collected on an Oxford XCALIBUR-S CCD diffractometer with Cu Kα radiation (λ = 1.54184 Å) at 195(100) K.

2.2

2.2 Synthesis of ethyl-(substituted)-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate (4a–4l)

2.2.1

2.2.1 Method A (microwave assisted method)

Mixture of pyridine 2-aldehydes 1 (4.668 mmol), β-diketone like Ethyl acetoacetate 2 (4.668 mmol) and various derivatives of 2-amino benzothiazole 3a–3l (4.668 mmol) in 10 ml microwave open vessel glass tube was placed in the cavity of a microwave oven (CEM Discover microwave) and irradiated for 2–5 min at 90 °C under solvent-free conditions by using PdCl₂ (10 mol%) as a catalyst with continuous monitoring of the reaction by TLC (30% Ethyl acetate: hexane). After completion of the reaction, the reaction mixture was cooled to room temperature and then poured into ice-water by solubilizing in ethanol solvent. The precipitated solid was filtered, and subjected to column chromatography using 20% (v/v) Ethyl acetate : hexane mixture as an eluent to afford ethyl-(substituted)-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5] thiazolo[3,2-a]pyrimidine-3-carboxylate (4a–4l) as pure product with 77–88% yield.

2.2.2

2.2.2 Method B (conventional method)

As mentioned in above method same chemical ratio was taken in a 25 ml round bottom flask. Resulting mixture was placed in oil bath for 20–70 min at 90 °C under solvent-free conditions using PdCl₂ (10 mol%) as a catalyst. The time taken by different derivatives of 2-amino benzothiazole in the reaction is shown in Table 3. After confirmation of each reaction using TLC (30% Ethyl acetate: hexane), the reaction mixture was allowed to cool at room temperature and poured in cold water after solubilizing in ethanol solvent with a requisite volume. The mixture was filtered continuously giving a wash of water. The solid crude product was simply purified by column chromatography over silica gel using solvent system (20% ethyl acetate: hexane) as an eluent to obtain subsequent pure product (4a–4l) with 69–80% yield.

2.2.3

2.2.3 Ethyl-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4a)

Yellow solid, mp 175–176 °C; Anal. Calcd for C₁₉H₁₇N₃O₂S: C, 64.94; H, 4.88; N, 11.96; O, 9.11; S, 9.12%; found C, 64.81; H, 4.75; N, 11.79; S, 8.95%; IR (KBr) (υ_max, cm⁻¹): 3072 (C—H_str), 2943 (C—H_str), 1745 (C⚌O_str), 1677 (C⚌C_str), 1688 (C⚌N_str), 1352 (C—N_str), 1240 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.59 (1H, dd, J = 7.5, 1.3 Hz, C₆—H), 7.74 (2H, dtd, J = 9.3, 7.5, 1.6 Hz, C_8&9—H), 7.42–7.08 (2H, m, C_7&4—H), 7.04 (1H, td, J = 7.5, 1.4 Hz, C₂—H), 6.73 (2H, dd, J = 11.8, 4.5 Hz, C_1&3—H), 6.11 (1H, s, C₅—H), 4.25 (2H, q, J = 5.9 Hz, —CH₂), 2.23 (3H, s, —CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.22, 146.82, 141.40, 139.47, 127.91, 127.44, 127.15, 124.57, 121.61, 114.02, 107.69, 61.50, 55.90, 18.12, 14.70; ESI-MS: m/z calculated 351.42, found [M+H]⁺ 352.4.

2.2.4

2.2.4 Ethyl-8-bromo-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4b)

Yellow solid, mp 165 °C; Anal. Calcd for C₁₉H₁₆BrN₃O₂S: C, 53.03; H, 3.75; Br, 18.57; N, 9.76; O, 7.44; S, 7.45%; found C, 53.22; H, 3.55; N, 9.58; S, 7.66%; IR (KBr) (υ_max, cm⁻¹): 3042 (C—H_str), 2933 (C—H_str), 1749 (C⚌O_str), 1656 (C⚌C_str), 1675 (C⚌N_str), 1277 (C—N_str), 1170 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.67 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.77 (1H, td, J = 7.4, 1.4 Hz, C₈—H), 7.72–7.62 (2H, m, C_9&4—H), 7.33 (1H, td, J = 7.4, 1.5 Hz, C₇—H), 7.19 (1H, dd, J = 7.5, 1.6 Hz, C₂—H), 6.47 (1H, d, J = 7.5 Hz, C₁—H), 5.74 (1H, s, C₅—H), 4.25 (2H, q, J = 5.9 Hz, —CH₂), 2.21 (3H, s, —CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.22, 146.82, 143.49, 139.47, 128.82, 127.91, 126.10, 124.22, 123.54, 121.56, 115.45, 107.69, 61.50, 55.90, 18.12, 14.70; ESI-MS: m/z calculated 430.32, found [M+H]⁺ 431.3.

2.2.5

2.2.5 Ethyl-2,8-dimethyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4c)

Yellow solid, mp 148 °C; Anal. Calcd for C₂₀H₁₉N₃O₂S: C, 65.73; H, 5.24; N, 11.50; O, 8.76; S, 8.77%; found C, 65.55; H, 5.41; N, 11.69; S, 8.56%; IR (KBr) (υ_max, cm⁻¹): 3046 (C—H_str), 2946 (C—H_str), 1741 (C⚌O_str), 1651 (C⚌C_str), 1610 (C⚌N_str), 1343 (C—N_str), 1230 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.60 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.74 (2H, dtd, J = 9.1, 7.5, 1.5 Hz, C_8&9—H), 7.44–7.28 (2H, m, C_4&7—H), 6.89 (1H, dd, J = 7.5, 1.4 Hz, C₂—H), 6.56 (1H, d, J = 7.5 Hz, C₁—H), 6.08 (1H, s, C₅—H), 4.25 (2H, q, J = 5.9 Hz, —CH₂), 2.34 (3H, s, —CH₃), 2.23 (3H, s, C₃—CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.22, 146.82, 140.75, 139.47, 135.27, 128.24, 127.91, 124.00, 122.43, 121.56, 112.72, 107.69, 61.50, 55.90, 21.21, 18.12, 14.70; ESI-MS: m/z calculated 365.45, found [M+H]⁺ 366.4.

2.2.6

2.2.6 Ethyl-2,6-dimethyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4d)

Off white solid, mp 260 °C; Anal. Calcd for C₂₀H₁₉N₃O₂S: C, 65.73; H, 5.24; N, 11.50; O, 8.76; S, 8.77%; found C, 65.48; H, 5.14; N, 11.35; S, 8.55%; IR (KBr) (υ_max, cm⁻¹): 3089 (C—H_str), 2923 (C—H_str), 1744 (C⚌O_str), 1625 (C⚌C_str), 1593 (C⚌N_str), 1289 (C—N_str), 1249 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.61 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.71 (2H, dtd, J = 8.9, 7.5, 1.4 Hz, C_8&9—H), 7.43–7.22 (2H, m, C_4&7—H), 6.88 (1H, dd, J = 7.5, 1.4 Hz, C₂—H), 6.67 (1H, t, J = 7.5 Hz, C₃—H), 5.94 (1H, s, C₅—H), 4.24 (2H, q, J = 5.9 Hz, —CH₂), 2.38 (3H, s, C₁-CH₃), 2.11 (3H, s, —CH₃), 1.37 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.11, 160.08, 157.22, 146.82, 139.50, 130.19, 128.61, 127.91, 127.02, 121.56, 118.62, 118.08, 107.69, 61.5, 55.58, 18.07, 14.70; ESI-MS: m/z calculated 365.45, found [M+H]⁺ 366.4.

2.2.7

2.2.7 Ethyl-2-methyl-8-nitro-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4e)

Yellow solid, mp 174–178 °C; Anal. Calcd for C₁₉H₁₆N₄O₄S: C, 57.57; H, 4.07; N, 14.13; O, 16.14; S, 8.09%; found C, 57.37; H, 4.27; N, 14.31; S, 8.22%; IR (KBr) (υ_max, cm⁻¹): 3050 (C—H_str), 2933 (C—H_str), 1737 (C⚌O_str), 1632 (C⚌C_str), 1597 (C⚌N_str), 1313 (C—N_str), 1256 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.61 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 8.41 (1H, d, J = 1.4 Hz, C₄—H), 7.97 (1H, dd, J = 7.5, 1.4 Hz, C₂—H), 7.77 (1H, td, J = 7.5, 1.4 Hz, C₈—H), 7.59 (1H, dd, J = 7.5, 1.6 Hz, C₉—H), 7.33 (1H, td, J = 7.5, 1.4 Hz, C₇—H), 6.85 (1H, d, J = 7.5 Hz, C₁—H), 5.57 (1H, s, C₅—H), 4.23 (2H, q, J = 5.9 Hz, —CH₂), 2.18 (3H, s, —CH₃), 1.37 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.22, 147.15, 146.82, 145.82, 139.47, 133.29, 127.91, 123.63, 121.56, 115.53, 112.10, 107.69, 61.50, 55.90, 18.12, 14.70; ESI-MS: m/z calculated 396.42, found [M+H]⁺ 397.4.

2.2.8

2.2.8 Ethyl-8-chloro-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4f)

Fluorescent Yellow solid, mp 177–178 °C; Anal. Calcd for C₁₉H₁₆ClN₃O₂S: C, 59.14; H, 4.18; Cl, 9.19; N, 10.89; O, 8.29; S, 8.31%; found C, 59.32; H, 4.29; N, 10.65; S, 8.13%; IR (KBr) (υ_max, cm⁻¹): 3075 (C—H_str), 2831 (C—H_str), 1741 (C⚌O_str), 1642 (C⚌C_str), 1610 (C⚌N_str), 1329 (C—N_str), 1287 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.67 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.77 (1H, td, J = 7.4, 1.4 Hz, C₈—H), 7.68 (1H, dd, J = 7.5, 1.4 Hz, C₉—H), 7.49 (1H, d, J = 1.6 Hz, C₄—H), 7.33 (1H, td, J = 7.4, 1.5 Hz, C₇—H), 7.03 (1H, dd, J = 7.5, 1.4 Hz, C₂—H), 6.52 (1H, d, J = 7.5 Hz, C₂—H), 5.75 (1H, s, C₅—H), 4.25 (2H, q, J = 5.9 Hz, —CH₂), 2.21 (3H, s, —CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.22, 146.82, 141.00, 139.47, 132.27, 129.15, 127.91, 127.13, 121.56, 120.29, 117.30, 107.69, 61.50, 55.90, 18.12, 14.70; ESI-MS: m/z calculated 385.87, found [M+H]⁺ 386.8.

2.2.9

2.2.9 Ethyl-6-chloro-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4g)

Pink solid, mp 195 °C; Anal. Calcd for C₁₉H₁₆ClN₃O₂S: C, 59.14; H, 4.18; Cl, 9.19; N, 10.89; O, 8.29; S, 8.31%; found C, 59.33; H, 4.39; N, 10.65; S, 8.24%; IR (KBr) (υ_max, cm⁻¹): 3012 (C—H_str), 2846 (C—H_str), 1740 (C⚌O_str), 1643 (C⚌C_str), 1669 (C⚌N_str), 1290 (C—N_str), 1283 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.70 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.72 (2H, dtd, J = 9.1, 7.5, 1.5 Hz, C_8&9—H), 7.44–7.25 (2H, m, C_4&7—H), 7.04 (1H, dd, J = 7.5, 1.4 Hz, C₂—H), 6.67 (1H, t, J = 7.5 Hz, C₃—H), 6.29 (1H, s, C₅—H), 4.23 (2H, q, J = 5.9 Hz, —CH₂), 2.40 (3H, s, —CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.11, 160.08, 157.22, 146.82, 139.47, 137.37, 129.08, 128.21, 127.91, 125.88, 123.72, 121.56, 121.02, 107.69, 61.50, 55.58, 18.12, 14.70; ESI-MS: m/z calculated 385.87, found [M+H]⁺ 386.8.

2.2.10

2.2.10 Ethyl-8-fluoro-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate (4h)

Greenish Yellow solid, mp 160 °C; Anal. Calcd for C₁₉H₁₆FN₃O₂S: C, 61.77; H, 4.37; F, 5.14; N, 11.37; O, 8.66; S, 8.68%; found C, 61.51; H, 4.25; N, 11.52; S, 8.83%; IR (KBr) (υ_max, cm⁻¹): 3077 (C—H_str), 2941 (C—H_str), 1743 (C⚌O_str), 1636 (C⚌C_str), 1641 (C⚌N_str), 1370 (C—N_str), 1183 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.65 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.74 (1H, td, J = 7.5, 1.5 Hz, C₈—H), 7.62 (1H, dd, J = 7.5, 1.4 Hz, C₉—H), 7.33 (1H, td, J = 7.5, 1.5 Hz, C₇—H), 7.20 (1H, dd, J = 8.1, 1.4 Hz, C₄—H), 6.78 (1H, td, J = 7.9, 1.5 Hz, C₂—H), 6.61 (1H, dd, J = 7.5, 5.1 Hz, C₁—H), 6.00 (1H, s, C₅—H), 4.23 (2H, q, J = 5.9 Hz, —CH₂), 2.44 (3H, s, —CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 159.48, 157.22, 146.82, 139.47, 138.13, 128.01–127.80, 121.56, 117.98, 115.88, 110.25–110.05, 107.69, 61.50, 55.90, 18.12, 14.70; ESI-MS: m/z calculated 369.09, found [M+H]⁺ 370.0.

2.2.11

2.2.11 Ethyl-8-methoxy-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4i)

Yellow solid, mp 150 °C; Anal. Calcd for C₂₀H₁₉N₃O₃S: C, 62.97; H, 5.02; N, 11.02; O, 12.58; S, 8.41%; found C, 62.75; H, 5.29; N, 11.27; S, 8.25%; IR (KBr) (υ_max, cm⁻¹): 3082 (C—H_str), 2932 (C—H_str), 1739 (C⚌O_str), 1629 (C⚌C_str), 1655 (C⚌N_str), 1269 (C—N_str), 1181 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.60 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.77 (1H, td, J = 7.5, 1.5 Hz, C₈—H), 7.68 (1H, dd, J = 7.5, 1.4 Hz, C₉—H), 7.33 (1H, td, J = 7.5, 1.6 Hz, C₇—H), 7.06 (1H, d, J = 1.4 Hz, C₄—H), 6.62 (1H, dd, J = 7.5, 1.4 Hz, C₂—H), 6.52 (1H, d, J = 7.5 Hz, C₁—H), 5.73 (1H, s, C₅—H), 4.23 (2H, q, J = 5.9 Hz, —CH₂), 3.80 (3H, s, —OCH₃), 2.58 (3H, s, —CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.59, 157.22, 146.82, 139.47, 136.72, 127.91, 127.49, 121.56, 118.44, 114.13, 111.15, 107.69, 61.50, 55.97, 18.12, 14.70; ESI-MS: m/z calculated 381.11, found [M+H]⁺ 382.1.

2.2.12

2.2.12 Ethyl-8-ethoxy-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4j)

Greenish Yellow solid, mp 141 °C; Anal. Calcd for C₂₁H₂₁N₃O₃S: C, 63.78; H, 5.35; N, 10.63; O, 12.14; S, 8.11%; found C, 63.57; H, 5.15; N, 10.41; S, 8.29%; IR (KBr) (υ_max, cm⁻¹): 3061 (C—H_str), 2939 (C—H_str), 1747 (C⚌O_str), 1645 (C⚌C_str), 1642 (C⚌N_str), 1288 (C—N_str), 1320 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.66 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.76 (1H, td, J = 7.5, 1.4 Hz, C₈—H), 7.62 (1H, dd, J = 7.5, 1.4 Hz, C₉—H), 7.33 (1H, td, J = 7.4, 1.5 Hz, C₇—H), 7.06 (1H, d, J = 1.4 Hz, C₄—H), 6.62 (1H, dd, J = 7.5, 1.4 Hz, C₂—H), 6.53 (1H, d, J = 7.4 Hz, C₁—H), 5.63 (1H, s, C₅—H), 4.21 (2H, q, J = 5.9 Hz, —CH₂), 4.03 (2H, q, J = 5.9 Hz, —OCH₂), 2.41 (3H, s, —CH₃), 1.35 (6H, dt, J = 31.7, 5.9 Hz, -2CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.29, 146.82, 139.47, 136.46, 127.91, 127.11, 121.56, 118.40, 115.70, 112.52, 107.69, 63.99, 61.50, 55.90, 18.12, 14.70, 13.83; ESI-MS: m/z calculated 395.47, found [M+H]⁺ 396.4.

2.2.13

2.2.13 Ethyl-2-methyl-4-(pyridin-2-yl)-8-(trifluoromethoxy)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate (4k)

Yellow solid, mp 177–180 °C; Anal. Calcd for C₂₀H₁₆F₃N₃O₃S: C, 55.17; H, 3.70; F, 13.09; N, 9.65; O, 11.02; S, 7.36%; found C, 55.35; H, 3.54; N, 9.48; S, 7.12%; IR (KBr) (υ_max, cm⁻¹): 3052 (C—H_str), 2925 (C—H_str), 1742 (C⚌O_str), 1657 (C⚌C_str), 1654 (C⚌N_str), 1370 (C—N_str), 1170 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.60 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.75 (2H, dtd, J = 9.1, 7.5, 1.5 Hz, C_8&9—H), 7.42–7.23 (1H, m, C₇—H), 7.09 (1H, d, J = 1.4 Hz, C₄—H), 6.74 (1H, dd, J = 7.5, 1.6 Hz, C₂—H), 6.59 (1H, d, J = 7.5 Hz, C₁—H), 5.63 (1H, s, C₅—H), 4.25 (2H, q, J = 5.9 Hz, —CH₂), 2.58 (3H, s, —CH₃), 1.36 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.22, 149.85, 146.82, 139.47, 131.49, 127.91, 125.37, 122.75, 122.34, 121.56, 114.16, 107.69, 61.50, 55.90, 18.12, 14.70; ESI-MS: m/z calculated 435.42, found [M+H]⁺ 436.4.

2.2.14

2.2.14 Ethyl-8-hydroxy-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4l)

Yellow solid, mp 177–178 °C; Anal. Calcd for C₁₉H₁₇N₃O₃S: C, 62.11; H, 4.66; N, 11.44; O, 13.06; S, 8.73%; found C, 62.39; H, 4.48; N, 11.68; S, 8.61%; IR (KBr) (υ_max, cm⁻¹): 3089 (C—H_str), 2947 (C—H_str), 1738 (C⚌O_str), 1640 (C⚌C_str), 1676 (C⚌N_str), 1393 (C—N_str), 1214 (C—O_str); ¹H NMR (400 MHz, DMSO) δ 8.59 (1H, dd, J = 7.5, 1.4 Hz, C₆—H), 7.77 (1H, td, J = 7.4, 1.4 Hz, C₈—H), 7.69 (1H, dd, J = 7.5, 1.6 Hz, C₉—H), 7.33 (1H, td, J = 7.5, 1.5 Hz, C₇—H), 6.95 (1H, d, J = 1.4 Hz, C₄—H), 6.59–6.49 (2H, m, C_1&2—H), 5.97 (1H, s, C₅—H), 4.81 (1H, s, —OH), 4.24 (2H, q, J = 5.9 Hz, —CH₂), 2.23 (3H, s, —CH₃), 1.35 (3H, t, J = 5.9 Hz, —CH₃); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.42, 166.12, 160.08, 157.22, 154.78, 146.82, 139.47, 134.83, 127.91, 126.04, 121.56, 118.57, 113.55, 111.39, 107.69, 61.50, 55.90, 18.12, 14.70; ESI-MS: m/z calculated 367.10, found [M+H]⁺ 368.1.

2.3

2.3 Determination of antibacterial activity

Antibacterial activities of 4a–4l were carried out in Nutrient-agar plates by well diffusion assay. Cultures were activated in Nutrient broth. Isolates were inoculated in Nutrient broth and incubated at 37 °C for 24 h for activation of cultures and then centrifuged at 3000 rpm for 15 min and the supernatant was collected to study antibacterial activity.

Using in vitro agar well diffusion method, antimicrobial activity experiments were carried out. The activity of 4a–4l against test microorganisms of activated test cultures (24 h old culture) (2 Gram negative and 2 Gram positive; viz. Enterobacter aerogens MTCC No. 8558, Escherichia coli MTCC No. 1610, Micrococcus luteus MTCC No. 11948 and Bacillus cereus MTCC No. 8557) was inoculated in molten agar and poured into sterile plates. The volume of test culture was 0.1 ml whose optical density is equal to 0.235 at 750 nm and was allowed to solidify. Wells with 5 mm diameter were prepared at equal distance in solidified agar plates using cup-borer. Various derivatives 4a–4l with 1000 μg/ml concentration were inoculated in the wells of nutrient agar whereas test microorganisms were inoculated by pour plate technique. The plates were incubated at 37 °C for 24 h. The inhibition zones were measured at the end of the incubation period and were compared and envisaged with positive and negative controls where streptomycin (1000 μg/ml) is a standard taken and the drug was dissolved in 95% DMF.

2.4

2.4 Antioxidant assay protocol

Each sample was dissolved in 95% DMF to make a concentration of 1 mg/ml and then diluted to prepare a series of concentrations for antioxidant assays. Reference chemicals were used for comparison in all assays.

2.4.1

2.4.1 DPPH radical scavenging activity assay

The free radical scavenging activity of the 4a–4l was measured in vitro by 2,2′-diphenyl-1-picrylhydrazyl (DPPH) assay according to the method described by Brand-Williams et al. (1995). The stock solution was prepared by dissolving 24 mg DPPH with 100 ml methanol and stored at 20 °C as required. The working solution was obtained by diluting DPPH solution with DMF to attain an absorbance of about 0.98 ± 0.02 at 517 nm using the spectrophotometer. A 3 ml aliquot of this solution was mixed with 100 μl of the sample at various concentrations (0–600 μg/ml) as shown in Table 5. The reaction mixture was shaken well and incubated in the dark for 15 min at room temperature. Then the absorbance was measured at 517 nm.

The control was prepared as aforesaid without any sample. The scavenging activity was estimated based on the percentage of DPPH radical scavenged as per the following equation: $Scavenging effect (%) = \frac{Control absorbance - Sample absorbance}{Control absorbance} \times 100$

2.4.2

2.4.2 Superoxide radical scavenging activity

The superoxide radical scavenging activity was assayed according to the reported method as shown by Nishikimi et al. (1972), Beauchamp and Fridovich (1971), and Pithawala and Jain (2014). Superoxide anions were generated in a phenazine methosulfate–nicotinamide adenine dinucleotide (PMS–NADH) system through the reaction of PMS–NADH and oxygen. It was assayed by the reduction of nitroblue tetrazolium (NBT). All the solutions used in this experiment were prepared in phosphate buffer (pH 7.4). 1 ml of NBT (156 μM), 1 ml of NADH (468 μM) and 1 ml of compound (0–600 μg/ml) were mixed. The reaction was initiated by adding 1 ml of PMS (60 μM) and the mixture was incubated at 25 °C for 5 min followed by measurement of absorbance at 560 nm spectrophotometrically. The decreased absorbance of the reaction mixture indicated increased superoxide anion scavenging activity. The percentage inhibition was calculated from the formula, $Inhibition (%) = \frac{Control absorbance - Sample absorbance}{Control absorbance} \times 100$ A percent inhibition versus concentration curve was plotted and the concentration of 4a–4l required for 50% inhibition was determined and expressed as IC₅₀ value. The lower IC₅₀ value indicates high antioxidant capacity (Raghavendra et al., 2013).

2.4.3

2.4.3 ABTS⁺ radical scavenging activity

The 2,2′-azinobis (3-ethylbenzthiazoline-6-sulfonic acid), commonly called ABTS⁺ cation scavenging activity was performed (Re et al., 1999). Briefly, ABTS solution (7 mM) was reacted with potassium persulfate (2.45 mM) solution and kept for overnight in the dark to yield a dark colored solution containing ABTS⁺ radical cations. Prior to use in the assay, the ABTS⁺ radical cation was diluted with 50% DMF for an initial absorbance of about 0.70 ± 0.02 at 745 nm, with temperature control set at 30 °C. Free radical scavenging activity was assessed by mixing 300 μl of test sample with 3.0 ml of ABTS working standard in a micro cuvette. Each concentration of sample was taken in range of 0–600 μg/ml as shown in Table 5. The decrease in absorbance was measured exactly one minute after mixing the solution, and then up to 6 min. The percentage inhibition was calculated according to the formula: $Scavenging effect (%) = \frac{Control absorbance - Sample absorbance}{Control absorbance} \times 100$ The antioxidant capacity of test samples was expressed as EC₅₀ (anti-radical activity), the concentration necessary for 50% reduction of ABTS⁺ (Gülçin et al., 2011).

2.5

2.5 In vitro anti-mycobacterial activity

Here we have performed in vitro anti-mycobacterial activity of all synthesized compounds. Antimicrobial assays performed in Lowenstein Jensen (L-J) medium. Determination of Colony forming units (c.f.u) on Lowenstein-Jensen (L-J) – The ten-fold dilution of standard 1 mg/ml M. tuberculosis suspension (Canetti et al., 1969) was streaked on L-J medium for determining c.f.u in the presence and absence of plant extracts. An M. tuberculosis suspension of 1 mg/ml is equivalent to MacFarland standard (Kent and Kubica, 1985). One loopful (6 μl) of this suspension was streaked on the L-J slants using 3 mm external diameter loop. Reagents of L-J media included potassium di hydrogen phosphate anhydrous (Qualigens), magnesium sulfate anhydrous (Qualigens), magnesium citrate (Loba Chemie), L-asparagine (Hi-media, Mumbai), glycerol (Fisher Scientific, Mumbai), and malachite green (Hi-Media, Mumbai). The drug was incorporated in the medium at concentration of 2 percent v/v and 4 percent v/v of drug was dissolved into 100 ml of culture medium prior to inspissation. The medium set inoculated with the standard bacterial suspension and incubated at 37 °C for 42 days. Reading was taken weekly. For comparison, drug free control slants were used. Susceptibility testing of MDR isolates was also performed against standard drugs such as rifampicin and isoniazid in the same batch of media for comparison of cfu on drug free controls. Each test was done in duplicate. Percentage inhibition was calculated by mean reduction in number of colonies on drug containing as compared to drug free controls. $% Inhibition = \frac{C_{c} - C_{t}}{C_{c}} \times 100$ where

c = control,
t = test.

2.6

2.6 Statistical analysis

The determination assay for each antioxidant parameter was carried out in thrice to minimize the percentage of error. Statistical analysis was performed using the Statistical Package for Social Science (IBM SPSS Statistics 20 for Windows, SPSS Inc., Chicago, IL, USA).

3 Result and discussion

In the present study it has been interpreted and observed that the catalysts have a major role in the organic synthesis, by either enhancing the rate of reaction or any other aspects in pertaining to the reaction. The same has been implemented but however it may not divulge any enhancement in the reaction which could give necessary prediction or their reactions as well.

Based on promising result as shown in Table 1, we have subsequently studied the effect of five solvents DMF (N,N-Dimethylformamide), ACN (Acetonitrile), Methanol, THF (Tetrahydrofuran), DCM (Dichloromethane) and various catalysts (30) in the reaction for the synthesis of 4a from 2-amino benzothiazole 3a as a basic structural configurator along with pyridine 2-aldehyde 1 and ethyl acetoacetate 2 by conventional method. Methanol was used as a solvent for this reaction at 65 °C temperature and it gave 46.2% yield in 96 h (entry 34), while the relative yield of 4a is 36.7% in 48 h by using DMF as solvent at 90 °C temperature (entry 32). But, to our surprise, we observed that the reaction did not proceed efficiently (Table 1, entries 33, 35–36) in other remaining solvents. The yield and time of product 4a could be improved to 64.8% (1.58 h) when the reaction was performed in DMF at 90 °C temperature by using PdCl₂ as catalyst (entry 10). But, the reaction proceeds efficiently when it was carried out under neat condition by conventionally heating for 0.75 h in the presence of PdCl₂ catalyst to afford compound 4a in 75.6% yield (entry 1). InCl₃ is known to have a second catalyst (with 73.2% yield at 1.3 h) of interest from rest of the catalysts used (entry 4). Thus, we conclude that PdCl₂ catalyst and solvent-free condition is the optimized condition for this transformation.

Table 1 Optimization of reaction by catalysts and solvents for the synthesis of 4a by conventional method.

No.	Time (h)	Catalyst	Solvent	Temp. (°C)	Yield^a (%)
1	0.75	PdCl₂	Neat	90	75.6
2	0.95	Zn(NO₃)₂	Neat	90	30.7
3	1.17	Na₂WO₄	Neat	90	63.0
4	1.25	InCl₃	Neat	90	73.2
5	1.30	Cr(NO₃)₂	Neat	90	51.3
6	1.33	Pb(NO₃)₂	Neat	90	59.7
7	1.40	Mg(NO₃)₂	Neat	90	44.6
8	1.42	Cu(NO₃)₂	Neat	90	43.5
9	1.42	CoCl₂	Neat	90	67.2
10	1.58	PdCl₂	DMF	90	64.8
11	1.58	BaCl₂	Neat	90	67.8
12	1.60	AlCl₃	Neat	90	59.3
13	1.60	LaCl₃	Neat	90	72.0
14	1.65	CaCl₂	Neat	90	45.6
15	1.67	ZnCl₂	Neat	90	56.6
16	1.72	Co(NO₃)₂	Neat	90	50.6
17	1.75	MgCl₂	Neat	90	43.1
18	1.77	NiCl₂	Neat	90	62.8
19	1.80	Cd(NO₃)₂	Neat	90	58.6
20	1.82	TiCl₃	Neat	90	64.6
21	1.83	Ca(NO₃)₂	Neat	90	47.7
22	1.83	Sr(NO₃)₂	Neat	90	54.7
23	1.83	CAN	Neat	90	71.8
24	1.87	CrCl₃	Neat	90	65.4
25	1.92	Ba(NO₃)₂	Neat	90	62.7
26	2.00	NH₄VO₃	Neat	90	61.1
27	2.00	Bi(NO₃)₂	Neat	90	71.6
28	2.33	MnCl₂	Neat	90	52.3
29	2.50	FeCl₃	Neat	90	51.2
30	2.50	ZnOCl₂	Neat	90	63.1
31	2.67	SnCl₂	Neat	90	65.7
32	48.0	–	DMF	90	36.7
33	72.0	–	ACN	90	39.3
34	96.0	–	Methanol	65	46.2
35	120	–	THF	70	40.2
36	240	–	DCM	45	20.5

Reaction conditions: Pyridine 2-aldehyde (1 mmol) 1, ethyl acetoacetate (1 mmol) 2, 2-aminobenzothiazole (1 mmol) 4a, various catalysts (10 mol%), solvent/solvent-free conditions, Δ.

a Isolated yield.

The selected catalyst (PdCl₂), was then optimized to work out the maximum yield with minimized reaction time by conventional method. Catalyst loading of 0, 1, 2, 5, 10, 15, and 20 (PdCl₂ mol%) was used. Out of all these varying concentrations, the maximum efficient value was obtained with 10 mol%, as shown in Fig. 1 with maximum yield of 75.6% in 0.75 h of reaction time. The value of yield increases in a gradual fashion, but is known to decrease its value in higher concentrations of catalysts beyond 15 mol%. This might be due to the interference of catalyst. However, it was also noticed to have a fixed reaction time (0.75 h) for the completion of the 4a product as revealed in Table 2.

Optimization study of PdCl2 as catalyst in the synthesis of 4a.

Table 2 Optimization study of PdCl₂ for the synthesis of 4a by conventional method.

No.	Catalyst (%)	Time (h)	Yield^a (%)
1	No catalyst	10.0	45.5
2	1	2.87	58.2
3	2	2.58	62.1
4	5	2.17	68.3
5	10	0.75	75.6
6	15	0.75	76.2
7	20	0.75	74.1

Reaction conditions: Pyridine 2-aldehyde (1 mmol) 1, ethyl acetoacetate (1 mmol) 2, 2-aminobenzothiazole (1 mmol) 4a, various catalysts (mol%), solvent-free conditions, 90 °C, Δ.

a Isolated yield.

By following the same criteria of conventional method, we have carried out the reaction under the CEM discover microwave reactor using an open vessel technique (Dallinger and Kappe, 2007; Stadler and Kappe, 2000; Stadler et al., 2002 ) for the synthesis of 4a as well as other derivatives (see Scheme 1). In Table 3, we have shown the comparison of reaction time and yield of product by both conventional and MW methods, comparative aspects with reference to yield and rate of reaction. MW method was found to be better than the conventional one as there is a remarkable reduction in reaction rate. MW is superior in terms of reaction rate, where there is a decrease from 12 to 15 folds and an increase in yield of product from 8% to 10%.

Synthetic route for the synthesis of title compounds under solvent-free condition.

The compound 4a was formed with 85.1% yield in 3 min of reaction time, which was confirmed by different spectroscopic techniques such as IR, ¹H NMR, ¹³C NMR, Mass and elemental analysis and X-ray analysis. The IR spectrum of 4a exhibited seven main peaks located at 3072, 2943, 1745, 1677, 1688, 1352, 1240 cm⁻¹, which were assigned to the (C—H_str) aromatic, (C—H_str) alkanes, (C⚌O_str) ester, (C⚌C_str) alkene, (C⚌N_str) imine, (C—N_str) aromatic amine, and (C—O_str) ester respectively. The mass spectrum of 4a showed a molecular ion peak at ESI-MS: m/z calculated 351.42, found [M+H]⁺ 352.4 which indicates the existence of 4a. The ¹H NMR spectrum of 4a revealed pyridine ring hydrogen (Py—H) peaks shown at 8.59 (1H, dd, J = 7.5, 1.3 Hz, C₆—H), 7.74 (2H, dtd, J = 9.3, 7.5, 1.6 Hz, C_8&9—H), 7.42–7.08 (1H, C₇—H) δ value. Chiral center hydrogen resonated at 6.11 δ and side chain of ester group contains —CH₂, —CH₃ shows peak at 4.25, 1.36 δ. Also aromatic hydrogens (Ar—H) resonated at 7.04 (J = 7.5, 1.4 Hz), 6.73 (J = 11.8, 4.5 Hz) δ value.

The structure of compound 4a was also confirmed from single-crystal X-ray diffraction studies. Fine crystals of compound 4a were obtained by slow evaporation of ethyl acetate: hexane (4:1, v/v) mixture with compound 4a. It revealed the formation of pure 4a, which crystallizes as a monoclinic crystal system with a space group P2₁/n. The results showed the proper structure and configuration as 4a, shown in ORTEP plot (CCDC No.: 1048059) along with the atom numbering in Fig. 2.

ORTEP diagram of compound 4a with the atom numbering.

Taking into consideration the entire outcome, a plausible mechanistic pathway for the synthesis of 4a is depicted in Scheme 2.

Plausible mechanism for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles 4a.

3.1

3.1 Antibacterial activity

Antibacterial activity of 4a–4l was carried out on Nutrient-agar plates by well-diffusion assay against test culture. Cultures were activated in Nutrient broth. The zone of inhibition was measured in terms of zone diameter and with the help of that zone index was calculated where streptomycin was used as standard.

3.1.1

3.1.1 Determination of activity index

The activity index of the probiotic culture was calculated as follows: $Activity index (A . I .) = \frac{Mean of zone of inhibition of derivative}{Zone of inhibition obtained for standard antibiotic drug}$ Note: Standard drug used was streptomycin with 1000 μg/ml concentration.

Table 4 shows that all synthesized derivatives 4a–4l have been screened with antimicrobial activity. Gram negative was more inhibited as compared to Gram Positive bacteria. Also 4c–4g were proved to have more antibacterial property as compared to the streptomycin while contrastingly 4a, 4b, 4j were having a lesser activity index, as shown in Fig. 3; however, it could be considered to have antimicrobial activity.

Table 4 Antibacterial activity of 4a–4l.

Derivatives	Enterobacter aerogens MTCC No. 8558		Escherichia coli MTCC No. 1610		Micrococcus luteus MTCC No. 11948		Bacillus cereus MTCC No. 8558
Derivatives	Mean value for zone of inhibition (mm)	Activity index (A.I.)	Mean value for zone of inhibition (mm)	Activity index (A.I.)	Mean value for zone of inhibition (mm)	Activity index (A.I.)	Mean value for zone of inhibition (mm)	Activity index (A.I.)
4a	22 ± 1.135	0.9166	21 ± 1.047	0.8750	19 ± 1.062	0.7916	18 ± 0.802	0.7500
4b	23 ± 1.112	0.9583	24 ± 1.124	1.0000	18 ± 1.045	0.7500	17 ± 0.812	0.7083
4c	34 ± 1.156	1.4166	31 ± 1.075	1.2910	27 ± 1.023	1.1250	19 ± 0.815	0.7916
4d	32 ± 1.142	1.3333	27 ± 1.072	1.1250	26 ± 1.047	1.0833	22 ± 0.842	0.9166
4e	30 ± 1.179	1.2500	30 ± 1.034	1.2500	28 ± 1.051	1.1666	24 ± 0.812	1.0000
4f	26 ± 1.120	1.0833	28 ± 1.159	1.1666	24 ± 1.078	1.0000	16 ± 0.823	0.6666
4g	27 ± 1.200	1.1250	25 ± 1.124	1.0416	29 ± 1.051	1.2083	19 ± 0.864	0.7916
4h	23 ± 1.141	0.9583	22 ± 1.118	0.9166	28 ± 1.023	1.1666	25 ± 0.846	1.0416
4i	24 ± 1.423	1.0000	21 ± 1.113	0.8750	21 ± 1.036	0.8750	22 ± 0.815	0.9166
4j	22 ± 1.231	0.9166	20 ± 1.076	0.8333	23 ± 1.061	0.9583	18 ± 0.841	0.7500
4k	26 ± 1.187	1.0833	25 ± 1.041	1.0416	27 ± 1.036	1.1250	21 ± 0.826	0.8750
4l	26 ± 1.154	1.0833	27 ± 1.023	1.1250	23 ± 1.078	0.9583	20 ± 0.831	0.8333
Std drug	24	–	24	–	24	–	24	–

3.2

3.2 Antioxidant activity

Free radical scavenging activity was evaluated by performing in vitro DPPH assay. As shown in Fig. 4 and Table 5, % DPPH activity was calculated and it is found that all derivatives 4a–4l have an antioxidant property.

% DPPH radical scavenging activity assay at various concentrations.

Table 5 % DPPH radical scavenging activity assay.

Antioxidant assays^⁎
% DPPH radical scavenging activity assay at various concentrations Mean ± S.E.
Derivatives	0.00 μg/ml	200 μg/ml	400 μg/ml	600 μg/ml
4a	0.00	30.00 ± 1.434	43.11 ± 2.627	76.63 ± 1.083
4b	0.00	36.71 ± 1.421	57.68 ± 2.645	76.77 ± 1.082
4c	0.00	29.48 ± 1.415	53.69 ± 2.678	79.83 ± 1.074
4d	0.00	32.75 ± 1.399	59.03 ± 2.632	84.75 ± 1.062
4e	0.00	36.18 ± 1.423	63.95 ± 2.641	86.51 ± 1.087
4f	0.00	37.75 ± 1.420	63.73 ± 2.678	84.66 ± 1.079
4g	0.00	38.47 ± 1.478	70.01 ± 2.694	79.92 ± 1.069
4h	0.00	47.02 ± 1.456	58.86 ± 2.632	75.55 ± 1.082
4i	0.00	32.06 ± 1.406	57.84 ± 2.591	83.84 ± 1.076
4j	0.00	36.71 ± 1.478	56.55 ± 2.589	82.21 ± 1.080
4k	0.00	39.99 ± 1.495	53.70 ± 2.645	79.03 ± 1.082
4l	0.00	40.73 ± 1.435	79.83 ± 2.632	84.88 ± 1.023

⁎ p value < 0.05, data significant.

Also the activity is dose dependent in order to scavenge the radical. The higher concentration indicates a greater amount of scavenging of the DPPH radical. The colored DPPH solution faded reducibly to half at an optimal concentration range from 200 μg/ml to 400 μg/ml. This may be induced and influenced due to electron transfer or the reduction of the DPPH molecules or thought to be due to their hydrogen donating ability (Baumann et al., 1979).

As an indirect indication it may also be a time dependent scavenging activity. As obtained almost all of the derivatives followed a general rule of dose dependent scavenging activity despite the degree of electron transfer. However 4l is known to have a more efficient scavenging property and the least known 4a with a gradual antioxidant property. At higher concentration 4h has a consistent reduction of DPPH scavenging activity to 50% with an optimal concentration range and relatively lesser % scavenging activity rate, as compared to its lower concentration. 4e and 4f showed a constant % scavenging activity following an accuracy as a dose dependent antioxidant reaction.

The scavenging of the ABTS⁺ radical by the derivatives 4a–4l was found to be higher than that of DPPH radical at higher concentration level, indicating that they may be useful as therapeutic agents for treating radical-related pathological damage. DPPH and ABTS⁺ radical scavenging activities are based on the ability of antioxidants to donate a hydrogen atom or an electron to stabilize radicals by converting them to the non-radical species (Binsan et al., 2008; Chandrasekara and Shahidi, 2010). DPPH is a radical having an odd electron and reacts with hydrogen donated from antioxidant.

In case of ABTS⁺ radical scavenging activity, the effectiveness depends on the molecular weight, the number of aromatic rings and the nature of hydroxyl group’s substitution than the specific functional groups (Hagerman et al., 1998). Superoxide anion scavenging assay at various concentrations in the range of 0–600 μg/ml was evaluated to calculate the IC₅₀ values of each of derivatives 4a–4l. The IC₅₀ values were correspondingly in between the range of 280–400 μg/ml. The values correspond to 4a (332 μg/ml), 4b (372 μg/ml), 4c (368 μg/ml), 4d (316 μg/ml), 4e (344 μg/ml), 4f (300 μg/ml), 4g (390 μg/ml), 4h (281 μg/ml), 4i (301 μg/ml), 4j (285 μg/ml), 4k (321 μg/ml), and 4l (301 μg/ml) respectively as shown in Fig. 5 and Table 6. ABTS⁺ radical scavenging activity at various concentrations indicated the EC₅₀ values as 4a (482 μg/ml), 4b (455 μg/ml), 4c (468 μg/ml), 4d (479 μg/ml), 4e (492 μg/ml), 4f (455 μg/ml), 4g (470 μg/ml), 4h (450 μg/ml), 4i (425 μg/ml), 4j (425 μg/ml), 4k (462 μg/ml), and 4l (405 μg/ml) respectively as shown in Fig. 6 and Table 7. This may shed a new horizon in pharmaceutical based drug development and an active drug moiety indicating the increased application of the synthesized compounds.

% Superoxide anion scavenging activity assay at various concentrations.

Table 6 % Superoxide anion scavenging activity assay.

Antioxidant assays^⁎
% Superoxide anion scavenging activity assay at various concentrations Mean ± S.E.
Derivatives	0.00 μg/ml	200 μg/ml	400 μg/ml	600 μg/ml
4a	0.00	37.94 ± 0.799	55.90 ± 1.681	83.15 ± 1.294
4b	0.00	31.93 ± 0.785	52.49 ± 1.675	81.16 ± 1.285
4c	0.00	32.66 ± 0.789	53.33 ± 1.684	78.31 ± 1.279
4d	0.00	35.61 ± 0.794	59.60 ± 1.672	88.88 ± 1.286
4e	0.00	36.95 ± 0.779	55.10 ± 1.689	83.47 ± 1.284
4f	0.00	38.83 ± 0.784	61.11 ± 1.628	79.06 ± 1.296
4g	0.00	31.00 ± 0.796	50.88 ± 1.678	84.45 ± 1.276
4h	0.00	37.66 ± 0.769	69.04 ± 1.684	88.29 ± 1.263
4i	0.00	36.74 ± 0.782	62.20 ± 1.689	79.04 ± 1.284
4j	0.00	39.00 ± 0.788	65.90 ± 1.691	74.37 ± 1.275
4k	0.00	36.66 ± 0.784	58.36 ± 1.693	81.11 ± 1.263
4l	0.00	33.30 ± 0.783	65.65 ± 1.682	88.03 ± 1.290

⁎ p value < 0.05, data significant.

% ABTS+ radical scavenging activity assay at various concentrations.

Table 7 % ABTS⁺ radical scavenging activity assay.

Antioxidant assays^⁎
% ABTS⁺ radical scavenging activity assay at various concentrations Mean ± S.E.
Derivatives	0.00 μg/ml	200 μg/ml	400 μg/ml	600 μg/ml
4a	0.00	12.33 ± 1.709	39.45 ± 1.227	64.93 ± 1.741
4b	0.00	22.83 ± 1.712	43.43 ± 1.233	66.83 ± 1.751
4c	0.00	25.76 ± 1.756	41.07 ± 1.2s45	68.33 ± 1.762
4d	0.00	13.94 ± 1.741	36.72 ± 1.245	70.84 ± 1.743
4e	0.00	18.42 ± 0.762	38.11 ± 1.224	63.63 ± 1.742
4f	0.00	11.63 ± 1.723	42.66 ± 1.278	68.92 ± 1.795
4g	0.00	19.20 ± 1.722	36.99 ± 1.235	73.94 ± 1.756
4h	0.00	26.39 ± 1.730	40.40 ± 1.212	76.76 ± 1.745
4i	0.00	28.23 ± 1.749	45.82 ± 1.254	84.69 ± 1.762
4j	0.00	27.46 ± 1.764	47.97 ± 1.252	65.92 ± 1.732
4k	0.00	17.30 ± 1.785	38.41 ± 1.274	75.41 ± 1.742
4l	0.00	22.73 ± 1.734	49.50 ± 1.262	68.43 ± 1.746

⁎ p value < 0.05, data significant.

3.3

3.3 In vitro anti-mycobacterial activity

In vitro anti-mycobacterial activity showed that all the compounds are bioactive against M. tuberculosis H₃₇Rv. From Table 8, Compounds 4c and 4e showed very good % inhibition 72.9% and 70.32% against M.tuberculosis H₃₇RV. Also compounds 4a, 4g, 4j, 4k and 4l exhibited good antimycobacterial activity (% inhibition 67.74%, 68.38%, 60%, 64.51% and 65.80%) against M.tuberculosis H₃₇RV. The compounds 4b, 4d and 4h showed moderate to good antimycobacterial activity (% inhibition 58%, 51.61% and 57.41%) against M.tuberculosis H₃₇RV. Compounds 4f, displayed less antimycobacterial activity (% inhibition 47.71%) toward M.tuberculosis H₃₇RV.

Table 8 In vitro anti-mycobacterial activity of newly synthesized compound.

Sample code	Mean c.f.u. on media		Percentage inhibition (%)
Sample code	Control	Treatment (100 μg/ml)	Percentage inhibition (%)
4a	155	50	67.74
4b	155	65	58.00
4c	155	42	72.90
4d	155	75	51.61
4e	155	46	70.32
4f	155	81	47.71
4g	155	49	68.38
4h	155	66	57.41
4i	155	71	54.19
4j	155	62	60.00
4k	155	55	64.51
4l	155	53	65.80

4 Conclusion

In summary, we have described a novel, efficient, one pot, three component reaction of some typical 2-aminobenzothiazole derivatives, pyridine 2-aldehyde and ethyl acetoacetate to develop ethyl 2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate derivatives using PdCl₂ as a prompt catalyst under microwave assisted solvent-free conditions. The simple one-pot nature of the reaction makes it an interesting approach. The main advantages of this current methodology are novel with operational simplicity, good yields, simple workup, superficial automation, without any anhydrous conditions. The reactions were carried out under solvent free condition which are considerably safer, nontoxic, and environmental friendly. Furthermore, the existence of three different important heterocyclic moieties such as benzothiazole, pyridine, and pyrimidine rings in one molecule is fantastic on account of the potential applications of these derivatives in biological and pharmacological activities. The synthesized compounds were evaluated in vitro antibacterial and antioxidant activities. The antibacterial data revealed that the all synthesized compounds proved to be active against the test organism, two gram negative and two gram positive reference strains compared to standard drugs. Moreover, all the synthesized compounds were evaluated as antioxidants according to a DPPH radical scavenging activity, superoxide anion scavenging assay and ABTS⁺ radical scavenging activity of our synthesized compound. All compounds showed moderate to good antioxidant activity. We have also screened antitubercular activity against Mycobacterium tuberculosis H₃₇RV strain. Compounds 4c and 4e showed very good % inhibition 72.9% and 70.32% respectively against M.tuberculosis H₃₇RV and other compound showed moderate to good antitubercular activity. Further optimization and pharmacokinetic characterization of this series are in progress in our laboratory.

Acknowledgment

The authors are thankful to UGC-Info net and INFLIBNET Gujarat University for providing e-source facilities. Both Manoj N. Bhoi and Mayuri A. Borad are thankful to UGC-BSR fellowship for financial assistance.

References

Alajarin R., Jordán P., Vaquero J.J., Alvarez-Builla J., . Synthesis of unsymmetrically substituted 1,4-dihydropyridines and analogous calcium antagonists by microwave heating. Synthesis 1995:389-391.
[Google Scholar]
Atar A.B., Jeong Y.T., . Catalyzed cascade C—C and C–N bond formation: synthesis of differentially substituted triheterocyclic benzothiazole functionalities under solvent-free condition. Molec. Diver.. 2014;18:389-401.
[Google Scholar]
Bastug G., Eviolitte C., Markó I.E., . Functionalized orthoesters as powerful building blocks for the efficient preparation of heteroaromatic bicycles. Organ. Lett.. 2012;14:3502-3505.
[Google Scholar]
Baumann J., Wurn G., Bruchlausen F., . Prostaglandin synthetase inhibiting O₂-radical scavenging properties of some flavonoids and related phenolic compounds, naunyn, schmiedebergs. Naunyn-Schmiedebergs Arch. Pharmacol.. 1979;308:R27-R32.
[Google Scholar]
Beauchamp C., Fridovich I., . Superoxide dismutase: improved assays and an assay applicable to acrylamide gels. Anal. Biochem.. 1971;44:276-287.
[Google Scholar]
Bejan V., Mantu D., Mangalagiu I.I., . Ultrasound and microwave assisted synthesis of isoindolo-1,2-diazine: a comparative study. Ultrason. Sonochem.. 2012;19:999-1002.
[Google Scholar]
Benetti S., Romagnoli R., De Risi C., Spalluto G., Zanirato V., . Mastering. beta.-keto esters. Chem. Rev.. 1995;95:1065-1114.
[Google Scholar]
Bhoi M.N., Borad M.A., Panchal N.K., Patel H.D., . 2-Aminobenzothiazole containing novel Schiff bases derivatives: search for new antibacterial agents. Int. Lett. Chem. Phys. Astron.. 2015;53:106-113.
[Google Scholar]
Bhoi M.N., Borad M.A., Parmar H.B., Patel H.D., . Synthesis and characterization of novel N-(benzo [d] thiazol-2-yl)-2-(2-(6-chloroquinolin-4-yl) hydrazinyl) acetamide derivatives containing Quinoline linkage as potent antibacterial agents. Int. Lett. Chem. Phys. Astron.. 2015;53:114-121.
[Google Scholar]
Bhoi M.N., Borad M.A., Patel H.D., . Synthetic strategies for fused benzothiazoles: past, present, and future. Synth. Commun.. 2014;44:2427-2457.
[Google Scholar]
Biginelli P., Gazz P., . Synthesis of 3,4-dihydropyrimidin-2 (1H)-ones. Chim. Ital.. 1893;23:360-416.
[Google Scholar]
Binsan W., Benjakul S., Visessanguan W., Roytrakul S., Tanaka M., Kishimura H., . Antioxidative activity of Mungoong, an extract paste, from the cephalothorax of white shrimp (Litopenaeus vannamei) Food Chem.. 2008;106:185-193.
[Google Scholar]
Boer R., Gekeler V., . Chemosensitizers in tumor therapy: new compounds promise better efficacy. Drugs Future. 1995;20:499-510.
[Google Scholar]
Borad M.A., Bhoi M.N., Parmar J.A., Patel H.D., . Microwave assisted synthesis of novel N-(benzo [d] thiazol-2-yl)-2-((2,4′-dioxospiro [indoline-3,2′-thiazolidin]-3′-yl) amino) acetamide derivatives as potent antibacterial agents. Int. Lett. Chem. Phys. Astron.. 2015;53:122-129.
[Google Scholar]
Brand-Williams W., Cuvelier M., Berset C., . Use of a free radical method to evaluate antioxidant activity. Food Sci. Technol.. 1995;28:25-30.
[Google Scholar]
Bretzel R.G., Bollen C.C., Maeser E., Federlin K.F., . Nephroprotective effects of nitrendipine in hypertensive Tune I and Type II diabetic patients. Am. J. Kidney Dis.. 1993;21:S53-S64.
[Google Scholar]
Canetti G., Fox W., Khomenko A.A., Mahler H.T., Menon N.K., Mitchison D.A., Šmelev N.A., . Advances in techniques of testing mycobacterial drug sensitivity, and the use of sensitivity tests in tuberculosis control programmes. Bull. World Health Organ.. 1969;41:21-43.
[Google Scholar]
Chandrasekara A., Shahidi F., . Inhibitory activities of soluble and bound millet seed phenolics on free radicals and reactive oxygen species. J. Agric. Food Chem.. 2010;59:428-436.
[Google Scholar]
Chebanov V.A., Gura K.A., Desenko S.M., . Aminoazoles as key reagents in multicomponent heterocyclizations Synthesis of Heterocycles via Multicomponent Reactions I. Springer; 2010. pp. 41–84
Choudhary S., Kini S.G., Mubeen M., . Antioxidant activity of novel coumarin substituted benzothiazole derivatives. Der Pharma Chemica. 2013;5:213-222.
[Google Scholar]
Chow C.P., Shea K.J., . Dual function catalysts. Dehydrogenation and asymmetric intramolecular diels-alder cycloaddition of N-hydroxy formate esters and hydroxamic acids: evidence for a ruthenium-acylnitroso intermediate. J. Am. Chem. Soc.. 2005;127:3678-3679.
[Google Scholar]
Dallinger D., Kappe C.O., . Microwave-assisted synthesis in water as solvent. Chem. Rev.. 2007;107:2563-2591.
[Google Scholar]
de Graaff C., Ruijter E., Orru R.V., . Recent developments in asymmetric multicomponent reactions. Chem. Soc. Rev.. 2012;41:3969-4009.
[Google Scholar]
De la Hoz A., Diaz-Ortiz A., Moreno A., . Microwaves in organic synthesis. Thermal and non-thermal microwave effects. Chem. Soc. Rev.. 2005;34:164-178.
[Google Scholar]
Desai B., Dallinger D., Kappe C.O., . Microwave-assisted solution phase synthesis of dihydropyrimidine C5 amides and esters. Tetrahedron. 2006;62:4651-4664.
[Google Scholar]
Devineni S.R., Doddaga S., Donka R., Chamarthi N.R., . CeCl₃·7H₂O–SiO₂: catalyst promoted microwave assisted neat synthesis, antifungal and antioxidant activities of α-diaminophosphonates. Chin. Chem. Lett.. 2013;24:759-763.
[Google Scholar]
Dömling A., . Recent developments in isocyanide based multicomponent reactions in applied chemistry. Chem. Rev.. 2006;106:17-89.
[Google Scholar]
Eynde J.J.V., Hecq N., Kataeva O., Kappe C.O., . Microwave-mediated regioselective synthesis of novel pyrimido [1,2-a] pyrimidines under solvent-free conditions. Tetrahedron. 2001;57:1785-1791.
[Google Scholar]
Gülçin İ., Topal F., Çakmakçı R., Bilsel M., Gören A.C., Erdogan U., . Pomological features, nutritional quality, polyphenol content analysis, and antioxidant properties of domesticated and 3 wild ecotype forms of raspberries (Rubus idaeus L.) J. Food Sci.. 2011;76:C585-C593.
[Google Scholar]
Hagerman A.E., Riedl K.M., Jones G.A., Sovik K.N., Ritchard N.T., Hartzfeld P.W., Riechel T.L., . High molecular weight plant polyphenolics (tannins) as biological antioxidants. J. Agri. Food Chem.. 1998;46:1887-1892.
[Google Scholar]
Hayes, B. L., 2002. Microwave Synthesis: Chemistry at the Speed of Light: Cem Corporation.
Hayes B.L., . Recent advances in microwave-assisted synthesis. Aldrichim. Acta. 2004;37:66-77.
[Google Scholar]
Hopper R.J., . 2-(Isopropylsulfinyl)-benzothiazole as a delayed action thiazole accelerator. Rubber Chem. Technol.. 1993;66:623-633.
[Google Scholar]
Huang W., Yang G.-F., . Microwave-assisted, one-pot syntheses and fungicidal activity of polyfluorinated 2-benzylthiobenzothiazoles. Bioorg. Med. Chem.. 2006;14:8280-8285.
[Google Scholar]
Hutchinson I., Chua M.-S., Browne H.L., Trapani V., Bradshaw T.D., Westwell A.D., Stevens M.F., . Antitumor benzothiazoles. 14. 1 synthesis and in vitro biological properties of fluorinated 2-(4-aminophenyl) benzothiazoles. J. Med. Chem.. 2001;44:1446-1455.
[Google Scholar]
Jaseer E., Prasad D., Dandapat A., Sekar G., . An efficient copper (II)-catalyzed synthesis of benzothiazoles through intramolecular coupling-cyclization of N-(2-chlorophenyl) benzothioamides. Tetrahedron Lett.. 2010;51:5009-5012.
[Google Scholar]
Jiang L.-L., Tan Y., Zhu X.-L., Wang Z.-F., Zuo Y., Chen Q., Yang G.-F., . Design, synthesis, and 3D-QSAR analysis of novel 1,3,4-oxadiazol-2 (3H)-ones as protoporphyrinogen oxidase inhibitors†. J. Agric. Food Chem.. 2009;58:2643-2651.
[Google Scholar]
Jiang L.-L., Zuo Y., Wang Z.-F., Tan Y., Wu Q.-Y., Xi Z., Yang G.-F., . Design and syntheses of novel N-(benzothiazol-5-yl)-4,5,6,7-tetrahydro-1H-isoindole-1,3(2H)-dione and N-(benzothiazol-5-yl) isoindoline-1,3-dione as potent protoporphyrinogen oxidase inhibitors. J. Agric. Food Chem.. 2011;59:6172-6179.
[Google Scholar]
Jiang X., Cao Y., Wang Y., Liu L., Shen F., Wang R., . A unique approach to the concise synthesis of highly optically active spirooxazolines and the discovery of a more potent oxindole-type phytoalexin analogue. J. Am. Chem. Soc.. 2010;132:15328-15333.
[Google Scholar]
Kappe C.O., . Recent advances in the Biginelli dihydropyrimidine synthesis. New tricks from an old dog. Acc. Chem. Res.. 2000;33:879-888.
[Google Scholar]
Kappe C.O., . High-speed combinatorial synthesis utilizing microwave irradiation. Curr. Opin. Chem. Biol.. 2002;6:314-320.
[Google Scholar]
Kappe C.O., . Controlled microwave heating in modern organic synthesis. Angew. Chem. Int. Ed. Engl.. 2004;43:6250-6284.
[Google Scholar]
Kappe C.O., . Microwave dielectric heating in synthetic organic chemistry. Chem. Soc. Rev.. 2008;37:1127-1139.
[Google Scholar]
Kappe C.O., Stadler A., . The Biginelli dihydropyrimidine synthesis. Organic reactions. John Wiley & Sons Inc.; 2004.
Kappe C.O., Stadler A., Dallinger D., . Microwaves in Organic and Medicinal Chemistry. John Wiley & Sons; 2012.
Karle M., Knecht W., Xue Y., . Discovery of benzothiazole guanidines as novel inhibitors of thrombin and trypsin IV. Bioorg. Med. Chem. Lett.. 2012;22:4839-4843.
[Google Scholar]
Kaur N., Kaur K., Raj T., Kaur G., Singh A., Aree T., Jang D.O., . One-pot synthesis of tricyclic dihydropyrimidine derivatives and their biological evaluation. Tetrahedron. 2015;71:332-337.
[Google Scholar]
Kent, P.T., Kubica, G.P., 1985. Antituberculosis chemotherapy and drug susceptibility testing. Public Health Mycobacteriology: A Guide for the Level III Laboratory. US Department of Health and Human Services, Centers for Disease Control and Prevention, pp. 159–184.
Khabazzadeh H., Saidi K., Sheibani H., . Microwave-assisted synthesis of dihydropyrimidin-2(1H)-ones using graphite supported lanthanum chloride as a mild and efficient catalyst. Bioorg. Med. Chem. Lett.. 2008;18:278-280.
[Google Scholar]
Klusa V., . Cerebrocrast. Neuroprotectant, cognition enhancer. Drugs Fut.. 1995;20:135-138.
[Google Scholar]
Langer P., . Regio-and diastereoselective cyclization reactions of free and masked 1,3-dicarbonyl dianions with 1,2-dielectrophiles. Chem. Eur. J.. 2001;7:3858-3866.
[Google Scholar]
Leadbeater N.E., Williams V.A., Barnard T.M., Collins M.J., . Open-vessel microwave-promoted Suzuki reactions using low levels of palladium catalyst: optimization and scale-up. Org. Process Res. Dev.. 2006;10:833-837.
[Google Scholar]
Nagarapu L., Gaikwad H.K., Palem J.D., Venkatesh R., Bantu R., Sridhar B., . Convenient approach for the one-pot, three-component synthesis of triheterocyclic 4 H-pyrimido [2,1-b] benzothiazole derivatives using TBAHS. Synth. Commun.. 2013;43:93-104.
[Google Scholar]
Nair V., Rajesh C., Vinod A., Bindu S., Sreekanth A., Mathen J., Balagopal L., . Strategies for heterocyclic construction via novel multicomponent reactions based on isocyanides and nucleophilic carbenes. Acc. Chem. Res.. 2003;36:899-907.
[Google Scholar]
Nishikimi M., Rao N.A., Yagi K., . The occurrence of superoxide anion in the reaction of reduced phenazine methosulfate and molecular oxygen. Biochem. Biophy. Res. Commun.. 1972;46:849-854.
[Google Scholar]
Noolvi M.N., Patel H.M., Kaur M., . Benzothiazoles: search for anticancer agents. Eur. J. Med. Chem.. 2012;54:447-462.
[Google Scholar]
Oliver Kappe C., . 100 years of the biginelli dihydropyrimidine synthesis. Tetrahedron. 1993;49:6937-6963.
[Google Scholar]
Pande A., Lokhande S., Patel M., Khadse B., . Synthesis and study of 2-substituted alkyl or aryl, amino 6-methyl benzothiazoles as antituberculosis agents. Indian Drugs. 1982;19:342-345.
[Google Scholar]
Perreux L., Loupy A., . Nonthermal effects of microwaves in organic synthesis.Microwaves in Organic Synthesis. Wiley Online Library; 2002. pp. 61–114
Pillai U.R., Sahle-Demessie E., Varma R.S., . Environmentally friendlier organic transformations on mineral supports under non-traditional conditions. J. Mater. Chem.. 2002;12:3199-3207.
[Google Scholar]
Pithawala E.A., Jain N.K., . In-vitro antioxidative property of water soluble bacterial melanin and evaluation on novel model system. IJRASET. 2014;2:325-333.
[Google Scholar]
Raghavendra M., Naik H.S.B., Sherigara B.S., . Microwave-assisted one-pot synthesis of some new furo [2,3-b] quinolines using potassium carbonate under solvent-free conditions. Can. J. Chem.. 2007;85:1041-1044.
[Google Scholar]
Raghavendra M., Reddy A.M., Yadav P.R., Raju A.S., Kumar L.S., . Comparative studies on the in vitro antioxidant properties of methanolic leafy extracts from six edible leafy vegetables of India. Asian. Pharm. Clin. Res.. 2013;6:96-99.
[Google Scholar]
Ramón D.J., Yus M., . Asymmetric multicomponent reactions (AMCRs): the new frontier. Angew. Chem. Int. Ed.. 2005;44:1602-1634.
[Google Scholar]
Ranu B.C., Hajra A., Jana U., . Indium (III) chloride-catalyzed one-pot synthesis of dihydropyrimidinones by a three-component coupling of 1,3-dicarbonyl compounds, aldehydes, and urea: an improved procedure for the Biginelli reaction. J. Org. Chem.. 2000;65:6270-6272.
[Google Scholar]
Rao G.D., Acharya B., Verma S., Kaushik M., . N,N′-Dichlorobis (2,4,6-trichlorophenyl) urea (CC-2) as a new reagent for the synthesis of pyrimidone and pyrimidine derivatives via Biginelli reaction. Tetrahedron Lett.. 2011;52:809-812.
[Google Scholar]
Re R., Pellegrini N., Proteggente A., Pannala A., Yang M., Rice-Evans C., . Antioxidant activity applying an improved ABTS radical cation decolorization assay. Free Radical Biol. Med.. 1999;26:1231-1237.
[Google Scholar]
Sahu P.K., Sahu P.K., Agarwal D.D., . Efficient and facile synthesis of heterocycles and their mechanistic consideration using kaolin. RSC Adv.. 2013;3:9854-9864.
[Google Scholar]
Sahu P.K., Sahu P.K., Gupta S., Thavaselvam D., Agarwal D., . Synthesis and evaluation of antimicrobial activity of 4H-pyrimido [2,1-b] benzothiazole, pyrazole and benzylidene derivatives of curcumin. Eur. J. Med. Chem.. 2012;54:366-378.
[Google Scholar]
Sahu P.K., Sahu P.K., Jain R., Yadav R., Agarwal D.D., . Hydrotalcite: recyclable, novel heterogeneous catalyst for facile, environmentally benign and high yielding multi-component synthesis and mechanistic study under solvent free conditions. Catal. Sci. Technol.. 2012;2:2465-2475.
[Google Scholar]
Sahu P.K., Sahu P.K., Lal J., Thavaselvam D., Agarwal D., . A facile green synthesis and in vitro antimicrobial activity 4H-pyrimido [2,1-b][1,3] benzothiazole derivatives using aluminum trichloride under solvent free conditions. Med. Chem. Res.. 2012;21:3826-3834.
[Google Scholar]
Saxena V., Chandra U., . Microwave synthesis: a physical concept. INTECH Open Access Publisher; 2011.
Sedash Y.V., Gorobets N.Y., Chebanov V.A., Konovalova I.S., Shishkin O.V., Desenko S.M., . Dotting the i’s in three-component Biginelli-like condensations using 3-amino-1,2,4-triazole as a 1,3-binucleophile. RSC Adv.. 2012;2:6719-6728.
[Google Scholar]
Shaabani A., Rahmati A., Naderi S., . A novel one-pot three-component reaction: synthesis of triheterocyclic 4H-pyrimido [2,1-b] benzazoles ring systems. Bioorg. Med. Chem. Lett.. 2005;15:5553-5557.
[Google Scholar]
Shiri M., . Indoles in multicomponent processes (MCPs) Chem. Rev.. 2012;112:3508-3549.
[Google Scholar]
Simon C., Constantieux T., Rodriguez J., . Utilisation of 1,3-dicarbonyl derivatives in multicomponent reactions. Eur. J. Org. Chem.. 2004;2004:4957-4980.
[Google Scholar]
Singh A., Raj T., Aree T., Singh N., . Fluorescent organic nanoparticles of biginelli-based molecules: recognition of Hg²⁺ and Cl⁻ in an aqueous medium. Inorg. Chem.. 2013;52:13830-13832.
[Google Scholar]
Singh S., Misra R., Parmar S.S., Brumleve S.J., . Synthesis of 2-(4-arylthiosemicarbazidocarbonylthio) benzthiazoles and their monamine oxidase inhibitory and anticonvulsant properties. J. Pharm. Sci.. 1975;64:1245-1247.
[Google Scholar]
Singh S., Vaid R., . Synthesis and antiinflammatory activity of some 2-(4′-butyl-3′, 5′-dimethylpyrazol-1′-yl)-6-substituted-benzothiazoles and 4-butyl-1-(6′-substituted-2′-benzothiazolyl)-3-methylpyrazol-5-ols. Ind. J. Chem. Sec. B. 1986;25:288-291.
[Google Scholar]
Stadler A., Kappe C.O., . Microwave-mediated Biginelli reactions revisited. On the nature of rate and yield enhancements. J. Chem. Soc. Perkin Trans.. 2000;2:1363-1368.
[Google Scholar]
Stadler A., Pichler S., Horeis G., Kappe C.O., . Microwave-enhanced reactions under open and closed vessel conditions. A case study. Tetrahedron. 2002;58:3177-3183.
[Google Scholar]
Stadler A., Yousefi B.H., Dallinger D., Walla P., Van der Eycken E., Kaval N., Kappe C.O., . Scalability of microwave-assisted organic synthesis. From single-mode to multimode parallel batch reactors. Org. Process Res. Dev.. 2003;7:707-716.
[Google Scholar]
Tale R., . Novel synthesis of 2-arylbenzothiazoles mediated by ceric ammonium nitrate (CAN) Organ. Lett.. 2002;4:1641-1642.
[Google Scholar]
Trapani G., Franco M., Latrofa A., Carotti A., Gerichi G., Serra M., Liso G., . Synthesis and benzodiazepine receptor binding of some imidazoand pyrimido [2,1-b] benzothiazoles. Eur. J. Med. Chem.. 1996;31:575-587.
[Google Scholar]
Wu Q.-Y., Jiang L.-L., Yang S.-G., Zuo Y., Wang Z.-F., Xi Z., Yang G.-F., . Hexahydrophthalimide–benzothiazole hybrids as a new class of protoporphyrinogen oxidase inhibitors: synthesis, structure–activity relationship, and DFT calculations. New J. Chem.. 2014;38:4510-4518.
[Google Scholar]
Wu Q.Y., Jiang L.L., Zuo Y., Wang Z.F., Xi Z., Yang G.F., . Synthesis, in vitro protoporphyrinogen oxidase inhibition, and herbicidal activity of N-(Benzothiazol-5-yl) hexahydro-1H-isoindole-1,3-diones and N-(Benzothiazol-5-yl) hexahydro-1H-isoindol-1-ones. Chem. Biol. Drug Des.. 2014;84:431-442.
[Google Scholar]
Xue W.-J., Guo Y.-Q., Gao F.-F., Li H.-Z., Wu A.-X., . A novel self-sequence reaction network involving a set of six reactions in one pot: the synthesis of substituted benzothiazoles from aromatic ketones and anilines. Organ. Lett.. 2013;15:890-893.
[Google Scholar]
Yang Z., Sheng-Gang Y., Yan-Ping L., Ying T., Ge-Fei H., Qiong-You W., Guang-Fu Y., . Design and synthesis of 1-(benzothiazol-5-yl)-1H-1, 2, 4-triazol-5-ones as protoporphyrinogen oxidase inhibitors. Bioorg. Med. Chem.. 2013;21:3245-3255.
[Google Scholar]
Zhang X., Wong O., Gao Z., Lee C., Kwong H., Lee S., Wu S., . A new blue-emitting benzothiazole derivative for organic electroluminescent devices. Mater. Sci. Eng., B. 2001;85:182-185.
[Google Scholar]
Zuo Y., Yang S.-G., Jiang L.-L., Hao G.-F., Wang Z.-F., Wu Q.-Y., Yang G.-F., . Quantitative structure–activity relationships of 1,3,4-thiadiazol-2(3H)-ones and 1,3,4-oxadiazol-2(3H)-ones as human protoporphyrinogen oxidase inhibitors. Bioorg. Med. Chem.. 2012;20:296-304.
[Google Scholar]

Appendix A

Supplementary material

Crystallographic data for the structures 4a have been deposited with Cambridge Crystallo graphic Data Centre as supplementary CCDC 1048059, respectively. These data can be obtained from http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2016.01.012.

Appendix A

Supplementary material

Supplementary data 1

Download

Introduction

Material and methods

Chemistry
Synthesis of ethyl-(substituted)-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate (4a–4l)
Determination of antibacterial activity
Antioxidant assay protocol
In vitro anti-mycobacterial activity
Statistical analysis

Result and discussion

Antibacterial activity
Antioxidant activity
In vitro anti-mycobacterial activity

Conclusion

Fulltext Views
438

PDF downloads
156

View/Download PDF
Download Citations

BibTeX
RIS

Show Sections

[1] Alajarin R., Jordán P., Vaquero J.J., Alvarez-Builla J., . Synthesis of unsymmetrically substituted 1,4-dihydropyridines and analogous calcium antagonists by microwave heating. Synthesis 1995:389-391.
[Google Scholar]

[2] Atar A.B., Jeong Y.T., . Catalyzed cascade C—C and C–N bond formation: synthesis of differentially substituted triheterocyclic benzothiazole functionalities under solvent-free condition. Molec. Diver.. 2014;18:389-401.
[Google Scholar]

[3] Bastug G., Eviolitte C., Markó I.E., . Functionalized orthoesters as powerful building blocks for the efficient preparation of heteroaromatic bicycles. Organ. Lett.. 2012;14:3502-3505.
[Google Scholar]

[4] Baumann J., Wurn G., Bruchlausen F., . Prostaglandin synthetase inhibiting O₂-radical scavenging properties of some flavonoids and related phenolic compounds, naunyn, schmiedebergs. Naunyn-Schmiedebergs Arch. Pharmacol.. 1979;308:R27-R32.
[Google Scholar]

[5] Beauchamp C., Fridovich I., . Superoxide dismutase: improved assays and an assay applicable to acrylamide gels. Anal. Biochem.. 1971;44:276-287.
[Google Scholar]

[6] Bejan V., Mantu D., Mangalagiu I.I., . Ultrasound and microwave assisted synthesis of isoindolo-1,2-diazine: a comparative study. Ultrason. Sonochem.. 2012;19:999-1002.
[Google Scholar]

[7] Benetti S., Romagnoli R., De Risi C., Spalluto G., Zanirato V., . Mastering. beta.-keto esters. Chem. Rev.. 1995;95:1065-1114.
[Google Scholar]

[8] Bhoi M.N., Borad M.A., Panchal N.K., Patel H.D., . 2-Aminobenzothiazole containing novel Schiff bases derivatives: search for new antibacterial agents. Int. Lett. Chem. Phys. Astron.. 2015;53:106-113.
[Google Scholar]

[9] Bhoi M.N., Borad M.A., Parmar H.B., Patel H.D., . Synthesis and characterization of novel N-(benzo [d] thiazol-2-yl)-2-(2-(6-chloroquinolin-4-yl) hydrazinyl) acetamide derivatives containing Quinoline linkage as potent antibacterial agents. Int. Lett. Chem. Phys. Astron.. 2015;53:114-121.
[Google Scholar]

[10] Bhoi M.N., Borad M.A., Patel H.D., . Synthetic strategies for fused benzothiazoles: past, present, and future. Synth. Commun.. 2014;44:2427-2457.
[Google Scholar]

[11] Biginelli P., Gazz P., . Synthesis of 3,4-dihydropyrimidin-2 (1H)-ones. Chim. Ital.. 1893;23:360-416.
[Google Scholar]

[12] Binsan W., Benjakul S., Visessanguan W., Roytrakul S., Tanaka M., Kishimura H., . Antioxidative activity of Mungoong, an extract paste, from the cephalothorax of white shrimp (Litopenaeus vannamei) Food Chem.. 2008;106:185-193.
[Google Scholar]

[13] Boer R., Gekeler V., . Chemosensitizers in tumor therapy: new compounds promise better efficacy. Drugs Future. 1995;20:499-510.
[Google Scholar]

[14] Borad M.A., Bhoi M.N., Parmar J.A., Patel H.D., . Microwave assisted synthesis of novel N-(benzo [d] thiazol-2-yl)-2-((2,4′-dioxospiro [indoline-3,2′-thiazolidin]-3′-yl) amino) acetamide derivatives as potent antibacterial agents. Int. Lett. Chem. Phys. Astron.. 2015;53:122-129.
[Google Scholar]

[15] Brand-Williams W., Cuvelier M., Berset C., . Use of a free radical method to evaluate antioxidant activity. Food Sci. Technol.. 1995;28:25-30.
[Google Scholar]

[16] Bretzel R.G., Bollen C.C., Maeser E., Federlin K.F., . Nephroprotective effects of nitrendipine in hypertensive Tune I and Type II diabetic patients. Am. J. Kidney Dis.. 1993;21:S53-S64.
[Google Scholar]

[17] Canetti G., Fox W., Khomenko A.A., Mahler H.T., Menon N.K., Mitchison D.A., Šmelev N.A., . Advances in techniques of testing mycobacterial drug sensitivity, and the use of sensitivity tests in tuberculosis control programmes. Bull. World Health Organ.. 1969;41:21-43.
[Google Scholar]

[18] Chandrasekara A., Shahidi F., . Inhibitory activities of soluble and bound millet seed phenolics on free radicals and reactive oxygen species. J. Agric. Food Chem.. 2010;59:428-436.
[Google Scholar]

[19] Chebanov V.A., Gura K.A., Desenko S.M., . Aminoazoles as key reagents in multicomponent heterocyclizations Synthesis of Heterocycles via Multicomponent Reactions I. Springer; 2010. pp. 41–84

[20] Choudhary S., Kini S.G., Mubeen M., . Antioxidant activity of novel coumarin substituted benzothiazole derivatives. Der Pharma Chemica. 2013;5:213-222.
[Google Scholar]

[21] Chow C.P., Shea K.J., . Dual function catalysts. Dehydrogenation and asymmetric intramolecular diels-alder cycloaddition of N-hydroxy formate esters and hydroxamic acids: evidence for a ruthenium-acylnitroso intermediate. J. Am. Chem. Soc.. 2005;127:3678-3679.
[Google Scholar]

[22] Dallinger D., Kappe C.O., . Microwave-assisted synthesis in water as solvent. Chem. Rev.. 2007;107:2563-2591.
[Google Scholar]

[23] de Graaff C., Ruijter E., Orru R.V., . Recent developments in asymmetric multicomponent reactions. Chem. Soc. Rev.. 2012;41:3969-4009.
[Google Scholar]

[24] De la Hoz A., Diaz-Ortiz A., Moreno A., . Microwaves in organic synthesis. Thermal and non-thermal microwave effects. Chem. Soc. Rev.. 2005;34:164-178.
[Google Scholar]

[25] Desai B., Dallinger D., Kappe C.O., . Microwave-assisted solution phase synthesis of dihydropyrimidine C5 amides and esters. Tetrahedron. 2006;62:4651-4664.
[Google Scholar]

[26] Devineni S.R., Doddaga S., Donka R., Chamarthi N.R., . CeCl₃·7H₂O–SiO₂: catalyst promoted microwave assisted neat synthesis, antifungal and antioxidant activities of α-diaminophosphonates. Chin. Chem. Lett.. 2013;24:759-763.
[Google Scholar]

[27] Dömling A., . Recent developments in isocyanide based multicomponent reactions in applied chemistry. Chem. Rev.. 2006;106:17-89.
[Google Scholar]

[28] Eynde J.J.V., Hecq N., Kataeva O., Kappe C.O., . Microwave-mediated regioselective synthesis of novel pyrimido [1,2-a] pyrimidines under solvent-free conditions. Tetrahedron. 2001;57:1785-1791.
[Google Scholar]

[29] Gülçin İ., Topal F., Çakmakçı R., Bilsel M., Gören A.C., Erdogan U., . Pomological features, nutritional quality, polyphenol content analysis, and antioxidant properties of domesticated and 3 wild ecotype forms of raspberries (Rubus idaeus L.) J. Food Sci.. 2011;76:C585-C593.
[Google Scholar]

[30] Hagerman A.E., Riedl K.M., Jones G.A., Sovik K.N., Ritchard N.T., Hartzfeld P.W., Riechel T.L., . High molecular weight plant polyphenolics (tannins) as biological antioxidants. J. Agri. Food Chem.. 1998;46:1887-1892.
[Google Scholar]

[31] Hayes, B. L., 2002. Microwave Synthesis: Chemistry at the Speed of Light: Cem Corporation.

[32] Hayes B.L., . Recent advances in microwave-assisted synthesis. Aldrichim. Acta. 2004;37:66-77.
[Google Scholar]

[33] Hopper R.J., . 2-(Isopropylsulfinyl)-benzothiazole as a delayed action thiazole accelerator. Rubber Chem. Technol.. 1993;66:623-633.
[Google Scholar]

[34] Huang W., Yang G.-F., . Microwave-assisted, one-pot syntheses and fungicidal activity of polyfluorinated 2-benzylthiobenzothiazoles. Bioorg. Med. Chem.. 2006;14:8280-8285.
[Google Scholar]

[35] Hutchinson I., Chua M.-S., Browne H.L., Trapani V., Bradshaw T.D., Westwell A.D., Stevens M.F., . Antitumor benzothiazoles. 14. 1 synthesis and in vitro biological properties of fluorinated 2-(4-aminophenyl) benzothiazoles. J. Med. Chem.. 2001;44:1446-1455.
[Google Scholar]

[36] Jaseer E., Prasad D., Dandapat A., Sekar G., . An efficient copper (II)-catalyzed synthesis of benzothiazoles through intramolecular coupling-cyclization of N-(2-chlorophenyl) benzothioamides. Tetrahedron Lett.. 2010;51:5009-5012.
[Google Scholar]

[37] Jiang L.-L., Tan Y., Zhu X.-L., Wang Z.-F., Zuo Y., Chen Q., Yang G.-F., . Design, synthesis, and 3D-QSAR analysis of novel 1,3,4-oxadiazol-2 (3H)-ones as protoporphyrinogen oxidase inhibitors†. J. Agric. Food Chem.. 2009;58:2643-2651.
[Google Scholar]

[38] Jiang L.-L., Zuo Y., Wang Z.-F., Tan Y., Wu Q.-Y., Xi Z., Yang G.-F., . Design and syntheses of novel N-(benzothiazol-5-yl)-4,5,6,7-tetrahydro-1H-isoindole-1,3(2H)-dione and N-(benzothiazol-5-yl) isoindoline-1,3-dione as potent protoporphyrinogen oxidase inhibitors. J. Agric. Food Chem.. 2011;59:6172-6179.
[Google Scholar]

[39] Jiang X., Cao Y., Wang Y., Liu L., Shen F., Wang R., . A unique approach to the concise synthesis of highly optically active spirooxazolines and the discovery of a more potent oxindole-type phytoalexin analogue. J. Am. Chem. Soc.. 2010;132:15328-15333.
[Google Scholar]

[40] Kappe C.O., . Recent advances in the Biginelli dihydropyrimidine synthesis. New tricks from an old dog. Acc. Chem. Res.. 2000;33:879-888.
[Google Scholar]

[41] Kappe C.O., . High-speed combinatorial synthesis utilizing microwave irradiation. Curr. Opin. Chem. Biol.. 2002;6:314-320.
[Google Scholar]

[42] Kappe C.O., . Controlled microwave heating in modern organic synthesis. Angew. Chem. Int. Ed. Engl.. 2004;43:6250-6284.
[Google Scholar]

[43] Kappe C.O., . Microwave dielectric heating in synthetic organic chemistry. Chem. Soc. Rev.. 2008;37:1127-1139.
[Google Scholar]

[44] Kappe C.O., Stadler A., . The Biginelli dihydropyrimidine synthesis. Organic reactions. John Wiley & Sons Inc.; 2004.

[45] Kappe C.O., Stadler A., Dallinger D., . Microwaves in Organic and Medicinal Chemistry. John Wiley & Sons; 2012.

[46] Karle M., Knecht W., Xue Y., . Discovery of benzothiazole guanidines as novel inhibitors of thrombin and trypsin IV. Bioorg. Med. Chem. Lett.. 2012;22:4839-4843.
[Google Scholar]

[47] Kaur N., Kaur K., Raj T., Kaur G., Singh A., Aree T., Jang D.O., . One-pot synthesis of tricyclic dihydropyrimidine derivatives and their biological evaluation. Tetrahedron. 2015;71:332-337.
[Google Scholar]

[48] Kent, P.T., Kubica, G.P., 1985. Antituberculosis chemotherapy and drug susceptibility testing. Public Health Mycobacteriology: A Guide for the Level III Laboratory. US Department of Health and Human Services, Centers for Disease Control and Prevention, pp. 159–184.

[49] Khabazzadeh H., Saidi K., Sheibani H., . Microwave-assisted synthesis of dihydropyrimidin-2(1H)-ones using graphite supported lanthanum chloride as a mild and efficient catalyst. Bioorg. Med. Chem. Lett.. 2008;18:278-280.
[Google Scholar]

[50] Klusa V., . Cerebrocrast. Neuroprotectant, cognition enhancer. Drugs Fut.. 1995;20:135-138.
[Google Scholar]

[51] Langer P., . Regio-and diastereoselective cyclization reactions of free and masked 1,3-dicarbonyl dianions with 1,2-dielectrophiles. Chem. Eur. J.. 2001;7:3858-3866.
[Google Scholar]

[52] Leadbeater N.E., Williams V.A., Barnard T.M., Collins M.J., . Open-vessel microwave-promoted Suzuki reactions using low levels of palladium catalyst: optimization and scale-up. Org. Process Res. Dev.. 2006;10:833-837.
[Google Scholar]

[53] Nagarapu L., Gaikwad H.K., Palem J.D., Venkatesh R., Bantu R., Sridhar B., . Convenient approach for the one-pot, three-component synthesis of triheterocyclic 4 H-pyrimido [2,1-b] benzothiazole derivatives using TBAHS. Synth. Commun.. 2013;43:93-104.
[Google Scholar]

[54] Nair V., Rajesh C., Vinod A., Bindu S., Sreekanth A., Mathen J., Balagopal L., . Strategies for heterocyclic construction via novel multicomponent reactions based on isocyanides and nucleophilic carbenes. Acc. Chem. Res.. 2003;36:899-907.
[Google Scholar]

[55] Nishikimi M., Rao N.A., Yagi K., . The occurrence of superoxide anion in the reaction of reduced phenazine methosulfate and molecular oxygen. Biochem. Biophy. Res. Commun.. 1972;46:849-854.
[Google Scholar]

[56] Noolvi M.N., Patel H.M., Kaur M., . Benzothiazoles: search for anticancer agents. Eur. J. Med. Chem.. 2012;54:447-462.
[Google Scholar]

[57] Oliver Kappe C., . 100 years of the biginelli dihydropyrimidine synthesis. Tetrahedron. 1993;49:6937-6963.
[Google Scholar]

[58] Pande A., Lokhande S., Patel M., Khadse B., . Synthesis and study of 2-substituted alkyl or aryl, amino 6-methyl benzothiazoles as antituberculosis agents. Indian Drugs. 1982;19:342-345.
[Google Scholar]

[59] Perreux L., Loupy A., . Nonthermal effects of microwaves in organic synthesis.Microwaves in Organic Synthesis. Wiley Online Library; 2002. pp. 61–114

[60] Pillai U.R., Sahle-Demessie E., Varma R.S., . Environmentally friendlier organic transformations on mineral supports under non-traditional conditions. J. Mater. Chem.. 2002;12:3199-3207.
[Google Scholar]

[61] Pithawala E.A., Jain N.K., . In-vitro antioxidative property of water soluble bacterial melanin and evaluation on novel model system. IJRASET. 2014;2:325-333.
[Google Scholar]

[62] Raghavendra M., Naik H.S.B., Sherigara B.S., . Microwave-assisted one-pot synthesis of some new furo [2,3-b] quinolines using potassium carbonate under solvent-free conditions. Can. J. Chem.. 2007;85:1041-1044.
[Google Scholar]

[63] Raghavendra M., Reddy A.M., Yadav P.R., Raju A.S., Kumar L.S., . Comparative studies on the in vitro antioxidant properties of methanolic leafy extracts from six edible leafy vegetables of India. Asian. Pharm. Clin. Res.. 2013;6:96-99.
[Google Scholar]

[64] Ramón D.J., Yus M., . Asymmetric multicomponent reactions (AMCRs): the new frontier. Angew. Chem. Int. Ed.. 2005;44:1602-1634.
[Google Scholar]

[65] Ranu B.C., Hajra A., Jana U., . Indium (III) chloride-catalyzed one-pot synthesis of dihydropyrimidinones by a three-component coupling of 1,3-dicarbonyl compounds, aldehydes, and urea: an improved procedure for the Biginelli reaction. J. Org. Chem.. 2000;65:6270-6272.
[Google Scholar]

[66] Rao G.D., Acharya B., Verma S., Kaushik M., . N,N′-Dichlorobis (2,4,6-trichlorophenyl) urea (CC-2) as a new reagent for the synthesis of pyrimidone and pyrimidine derivatives via Biginelli reaction. Tetrahedron Lett.. 2011;52:809-812.
[Google Scholar]

[67] Re R., Pellegrini N., Proteggente A., Pannala A., Yang M., Rice-Evans C., . Antioxidant activity applying an improved ABTS radical cation decolorization assay. Free Radical Biol. Med.. 1999;26:1231-1237.
[Google Scholar]

[68] Sahu P.K., Sahu P.K., Agarwal D.D., . Efficient and facile synthesis of heterocycles and their mechanistic consideration using kaolin. RSC Adv.. 2013;3:9854-9864.
[Google Scholar]

[69] Sahu P.K., Sahu P.K., Gupta S., Thavaselvam D., Agarwal D., . Synthesis and evaluation of antimicrobial activity of 4H-pyrimido [2,1-b] benzothiazole, pyrazole and benzylidene derivatives of curcumin. Eur. J. Med. Chem.. 2012;54:366-378.
[Google Scholar]

[70] Sahu P.K., Sahu P.K., Jain R., Yadav R., Agarwal D.D., . Hydrotalcite: recyclable, novel heterogeneous catalyst for facile, environmentally benign and high yielding multi-component synthesis and mechanistic study under solvent free conditions. Catal. Sci. Technol.. 2012;2:2465-2475.
[Google Scholar]

[71] Sahu P.K., Sahu P.K., Lal J., Thavaselvam D., Agarwal D., . A facile green synthesis and in vitro antimicrobial activity 4H-pyrimido [2,1-b][1,3] benzothiazole derivatives using aluminum trichloride under solvent free conditions. Med. Chem. Res.. 2012;21:3826-3834.
[Google Scholar]

[72] Saxena V., Chandra U., . Microwave synthesis: a physical concept. INTECH Open Access Publisher; 2011.

[73] Sedash Y.V., Gorobets N.Y., Chebanov V.A., Konovalova I.S., Shishkin O.V., Desenko S.M., . Dotting the i’s in three-component Biginelli-like condensations using 3-amino-1,2,4-triazole as a 1,3-binucleophile. RSC Adv.. 2012;2:6719-6728.
[Google Scholar]

[74] Shaabani A., Rahmati A., Naderi S., . A novel one-pot three-component reaction: synthesis of triheterocyclic 4H-pyrimido [2,1-b] benzazoles ring systems. Bioorg. Med. Chem. Lett.. 2005;15:5553-5557.
[Google Scholar]

[75] Shiri M., . Indoles in multicomponent processes (MCPs) Chem. Rev.. 2012;112:3508-3549.
[Google Scholar]

[76] Simon C., Constantieux T., Rodriguez J., . Utilisation of 1,3-dicarbonyl derivatives in multicomponent reactions. Eur. J. Org. Chem.. 2004;2004:4957-4980.
[Google Scholar]

[77] Singh A., Raj T., Aree T., Singh N., . Fluorescent organic nanoparticles of biginelli-based molecules: recognition of Hg²⁺ and Cl⁻ in an aqueous medium. Inorg. Chem.. 2013;52:13830-13832.
[Google Scholar]

[78] Singh S., Misra R., Parmar S.S., Brumleve S.J., . Synthesis of 2-(4-arylthiosemicarbazidocarbonylthio) benzthiazoles and their monamine oxidase inhibitory and anticonvulsant properties. J. Pharm. Sci.. 1975;64:1245-1247.
[Google Scholar]

[79] Singh S., Vaid R., . Synthesis and antiinflammatory activity of some 2-(4′-butyl-3′, 5′-dimethylpyrazol-1′-yl)-6-substituted-benzothiazoles and 4-butyl-1-(6′-substituted-2′-benzothiazolyl)-3-methylpyrazol-5-ols. Ind. J. Chem. Sec. B. 1986;25:288-291.
[Google Scholar]

[80] Stadler A., Kappe C.O., . Microwave-mediated Biginelli reactions revisited. On the nature of rate and yield enhancements. J. Chem. Soc. Perkin Trans.. 2000;2:1363-1368.
[Google Scholar]

[81] Stadler A., Pichler S., Horeis G., Kappe C.O., . Microwave-enhanced reactions under open and closed vessel conditions. A case study. Tetrahedron. 2002;58:3177-3183.
[Google Scholar]

[82] Stadler A., Yousefi B.H., Dallinger D., Walla P., Van der Eycken E., Kaval N., Kappe C.O., . Scalability of microwave-assisted organic synthesis. From single-mode to multimode parallel batch reactors. Org. Process Res. Dev.. 2003;7:707-716.
[Google Scholar]

[83] Tale R., . Novel synthesis of 2-arylbenzothiazoles mediated by ceric ammonium nitrate (CAN) Organ. Lett.. 2002;4:1641-1642.
[Google Scholar]

[84] Trapani G., Franco M., Latrofa A., Carotti A., Gerichi G., Serra M., Liso G., . Synthesis and benzodiazepine receptor binding of some imidazoand pyrimido [2,1-b] benzothiazoles. Eur. J. Med. Chem.. 1996;31:575-587.
[Google Scholar]

[85] Wu Q.-Y., Jiang L.-L., Yang S.-G., Zuo Y., Wang Z.-F., Xi Z., Yang G.-F., . Hexahydrophthalimide–benzothiazole hybrids as a new class of protoporphyrinogen oxidase inhibitors: synthesis, structure–activity relationship, and DFT calculations. New J. Chem.. 2014;38:4510-4518.
[Google Scholar]

[86] Wu Q.Y., Jiang L.L., Zuo Y., Wang Z.F., Xi Z., Yang G.F., . Synthesis, in vitro protoporphyrinogen oxidase inhibition, and herbicidal activity of N-(Benzothiazol-5-yl) hexahydro-1H-isoindole-1,3-diones and N-(Benzothiazol-5-yl) hexahydro-1H-isoindol-1-ones. Chem. Biol. Drug Des.. 2014;84:431-442.
[Google Scholar]

[87] Xue W.-J., Guo Y.-Q., Gao F.-F., Li H.-Z., Wu A.-X., . A novel self-sequence reaction network involving a set of six reactions in one pot: the synthesis of substituted benzothiazoles from aromatic ketones and anilines. Organ. Lett.. 2013;15:890-893.
[Google Scholar]

[88] Yang Z., Sheng-Gang Y., Yan-Ping L., Ying T., Ge-Fei H., Qiong-You W., Guang-Fu Y., . Design and synthesis of 1-(benzothiazol-5-yl)-1H-1, 2, 4-triazol-5-ones as protoporphyrinogen oxidase inhibitors. Bioorg. Med. Chem.. 2013;21:3245-3255.
[Google Scholar]

[89] Zhang X., Wong O., Gao Z., Lee C., Kwong H., Lee S., Wu S., . A new blue-emitting benzothiazole derivative for organic electroluminescent devices. Mater. Sci. Eng., B. 2001;85:182-185.
[Google Scholar]

[90] Zuo Y., Yang S.-G., Jiang L.-L., Hao G.-F., Wang Z.-F., Wu Q.-Y., Yang G.-F., . Quantitative structure–activity relationships of 1,3,4-thiadiazol-2(3H)-ones and 1,3,4-oxadiazol-2(3H)-ones as human protoporphyrinogen oxidase inhibitors. Bioorg. Med. Chem.. 2012;20:296-304.
[Google Scholar]

Entry	Benzothiazole	Product	Δ		MW
Entry	Benzothiazole	Product	Time (min)	Yield^a (%)	Time (min)	Yield^a (%)
1			45	75.6	3	85.1

2			51	74.7	3	82.3

3			47	78.2	3	87.4

4			55	71.6	5	80.6

5			69	70.6	5	80.1

6			53	72.3	3	82.6

7			44	70.3	3	81.4

8			40	75.6	3	85.3

9			57	73.3	3	82.5

10			20	79.4	3	87.2

11			52	71.2	4	80.8

12			58	69.3	4	77.8

Novel benzothiazole containing 4H-pyrimido[2,1-b]benzothiazoles derivatives: One pot, solvent-free microwave assisted synthesis and their biological evaluation

Abstract

Abstract

Keywords

Antioxidant

2-Amino benzothiazole

Microwave irradiation

4H-pyrimido[2,1-b]benzothiazoles

Solvent-free reaction

Mycobacterium tuberculosis

1 Introduction

2 Material and methods

2.1 Chemistry

2.2 Synthesis of ethyl-(substituted)-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate (4a–4l)

2.2.1 Method A (microwave assisted method)

2.2.2 Method B (conventional method)

2.2.3 Ethyl-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4a)

2.2.4 Ethyl-8-bromo-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4b)

2.2.5 Ethyl-2,8-dimethyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4c)

2.2.6 Ethyl-2,6-dimethyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4d)

2.2.7 Ethyl-2-methyl-8-nitro-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4e)

2.2.8 Ethyl-8-chloro-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4f)

2.2.9 Ethyl-6-chloro-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4g)

2.2.10 Ethyl-8-fluoro-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate (4h)

2.2.11 Ethyl-8-methoxy-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4i)

2.2.12 Ethyl-8-ethoxy-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4j)

2.2.13 Ethyl-2-methyl-4-(pyridin-2-yl)-8-(trifluoromethoxy)-4H-benzo[4,5]thiazolo[3,2-a] pyrimidine-3-carboxylate (4k)

2.2.14 Ethyl-8-hydroxy-2-methyl-4-(pyridin-2-yl)-4H-benzo[4,5]thiazolo[3,2-a]pyrimidine-3-carboxylate (4l)

2.3 Determination of antibacterial activity

2.4 Antioxidant assay protocol

2.4.1 DPPH radical scavenging activity assay

2.4.2 Superoxide radical scavenging activity

2.4.3 ABTS+ radical scavenging activity

2.5 In vitro anti-mycobacterial activity

2.6 Statistical analysis

3 Result and discussion

3.1 Antibacterial activity

3.1.1 Determination of activity index

3.2 Antioxidant activity

3.3 In vitro anti-mycobacterial activity

4 Conclusion

Acknowledgment

References

Appendix A

Supplementary material

Appendix A

Supplementary material

Supplementary data 1

Suggested read for related articles:

2.4.3 ABTS⁺ radical scavenging activity