Introduction

Phenothiazines are an important class of drugs exhibiting antipsychotic, antihistaminic, antitussive, and anti-emetic activities (Gupta and Kumar, 1988). The most significant modifications of the phenothiazine structure are the introduction of new pharmacophoric substituents at the thiazine nitrogen atom and the substitution of the benzene rings with other homoaromatic or heteroaromatic ones. Recently studied phenothiazines exhibit promising antibacterial, antifungal, anticancer, antiviral, anti-inflammatory, antimalarial, antifilarial, trypanocidal, anticonvulsant, analgesic, immunosuppressive, and multidrug resistance reversal properties (Aaron et al., 2009; Dasgupta et al., 2008; Motohashi et al., 2006; Pluta et al., 2011). In our study of new azaphenothiazines, we elaborated the synthesis of new types of phenothiazines containing the heterocyclic rings of pyridine or quinoline. Some of those azaphenothiazines exhibited promising immunosuppressive and anticancer activities against cell lines of ten types of human cancer in vitro: leukemia, non-small cell lung cancer, melanoma, as well as colon, CNS, ovarian, renal, prostate, breast, and skin cancer (Jeleń et al., 2013; Pluta et al., 2010; Zimecki et al., 2009).

Free radicals, generated in many redox processes, may induce oxidative damage of proteins, lipids, and DNA. They affect living cells and mediate the pathogenesis of many chronic diseases, such as atherosclerosis, Parkinson’s and Alzheimer’s diseases, stroke, and arthritis, acting by various mechanisms. A recent trend in the field of antioxidant development focuses on multipotent antioxidant agents that not only can prevent biological substrates from radical induced oxidative damage but also possess additional pharmacological properties (Zhang et al., 2006). The study of antioxidant activity among N-heterocycles has attracted attention. One such heterocyclic structural scaffold is the 1,4-thiazine ring present in the multi-target phenothiazines. Therefore, recent reports on promising antioxidant compounds deal with classical and new phenothiazines (Asghar et al., 2012; Borges et al., 2010; Liu et al., 2009; Naik et al., 2012;) and their derivatives, benzothiazines (Matralis et al., 2011), and azaphenothiazines (Kumar et al., 2010; Morak-Młodawska et al., 2010).

Our previous work (Morak-Młodawska et al., 2010) revealed that tricyclic azaphenothiazines being dipyridothiazines have a variable degree of antioxidant activity depending on substitution at the thiazine nitrogen atom, with the unsubstituted compound being the most active. In this study, we obtained eleven tetracyclic and pentacyclic (linearly and angularly fused) azaphenothiazines containing one or two quinoline rings instead of the benzene rings and determined their antioxidant properties to find an influence of the number of rings, their type of fusion, and their substituents.

Materials and methods

General techniques

Melting points were determined in open capillary tubes on a Boetius melting point apparatus and were uncorrected. The 1H NMR spectra were recorded on a Bruker Fourier 300 and a Bruker DRX spectrometer at 500 MHz in CDCl3 and DMSO-d 6 with tetramethylsilane as the internal standard. The 13C NMR spectra were recorded at 75 MHz. Electron impact (EI MS) mass spectra were run on a Finnigan MAT 95 spectrometer at 70 eV. The thin-layer chromatography was performed on aluminum oxide 60 F254 neutral (type E, Merck 1.05581) with CH2Cl2 and on silica gel 60 F254 (Merck 1.05735) with CHCl3-EtOH (10:1 v/v) as eluents.

Synthesis of substrates 1, 2, 7, 8, 10, and 11

The substrates for the title compounds, i.e., diquinodithiins 1, 7, 10, sulfides 8, 11, and disulfide 2, were obtained as described previously (Nowak et al., 2002, 2003, 2007; Pluta, 1994).

Quino[3,2-b]benzo[1,4]thiazines (3a–c)

From diquino-1,4-dithiin 1

A mixture of diquino-1,4-dithiin 1 (0.16 g, 0.5 mmol) and hydrochloride of aniline, or p-chloroaniline or p-methoxyaniline (2.5 mmol) was finely powdered together and then heated on an oil bath at 200–205 °C for 4 h and after cooling water was added (10 ml) and the insoluble solid was filtered off. The filtrate was alkalized with 5 % aqueous sodium hydroxide to pH 10, and the resulting solid was filtered off and washed with water. The combined solids were purified by column chromatography (silica gel, CHCl3) to give quinobenzothiazines 3a–c.

6H-Quinobenzothiazine (3a)

0.06 g (24 %), yellow, mp 169–170 °C (mp 169–170 °C, Jeleń and Pluta, 2009). 1H NMR (CDCl3) δ: 6.62 (m, 1H, H-7), 6.87 (m, 1H, H-9), 7.03 (m, 2H, H-8, H-10), 7.26 (t, 1H, H-2), 7.47 (m, 2H, H-1, H-3), 7.53 (s, 1H, H-12), 7.56 (d, 1H, H-4). 13C NMR (CDCl3) δ: 115.57 (C-7), 116.49 and 116.69 (C-10a, C-11a), 122.95 (C-9), 124.19 (C-2), 125.86 (C-10), 126.04 and 126.45 (C-1, C-8), 126.56 (C-12a), 127.57 (C-4), 129.52 (C-3), 131.69 (C-12), 138.45 (C-6a), 145.40 (C-4a), 150.98 (C-5a).

6H-9-Chloroquinobenzothiazine (3b)

0.08 g (28 %), yellow, mp 224–225 °C (mp 224–225 °C, Jeleń and Pluta, 2009). 1H NMR (CDCl3) δ: 6.63 (d, 1H, H-7), 6.99 (s, 1H, H-10), 7.01 (d, 1H, H-8), 7.33 (t, 1H, H-2), 7.51 (d, 1H, H-1), 7.52 (t, 1H, H-3), 7.59 (d, 1H, H-4), 7.60 (s, 1H, H-12). 13C NMR (CDCl3) δ: 115.80 (C-11a), 116.71 (C-7), 118.19 (C-10a), 124.84 and 124.91 (C-8, C-10), 125.65 (C-2), 126.13 (C-12a), 126.61 (C-1), 127.59 (C-4), 128.56 (C-9), 130.31 (C-3), 132.35 (C-12), 136.29 (C-6a), 143.81 (C-4a), 150.04 (C-5a),

6H-9-Methoxyquinobenzothiazine (3c)

0.09 g (32 %), orange, mp 159–160 °C.

1H NMR (CDCl3) δ 3.76 (s, 3H, CH3), 6.54 (d, 1H, H-7), 6.63 (d, 1H, H-10), 6.76 (d, 1H, H-8), 7.29 (t, 1H, H-2), 7.46 (d, 1H, H-1), 7.52 (t, 1H, H-3), 7.55 (s, 1H, H-12), 7.57 (d, 1H, H-4). 13C NMR (CDCl3) δ: 111.59 (C-10), 113.22 (C-8), 116.41 (C-11a), 116.82 (C-7), 117.39 (C-10a), 124.36 and 124.49 (C-1, C-2), 125.80 (C-12a), 126.55 (C-4), 130.10 (C-3), 130.60 (C-6a), 132.07 (C-12), 143.40 (C-4a), 150.36 (C-5a), 156.12 (C-9). EIMS m/z: 280 (M+, 100), 265 (M-CH3, 90). Anal. Calcd. for C16H12N2OS: C, 68.55; H, 4.31; N, 9.99. Found: C, 68.45; H, 4.36; N, 9.82.

From 2,2′-dichloro-3,3′-diquinolinyl disulfide (2)

A solution of disulfide 2 (0.20 g, 0.5 mmol) and p-methoxyaniline (0.25 g, 2 mmol) in monomethyl ether of diethylene glycol (MEDG) (5 ml) was refluxed for 3 h. After cooling, the solution was poured into water (20 ml) and alkalized with 5 % aqueous sodium hydroxide to pH 10. The resulting solid was filtered off, washed with water, and purified by column chromatography (silica gel, CHCl3) to give 0.18 g (64 %) of 6H-9-methoxyquinobenzothiazine (3c).

Quino[3,2-b]naphtho[1′,2′-e][1,4]thiazine (4)

Diquinodithiin 1 (0.16 g, 0.5 mmol) was finely powdered together with 1-naphthylamine hydrochloride (0.45 g, 2.5 mmol) on an oil bath at 200–205 °C for 4 h. After cooling, the solution was poured into water (10 ml) and alkalized with 5 % aqueous sodium hydroxide to pH 10. The resulting solid was filtered off, washed with water, and purified by column chromatography (Al2O3, CHCl3) to give 0.08 g (27 %) of 14H-quinonaphthothiazine (4), orange, mp 147-148 °C.

1H NMR (CDCl3) δ: 7.01 (d,1H, H-6), 7.30 (t, 1H, H-10), 7.47 (m, 4H, H-3, H-4, H-5, H-9), 7.52 (t, 1H, H-2), 7.56 (s, 1H, H-8), 7.60 (t, 1H, H-11), 7.64 (d, 1H, H-12), 7.75 (d, 1H, H-1). 13C NMR (CDCl3) δ: 110.98 (C-6a), 116.91 (C-7a), 118.43 (C-1), 121.89 (C-14b), 122.87 (C-6), 123.70 (C-5), 124.49 (C-10), 125.93, 126.45 and 126.83 (C-2, C-3, C-9), 126.90 (C-8a), 128.92 and 129.65 (C-4, C-12), 131.54 (C-11), 132.55 (C-4a), 133.04 (C-8), 135.07 (C-14a), 145.23 (C-12a), 150.98 (C-13a). EIMS m/z: 300 (M+, 100), 268 (M-S, 45). Anal. Calcd. for C19H12N2S: C, 75.97; H, 4.03; N, 9.33. Found: C, 75.82; H, 4.07; N, 9.21.

Quino[3,2-b]naphtho[2′,1′-e][1,4]thiazine (5)

Diquinodithiin 1 (0.16 g, 0.5 mmol) was finely powdered together with 2-naphthylamine hydrochloride (0.45 g, 2.5 mmol) on an oil bath at 200–205 °C for 4 h. After cooling, the solution was poured into water (10 ml) and alkalized with 5 % aqueous sodium hydroxide to pH 10. The resulting solid was filtered off, washed with water, and purified by column chromatography (Al2O3, CHCl3) to give 0.12 g (40 %) of 7H-quinonaphthothiazine (5), greenish, mp 244-245 °C.

1H NMR (CDCl3) δ: 7.06 (d, 1H, H-6), 7.37 (t, 1H, H-11), 7.47 (t, 1H, H-3), 7,57 (m, 3H, H-2, H-10, H-12), 7.65 (d, 1H, H-5), 7.66 (d, 1H, H-4), 7.72 (s, 1H, H-13), 7.80 (m, 2H, H-9, H-1). 13C NMR (CDCl3) δ: 107.94 (C-14a), 115.77 (C-13a), 116.04 (C-6), 121.32 (C-1), 123.33, 123.66 and 123.89 (C-3, C-9, C-11), 125.23 (C-12a), 125.62 (C-2), 126.36, 126.99 and 127.56 (C-4, C-5, C-12), 128.73 (C-4a), 129.22 (C-10), 129.62 (C-14b), 131.51 (C-13), 133.54 (C-6a), 142.13 (C-8a), 149.64 (C-7a). EIMS m/z: 300 (M+, 100), 268 (M-S, 50). Anal. Calcd. for C19H12N2S: C, 75.97; H, 4.03; N, 9.33. Found: C, 75.88; H, 4.05; N, 9.19.

Diquino[3,2-b;6′,5′-e][1,4]thiazine (6)

Diquinodithiin 1 (0.16 g, 0.5 mmol) was finely powdered together with 6-aminoquinoline hydrochloride (0.46 g, 2.5 mmol) on an oil bath at 200–205 °C for 4 h. After cooling, the solution was poured into water (10 ml) and alkalized with 5 % aqueous sodium hydroxide to pH 10. The resulting solid was filtered off, washed with water, and purified by column chromatography (Al2O3, CHCl3) to give 0.10 g (33 %) of 7H-diquinothiazine (6), brown, mp 260–261 °C.

1H NMR (CDCl3) δ: 7.44 (t, 1H, H-11), 7.49 (d, 1H, H-6), 7.57 (m, 2H, H-2, H-12), 7.64 (t, 1H, H-10), 7.70 (d, 1H, H-9), 7.75 (s, 1H, H-13), 8.10 (d, 1H, H-5), 8.19 (d, 1H, H-1), 8.90 (d, 1H, H-3). 13C NMR (CDCl3) δ: 107.62 (C-14a), 114.59 (C-13a), 119.33 (C-6), 120.76 (C-2), 124.05 (C-11), 124.37 and 125.45 (C-12a, C-14b), 125.65 (C-12), 128.27, 129.24, 129.62 and 129.64 (C-1, C-5, C-9, C-10), 131.80 (C-13), 134.54 (C-6a), 144.53 (C-7a), 147.55 (C-3), 149.49 and 149.55 (C-4a, C-8a). EIMS m/z: 301 (M+, 100), 269 (M-S, 45). Anal. Calcd. for C18H11N3S: C, 71.74; H, 3.68; N, 13.94. Found: C, 71.59; H, 3.71; N, 13.72.

Diquino[3,2-b;2′,3′-e][1,4]thiazines (9)

6H-Diquinothiazine 9a

This compound was obtained in the reaction of diquinodithiin 7 with acetamide (Nowak et al., 2007), orange, mp > 300 °C (mp > 300 °C, Nowak et al., 2007). 1H NMR (CDCl3) δ: 7.42 (t, 2H, H-2, H-10), 7.55 (d, 2H, H-1, H-11), 7.62 (t, 2H, H-3, H-9), 7.72 (s, 2H, H-12, H-14), 7.86 (d, 2H, H-4, H-8). 13C NMR (DMSO-d 6) δ: 124.83 (C-12a, C-13a), 127.29 (C-2, C-10), 128.00 (C-11a, C-14a), 128.16 and 128.28 (C-1, C-11 and C-4, C-8), 131.29 (C-3, C-9), 135.26 (C-12, C-14), 146.58 (C-4a, C-7a), 156.22 (C-5a, C-6a).

6-(p-Fluorophenyl)diquinothiazine (9b)

From diquinodithiin 7

Diquinodithiin 7 (0.16 g, 0.5 mmol) was finely powdered together with p-fluoroaniline hydrochloride (0.37 g, 2.5 mmol), and the mixture was heated on an oil bath at 200–205 °C for 3 h. After cooling, water (10 ml) was added to the reaction mixture and the resulting solid was filtered off, washed with water, air-dried, and purified by column chromatography (Al2O3, CH2Cl2) to give 0.14 g (35 %) of 6-(p-fluorophenyl)diquinothiazine (9b), yellow, mp 248–249 °C.

From 2,2′-dichloro-3,3′-diquinolinyl sulfide 8

A solution of sulfide 8 (0.18 g, 0.5 mmol) and p-fluoroaniline (0.17 g, 1.5 mmol) in MEDG (5 ml) was refluxed for 3 h. After cooling, the solution was poured into water (20 ml) and alkalized with 5 % aqueous sodium hydroxide to pH = 10. The resulting solid was filtered off, washed with water, and purified by column chromatography (Al2O3, CH2Cl2) to give 0.16 g (81 %) 6-(p-fluorophenyldiquinothiazine (9b), yellow, mp 248–249 °C.

1H NMR (CDCl3) δ: 7.31 (m, 4H, H-2, H-10, C6H2), 7.47 (m, 4H, H-3, H-9, C6H2), 7.56 (d, 2H, H-1, H-11), 7.67 (d, 2H, H-4, H-8), 7.83 (s, 2H, H-12, H-14). 13C NMR (CDCl3) δ: 115.85 (J = 22.6 Hz, m-C of C6H4F), 115.98 (C-12a, C-13a), 125.16 (C-2, C-10), 125.78 (C-11a, C-14a), 125.96 (C-1, C-11), 128.07 (C-4, C-8), 129.37 (C-3, C-9), 132.07 (C-12, C-14), 132.40 (J = 7.5 Hz, o-C of C6H4F), 135.59 (J = 2.5 Hz, ipso-C of C6H4F), 145.13 (C-4a, C-7a), 150.98 (C-5a, C-6a), 161.83 (J = 244.6 Hz, p–C of C6H4F).

EIMS m/z: 395 (M+, 75), 394 (M-1, 100), 363 (M-S, 5). Anal. Calcd. for C24H14FN3S: C, 72.89; H, 3.57; N, 10.63. Found: C, 72.80; H, 3.55; N, 10.41.

Diquino[3,4-b;4′,3′-e][1,4]thiazines (12a–c)

6H-Diquinothiazine (12a) and 6-methyldiquinothiazine (12b) were obtained from the reaction of sulfide 11 with ammonia and methylamine in hot phenol (Pluta, 1997).

6H-Diquinothiazine (12a)

Beige, mp 200–201 °C (mp 200–201 °C, Pluta, 1997). 1H NMR (CDCl3) δ: 7.64 (t, 2H, H-2, H-12), 7.71 (t, 2H, H-3, H-11), 7.81 (d, 2H, H-4, H-10), 8.04 (d, 2H, H-1, H-13), 8.40 (s, 2H, H-6, H-8). 13C NMR (CDCl3) δ: 109.10 (C-6a, C-7a), 117.18 (C-13a, C-14b), 117.41 (C-1, C-13), 127.25 (C-2, C-12), 129.49 (C-3, C-11), 130.78 (C-4, C-10), 142.21 (C-4a, C-9a), 147.94 (C-6, C-8), 148.07 (C-13b, C-14a).

6-Methyldiquinothiazine (12b)

Yellow, mp 156–157 °C (mp 156–157 °C, Pluta, 1997). 1H NMR (CDCl3) δ: 3.54 (s, 3H, CH3), 7.66 (t, 2H, H-2, H-12), 7.72 (t, 2H, H-3, H-11), 8.11 (d, 2H, H-4, H-10), 8.34 (d, 2H, H-1, H-13), 8.66 (s, 2H, H-6, H-8). 13C NMR (CDCl3) δ: 43.63 (CH3), 122.09 (C-1, C-13), 124.17 and 124.42 (C-6a, C-7a and C-13a, C-14b), 127.46 (C-2, C-12), 129.44 (C-3, C-11), 130.11 (C-4, C-10), 148.33 (C-6, C-8), 148.76 and 148.85 (C-4a, C-9a and C-13b, C-14a).

14-(p-Fluorophenyl)diquinothiazine (12c)

From diquinodithiin 10

Diquinodithiin 10 (0.16 g, 0.5 mmol) was finely powdered together with p-fluoroaniline hydrochloride (0.37 g, 2.5 mmol), and the mixture was heated on an oil bath at 200–205 °C for 3 h. After cooling, water (10 ml) was added to the reaction mixture and the resulting solid was filtered off, washed with water, air-dried, and purified by column chromatography (Al2O3, CH2Cl2) to give 0.12 g (30 %) of 14-(p-fluorophenyl)diquinothiazine (12c), beige, mp 315-316 °C.

From 4,4′-dichloro-3,3′-diquinolinyl sulfide (11)

A solution of sulfide 11 (0.18 g, 0.5 mmol) and p-fluoroaniline (0.17 g, 1.5 mmol) in MEDG (5 mL) was refluxed for 3 h. After cooling, the solution was poured into water (20 ml) and alkalized with 5 % aqueous sodium hydroxide to pH 10. The resulting solid was filtered off, washed with water and purified by column chromatography (Al2O3, CHCl3) to give 0.17 g (86 %) of 14-(p-fluorophenyl)diquinothiazine (12c), beige, mp 315–316 °C.

1H NMR (CDCl3) δ: 6.43 (dd, 2H, C6H2), 6.77 (m, 2H, C6H2), 7.75 (t, 2H, H-2, H-12), 7.85 (t, 2H, H-3, H-11), 8.34 (d, 2H, H-4, H-10), 8.39 (d, 2H, H-1, H-13), 9,06 (s, 2H, H-6, H-8). 13C NMR (CDCl3) δ: 115.75 (J = 22.5 Hz, m-C of C6H4F), 116.30 (J = 7.5 Hz, o-C of C6H4F), 122.87 (C-1, C-13), 126.82 (C-13a, C-14b), 128.51 (C-2, C-12), 129.89 (C-6a, C-7a), 130.13 (C-3, C-11), 130.25 (C-4, C-10), 140.57 (J = 2.5 Hz, ipso-C of C6H4F), 145.54 (C-13b, C-14a), 147.98 (C-4a, C-9a), 149.49 (C-6, C-8), 158.07 (J = 238.5 Hz, p–C of C6H4F). EIMS m/z: 395 (M+, 100), 363 (M-S,20), 300 (M-C6H4F, 17). Anal. Calcd. for C24H14FN3S: C, 72.89; H, 3.57; N, 10.63. Found: C, 72.77; H, 3.59; N, 10.46.

In vitro lipid peroxidation

Heat-inactivated hepatic microsomes from untreated rats were prepared as described (Rekka et al., 1989). The incubation mixture contained microsomal fraction (corresponding to 2.5 mg of hepatic protein per ml or 4 mM fatty acid residues), ascorbic acid (0.2 mM) in Tris–HCl/KCl buffer (50 mM/150 mM, pH 7.4), and the studied compounds (50–1 μM) dissolved in DMSO. The reaction was initiated by addition of a freshly prepared FeSO4 solution (10 μΜ), and the mixture was incubated at 37 °C for 45 min. Lipid peroxidation of aliquots was assessed spectrophotometrically (535 against 600 nm) as TBAR. Both compounds and solvents were found not to interfere with the assay. Each assay was performed in duplicate, and IC50 values represent the mean concentration of compounds that inhibit the peroxidation of control microsomes by 50 % after 45 min of incubation. All standard errors are within 10 % of the respective reported values.

Calculation of lipophilicity, molecular mass, surface area, and molecular volume

Lipophilicity (as cLogP), molecular mass (M), surface area (S), and molecular volume (VM) were calculated using CS Chem 3D Ultra 7.0 (CambridgeSoft) and Spartan’04 (Wavefunction, Inc. Irvine, CA).

Results and discussion

Synthesis

The synthesis of the title azaphenothiazines was based on the reactions of isomeric diquinodithiins, dichlorodiquinolinyl sulfides, and disulfide with amines, ammonia, and acetamide. The fusion reactions of linearly condensed diquinodithiin 1 with hydrochlorides of aniline and its p-substituted derivatives such as p-chloroaniline and p-methoxyaniline led to tetracyclic 9-substituted 6H-quinobenzothiazines 3a–c (Scheme 1). 9-Methoxy-6H-quinobenzothiazine 3c was obtained in better yield in the reaction of 2,2′-dichloro-3,3′-diquinolinyl disulfide 2 with p-methoxyaniline in monomethyl ether of diethylene glycol. The similar reaction of diquinodithiin 1 with hydrochlorides of 1-naphthylamine, 2-naphthylamine, and 6-aminoquinoline gave pentacyclic 7H-quinonaphthothiazine 4, 14H-quinonaphthothiazine 5, and 7H-diquinothiazine 6. The reaction of isomeric diquinodithiin 7 with acetamide and p-fluoroaniline hydrochloride gave linearly condensed pentacyclic 6H-diquinothiazines 9a and 6-(p-fluorophenyl)diquinothiazine 9b (Scheme 2). Analogous reaction of another isomeric diquinodithiin 10 with p-fluoroaniline hydrochloride led to angularly condensed diquinothiazine 12c. Better yields of the fluoroaniline products 9b and 12c were achieved when x,x’-dichloro-3,3′-diquinolinyl sulfides 8 and 11 (x = 2 and 4) were used. Sulfide 11 reacted also with ammonia or methylamine in hot phenol to give diquinothiazines 12a, b.

Scheme 1
scheme 1

Reactans: a C6H5NH2·HCl (p-ClC6H4NH2·HCl, p-CH3OC6H4NH2·HCl), 200–205 °C, 4 h; b p-CH3OC6H4NH2, MEDG, reflux, 3 h; c 1-naphthylamine·HCl, 200–205 °C, 4 h; d 2-naphthylamine·HCl, 200–205 °C, 4 h; e 6-aminoquinoline·HCl, 200–205 °C, 4 h

Full size image
Scheme 2
scheme 2

Reactans: a CH3CONH2, K2CO3, 180 °C, 0.5 h; b pF-C6H4NH2·HCl), 200–205 °C, 3 h; c p-FC6H4NH2, MEDG, reflux, 3 h; d NH3 (CH3NH2), phenol, 180 °C, 1 h

Full size image

The described syntheses were monitored by TLC analysis. All chromatograms of new compounds showed characteristic for azaphenothiazines (Jeleń et al., 2011) color changing during irradiation with UV light from blue to yellow (4, 9b), from yellow to green (5, 6), from orange to yellow (12c), and from yellow to orange (7c).

Structure

It is well known that the synthesis of phenothiazines can proceed via the Smiles rearrangement of the S–N type of the appropriate sulfide (Pluta et al., 2009). The identification of the product structures was based on the spectroscopic 1H NMR and MS analysis. In the case of the reactions of sulfides 7 and 11, the products 9 and 12 possessed the C2v symmetry (the left part was a mirror image of the right one) what excluded the stage of rearrangement. The reactions of diquinodithiin 1 and disulfide 2 with anilines proceeded similarly without the stage of rearrangement to give tetracyclic quinobenzothiazines 3a–c (Jeleń and Pluta, 2009). The reaction with 1-naphthylamine gave pentacyclic quinonaphthothiazine 4. On the contrary, the reactions with 2-naphthylamine and 6-aminoquinoline were more complex as there were two possibilities of the thiazine ring formation. The 1H NMR analysis of the reaction products pointed at compounds 5 and 6 excluding compounds 13 and 14, as evidenced from coupling constants; the H-5 and H-6 protons in compounds 5 and 6 showed a coupling constant J ortho, whereas analogous protons in compounds 13 and 14 (H-7/H-12 and H-5/H-14, respectively) would have shown a coupling constant J para, which is very small (i.e., J 1,4 = 0.6-0.8 Hz in naphthalene (Hamm and von Philipsborn, 1971; Lucchini and Wells, 1976) and J 5,8 = 0.5-0.8 Hz in quinoline (Hamm and von Philipsborn, 1971; Jones, 1977). We did not observe such small values of coupling constants in the reaction products 5 and 6.

Antioxidant activity

The effect of the new derivatives on non-enzymatic lipid peroxidation of rat hepatic microsomal membrane lipids was investigated in vitro. Most of the studied derivatives demonstrated significant antioxidant activity, with IC50 values between 1 and 23 μM (Table 1). It is worthwhile to mention that under the same experimental conditions known potent antioxidants, trolox ((S)-(-)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid) and probucol (4,4′-[(1-methylethylidene)bis(thio)]bis[2,6-bis(1,1-dimethylethyl)phenol]), exhibited IC50 values of 25 μM and >1 mM, respectively (Kourounakis et al., 2008). Further, all of the active new derivatives were significantly much more potent than previously studied tricyclic dipyridothiazines (IC50 of most active compounds was between 64 and 470 μM) (Morak-Młodawska et al., 2010). The time course of lipid peroxidation, as affected by various concentrations of representative compounds, is depicted in Fig. 1.

Table 1 IC50 values for in vitro lipid peroxidation (LP), LogP, molecular volume (VM), and molecular mass (M) as well as surface area (S) of the tested compounds
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Fig. 1
figure 1

Representative graphs of the time course of lipid peroxidation as affected by various concentrations of compounds 3a–c, 5, 6, and 12a. IC50 values are calculated according to these results as the concentration showing 50 % inhibition of the lipid peroxidation reaction at 45 min incubation time

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Tetracyclic NH-azaphenothiazines 3a–c exhibited significant activity dependent on the substitution (H, Cl, and OCH3) on the benzene ring (Table 1). From the pentacyclic compounds, the angularly fused with unsubstituted, the thiazine nitrogen atom (4–6 and 12a) exhibited very significant activity with most active compound 12a, which showed an IC50 of 1 μM. The change of the quinoline moiety into naphthalene (compare compounds 4 and 5 with 6) marginally increased activity. However, compounds with a linearly fused ring system (9a and 9b) and/or a large aryl substituent at the thiazine nitrogen atom (9b and 12c) did not show any antioxidant activity, while compound 12b, with a small substituent, exhibited very weak activity.

Considering three isomers (6, 9a, and 12a), one can find that their antioxidant activity increased with decreasing lipophilic character represented by the logP values. On the other hand, the least active compounds (9b, 12b, and 12c) exhibited high values of molecular descriptors such as molecular mass (M > 315), molecular volume (V M > 321), and surface area (S > 317, Table 1). However, attempts to the correlate the activity with those properties turned out to be unsatisfactory.

In conclusion, eleven tetracyclic and pentacyclic (linearly or angularly condensed) azaphenothiazines were synthesized, and structure–(antioxidant)activity relationships were investigated. The type of the ring fusion was concluded from the 1H NMR spectra. The degree of antioxidant activity of these derivatives seems to depend on their lipophilicity and molecular mass. The non-substitution of the thiazine nitrogen atom, the type of ring system fusion, and the nature of substituents promote activity. Finally, it is the first time to our knowledge that azaphenothiazines are shown to exhibit such potent antioxidant activity.