Synthesis and biological activity of novel series of 4-methoxy, and 4,9-dimethoxy-5-substituted furo[2,3-g]-1,2,3-benzoxathiazine-7,7-dioxide derivatives.

A novel series of 4-methoxy, and 4,9-dimethoxy-5-substituted furo[2,3-g]-1,2,3-benzoxathiazine-7,7-dioxide derivatives 3a,b, 10a-g and 11a-g were prepared in good yields via the reaction of 4-methoxy (1a) and 4,7-dimethoxy-5-acetyl-6-hydroxybenzofurans (1b) and their α,β-unsaturated keto derivatives 6a-g and 7a-g with chlorosulfonyl isocyanate (CSI). On the other hand, N-chlorosulfonyl carbamate derivatives 4a,b, 12a,b and 13a,b were prepared and allowed to react with piperidine to give the corresponding N-piperidinosulfonyl carbamate derivatives 5a,b, 14a,b and 15a,b, respectively. Sixteen new target compounds 3a,b, 10a-g, and 11a-g were tested for their DPPH radical-scavenging, and in vitro antiproliferative activity against A-549, MCF7 and HCT-116 cancer cell lines. Compounds 10a, 11c, 11e, and 11g showed moderate DPPH radical-scavenging activity compared to ascorbic acid at 100 μg/mL. 4,9-Dimethoxy-5-substituted styrylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxides 11a, 11b, and 11c were found to be highly active against A-549 and HCT-116 cancer cell lines with IC50 values ranging from 0.02 to 0.08 μmol/mL compared to doxorubicin with IC50 = 0.04 and 0.06 μmol/mL, respectively.

Introduction

Chlorosulfonyl isocyanate (CSI) is a remarkable reagent of exceptional reactivity [1], [2]. This reagent as a uniparticulate electrophile and as a versatile heterocumulene is useful in many synthetic transformations and in the synthesis of several heterocyclic systems [3], [4], [5], [6], [7]. CSI undergoes nucleophilic addition reaction with salicylaldehydes, 2-hydroxybenzophenones, and 2-hydroxychalcones in dry benzene at 0–5 °C to afford N-chlorosulfonyl carbamate derivatives, whereas cycloaddition of CSI with the above moieties in dry toluene at 100 °C produces 1,2,3-benzoxathiazine 2,2-dioxides [5], [7]. On the other hand, benzofuran derivatives occupy significance position due to their widespread occurrence in plants [8] and for their biological activities as antioxidant [9], [10] and anticancer [11], [12]. Based on the above observations, our goal deals with study of the reaction of CSI with 4-methoxy-5-acetyl-6-hydroxybenzofuran (visnaginone) (1a), 4,7-dimethoxy-5-acetyl-6-hydroxybenzofuran (khellinone) (1b), and their α,β-unsaturated keto derivatives 6a–g and 7a–g to obtain novel furo[2,3-g]-1,2,3-benzoxathiazine-7,7-dioxide derivatives and evaluating their DPPH radical-scavenging and anticancer activities.

Experimental

Synthesis

Melting points were determined in open capillary tubes on an Electrothermal 9100 digital melting point apparatus (Mount Holly, New Jersey, USA) and were uncorrected. Elemental analyses were performed on a Perkin-Elmer 2400 analyzer (USA) and were found within ±0.4% of the theoretical values. IR spectra were recorded on a Perkin-Elmer 1600 FTIR (USA) in KBr disks. The NMR spectra were measured with a Bruker Avance digital spectrometer (Germany) (500 and 125 MHz) in DMSO-d6 and chemical shifts was recorded in δ ppm relative to TMS as internal standard. Mass spectra (EI) were run at 70 eV with a JEOL-JMS-AX500 mass spectrometer (Japan). 4-methoxy-5-acetyl-6-hydroxy benzofuran (visnaginone) (1a) [13], 4,7-dimethoxy-5-acetyl-6-hydroxy benzofuran (khellinone) (1b) [14], α,β-unsaturated keto derivatives 6a–g and 7a–g [10], [15], [16] were prepared as reported.

Synthesis of compounds 2a and 2b

To a stirred solution of compound 1a or 1b (5 mmol) in dry benzene (10 mL), was added a solution of chlorosulfonyl isocyanate (0.87 mL, 10 mmol) in dry benzene (5 mL) at 0–5 °C during 20 min and the stirring was continued for additional 1 h at the same temperature and then for 30 min at room temperature. The reaction mixture was set aside at refrigerator overnight. The solid that formed was filtered off, air-dried, and crystallized from benzene.

N-(4-Methoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl) ethylidene)chlorosulfonyl amine 2a

R = H; m.p. 112–4 °C; yield 44%. – IR (KBr): υ = 3200 (NH), 1645 (CO), 1620 (CN), 1585 (CC), 1371, 1136 (SO2), 1121, 1119, 1110 (C—O—C), 740 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 1.61 (s, 3H, CH3), 4.20 (s, 3H, OCH3), 6.77 (s, 1H, H-7), 7.52 (d, 1H, H-3), 8.02 (s, 1H, NH), 8.21 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 20.2 (CH3), 60.9 (OCH3), 92.8–159.0 (Ar—C), 166.6 (CN). – C12H10Cl2N2O8S2 (445.25): calcd. C 32.37; H 2.26; N 6.29; found C 32.15; H 2.11; N 6.47.

N-(4,7-Dimethoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl) ethylidine)chlorosulfonyl amine 2b

R = OCH3; m.p 107–9 °C; yield 32%. – IR (KBr): υ = 3218 (NH), 1665 (CO), 1620 (CN), 1575 (CC), 1372, 1136 (SO2), 1121, 1120, 1119, 1110 (C—O—C), 745 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 1.99 (s, 3H, CH3), 4.01, 4.22 (2s, 6H, 2OCH3), 7.81 (d, 1H, H-3), 8.01 (s, 1H, NH), 8.23 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 21.8 (CH3), 61.0, 61.8 (OCH3), 105.6–151.3 (Ar—C), 170.0 (CN). – C13H12Cl2N2O9S2(475.28): calcd. C 32.85; H 2.54; N 5.89; found C 32.61; H 2.30; N 5.64.

Synthesis of compounds 3a and 3b

Method A: cyclization of 2a and 2b

To a cold stirred solution of compounds 2a or 2b (10 mmol) in dry dichloromethane (10 mL), a solution of triethylamine (0.5 mL) in dry dichloromethane (2.5 mL) was added dropwise during 5 min. The reaction mixture was stirred for 5 h at room temperature. The solvent was evaporated in vacuo, and the residue was triturated with acetone–water (1:10, 11 mL) and allowed to stay for 1 h. The solution was neutralized by addition of 5% sodium hydrogen carbonate and the separated solid was filtered off, washed with water, air-dried, and crystallized from absolute ethanol to give 3a (33%) or 3b (35%), respectively.

Method B

To a stirred solution of compound 1a or 1b (10 mmol) in dry toluene (40 mL), a solution of chlorosulfonyl isocyanate (0.87 mL, 10 mmol) in dry toluene (5 mL) was added within 15 min. The reaction mixture was heated at 100–105 °C for 3 h. Toluene was evaporated under vacuo, and the residue was treated with cold water (50 mL). The solid that formed was filtered off, washed with water, air-dried, and crystallized from absolute ethanol to give 3a (45%) or 3b (52%), respectively.

4-Methoxy-5-methylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 3a

R = H; m.p. 161–3 °C. – IR (KBr): υ = 1620 (CN), 1579 (CC), 1375, 1165 (SO2), 1120, 1119, 1110 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 2.89 (s, 3H, CH3), 4.40 (s, 3H, OCH3), 6.77 (s, 1H, H-9), 7.62 (d, 1H, H-3), 8.22 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 18.5 (CH3), 60.9 (OCH3), 92.8–159.0 (Ar—C), 166.6 (CN). – EI-MS: m/z (%) = 267 (M+, 100). – C11H9NO5S(267.26): calcd. C 49.43; H 3.39; N 5.24; found C 49.22; H 3.11; N 5.02.

4,9-Dimethoxy-5-methylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 3b

R = OCH3; m.p. 120–2 °C. – IR (KBr): υ = 1618 (CN), 1575 (CC), 1365, 1135 (SO2), 1120, 1119, 1110, 1009 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 2.81 (s, 3H, CH3), 4.11, 4.32 (2s, 6H, 2OCH3), 7.61 (d, 1H, H-3), 8.22 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 28.8 (CH3), 61.0 (OCH3), 61.8 (OCH3), 105.6-151.3 (Ar—C), 170.0 (CN). – EI-MS: m/z (%) = 297 (M+, 100). – C12H11NO6S(297.28): calcd. C 48.48; H 3.73; N 4.71; found C 48.23; H 3.69; N 4.55.

Synthesis of compounds 4a and 4b

To a stirred solution of compound 1a or 1b (10 mmol) in dry benzene (10 mL), was added a solution of chlorosulfonyl isocyanate (0.87 mL, 10 mmol) in dry benzene (5 mL) at 0–5 °C during 20 min and the stirring was continued for additional 1 h at the same temperature and for 30 min at room temperature. The reaction mixture was set aside at refrigerator overnight. The solid that formed was filtered off, air-dried, and crystallized from ethanol–water (10:1).

1-(4-Methoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl) ethanone 4a

R = H; m.p. 100–2 °C; yield 65%. – IR (KBr): υ = 3128 (NH), 1730, 1645 (CO), 1600 (CC), 1375, 1157 (SO2), 1110, 1066, 1009 (C—O—C), 740 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 2.91 (s, 3H, CH3), 3.99 (s, 3H, OCH3), 7.21 (s, 1H, H-7), 7.99 (s, 1H, H-3), 8.21 (d, 1H, H-2), 9.11 (s, 1H, NH).-C12H10ClNO7S(346.73): calcd. C 41.45; H 2.90; N 4.03; found C 41.22; H 3.01; N 3.99.

1-(4,7-Dimethoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl) ethanone 4b

R = OCH3; m.p. 122–4 °C; yield 55%. – IR (KBr): υ = 3200 (NH), 1732, 1686 (CO), 1578 (CC), 1372, 1153 (SO2), 1111, 1099, 1066, 1001 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 2.69 (s, 3H, CH3), 3.91, 4.21 (2s, 6H, 2OCH3), 7.01 (d, 1H, H-3), 8.22 (d, 1H, H-2), 8.91 (s, 1H, NH). – C13H12ClNO8S(377.75): calcd. C 41.33; H 3.20; N 3.71; found C 41.11; H 3.22; N 3.56.

Synthesis of compounds 5a and 5b

A mixture of compounds 4a or 4b (10 mmol) and piperidine (0.85 mL, 10 mmol) in dry 1,4-dioxane (20 mL) containing triethylamine (0.5 mL) was stirred at room temperature for 5 h for compound 4a, 7 h for compound 4b, and then left overnight at room temperature. The reaction mixture was poured onto water and the precipitate that formed was filtered off, dried, and crystallized from methanol.

N-Piperidinosulfonyl-(4-methoxy-5-acetylbenzofuran-6-yl) carbamate 5a

R = H; m.p. 220–2 °C; yield 35%. – IR (KBr): υ = 3128 (NH), 1725, 1678 (CO), 1575 (CC), 1370, 1152 (SO2), 1101, 1099, 1009 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 2.67 (s, 3H, CH3), 2.99–3.22 (m, 10H, piperidinyl), 3.91 (s, 3H, OCH3), 6.66 (s, 1H, H-7), 7.01 (d, 1H, H-3), 8.12 (d, 1H, H-2), 8.71 (s, 1H, NH). – C17H20N2O7S(396.41): calcd. C 51.51; H 5.09; N 7.07; found C 51.32; H 5.21; N 6.99.

N-Piperidinosulfonyl-(4,7-dimethoxy-5-acetylbenzofuran-6-yl) carbamate 5b

R = OCH3; m.p. 166–9 °C; yield 42%. – IR (KBr): υ = 3161 (NH), 1720, 1645 (CO), 1601 (CC), 1370, 1152 (SO2), 1112, 1099, 1066, 1009 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 2.61 (s, 3H, CH3), 2.81–3.31 (m, 10H, piperidinyl), 3.91, 4.21 (2s, 6H, 2OCH3), 7.61 (d, 1H, H-3), 8.12 (d, 1H, H-2), 9.51 (s, 1H, NH). – C18H22N2O8S(426.44): calcd. C 50.70; H 5.20; N 6.57; found C 50.55; H 5.42; N 6.36.

Synthesis of compounds 8a and 9a

To a stirred solution of compound 6a or 7a (5 mmol) in dry benzene (10 mL), was added a solution of chlorosulfonyl isocyanate (0.87 mL, 10 mmol) in dry benzene (5 mL) at 0–5 °C during 20 min and the stirring was continued for additional 1 h at the same temperature and then for 30 min at room temperature. The reaction mixture was set aside at refrigerator overnight. The solid that formed was filtered off, air-dried, and crystallized from benzene.

N-[4-Methoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl)-3-phenyl-prop-2-ene]chlorosulfonyl amine 8a

R = H; ArC6H5; m.p. 120–2 °C; yield 65%. – IR (KBr): υ = 3182 (NH), 1678 (CO), 1620 (CN), 1567 (CC), 1375, 1152 (SO2), 1111, 1066, 1009 (C—O—C), 750 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 3.99 (s, 3H, OCH3), 6.66 (s, 1H, H-7), 7.11–7.31 (m, 5H, Ar—H), 7.01, 7.66 (2d, 2H, CHCH), 7.98 (d, 1H, H-3), 8.12 (d, 1H, H-2), 8.57 (s, 1H, NH). – C19H14Cl2N2O8S2(533.36): calcd. C 42.79; H 2.65; N 5.25; found C 42.55; H 2.88; N 5.35.

N-[4,7-Dimethoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl)-3-phenylprop-2-ene]chlorosulfonyl amine 9a

R = OCH3; ArC6H5; m.p. 100–2 °C; yield 60%. – IR (KBr): υ = 3200 (NH), 1645 (CO), 1618 (CN), 1575 (CC), 1370, 1150 (SO2), 1110, 1066, 1009, 1001 (C—O—C), 748 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 3.99, 4.22 (2s, 6H, 2OCH3), 7.01–7.30 (m, 5H, phenyl), 7.66, 7.56 (2d, 2H, CHCH), 7.98 (d, 1H, H-3), 8.12 (d, 1H, H-2), 9.55 (s, 1H, NH). – C20H16Cl2N2O9S2(563.39): calcd. C 42.64; H 2.86; N 4.97; found C 42.44; H 3.01; N 5.10.

Synthesis of compounds 10a and 11a

Method A: cyclization of 8a and 9a

To a cold stirred solution of compound 8a or 9a (10 mmol) in dry dichloromethane (10 mL), a solution of triethylamine (0.5 mL) in dry dichloromethane (2.5 mL) was added dropwise during 5 min. The reaction mixture was stirred for 5 h at room temperature. The solvent was evaporated under vacuo, and the residue was triturated with acetone–water (1:10, 11 mL) and allowed to stay for 1 h. The solution was neutralized by addition of 5% sodium hydrogen carbonate, and the separated solid was filtered off, washed with water, air-dried, and crystallized from absolute ethanol to give 10a (30%) or 11a (32%), respectively.

To a stirred solution of compound 6a or 7a (10 mmol) in dry toluene (40 mL), a solution of chlorosulfonyl isocyanate (0.87 mL, 10 mmol) in dry toluene (5 mL) was added within 15 min. The reaction mixture was heated at 100–105 °C for 3 h. Toluene was evaporated under vacuo and the residue was triturated with cold water (50 mL). The solid that formed was filtered off, washed with water, air-dried, and crystallized from absolute ethanol to give 10a (77%) or 11a (78%), respectively.

4-Methoxy-5-styrylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 10a

R = H; ArC6H5; m.p. 92–4 °C. – IR (KBr): υ = 1620 (CN), 1569 (CC), 1375,1145 (SO2), 1120, 1119, 1110 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 4.32 (s, 3H, OCH3), 6.88 (s, 1H, H-9), 7.01, 7.12 (2d, 2H, CHCH), 7.32–7.68 (m, 5H, Ar—H), 7.90 (d, 1H, H-3), 8.12 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 61.2 (OCH3), 96.3–155.2 (Ar—C), 159.3 (CN). – EI-MS: m/z (%)=355 (M+, 53). – C18H13NO5S(355.36): calcd. C 60.84; H 3.69; N 3.94; found C 60.66; H 3.54; N 3.77.

4,9-Dimethoxy-5-styrylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 11a

R = OCH3; ArC6H5; m.p. 95–7 °C. – IR (KBr): υ = 1620 (CN), 1598 (CC), 1375, 1135 (SO2), 1120, 1119, 1110, 1109 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 4.01, 4.22 (2s, 6H, 2OCH3), 7.01–733 (m, 5H, Ar—H), 7.51, 7.61 (2d, 2H, CHCH), 7.89 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 61.6, 61.9 (2OCH3), 105.0–151.3 (Ar—C), 172.0 (CN).-EI-MS: m/z (%) = 385 (M+, 75). – C19H15NO6S(385.39): calcd. C 59.21; H 3.92; N 3.63; found C 59.44; H 3.64; N 3.51.

Synthesis of compounds 10b–g and 11a–g

To a stirred solution of the appropriate α,β-unsaturated keto derivatives 6b–g or 7b–g (10 mmol) in dry toluene (40 mL), a solution of chlorosulfonyl isocyanate (0.87 mL, 10 mmol in dry toluene 5 mL) was added during 15 min. The reaction mixture was heated at 100–105 °C for 3–4 h. Toluene was evaporated under vacuo, and the residue was triturated with cold water (50 mL). The solid that formed was filtered off, washed with water, air-dried, and crystallized from absolute ethanol.

4-Methoxy-5-(4-chlorostyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 10b

R = H; ArC6H4Cl-p; m.p. 125–7 °C; yield 52%. – IR (KBr): υ = 1620 (CN), 1599 (CC), 1375, 1155 (SO2), 1120, 1119, 1110 (C—O—C), 740 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 4.32 (s, 3H, OCH3), 6.77 (s, 1H, H-9), 7.00, 7.12 (2d, 2H, CHCH), 7.21–7.76 (m, 4H, Ar—H), 7.99 (d, 1H, H-3), 8.12 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 61.2 (OCH3), 96.2–131.1 (Ar—C).-EI-MS: m/z (%) = 389/391 (M+/M++2, 8/2). – C18H12ClNO5S (389.81): calcd. C 55.46; H 3.10; N 3.59; found C 55.33; H 3.22; N 3.34.

4-Methoxy-5-(4-fluorostyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 10c

R = H; ArC6H4F-p; m.p. 93–5 °C; yield 42%. – IR (KBr): υ = 1620 (CN), 1601 (CC), 1375, 1145 (SO2), 1120, 1119, 1110 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 4.32 (s, 3H, OCH3), 6.77 (s, 1H, H-9), 7.01, 7.12 (2d, 2H, CHCH), 7.31–7.68 (m, 4H, Ar—H), 7.99 (d, 1H, H-3), 8.22 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 60.2 (OCH3), 92.8– 151.9 (Ar—C).-EI-MS: m/z (%) = 373 (M+, 8). – C18H12FNO5S(373.35): calcd. C 57.91; H 3.24; N 3.75; found C 58.05; H 3.11; N 3.50.

4-Methoxy-5-(4-methoxystyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 10d

R = H; ArC6H4OCH3-p; m.p. 162–4 °C; yield 90%. – IR (KBr): υ = 1620 (CN), 1575 (CC), 1375, 1175 (SO2), 1120, 1119, 1110, 1009 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 3.99, 4.30 (2s, 6H, 2OCH3), 6.77 (s, 1H, H-9), 7.01, 7.12 (2d, 2H, CHCH), 7.35–7.89 (m, 4H, Ar—H), 7.99 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 55.4, 61.2 (OCH3), 96.3–161.8 (Ar—C), 172.3 (CN). – EI-MS: m/z (%)=385 (M+, 40). – C19H15NO6S(385.39): calcd. C 59.21; H 3.92; N 3.63; found C 59.01; H 4.00; N 3.55.

4-Methoxy-5-(3,4,5-trimethoxystyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 10e

R = H; ArC6H2(OCH3)3-3,4,5; m.p. 104–6 °C; yield 97%. – IR (KBr): υ = 1620 (CN), 1589 (CC), 1375, 1175 (SO2), 1120, 1119, 1110, 1109, 1008 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 3.89, 4.01, 4.12, 4.34 (4s, 12H, 4OCH3), 6.77 (s, 1H, H-9), 7.01, 7.32 (2d, 2H, CHCH), 7.55–7.89 (2d, 2H, Ar—H), 7.99 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 55.9, 59.9, 60.0, 61.0 (OCH3), 96.0–159.2 (Ar—C), 172.4 (CN). – EI-MS: m/z (%)=445 (M+, 43).-C21H19NO8S (445.44): calcd. C 56.62; H 4.30; N 3.14; found C 56.44; H 4.11; N 3.30.

4-Methoxy-5-(4-N,N-dimethylstyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 10f

R = H; ArC6H4N(CH3)2-p; m.p. 92–4 °C; yield 98%. – IR (KBr): υ = 1618 (CN), 1599 (CC), 1375, 1135 (SO2), 1120, 1119, 1110 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 3.01, 3.12 (2s, 6H, 2CH3), 4.02 (s, 3H, OCH3), 6.77 (s, 1H, H-9), 6.99, 7.12 (2d, 2H, CHCH), 7.24–7.57 (m, 4H, Ar—H), 7.89 (d, 1H, H-3), 8.09 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 60.0 (CH3), 61.1 (OCH3), 92.7–157.2 (Ar—C), 170.0 (CN). – EI-MS: m/z (%) = 398 (M+, 10). – C20H18N2O5S(398.43): calcd. C 60.29; H 4.55; N 7.03; found C 60.11; H 4.35; N 7.21.

4-Methoxy-5-(2-(3-indolyl)vinyl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 10g

R = H; Ar = 3-indolyl; m.p. 134–6 °C; yield 97%. – IR (KBr): υ = 3350 (NH), 1620 (CN), 1589 (CC), 1375, 1135 (SO2), 1120, 1119, 1110 cm−1 (C—O—C). – 1H NMR (DMSO-d) δ: 4.22 (s, 3H, OCH3), 6.77 (s, 1H, H-9), 7.01, 7.12 (2d, 2H, CHCH), 7.24–7.57 (m, 4H, Ar—H), 7.99 (s, 1H, indolyl 2-H), 8.12 (d, 1H, H-3), 8.34 (d, 1H, H-2), 9.91 (s, 1H, NH). – 13C NMR (DMSO-d) δ: 60.8 (OCH3), 105.8–158.5 (Ar—C), 107.0 (CN). – EI-MS: m/z (%) = 394 (M+, 1). – C20H14N2O5S(394.4): calcd. C 60.91; H 3.58; N 7.10; found C 60.87; H 3.42; N 7.22.

4,9-Dimethoxy-5-(4-chlorostyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 11b

R = OCH3; ArC6H4Cl-p; m.p. 113–5 °C; yield 60%. – IR (KBr): υ = 1620 (CN), 1585 (CC), 1375, 1135 (SO2), 1120, 1119, 1110, 1009 (C—O—C), 740 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 4.01, 4.22 (2s, 6H, 2OCH3), 6.97, 7.11 (2d, 2H, CHCH), 7.24–7.67 (m, 4H, ArH), 7.99 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 60.6, 60.9 (2OCH3), 105.0–152.8 (ArC), 170.0 (CN). – EI-MS: m/z (%)=419 (M+, 34). – C19H14ClNO6S(419.84): calcd. C 54.36; H 3.36; N 3.34; found C 54.44; H 3.11; N 3.12.

4,9-Dimethoxy-5-(4-fluorostyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 11c

R = OCH3; ArC6H4F-p; m.p. 130–2 °C; yield 82%. – IR (KBr): υ = 1620 (CN), 1596 (CC), 1385, 1135 (SO2), 1120, 1119, 1110, 1009 cm−1 (COC). – 1H NMR (DMSO-d) δ: 4.01, 4.22 (2s, 6H, 2OCH3), 7.21–7.37 (m, 4H, ArH), 7.51, 7.62 (2d, 2H, CHCH), 7.99 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 60.4, 60.9 (2OCH3), 105.5–152.6 (ArC), 164.6 (CN). – EI-MS: m/z (%)=403 (M+, 11). – C19H14FNO6S(403.38): calcd. C 56.57; H 3.50; N 3.47; found C 56.44; H 3.35; N 3.24.

4,9-dimethoxy-5-(4-methoxystyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 11d

R = OCH3; ArC6H4OCH3-p; m.p. 150 °C dec.; yield 68%. – IR (KBr): υ = 1620 (CN), 1596 (CC), 1385, 1135 (SO2), 1120, 1119, 1110, 1009 cm−1 (COC). – 1H NMR (DMSO-d) δ: 3.88, 3.99, 4.20 (3s, 9H, 3OCH3), 7.01–7.39 (m, 4H, ArH), 7.66, 7.72 (2d, 2H, CHCH), 7.99 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 55.4, 61.6, 61.9 (OCH3), 106.2–151.1 (ArC), 172.4 (CN). – EI-MS: m/z (%) = 415 (M+, 62). – C20H17NO7S(415.42): calcd. C 57.82; H 4.12; N 3.37; found C 57.65; H 4.22; N 3.11.

4,9-Dimethoxy-5-(3,4,5-trimethoxystyryl)furo[3,2-g]-1,2,3-benzoxathi-azine-7,7-dioxide 11e

R = OCH3; ArC6H2(OCH3)3-3,4,5; m.p. 120 °C dec.; yield 72%. – IR (KBr): υ = 1621 (CN), 1585 (CC), 1375, 1135 (SO2), 1120, 1118, 1110, 1109, 1009 cm−1 (COC). – 1H NMR (DMSO-d) δ: 3.88, 3.99, 3.99, 4.01, 4.22 (5s, 15H, 5OCH3), 7.01 (s, 2H, ArH), 7.52, 7.82 (2d, 2H, CHCH), 7.99 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 55.6, 59.9, 60.0, 61.5, 62.7 (OCH3), 103.9–153.1 (ArC). – EI-MS: m/z (%) = 475 (M+, 11). – C22H21NO9S(475.47): calcd. C 55.57; H 4.45; N 2.95; found C 55.44; H 4.22; N 2.77.

4,9-Dimethoxy-5-(4-N,N-dimethylstyryl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 11f

R = OCH3; ArC6H4N(CH3)2-p; m.p. 86 °C dec.; yield 93%. – IR (KBr): υ = 1620 (CN), 1589 (CC), 1375, 1135 (SO2), 1121, 1119, 1110, 1009 cm−1 (COC). – 1H NMR (DMSO-d) δ: 2.31, 2.67 (2s, 6H, 2CH3), 4.01, 4.22 (2s, 6H, 2OCH3), 6.10, 6.31 (2d, 2H, CHCH), 7.67 (d, 1H, H-3), 8.24 (d, 1H, H-2). – 13C NMR (DMSO-d) δ: 21.0, 29.8 (CH3), 60.5, 61.8 (OCH3), 105.9–150.3 (ArC), 170.0 (CN).-EI-MS: m/z (%) = 428 (M+, 17). – C21H20N2O6S(428.46): calcd. C 58.87; H 4.70; N 6.54; found C 58.74; H 4.65; N 6.44.

4,9-Dimethoxy-5-(2-(3-indolyl)vinyl)furo[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 11g

R = OCH3; Ar = 3-indolyl; m.p. 129–31 °C; yield 86%. – IR (KBr): υ = 3332 (NH), 1618 (CN), 1585 (CC), 1375, 1135 (SO2), 1121, 1119, 1110 cm−1 (COC). – 1H NMR (DMSO-d) δ: 3.99, 4.12 (2s, 6H, 2OCH3), 6.99, 7.12 (2d, 2H, CHCH), 7.21–7.47 (m, 4H, ArH), 7.9 (s, indolyl 2-H), 8.12 (d, 1H, H-3), 8.34 (d, 1H, H-2), 9.91 (s, 1H, NH). – 13C NMR (DMSO-d) δ: 60.5, 61.8 (OCH3), 105.9–150.3 (ArC), 166.3 (CN). – EI-MS: m/z (%) = 424(M+, 1). – C21H16N2O6S (424.43): calcd. C 59.43; H 3.80; N 6.60; found C 59.22; H 3.65; N 6.45.

Synthesis of compounds 12a,b and 13a,b

To a stirred solution of the appropriate α,β-unsaturated keto derivatives 6a, 6g, 7a or 7g (10 mmol) in dry benzene (10 mL), was added a solution of chlorosulfonyl isocyanate (0.87 mL, 10 mmol in dry benzene 5 mL) at 0–5 °C during 20 min and the stirring was continued for additional 1 h at the same temperature and for 30 min at room temperature. The reaction mixture was set aside at refrigerator overnight. The solid that formed was filtered off, air-dried, and crystallized from ethanol–water (10:1).

N-Chlorosulfonyl 4-methoxy-5-(3-phenylacryloyl)benzofuran-6-yl carbamates 12a

R = H; ArC6H5; m.p. 202–4 °C; yield 70%. – IR (KBr): υ = 3180 (NH), 1720, 1645 (CO), 1545 (CC), 1370, 1150 (SO2), 1110, 1109, 1009 (COC), 742 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 3.99 (s, 3H, OCH3), 6.66 (s, 1H, H-7), 7.21–7.45 (m, 5H, phenyl), 7.92 (d, 1H, H-3), 7.69, 7.71 (2d, 2H, CHCH), 8.12 (d, 1H, H-2), 9.71 (s, 1H, NH). – C19H14ClNO7S(435.83): calcd. C 52.36; H 3.24; N 3.21; found C 52.14; H 3.07; N 3.03.

N-Chlorosulfonyl 4-methoxy-5-(3-(3-indolyl)acryloyl)benzofuran-6-yl carbamates 12b

R = H; Ar = 3-indolyl; m.p. 194–6 °C; yield 75%. – IR (KBr): υ = 3280, 3218 (NH), 1702, 1665 (CO), 1575 (CC), 1370, 1151 (SO2), 1111, 1110, 1009 (COC), 745 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 4.21 (s, 3H, OCH3), 6.66 (s, 1H, H-7), 7.21–7.45 (m, 4H, indolyl), 7.12, 7.66 (2d, 2H, CHCH), 7.98 (d, 1H, H-3), 8.12 (d, 1H, H-2), 8.25 (s, 1H, indolyl 2-H), 8.70 (s, 1H, NH), 9.71 (s, 1H, indolyl NH). – C21H15ClN2O7S (474.87): calcd. C 53.11; H 3.18; N 5.90; found C 53.27; H 3.01; N 5.72.

N-Chlorosulfonyl 4,7-dimethoxy-5-(3-phenylacryloyl)benzofuran-6-yl carbamates 13a

R = OCH3; ArC6H5; m.p. 238–40 °C; yield 70%. – IR (KBr): υ = 3180 (NH), 1730, 1670 (CO), 1575 (CC), 1370, 1157 (SO2), 1111, 1066, 1009, 1001 (COC), 745 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 3.99, 4.21 (2s, 6H, 2OCH3), 7.11–7.54 (m, 5H, phenyl), 7.66, 7.76 (2d, 2H, CHCH), 7.91(d, 1H, H-3), 8.12 (d, 1H, H-2), 8.75 (s, 1H, NH). – C20H16ClNO8S(465.86): calcd. C 51.56; H 3.46; N 3.01; found C 51.46; H 3.31; N 3.11.

N-Chlorosulfonyl 4,7-dimethoxy-5-(3-(3-indolyl)acryloyl) benzofuran-6-yl carbamates 13b

R = OCH3; Ar = 3-indolyl; m.p. 141–3 °C; yield 80%. – IR (KBr): υ = 3200, 3128 (NH), 1701, 1654 (CO), 1570 (CC), 1375, 1157 (SO2), 1111, 1099, 1066, 1001 (COC), 745 cm−1 (Cl). – 1H NMR (DMSO-d) δ: 3.91, 4.21(2s, 6H, 2OCH3), 7.21–7.45 (m, 4H, indolyl), 7.66, 7.79 (2d, 2H, CHCH), 7.91 (d, 1H, H-3), 8.12 (d, 1H, H-2), 8.57 (s, 1H, indolyl 2-H), 9.50 (s, 1H, NH), 10.51 (s, 1H, indolyl NH). – C22H17ClN2O8S(504.90): calcd. C 52.33; H 3.39; N 5.55; found C 52.21; H 3.30; N 5.42.

Synthesis of compounds 14a,b and 15a,b

A mixture of compounds 12a,b or 13a,b (10 mmol) and piperidine (0.85 mL, 10 mmol) in dry 1,4-dioxane (20 mL) containing triethylamine (0.5 mL) was stirred at room temperature for 5–7 h and then left overnight at room temperature. The reaction mixture was poured onto water and the precipitate that formed was filtered off, dried, and crystallized from methanol.

N-Piperidinosulfonyl 4-methoxy-5-(3-phenylacryloyl)benzofuran-6-yl carbamates 14a

R = H; ArC6H5; m.p. 78–80 °C; yield 40%. – IR (KBr): υ = 3180 (NH), 1703, 1671 (CO), 1575 (CC), 1370, 1152 (SO2), 1009, 1000 cm−1 (COC). – 1H NMR (DMSO-d) δ: 2.66–3.21 (m, 10H, piperidinyl), 3.91 (s, 3H, OCH3), 6.67 (s, 1H, H-7), 7.01–7.22 (m, 5H, phenyl), 7.71, 7.79 (2d, 2H, CHCH), 7.98 (d, 1H, H-3), 8.12 (d, 1H, H-2), 9.71 (s, 1H, NH). – C24H24N2O7S(484.52): calcd. C 59.49; H 4.99; N 5.78; found C 59.32; H 4.85; N 5.64.

N-Piperidinosulfonyl 4-methoxy-5-(3-(3-indolyl)acryloyl) benzofuran-6-yl carbamates 14b

R = H; Ar = 3-indolyl; m.p. 262–4 °C; yield 42%. – IR (KBr): υ = 3200, 3120 (NH), 1700, 1651 (CO), 1545 (CC), 1730, 1152 (SO2), 1111, 1009, 1000 cm−1 (COC). – 1H NMR (DMSO-d) δ: 2.61–3.22 (m, 10H, piperidinyl), 3.91 (s, 3H, OCH3), 6.61 (s, 1H, H-7), 7.11–7.45 (m, 4H, indolyl), 7.71, 7.79 (2d, 2H, CHCH), 7.98 (d, 1H, H-3), 8.12 (d, 1H, H-2), 8.25 (s, 1H, indolyl 2-H), 8.71 (s, 1H, NH), 9.71 (s, 1H, indolyl NH). – C26H25N3O7S(523.56): calcd. C 59.65; H 4.81; N 8.03; found C 59.51; H 4.67; N 8.14.

N-Piperidinosulfonyl 4,7-dimethoxy-5-(3-phenylacryloyl) benzofuran-6-yl carbamates 15a

R = OCH3; ArC6H5; m.p. 147–9 °C; yield 55%. – IR (KBr): υ = 3187 (NH), 1701, 1645 (CO), 1560 (CC), 1375, 1152 (SO2), 1099, 1066, 1001 cm−1 (COC). – 1H NMR (DMSO-d) δ: 2.66–3.21 (m, 10H, piperidinyl), 3.91, 4.22 (2s, 6H, 2OCH3), 7.00–7.09 (m, 5H, phenyl), 7.71, 7.91 (2d, 2H, CHCH), 7.98 (d, 1H, H-3), 8.12 (d, 1H, H-2), 9.71 (s, 1H, NH). – C25H26N2O8S(514.55): calcd. C 58.36; H 5.09; N 5.44; found C 58.11; H 5.22; N 5.26.

N-Piperidinosulfonyl 4,7-dimethoxy-5-(3-(3-indolyl)acryloyl) benzofuran -6-yl carbamates 15b

R = OCH3; Ar = 3-indolyl; m.p. 91–3 °C; yield 45%. – IR (KBr): υ = 3280, 3181 (NH), 1699, 1671 (CO), 1545 (CC), 1371, 1150 (SO2), 1111, 1009, 1000 cm−1 (COC). – 1H NMR (DMSO-d) δ: 2.61–3.22 (m, 10H, piperidinyl), 3.81, 4.11 (2s, 6H, 2OCH3), 7.11–7.45 (m, 4H, indolyl), 7.73, 7.71 (2d, 2H, CHCH), 7.98 (d, 1H, H-3), 8.12 (d, 1H, H-2), 8.35 (s, 1H, indolyl 2-H), 8.79 (s, 1H, NH), 10.20 (s, 1H, indolyl NH). – C27H27N3O8S(553.58): calcd. C 58.58; H 4.92; N 7.59; found C 58.31; H 4.86; N 7.45.

Biological assay

DPPH radical-scavenging activity

Sixteen new target synthesized compounds 3a,b, 10a–g, and 11a–g were screened for their DPPH radical-scavenging activity using the procedure of Viuda-Martos et al. [17]. A volume of 20 μL of methanolic solution of test compounds of 100 μg/mL was added to 2 mL of 6 × 10−5 mol L−1 methanolic solution of DPPH (2.3659 mg DPPH in 100 mL methanol). The mixture was shaken vigorously and allowed to stand for 1 h in a dark room. Ascorbic acid (Sigma–Aldrich Chemie GmeH, Taufkirchen, Germany) was used as a reference. The decrease in absorbance at 517 nm was determined using microplate ELIZA reader (ASYS Hitech GmbH, Austria). Absorbance of DPPH radical without sample was used as negative control. The percentage of scavenging activity was calculated according to the formula, % I = [(AB − As)/AB] × 100, where I = DPPH inhibition%, AB = absorbance of control (t = 0 h) and A = absorbance of a tested sample at the end of the reaction (t = 1 h). All tests and analyses were done in triplicate and the results were averaged.

Cell culture

A-549 (human lung carcinoma), MCF7 (human breast carcinoma), and HCT-116 (human colon carcinoma) cell lines were obtained from Karolinska Institute, Stockholm, Sweden. All cells were maintained in RPMI 1640 medium, except for A-549 cancer cells which were maintained in DMEM medium (Lonza Biowahittkar, Belgium). All the media were supplemented with 1% antibiotic–antimycotic mixture (10,000 U mL−1 potassium penicillin, 10,000 μg/mL streptomycin sulfate, 25 μg/mL amphotericin B, and 1% L-glutamine (Biowest, USA).

MTT cytotoxicity assay

Cell viability was investigated using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] (Bio Basic Canada Inc., Canada) assay [18]. This reaction depends on the mitochondrial reduction of yellow MTT into purple formazan. All the preceding steps were carried out in sterile laminar air flow cabinet Biosafety class II level (Baker, SG403INT, Sanford, ME, USA). All incubations were done at 37 °C in 5% CO2 incubator in humidified atmosphere (Sheldon, TC2323, Cornelius, OR, USA). Cells were seeded into 96-well microtiter plastic plates at the concentration of (104 cells per well) and allowed to adhere for 24 h. Medium was aspirated and fresh medium (without serum) was added to the cells with various concentrations of the test compounds (100, 50, 25, 12.5, 6.25, 3.12, 1.56, and 0.78 μg/mL in DMSO) and incubated for 48 h. Medium was aspirated and 40 μL MTT salt (2.5 μg/mL) was added to each well and incubated for further 4 h. To stop the reaction and dissolve any formed formazan crystals, 200 μL of 10% sodium dodecyl sulfate (SDS) was added to each well and incubated overnight at 37 °C. The amount of formazan product was measured at 595 nm with a reference wavelength of 620 nm as a background using a microplate reader (Bio-Rad Laboratories, model 3350, USA). For the untreated cells (negative control), medium was added instead of the test compounds. A positive control Adrinamycin® (doxorubicin) (Mr = 579.9) was used as a known cytotoxic natural agent giving 100% inhibition. Dimethyl sulfoxide (DMSO) was the vehicle used for dissolution of tested compound, and its final concentration on the cells was less than 0.2%.

IC50 was calculated for the samples and negative control (cells with vehicle) by the probit analysis using simple t-test (SPSS statistical analysis software package/version 11.0, SPSS Inc., (IL), Chicago, USA).

Results and discussion

Chemistry

The synthetic routes of the titled compounds are outlined in Scheme 1, Scheme 2. Addition of two equivalent of chlorosulfonyl isocyanate (CSI) to 4-methoxy-5-acetyl-6-hydroxybenzofuran (1a) and 4,7-dimethoxy-5-acetyl-6-hydroxybenzofuran (1b) in dry benzene at 0–5 °C led to the formation of N-(4-methoxy-6-(N-chlorosulfonyl carbamate-benzofuran-5-yl)ethylidene)chlorosulfonyl amine (2a) and N-(4,7-dimethoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl)ethylidine) chlorosulfonyl amine (2b). Treatment of compound 2a or 2b with triethylamine under stirring afforded the cyclized, 4-methoxy-5-methylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide (3a) and 4,9-dimethoxy-5-methylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide (3b) with overall yield 33 and 35%, respectively (method A). This reaction probably proceeded through the intermediate A followed by losing a molecule of (ClSO2NCO) as reported in similar reactions [7] (Scheme 1).

Scheme 1

Compounds 1–5, R, a = H; b = OCH3; reagents and conditions, (i) chlorosulfonyl isocyanate (CSI) (1:2 adduct), dry benzene, 0–5 °C; (ii) triethylamine, stirring; (iii) CSI (1:1 adduct), dry toluene, 100–105 °C; (iv) CSI (1:1 adduct), dry benzene, 0–5 °C; (v) dry1,4-dioxane, triethylamine, r.t.

Scheme 2

Compounds 6 and 10, R = H; Ar, a = C6H5, b = C6H4Cl-p, c = C6H4F-p, d = C6H4OCH3-p, e = C6H2(OCH3)3-3,4,5, f = C6H4N(CH3)2-p, g = 3-indolyl; compounds 7 and 11, R = OCH3; Ar, a = C6H5, b = C6H4Cl-p, c = C6H4F-p, d = C6H4OCH3-p, e = C6H2(OCH3)3-3,4,5, f = C6H4N(CH3)2-p, g = 3-indolyl; reagents and conditions; (i) chlorosulfonyl isocyanate (CSI) (1:2 adduct), dry benzene, 0–5 °C; (ii) triethylamine, stirring; (iii) CSI (1:1 adduct), dry toluene, 100–105 °C; (iv) CSI (1:1 adduct), dry benzene, 0–5 °C; (v) dry1,4-dioxane, triethylamine, r.t.

On the other hand, cycloaddition reaction of CSI with compounds 1a or 1b in dry toluene at 100–105 °C (1:1 adduct) gave a product identical in all aspects (mp, TLC and spectra) with the cyclic 1,2,3-benzoxathiazine-7,7-dioxide derivatives 3a and 3b with good yield of 45% and 52%, respectively (method B). This reaction probably proceeded through the formation of aryloxyisocyanate B followed by intramolecular [2 + 2] addition of CN group of the isocyanate to the CO group of COCH3 with subsequent loss of carbon dioxide as reported in similar reactions [7] (Scheme 1).

IR spectra of 3a and 3b showed no absorption bands for CO and OH groups but showed absorption bands from 1135 to 1375 for SO2 group (c.f. experimental section). The IR spectrum of compound 3a as an example showed absorption bands at 1620, 1579, 1375, 1165, 1120, 1119, 1110 cm−1. Its 1HNMR spectrum revealed signals at 2.89 (s, 3H, CH3), 4.40 (s, 3H, OCH3), 6.77 (s, 1H, H-9), 7.62 (d, 1H, H-3), 8.22 ppm (d, 1H, H-2). Its 13C NMR spectrum revealed signals at 18.5 (CH3), 60.9 (OCH3), 92.8–159.0 (Ar—C), 166.6 ppm (CN).

Moreover, reaction of CSI with compounds 1a or 1b in equimolar amounts in dry benzene (1:1 adduct) at 0–5 °C led to the formation of 1-(4-methoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl) ethanone 4a and 1-(4,7-dimethoxy-6-(N-chlorosulfonyl carbamatobenzofuran-5-yl) ethanone 4b, respectively (Scheme 1). The structures of compounds 4a and 4b were confirmed upon the bases of their correct elemental analyses and spectral data (c.f. experimental section), besides the chemical evidences that; (a) compounds 4a,b showed negative ferric chloride test and positive sulfur, nitrogen, and halogen tests and (b) compounds 4a,b reacted with piperidine in presence of triethylamine and afforded the corresponding N-piperidinosulfonyl carbamates 5a and 5b (Scheme 1).

In a similar manner, α,β-unsaturated keto derivative 6a or 7a reacted with two equivalent of CSI in dry benzene at 0–5 °C and gave N-(4-methoxy (8a) and (4,7-dimethoxy)-6-(N-chlorosulfonylcarbamato-benzofuran-5-yl)-3-phenylprop-2-ene)chlorosulfonyl amines (9a) with yields of 52% and 60%, respectively. Cyclization of the latter compounds via their reaction with triethylamine led to the formation of 4-methoxy 10a and 4,9-dimethoxy-5-styrylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide 11a with over all yields 30 and 32%, respectively (Scheme 2).

Due to the low yield of compounds 10a and 11a via the above two step reactions, we used the direct reaction of CSI with compounds 6a or 7a in dry toluene at 100–105 °C (1:1 adduct) to give 10a and 11a with good yields 77% and 78%, respectively (Scheme 2). Also, α,β-unsaturated keto derivatives 6b–g and 7b–g allowed to react with CSI under the above mentioned conditions to give the corresponding 4-methoxy-5-arylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide derivatives (10b–g) and 4,9-dimethoxy-5-arylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxide derivatives (11b–g), respectively (Scheme 2) with good yields ranging from 60% to 98%. IR (KBr) spectrum of 10a as an example showed characteristic absorption bands at 1620, 1569, 1375, 1145, 1120, 1119 and 1110 cm−1. Its 1H NMR spectrum revealed signals at 4.32 (s, 3H, OCH3), 6.88 (s, 1H, H-9), 7.01, 7.12 (2d, 2H, CHCH), 7.32–7.68 (m, 5H, Ar—H), 7.90 (d, 1H, H-3), 8.12 ppm (d, 1H, H-2). Its 13C NMR spectrum revealed signals at 61.2 (OCH3), 96.3–155.2 (Ar—C), 159.3 ppm (CN). Mass spectrum of 10a showed molecular ion peak at m/z = 355 (53%) and base peak at m/z = 190.

On the other hand, reaction of compounds 6a, 6g, 7a or 7g with CSI in dry benzene (1:1 adduct) at 0–5 °C afforded N-chlorosulfonyl carbamato-α,β-unsaturated keto derivatives 12a,b and 13a,b, respectively (Scheme 2). The reaction of the latter compounds 12a,b and 13a,b with piperidine in presence of triethylamine afforded the corresponding N-piperidinosulfonyl carbamates 14a,b and 15a,b, respectively (Scheme 2).

The structure of all the new compounds was confirmed based on their correct elemental analyses and spectral data (c.f. experimental section).

Biological activity

DPPH radical-scavenging activity

Compounds 3a,b, 10a–g, and 11a–g were screened for their DPPH radical-scavenging activity using ascorbic acid as a reference. Antioxidant reacts with DPPH, which is stable free radical and converts it to 1,1-diphenyl-2-picrylhydrazine. The degree of discoloration indicates the scavenging potential of the antioxidant compounds. From the data obtained, only compounds 10a, 11c, 11e, and 11g showed moderate DPPH radical-scavenging activity of 59.7%, 50.3%, 50.0% and 53.8%, respectively, than the rest of the screened compounds, which showed slightly activity compared to ascorbic acid of 96.0% at 100 μg/mL, Table 1.

Table 1

DPPH radical-scavenging assay of the synthesized compounds.

Compd.	Scavenging activity (%)a at 100 μg/mL
3a	10.6
3b	30.4
10a	59.7
10b	12.7
10c	11.1
10d	7.5
10e	11.8
10f	0.0
10g	20.4
11a	17.4
11b	28.5
11c	50.3
11d	17.3
11e	50.0
11f	9.7
11g	53.8
Negative control	–
Ascorbic acid	96.0

Results are the mean of three independent experiments.

Antiproliferative activity

Compounds 3a,b, 10a–g, and 11a–g were preliminary screened for their in vitro antiproliferative activity against human lung carcinoma (A-549), human breast cancer (MCF7), and human colon cancer (HCT-116) cell lines at a concentration of 100 μg/mL, Table 2. Compounds 10a, 10b, 11a, 11b, and 11c were found to be the most active compounds with antiproliferative activity of 100% against HCT-116 cancer cell line, whereas the most active compounds against MCF7 cancer cell line was in the descending order of 10b > 11b > 10c > 11c > 10a > 10d and 10e with antiproliferative activity of 99.2%, 97.5%, 95.4%, 92.2%, 91.2%, 85.4% and 85.4%, respectively. On the other hand, compounds 10a, 10b,11a, 11b, and 11c were found to be the most active one with antiproliferative activity of 73.3%, 82.7%, 81.0%, 82.7%, and 73.8%, respectively, against A-549 cancer cell line.

Table 2

Antiproliferative activity of the newly synthesized compounds against human carcinoma cell lines.

Compd.a	Inhibition growth (%)
	A549	MCF7	HCT-116
3a	15.8	28.0	45.7
3b	4.2	36.0	0
10a	73.3	91.2	100
10b	82.7	99.2	100
10c	8.2	95.4	69.9
10d	4.9	85.4	74.8
10e	68.7	85.4	81.6
10f	21.4	27.2	4.3
10g	29.6	16.7	0
11a	81.0	84.4	100
11b	82.7	97.5	100
11c	73.8	92.2	100
11d	1.5	6.1	16.7
11e	55.2	80.2	88.0
11f	0	20.7	0
11g	0	0	1.4
Negativeb control	–	–	–
Doxorubicin	100.0	100.0	100.0

Concentration of test compounds and positive control (doxorubicin) 100 μg/mL.

Untreated cells in DMSO and its final concentration on the cells was less than 0.2%.

The compounds that showed antiproliferative activity higher than 70% at concentration of 100 μg/mL were used to calculate their IC50 value, which corresponds to the concentration required for 50% inhibition of cell viability. Doxorubicin was used as a reference drug, Table 3. From the data obtained, compound 11a showed potent inhibition of IC50 = 0.05 and 0.08 μmol/mL against A-549 and HCT-116, respectively, nearly as active as doxorubicin of IC50 = 0.04 and 0.06 μmol/mL, respectively.

Table 3

IC50 of the highly antiproliferative active compounds against human cancer cell lines.

Compd.	IC50 (μmol/mL)
	A549	MCF7	HCT-116
10a	>0.78	>0.78	>0.78
10b	>0.78	>0.78	>0.78
11a	0.05	>0.78	0.08
11b	0.03	>0.78	0.05
11c	0.02	>0.78	0.03
11e	–	>0.78	>0.78
Doxorubicin	0.04	0.07	0.06

IC50. – Concentration required inhibiting cell viability by 50%.

IC50. – >0.78 is considered inactive.

On the other hand, compounds 11b and 11c showed higher activity with inhibition of IC50 = 0.03 and 0.02 μmol/mL, respectively, against A-549 compared to doxorubicin (IC50 = 0.04 μmol/mL). Also, compounds 11b and 11c showed higher activity with inhibition of IC50 = 0.05 and 0.03 μmol/mL, respectively, against HCT-116 compared to doxorubicin (IC50 = 0.06 μmol/mL.

From the data obtained, it is clear that compounds 11a, 11b, and 11c found to be the most active compounds against A-549 and HCT-116 cancer cell lines and their activity may be due to the presence of the methoxy donating group at the position-9 of furobenzoxathiazine. Besides that, the presence of withdrawing chlorine atom in 11b and fluorine atom in 11c at the Para position of phenyl ring improved their activities toward A-549 and HCT-116 cancer cell lines.

Conclusions

A novel series of 4-methoxy, and 4,9-dimethoxy-5-substituted furo[2,3-g]-1,2,3-benzoxathiazine-7,7-dioxide derivatives 3a,b, 10a–g and 11a–g were prepared via reaction of 4-methoxy (1a) and 4,7-dimethoxy-5-acetyl-6-hydroxy benzofurans (1b) and their α,β-unsaturated keto derivatives 6a–g and 7a–g with chlorosulfonyl isocyanate (CSI). Compounds 10a, 11c, 11e, and 11g showed moderate DPPH radical-scavenging activity compared to ascorbic acid at 100 μg/mL. 4,9-Dimethoxy-5-substituted styrylfuro[3,2-g]-1,2,3-benzoxathiazine-7,7-dioxides 11a, 11b, and 11c were found to be highly active against A-549 and HCT-116 cancer cell lines with IC50 values ranging from 0.02 to 0.08 μmol/mL compared to doxorubicin of IC50 = 0.04 and 0.06 μmol/mL, respectively.

Conflict of interest

The authors have declared no conflict of interest.