Lignans are a very interesting class of natural products due to their pharmacological properties, their great number of structural possibilities, and the chemical approaches to their synthesis [1]. Among the lignan family, the cytotoxic podophyllotoxin has been the most studied is a potent tubulin binding antimitotic agent, and its derivative etoposide and Teniposide is currently used in cancer chemotherapy [2]. Although the natural podophyllin resin was used in folk medicine, it was not until the 1940s that its antitumor activity was confirmed and this triggered intense studies toward synthetic routes led mainly by the late Professor Walter Gensler. Gensler’s contributions in the 1950s and 1960s on synthetic, structural, and mechanistic aspects of podophyllotoxin provided much of the basis for the synthetic studies that followed. All the reported synthetic routes to 1 produced racemic material or involved classical resolution techniques [3].

In view of the above facts, it was decided to modify the structure of podophyllotoxin (Figure 1) and synthesized podophyllotoxin using tetralone analogues (Table 1). They were synthesized by replacing 1, 3‐methylene dioxy ring with dimethoxy, hydroxy, methyl, chlorine, hydrogen and methoxy group. The synthesized podophyllotoxin analogues were screened for their biological activity [4].

Figure 1: The structure of podophyllotoxin.

Table 1: Protocol for the synthesis of podophyllotoxin analogues.

All the reagents and chemicals were purchased from Merck chemicals used without further purification. Melting points were taken in open capillary tubes and are uncorrected. TLC is performed with E. Merck precoated silica gel plates (60F-254). Acme, India silica gel, 60–120 mesh is used for column chromatography. IR spectra in KBr were recorded on Perkin-Elmer model 683 spectrometers [5]. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded CDCl₃ solvent containing tetra methyl silane (TMS) as internal references were recorded on Bruker spectrometer; Elemental analyses were performed on a Perkin-Elmer 2400. Mass spectra were obtained by Water-Q-TOF ultima spectrometer.

Synthesis

General procedure for the synthesis of α-bromo derivative of 1-tetralone tetralone 3(a-f): The starting material 2a-f (4 mmol) in acetonitrile (50 mL) and p-toluene sulphonic acid (1.14 g, 6 mmol) was heated at 65°C, N-bromosuccinimide (0.71 g, 4 mmol) was added slowly in lots over a period of 1 hour and the reaction mixture was stirred for 4-5 h. After completion of the reaction, reaction mixture was diluted with ethyl acetate (100 mL) and washed with water (2 × 50 mL) [6]. The ethyl acetate layer was separated and evaporated to get the crude product. The obtained product again recrystallized with ethanol to get a pure product.

2-bromo-6, 7-dimethoxy-4-(3, 4, 5-trimethoxyphenyl)-3, 4-dihydronaphthalen-1(2H)-one 3a: Color: light brown solid; M.p 168°C; Yield: 75.18%.

¹H NMR: 7.58–7.05 (2 H, s, Ar-H), 6.52(2 H, s, Ar-H), 4.93(1 H, t, CH-Br), 4.04 (1 H, t, J = 4.7, CH), 3.80(15 H, s, OCH₃), 2.69–2.45 (2 H, m, J = 6.4, CH₂);

¹³C NMR: 186.1, 154.7, 153.4, 147.2, 137.3, 136.7, 133.8, 127.3, 110.5, 109.2, 106.6, 60.8, 56.1, 53.9, 42.1, 39.9;

MS, m/z: 452.15 (M+).

Analytically calculated for C₂₁H₂₃BrO₆: C, 55.89; H, 5.14; Br 17.70; O, 21.26 Found: C, 55.87; H, 5.04; Br 17.71; O, 21.29%.

2-bromo-6-hydroxy-4-(3, 4, 5-trimethoxyphenyl)-3, 4-dihydronaphthalen-1(2H)-one 3b: Color: dark brown solid; M.p 171°C; Yield: 75.18%. ¹H NMR: 8.15 (1 H, d, Ar-H), 7.12(1 H, d, Ar-H), 6.60-6.52(3 H, m, Ar-H),5.35(1 H, t, OH), 4.93(1 H, t, CH-Br), 4.04 (1 H, t, J = 4.7, CH), 3.80(12 H, s, OCH₃), 2.69–2.45 (2 H, m, J = 6.4, CH₂); ¹³C NMR: 186.1, 161.9, 153.4, 141.9, 137.3, 136.7, 130.7, 126.6, 120.6, 113.3, 106.6, 60.8, 56.1, 53.9, 42.1, 39.9;MS, m/z: 408.15 (M+).

Analytically calculated for C₁₉H₁₉BrO₅: C, 56.03; H, 4.70; Br 19.62; O, 19.64 Found: C, 56.07; H, 4.71; Br 19.62; O, 19.64%.

2-bromo-6-methyl-4-(3, 4, 5-trimethoxyphenyl)-3, 4-dihydronaphthalen-1(2H)-one 3c: Color: dark brown solid. Yield: 65.18%. ¹H NMR: 7.80– 7.13 (3 H, d, Ar-H), 6.52(2 H, s, Ar-H), 4.93(1 H, t, CH-Br), 4.04 (1 H, t, J = 4.7, CH), 3.80(12 H, s, OCH₃), 2.69–2.45 (2 H, m, J = 6.4, CH₂) 2.34 (3 H, s, CH₃); ¹³C NMR: 186.1, 153.4, 143.3, 140.4, 137.3, 136.7, 131.0, 128.0, 126.4, 125.2, 106.6, 60.8, 56.1, 53.9, 42.1, 39.9; MS, m/z: 406.15 (M+).

Analytically calculated for C20H21BrO4: C, 59.27; H, 5.22; Br 19.72; O, 15.79 Found: C, 59.28; H, 5.21; Br 19.76; O, 15.76%.

2-bromo-6-chloro-4-(3, 4, 5-trimethoxyphenyl)-3, 4-dihydronaphthalen-1(2H)-one 3d: Color: light brown solid. Yield: 71.18%. ¹H NMR: 7.86– 7.39 (3 H, q, Ar-H), 6.52(2 H, s, Ar-H), 4.93(1 H, t, CH-Br), 4.04 (1 H, t, J = 4.7, CH), 3.80(12 H, s, OCH₃), 2.69–2.45 (2 H, m, J = 6.4, CH₂); ¹³C NMR: 186.1, 165.5, 153.4, 141.9, 139.2, 137.3, 136.7, 132.1, 130.7, 127.9, 126.2, 106.6, 60.8, 56.1, 53.9, 41.1, 39.9; MS, m/z: 426.15 (M+).

Analytically calculated for C₁₉H₁₈BrClO₄: C, 53.61; H, 4.26; Br 18.77; Cl, 8.33, O, 15.03 Found C, 53.60; H, 4.25; Br 18.79; Cl, 8.31, O, 15.07%.

2-bromo-4-(3, 4, 5-trimethoxyphenyl)-3, 4-dihydronaphthalen- 1(2H)-one 3e: Color: brown coloured solid. Yield: 67.78%. ¹H NMR: 7.95– 7.35 (6 H, m, Ar-H), 6.52(2 H, s, Ar-H), 4.93(1 H, t, CH-Br), 4.04 (1 H, t, J = 4.7, CH), 3.80(12 H, s, OCH₃), 2.69–2.45 (2 H, m, J = 6.4, CH₂);

¹³C NMR: 186.1, 153.4, 140.5, 137.3, 136.7, 134.0, 133.6, 128.1, 126.1, 106.6, 60.8, 56.1, 53.9, 41.8, 39.9; MS, m/z: 390.15 (M+).

Analytically calculated for C₁₉H₁₉BrO₄: C, 58.33; H, 4.89; Br 20.42; O, 16.36 Found: C, 58.34; H, 4.83; Br 20.45; O, 16.37%.

2-bromo-6-methoxy-4-(3, 4, 5-trimethoxyphenyl)-3, 4-dihydronaphthalen-1(2H)-one 3f: Color: dark brown solid. Yield: 69.98%. ¹H NMR: 8.25– 6.89 (3 H, s, Ar-H), 6.52(2 H, s, Ar-H), 4.93(1 H, t, CH-Br), 4.04 (1 H, t, J = 4.7, CH), 3.80(12 H, s, OCH₃), 2.69–2.45 (2 H, m, J = 6.4, CH₂); ¹³C NMR: 186.1, 165.5, 153.4, 147.2, 137.3, 136.7, 133.8, 127.3, 110.5, 109.2, 106.6, 60.8, 56.1, 53.9, 42.1, 39.9; MS, m/z: 422.15 (M+).

Analytically calculated for C₂₀H₂₁BrO₅: C, 57.09; H, 5.04; Br 18.97; O, 18.99 Found: C, 57.07; H, 5.01; Br 18.99; O, 18.97%.

General procedure for the preparation of 4-(3, 4, 5-trimethoxyphenyl) naphthalen-1(4h)-one 4(a-f): A mixture of 3a-f (3 mmol) and Sodium-tertiary-butoxide (0.317g, 3.3 mmol) in THF (25 mL) was refluxed at room temperature under nitrogen atmosphere for 10 hours. The progress of the reaction was monitored by TLC which showed the absence of starting material in the reaction mixture [7]. After completion of the reaction, reaction mixture was poured into water (100 mL) and extracted with ethyl acetate (50 mL). The evaporation of the ethyl acetate layer gave crude product. This was purified by ethyl acetate/hexane (90: 10) on silica gel to get pure product.

6, 7-dimethoxy-4-(3, 4, 5-trimethoxyphenyl) naphthalen-1(4H)- one 4a: Color: pale yellow solid; M.p. 118°C; Yield: 70.18%. ¹H NMR: 7.21– 7.09 (2 H, d, Ar-H), 6.88 (1 H, d, α-CH), 6.86(1 H, d, β-CH), 6.46(2 H, s, Ar-H), 4.74 (1 H, t, J = 4.7, CH), 3.89(15 H, s, OCH₃);

¹³C NMR: 183.7, 156.1, 153.4, 147.8, 139.7, 136.7, 136.2, 134.6, 131.9, 126.3, 113.8, 106.7, 104.3, 60.8, 56.1, 47.4; MS, m/z: 372.15 (M+).

Analytically calculated for C₂₀H₂₂O₆: C, 68.10; H, 5.94; O, 25.99 Found: C, 68.07; H, 5.91; O, 25.97%.

6-hydroxy-4-(3, 4, 5-trimethoxyphenyl) naphthalen-1(4H)-one 4b: Color: cream coloured solid; M.p. 128°C; Yield: 65.08%. ¹H NMR: 7.61-7.10 (2 H, d, Ar-H), 6.86 (1 H, d, α-CH), 6.82(1 H, d, β-CH), 6.46(2 H, s, Ar-H), 6.79 (1 H, d, Ar-H), 5.30(1 H, s, OH), 4.79 (1 H, t, J = 4.7, CH), 3.89(9 H, s, OCH₃); ¹³C NMR: 183.7, 163.3, 153.4, 140.0, 139.7, 136.7, 136.2, 134.6, 130.2, 125.6, 114.4, 113.9, 104.3, 60.8, 56.1, 47.4; MS, m/z: 342.15 (M+).

Analytically calculated for C₁₈H₁₉O₅: C, 69.95; H, 5.57; O, 24.51 Found: C, 69.97; H, 5.56; O, 24.50%.

6-methyl-4-(3, 4, 5-trimethoxyphenyl) naphthalen-1(4H)-one 4c: Color: cream coloured solid. Yield: 75.18%. ¹H NMR: 7.65-7.23 (3 H, m, Ar-H), 6.89 (1 H, d, α-CH), 6.86(1 H, d, β-CH), 6.46(2 H, s, Ar-H), 4.76 (1 H, t, J = 4.7, CH), 3.89(9 H, s, OCH₃), 2.35(3 H, s, CH₃);

¹³C NMR: 183.7, 153.4, 144.7, 139.7, 138.5, 136.7, 136.2, 134.6, 130.6, 130.3, 130.0, 127.0, 104.3, 60.8, 56.1, 47.4, 21.6; MS, m/z: 324.15 (M+).

Analytically calculated for C₂₀H₂₀O₄: C, 74.07; H, 6.21; O, 19.73 Found: C, 74.09; H, 6.19; O, 19.75%.

6-chloro-4-(3, 4, 5-trimethoxyphenyl) naphthalen-1(4H)-one 4d: Color: light yellow coloured solid. Yield: 75.18%. ¹H NMR: 7.71- 7.49 (3 H, m, Ar-H), 6.89 (1 H, d, α-CH), 6.86(1 H, d, β-CH), 6.46(2 H, s, Ar-H), 4.86 (1 H, t, J = 4.7, CH), 3.92(9 H, s, OCH₃);

¹³C NMR: 183.7, 153.4, 140.7, 140.0, 139.7, 136.7, 136.2, 134.6, 131.8, 131.1, 128.5, 126.8, 104.3, 60.8, 56.1, 46.4; MS, m/z: 344.15 (M+).

Analytically calculated for C₁₉H₁₇ ClO₄: C, 66.19; H, 4.97; Cl, 10.28; O, 18.56 Found: C, 66.17; H, 4.99; Cl, 10.26; O, 18.58%.

4-(3, 4, 5-trimethoxyphenyl) naphthalen-1(4H)-one 4e: Color: cream coloured solid. Yield: 75.18%. ¹H NMR: 7.71- 7.49 (3 H, m, Ar- H), 6.89 (1 H, d, α-CH), 6.86(1 H, d, β-CH), 6.46(2 H, s, Ar-H), 4.86 (1 H, t, J = 4.7, CH), 3.92(9 H, s, OCH₃);

¹³C NMR: 183.7, 153.4, 139.7, 138.6, 136.7, 136.2, 135.0, 134.6, 133.0, 128.7, 127.9, 126.7, 104.3, 60.8, 56.1, 47.4; MS, m/z: 311.15 (M+).

Analytically calculated for C₁₉H₁₈O₄: C, 73.54; H, 5.85; O, 20.62 Found: C, 73.53; H, 5.86; O, 20.64%.

6-methoxy-4-(3, 4, 5-trimethoxyphenyl) naphthalen-1(4H)-one 4f: Color: cream coloured solid. Yield: 75.18%. ¹H NMR: 7.61-7.20 (1 H, d, Ar-H), 6.99 (1H d Ar-H), 6.86 (1 H, d, α-CH), 6.82(1 H, d, β-CH), 6.46(2 H, s, Ar-H), 4.70 (1 H, t, J = 4.7, CH), 3.79(12 H, s, OCH₃);

¹³C NMR: 183.7, 166.9, 153.4, 139.7, 139.6, 136.7, 136.2, 134.6, 129.8, 125.3, 112.8, 112.3, 104.3, 60.8, 56.1, 47.4; MS, m/z: 342.15 (M+).

Analytically calculated for C₂₀H₂₀O₅: C, 70.57; H, 5.92; O, 23.50 Found: C, 70.58; H, 5.91; O, 23.52%.

General procedure for the preparation of 3-bromomethyl-4- oxo-1-(3, 4, 5-trimethoxyphenyl)-1, 2, 3, 4-tetrahydronaphthalene- 2-carbonitrile 5(a-f): Potassium cyanide (0.1625g, 2.5 mmol) was added to the reaction mixture of compound 4a-f (2.5 mmol), tetra butyl ammonium bromide (0.08 g, 0.25 mmol) and dibromomethane (0.64 g, 3.75 mmol) which were stirred under nitrogen gas atmosphere at room temperature using dried THF solvent (25 mL) and refluxed for 5 hours at 60°C. After completion of the reaction which was monitored by TLC, the reaction mixture was evaporated to dryness and extracted with water (100 mL) followed by Diethyl ether (50 mL). The organic layer was separated and was evaporated to dryness. The obtained solid compound was purified by column chromatography [8] on silica gel using hexane/ethyl acetate (80:20).

3-(bromomethyl)-6, 7-dimethoxy-4-oxo-1-(3, 4, 5-trimethoxyphenyl)-1, 2, 3, 4-tetrahydronaphthalene-2-carbonitrile 5a: Color: yellowish solid; M.p 152°C; Yield: 62.88%.

¹H NMR: 8.20 (1 H, d, Ar-H), 7.13 (2 H, s, Ar-H), 6.52-6.60 (2 H, t, Ar-H), 5.32(1 H, s OH), 4. 07 (1 H, d, J = 4.0, CH), 3.83(9 H, s, OCH₃), 3.80(1 H, t, CH-CN), 3.58-3.33(3 H, m, CH₂-Br), 2.35(3 H, s, CH₃); ¹³C NMR: 199.1, 161.9, 153.4, 141.9, 137.3, 136.7, 130.7, 126.6, 120.6, 119.2, 113.3, 106.6, 60.8, 56.1, 49.0, 36.7, 30.4, 26.8; MS, m/z: 492.15 (M+).

Analytically calculated for C₂₃H₂₄BrNO₆: C, 56.34; H, 4.93; Br, 16.30; N, 2.86; O, 19.58 Found: C, 56.38; H, 4.92; Br, 16.31; N, 2.15; O, 17.92%.

3-(bromomethyl)-7-hydroxy-4-oxo-1-(3, 4, 5-trimethoxyphenyl)-1, 2, 3, 4-tetrahydronaphthalene-2-carbonitrile 5b: Color: buff coloured solid; M.p. 164°C. Yield: 63.18%. ¹H NMR: 8.20 (1 H, d, Ar-H), 7.13 (2 H, s, Ar-H), 6.52-6.60 (2 H, t, Ar-H), 5.32(1 H, s OH), 4. 07 (1 H, d, J = 4.0, CH), 3.83(9 H, s, OCH₃), 3.80(1 H, t, CH-CN), 3.58-3.33(3 H, m, CH₂-Br), 2.35(3 H, s, CH₃); ¹³C NMR: 199.1, 161.9, 153.4, 141.9, 137.3, 136.7, 130.7, 126.6, 120.6, 119.2, 113.3, 106.6, 60.8, 56.1, 49.0, 36.7, 30.4, 26.8; MS, m/z: 447.15 (M+).

Analytically calculated for C₂₁H₂₀BrNO₅: C, 56.52; H, 4.52; Br, 17.90; N, 3.14; O, 17.97 Found: C, 56.58; H, 4.58; Br, 17.93; N, 3.15; O, 17.92%.

3-(bromomethyl)-7-methyl-4-oxo-1-(3, 4, 5-trimethoxyphenyl)-1, 2, 3, 4-tetrahydronaphthalene-2-carbonitrile 5c: Color: yellow coloured solid; M.p. 174°C; Yield: 75.18%; ¹H NMR: 7.80 (1 H, d, Ar- H), 7.24-7.13 (2 H, d, Ar-H), 6.52 (2 H, s, Ar-H), 4. 05 (1 H, d, J = 4.0, CH), 3.93(9 H, s, OCH₃), 3.80(1 H, t, CH-CN), 3.58-3.33(2 H, m, CH₂- Br), 3.4(1 H, q, CH), 2.35(3 H, s, CH₃); ¹³C NMR: 199.1, 153.4, 143.3, 140.4, 137.3, 136.7, 131.0, 128.0, 126.4, 125.2, 119.2, 106.6, 60.8, 56.1, 49.0, 36.7, 30.4, 26.8, 21.6; MS, m/z: 432.15 (M+).

Analytically calculated for C₂₂H₂₂BrNO₄: C, 59.47; H, 4.99; Br, 17.98; N, 3.16; O, 14.47 Found: C, 59.48; H, 4.98; Br, 17.99; N, 3.15; O, 14.40%.

3-(bromomethyl)-7-chloro-4-oxo-1-(3, 4, 5-trimethoxyphenyl)-1, 2, 3, 4-tetrahydronaphthalene-2-carbonitrile 5d: Color: light yellow solid; M.p. 154°C; Yield: 75.18%; ¹H NMR: 7.80 (1 H, d, Ar-H), 7.24- 7.13 (2 H, d, Ar-H), 6.52 (2 H, s, Ar-H), 4. 05 (1 H, d, J = 4.0, CH), 3.93(9 H, s, OCH₃), 3.80(1 H, t, CH-CN), 3.58-3.33(2 H, m, CH₂-Br), 3.4(1 H, q, CH), 2.35(3 H, s, CH₃); ¹³C NMR: 199.1, 153.4, 143.3, 140.4, 137.3, 136.7, 131.0, 128.0, 126.4, 125.2, 119.2, 106.6, 60.8, 56.1, 49.0, 36.7, 30.4, 26.8, 21.6; MS, m/z: 432.15 (M+).

Analytically calculated for C₂₂H₂₂BrNO₄: C, 59.47; H, 4.99; Br, 17.98; N, 3.16; O, 14.47 Found: C, 59.48; H, 4.98; Br, 17.99; N, 3.15; O, 14.40%.

3-(bromomethyl)-4-oxo-1-(3, 4, 5-trimethoxyphenyl)-1, 2, 3, 4-tetrahydronaphthalene-2-carbonitrile 5e: Color: dark yellow coloured solid; M.p. 169°C; Yield: 75.18%; ¹H NMR: 7.92-7.35 (4 H, m, Ar-H), 6.56(2 H, s, Ar-H), 4. 0 (1 H, d, J = 4.0, CH), 3.92(9 H, s, OCH₃), 3.78(1 H, t, CH-CN), 3.58-3.33(2 H, m, CH₂-Br), 3.4(1 H, q, CH); ¹³C NMR: 199.1, 153.4, 140.5, 137.3, 136.7, 134.0, 133.6, 128.1, 126.1, 119.2, 106.6, 60.8, 56.1, 49.0, 36.4, 30.4, 26.8; MS, m/z: 432.15 (M+).

Analytically calculated for C₂₁H₂₀BrNO₄: C, 58.62; H, 4.68; Br, 18.57; N, 3.26; O, 14.87 Found: C, 58.65; H, 4.65; Br, 18.58; N, 3.25; O, 14.89%.

3-(bromomethyl)-7-methoxy-4-oxo-1-(3, 4, 5-trimethoxyphenyl)-1, 2, 3, 4-tetrahydronaphthalene-2-carbonitrile 5f: Color: dark yellow coloured solid; M.p. 179°C; Yield: 75.18%; ¹H NMR: 7.88-7.39 (3 H, m, Ar-H), 6.58(2 H, s, Ar-H), 4. 08 (1 H, d, J = 4.0, CH), 3.82(9 H, s, OCH₃), 3.76(1 H, t, CH-CN), 3.58-3.33(2 H, m, CH₂-Br), 3.45(1 H, m, CH); ¹³C NMR: 199.1, 153.4, 141.9, 139.2, 137.3, 136.7, 132.1, 130.7, 127.9, 127.9, 119.2, 106.6, 60.8, 56.1, 49.0, 35.9, 30.4, 26.8; MS, m/z: 466.15 (M+).

Analytically calculated for C₂₁H₁₉BrClNO₄: C, 54.27; H, 4.12; Br, 17.19; Cl, 7.63; N, 3.01; O, 13.77 Found: C, 54.24; H, 4.11; Br, 17.18; Cl, 7.63; N, 3.05; O, 13.78%.

General procedure for the synthesis of 9-(3, 4, 5-trimethoxyphenyl)-3, 3a, 9, 9a-tetrahydronaphtho [2, 3-c] furan-1, 4-dione 6(a-f): To a stirred solution of compound 5a-f (0.980 g, 2 mmol) 20% sulphuric acid was added and refluxed at 90°C for 15 hours. After completion of the reaction, the reaction mixture was poured into ice, extracted with ethyl acetate and then ethyl acetate was evaporated to get crude solid which was purified by column chromatography [9] on silica gel using hexane/ethyl acetate (80: 20).

6,7-dimethoxy-9-(3,4,5-trimethoxyphenyl)-3,3a,9,9atetrahydronaphtho[ 2,3-c]furan-1,4-dione 6a: Color: white coloured solid; M.p. 151°C; Yield: 68.98%;¹H NMR: 7.98-7.35 (4 H, m, Ar-H), 6.58(2 H, s, Ar-H), 4. 68 (2 H, d, J = 4.0, CH), 3.83(9 H, s, OCH₃), 4.58- 3.59 (6 H, t, CH₂); ¹³C NMR: 193.3, 175.4, 153.4, 140.5, 136.7, 136.1, 133.6, 134.0, 128.1, 126.1, 106.6, 69.3, 60.8, 56.1, 47.0, 46.7, 43.5;MS, m/z: 369.15 (M+). Analytically calculated for C₂₁H₂₀O₆: C, 68.47; H, 5.47; O, 26.06 Found: C, 68.49; H, 5.44; O, 26.08%.

7-hydroxy-9-(3, 4, 5-trimethoxyphenyl)-3, 3a, 9, 9a-tetrahydronaphtho [2, 3-c] furan-1, 4-dione 6b: Color: white coloured solid; M.p. 158°C; Yield: 68.09%; ¹H NMR: 7.98-7.35 (4 H, m, Ar-H), 6.58(2 H, s, Ar-H), 4. 68 (2 H, d, J = 4.0, CH), 3.83(9 H, s, OCH₃), 4.58-3.59 (6 H, t, CH₂); ¹³C NMR: 193.3, 175.4, 153.4, 140.5, 136.7, 136.1, 133.6, 134.0, 128.1, 126.1, 106.6, 69.3, 60.8, 56.1, 47.0, 46.7, 43.5; MS, m/z: 369.15 (M+). Analytically calculated f or C₂₁H₂₀O₆: C, 68.47; H, 5.47; O, 26.06 Found: C, 68.49; H, 5.44; O, 26.08%.

7-methyl-9-(3, 4, 5-trimethoxyphenyl)-3, 3a, 9, 9a-tetrahydronaphtho [2, 3-c] furan-1, 4-dione 6c: Color: white coloured solid; M.p. 167°C; Yield: 65.68%; ¹H NMR: 7.98-7.35 (4 H, m, Ar-H), 6.58(2 H, s, Ar-H), 4. 68 (2 H, d, J = 4.0, CH), 3.83(9 H, s, OCH₃), 4.58-3.59 (6 H, t, CH₂); ¹³C NMR: 193.3, 175.4, 153.4, 140.5, 136.7, 136.1, 133.6, 134.0, 128.1, 126.1, 106.6, 69.3, 60.8, 56.1, 47.0, 46.7, 43.5; MS, m/z: 369.15 (M+). Analytically calculated for C₂₁H₂₀O₆: C, 68.47; H, 5.47; O, 26.06 Found: C, 68.49; H, 5.44; O, 26.08%.

7-chloro-9-(3, 4, 5-trimethoxyphenyl)-3, 3a, 9, 9a-tetrahydronaphtho [2, 3-c] furan-1, 4-dione 6d: Color: white coloured solid; M.p. 161°C; Yield: 86.78%; ¹H NMR: 7.98-7.35 (4 H, m, Ar-H), 6.58(2 H, s, Ar-H), 4. 68 (2 H, d, J = 4.0, CH), 3.83(9 H, s, OCH₃), 4.58-3.59 (6 H, t, CH₂); ¹³C NMR: 193.3, 175.4, 153.4, 140.5, 136.7, 136.1, 133.6, 134.0, 128.1, 126.1, 106.6, 69.3, 60.8, 56.1, 47.0, 46.7, 43.5; MS, m/z: 369.15 (M+). Analytically calculated for C₂₁H₂₀O₆: C, 68.47; H, 5.47; O, 26.06 Found: C, 68.49; H, 5.44; O, 26.08%.

9-(3, 4, 5-trimethoxyphenyl)-3, 3a, 9, 9a-tetrahydronaphtho [2, 3-c] furan-1, 4-dione 6e: Color: white coloured solid; M.p. 171°C; Yield: 75.78%; ¹H NMR: 7.98-7.35 (4 H, m, Ar-H), 6.58(2 H, s, Ar-H), 4. 68 (2 H, d, J = 4.0, CH), 3.83(9 H, s, OCH₃), 4.58-3.59 (6 H, t, CH₂); ¹³C NMR: 193.3, 175.4, 153.4, 140.5, 136.7, 136.1, 133.6, 134.0, 128.1, 126.1, 106.6, 69.3, 60.8, 56.1, 47.0, 46.7, 43.5; MS, m/z: 369.15 (M+). Analytically calculated for C₂₁H₂₀O₆: C, 68.47; H, 5.47; O, 26.06 Found: C, 68.49; H, 5.44; O, 26.08%.

7-methoxy-9-(3, 4, 5-trimethoxyphenyl)-3, 3a, 9, 9a-tetrahydronaphtho [2, 3-c] furan-1, 4-dione 6f: Color: white coloured solid; M.p. 167°C; Yield: 95.28%; ¹H NMR: 8.21-8.30 (1 H, d, Ar-H), 7.16(1 H, s, Ar-H), 7.05-7.07(1 H, d, Ar-H), 6.70(2 H, s, Ar- H), 4. 70- 4.48 (4 H, m, Ar-CH), 3.83(12 H, s, OCH₃), 3.65-3.63 (4 H, t, CH₂); ¹³C NMR: 193.7, 175.0, 165.5, 153.4, 149.3, 136.7, 136.1, 130.3, 126.3, 111.7, 106.6, 104.6, 69.3, 60.8, 56.1, 55.8, 46.7, 44.1, 43.5; MS, m/z: 398.45 (M+). Analytically calculated for C₂₂H₂₂O₇: C, 66.32; H, 5.57; O, 28.11 found: C, 66.33; H, 5.59; O, 28.09%.

Synthesis

Bromination of the compound 2(a-f) was carried out in acetonitrile using N-bromo succinimide as the brominating agent. The conditions adopted are a modification of the reported procedure. The bromination happened selectively at the α-position to the carbonyl group of the tetralone (Figure 2). The selective bromination of the compound 2(af) was performed using N-bromosuccinimide in acidic medium by maintaining optimum temperature of 70°C in acetonitrile solvent. Using of N-bromosuccinimide is to activate the p-toluene sulphonic acid at the carbonyl oxygen to facilitate the formation of bromonium ions that leads to the formation of α-bromo tetralones. Single time addition of NBS resulted in the formation of more dibromo derivative. An excess of NBS also resulted in the formation of dibromo derivative. Hence one equivalent of NBS was added slowly in lots. The progress of the reaction was monitored by TLC [10]. The impurities were removed during crystallization. The reaction mixture after the completion of the reaction was washed with water and extracted with ethyl acetate. The ethyl acetate layer was evaporated to get the product which was recrystallized from 10% ethanol in hexane to give the product. The products were identified using 1H and 13C-NMR. ¹H NMR showed characteristic triplet at δ 4.7-4.93 ppm for hydrogen at the carbon bearing the bromo group. The tertiary hydrogen at γ-carbon to the carbonyl showed a triplet at δ 4.05-4.48 ppm. 13C-NMR showed carbonyl carbon at δ 186.10 ppm. The mass spectrums of M-1, M+1 peak are observed in the ratio of 50.5: 49.5 indicating the presence of bromine atom in the compound.

Figure 2: Protocol for the synthesis of podophyllotoxin analogues.

The brominated tetralone was dehydrohalogenated to the compound 4(a-f). The reaction mixture was refluxed using sodiumtertiary- butoxide in THF as a solvent for 10 hours under nitrogen atmosphere. Use of milder bases like triethyl amine and sodium carbonate led to lesser yield and incomplete reaction. Slight excess of sodium-tertiary-butoxide caused faster reaction but lesser yield after purification, the optimum amount of base was critical for the reaction. Hence THF was found to be the ideal solvent for the reaction. The product was identified by 1H-NMR and 13C-NMR and mass spectra and IR spectra. Mass spectra showed absence of peaks associated with bromine atom. The 1H-NMR showed signal at δ 6.8-7.1 ppm for the protons across the double bond. The absence of multiplet also at δ 2.6 ppm also confirms the formation of the product.

Potassium cyanide solution was added to the mixture of dibromomethane and tetra-n-butylammonium bromide in dry THF under nitrogen atmosphere which was kept at 60°C. The progress of the reaction was monitored by TLC. The products were identified by 1H-NMR and mass spectra.1H-NMR did not show signals at δ 6.8 ppm indicating the absence of double bond protons. It showed a doublet at δ 3.5 ppm for the protons on carbon bearing the bromine atom. 13C-NMR shows signals at δ 119.3 ppm indicates the presence of the cyanide group. The carbonyl group is intact at δ 200.4 ppm.

The 3-bromomethyl-1-phenyl-1, 2, 3, 4-tetrahydronaphthalene-2- carbonitrile was added to 20% sulphuric acid and refluxed for 15 hours. The reaction mixture was poured into ice and extracted with ethyl acetate. The crude compound is purified by column chromatography on silica gel using ethyacetate/hexane (80: 20). IR spectra showed two strong carbonyl absorptions, one at 1755 cm-1 attributed to the lactone ring and the other at 1635 cm-1 attributed to a benzylic carbonyl group. 1H-NMR of the compound shows signals of doublet at δ 4.5 ppm and at δ 4.7 ppm indicating the protons at C-1 and C-9. Multiplets are seen at δ 3.5 ppm which indicates the protons on the carbon in the tetrahydro naphthalene ring system. 13C-NMR shows signals as singlet at δ 176 ppm pertaining to the carbonyl group of the lactone moiety while the carbonyl on the tetralone ring shows as singlet at δ 200.7 ppm.

Anti diabetic activity

The new synthetic analogues were screened for antidiabetic activity by determining the ability of compounds to inhibit alpha amylase by DNS method and by measuring the non-enzymatic albumin glycosilation. In this study, amylase activity for synthesized compounds was assayed according to Bernfeld method. This assay is based on the oxidation of ketone functional group in synthesized compounds. The principal involved is the test for the presence of free carbonyl group (C=O), the so called reducing sugar. One mole of sugar will react with one mole of 3, 5-dinitrosalicylic acid. Simultaneously 3, 5 -dinitrosalicylic acid (DNS) is reduced to 3-amino, 5- nitro salicylic acid under alkaline conditions.

The Pancreatic α-amylase inhibition of podophyllotoxin analogues (Figure 3) were studied in terms of oxidation of ketone functional group using DNS method (Table 2). All the analogues inhibited α- amylase enzyme significantly with an IC₅₀ values ranging between 23.25 μg/mL and 46.5 μg/mL compared to the reference compound acarbose with an IC₅₀ value of 95.8 μg/mL (Table 3). The compounds were taken in micro molar concentration; the above said method is followed. The reference compound was treated as same without the compound treatment and taken as 100%.

Figure 3: Pancreatic α-amylase inhibition of aryl tetralin derivatives.

Table 2: Pancreatic α-amylase inhibition of aryl tetralin derivatives 6(a-f).

Table 3: IC₅₀ values of tetralin derivatives in Nitric oxide radicals.

Antibacterial activity

Antibacterial activity of the synthesized compounds (Table 4) was determined against Gram-positive bacteria (Bacilus subtilius, Streptococcus) and Gram-negative bacteria (Escherichia coli, Proteus) in DMF by disc diffusion method on nutrient agar medium. Gentamycin is used as positive control for comparison. For each treatment, three replicates were maintained. The plates were incubated at 37°C for 24 h and the zone of inhibition was determined.

Table 4: Anti-bacterial activities of synthesized compounds.

The Table 4 reveals that 5f is potent compound of all the compounds which were under study with the MIC values ranging from 1.2 μg to 7 μg. Compounds 6b and 6c are not acting on Gram-positive bacteria such as B. subtilis. Compared to the reference compounds the activity of the tetralin derivatives is not significant.

Antioxidant assays

DPPH radical scavenging assay: DPPH radical reacts with an Anti-oxidant compound that can donate proton and get reduced. DPPH when acted upon by an Anti-oxidant is converted into diphenyl picryl hydrazine (Figure 4). This can be identified by the conversion of purple to light yellow (Table 5).

Figure 4: DPPH radical scavenging activity of compounds.

Table 5: DPPH radical scavenging activity of synthesized compounds.

Nitric oxide radical scavenging assay: Nitric oxide was generated from sodium nitroprusside and measured by the Griess reaction. Sodium nitroprusside, in aqueous solution at physiological pH, spontaneously generates nitric-oxide, which in turn reacts with oxygen to produce nitric ions that can be estimated by the Griess reagent spectrophotometrically at 540 nm (Figure 5).

Figure 5: Nitric oxide radical scavenging activity of compounds.

Scavengers of nitric oxide compete with oxygen leading to reduced production of nitric oxide. Sodium nitroprusside (5 mM) in phosphate buffer saline was mixed with the compounds and incubated at 25°C for 120 minutes. The samples from the above were reacted with Griess reagent. The absorbance of the chromophore form during the diazotization of nitrate with sulphonyl amide and subsequent coupling with naphthyl ethylene diamine was read at 540 nm and refer to the absorbance standard solutions of BHT, treated in the same way with Griess reagent. The radical scavenging activity was measured using the equation described below:

Scavenging effect (%)=A control (540 nm)-A sample (540 nm)/A control (540 nm) × 100

All the tetralin derivatives showed the scavenging activity for nitric oxide radicals where scavenging activity was not significant compared to the reference compound BHT 6.4 μg/mL.

In conclusion, the new podophyllotoxin analogues has been synthesized successively in good yield and The structures of synthesized compounds were confirmed and characterized by analytical data’s such as IR, ¹H NMR, ¹³C NMR, Mass spectra and elemental analysis. They were screened for biological activity and showed good alpha amylase inhibition assay.