Total Synthesis of Ningalin C

Ningalin C is a highly colored pyrrole alkaloid isolated from an unidentified Western Australian ascidian of the genus Didemnum by Fenical et al., together with biogenetically related congeners, ningalins A, B, and D, all of which are derived from 3,4-dihydroxyphenylalanine in biosynthetic pathway.1 Structurally, it is composed of a densely substituted benzo[e]indole-2,5(3H)-dione core, two o-catechol subunits and 3,4-dihydroxyphenethyl chain on the pyrrole nitrogen (Figure 1). The first synthesis was reported by Steglich et al. in 2000, starting from tetra-substituted pyrrole by following a sequence of reactions involving a Rh(II)acetate catalyzed carbene insertion and an intramolecular Friedel-Crafts acylation for the construction of the benzo[e]indole-dione ningalin C core.2 Two years later, Ruchirawat et al. reported the second synthesis in which a base catalyzed cyclization of tetrasubstituted 1,4-naphthoquinone was utilized.3 As a part of our ongoing research program on aryl hydrazides,4 we have previously reported total syntheses of ningalin D and G, utilizing [3,3]-sigmatropic rearrangement reactions of dinaphthyl hydrazides and cyclizations.4 In this account, we report a new synthesis of ningalin C as an extension of our investigations on bioactive pyrrole marine alkaloids. Our retrosynthesis of the titled compound is delineated in Scheme 1. It was anticipated that the densely functionalized pyrrolidinone system could be synthesized from benzo[e]indole 2 through oxidation and subsequent installation of 3,4-dimethoxyphenyl group. The key benzo[e]indole 3 could be accessed from aryl vinyl hydrazide 4 through a [3,3]-sigmatropic rearrangement and cyclization cascade.4 Further analysis would require aryl bromide 5, styryl bromide 6, and bis-Boc-hydrazine 7. The synthesis began with the assembly of hydrazide 4 (Scheme 2). The CN coupling reaction between aryl bromide 54 and bis-Boc hydrazine 7 gave aryl hydrazide 8 in 87% yield. The resulting coupling product 8 was then coupled with vinyl bromide 6 to afford aryl vinyl hydrazide 4 in 98% yield. Subsequent acid-catalyzed [3,3]-sigmatropic rearrangement and cyclization cascade reaction produced the corresponding benzo[e]indole 3 in 83% yield, when heated with ZnCl2 in 1,4-dioxane under reflux. Treatment with dimethoxyphenethyl iodide 9 in the presence of KOH followed by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-mediated oxidation delivered aldehyde 2 in 60% total yields over two steps. Baeyer–Villiger oxidation, hydrolysis, and air oxidation gave rise to benzo[e]indole-dione 11 in 37% overall yield. Efforts to improve the yield were not successful. In fact, a similar yield was reported for the similar transformation. We then screened the methods and conditions for the selective bromination at C4-position. After extensive optimization, we learned that bromination with HBr in dimethyl sulfoxide (DMSO) under the conditions reported by Jiao et al.5 were most effective, providing bromide 12 in 51% yield. Suzuki–Miyaura reaction with (3,4-dimethoxyphenyl)-boronic acid (13) provided 1 in 86% yield. BBr3-mediated demethylation reactions completed the synthesis of ningaln C over nine steps from the known naphthalene 5 in 4.8% overall yield. The spectral data of our synthetic ningalin C are consistent with the literature values. In summary, we have achieved a total synthesis of ningalin C using [3,3]-sigmatropic rearrangement and cyclization cascade as key reaction. Unless otherwise stated, reactions were performed under an argon atmosphere using freshly dried solvents. Tetrahydrofuran (THF), dichloromethane, toluene, and ethyl ether were dried by passing through activated alumina columns. All other commercially obtained reagents were used as received. All reactions were monitored by thin-layer chromatography using EMD/Merck silica gel 60 F254 pre-coated plates (0.25 mm; Darmstadt, Germany). The TLC spots were visualized under ultraviolet lamp or using staining solutions such as ceric ammonium molybdate solution, p-anisaldehyde solution, ninhydrin solution, or potassium permanganate solution. Flash column chromatography was performed with indicated solvents using silica gel (GS60–40/75) purchased from Fuji Silysia Chemical (Kasugai, Aichi JAPAN). A CEM Discover microwave reaction system was used for the carbazole synthesis. 1H nuclear magnetic resonance (NMR) and 13C spectra were recorded on 400 MHz (Bruker Avance III HD, Billerica, MA, USA) spectrometer at 400 and 100 MHz, respectively. Chemical shifts are reported relative to tetramethylsilane (0 ppm) or internal chloroform (1H, δ = 7.26, 13C, δ = 77.16) as indicated. Infrared spectra were recorded on an ABB FTLA2000 FTIR (ABB, Québec, Canada) spectrometer. High-resolution mass spectra were measured by using ESI-TOF method at Korean Basic Research Center, O-Chang, Republic of Korea. A solution of 3-bromo-6,7-dimethoxy-1-methylnaphthalene 5 (400 mg, 1.42 mmol), di-tert-butyl hydrazodicarboxylate (7, 662 mg, 2.85 mmol), CuI (542 mg 2.85 mmol), 1,10-phenonthroline (513 mg, 2.85 mmol), Cs2CO3 (928 mg, 2.85 mmol), in dimethylformamide (DMF) (3 mL) was stirred under Ar atmosphere in a sealed tube at 80°C for 24 h. The reaction mixture was then cooled to room temperature and filtered through a pad of silica gel with the aid of EtOAc. The reaction mixture was concentrated under reduced pressure and purified by flash column chromatography (hexane/EtOAc = 2/1) to give naphthyl hydrazide 8 as a white solid (535 mg, 87% yield). 1H NMR (400 MHz, CDCl3) δ 1.51 (s, 18H), 2.59 (s, 3H), 3.95 (s, 3H), 3.99 (s, 3H), 6.96 (s, 1H), 7.06 (s, 1H), 7.12 (s, 1H), 7.28 (s, 1H), 7.59 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 19.7, 28.28, 28.31, 55.8, 81.5, 82.1, 102.9, 106.1, 107.2, 119.3, 122.2, 126.4, 129.3, 133.2, 138.0, 149.2, 149.4, 154.0, 155.51; Fourier-transform infrared spectroscopy (FT-IR) (CH2Cl2, cm−1) 1714, 2976, 3326; electrospray ionization high resolution mass spectrometry (ESI-HRMS) calcd for C23H32N2NaO6 [M + Na]+ 455.2153, found 455.2158. A mixture of aryl hydrazide 8 (2.533 g, 5.86 mmol), (E)-4-(2-bromo-vinyl)-1,2-dimethoxybenzene (6, 2.847 g, 11.7 mmol), CuI (0.223 g, 1.17 mmol), Cs2CO3 (3.816 g, 11.7 mmol), N,N′-dimethylethylenediamine (0.25 mL, 2.34 mmol) in THF (29 mL) was stirred under Ar atmosphere in sealed tube at 70°C overnight. The reaction was then cooled to room temperature and filtered through a pad of silica gel with the aid of EtOAc. The reaction mixture was concentrated under reduced pressure and purified by flash column chromatography (hexane/EtOAc = 3/1) to naphthyl styryl hydrazide 4 as a white solid (3.399 g, 98% yield). 1H NMR (400 MHz, CDCl3) δ 1.48 (s, 9H), 1.54 (s, 9H), 2.63 (s, 3H), 3.85 (s, 3H), 3.89 (s, 3H), 3.98 (s, 3H), 4.00 (s, 3H), 6.06 (d, 1H, J = 14.4 Hz), 6.79 (d, 1H, J = 8.3 Hz), 6.85 (dd, 1H, J = 8.3, 1.8 Hz), 6.91 (d, 1H, J = 1.8 Hz), 7.09 (s, 1H), 7.15 (s, 1H), 7.37 (s, 1H), 7.67 (d, 1H, J = 14.4 Hz); 13C NMR (100 MHz, CDCl3) δ 19.9, 28.1, 28.2, 28.3, 55.8, 55.9, 56.0, 82.3, 82.8, 83.3, 102.8, 106.0, 106.4, 107.1, 107.2, 108.1, 109.1, 109.2, 109.6, 111.4, 118.8, 124.4, 125.6, 125.9, 126.2, 129.3, 133.3, 136.6; FT-IR (CH2Cl2, cm−1) 1654, 1722, 3058; ESI-HRMS calcd for C33H42N2NaO8 [M + Na]+ 617.2833, found 617.2839. A mixture of naphthyl styryl hydrazide 4 (1.320 g, 2.22 mmol), ZnCl2 (0.666 g, 4.89 mmol) in anhydrous 1,4-dioxane (22 mL) was heated at 90°C in a sealed tube under Ar atmosphere. After 4.5 h, the reaction mixture was cooled to room temperature, quenched by adding saturated aqueous Na2CO3 and saturated aqueous NH4Cl, and the resulting mixture was extracted with dichloromethane (DCM) a few times. The combined organic solution was dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure and purified by column chromatography (CH2Cl2/EtOAc = 50/1) to give benzo[e]indole 3 as a yellow solid (0.692 g, 83% yield). 1H NMR (400 MHz, CDCl3) δ 2.67 (s, 3H), 3.60 (s, 3H), 3.83 (s, 3H), 3.92 (s, 3H), 3.99 (s, 3H), 6.95 (s, 1H), 6.97 (s, 1H), 7.01 (d, 1H, J = 2.4 Hz), 7.12–7.15 (m, 2H), 7.19 (d, 1H, J = 0.8 Hz), 7.31 (s, 1H), 7.55 (s, 1H), 8.46 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 20.5, 55.4, 55.8, 55.9, 56.1, 104.4, 105.0, 111.1, 111.6, 113.8, 118.3, 119.8, 120.5, 122.5, 123.9, 124.0, 128.3, 130.1, 132.3, 146.6, 148.0, 148.2, 148.6; FT-IR (CH2Cl2, cm−1) 1023, 2360, 3357; ESI-HRMS calcd for C23H23NNaO4 [M + Na]+ 400.1519, found 400.1525. To a stirred solution of benzo[e]indole 3 (0.669 g, 1.77 mmol) in anhydrous DMF (7 mL) was added solid KOH (1.990 g, 35.5 mmol) at room temperature. The resulting suspension was stirred at room temperature for 30 min. To this mixture, 4-(2-chloroethyl)-1,2-dimethoxybenzene (9, 2.313 g, 11.5 mmol) was added in portions at room temperature. The resulting mixture was stirred overnight, quenched with saturated aqueous NH4Cl, diluted with water and extracted with EtOAc. The combined organic solution was washed with brine, dried over anhydrous Na2SO4, and filtered. The filtrate was concentrated under reduced pressure and purified by column chromatography (hexane/EtOAc = 3/1) to give N-alkylated benzo[e]indole 10 as a yellow solid (0.747 g, 78% yield). 1H NMR (400 MHz, CDCl3) δ 2.73 (s, 3H), 3.10 (t, 2H, J = 7.0 Hz), 3.61 (s, 3H), 3.66 (s, 3H), 3.84 (s, 3H), 3.86 (s, 3H), 3.93 (s, 3H), 4.01 (s, 3H), 4.38 (t, 2H, J = 7.0 Hz), 6.39 (d, 1H, J = 1.9 Hz), 6.69 (dd, 1H, J = 8.1, 1.9 Hz), 6.75 (s, 1H), 6.78 (d, 1H, J = 8.2 Hz), 6.80 (d, 1H, J = 8.1 Hz), 7.04–7.09 (m, 2H), 7.23 (s, 1H), 7.31 (s, 1H), 7.55 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 20.8, 36.7, 48.5, 55.5, 55.8, 55.96, 55.91, 55.98, 56.1, 104.3, 105.0, 109.9, 111.1, 111.4, 112.3, 113.8, 118.3, 119.0, 120.8, 122.4, 123.8, 124.2, 124.6, 127.8, 130.1, 131.3, 132.1, 146.7, 147.90, 147.91, 148.3, 148.7, 149.0; FT-IR (CH2Cl2, cm−1) 1027, 2832 2938; ESI-HRMS calcd for C33H35NNaO6 [M + Na]+ 564.2357, found 564.2362. To a solution of N-alkylated benzo[e]indole 10 (50 mg, 0.09 mmol) dissolved in 1,4-dioxane (0.18 mL) were added DDQ (74 mg, 0.32 mmol), HOAc (1 drop) and H2O (1 drop) at room temperature. The reaction mixture was stirred for 5 min, quenched by adding brine and extracted with DCM. The combined organic solution was washed with brine, dried over anhydrous MgSO4 and filtered. The filtrate was concentrated under reduced pressure and purified by column chromatography (hexane/EtOAc = 1/1) to give aldehyde 2 as a brown solid (39 mg, 77%). 1H NMR (400 MHz, CDCl3) δ 3.15 (t, 2H, J = 6.7 Hz), 3.58 (s, 3H), 3.64 (s, 3H), 3.84 (s, 3H), 3.87 (s, 3H), 3.95 (s, 3H), 4.06 (s, 3H), 4.52 (t, 2H, J = 6.7 Hz), 6.35 (d, 1H, J = 1.8 Hz), 6.66 (dd, 1H, J = 1.9 Hz, 8.1 Hz), 6.78 (d, 1H, J = 8.2 Hz), 6.97–7.05 (m, 4H), 7.52 (s, 1H), 7.78 (s, 1H), 8.94 (s, 1H), 10.23 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 36.9, 48.9, 55.4, 55.8, 56.0, 56.0, 56.1, 56.2, 103.9, 105.9, 111.2, 111.6, 112.2, 113.7, 119.2, 120.9, 122.1, 122.1, 124.4, 125.1, 125.7, 128.6, 130.7, 131.1, 148.2, 148.4, 148.86, 148.91, 149.1, 193.3; FT-IR (CH2Cl2, cm−1) 1218, 1664, 2360; ESI-HRMS calcd for C33H33NNaO7 [M + Na]+ 578.2149, found 578.2155. To a flask charged with indole aldehyde 2 (428 mg, 0.77 mmol) and AcOH (4.3 mL) was added 30% H2O2 (1.4 mL) at room temperature. After 5 h, reaction was quenched with H2O (4.5 mL) and extracted with DCM a few times. The combined organic solution was washed with brine, dried over anhydrous MgSO4 and filtered. The filtrate was concentrated under reduced pressure and purified by column chromatography (hexane/EtOAc = 1/1) to give 11 as red amorphous solid (159 mg, 37%). 1H NMR (400 MHz, CDCl3) δ 2.92 (t, 2H, J = 7.6 Hz), 3.55 (s, 3H), 3.85 (s, 3H), 3.86 (s, 3H), 3.87 (t, 2H, J = 1.36 Hz), 3.89 (s, 3H), 3.95 (s, 3H), 3.97 (s, 3H), 5.97 (s 1H), 6.75 (s, 1H), 6.808 (s, 1H), 6.814 (s, 1H), 7.00 (d, 1H, J = 8.4 Hz), 7.05 (d, 1H, J = 2.0 Hz), 7.17 (dd, 1H, J = 8.4 Hz, 2.0 Hz), 7.30 (s, 1H), 7.62 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 34.1, 41.7, 56.0, 56.06, 56.07, 56.2, 56.3, 104.3, 108.0, 109.3, 111.3, 111.6, 112.1, 112.8, 121.0, 122.8, 123.2, 123.9, 125.1, 130.3, 130.8, 132.5, 148.1, 149.2, 149.3, 150.5, 150.8, 152.1, 152.4, 170.1, 183.9; FT-IR (CH2Cl2, cm−1) 1143, 1633, 1712; ESI-HRMS calcd for C32H31NNaO8 [M + Na]+ 580.1942, found 580.1947. To a solution of benzo[e]indole-dione 11 (30 mg, 0.06 mmol), DMSO (5 μL, 0.07 mmol) in DCM (2 mL) was added hydrobromic acid (48%, 7 μL, 0.07 mmol) under air. The resulting solution was heated under reflux at 60°C for 5 h and cooled down to room temperature. Reaction mixture was washed with brine, dried over anhydrous MgSO4, filtered, concentrated under reduced pressure and purified by column chromatography (DCM/EtOAc = 20/1) to give bromide 12 (18 mg, 51%). 1H NMR (400 MHz, CDCl3) δ 2.98 (t, 2H, J = 7.8 Hz), 3.50 (s, 3H), 3.83 (s, 3H), 3.85 (s, 3H), 3.89 (s, 3H), 3.94 (s, 3H), 3.97 (s, 3H), 4.42 (t, 2H, J = 7.8 Hz), 6.789 (s, 1H), 6.79 (s, 1H), 6.82 (s, 1H), 6.84 (dd, 1H, J = 1.8 Hz, 8.1 Hz), 6.97 (d, 1H, J = 1.9 Hz), 7.00 (d, 1H, J = 8.3 Hz), 7.10 (dd, 1H, J = 8.3, 1.9 Hz), 7.20 (s, 1H), 7.64 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 34.0, 42.8, 55.9, 55.92, 56.0, 56.2, 56.4, 103.9, 108.0, 110.0, 111.5, 111.5, 112.3, 112.6, 121.2, 122.5, 123.0, 123.5, 124.1, 130.2, 130.4, 134.7, 148.0, 149.0, 149.4, 149.5, 150.6, 150.9, 152.4, 170.6, 177.8; FT-IR (CH2Cl2, cm−1) 1073, 1588, 1713; ESI-HRMS calcd for C32H30BrNNaO8 [M + Na]+ 658.1047, found 658.1052. To a sealed tube bromide 12 (36 mg, 0.2 mmol), (PPh3)4Pd (7.7 mg, 0.007 mmol), K3PO4 (169 mg, 0.8 mmol), and 1,4-dioxane (1.3 mL) were added under Ar atmosphere. The sealed tube was capped and heated at 100°C overnight. The reaction was then cooled to room temperature and filtered through a pad of silica gel with the aid of EtOAc. The reaction mixture was concentrated under reduced pressure and purified by flash column chromatography (100% DCM gives DCM/MeOH = 100/1) to give permethyl ningalin C (1, 40 mg, 86%). 1H NMR (400 MHz, CDCl3) δ 2.54 (s, 2H, J = 8.0 Hz), 3.48–3.64 (m, 2H), 3.56 (s, 3H), 3.78 (s, 3H), 3.82 (s, 3H), 3.93 (s, 3H), 3.94 (s, 3H), 3.96 (s, 3H), 3.970 (s, 3H) 3.972 (s, 3H), 6.31 (d, 1H, J = 1.6 Hz), 6.42 (dd, 1H, J = 8.0, 2.0 Hz), 6.70 (d, 1H, J = 8.4 Hz), 6.99 (s, 1H), 7.00 (s, 1H), 7.04 (d, 1H, J = 8.0 Hz), 7.07 (d, 1H, J = 2.0 Hz), 7.19 (dd, 1H, J = 8.4, 2.0 Hz), 7.64 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 34.9, 43.2, 55.7, 55.8, 55.9, 55.9, 56.0, 56.1, 107.8, 109.4, 110.9, 111.2, 111.3, 111.7, 112.7, 114.2, 119.0, 120.8, 122.9, 123.1, 123.7, 123.8, 124.5, 125.0, 130.3, 130.5, 134.3, 147.2, 147.6, 148.6, 148.8, 149.3, 149.5, 150.2, 150.7, 152.1, 170.9, 183.7. To a flask charged with permethyl ningalin C (1, 24 mg, 0.035 mmol), anhydrous CH2Cl2 (3.7 mL) was added and the solution was cooled to −78°C. To the solution, BBr3 dissolved in CH2Cl2 (1.0 M, 0.46 mL, 0.46 mmol) was added and the resulting mixture was warmed to room temperature and stirred overnight. The solution was then cooled with an ice bath before adding water (2.5 mL) and the mixture was stirred for 30 min at room temperature. The organic phase was separated, and the remaining aqueous phase was saturated with NaCl (s) and extracted with ethyl acetate (2 × 5 mL). The combined organic solution was washed with brine and dried over anhydrous Na2SO4. Removal of the solvent in vacuo gave a red solid, which was purified by preparative thin layered chromatography (5% methanol in EtOAc) to give ningalin C as an amorphous red solid (14 mg, 70% yield). 1H NMR (400 MHz, CDCl3) δ 2.38 (t, 2H, J = 7.8 Hz), 3.35–3.60 (m, 2H), 6.16 (dd, 1H, J = 2.0 Hz, 8.1 Hz), 6.38 (d, 1H, J = 2.0 Hz), 6.56 (d, 1H, J = 8.0 Hz), 6.68 (dd, 1H, J = 8.0, 2.0 Hz), 6.81–6.93 (m, 5H), 7.20 (s, 1H), 7.41 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 35.4, 44.2, 113.7, 114.6, 116.2, 116.3, 116.9, 117.1, 117.7, 119.4, 120.9, 121.3, 123.0, 123.5, 124.0, 124.6, 124.7, 125.3, 131.0, 131.5, 135.5, 144.7, 146.0, 146.1, 146.8, 147.1, 148.1, 148.9, 149.4, 151.8, 173.1, 185.8. This work was supported by the grants from the National Research Foundation of Korea (2012M3A7B4049657). Additional supporting information is available in the online version of this article. 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