HETEROCYCLES

Paper | Regular issue | Vol. 87, No. 2, 2013, pp. 357-367
Received, 19th November, 2012, Accepted, 6th December, 2012, Published online, 17th December, 2012.
DOI: 10.3987/COM-12-12630

■ First Total Syntheses of 1,3-Disubstituted β-Carboline Alkaloids, Dichotomide I and Marinacarbolines A-D

Shinji Tagawa, Tominari Choshi,* Asuka Okamoto, Takashi Nishiyama, Shiroh Watanabe, Noriyuki Hatae, and Satoshi Hibino*

Faculty of Pharmacy and Pharmaceutical Sciences, Fukuyama University, Fukuyama, Hiroshima 729-0292, Japan

Abstract

The first total syntheses of dichotomide I (1), and marinacarbolines A-D (3-6) were achieved in four steps from methyl 1-chloro-β-carboline-3-carboxlyate (9), which was previously used as a synthetic intermediate of dichotomine C. The required compound 9 was prepared in a six-step sequence including a microwave-assisted thermal electrocyclic reaction of a 1-azahexatriene system.

INTRODUCTION
1,3-Disubstituted β-carboline alkaloids dichotomide I (1), II (2) and dichotomines A-D were isolated from Stellaria dichotoma by Yoshikawa and co-workers in 2004 (Figure 1).1 Dichotomines A-D (3-6) have antiallergic effects by inhibiting the release of β-hexosaminidase in RBL-2H3 cells. Whether dichotomides I (1) and II (2) have similar biological activities, however, is unclear.1 We previously reported the first total synthesis of dichotomine C by construction of a β-carboline framework based on a microwave-assisted thermal electrocyclic reaction of a 1-azahexatriene system involving the indole 2,3-bond, followed by enantioselective 1,2-dihydroxylation.2 Nemet and co-workers reported the synthesis of racemic dichotomine A from L-tryptophan with methylglyoxal under acidic conditions.3 Shi and co-workers recently reported the asymmetric syntheses of dichotomines A-D were achieved by using L-tryptophan methyl ester and 2,3-O-isopropylidene-D-glyceraldehyde.4
Closely related β-carboline alkaloids, marinacarbolines A-D (3-6) were recently isolated from the fermentation broth of the actinomycete Marinactinospora thermotolerans SCSIO 00652, belonging to the family Nocardiopasaceae, that was newly discovered by Ju and co-workers in 2011 (Figure 1).5 The compounds were elucidated by NMR spectral analyses and other spectroscopic experiments. These 1,3-disubstituted β-carboline alkaloids, having an acetyl group at the C-1 position and a carbamoyl group at the C-3 position, exhibit antiplasmodial activities against Plasmodium falciparum lines 3D7 and Dd2. Synthetic studies of these compounds have not yet been reported.

We have focused our efforts on developing the synthesis of bioactive nitrogen containing fused-heteroaromatic compounds including natural products based on a thermal electrocyclic reaction of either a 6π- or an aza 6π-electron system involving an aromatic or heteroaromatic double bond in principle.6 Recently, we reported the total synthesis of furoisoquinoline,7 phenanthridine,8 azaanthraquinone,9 benzo[c]phenanthridine,10 and indoloquinoline alkaloids11 based on a microwave (MW)-assisted electrocyclic reaction of the aza 6π-electron system. Herein, we describe the details of the first total syntheses of dichotomide I (1) and marinacarbolines A-D (3-6). In the retrosynthetic analysis shown in Scheme 1, we planned to derive β-carboline alkaloids 8 from methyl 1-chloro-β-carboline-3-carboxylate (9) for use in the asymmetric synthesis of dichotomine C.2

RESULTS AND DISCUSSION
The required β-carboline 9 was prepared in a six-step sequence starting from N-MOM-3-iodoindole-2-carbaldehyde 1212 (Scheme 2). The Heck reaction between 12 and methyl acrylate in the presence of Pd(OAc)2 gave the 3-alkenylindole 13 (97%). Subsequent treatment of 13 with hydroxylamine produced the oxime 11 (90%), which was subjected to the microwave-assisted thermal electrocylic reaction in 1,2-dichlorobenzene to yield the β-carboline 10 (98%). Oxidation of 10 with m-chloroperbenzoic acid (mCPBA) followed by treatment with oxalyl chloride13 and DMF in CH2Cl2 yielded the 1-chloro-β-carboline 15 (89% from 10), which was heated with trifluoromethanesulfonic acid, trimethyl orthoformate, and MeOH in nitromethane to produce the desired methyl 1-chloro-β-carboline-3-carboxylate (9) (99%).14 The yield of 9 from 12 significantly increased to 75% from the previously reported 45%.2
As shown in Scheme 3, we initially attempted the synthesis of dichotomide I (1). Hydrolysis of the β-carboline 9 with aqueous 3% NaOH in MeOH afforded the β-carboline-3-carboxylic acid 16 in excellent yield (99%). The carboxylic acid 16 was treated with β-alanine methyl ester in the presence of DCC and DMAP to give the amide 17 (68%). Subsequently, the Stille reaction of the amide 17 and 1-ethoxyvinyltin in the presence of PdCl2(PPh3)2 provided dichotomide I (1) in low yield (34%) without the isolation of 1-(1-ethoxyvinyl)-β-carboline.

Therefore, we next introduced the acetyl group at the C-1 position of β-carboline at first (Scheme 4). The Stille reaction of the β-carboline 9 and 1-ethoxyvinyltin in the presence of a Pd-catalyst, followed by treatment with 2 M HCl in MeOH afforded the 1-acetyl-β-carboline 19 (99% from 9). Hydrolysis of 1-acetyl-β-carboline 19 with aqueous 3% NaOH in MeOH afforded the carboxylic acid 20 in 84% yield. Subsequent treatment of 20 with β-alanine methyl ester in the presence of dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) gave dichotomide I (1) in excellent yield (91%). In the case of marinacarbolines, the carboxylic acid 20 was treated with amines (three type of phenylethylamines and tryptamine) in the presence of diethylphosphorylcyanide (DEPC) and triethylamine to provide marinacarbolines A (3) (98%), B (4) (97%), C (5) (99%), and D (6) (97%). Physical and spectroscopic data of our synthetic dichotomide I (1) and marinacarbolines A-D (3-6) were identical with those of previously reported data.1,5

CONCLUSION
In conclusion, the first total syntheses of dichotomide I (1) and marinacarbolines A-D (3-6), having an acetyl group at the C-1 position and a carbamoyl group at the C-3 position of the β-carboline nucleus, were achieved from methyl 1-chloro-β-carboline-3-carboxylate (9) in four steps. Dichotomide I (1) and marinacarbolines A-D (3-6) were obtained in an overall yield of 57%, 61%, 61%, 62%, and 61%, respectively. Further studies in this series are now in progress.

EXPERIMENTAL
All non-aqueous reactions were carried out under an atmosphere of nitrogen in dried glassware unless otherwise noted. Solvents were dried and distilled according to standard protocols. Analytical thin-layer chromatography was performed with Silica gel 60PF254 (Merck). Silica gel column chromatography was performed with Silica gel 60N (63-210 µm, KANTO CHEMICAL Co. Ltd.). All melting points were determined on Yanagimoto micro melting point apparatus and are uncorrected. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a JEOL AL-300 at 300 MHz. Chemical shifts are reported relative to Me4Si (δ 0.00). NMR spectra was measured with CDCl3 unless otherwise noted. Multiplicity is indicated by one or more of the following: s (singlet); d (doublet); t (triplet); q (quartet); m (multiplet); br (broad). Carbon nuclear magnetic resonance (13C NMR) spectra were recorded on a JEOL AL-300 at 75 MHz. Chemical shifts are reported relative to CDCl3 (δ 77.0) and DMSO-d6 (δ 39.7). Infrared spectra were recorded with ATR method using a Shimadzu FTIR-8000 spectrophotometer and technologies DuraScop. Low and high resolution mass spectra were recorded on JEOL JMS-700 spectrometers by direct inlet system.
Methyl 3-(N-methoxymethyl-2-formylindol-3-yl)acrylate (13)
A mixture of the 3-iodoindole 1212 (6.6 g, 20.9 mmol), methyl acrylate (3.78 mL, 41.9 mmol), Et3N (5.78 mL, 41.9 mmol), PPh3 (164 mg, 0.63 mmol), and Pd(OAc)2 (13.5 mg, 0.42 mmol) in DMF (100 mL) was heated at 100 oC for 2 h under an argon atmosphere. After being cooled to rt, the reaction mixture was quenched with water, and then the mixture was extracted with EtOAc. The EtOAc layer was washed with water and brine, dried over Na2SO4, and concentrated in vacuo. The residue was purified by column chromatography (silica gel, 40 g) using EtOAc-hexane (1:4 v/v) as an eluent to give the acrylate 13 (5.6 g, 97%), mp 104-106 oC (EtOAc). IR (ATR) ν: 1710, 1644 cm-1. 1H NMR (300 MHz CDCl3) δ: 3.32 (3H, s), 3.86 (3H, s), 6.01 (2H, s), 6.69 (1H, d, J=16.0 Hz), 7.35 (1H, t, J=8.1 Hz), 7.52 (1H, t, J=8.1 Hz), 7.62 (1H, d, J=8.1 Hz), 8.01 (1H, d, J=8.1 Hz), 8.32 (1H, d, J=16.0 Hz), 10.38 (1H, s). 13C NMR (75 MHz CDCl3) δ: 51.9, 56.1, 74.9, 111.9, 121.3, 122.5, 123.3, 124.3, 124.8, 128.1, 132.5, 134.0, 139.9, 167.2, 181.4. MS (EI) m/z: 273 (M+); HRMS (EI) Calcd for C15H15NO4: 273.1001. Found: 273.1016.
Methyl 3-(N-methoxymethyl-2-hydroxyiminoindol-3-yl)acrylate (11)
A mixture of the acrylate 13 (3.7 g, 13.5 mmol), NH2OH・HCl (1.88 g, 27.1 mmol), and AcONa (2.27 g, 27.1 mmol) in EtOH (100 mL) was heated at 80 oC for 2 h. After removal of solvent followed by addition of water, the mixture was extracted with EtOAc. The EtOAc layer was washed with water and brine, dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography (silica gel, 40 g) using EtOAc-hexane (1:4 v/v) as an eluent to give the oxime 11 (3.5 g, 90%), mp 182-184 oC (EtOAc). IR (ATR) ν: 3349, 1681 cm-1. 1H NMR (300 MHz CDCl3) δ: 3.30 (3H, s), 3.84 (3H, s), 5.82 (2H, s), 6.60 (1H, d, J=15.8 Hz), 7.30 (1H, d, J=8.1 Hz), 7.39 (1H, t, J=8.1 Hz), 7.54 (1H, d, J=8.1 Hz), 7.95 (1H, d, J=8.1 Hz), 8.13 (1H, d, J=15.8 Hz), 8.16 (1H, br s), 8.61 (1H, s). 13C NMR (75 MHz CDCl3) δ: 51.7, 56.1, 75.2, 110.9, 116.1, 117.0, 121.2, 122.5, 125.3, 125.6, 132.0, 136.2, 139.3, 141.8, 168.2. MS (EI) m/z: 288 (M+); HRMS (EI) Calcd for C15H16N2O4: 288.1110. Found: 288.1117.
Methyl N-(methoxymethyl)pyrido[3,4-b]indole-3-carboxylate (10)
A solution of the oxime 11 (100 mg, 0.35 mmol) in 1,2-dichlorobenzene (8 mL) was stirred at 180 oC for 1.5 h under N2 atmosphere under microwave irradiation. After removal of solvent, the residue was purified by column chromatography (silica gel) using EtOAc-hexane (3:7 v/v) as an eluent to give the β-carboline 10 (92 mg, 98%), mp 148-149 oC (EtOAc). IR (ATR) ν: 1704 cm-1. 1H NMR (300 MHz CDCl3) δ: 3.33 (3H, s), 4.08 (3H, s), 5.81 (2H, s), 7.38-7.46 (1H, m), 7.66-7.69 (2H, m), 8.22 (1H, d, J=7.7 Hz), 8.91 (1H, s), 9.08 (1H, s). 13C NMR (75 MHz CDCl3) δ: 52.8, 56.6, 74.6, 110.5, 117.8, 121.7, 121.8, 122.1, 129.3, 129.4, 132.2, 138.2, 138.6, 141.7, 166.6. MS (EI) m/z: 270 (M+); HRMS (EI) Calcd for C15H14N2O3: 270.1004. Found: 270.0994.
Methyl 1-chloro-N-(methoxymethyl)pyrido[3,4-b]indole-3-carboxylate (15)
mCPBA (1.92 g, 11.1 mmol) was added to a solution of the β-carboline 10 (1.0 g, 3.70 mmol) in CH2Cl2 (100 mL) under cooling with ice under N2 atmosphere. After being stirred at rt for 5 h, the reaction mixture was quenched with water, and then the mixture was extracted with CH2Cl2. The CH2Cl2 layer was washed with aqueous NaHCO3 (saturated) solution, water and brine, dried over Na2SO4, and concentrated in vacuo to give the N-oxide 14. The N-oxide 14 was used without further purification. Oxalyl chloride (0.97 mL, 11.1 mmol) was added dropwise to a solution of the N-oxide 14 in CH2Cl2 (100 mL) under cooling with ice, followed by slow addition of DMF (2 mL, 25.9 mmol). After being stirred at rt for 1 h, the reaction mixture was quenched with water, and then the mixture was extracted with CH2Cl2. The CH2Cl2 layer was washed with water and brine, dried over Na2SO4, and concentrated in vacuo. The residue was purified by column chromatography (silica gel, 40 g) using EtOAc (1:4 v/v) as an eluent to give the N-MOM-1-chloro-β-carboline 15 (1.0 g, 89%), mp 154-155 oC (EtOAc). IR (ATR) ν: 1714 cm-1. 1H NMR (300 MHz CDCl3) δ: 3.38 (3H, s), 4.06 (3H, s), 6.19 (2H, s), 7.40-7.48 (1H, m), 7.66-7.74 (2H, m), 8.18 (1H, d, J=7.0 Hz), 8.84 (1H, s). 13C NMR (75 MHz CDCl3) δ: 52.9, 55.9, 74.3, 111.3, 117.0, 121.5, 121.7, 122.3, 129.8, 129.8, 132.7, 134.0, 137.7, 142.5, 165.3. MS (EI) m/z: 304 (M+), 306 (M++2); HRMS (EI) Calcd for C15H13ClN2O3: 304.0615. Found: 304.0625.
Methyl 1-chloropyrido[3,4-b]indole-3-carboxylate (9)
CF3SO3H (5.4 mL, 0.06 mmol) was added to a solution of the N-MOM-1-chloro-β-carboline 15 (3.7 g, 12.1 mmol), MeOH (19 mL, 150 mmol) and CH(OMe)3 (15.9 mL, 150 mmol) in MeNO2 (100 mL) under cooling with ice, and then the mixture was heated at 100 oC for 2 h. After cooling to rt, the reaction mixture was quenched with aqueous Na2CO3 (saturated) solution, and then the mixture was extracted with EtOAc. The EtOAc layer was washed with brine, dried over Na2SO4 and concentrated in vacuo. The residue was purified by column chromatography (silica gel, 40 g) using EtOAc-hexane (3:7 v/v) as an eluent to give the 1-chloro-β-carboline 9 (3.2 g, 99%), mp 238-240 oC (EtOAc). IR (ATR) ν: 1724 cm-1. 1H NMR (300 MHz CDCl3) δ: 4.06 (3H, s), 7.37-7.45 (1H, m), 7.60-7.72 (2H, m), 8.19 (1H, d, J=8.1 Hz), 8.69 (1H, br s), 8.85 (1H, s). 13C NMR (75 MHz CDCl3) δ: 52.90, 112.2, 117.5, 121.8, 121.8, 122.3, 129.7, 130.5, 133.4, 134.5, 137.6, 140.2, 165.6. MS (EI) m/z: 260 (M+), 262 (M++2); HRMS (EI) Calcd for C13H9ClN2O2: 260.0353. Found: 260.0339.
1-Chloropyrido[3,4-b]indole-3-carboxylic acid (16)
A mixture of methyl ester 9 (89 mg, 0.34 mmol) and 3% NaOH (10 mL) in MeOH (10 mL) was stirred at rt for 12 h. The mixture was adjusted with AcOH to pH 5, and then the resulting mixture was extracted with CHCl3-MeOH (9:1). The organic layer was washed with brine, dried over Na2SO4 and concentrated in vacuo to give the carboxylic acid 16 (84 mg, 99%), mp 226-228 oC (EtOAc). IR (ATR) ν: 3120, 1689 cm-1. 1H NMR (300 MHz DMSO-d6) δ: 7.37 (1H, t, J=7.9 Hz), 7.66 (1H, t, J=7.9 Hz), 7.70 (1H, d, J=7.9 Hz), 8.42 (1H, d, J=7.9 Hz), 8.94 (1H, s), 12.41 (1H, s). 13C NMR (75 MHz DMSO-d6) δ: 112.8, 117.5, 120.9, 121.4, 122.5, 129.3, 129.9, 132.4, 134.2, 137.0, 141.1, 165.8. MS (EI) m/z: 246 (M+), 248 (M++2); HRMS (EI) Calcd for C12H7ClN2O2: 246.0196. Found: 246.0213.
Methyl 3-[(1-chloropyrido[3,4-b]indole-3-carbonyl)animo]propanoate (17)
A solution of DCC (94 mg, 0.46 mmol), and DMAP (4 mg, 0.03 mmol) in CH2Cl2 (20 mL) was added to a solution of carboxylic acid 20 (100 mg, 0.41 mmol) and methyl β-alanine (71 mg, 0.51 mmol) in CH2Cl2 (20 mL) at -20 oC, and then the mixture was stirred at the same temperature for 30 min. After being gradually raised up to rt, the reaction mixture was stirred at rt for 8 h. The mixture was filtered off through Celite pad and the filtrate was concentrated in vacuo. The residue was purified by column chromatography (silica gel, 20 g) using EtOAc-hexane (1:4 v/v) as an eluent to give the amide 17 (92 mg, 68%), mp 154-155 oC (EtOAc). IR (ATR) ν: 1724 cm-1. 1H NMR (300 MHz DMSO-d6) δ: 2.65 (2H, t, J=6.6 Hz), 3.59 (2H, q, J=6.2 Hz), 3.61 (3H, s), 7.33 (1H, t, J=7.8 Hz), 7.68 (1H, d, J=7.8 Hz), 8.40 (1H, d, J=7.8 Hz), 8.63 (1H, t, J=6.0 Hz), 8.84 (1H, s), 12.33 (1H, br s). 13C NMR (75 MHz DMSO-d6) δ: 33.7, 35.1, 51.4, 112.7, 114.2, 120.8, 121.4, 122.6, 129.3, 130.4, 131.4, 133.9, 139.4, 141.2, 163.6, 172.0. MS (EI) m/z: 331 (M+), 333 (M++2); HRMS (EI) Calcd for C16H14ClN3O3: 331.0724. Found: 331.0724.
Dichotomide I (1)
A solution of tributyl(1-ethoxyvinyl)tin (0.19 mL, 0.55 mmol) was added to a mixture of the amide 17 (92 mg, 0.28 mmol), PdCl2(PPh3)2 (3 mg, 4.7 mmol), and Et4NCl (50 mg, 0.31 mmol) in DMF (10 mL) at rt under an argon atmosphere. The stirred mixture was heated at 100 oC for 2 h, which was cooled to rt. After being quenched with an aqueous solution of 30% KF (10 mL), and then the mixture was stirred at rt for 30 min. The mixture was filtered off through Celite pad and the filtrate was extracted with EtOAc. The EtOAc layer was washed with water and brine, dried over Na2SO4, and concentrated in vacuo. The residue was purified by column chromatography (silica gel, 10 g) using EtOAc-hexane (3:7 v/v) as an eluent to give dichotomide I (1) (32 mg, 34%), mp 199-201 oC (EtOAc). IR (ATR) ν: 3376, 1727 cm-1. 1H NMR (300 MHz CDCl3) δ: 2.74 (2H, t, J=7.0 Hz), 2.94 (3H, s), 3.75 (3H, s), 3.86 (2H, q, J=7.0 Hz), 7.38-7.42 (1H, m), 7.61-7.64 (2H, m), 8.23 (1H, d, J=8.0 Hz), 8.63 (1H, br s), 9.10 (1H, s), 10.41 (1H, br s). 13C NMR (75 MHz CDCl3) δ: 25.6, 34.2, 34.9, 51.9, 112.2, 118.4, 121.1, 121.6, 122.3, 129.8, 132.7, 133.7, 136.4, 139.2, 141.6, 164.7, 173.0, 202.4. MS (EI) m/z: 339 (M+); HRMS (EI) Calcd for C18H17N3O4: 339.1219. Found: 339.1216.
Methyl 1-acetylpyrido[3,4-b]indole-3-carboxylate (19)
A solution of tributyl(1-ethoxyvinyl)tin (0.16 mL, 0.48 mmol) was added to a mixture of the 1-chloro-β-carboline 9 (100 mg, 0.38 mmol), PdCl2(PPh3)2 (4 mg, 6.3 mmol), and Et4NCl (70 mg, 0.42 mmol) in DMF (5 mL) at rt under an argon atmosphere. The stirred mixture was heated at 100 oC for 3 h, which was cooled to rt. After being quenched with aqueous solution of 30% KF (10 mL), and then the mixture was stirred at rt for 2 h. The mixture was filtered off through Celite pad and the filtrate was extracted with EtOAc. The EtOAc layer was washed with water and brine, dried over Na2SO4, and concentrated in vacuo. 2 M HCl (2 mL) and MeOH (4 mL) was added to the residue without purification, and then the resulting mixture was stirred at rt for 1 h. The mixture was extracted with EtOAc. The EtOAc layer was washed with brine, dried over Na2SO4 and concentrated in vacuo. The residue was purified by column chromatography (silica gel, g) using EtOAc-hexane (1:9 v/v) as an eluent to give the 1-acetyl-β-carboline 19 (102 mg, 99%), mp 226-227 oC (EtOAc) (lit.,15 mp 234-236 oC). IR (ATR) ν: 1708, 1670 cm-1. 1H NMR (300 MHz CDCl3) δ: 2.97 (3H, s), 4.09 (3H, s), 7.37-7.43 (1H, m), 7.64-7.66 (2H, m), 8.22 (1H, J=8.0 Hz), 9.04 (1H, s), 10.49 (1H, br s). 13C NMR (75 MHz CDCl3) δ: 25.7, 52.8, 112.4, 121.0, 121.3, 121.7, 122.1, 129.8, 131.9, 135.3, 136.5, 136.9, 141.4, 166.1, 203.2. MS (EI) m/z: 268 (M+); HRMS (EI) Calcd for C15H12N2O3: 268.0848. Found: 268.0877.
1-Acetylpyrido[3,4-b]indole-3-carboxylic acid (20)
A mixture of 1-acetyl-β-carboline 19 (100 mg, 0.37 mmol) and 3% NaOH (12 mL) in MeOH (12 mL) was heated at rt for 12 h. The mixture was adjusted with AcOH to pH 5, and then the resulting mixture was extracted with CHCl3-MeOH (9:1). The organic layer was washed with brine, dried over Na2SO4 and concentrated in vacuo to give the carboxylic acid 20 (84 mg, 84%), mp 265-266 oC (EtOAc). IR (ATR) ν: 3325, 1701, 1670 cm-1. 1H NMR (300 MHz DMSO-d6) δ: 2.91 (3H, s), 7.40 (1H, t, J=8.0 Hz), 7.68 (1H, t, J=8.0 Hz), 7.90 (1H, d, J=8.0 Hz), 8.49 (1H, d, J=8.0 Hz), 9.20 (1H, s), 12.3 (1H, s). 13C NMR (75 MHz DMSO-d6) δ: 25.8, 113.4, 120.2, 121.0, 121.0, 122.2, 129.3, 131.5, 135.0, 135.1, 136.3, 142.3, 166.3, 201.1. MS (EI) m/z: 254 (M+); HRMS (EI) Calcd for C14H10N2O3: 254.0691. Found: 254.0720.
Dichotomide I (1)
A solution of DCC (47 mg, 0.22 mmol), and DMAP (2 mg, 0.015 mmol) in CH2Cl2 (10 mL) was added to a solution of carboxylic acid 20 (50 mg, 0.20 mmol) and methyl β-alanine (36 mg, 0.25 mmol) at -20 oC, and then the mixture was stirred at same temperature for 30 min. After being gradually raised up to rt, the reaction mixture was stirred at rt for 8 h. The mixture was filtered off through Celite pad and the filtrate was concentrated in vacuo. The residue was purified by column chromatography (silica gel, 20 g) using EtOAc-hexane (1:3 v/v) as an eluent to give dichotomide I (1) (61 mg, 91%).
Marinacarboline A (3)
DEPC (14 mL, 0.088 mmol) and Et3N (23 mL, 0.16 mmol) were added to a solution of carboxylic acid 20 (20 mg, 0.08 mmol) and 2-(4-methoxyphenyl)ethylamine (15 mg, 0.10 mmol) in DMF (5 mL) at -10 oC, and then the mixture was stirred at the same temperature for 30 min. After being gradually raised up to rt, the mixture was stirred at rt for 12 h. The reaction mixture was quenched with water, and then the mixture was extracted with EtOAc. The EtOAc layer was washed with brine, dried over Na2SO4 and concentrated in vacuo. The residue was purified by column chromatography (silica gel, 20 g) using EtOAc-hexane (1:3 v/v) as an eluent to give the marinacarboline A (3) (30 mg, 98%), mp 177-178 oC (EtOAc). IR (ATR) ν: 3232, 1695, 1654 cm-1. 1H NMR (300 MHz CDCl3) δ: 2.78 (3H, s), 2.96 (2H, t, J=7.0 Hz), 3.81 (3H, s), 3.82 (2H, q, J=7.0 Hz), 6.90 (2H, d, J=8.5 Hz), 7.23-7.26 (2H, m), 7.37-7.42 (1H, m), 7.58-7.64 (2H, m), 8.06 (1H, br s), 8.23 (1H, d, J=8.0 Hz), 9.10 (1H, s), 10.37 (1H, br s). 13C NMR (75 MHz CDCl3) δ: 25.7, 35.0, 40.7, 55.3, 112.2, 114.2(x2), 118.4, 121.1, 121.6, 122.3, 129.7, 129.9(x2), 131.0, 132.7, 133.5, 136.3, 139.3, 141.5, 158.4, 164.5, 202.3. MS (EI) m/z: 387 (M+); HRMS (EI) Calcd for C23H21N3O3: 387.1583. Found: 387.1594.
Marinacarboline B (4)
The same procedure as above was carried out using the carboxylic acid 20 (30 mg, 0.12 mmol) and 2-(4hydroxyphenyl)ethylamine (20 mg, 0.15 mmol) to give the marinacarboline B (4) (43 mg, 97%), mp 271-273 oC (EtOAc). IR (ATR) ν: 3332, 1693, 1643 cm-1. 1H NMR (300 MHz DMSO-d6) δ: 2.82 (2H, t, J=7.0 Hz), 2.87 (3H, s), 3.59 (2H, q, J=7.0 Hz), 6.72 (2H, d, J=8.0 Hz), 7.12 (2H, d, J=8.0 Hz), 7.34 (1H, t, J=8.0 Hz), 7.63 (1H, t, J=8.0 Hz), 7.84 (1H, d, J=8.0 Hz), 8.45 (1H, d, J=8.0 Hz), 8.68 (1H, t, J=6.0 Hz), 9.06 (1H, s), 9.23 (1H, br s), 12.16 (1H, br s). 13C NMR (75 MHz DMSO-d6) δ: 26.0, 34.5, 40.8, 113.3, 115.3(x2), 117.8, 120.3, 120.8, 122.3, 129.3, 129.5, 129.6, 132.0, 1339.9, 134.8, 138.7, 142.4, 155.7, 164.0, 201.0. MS (EI) m/z: 373 (M+); HRMS (EI) Calcd for C22H19N3O3: 373.1426. Found: 373.1433.
Marinacarboline C (5)
The same procedure as above was carried out using the carboxylic acid 20 (20 mg, 0.08 mmol) and phenylethylamine (13 mL, 0.088 mmol) to give the marinacarboline C (5) (28 mg, 99%), mp 195-196 oC (EtOAc). IR (ATR) ν: 3239, 1695, 1658 cm-1. 1H NMR (300 MHz CDCl3) δ: 2.76 (3H, s), 3.02 (2H, t, J=7.0 Hz), 3.86 (2H, q, J=7.0 Hz), 7.34-7.42 (6H, m), 7.59-7.67 (2H, m), 8.08 (1H, br s), 8.23 (1H, d, J=8.0 Hz), 9.10 (1H, s), 10.38 (1H, br s). 13C NMR (75 MHz CDCl3) δ: 25.7, 35.9, 40.5, 112.2, 118.4, 121.1, 121.6, 122.3, 126.6, 128.8(x2), 129.0(x2), 129.7, 132.6, 133.5, 136.3, 139.1, 139.3, 141.5, 164.5, 202.3. MS (EI) m/z: 357 (M+); HRMS (EI) Calcd for C22H19N3O2: 357.1477. Found: 357.1455.
Marinacarboline D (6)
The same procedure as above was carried out using the carboxylic acid 20 (20 mg, 0.08 mmol) and the tryptamine (14 mg, 0.088 mmol) to give the marinacarboline D (6) (30 mg, 97%), mp 244-245 oC (EtOAc). IR (ATR) ν: 3421, 3343, 1670, 1650 cm-1. 1H NMR (300 MHz CDCl3) δ: 2.63 (3H, s), 3.19 (2H, t, J=7.0 Hz), 3.94 (2H, q, J=7.0 Hz), 7.12 (1H, t, J=8.0 Hz), 7.18 (1H, d, J=2.5 Hz), 7.22 (1H, t, J=8.0 Hz), 7.36-7.42 (2H, m), 7.58-7.63 (2H, m), 7.68 (1H, d, J=8.0 Hz), 8.08 (1h, br s), 8.15 (1H, t, J=6.0 Hz), 8.23 (1H, d, J=8.0 Hz), 9.11 (1H, s). 13C NMR (75 MHz CDCl3) δ: 25.5(x2), 39.8, 111.2, 112.2(x2), 113.4, 118.4, 118.9, 119.6, 121.1, 121.5, 122.1, 122.3(x2), 127.5, 129.7, 132.6, 136.3, 136.5, 139.4, 141.5, 164.6, 202.3. MS (EI) m/z: 396 (M+); HRMS (EI) Calcd for C24H20N4O2: 396.1586. Found: 396.1565.

ACKNOWLEDGEMENTS
This work was partly supported by JSPS KAKENHI Grant Number 24590040.

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