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Open AccessArticle

Palladium-Catalyzed Synthesis of Natural and Unnatural 2-, 5-, and 7-Oxygenated Carbazole Alkaloids from N-Arylcyclohexane Enaminones

Departamento de Química Orgánica, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional. Prol. Carpio y Plan de Ayala, México D.F. 11340, Mexico
*
Author to whom correspondence should be addressed.
Molecules 2013, 18(9), 10334-10351; https://doi.org/10.3390/molecules180910334
Received: 5 July 2013 / Revised: 5 August 2013 / Accepted: 12 August 2013 / Published: 26 August 2013
(This article belongs to the Special Issue Palladium Catalysts)

Abstract

A palladium-catalyzed synthesis of the carbazole framework is described, including the preparation of 2-, 5-, and 7-oxygenated natural and unnatural carbazole alkaloids. A series of N-arylcyclohexane enaminones, generated by condensation of cyclohexane-1,3-dione with diverse anilines, were aromatized by a Pd(0)-catalyzed thermal treatment to afford the corresponding diarylamines. The latter were submitted to a Pd(II)-catalyzed cyclization and methylation processes to provide the desired carbazoles, including clausine V. Following an inverse strategy, a new and short total synthesis of glycoborine is also reported.
Keywords: 2-oxygenated carbazoles; enaminones; palladium(II) cyclization; clausine V; glycoborine 2-oxygenated carbazoles; enaminones; palladium(II) cyclization; clausine V; glycoborine

1. Introduction

Biologically active carbazole alkaloids, a family of natural products with a variety of molecular structures, are isolated from higher order plants of the genera Clausena, Glycosmis, Micromelum, and Murraya (Rutaceae), among other sources [1,2,3,4,5]. Specifically, a great number of 2-, 5-, 6-, 7-mono- and bis-oxygenated tricyclic carbazoles isolated from these genera [1,2,3] exhibit a broad range of significant biological activities, including compounds with anti-tumor [6,7], antiplatelet aggregative [8], antibiotic [6,9,10], anti-viral [11,12,13], anti-plasmodial [14], anti-convulsant [15], and sigma receptor antagonist [16,17] properties. Carbazole derivatives 1ag are examples of these natural alkaloids [8,18,19,20,21,22,23] (Figure 1).
Figure 1. Examples of naturally occurring 2-, 7-, and 2,7-oxygenated tricyclic carbazoles.
Figure 1. Examples of naturally occurring 2-, 7-, and 2,7-oxygenated tricyclic carbazoles.
Molecules 18 10334 g001
In spite of the large number of 2-, 5-, 6-, 7-mono- and bis-oxygenated tricyclic natural carbazoles that have been isolated, the wide range of functional groups and substitution patterns that exists among these compounds, and their important pharmacological activity, only recently a considerable number of synthetic approaches for their efficient preparation have been published [1,2,3,24,25,26,27,28,29,30,31,32,33,34,35,36].
We recently described a general synthetic approach for the construction of 1-methoxycarbazoles, including the naturally occurring alkaloid glycozolicine, which was accomplished with high overall yields through a three-step reaction sequence [37]. Based on this approach, we describe herein a new synthetic route for the preparation of 2-, 7-, and 2,7-oxygenated carbazoles 1. Starting from cyclohexene-1,3-dione (2) and the respective anilines 3ae, enaminones 4ae were prepared (Scheme 1). The latter were converted into diarylamines 5ae and then cyclized to the desired carbazoles 1, via an efficient Pd-catalyzed aromatization and cyclization sequence of reactions.
Scheme 1. Synthetic approach for the preparation of 2-oxygenated tricyclic carbazoles 1.
Scheme 1. Synthetic approach for the preparation of 2-oxygenated tricyclic carbazoles 1.
Molecules 18 10334 g002

2. Results and Discussion

2.1. Synthesis of Diarylamines

The catalyst-free condensation of cyclohexane-1,3-dione (2) with anilines 3ae provided 3-anilino-2-cyclohexen-1-ones 4ae in high yields (Table 1). However, the use of deactivated anilines, such as 3-nitro- and 4-nitroanilines, failed to provide the desired enaminones, thus limiting this procedure to anilines substituted with electron-donating groups. Applying our previous procedure for aromatization using Pd(OAc)2 (30% mol) [37], derivatives 4ab did not lead to the desired diarylamines 6ab, but instead furnished the carbazole frame compounds 7ab in good yields (Scheme 2). Similar results via Pd-mediated procedures have been reported for analogous substrates [1,38,39,40,41], which in turn have been transformed into the 4-oxygenated carbazoles [42]. When using other catalysts, such as mercuric acetate [43] and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) [44,45], diarylamines 6 were indeed produced, but in very low yields (15%–20%).
Table 1. Scope of the reaction between cyclohexane-1,3-dione (2) and anilines 3ae a. Molecules 18 10334 i001
Table 1. Scope of the reaction between cyclohexane-1,3-dione (2) and anilines 3ae a. Molecules 18 10334 i001
Entry3 (Ar)4 (%) b
13a (C6H4-4-Me)4a (92)
23b (C6H4-4-OMe)4b (95)
33c (C6H4-3-Me)4c (90)
43d (C6H4-3-OMe)4d (93)
53e (C6H3-3,5-(OMe)24e (96)
a Standard conditions: 2 (3.57 mmol), 3 (3.57 mmol), toluene (150 mL), reflux, 12 h.
b Isolated yields.
Scheme 2. Pd(II)-catalyzed treatment of 3-anilino-2-cyclohexen-1-ones 4ab.
Scheme 2. Pd(II)-catalyzed treatment of 3-anilino-2-cyclohexen-1-ones 4ab.
Molecules 18 10334 g003
Due to the fact that the insertion of the aryl and cyclohexenone rings takes place via a Pd(II)-catalyzed pathway [1,2,38,39,40,41,46], we chose a Pd(0)-mediated method for carrying out such an aromatization. Initially, when 4b was treated with Pd/C (5%) at different concentrations (1–6 mol%) with MeOH as the solvent and heating to 50–200 °C in a sealed vessel, diarylamine 6b was not obtained and the starting material was recovered. However, the desired transformation was achieved by increasing both the palladium(0) loading on charcoal (10%) (1.9%–5.7% mol) and the reaction temperature (Table 2, entries 1–3). The use of the Pd(0)-mediated aromatization method for similar substrates or carbazole derivatives in moderate to good yields has been reported [42,47,48,49,50,51,52]. Reagents such as DDQ [44,45,53] and chloranil [54] have also been successfully applied to achieve analogous conversions [43,55].
Although the preparation and purification of diarylamines 6ab and 6de resulted in high yields (Table 2, entries 3–4 and 6–7), the relative instability of these compounds under the conditions of the following cyclization reaction made it necessary to protect the phenol moiety. In order to achieve this protection and taking into account that there are many naturally occurring methoxy-containing oxygenated carbazoles, we decided to obtain the methylated derivatives 5ae. For this purpose, we employed a direct sequential procedure for the dehydrogenation and methylation of phenols 6 without purification (Table 2, entries 3–7). Thus, the series of compounds 5ae was prepared in high yields (81%–87%).
Table 2. Conversion of 3-anilino-2-cyclohexen-1-ones 4ae into diarylamines 6ab, 6de and 5ae a. Molecules 18 10334 i002
Table 2. Conversion of 3-anilino-2-cyclohexen-1-ones 4ae into diarylamines 6ab, 6de and 5ae a. Molecules 18 10334 i002
Entry4 (R)Pd/C (10%) (mol%) b6 (%) c5 (%) d
1 e4b (4-OMe)1.96b (65)----
2 f4b (4-OMe)3.86b (75)----
34b (4-OMe)5.76b (87)5b (85)
44a (4-Me)5.76a (85)5a (83)
54c (3-Me)5.7(g)5c (87)
64d (3-OMe)5.76d (84)5d (81)
74e (3,5-(OMe)25.76e (88)5e (86)
a Standard conditions: (a) Preparation of diarylamines 6: 4 (0.81–1.00 mmol), Pd/C (10%), MeOH, 210 °C, 48 h; (b) Preparation of diarylamines 5ae: Aromatization step: 4 (0.82–1.00 mmol), Pd/C (10%), MeOH, 210 °C, 24 h; Methylation step: 6 (1.0 mol equiv.), MeI (2.0 mol equiv.), K2CO3 (1.5 mol equiv.), acetone, reflux, 12 h. b Calculated for Pd(0). c Isolated yields. d Isolated yields for the two steps. e At 180 °C for 12 h. f At 200 °C for 48 h. g Not isolated.

2.2. Synthesis of Carbazoles

The final cyclization step of diarylamines 5ae was successfully carried out by following the protocol originally developed by Knölker and coworkers [40,56,57], later applied by others [28,29], and optimized in our syntheses of natural carbazoles [37,44]. Thus, the conversion of the series 5ac and 5e into the carbazole derivatives 1hk resulted in good yields (80%–92%) (Table 3). It is noteworthy that the cyclization of 5d provided clausine V (1d) in high yield (90%) [22,33].
With the aim of testing the utility of this methodology for the total synthesis of natural 7-oxygenated tricyclic carbazoles, we carried out the conversion of derivative 1h into clauszoline-K (1f) and clauszoline-L (clausine C, 1g). Thus, upon applying the well-known procedure [58,59] for the synthesis of these [32] and other natural carbazoles [44], carbazole 1h was treated with DDQ in a mixture of MeOH/H2O/acetone (1:1:1) at room temperature for 45 min to give 1f in 70% yield (Scheme 3).
Table 3. Preparation of carbazoles 1d and 1hk via Pd(II)-catalyzed cyclization of diarylamines 5ae a. Molecules 18 10334 i003
Table 3. Preparation of carbazoles 1d and 1hk via Pd(II)-catalyzed cyclization of diarylamines 5ae a. Molecules 18 10334 i003
Entry5 (R)1 Isolated yield (%) b
15a (4-Me) Molecules 18 10334 i00480
25b (4-OMe) Molecules 18 10334 i00587
35c (3-Me) Molecules 18 10334 i00682
45d (3-OMe) Molecules 18 10334 i00790
55e (3,5-(OMe)2 Molecules 18 10334 i00892
a Standard conditions: 5 (0.32 –0.47 mmol), Pd(OAc)2 (10 mol%), Cu(OAc)2 (2.5 mol equiv.), DMF, MW (100 W), 130 °C, 70 min. b Isolated yields.
Scheme 3. Preparation of natural carbazoles clauszoline-K (1f) and clauszoline-L (1g).
Scheme 3. Preparation of natural carbazoles clauszoline-K (1f) and clauszoline-L (1g).
Molecules 18 10334 g004
The latter was oxidized with a mixture of MnO2/KCN in MeOH [58] to furnish clauszoline-L (1g) in almost quantitative yield. The spectral data of the products obtained agree with those described for the natural [20,23] and synthetic [58] products.

2.3. Total Synthesis of Glycoborine (Glycrophylamine, 9)

Recently, 5-methoxy-3-methylcarbazole (9) was isolated from the roots and branches of Glycosmis macrophylla and named glycrophylamine. This compound showed cytotoxic activity against NC1-H187 cancerigene cells [60]. However, the same carbazole had been isolated from Glycosmis arborea a decade earlier, and named glycoborine. This was the first 5-oxygenated tricyclic natural carbazole ever isolated [61]. Nowadays, three routes of synthesis have been developed for 9 based on Fischer [61], Japp-Klingemann [62], and Cadogan cyclizations [33] as the key step. We herein describe a new total synthesis of 9 starting from the key precursor tetrahydrocarbazole 7a (Scheme 4), which was efficiently prepared from 4a (Scheme 2).
When a mixture of 7a, Pd/C (10%) (5.7% mol) and anhydrous MeOH was heated in a sealed vessel to 270 °C for 48 h, 5-hydroxy-3-methylcarbazole (8) was isolated and then purified in good yield (Scheme 4). Methylation of the latter under the usual reaction conditions provided the desired natural carbazole 9, which was synthesized in four steps with high overall yield (53%). The spectral data of 9 agree with those described for the natural [60,61] and synthetic [33,62] products.
Scheme 4. Preparation of natural carbazole glycoborine (9).
Scheme 4. Preparation of natural carbazole glycoborine (9).
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All the structures of intermediates and products described in these synthetic sequences were characterized by 1H- and 13C-NMR spectroscopy, with the help of 2D (HMQC and HMBC) experiments and mass spectrometric techniques (MS and HRMS).

3. Experimental

3.1. General

Melting points (uncorrected) were determined with an Electrothermal capillary melting point apparatus. IR spectra were recorded on a Perkin-Elmer 2000 spectrophotometer. 1H (300 or 500 MHz) and 13C-NMR (75 or 125 MHz) spectra were recorded on Varian Mercury-300 or Varian VNMR System instruments, with TMS as internal standard. Mass spectra (MS) and high-resolution mass spectra (HRMS) were obtained, in electron impact (EI) (70 eV) mode, on Thermo-Finnigan Polaris Q and Jeol JSM-GcMateII spectrometers, respectively. Microwave (MW) irradiation was performed on a CEM MW reactor. Analytical thin-layer chromatography was carried out using E. Merck silica gel 60 F254 coated 0.25 plates, visualized by a long- and short-wavelength UV lamp. Flash column chromatography was performed over Natland International Co. silica gel (230–400 mesh). All air moisture sensitive reactions were carried out under nitrogen using oven-dried glassware. Toluene, MeOH, and MeCN were freshly distilled over sodium and DMF over calcium hydride prior to use. Acetone was dried by distillation after treatment with potassium permanganate. K2CO3 was dried overnight at 200 °C prior to use. All other reagents were used without further purification.

3.2. Synthesis and Characterization

3-(p-Tolylamino)cyclohex-2-en-1-one (4a) [63]. In a 250 mL, three necked, round-bottomed flask equipped with a magnetic stirring bar, rubber septum, a water condenser and a Dean-Stark trap, under N2 atmosphere, a mixture of 2 (0.400 g, 3.57 mmol) and 3a (0.382 g, 3.37 mmol) in dry toluene (150 mL) was stirred at reflux for 12 h. The solvent was removed under vacuum, and the residue purified by column chromatography over silica gel (10 g/g of crude, hexane/EtOAc, 1:1) to give 4a (0.66 g, 92%) as a pale yellow solid. Rf 0.15 (hexane/EtOAc, 1:1); mp 248–249 °C. IR (KBr): νmax 3214, 3029, 2937, 1573, 1512, 1361, 1311, 1245, 1183, 1141, 818 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 1.98 (qu, J = 6.5 Hz, 2H, H-5), 2.31 (br t, J = 6.5 Hz, 2H, H-6), 2.32 (s, 3H, CH3), 2.48 (t, J = 6.5 Hz, 2H, H-4), 5.48 (s, 1H, H-2), 6.99–7.03 (m, 2H, H-2′), 7.07–7.11 (m, 2H, H-3′), 7.12 (br s, 1H, NH). 13C-NMR (125 MHz, CDCl3): δ = 20.9 (CH3), 21.8 (C-5), 29.5 (C-4), 36.4 (C-6), 99.0 (C-2), 124.0 (C-2′), 129.7 (C-3′), 135.3 (C-4′), 135.5 (C-1′), 163.1 (C-3), 198.1 (C-1). MS (70 eV): m/z (%) 201 (M+, 74), 184 (26), 173 (100), 144 (53), 130 (29), 106 (13), 91 (12), 77 (10). HRMS (EI): m/z [M+] calcd for C13H15NO: 201.1154; found: 201.1156.
3-(4-Methoxyphenylamino)cyclohex-2-en-1-one (4b). Following the procedure described for 4a, using 2 (0.400 g, 3.57 mmol) and 3b (0.439 g, 3.57 mmol), 4b (0.74 g, 95%) was obtained as a pale yellow solid. Rf 0.12 (hexane/EtOAc, 1:1); mp 166–167 °C [Lit. [64] 164–166 °C]. IR (KBr): νmax 3218, 3039, 2946, 1513, 1412, 1365, 1243, 1180, 1135, 1032, 834, 716 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 1.97 (qu, J = 6.5 Hz, 2H, H-5), 2.30 (t, J = 6.5 Hz, 2H, H-6), 2.47 (t, J = 6.5 Hz, 2H, H-4), 3.78 (s, 3H, CH3O), 5.34 (s, 1H, H-2), 6.80–6.84 (m, 2H, H-3′), 7.02–7.06 (m, 2H, H-2′), 7.13 (br s, 1H, NH). 13C-NMR (125 MHz, CDCl3): δ = 21.8 (C-5), 29.3 (C-4), 36.4 (C-6), 55.4 (CH3O), 98.6 (C-2), 114.4 (C-3′), 126.1 (C-2′), 130.8 (C-1′), 157.5 (C-4′), 164.0 (C-3), 198.0 (C-1). MS (70 eV): m/z (%) 217 (M+, 100), 200 (55), 189 (43), 174 (20), 160 (98), 146 (30), 130 (23), 117 (12), 77 (10). HRMS (EI): m/z [M+] calcd for C13H15NO2: 217.1103; found: 217.1110.
3-(m-Tolylamino)cyclohex-2-en-1-one (4c). Following the procedure described for 4a, 4c (0.65 g, 90%) was obtained as a pale yellow oil from 2 (0.400 g, 3.57 mmol) and 3c (0.382 g, 3.57 mmol). Rf 0.15 (hexane/EtOAc, 1:1). IR (film): νmax 3256, 3066, 2959, 1546, 1453, 1360, 1244, 1185, 1136, 829, 795, 728 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 1.98 (qu, J = 6.5 Hz, 2H, H-5), 2.29 (s, 3H, CH3), 2.32 (t, J = 6.5 Hz, 2H, H-6), 2.49 (t, J = 6.5 Hz, 2H, H-4), 5.55 (s, 1H, H-2), 6.91-6.95 (m, 2H, H-4′, H-6′), 6.96 (br s, 1H, H-2′), 7.17 (t, J = 8.0 Hz, 1H, H-5′), 7.18 (br s, 1H, NH). 13C-NMR (125 MHz, CDCl3): δ 21.3 (CH3), 21.8 (C-5), 29.6 (C-4), 36.4 (C-6), 99.3 (C-2), 120.9 (C-6′), 124.4 (C-2′), 126.2 (C-4′), 129.0 (C-5′), 138.0 (C-3′), 139.1 (C-1′), 162.9 (C-3), 198.3 (C-1). MS (70 eV): m/z (%) 201 (M+, 79), 184 (49), 173 (100), 158 (16), 144 (88), 130 (42), 106 (13), 91 (19), 77 (16). HRMS (EI): m/z [M+] calcd for C13H15NO: 201.1154; found: 201.1160.
3-(3-Methoxyphenylamino)cyclohex-2-en-1-one (4d). Following the procedure described for 4a, using 2 (0.400 g, 3.57 mmol) and 3d (0.439 g, 3.57 mmol), 4d (0.72 g, 93%) was obtained as a pale yellow solid. Rf 0.11 (hexane/EtOAc, 1:1); mp 126–127 °C [Lit. [64] 122.5–124 °C; [65] 126–128 °C]. IR (KBr): νmax 3278, 3196, 3133, 2937, 1540, 1425, 1357, 1318, 1245, 1191, 1143, 1051, 871, 733 cm−1. 1H-NMR (500 MHz, CDCl3): δ 1.98 (qu, J = 6.5 Hz, 2H, H-5), 2.32 (t, J = 6.5 Hz, 2H, H-6), 2.50 (t, J = 6.5 Hz, 2H, H-4), 3.75 (s, 3H, CH3O), 5.59 (s, 1H, H-2), 6.66–6.70 (m, 2H, H-2′, H-6′), 6.72 (dm, J = 8.0 Hz, 1H, H-4′), 7.16–7.21 (m, 1H, H-5′), 7.29 (br s, 1H, NH). 13C-NMR (125 MHz, CDCl3): δ 21.8 (C-5), 29.5 (C-4), 36.4 (C-6), 55.2 (CH3O), 99.6 (C-2), 109.7 (C-2′), 110.8 (C-6′), 116.0 (C-4′), 129.9 (C-5′), 139.4 (C-1′), 160.2 (C-3′), 162.7 (C-3), 198.4 (C-1). MS (70 eV): m/z (%) 217 (M+, 99), 200 (59), 189 (46), 160 (100), 146 (32), 130 (23), 117 (11), 77 (7). HRMS (EI): m/z [M+] calcd for C13H15NO2: 217.1103; found: 217.1101.
3-(3,5-Dimethoxyphenylamino)cyclohex-2-en-1-one (4e). Following the procedure described for 4a, with 2 (0.400 g, 3.57 mmol) and 3e (0.546 g, 3.57 mmol), 4e (0.85 g, 96%) was obtained as a pale yellow solid. Rf 0.12 (hexane/EtOAc, 1:1); mp 139–140 °C. IR (KBr): νmax 3272, 2940, 1598, 1582, 1538, 1462, 1423, 1361, 1253, 1186, 1153, 1055, 824 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 1.99 (qu, J = 6.5 Hz, 2H, H-5), 2.33 (t, J = 6.5 Hz, 2H, H-6), 2.49 (t, J = 6.5 Hz, 2H, H-4), 3.73 (s, 6H, 2CH3O), 5.64 (s, 1H, H-2), 6.24 (t, J = 2.0 Hz, 1H, H-4′), 6.30 (d, J = 2.0 Hz, 2H, H-2′, H-6′), 7.06 (br s, 1H, NH). 13C-NMR (125 MHz, CDCl3): δ = 21.8 (C-5), 29.6 (C-4), 36.4 (C-6), 55.3 (CH3O), 97.4 (C-4′), 100.2 (C-2), 102.1 (C-2′, C-6′), 139.9 (C-1′), 161.2 (C-3′, C-5′), 162.3 (C-3), 198.3 (C-1). MS (70 eV): m/z (%) 247 (M+, 37), 230 (100), 219 (25), 190 (83), 160 (18), 135 (30), 120 (14), 77 (7). HRMS (EI): m/z [M+] calcd for C14H17NO3: 247.1208; found: 247.1207.
3-(p-Tolylamino)phenol (6a). In a threaded ACE glass pressure tube with a sealed Teflon screw cap, under N2 atmosphere, a mixture of 4a (0.20 g, 1.0 mmol) and Pd/C (10%) (0.060 g, 0.057 mmol) in dry MeOH (2.5 mL) was stirred at 210 °C for 48 h. The solvent was removed under vacuum, and the residue purified by column chromatography over silica gel (20 g/g of crude, hexane/EtOAc, 80:20), to give 6a (0.168 g, 85%) as a pale grey solid. Rf 0.55 (hexane/EtOAc, 7:3); mp 81–82 °C [Lit. [66] 82 °C]. IR (film): νmax 3394, 1606, 1512, 1493, 1332, 1243, 1155, 969, 815, 766 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 2.27 (s, 3H, CH3), 5.58 (br s, 1H, NH), 6.31 (dd, J = 7.8, 2.1 Hz, 1H, H-4), 6.44 (t, J = 2.1 Hz, 1H, H-2), 6.51 (dd, J = 7.8, 2.1 Hz, 1H, H-6), 6.92–6.99 (m, 2H, H-2′), 6.99–7.07 (m, 3H, H-3′, H-5). 13C-NMR (75.4 MHz, CDCl3): δ = 20.6 (CH3), 103.2 (C-2), 107.1 (C-4), 108.9 (C-6), 119.5 (C-2′), 129.8 (C-3′), 130.2 (C-5), 131.2 (C-4′), 139.7 (C-1′), 145.6 (C-1), 156.6 (C-3). MS (70 eV): m/z (%) 199 (M+, 100), 183 (19), 170 (22), 154 (35), 128 (14), 91 (83), 65 (18). HRMS (EI): m/z [M+] calcd for C13H13NO: 199.0997; found: 199.0998.
3-(4-Methoxyphenylamino)phenol (6b). Following the procedure described for 6a, with 4b (0.200 g, 0.92 mmol) and Pd/C (10%) (0.060 g, 0.057 mmol), 6b (0.172 g, 87%) was obtained as a pale grey solid. Rf 0.50 (hexane/EtOAc, 8:2); mp 66–67 °C [Lit. [66] 67–68 °C]. IR (KBr): νmax 3379, 1601, 1526, 1504, 1459, 1291, 1239, 1174, 1110, 1027, 735 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 3.79 (s, 3H, CH3O), 6.50 (br s, 1H, NH), 6.28 (ddd, J = 8.0, 2.4, 0.6 Hz, 1H, H-4), 6.37 (t, J = 2.4 Hz, 1H, H-2), 6.44 (ddd, J = 8.0, 2.4, 0.6 Hz, 1H, H-6), 6.82–6.89 (m, 2H, H-3′), 7.04 (t, J = 8.0 Hz, 1H, H-5), 7.04–7.10 (m, 2H, H-2′). 13C-NMR (75.4 MHz, CDCl3): δ = 55.5 (CH3O), 101.9 (C-2), 106.3 (C-4), 108.0 (C-6), 114.6 (C-3′), 122.9 (C-2′), 130.3 (C-5), 135.1 (C-1′), 146.9 (C-1), 155.4 (C-4′), 156.7 (C-3). MS (70 eV): m/z (%) 215 (M+, 100), 201 (6), 185 (7), 172 (5), 146 (4), 132 (5), 91 (11). HRMS (EI): m/z [M+] calcd for C13H13NO2: 215.0946; found: 215.0952.
3-(3-Methoxyphenylamino)phenol (6d) [66]. Following the procedure described for 6a, with 4d (0.200 g, 0.92 mmol) and Pd/C (10%) (0.060 g, 0.057 mmol), 6d (0.166 g, 84%) was obtained as a purple oil. Rf 0.51 (hexane/EtOAc, 8:2). IR (film): νmax 3411, 1645, 1489, 1156, 764 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 3.71 (s, 3H, CH3O), 5.76 (br s, 1H, NH), 6.38 (ddd, J = 8.1, 2.4, 0.9 Hz, 1H, H-4), 6.46 (dm, J = 7.8 Hz, 1H, H-4′), 6.54 (t, J = 2.4 Hz, 1H, H-2), 6.56–6.65 (m, 3H, H-2′, H-6, H-6′), 7.05 (t, J = 8.1 Hz, 1H, H-5), 7.11 (t, J = 7.8 Hz, 1H, H-5′). 13C-NMR (75.4 MHz, CDCl3): δ = 55.1 (CH3O), 103.8 (C-2′), 104.6 (C-2), 106.3 (C-4′), 108.0 (C-4), 110.1 (C-6′), 110.7 (C-6), 130.0 (C-5′), 130.2 (C-5), 144.0 (C-1′), 144.3 (C-1), 156.6 (C-3), 160.3 (C-3′). MS (70 eV): m/z (%) 215 (M+, 26), 199 (21), 182 (25), 160 (31), 146 (45), 130 (25), 109 (23), 51 (100). HRMS (EI): m/z [M+] calcd for C13H13NO2: 215.0946; found: 215.0952.
3-(3,5-Dimethoxyphenylamino)phenol (6e). Following the procedure described for 6a, with 4e (0.200 g, 0.81 mmol) and Pd/C (10%) (0.060 g, 0.057 mmol), 6e (0.174 g, 88%) was obtained as a yellow oil. Rf 0.49 (hexane/EtOAc, 8:2). IR (film): νmax 3379, 2917, 1594, 1481, 1203, 1152, 1065, 821 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 3.73 (s, 6H, 2CH3O), 5.74 (br s, 1H, NH), 6.07 (t, J = 2.1 Hz, 1H, H-4′), 6.23 (d, J = 2.1 Hz, 2H, H-2′, H-6′), 6.39 (dd, J = 8.1, 2.4 Hz, 1H, H-4), 6.55 (dd, J = 2.4, 2.1 Hz, 1H, H-2), 6.62 (ddd, J = 8.1, 2.1, 0.9 Hz, 1H, H-6), 7.08 (t, J = 8.1 Hz, 1H, H-5). 13C-NMR (75.4 MHz, CDCl3): δ = 55.3 (2CH3O), 93.2 (C-4′), 96.3 (C-2′, C-6′), 105.0 (C-2), 108.2 (C-4), 110.6 (C-6), 130.3 (C-5), 144.1 (C-1′), 144.7 (C-1), 156.6 (C-3), 161.4 (C-3′, C-5′). MS (70 eV): m/z (%) 245 (M+, 3), 154 (14), 153 (96), 125 (15), 124 (100), 94 (25), 92 (22). HRMS (EI): m/z [M+] calcd for C14H15NO3: 245.1052; found: 245.1059.
3-Methoxy-N-(p-tolyl)aniline (5a) [67]. In a threaded ACE glass pressure tube with a sealed Teflon screw cap, under N2 atmosphere, a mixture of 4a (0.20 g, 1.0 mmol) and Pd/C (10%) (0.060 g, 0.057 mmol) in dry MeOH (2.5 mL) was stirred at 210 °C for 24 h. After removing the solvent under vacuum, K2CO3, (0.200 g, 1.45 mmol) and CH3I (0.281 g, 1.98 mmol) in dry acetone (20 mL) were added, and the mixture was heated to reflux for 12 h. The solvent was removed under vacuum and the residue purified by column chromatography over silica gel (20 g/g of crude, hexane/EtOAc, 90:10), to give 5a (0.176 g, 83%) as a white solid. Rf 0.60 (hexane/EtOAc, 7:3); mp 49–50 °C. IR (KBr): νmax 3367, 1598, 1493, 1462, 1256, 1157, 1032, 950, 832, 774 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 2.30 (s, 3H, CH3), 3.75 (s, 3H, CH3O), 6.00 (br s, 1H, NH), 6.42 (ddd, J = 8.5, 2.5, 0.5 Hz, 1H, H-4), 6.55-6.59 (m, 2H, H-2, H-6), 6.98–7.02 (m, 2H, H-2′), 7.06-7.10 (m, 2H, H-3′), 7.12 (tm, J = 8.5 Hz, 1H, H-5). 13C-NMR (125 MHz, CDCl3): δ = 20.7 (CH3), 55.1 (CH3O), 102.4 (C-2), 105.5 (C-4), 109.4 (C-6), 119.4 (C-2′), 129.8 (C-3′), 130.0 (C-5), 131.2 (C-4′), 140.0 (C-1′), 145.4 (C-1), 160.7 (C-3). MS (70 eV): m/z (%) 213 (M+, 100), 200 (23), 189 (24), 174 (21), 160 (39), 130 (11), 91 (12), 84 (9). HRMS (EI): m/z [M+] calcd for C14H15NO: 213.1154; found: 213.1153.
3-Methoxy-N-(4-methoxyphenyl)aniline (5b) [29]. Following the procedure described for 5a using 4b (0.200 g, 0.92 mmol), Pd/C (10%) (0.055 g, 0.052 mmol), K2CO3 (0.190 g, 1.38 mmol) and MeI (0.261 g, 1.84 mmol), 5b (0.179 g, 85%) was obtained as a white solid. Rf 0.55 (hexane/EtOAc, 7:3); mp 99–100 °C. IR (KBr): νmax 3400, 1597, 1509, 1460, 1239, 1157, 1035, 825, 768 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 3.75 (s, 3H, CH3O), 3.79 (s, 3H, CH3O), 6.51 (br s, 1H, NH), 6.38 (br dd, J = 7.8, 2.1 Hz, 1H, H-4), 6.44–6.51 (m, 2H, H-2, H-6), 6.83–6.89 (m, 2H, H-3′), 7.04–7.10 (m, 2H, H-2′), 7.11 (t, J = 7.8 Hz, 1H, H-5). 13C-NMR (75.4 MHz, CDCl3): δ = 55.1 (CH3O-C3), 55.5 (CH3O-C4′), 101.1 (C-2), 104.6 (C-4), 108.2 (C-6), 114.6 (C-3′), 122.7 (C-2′), 130.0 (C-5), 135.3 (C-1′), 146.6 (C-1), 155.4 (C-4′), 160.7 (C-3). MS (70 eV): m/z (%) 229 (M+, 90), 216 (47), 214 (100), 186 (19), 171 (21), 142 (15), 115 (21). HRMS (EI): m/z [M+] calcd for C14H15NO2: 229.1103; found: 229.1111.
3-Methoxy-N-(m-tolyl)aniline (5c). Following the procedure described for 5a, with 4c (0.20 g, 1.0 mmol), Pd/C (10%) (0.060 g, 0.057 mmol), K2CO3 (0.200 g, 1.45 mmol) and MeI (0.281 g, 1.98 mmol), 5c (0.18 g, 87%) was obtained as a yellow oil. Rf 0.59 (hexane/EtOAc, 7:3). IR (film): νmax 3391, 1589, 1490, 1266, 1203, 1155, 1042, 765, 687 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 2.31 (s, 3H, CH3), 3.77 (s, 3H, CH3O), 5.67 (br s, 1H, NH), 6.47 (br dd, J = 7.8, 2.4 Hz, 1H, H-4), 6.62-6.67 (m, 2H, H-2, H-6), 6.76 (br d, J = 7.2 Hz, 1H, H-4′), 6.88–6.94 (m, 2H, H-2′, H-6′), 7.12–7.20 (m, 2H, H-5, H-5′). 13C-NMR (75.4 MHz, CDCl3): δ = 21.5 (CH3), 55.2 (CH3O), 103.2 (C-2), 105.9 (C-4), 110.2 (C-6), 115.4 (C-6′), 119.0 (C-2′), 122.1 (C-4′), 129.1 (C-5′), 130.0 (C-5), 139.2 (C-3′), 142.7 (C-1′), 144.6 (C-1), 160.6 (C-3). MS (70 eV): m/z (%) 213 (M+, 100), 200 (32), 189 (35), 174 (26), 160 (44), 130 (13), 92 (11), 77 (11). HRMS (EI): m/z [M+] calcd for C14H15NO: 213.1154; found: 213.1161.
bis(3-Methoxyphenyl)amine (5d) [26]. Following the procedure described for 5a, with 4d (0.20 g, 0.92 mmol), Pd/C (10%) (0.055 g, 0.052 mmol), K2CO3 (0.190 g, 1.38 mmol) and MeI (0.261 g, 1.84 mmol), 5d (0.171 g, 81%) was obtained as a white solid. Rf 0.55 (hexane/EtOAc, 7:3); mp 154–155 °C. IR (film): νmax 3393, 1592, 1490, 1270, 1207, 1155, 1040, 832, 760, 685 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 3.74 (s, 6H, 2CH3O), 5.78 (br s, 1H, NH), 6.47 (ddm, J = 8.1, 2.4 Hz, 2H, H-4, H-4′), 6.61–6.68 (m, 4H, H-2, H-2′, H-6, H-6′), 7.14 (t, J = 8.1 Hz, 2H, H-5, H-5′). 13C-NMR (75.4 MHz, CDCl3): δ = 55.1 (2CH3O), 103.6 (C-2, C-2′), 106.3 (C-4, C-4′), 110.4 (C-6, C-6′), 130.0 (C-5, C-5′), 144.1 (C-1, C-1′), 160.5 (C-3, C-3′). MS (70 eV): m/z (%) 229 (M+, 100), 217 (10), 200 (12), 189 (6), 170 (11), 160 (9), 154 (12), 142 (9), 115 (5). HRMS (EI): m/z [M+] calcd for C14H15NO2: 229.1103; found: 229.1104.
3,5-Dimethoxy-N-(3-methoxyphenyl)aniline (5e). Following the procedure described for 5a, with 4e (0.200 g, 0.818 mmol), Pd/C (10%) (0.050 g, 0.047 mmol), K2CO3 (0.167 g, 1.21 mmol) and MeI (0.230 g, 1.62 mmol), 5e (0.18 g, 86%) was obtained as a colorless oil. Rf 0.52 (hexane/EtOAc, 7:3). IR (film): νmax 3735, 1590, 1541, 1457, 1203, 1150, 1057 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 3.74 (s, 6H, 2CH3O), 3.76 (s, 3H, CH3O-3), 5.73 (br s, 1H, NH), 6.07 (t, J = 2.0 Hz, 1H, H-4), 6.24 (d, J = 2.0 Hz, 2H, H-2, H-6), 6.49 (ddd, J = 8.0, 2.0, 1.0 Hz, 1H, H-4′), 6.65 (t, J = 2.0 Hz, 1H, H-2′), 6.67 (ddd, J = 8.0, 2.0, 1.0 Hz, 1H, H-6′), 7.15 (t, J = 8.0 Hz, 1H, H-5′). 13C-NMR (125 MHz, CDCl3): δ = 55.1 (CH3O-3′), 55.2 (2CH3O), 93.3 (C-4), 96.2 (C-2, C-6), 104.2 (C-2′), 106.7 (C-4′), 111.0 (C-6′), 130.0 (C-5′), 144.0 (C-1), 144.9 (C-1′), 160.6 (C-3′), 161.6 (C-3, C-5). MS (70 eV): m/z (%) 259 (M+, 7), 257 (97), 242 (100), 214 (49), 199 (42), 184 (13), 156 (8), 128 (7). HRMS (EI): m/z [M+] calcd for C15H17NO3: 259.1208; found: 259.1209.
2-Methoxy-6-methyl-9H-carbazole (1h). A mixture of 5a (0.100 g, 0.47 mmol), Pd(AcO)2 (0.0105 g, 0.047 mmol) and Cu(AcO)2 (0.211 g, 1.17 mmol) in dry DMF (0.5 mL), under N2 atmosphere, was stirred and heated at 130 °C for 70 min under MW irradiation (100 W). The solvent was removed under vacuum by adding toluene, and the azeotropic distillation was continued until no solvent remained. The residue was purified by column chromatography over silica gel (10 g/g of crude, hexane/EtOAc, 95:5), to give 1h (0.079 g, 80%) as a white solid. Rf 0.60 (hexane/EtOAc, 7:3); mp 226–227 °C [Lit. [32] 227–228 °C]. IR (film): νmax 3392, 1659, 1026, 826, 764, 687 cm−1. 1H-NMR (500 MHz, DMSO-d6/acetone-d6, 3:7): δ = 2.46 (s, 3H, CH3), 3.86 (s, 3H, CH3O), 6.76 (dd, J = 8.4, 2.4 Hz, 1H, H-3), 6.99 (d, J = 2.4 Hz, 1H, H-1), 7.11 (dd, J = 8.2, 1.5 Hz, 1H, H-7), 7.33 (d, J = 8.2 Hz, 1H, H-8), 7.78 (br s, 1H, H-5), 7.91 (d, J = 8.4 Hz, 1H, H-4), 10.6 (br s, 1H, NH). 13C-NMR (125 MHz, DMSO-d6/acetone-d6, 3:7): δ = 21.2 (CH3), 55.4 (CH3O), 95.0 (C-1), 108.1 (C-3), 110.8 (C-8), 117.2 (C-4a), 119.6 (C-5), 121.1 (C-4), 124.0 (C-4a), 125.9 (C-7), 128.0 (C-6), 139.0 (C-8a), 142.4 (C-9a), 159.5 (C-2). MS (70 eV): m/z (%) 211 (M+, 100), 196 (51), 168 (76), 139 (10), 86 (6). HRMS (EI): m/z [M+] calcd for C14H13NO: 211.0997; found: 211.0994.
2,6-Dimethoxy-9H-carbazole (1i). Following the procedure described for 1h, with 5b (0.100 g, 0.44 mmol), Pd(AcO)2 (0.0099 g, 0.044 mmol) and Cu(AcO)2 (0.198 g, 1.1 mmol), 1i (0.086 g, 87%) was obtained as a white solid. Rf 0.55 (hexane/EtOAc, 7:3); mp 162–163 °C [Lit. [29] 163–164 °C]. IR (KBr): νmax 3398, 1626, 1491, 1465, 1284, 1221, 1202, 1162, 1029, 820 cm−1. 1H-NMR (300 MHz, CDCl3): δ = 3.85 (s, 3H, CH3O), 3.86 (s, 3H, CH3O), 6.75 (dd, J = 8.4, 2.4 Hz, 1H, H-3), 6.92 (dd, J = 8.7, 2.5 Hz, 1H, H-7), 6.95 (d, J = 2.4 Hz, 1H, H-1), 7.31 (d, J = 8.7 Hz, 1H, H-8), 7.51 (d, J = 2.5 Hz, 1H, H-5), 7.89 (d, J = 8.4 Hz, 1H, H-4), 9.90 (br s, 1H, NH). 13C-NMR (75.4 MHz, CDCl3): δ = 53.9 (CH3O), 54.3 (CH3O), 93.3 (C-1), 101.2 (C-5), 106.6 (C-3), 110.0 (C-8), 112.0 (C-7), 115.9 (C-4a), 119.7 (C-4), 122.7 (C-4b), 133.8 (C-8a), 141.1 (C-9a), 152.8 (C-6), 158.0 (C-2). MS (70 eV): m/z (%) 227 (M+, 100), 212 (86), 184 (69), 169 (28), 141 (19), 114 (7). HRMS (EI): m/z [M+] calcd for C14H13NO2: 227.0946; found: 227.0951.
2-Methoxy-7-methyl-9H-carbazole (1j). Following the procedure described for 1h, with 5c (0.100 g, 0.47 mmol), Pd(AcO)2 (0.0105 g, 0.047 mmol) and Cu(AcO)2 (0.211 g, 1.17 mmol), 1j (0.081 g, 82%) was obtained as a white solid. Rf 0.61 (hexane/EtOAc, 7:3); mp 162–163 °C [Lit. [68] 280 °C]. IR (film): νmax 3399, 1654, 1047, 1025, 995, 827, 766 cm−1. 1H-NMR (500 MHz, DMSO-d6/acetone-d6, 3:7): δ = 2.46 (s, 3H, CH3), 3.85 (s, 3H, CH3O), 6.75 (dd, J = 8.5, 2.5 Hz, 1H, H-3), 6.95 (dd, J = 8.3, 1.0 Hz, 1H, H-6), 6.99 (d, J = 2.5 Hz, 1H, H-1), 7.25 (br s, 1H, H-8), 7.84 (d, J = 8.3 Hz, 1H, H-5), 7.88 (d, J = 8.5 Hz, 1H, H-4), 10.7 (br s, 1H, NH). 13C-NMR (125 MHz, DMSO-d6/acetone-d6, 3:7): δ = 21.4 (CH3), 54.9 (CH3O), 94.2 (C-1), 106.9 (C-3), 110.3 (C-8), 116.1 (C-4a), 118.4 (C-5), 119.6 (C-6), 119.9 (C-4), 120.2 (C-4b), 133.1 (C-7), 140.0 (C-8a), 140.8 (C-9a), 157.8 (C-2). MS (70 eV): m/z (%) 211 (M+, 100), 196 (58), 168 (66), 139 (10), 86 (6). HRMS (EI): m/z [M+] calcd for C14H13NO: 211.0997; found: 211.1000.
2,7-Dimethoxy-9H-carbazole (Clausine V, 1d). Following the procedure described for 1h, with 5d (0.100 g, 0.44 mmol), Pd(AcO)2 (0.0099 g, 0.044 mmol) and Cu(AcO)2 (0.198 g, 1.10 mmol), 1d (0.089 g, 90%) was obtained as a white solid. Rf 0.56 (hexane/EtOAc, 7:3); mp 229–230 °C [Lit. [22] 228–230 °C]. IR (KBr): νmax 3382, 2927, 1608, 1575, 1502, 1457, 1322, 1265, 1233, 1160, 1118, 1026, 825, 805 cm−1. 1H-NMR (300 MHz, DMSO-d6/acetone-d6, 3:7): δ = 3.85 (s, 6H, 2CH3O), 6.75 (dd, J = 8.4, 2.4 Hz, 2H, H-3, H-6), 6.99 (d, J = 2.4 Hz, 2H, H-1, H-8), 7.85 (d, J = 8.4 Hz, 2H, H-4, H-5), 10.81 (br s, 1H, NH). 13C-NMR (75.4 MHz, DMSO-d6/acetone-d6, 3:7): δ = 54.9 (2CH3O), 94.6 (C-1, C8), 107.3 (C-3, C-6), 116.8 (C-4a, C-4b), 119.7 (C-4, C-5), 141.4 (C-8a, C-9a), 157.9 (C-2, C-7). MS (70 eV): m/z (%) 227 (M+, 77), 212 (100), 184 (42), 169 (54), 153 (13), 141 (27), 114 (5). HRMS (EI): m/z [M+] calcd for C14H13NO2: 227.0946; found: 227.0946.
2,4,7-Trimethoxy-9H-carbazole (1k). Following the procedure described for 1h, with 5e (0.101 g, 0.39 mmol), Pd(AcO)2 (0.0087 g, 0.039 mmol) and Cu(AcO)2 (0.175 g, 0.97 mmol), 1k (0.092 g, 92%) was obtained as a white solid. Rf 0.20 (hexane/EtOAc, 7:3); mp 167–168 °C. IR (KBr): νmax 3383, 1617, 1580, 1510, 1453, 1428, 1260, 1213, 1149, 1119, 1032, 803 cm−1. 1H-NMR (500 MHz, CDCl3/acetone-d6, 7:3): δ = 3.83 (s, 6H, 2CH3O), 3.98 (s, 3H, CH3O), 6.27 (d, J = 1.3 Hz, 1H, H-3), 6.46 (d, J = 1.3 Hz, 1H, H-1), 6.76 (dd, J = 8.5, 2.2 Hz, 1H, H-6), 6.82 (d, J = 2.2 Hz, 1H, H-8), 7.99 (d, J = 8.5 Hz, 1H, H-5), 9.32 (br s, 1H, NH). 13C-NMR (125 MHz, CDCl3/acetone-d6, 7:3): δ = 54.7 (CH3O), 54.9 (CH3O), 55.0 (CH3O), 86.7 (C-1), 90.2 (C-3), 94.0 (C-8), 106.0 (C-4a), 107.0 (C-6), 116.1 (C-4b), 121.7 (C-5), 139.7 (C-8a), 141.3 (C-9a), 155.2 (C-4), 156.8 (C-7), 158.7 (C-2). MS (70 eV): m/z (%) 257 (M+, 39), 247 (36), 230 (100), 219 (24), 214 (22), 190 (82), 176 (21), 160 (19), 117 (7). HRMS (EI): m/z [M+] calcd for C15H15NO3: 257.1052; found: 257.1052.
7-Methoxy-9H-carbazole-3-carbaldehyde (Clauszoline-K) (1f). A mixture of 1h (0.030 g, 0.14 mmol) and DDQ (0.129 g, 0.57 mmol) in acetone/MeOH/H2O (1:1:1) (10 mL) was stirred at 25 °C for 45 min. The solvent was removed under vacuum and the residue purified by column chromatography over silica gel (10 g/g of crude, hexane/EtOAc, 8:2), to give 1f (0.022 g, 70%) as a white solid. Rf 0.25 (hexane/EtOAc, 8:2); mp 184–185 °C [Lit. [32] 183–186 °C]. IR (KBr): νmax 3296, 1670, 1604, 1570, 1479, 1322, 1237, 1160, 1026, 821 cm−1. 1H-NMR (500 MHz, DMSO-d6/CDCl3, 3:7): δ = 3.88 (s, 3H, CH3O), 6.84 (dd, J = 8.5, 2.0 Hz, 1H, H-6), 7.00 (d, J = 2.0 Hz, 1H, H-8), 7.51 (d, J = 8.0, 1H, H-1), 7.83 (dd, J = 8.0, 1.0 Hz, 1H, H-2), 7.96 (d, J = 8.5 Hz, 1H, H-5), 8.45 (s, 1H, H-4), 10.05 (s, 1H, CHO), 11.40 (br s, 1H, NH). 13C-NMR (125 MHz, DMSO-d6/CDCl3, 3:7): δ = 54.0 (CH3O), 93.8 (C-8), 107.6 (C-6), 109.5 (C-1), 115.0 (C-4b), 119.7 (C-5), 121.3 (C-4), 121.8 (C-4a), 124.1 (C-2), 126.9 (C-3), 140.8 (C-8a), 142.6 (C-9a), 158.0 (C-7), 190.2 (CHO). MS (70 eV): m/z (%) 225 (M+, 40), 210 (28), 180 (72), 167 (97), 160 (44), 146 (30), 130 (32), 115 (28), 77 (29), 51 (100). HRMS (EI): m/z [M+] calcd for C14H11NO2: 225.0790; found: 225.0796.
Methyl 7-methoxy-9H-carbazole-3-carboxylate (Clauszoline-L, Clausine C) (1g). A mixture of 1f(0.200 g, 0.89 mmol), MnO2 (0.20 g, 2.3 mmol), and KCN (0.028 g, 0.43 mmol) in MeOH (10 mL) was stirred at 25 °C for 24 h. The solvent was removed under vacuum and the residue purified by column chromatography over silica gel (10 g/g of crude, hexane/EtOAc, 8:2), to give 1g (0.22 g, 97%) as a white solid. Rf 0.29 (hexane/EtOAc, 8:2); mp 194–195 °C [Lit. [20] 195–197 °C; [32] 195 °C; [33] 194–195 °C]. IR (KBr): νmax 3288, 1698, 1605, 1439, 1327, 1259, 1195, 1159, 1094, 815, 728 cm−1. 1H-NMR (500 MHz, acetone-d6): δ = 3.88 (s, 3H, CH3O), 3.91 (s, 3H, CH3O), 6.88 (dd, J = 8.5, 2.0 Hz, 1H, H-6), 7.08 (d, J = 2.0 Hz, 1H, H-8), 7.51 (d, J = 8.5, 1H, H-1), 7.99 (dd, J = 8.5, 1.5 Hz, 1H, H-2), 8.09 (d, J = 8.5 Hz, 1H, H-5), 8.69 (d, J = 1.5 Hz, 1H, H-4), 10.80 (br s, 1H, NH). 13C-NMR (125 MHz, acetone-d6): δ = 51.8 (CH3O), 55.7 (CH3O), 95.7 (C-8), 109.6 (C-6), 110.9 (C-1), 117.4 (C-4b), 121.6 (C-5), 121.9 (C-4a), 122.1 (C-4), 123.9 (C-3), 126.4 (C-2), 143.0 (C-8a), 144.0 (C-9a), 160.5 (C-2), 167.9 (CO2Me). MS (70 eV): m/z (%) 255 (M+, 100), 240 (22), 224 (44), 212 (61), 196 (38), 181 (33), 153 (67), 126 (15), 84 (20), 51 (21). HRMS (EI): m/z [M+] calcd for C15H13NO3: 255.0895; found: 255.0900.
6-Methyl-2,3-dihydro-1H-carbazol-4(9H)-one (7a). In a threaded ACE glass pressure tube with a sealed Teflon screw cap, under N2 atmosphere, a mixture of 4a (0.10 g, 0.5 mmol) and Pd(AcO)2 (0.034 g, 0.15 mmol) in dry MeCN (2.5 mL) was stirred at 80 °C for 24 h. The solvent was removed under vacuum and the residue purified by column chromatography over silica gel (20 g/g of crude, hexane/EtOAc, 80:20), to give 7a (0.077 g, 78%) as a white solid. Rf 0.15 (hexane/EtOAc, 1:1); mp 281–282 °C [Lit. [69] 280–282 °C]. IR (film): νmax 3154, 2934, 1615, 1469, 1406, 1375, 1213, 1183, 1122, 1070, 1016, 797 cm−1. 1H-NMR (300 MHz, DMSO-d6): δ = 2.09 (qu, J = 6.3 Hz, 2H, H-2), 2.38 (s, 3H, CH3), 2.41 (t, J = 6.3 Hz, 2H, H-3), 2.93 (t, J = 6.3 Hz, 2H, H-1), 6.97 (br d, J = 8.1 Hz, 1H, H-7), 7.27 (d, J = 8.1 Hz, 1H, H-8), 7.77 (br s, 1H, H-5), 11.75 (br s, 1H, NH) 13C-NMR (75.4 MHz, DMSO-d6): δ = 21.2 (CH3), 22.7 (C-1), 23.4 (C-2), 37.8 (C-3), 111.1 (C-8), 111.4 (C-4a), 120.1 (C-5), 123.7 (C-7), 124.7 (C-4b), 130.2 (C-6), 134.1 (C-8a), 152.2 (C-9a), 192.8 (C-4). MS (70 eV): m/z (%) 199 (M+, 100), 198 (45), 183 (13), 170 (11), 154 (20), 128 (8), 91 (40).
6-Methoxy-2,3-dihydro-1H-carbazol-4(9H)-one (7b). Following the procedure described for 7a, with 4b (0.100 g, 0.46 mmol) and Pd(AcO)2 (0.0309 g, 0.138 mmol), 7b (0.08 g, 80%) was obtained as a white solid. Rf 0.13 (hexane/EtOAc, 1:1); mp 252–253 °C [Lit. [70] 250–254 °C]. IR (KBr): νmax 3416, 1578, 1482, 1459, 1259, 1217, 1175, 1031, 796, 780 cm−1. 1H-NMR (300 MHz, DMSO-d6): δ = 2.09 (qu, J = 6.3 Hz, 2H, H-2), 2.41 (t, J = 6.3 Hz, 2H, H-3), 2.92 (t, J = 6.3 Hz, 2H, H-1), 3.76 (s, 3H, CH3O), 6.77 (dd, J = 8.7, 2.7 Hz, 1H, H-7), 7.28 (d, J = 8.7 Hz, 1H, H-8), 7.45 (d, J = 2.7 Hz, 1H, H-5), 11.74 (br s, 1H, NH) 13C-NMR (75.4 MHz, DMSO-d6): δ = 22.8 (C-1), 23.4 (C-2), 37.7 (C-3), 55.2 (CH3O), 102.4 (C-5), 111.6 (C-7), 111.7 (C-4a), 112.2 (C-8), 125.2 (C-4b), 130.5 (C-8a), 152.3 (C-9a), 155.1 (C-6), 192.8 (C-4). MS (70 eV): m/z (%) 215 (M+, 2), 155 (37), 153 (100), 127 (12), 125 (35), 90 (23).
6-Methyl-9H-carbazol-4-ol (8). In a threaded ACE glass pressure tube with a sealed Teflon screw cap, under N2 atmosphere, a mixture of 7a (0.20 g, 1.0 mmol) and Pd/C (10%) (0.060 g, 0.057 mmol) in dry MeOH (2.5 mL) was stirred at 270 °C for 48 h. The solvent was removed under vacuum and the residue purified by column chromatography over silica gel (20 g/g of crude, hexane/EtOAc, 80:20), to give 8 (0.168 g, 85%) as a white solid. Rf 0.30 (hexane/EtOAc, 7:3); mp 125–126 °C. IR (film): νmax 3404, 1615, 1589, 1455, 1341, 1297, 1267, 1047, 803, 752, 724 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 2.52 (s, 3H, CH3), 5.38 (br s, 1H, OH), 6.53 (d, J = 8.5 Hz, 1H, H-3), 6.95 (d, J = 8.5 Hz, 1H, H-1), 7.17–7.22 (m, 2H, H-2, H-7), 7.26 (t, J = 8.5 Hz, 1H, H-8), 7.89 (br s, 1H, NH), 8.06 (br s, 1H, H-5). 13C-NMR (125 MHz, CDCl3): δ = 21.4 (CH3), 103.3 (C-1), 104.9 (C-3), 109.7 (C-8), 111.6 (C-4a), 122.5 (C-4b), 122.7 (C-5), 126.3 (C-2 or C-7), 126.4 (C-7 or C-2), 129.0 (C-6), 137.0 (C-8a), 141.7 (C-9a), 151.8 (C-4). HRMS (EI): m/z [M+] calcd for C13H11NO: 197.0841; found: 197.0844.
5-Methoxy-3-methyl-9H-carbazole (Glycoborine, Glycrophylamine, 9). A mixture of 8 (0.150 g, 0.76 mmol), MeI (0.216 g, 1.52 mmol) and K2CO3 (0.157 g, 1.14 mmol) in dry acetone (10 mL) was heated to reflux for 2 h. The solvent was removed under vacuum and the residue purified by column chromatography over silica gel (10 g/g of crude, hexane/EtOAc, 95:5), to give 9 (0.151 g, 94%) as a white solid. Rf 0.35 (hexane/EtOAc, 8:2); mp 133–134 °C [Lit. [33] 154–156 °C; [60] 132–134.6 °C; [61] 155–156 °C; [62] 135 °C]. IR (KBr): νmax 3402, 1586, 1508, 1458, 1346, 1261, 1103, 804, 719 cm−1. 1H-NMR (500 MHz, CDCl3): δ = 2.52 (s, 3H, CH3), 4.06 (s, 3H, CH3O), 6.64 (d, J = 8.0 Hz, 1H, H-6), 6.97 (d, J = 8.0 Hz, 1H, H-8), 7.18 (dd, J = 8.0, 1.2 Hz, 1H, H-2), 7.24 (d, J = 8.0 Hz, 1H, H-1), 7.29 (t, J = 8.0 Hz, 1H, H-7), 8.00 (br s, 1H, NH), 8.11 (br s, 1H, H-4). 13C-NMR (125 MHz, CDCl3): δ = 21.4 (CH3), 55.3 (CH3O), 100.1 (C-6), 103.5 (C-8), 109.5 (C-1), 112.4 (C-4b), 122.8 (C-4a), 122.9 (C-4), 126.1 (C-2), 126.4 (C-7), 128.8 (C-3), 136.9 (C-9a), 141.2 (C-8a), 156.2 (C-5). HRMS (EI): m/z [M+] calcd for C14H13NO: 211.0997; found: 211.0995.

4. Conclusions

In this work, a short and efficient synthetic route for the construction of 2-, 5-, and 7-oxygenated carbazole alkaloids including natural clausine V (1d) is described. As the key steps, this approach includes a palladium(0)-catalyzed aromatization and a palladium(II)-catalyzed cyclization to provide the 2- and 7-oxygenated tricyclic carbazole framework. In the case of the natural 5-oxygenated carbazole glycoborine (glycrophylamine, 9), the palladium-catalyzed sequence was inverted, with cyclization performed before aromatization. The preparation of natural carbazoles clauszoline-K (1f) and clauszoline-L (1g) was also carried out by transformation of carbazole 1h. This methodology is currently being applied to the synthesis of diverse carbazoles, and the results will be reported in due course.

Acknowledgments

We thank Professors Francisco Delgado and Gerardo Zepeda for their help in spectrometric analyses, and Bruce A. Larsen for reviewing the use of English in the manuscript. J.T. acknowledges SIP/IPN (Grants 20110172, 20120830, and 20130686) and CONACYT (Grants 83446 and 178319) for financial support. R.B., P.A.M., and A.R. thank CONACYT for awarding them graduate scholarships, and SIP/IPN (PIFI) for scholarship complements. J.T. and E.B. are fellows of the EDI-IPN and COFAA-IPN programs.

Conflicts of Interest

The authors declare no conflict of interest.

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  • Sample Availability: Samples of the compounds 4ae, 5a, 5d, 5e, 1d, 1fg, 1hi, and 1k are available from the authors.
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