HETEROCYCLES

Paper | Regular issue | Vol. 78, No. 12, 2009, pp. 3011-3021
Received, 5th August, 2009, Accepted, 24th September, 2009, Published online, 25th September, 2009.
DOI: 10.3987/COM-09-11813

■ Benzylic Oxidation of N-Acyl-1,2,3,4-tetrahydroquinolines

Roxanne C. Higgins, Norman O. Townsend, and Yvette A. Jackson*

Department of Chemistry, Mona Campus, University of the West Indies, Mona, Kingston 7, Jamaica

Abstract

Chromium hexacarbonyl/tert-butyl hydroperoxide has been identified as a general reagent for the benzylic oxidation of N-acyl-1,2,3,4-tetrahydroquinolines.

INTRODUCTION
During the synthesis of compound 1, a nitrogen analogue of eleutherol (2)1 the benzylic oxidation of tetrahydroquinoline 3 was required. A review of the literature showed that except for the work of Bonvin et al. 2 and Bolm and Nakanishi,3 who investigated the bismuth catalyzed oxidation of N-acyl tetrahydroquinoline (4) and the iron-catalyzed benzylic oxidation of N-tosyl tetrahydroquinoline respectively, no oxidation studies have been reported for tetrahydroquinolines.

This led to the exploration of benzylic oxidation of compounds of this type using well known oxidants such as selenium dioxide, pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), chromium trioxide, ceric ammonium nitrate (CAN)4 and chromium hexacarbonyl (Cr(CO)6).5

RESULTS AND DISCUSSION
Benzylic oxidation of compounds 4 and 5, which were easily obtained from readily available quinoline (8) and quinaldine (9) by NaCNBH3/HOAc reduction and subsequent N-acylation with acetic anhydride (Scheme 1), was first investigated.

As shown in Table 1, the overall yields of the oxidation reactions were usually quite low. Use of CAN and SeO2 resulted in little or no conversion of starting material. Better results were obtained from the chromium oxide-derived oxidants. In fact, a slight increase in the yield and a decrease in reaction time were observed when tert-butyl hydroperoxide (t-BuO2H) was used as a co-oxidant.6

Pearson et al.5 investigated the oxidation of cyclic alkenes to enones and reported reasonable yields when Cr(CO)­­6 and t-BuO2H in refluxing acetonitrile was used. We attempted this reaction on substrates 4 and 5 with good results, obtaining yields of 65% and 90% respectively (Table 2). Percentage conversions were over 70%, whereas conversion was usually below 60% with use of other oxidants. We therefore synthesized compounds 3, 6, 7 and 17, and explored the use of Cr(CO)6 and t-BuO2H for their oxidation. Reissert reaction7-9 on quinoline (8) using the heterogenous phase modification10 produced compound 14. This was hydrolyzed using hydrobromic acid to give quinaldic acid hydrobromide (15)11 which on treatment with MeI and K2CO3 yielded methyl quinaldate (16), (Scheme 2).

Subsequent reduction with sodium cyanoborohydride (NaCNBH3) in acetic acid, followed by N-acetylation, with acetic anhydride gave compound 3 (55% over two steps). [Reduction of 16 with hydrogen and 10% Pd/C resulted in lower yields and even after 52 h the reaction had not gone to completion.] Hydrolysis of 3 using K2CO3 in MeOH provided acid 7. Amide 17 was obtained by coupling 2-bromoaniline with acid 7 using dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) in dichloromethane.

For the synthesis of compound 6, the Boger protocol12 was used to install the nitrile group at position 2 (Scheme 3). The heterocyclic ring of 18 was then reduced and acetylated utilizing the same methods as before, to produce 6.
The results obtained from oxidation of compounds 3, 6, 7 and 17 (Scheme 4) are shown in Table 2. The reaction of 7 was monitored by NMR-analysis as acids 7 and 21 are inseparable by column chromatography.

Requiring the 2-carboxamide of compound 19, we proceeded with hydrolysis of the ester using K2CO3 in refluxing MeOH, and obtained the corresponding deacylated carboxylic acid 23 in 80% yield (Scheme 5). Deacylation under these mild reaction conditions was quite unexpected. The keto group at position 4 may be facilitating a weakening of the N-acyl bond. This will be explored further.

Coupling of 23 with 2-bromoaniline using DCC and DMAP in dichloromethane resulted not in the desired amide but impure 24. The reaction was repeated in the absence of 2-bromoaniline and produced the novel diketopiperazine (DKP) 24 in about 40% yield. All attempts at purification of 24 were, however, unsuccessful.
When the reaction was done using 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) with triethylamine in acetonitrile or 1-ethyl-3-(3’-dimethylaminopropyl)- carbodiimide hydrochloride (EDCI) with DMAP in dichloromethane at room temperature for 4 h, DKP 24 was obtained in 33% and 45% yields respectively. The NMR spectra of starting material and product were quite similar. The product, however, was much less polar than the starting material (Rf 0.45 and 0.07 respectively in hexane: EtOAc - 1:1), had a much higher melting point (299-302 °C vs. 183-185 °C (sm)), and analyzed well for C20H14N2O4.
DKPs of general structure 25 occur widely in nature13 and exhibit a wide spectrum of biological properties, including antiviral,14 antifungal, antibacterial and cytotoxic activities with antithrombotic effects.14-17 DKPs have also shown affinities for calcium channels, serotoninergic 5-HT1A, GABAergic, oxytocin and DNA topoisomerase I receptors14-16,18,19 and have even been used as a drug delivery system for oral administration.20 These biological properties and their interesting chemical properties have made DKP substrates useful in medicinal chemistry and promising agents for drug development and potential lead compounds.14,18

We have thus identified a new general method for the benzylic oxidation of N-acyl-1,2,3,4- tetrahydroquinolines using chromium hexacarbonyl and tert-butyl hydroperoxide. This has led to the synthesis of novel DKP 24.

EXPERIMENTAL
Unless otherwise stated the following generalizations apply. All melting points are uncorrected. 1H-NMR and 13C-NMR spectra were determined in deuteriochloroform (CDCl3) solution on a 200 MHz or 500 MHz Bruker ACE 200 instruments. Resonances are in δ units (ppm) downfield from TMS. J values are given in Hz. Elemental analysis was carried out by MEDAC Ltd. All chromatography was carried out using silica as support.

General method for preparation of N-Acyl-1,2,3,4-tetrahydroquinolines
A solution of the quinoline (2.5 mmol) in glacial AcOH was cooled to below 30 °C and sodium cyanoborohydride (NaCNBH3) (3.0 mmol) added in small portions. The mixture was then allowed to stir at room temperature for 3 h, neutralized with saturated aqueous NaHCO3 then extracted with Et2O (3 × 15 mL). The combined extract was dried over Na2SO4 then concentrated under reduced pressure. The crude product was treated with Ac2O (10 mL) and the resulting solution heated at 90 – 100 °C for 4 h. After allowing the mixture to cool to room temperature it was diluted with water (50 mL) and solid NaHCO3 was added until no further evolution of CO2 was observed. The mixture was then extracted with Et2O, and the organic extract dried over Na2SO4 then evaporated under vacuum. The crude product was purified by flash column chromatography (hexane: EtOAc – 3:1) to give the product.
From quinoline: 1-Acetyl-1,2,3,4-tetrahydroquinoline (4),21 yellow oil, 82%.
From quinaldine: 1-Acetyl-2-methyl-1,2,3,4-tetrahydroquinoline (5),21 yellow oil, 82%.
From 16: Methyl 1-Acetyl-1,2,3,4-tetrahydroquinoline-2-carboxylate (3),1 yellow oil, 55%.

Quinaldic Acid Hydrobromide (15)
Reissert compound10 (14) (3.00 g, 11.6 mmol) was suspended in glacial AcOH (7.5 mL) and 49% aqueous HBr (3.5 mL, 63.2 mmol) was added. The mixture was heated at reflux for 30 min. On cooling, the monohydrate of quinaldic acid hydrobromide crystallized. The solid was collected by filtration and washed with glacial AcOH followed by Et2O, to give 15 as orange crystals (3.00 g, 100%): mp 225 – 230 °C [lit.,11 220 – 224 °C]; 1H-NMR (DMSO-d6) δ 7.20 (1H, s), 7.50 (1H, s, NH ), 7.81 (1H, t, J 8.4, 5-H), 7.94 (1H, t, J 8.4, 6-H), 8.05-8.25 (3H, m, 3,8,7-H), 8.56 (1H, d, J 8.5, 4-H); 13C-NMR δ 120.6, 126.6, 128.1, 129.0, 129.4, 132.3, 141.0, 143.6, 146.8, 164.8.

Methyl Quinaldate (16)
Quinaldic acid hydrobromide 15, (2.00 g, 7.85 mmol) was dissolved in acetone/water (80 mL: 8 mL) and K2CO3 (6.09 g, 44.1 mmol) and MeI (3 mL) added. The mixture was heated at reflux for 2 h, filtered hot and concentrated under reduced pressure. The product was extracted with EtOAc (3 × 20 mL), dried over Na2SO4 and concentrated to give the crude methyl quinaldate (1.25 g, 6.68 mmol) which was recrystallized from aqueous MeOH to give compound 16 as white crystals (0.86 g, 59%); mp 79-80 °C [lit.,22 79 °C].

1-Acetyl-1,2,3,4-tetrahydroquinoline-2-carboxylic acid (7)
Methyl 1-acetyl-1,2,3,4-tetrahydroquinoline-2-carboxylate1 (3) (600 mg, 2.5 mmol) was dissolved in MeOH (15 mL) and K2CO3 added. The mixture was heated at reflux for 2 h, concentrated under reduced pressure and the residue diluted with water (20 mL). The resulting solution was acidified with dilute HCl (5% v/v) until effervescence ceased, and extracted with EtOAc (3 × 15 mL). The combined organic layer was concentrated under reduced pressure to yield 723 (546 mg, 100%).

1-Acetyl-1,2,3,4-tetrahydroquinoline-2-carbonitrile (6)
Quinoline-2-carbonitrile 1812 (900 mg, 6.42 mmol) was dissolved in glacial HOAc (10 mL) and the solution cooled in an ice-water bath until it began to solidify. NaCNBH3 (610 mg, 9.7 mmol) was added to the mixture over 2 min. and the whole stirred at room temperature for 2 h. The mixture was neutralized with saturated aqueous NaHCO3 (20 mL) then extracted with EtOAc (3 × 10 mL). The combined extract was washed with water (2 × 5 mL) dried over Na2SO4 then evaporated in vacuo. The residue was purified by column chromatography (hexane: EtOAc – 4:1) to give 1,2,3,4-tetrahydroquinoline-2-carbonitrile 1812 (620 mg, 60%). The product (620 mg, 3.92 mmol) was then dissolved in Ac2O (10 mL) and the solution heated at 90 °C for 3 h. The mixture was allowed to cool to room temperature, neutralized with saturated aqueous NaHCO3 solution then extracted with EtOAc (3 × 10 mL). The combined organic extract was washed with water (3 × 10 mL) then dried over anhydrous Na2SO4. After removing the solvent the crude product was purified by column chromatography (hexane: EtOAc – 2:1) to give 624 as a brown oil (720 mg, 92%; 55% overall yield).

1-Acetyl-N-(2-bromophenyl)-1,2,3,4-tetrahydroquinoline-2-carboxamide (17)
1-Acetyl-1,2,3,4-tetrahydroquinoline-2-carboxylic acid (7) (250 mg, 1.14 mmol) was dissolved in CH­2Cl2 (20 mL), and DCC (0.21 g, 1.20 mmol), DMAP (0.01 g, 0.1 mmol) and 2-bromoaniline (210 mg, 1.20 mmol) added. The mixture was stirred at room temperature overnight. The mixture was concentrated and the crude product was purified by column chromatography (hexane: EtOAc - 3:1) to give 17 as a viscous orange oil (0.29 g, 66%); 1H-NMR 2.18 (4H, m, -CH3, 3-Ha), 2.43 (2H, m, 3-H b, 4-Ha ), 2.77 (1H, m, 4-Hb), 5.40 (1H, t, J 9.7 2-H), 6.92 (1H, t, 6-H) 7.20 (5H, m, 5,7-H and 3,4,5-H), 7.49 (1H, d, 6-H), 8.28 (1H, d, H-8), 8.89 (1H, s, NH); 13C-NMR δ 22.8, 26.0, 27.0, 56.9, 113.7,122.1, 125.2, 125.6, 126.9, 127.8, 128.2, 132.3, 135.7, 137.0, 169.2, 171.9.

General Procedure for Oxidation using Pyridinium Dichromate (PDC)
N-Acetyl-1,2,3,4-tetrahydroquinoline (3.14 mmol) was dissolved in CH2Cl2 (15 mL). PDC (130 mg, 0.345 mmol) and 70% t-BuO2H (6 mL, 46.4 mmol) were added to the solution and the resulting mixture heated at reflux for 16 h. The solvent was removed in vacuo and the residue purified by column chromatography (hexane: EtOAc - 3:1) to give the product.
From 4: 1-Acetyl-4-oxo-1,2,3,4-tetrahydroquinoline25 (12), viscous oil that solidified on standing, 37 %;
From 5: 1-Acetyl-2-methyl-4-oxo-1,2,3,4-tetrahydroquinoline (13), cream solid, mp 120-122 °C, 36%; 1H-NMR δ 1.25 (3H, d, J 8.4, -CH3), 2.34 (3H, s, -COCH3), 2.60 (1H, d, J 15.7, 3-Ha), 3.02 (1H, dd, J 15.7 & 8.4, 3-Hb), 5.38 (1H, m, 2-H), 7.29 (1H, t, J 9.7, 6-H), 7.38 (1H, m, 5-H) , 7.57 (1H, t, J 9.7, 7-H), 8.02 (1H, d, J 9.7, 8-H); 13C-NMR δ 17.9, 23.5, 45.1, 48.5, 125.2, 125.4, 125.6, 127.3, 134.3, 141.3, 169.4, 193.5. Anal. Calcd For C12H13NO2: C, 70.92; H, 6.45; N, 6.89 %. Found C, 70.79; H, 6.63; N, 7.00%.

General Procedure for Oxidation using Selenium Dioxide
Selenium dioxide (150 mg, 1.35 mmol) and 90% t--B t-BuO2H (1 mL) were added to a round bottom flask containing CH2Cl2 (3 mL). The mixture was stirred at room temperature for 30 min. A solution of N-acetyl-1,2,3,4-tetrahydroquinoline (2.28 mmol) in CH2Cl2 (9 mL) was added dropwise to the reaction mixture and the whole stirred at room temperature for 24 h. The filtrate was evaporated under reduced pressure and the crude product purified by column chromatography (hexane: EtOAc - 3:1).
From 4: 1-Acetyl-4-oxo-1,2,3,4-tetrahydroquinoline25 (12), 7%;
From 5: 1-Acetyl-2-methyl-4-oxo-1,2,3,4-tetrahydroquinoline (13), 6%.

General Procedure for Oxidation using Pyridinium Chlorochromate (PCC)
Celite (2.2 g) was heated at 140 ºC for 2 h then cooled. Dry benzene (25 mL) was added followed by PCC (2.0 g, 9.27 mmol) and the mixture stirred for 15 min. A solution of N-acetyl-1,2,3,4-tetrahydroquinoline (2.28 mmol) in dry benzene (25 mL) was added and the mixture heated at reflux for 12 h. Upon cooling the reaction mixture was filtered through a pad of celite and the bed washed with benzene. The combined organic extracts were washed with water (15 mL) and brine (15 mL) then dried over Na2SO4. The solvent was removed in vacuo and the crude product purified by column chromatography (hexane: EtOAc - 3:1).
From 4: 1-Acetyl-4-oxo-1,2,3,4-tetrahydroquinoline25 (12), 30%.
From 5: 1-Acetyl-2-methyl-4-oxo-1,2,3,4-tetrahydroquinoline (13), 30%.

Using PCC/ t-BuO2H
Repeating the reaction but adding 70% t-BuO2H (3 mL) after the tetrahydroquinoline.
From 4: 1-Acetyl-4-oxo-1,2,3,4-tetrahydroquinoline25 (12), 40%.
From 5: 1-Acetyl-2-methyl-4-oxo-1,2,3,4-tetrahydroquinoline (13), 38%.

General Procedure for Oxidation using Chromium Trioxide
A solution of chromium trioxide (1.15 g, 11.5 mmol) in AcOH (2.5 mL) and water (0.5 mL) was added dropwise to a cooled (~10 °C) solution of N-acetyl-1,2,3,4-tetrahydroquinoline (2.85 mmol) in AcOH (2.0 mL). The reaction mixture was warmed to room temperature, stirred for 8 h, and cautiously quenched with saturated aqueous NaHCO3 solution, and extracted with Et2O. The combined organic layers were dried and concentrated to leave a residue, which was purified by column chromatography on silica gel (EtOAc: hexane - 1:1).
From 4: 1-Acetyl-4-oxo-1,2,3,4-tetrahydroquinoline25 (12), 30%.
From 5: 1-Acetyl-2-methyl-4-oxo-1,2,3,4-tetrahydroquinoline (13), 25%.

General Procedure for Oxidation using Chromium Hexacarbonyl / t-Butyl Hydroperoxide
N-Acetyl-1,2,3,4-tetrahydroquinoline (4.50 mmol), Cr(CO)6 (0.54 g, 2.34 mmol) and t-BuO2H (70 wt% in water, 6.0 mL) were added to a round bottom flask containing MeCN (30.0 mL). The mixture was heated at reflux for 48 h, then cooled to room temperature, filtered and concentrated under reduced pressure. The residue was purified by column chromatography (hexane: EtOAc - 3:1) to give the product. (Yields below are based on recovered starting material. See Table 2.)
From 4: 1-Acetyl-4-oxo-1,2,3,4-tetrahydroquinoline25 (12), 65%.
From 5: 1-Acetyl-2-methyl-4-oxo-1,2,3,4-tetrahydroquinoline (13), 90%.
From 3: Methyl 1-acetyl-4-oxo-1,2,3,4-tetrahydroquinoline-2-carboxylate (19), a white crystalline solid (0.66 g, 60%), mp 119-120 °C, [lit.,1 120-122 °C];
From 6: 1-Acetyl-4-oxo-1,2,3,4-tetrahydroquinoline-2-carbonitrile (20), a white solid (50%), mp 123-124 °C, 1H-NMR 2.36 (3H, s, -CH3), 3.08 (2H, m, 3-Ha and b), 6.44 (1H, m, 2-H), 7.39 (2H, m, 5,6-H), 7.68 (1H, m, 7-H), 8.08 (1H, d, H-8); 13C-NMR δ 22.6, 42.2, 43.3, 116.9, 124.7, 125.4, 127.3, 128.3, 135.3, 140.5, 168.8, 189.5. Anal. Calcd For C12H10N2O2: C, 67.28; H, 4.70; N, 13.07 %. Found C, 67.64; H, 4.80; N, 13.14 %.
From 18: 1-Acetyl-N-(2-bromophenyl)-4-oxo-1,2,3,4-tetrahydroquinoline-2-carboxamide (22), a white solid (50%), mp 181-182 °C, 1H-NMR 2.51 (3H, s, -CH3), 2.99 (1H, dd, J 16.9 and 8.4, 3-Ha), 3.47 (1H, d, J 16.9, 3-Hb), 6.03 (1H, d, J 8.4 2-H), 6.92 (1H, t, 6-H) 7.28 (3H, m, 7-H and 4,5-H), 7.55 (2H, m, 7-H and 3-H), 8.04 (2H, m, H-8 and 6-H), 8.44 (1H, s, NH); 13C-NMR δ 23.1, 39.0, 56.0, 121.7, 124.0, 125.5, 126.5, 128.2, 128.4, 132.3, 134.5, 135.0, 140.3, 167.1, 171.4, 191.3. Anal. Calcd For C18H15N2O3Br: C, 55.83; H, 3.90; N, 7.23%. Found C, 55.88; H, 3.66; N, 7.20 %.

4-Oxo-1,2,3,4-tetrahydroquinoline-2-carboxylic acid (23)
Methyl 1-acetyl-4-oxo-1,2,3,4-tetrahydroquinoline-2-carboxylate (19) (0.51 g, 2.05 mmol) was added to a round bottom flask containing MeOH (15 mL). K2CO3 (0.57 g, 1.34 mmol) was then added and the mixture heated at reflux for 4 h. MeOH was removed in vacuo, then water (5.0 mL) was added to the residue. Dilute HCl was added dropwise until effervescence ceased. The mixture was extracted with EtOAc (3 × 15 mL), dried over anhydrous Na2SO4 and the solvent concentrated under reduced pressure to give a yellow solid. The residue was triturated with CH2Cl2 to give 23, a yellow powdery solid (80%); mp 183-185 °C, [lit.,26 183-184 °C].

Coupling Reactions
5,6,6a,7,13,14,14a,15-Octahydrodiquino[1,2-a:1′,2′-d]pyrazine-5,7,13,15-tetraone (24)
i) Using Dicyclohexylcarbodiimide (DCC)/4-Dimethylaminopyridine (DMAP)
4-Oxo-1,2,3,4-tetrahydroquinoline-2-carboxylic acid (23) (100 mg, 0.52 mmol) was dissolved in CH2Cl2 (10 mL), and DCC (70 mg, 0.39 mmol) and DMAP (4 mg, 0.04 mmol) added. The mixture was stirred at room temperature overnight. The product was isolated by filtering and concentrating the filtrate under reduced pressure. The crude product was purified using column chromatography (hexane: EtOAc - 1:1). An impure sample of compound 24 was obtained as a cream solid (41 mg).

ii) Using 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU)
4-Oxo-1,2,3,4-tetrahydroquinoline-2-carboxylic acid (23) (100 mg, 0.53 mmol) was dissolved in MeCN (1.5 mL). Triethylamine (110 mg, 0.11 mmol) and HBTU (200 mg, 0.53 mmol) were then added and the mixture stirred at room temperature for 4 h. Compound 24, (30 mg, 33%) precipitated as a white solid, mp 299-302 °C. 1H-NMR (DMSO, d6) δ 3.12 (2H, dd, J 17.5 and 3.0, H-14a, 6a), 3.59 (2H, dd, J 17.5 and 3.8, H-14b, 6b), 5.04 (2H, dd, J 13.5 and 3.1, H-6a,14a), 7.40 (2H, t, J 7.5, H-3,10), 7.70 (2H, t, J 7.3, H-2,10), 7.98 (4H, m, H-1,4,9,12). 13C-NMR δ 57.3, 124.8, 125.7, 126.6, 134.0, 141.1, 162.3, 191.7. Anal. Calcd For C20H14N2O4·½ H2O: C, 67.60; H, 4.22; N, 7.89%. Found C, 67.46; H, 3.92; N, 7.73%.

iii) Using 1-ethyl-3-(3′-dimethylaminopropyl)carbodiimide hydrochloride (EDCI)
4-Oxo-1,2,3,4-tetrahydroquinoline-2-carboxylic acid (23) (100 mg, 0.52 mmol) was dissolved in CH2Cl2 (5.0 mL), and EDCI (330 mg, 0.17 mmol) and DMAP (10 mg, 0.08 mmol) added. The mixture was stirred at room temperature for 4 h. The solution was diluted with EtOAc (10.0 mL), washed with brine, and the organic layer concentrated under reduced pressure. The crude product was purified using column chromatography (hexane: EtOAc - 1:1). Compound 24 was obtained as a cream solid (40 mg, 45%).

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