Research Article Cite This: ACS Comb. Sci. 2017, 19, 708-713
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Microwave-Assisted Regioselective Synthesis of 3‑Functionalized Indole Derivatives via Three-Component Domino Reaction Wei Lin,†,‡ Yong-Xiang Zheng,† Zhan Xun,† Zhi-Bin Huang,*,† and Da-Qing Shi*,† †
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China School of Chemistry and Environmental Engineering, Jiangsu University of Technology, Changzhou 213001, P. R. China
‡
S Supporting Information *
ABSTRACT: A microwave-assisted regioselective synthesis of 3-functionalized indole derivatives via a three-component domino reaction of anilines, arylglyoxal monohydrates, and cyclic 1,3-dicarbonyl compounds is described. The main advantages of this protocol are short reaction times, practical simplicity, its metal-free nature, the availability of starting materials, green solvents, and high regioselectivity. KEYWORDS: indole, three-component reaction, microwave-assisted
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INTRODUCTION Indole motifs are present in many natural products and biologically active compounds.1 Tryptophan is a significant indole derivative, while serotonin and melatonin are biochemically active indole molecules. Many indole derivatives have important biological activities, including anticancer,2 antioxidant,3 antirheumatoidal,4 anti-HIV,5 antiproliferative,6 antineoplastic, and antiestrogenic activities.7 In addition, various 3substituted indoles have been used as starting materials for the synthesis of a number of alkaloids, pharmaceuticals, and agrochemicals. Over the last hundred years, many powerful approaches have been developed for the construction of the indole moiety,8 including the classical Fischer indole synthesis.9 Recently, transition-metal-catalyzed cross-dehydrogenative coupling reactions,10 directing-group-assisted intermolecular C−H annulation,11 and other methods have been emerging as powerful tools for the synthesis of this versatile molecule.12 However, there is an enormous need to develop new and efficient methods for the synthesis of indoles and their functionalized derivatives using readily available starting materials. The development of environmentally friendly synthetic methods is a challenge in modern organic synthesis. Multicomponent reactions (MCRs) are promising powerful tools in organic,13 combinatorial, and medical chemistry on account of their atom economy, high complexity and diversity of products, multiple bond formation efficiency, and environmental friendliness.14 These processes avoid the isolation and purification of intermediates, maximize the yield of the final product, minimize solvent waste, and enhance the greenness of the transformations. Consequently, MCRs have been significantly used for the construction of complex heterocycles and natural products.15 Recently, our group has developed a series of new MCRs that offer convenient and efficient access to some heterocycles of chemical and pharmaceutical interest using arylglyoxals as a reaction partner.16 In this study, we found that when anilines (1), arylglyoxal monohydrates (2), and cyclic 1,3-dicarbonyl © 2017 American Chemical Society
compounds (3) in a mixture of ethanol and water were subjected to microwave irradiation (MWI), the reaction underwent a regioselective three-component domino reaction to give 3functionalized indole derivatives (4) without observation of hydroquinolines (5) (Scheme 1).17 Herein we report these useful transformations.
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RESULTS AND DISCUSSION We initially selected the three-component domino reaction of 4chloroaniline (1{1}), phenylglyoxal monohydrate (2{1}), and 4hydroxy-6-methyl-2H-pyran-2-one (3{1}) as the model reaction for optimizing the reaction conditions. The reaction was conducted using a 1:1:1 mixture of 1{1}, 2{1}, and 3{1} under microwave irradiation and different conditions. Table 1 summarizes the results. When the reaction was carried out in ethanol at 100 °C for 30 min under microwave irradiation and catalyst-free conditions, the desired product 3-(6-chloro-2phenyl-1H-indol-3-yl)-4-hydroxy-6-methyl-2H-pyran-2-one (4{1,1,1}) was obtained in 44% yield without observation of the corresponding hydroquinoline 5{1,1,1} (Table 1, entry 1). Various solvents were evaluated to determine the impact of the solvent on the yield. Of all the solvents tested−ethanol, acetonitrile, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), toluene, acetic acid, and water−ethanol gave the best result (Table 1, entries 1−8). Water is a greener solvent; therefore, we evaluated the effect on the yield of mixing ethanol and water in different ratios. Screening experiments showed that a 1:1 (v/v) mixture of ethanol and water was the best solvent for this particular transformation (Table 1, entries 9−11). To improve the yield, several catalysts were evaluated: sodium carbonate, ferric chloride, acetic acid, trifluoroacetic acid (TFA), p-toluenesulfonic acid (p-TSA) and LReceived: August 18, 2017 Revised: September 29, 2017 Published: October 6, 2017 708
DOI: 10.1021/acscombsci.7b00126 ACS Comb. Sci. 2017, 19, 708−713
Research Article
ACS Combinatorial Science Scheme 1. Synthesis of Functionalized Indoles under Microwave Irradiation
a Reaction conditions: 1{1} (1 mmol), 2{1} (1 mmol), 3{1} (1 mmol), and solvent (4 mL). bYields were determined by HPLC−MS.
transformation was 90 °C. Finally, the reaction was performed at different reaction times to determine the optimum reaction time. The results showed that the best reaction time was 40 min (Table 1, entries 15 and 23−26). On the basis of all of these experiments, the optimum reaction conditions were identified as 1:1 (v/v) ethanol/water at 90 °C for 40 min under microwave irradiation catalyzed by TFA. With the optimal reaction conditions in hand, the substrate scope of the transformation was then evaluated using 16 substituted anilines 1, five arylglyoxal monohydrates 2, and four cyclic 1,3-dicarbonyl compounds 3 (4-hydroxy-6-methyl2H-pyran-2-one, tetronic acid, 4-hydroxycoumarin, and dimedione) (Figure 1). The corresponding 3-functionalized indoles 4 were obtained, and the results are summarized in Table 2. As displayed in Table 2, the anilines and arylglyoxals bearing either electron-withdrawing or electron-donating groups were tolerated under the reaction conditions, leading to the final products in satisfactory yields. However, when the 2-substituted anilines (Table 2, 4{12,1,1} and 4{13,1,1}) were used, the yields of the products were very low (27% and 38%, respectively). This fact indicated that steric hindrance of the aniline can affect the yield in this transformation. Recently, Bhuyan’s group reported the synthesis of 3-aryl-substituted indoles via a three-component reaction of arylamines, arylglyoxals, and cyclic 1,3-dicarbonyl compounds catalyzed by PTSA.18 This method provided an efficient and regioselective synthesis of 2-aryl-substituted indoles. The structures of the products synthesized in the current study were identified using IR, 1H NMR, and 13C NMR spectroscopies as well as HRMS analysis. The structure of compound 4{1,1,1} was confirmed using single-crystal X-ray diffraction analysis (Figure 2). On the basis of our results, we propose the reaction mechanism shown in Scheme 2 for this novel three-component reaction. Intermediate A is obtained through Knoevenagel condensation of arylglyoxal 2 with 4-hydroxy-6-methyl-2Hpyran-2-one (3) catalyzed by TFA. The subsequent Michael addition of aniline 1 to A gives intermediate B, which undergoes an intramolecular nucleophilic addition reaction followed by loss of water to form the product 4.
proline (Table 1, entries 12−17). The results revealed that the catalytic efficiency of TFA was the highest. The reaction was then conducted at different temperatures, such as 60, 70, 80, 90, 100, and 110 °C, to determine the optimum temperature for the transformation. All of these experiments were conducted in 1:1 (v/v) ethanol/water under microwave irradiation and catalyzed by TFA, and the desired product 4{1,1,1} was obtained in yields of 27%, 37%, 48%, 69%, 66%, and 65%, respectively (Table 1, entries 15 and 18−22). Accordingly, the best temperature for this
CONCLUSION We have developed a clean and efficient protocol for the construction of 3-functionalized indole derivatives via a novel three-component domino reaction using anilines, arylglyoxal monohydrates, and cyclic 1,3-dicarbonyl compounds as starting materials. This protocol has the advantages of mild reaction conditions, short reaction times, convenient operation, metalfree nature, cheap and green solvents, high regioselectivity, and environmental friendliness.
Table 1. Optimization of the Reaction Conditions for the Synthesis of Compound 4{1,1,1}a
entry
solvent
catalyst (20 mol %)
T (°C)
time (min)
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
EtOH CH3CN DMSO DMF THF toluene HOAc H2O EtOH/H2O(1:1) EtOH/H2O (2:1) EtOH/H2O (1:2) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1) EtOH/H2O (1:1)
− − − − − − − − − − − Na2CO3 FeCl3 HOAc TFA p-TSA L-proline TFA TFA TFA TFA TFA TFA TFA TFA TFA
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 60 70 80 90 110 90 90 90 90
30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 10 20 40 50
44 32 12 trace 14 30 43 19 47 44 41 33 39 62 66 42 47 27 37 48 69 65 56 66 74 70
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DOI: 10.1021/acscombsci.7b00126 ACS Comb. Sci. 2017, 19, 708−713
Research Article
ACS Combinatorial Science
Figure 1. Substrates used in the synthesis of compounds 4.
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3-(6-Chloro-2-phenyl-1H-indol-3-yl)-4-hydroxy-6-methyl2H-pyran-2-one (4{1,1,1}). White solid; IR (KBr, ν, cm−1) 3292, 3063, 2921, 1667, 1624, 1571, 1446, 1238, 1217, 992, 921, 786, 692; 1H NMR (400 MHz, DMSO-d6) δ (ppm) 11.62 (s, 1H, NH), 11.10 (s, 1H, OH), 7.58 (d, J = 7.6 Hz, 2H, ArH), 7.44− 7.40 (m, 3H, ArH), 7.30 (t, J = 7.2 Hz, 1H, ArH), 7.16 (d, J = 8.4 Hz, 1H, ArH), 7.00−6.97 (m, 1H, ArH), 6.12 (s, 1H, ArH), 2.25 (s, 3H, CH3); 13C NMR (75 MHz, DMSO-d6) δ (ppm) 167.2, 163.7, 161.6, 136.5, 136.4, 132.8, 128.6, 127.8, 127.5, 126.5, 125.9, 120.6, 119.2, 110.6, 103.6, 100.1, 95.2, 19.5; HRMS (ESI) calcd for C20H15ClNO3 [M + H]+ 352.0740, found 352.0749. 4-Hydroxy-3-(5-methyl-2-phenyl-1H-indol-3-yl)furan2(5H)-one (4{2,1,2}). White solid; IR (KBr, ν, cm−1) 3418, 1719, 1610, 1479, 1453, 1431, 1076, 1052, 1029, 800, 774, 739, 696; 1H NMR (400 MHz, DMSO-d6) δ (ppm) 11.95 (s, 1H, OH), 11.39 (s, 1H, NH), 7.65 (d, J = 7.6 Hz, 2H, ArH), 7.42 (t, J = 7.6 Hz, 2H, ArH), 7.30 (t, J = 8.4 Hz, 2H, ArH), 7.09 (s, 1H, ArH), 6.96 (d, J = 8.4 Hz, 1H, ArH), 4.85 (s, 2H, CH2), 2.37 (s, 3H, CH3); 13 C NMR (100 MHz, DMSO-d6) δ (ppm) 175.3, 173.6, 135.7, 134.4, 133.0, 129.2, 128.5, 127.5, 127.2, 126.6, 123.4, 118.9, 110.9, 99.9, 94.9, 67.0, 21.3; HRMS (ESI) calcd for C19H15NaNO3 [M + Na]+ 328.0950, found 328.0940. 3-(5-Chloro-2-phenyl-1H-indol-3-yl)-2-hydroxy-4H-chromen-4-one (4{1,1,3}). Yellow solid; IR (KBr, ν, cm−1) 3440, 3424, 3283, 1689, 1618, 1495, 1469, 1457, 1414, 1251, 1230,
EXPERIMENTAL PROCEDURES Instrumentation. Melting points were measured using an XT-5 micro melting point apparatus and are uncorrected. IR spectra were recorded with a Varian F-1000 spectrometer using KBr disks; absorptions are reported in cm−1. 1H and 13C NMR spectra were obtained in DMSO-d6 solution using a Varian Inova 400 or 300 MHz spectrometer. J values are reported in hertz, and chemical shifts are expressed in parts per million downfield from tetramethylsilane as the internal standard. HRMS analyses were carried out using a Bruker micrOTOF-QII instrument. General Procedure for the Synthesis of Functionalized Indole Derivatives 4. Aniline 1 (1 mmol), arylglyoxal monohydrate 2 (1 mmol), cyclic 1,3-dicarbonyl compound 3 (1 mmol) and CF3CO2H (0.2 mmol) were placed in a 10 mL initiator reactor vial, followed by ethanol (2 mL) and water (2 mL). The reaction vial was then sealed, prestirred for 10 s, and microwave-irradiated (time, 40 min; temperature, 90 °C; absorption leave, high; fixed hold time) until thin-layer chromatography (3:1 mixture of petroleum ether and ethyl acetate) revealed the complete consumption of the starting materials. The reaction mixture was then cooled to room temperature. The solvent was evaporated under reduced pressure, and the crude product was purified by column chromatography using petroleum ether and ethyl acetate as the eluent, giving pure compound 4. 710
DOI: 10.1021/acscombsci.7b00126 ACS Comb. Sci. 2017, 19, 708−713
Research Article
ACS Combinatorial Science Table 2. Synthesis of Functionalized Indole Derivatives 4
entry
product
isolated yield (%)
mp (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
4{1,1,1} 4{2,1,1} 4{3,1,1} 4{4,1,1} 4{5,1,1} 4{6,1,1} 4{8,1,1} 4{9,1,1} 4{10,1,1} 4{11,1,1} 4{12,1,1} 4{13,1,1} 4{14,1,1} 4{15,1,1} 4{16,1,1} 4{1,2,1} 4{2,2,1} 4{3,2,1} 4{4,2,1} 4{5,2,1} 4{6,2,1} 4{7,2,1} 4{9,2,1} 4{14,2,1} 4{15,2,1} 4{3,3,1} 4{4,3,1} 4{5,3,1} 4{7,3,1} 4{1,4,1} 4{3,4,1} 4{4,4,1} 4{5,4,1} 4{6,4,1} 4{7,4,1} 4{4,5,1} 4{5,5,1} 4{2,1,2} 4{4,1,2} 4{5,1,2} 4{16,1,2} 4{4,2,2} 4{5,2,2} 4{16,2,2} 4{1,1,3} 4{4,1,3} 4{3,2,3} 4{1,4,3} 4{14,4,3} 4{2,1,4} 4{4,1,4}
72 60 51 72 81 76 57 64 66 72 27 38 80 71 69 81 62 58 76 84 72 73 66 89 86 64 65 72 62 73 62 71 74 67 71 66 72 57 71 65 72 69 64 78 57 75 65 74 74 72 67
275−277 261−263 276−278 259−261 218−220 210−212 236−238 >300 275−277 285−287 214−216 210−212 288−290 276−278 247−249 293−295 266−268 256−258 248−250 >300 272−274 227−229 280−282 290−292 281−283 218−220 228−230 >300 231−233 >300 266−268 234−236 >300 282−284 234−236 278−280 >300 276−278 250−252 280−282 244−246 268−270 266−268 280−282 235−236 174−175 249−250 >300 265−266 225−227 214−216
1155, 1132, 969, 799, 753, 695; 1H NMR (400 MHz, DMSO-d6) δ (ppm) 11.83 (s, 1H, OH), 11.03 (s, 1H, NH), 7.91 (d, J = 8.0
Hz, 1H, ArH), 7.67 (t, J = 7.2 Hz, 1H, ArH), 7.61 (d, J = 7.6 Hz, 2H, ArH), 7.48−7.36 (m, 5H, ArH), 7.32−7.26 (m, 2H, ArH), 711
DOI: 10.1021/acscombsci.7b00126 ACS Comb. Sci. 2017, 19, 708−713
Research Article
ACS Combinatorial Science Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support from the Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (15KJA150006), a project fund from the Priority Academic Project Development of the Jiangsu Higher Education Institutions, and the Key Laboratory of Organic Synthesis of Jiangsu Province (JSK1210).
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Scheme 2. Proposed Reaction Mechanism for the Formation of 4
7.16−7.14 (m, 1H, ArH); 13C NMR (100 MHz, DMSO-d6) δ (ppm) 162.2, 161.8, 152.8, 138.4, 134.8, 132.4, 132.2, 130.4, 128.7, 127.8, 126.8, 123.9, 123.8, 123.7, 121.5, 116.3, 102.1, 98.6; HRMS (ESI) calcd for C23H14ClNNaO3 [M + Na]+ 410.0547, found 410.0560. 3-Hydroxy-5,5-dimethyl-2-(5-methyl-2-phenyl-1H-indol-3yl)cyclohex-2-enone (4{2,1,4}). White solid; 1H NMR (400 MHz, DMSO-d6) δ (ppm) 11.13 (s, 1H, OH), 10.00 (s, 1H, NH), 7.57 (d, J = 7.5 Hz, 2H, ArH), 7.36 (t, J = 7.7 Hz, 2H, ArH), 7.24 (t, J = 7.7 Hz, 2H, ArH), 6.93−6.82 (m, 2H, ArH), 2.33 (s, 7H, CH3 + 2 × CH2), 1.17 (s, 3H, CH3), 1.12 (s, 3H, CH3); 13C NMR (100 MHz, DMSO-d6) δ (ppm) 135.2, 134.6, 133.6, 129.9, 128.2, 126.7, 126.5, 122.7, 118.8, 110.7, 108.8, 104.6, 31.6, 28.4, 28.0, 21.3; HRMS (ESI) calcd for C23H23NO2 [M + H]+ 346.1807, found 346.1809.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscombsci.7b00126. Experimental details and spectroscopic characterization of 4 (PDF) Crystallographic data for 4{1,1,1} (CIF)
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REFERENCES
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Figure 2. Crystal structure of compound 4{1,1,1}.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Da-Qing Shi: 0000-0003-2262-8491 712
DOI: 10.1021/acscombsci.7b00126 ACS Comb. Sci. 2017, 19, 708−713
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on October 18, 2017 with an error in Figure 1. The corrected version was reposted on November 2, 2017.
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DOI: 10.1021/acscombsci.7b00126 ACS Comb. Sci. 2017, 19, 708−713
