Synthesis of 7-cyanoindolizine derivatives via a tandem reaction
-
Xian-Sheng Zhang
and
Jianwu Wang
Abstract:
A series of 2-substituted and 2,3-disubstituted 5-cyanoindolizine derivatives were conveniently synthesized by a one-pot tandem reaction under mild conditions in moderate yields. The reaction mechanism was proposed.
Indolizine (pyrrolo[1,2-a]pyridine), one of the most important N-fused heterocycles, plays key roles in medicinal and materials chemistry [1], [2], [3]. The partially or more often wholly hydrogenated derivatives of indolizine can be frequently found as backbones in many bioactive natural products such as swainsonine or cryptowoline [4], [5]. Synthetic indolizines are potential inhibitors of vascular endothelial growth factor (VEGF), calcium entry blockers, potential central nervous system depressants, 5-HT3 receptor antagonists, phosphodiesterase V inhibitors, histamine H3 receptor antagonists, cardiovascular agents, and PLA2 inhibitors [6], [7], [8], [9], [10], [11], [12], [13]. They have also drawn much attention owing to their possible usage as dyes for dye-sensitized solar cells (DSSC) or organic light-emitting devices (OLEDs) [14], [15], [16], [17], [18], [19], [20], [21], [22].
During the last decade, many methods for the synthesis of indolizines had been explored [1], [2], [3]. Most strategies are still based on pyridine derivatives as starting materials because the pyridines are easily available via synthetic methods and more than 2000 of them are commercial products. In principle, the alternative approach toward assembling the indolizine core starting from a pyrrole derivative and building a six-membered ring is simply a mirror image of the previous concept. However, such a synthesis has been a challenge to realize. Numerous difficulties, including significantly fewer commercially available pyrrole derivatives, their low oxidation potential and, hence, their stability, are reflected in an overall smaller number of such synthetic strategies.
Cyano-substituted indoles are key building units embedded in lead compounds currently being developed as estrogen receptor ligands, hepatitis C virus inhibitors, or therapeutic agents for cardiovascular diseases [23], [24], [25]. Thus, the synthesis of compounds containing a cyano group is important. Previously, we reported a novel tandem reaction of α,β-unsaturated esters with aldehydes to synthesize indolizine, imidazo[1,2-a]pyridine, imidazo[1,5-a]pyridine, and pyrido[1,2-a]benzimidazole derivatives [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. Herein, we expand the tandem reaction to synthesize 2-substituted and 2,3-disubstituted 7-cyanoindolizines under mild conditions.
4,5-Disubstituted 1H-pyrrole-2-carbaldehydes 1a–j were prepared per a literature method (Scheme 1) [28]. The desired 2-substituted and 2,3-disubstituted 7-cyanoindolizines 3a–j were obtained by the reaction of pyrrole-2-carbaldehydes 1a–j and 4-bromobut-2-enenitrile 2 in the presence of K2CO3 in dry DMF at 70°C for 6–10 h. A variety of products 3a–j were obtained in good yields starting with substituted pyrrole-2-carbaldehydes 1a–j (Scheme 1). The structures of products 3 were characterized by 1H NMR, 13C NMR, IR and HR-MS.
The suggested mechanism is shown in Scheme 2. First, in the presence of base B−, an intermolecular displacement reaction occurs between pyrrole-2-carbaldehyde 1 and 4-bromobut-2-enenitrile 2. Then, deprotonation of the resultant adduct 4 followed by cyclization of the anion 5 generate intermediate product 6, which is a direct precursor to the final indolizine 3.
Experimental
All reagents and solvents were purchased from Sinopharm Chemical Reagent Co. Ltd. Thin-layer chromatography (TLC) was conducted on silica gel GF254 plates (Merck KGaA). 1H NMR spectra (300 MHz) and 13C NMR spectra (75 MHz) were recorded on a Bruker Avance 300 spectrometer, using CDCl3 or DMSO-d6 as solvents and tetramethylsilane (TMS) as an internal standard. IR spectra were recorded with an Avtar 370 FT-IR spectrophotometer (Termo Nicolet). Elemental analyses were performed on a Vario EL III (Elementar Analysensysteme GmbH) analyzer. Mass spectra were recorded on a Trace DSQ mass spectrometer. The high resolution mass spectra (HRMS) were recorded on an Agilent Q-TOF6510 spectrometer. Pyrrole-2-carbaldehydes 1a–j were prepared per the literature [36], [37].
General procedure for the synthesis of 3a–j
To a 100-mL round-bottom flask were added 1a–j (6.0 mmol), enoate 2 (7.2 mmol), potassium carbonate (1.60 g, 12.5 mmol), and DMF (30 mL). The mixture was stirred at 70°C for 6–10 h and then filtered. The filtrate was concentrated by rotary evaporation. The crude products were purified by column chromatography eluting with EtOAc/hexane (1:5).
2-Propionylindolizine-7-carbonitrile (3a) Yellow solid; mp 161–162°C; yield 68%; 1H NMR (CDCl3): δ 7.97–7.89 (m, 2H), 7.84 (s, 1H), 7.09 (s, 1H), 6.65 (dd, 1H, J=7.3, 1.7 Hz), 2.95 (q, 2H, J=7.3 Hz), 1.24 (t, 3H, J=7.3 Hz); 13C NMR (CDCl3): δ 197.2, 130.8, 129.9, 128.1, 126.1, 118.2, 117.7, 111.9, 104.7, 101.4, 33.4, 8.2: IR (neat): v (cm−1) 3425, 3140, 2979, 2934, 2218, 1673, 1628, 1521, 1467, 1414, 1333, 1202, 1167, 901, 797, 613, 558, 478, 419. HR-MS. Calcd for (C12H10N2O+H)+: m/z 199.0871. Found: m/z 199.0862.
2-Benzoylindolizine-7-carbonitrile (3b) Yellow solid; mp 192–193°C; yield 76%; 1H NMR (CDCl3): δ 7.98–7.84 (m, 5H), 7.66–7.57 (m, 1H), 7.57–7.47 (m, 2H), 7.14 (s, 1H), 6.68 (dd, 1H, J=7.3, 1.7 Hz; 13C NMR (CDCl3) δ 191.0, 138.6, 132.5, 130.6, 129.4, 129.1, 128.5, 128.1, 126.0, 119.6, 118.2, 112.0, 106.7, 101.5; IR (neat): v (cm−1) 3075, 2227, 1633, 1528, 1478, 1402, 1359, 1328, 1243, 1113, 883, 796, 717, 684, 424. HR-MS. Calcd for (C16H10N2O+H)+: m/z 247.0871. Found: m/z 247.0861.
2-(4-Nitrobenzoyl)indolizine-7-carbonitrile (3c) Yellow solid; mp>220°C; yield 65%; 1H NMR (DMSO-d6): δ 8.49–8.29 (m, 5H), 8.15–8.05 (m, 2H), 7.18 (s, 1H), 6.95 (dd, 1H, J=7.3, 1.7 Hz); 13C NMR (DMSO-d6): δ 189.0, 149.4, 143.6, 130.5, 130.1, 128.4, 127.4, 127.1, 123.7, 121.5, 118.4, 111.6, 105.8, 101.0; IR (neat): v (cm−1) 3444, 3124, 2927, 2852, 2220, 1651, 1600, 1519, 1350, 1236, 1110, 1007, 848, 720, 622, 428. HR-MS. Calcd for (C16H9N3O3+H)+: m/z 292.0722. Found: m/z 292.0726.
2-(4-Methylbenzoyl)indolizine-7-carbonitrile (3d) Yellow solid; mp 215–216°C; yield 66%; 1H NMR (CDCl3): δ 7.98–7.89 (m, 2H), 7.89–7.79 (m, 3H), 7.32 (d, 2H, J=7.9 Hz), 7.13 (s, 1H), 6.71–6.63 (m, 1H), 2.46 (s, 3H); 13C NMR (CDCl3): δ 190.6, 143.3, 136.0, 130.6, 129.6, 129.4, 129.2, 128.1, 126.0, 119.5, 118.3, 111.9, 106.7, 101.4, 21.7; IR (neat): v (cm−1) 2222, 1631, 1526, 1471, 1328, 1247, 1114, 885, 744. HR-MS. Calcd for (C17H12N2O+H)+: m/z 261.1028. Found: m/z 261.1023.
2-(4-Methoxybenzoyl)indolizine-7-carbonitrile (3e) Yellow solid; mp 216–217°C; yield 65%; 1H NMR (CDCl3) δ 8.00–7.83 (m, 5H), 7.12 (s, 1H), 7.00 (d, 2H, J=8.9 Hz), 6.67 (dd, 1H, J=7.3, 1.6 Hz), 3.91 (s, 3H); 13C NMR (CDCl3): δ 189.5, 163.3, 131.8, 131.3, 130.5, 129.5, 128.0, 126.0, 119.3, 118.3, 113.8, 111.8, 106.6, 101.3, 55.5; IR (neat): v (cm−1) 3445, 3127, 3074, 2227, 1628, 1465, 1394, 1321, 1250, 1174, 1112, 885, 846, 806, 706, 471. HR-MS. Calcd for (C17H12N2O2+H)+: m/z 277.0977. Found: m/z 277.0977.
2-(4-Fluorobenzoyl)indolizine-7-carbonitrile (3f) Yellow solid; mp 233–234°C; yield 62%; 1H NMR (CDCl3): δ 7.99–7.93 (m, 3H), 7.92–7.88 (m, 2H), 7.23–7.17 (m, 2H), 7.11 (s, 1H), 6.69 (dd, 1H, J=7.2, 1.8 Hz); 13C NMR (CDCl3): δ 189.4, 134.8, 132.0, 131.9, 130.7, 128.9, 128.1, 126.0, 119.4, 118.1, 115.8, 115.5, 112.1, 106.6, 101.7; IR (neat): v (cm−1) 3072, 2227, 1633, 1603, 1510, 1478, 1404, 1359, 1329, 1246, 1154, 1113, 891, 843, 801, 742, 702, 607, 426. HR-MS. Calcd for (C16H9N2OF+H)+: m/z 265.0777. Found: m/z 265.0780.
2-(4-Chlorobenzoyl)indolizine-7-carbonitrile (3g) Yellow solid; mp 230–231°C; yield 72%; 1H NMR (CDCl3) δ 8.05–7.78 (m, 5H), 7.50 (d, 2H, J=8.4 Hz), 7.11 (s, 1H), 6.70 (d, 1H, J=7.2 Hz); 13C NMR (CDCl3): δ 189. 7, 139.0, 136.9, 130.8, 130.7, 128.8, 128.8, 128.1, 126.1, 119.5, 118.1, 112.1, 106.6, 101.8; IR (neat): v (cm−1) 3328, 3069, 2928, 2852, 2229, 1631, 1587, 1527, 1478, 1401, 1355, 1329, 1242, 1095, 1009, 884, 746, 696, 573, 477, 424. HR-MS. Calcd for (C16H9N2OCl+H)+: m/z 281.0482. Found: m/z 281.0485.
2-(2-Fluorobenzoyl)indolizine-7-carbonitrile (3h) Yellow solid; mp 204–205°C; yield 76%; 1H NMR (CDCl3): δ 7.95–7.81 (m, 3H), 7.66–7.49 (m, 2H), 7.33–7.27 (m, 1H), 7.26–7.15 (m, 1H), 7.09 (d, 1H, J=0.9 Hz), 6.66 (dd, 1H, J=7.3, 1.7 Hz); 13C NMR (CDCl3) δ 188.0, 161.5, 158.2, 133.1, 133.0, 130.8, 130.4, 130.4, 129.8, 128.3, 126.1, 124.3, 124.3, 119.7, 119.7, 118.1, 116.6, 116.3, 112.1, 106.3, 106.3, 101.7; IR (neat): v (cm−1) 3134, 3077, 2225, 1644, 1527, 1482, 1444, 1400, 1359, 1327, 1234, 1148, 1094, 891, 819, 744, 712, 662, 600, 423. HR-MS. Calcd for (C16H9N2OF+H)+: m/z 265.0777. Found: m/z 265.0775.
2-(2,4-Dichlorobenzoyl)indolizine-7-carbonitrile (3i) Yellow solid; mp 198–199°C; yield 71%; 1H NMR (CDCl3): δ 7.90 (d, 1H, J=7.3 Hz), 7.84 (s, 1H), 7.77 (d, 1H, J=0.8 Hz), 7.52 (d, 1H, J=1.7 Hz), 7.44–7.34 (m, 2H), 7.01 (s, 1H), 6.67 (dd, 1H, J=7.3, 1.7 Hz); 13C NMR (CDCl3): δ 189.1, 137.4, 136.9, 132.2, 131.1, 130.3, 129.8, 129.2, 128.3, 127.1, 126.2, 119. 8, 118.0, 112.4, 106.2, 102.0; IR (neat): v (cm−1) 3446, 3137, 2219, 1655, 1583, 1521, 1466, 1371, 1330, 1230, 1109, 885, 825, 775, 595, 512, 416. HR-MS. Calcd for (C16H8N2OCl2+H)+: m/z 315.0092. Found: m/z 315.0095.
2,3-Dibromoindolizine-7-carbonitrile (3j) White solid; mp 186–187°C; yield 62%; 1H NMR (CDCl3): δ 7.98 (d, 1H, J=7.3 Hz), 7.73 (s, 1H), 6.91 (s, 1H), 6.76 (dd, 1H, J=7.3, 1.6 Hz); 13C NMR (CDCl3): δ 131.6, 124.9, 123.9, 118.2, 111.4, 108.9, 107.2, 100.9, 99.6; IR (neat): v (cm−1) 3424, 3121, 3073, 2222, 1628, 1511, 1460, 1425, 1345, 1285, 1246, 1142, 990, 898, 757, 697, 599, 480, 420. HR-MS. Calcd for (C9H4N2Br2+H)+: m/z 298.8819. Found: m/z 298.8820.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21602153), the Shandong Natural Science Foundation (ZR2015BM028), and Taishan Medical University (2013GCC02).
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- Research Articles
- Synthesis and spectral evaluation of 5,10,15,20-tetrakis(3,4-dibenzyloxyphenyl)porphyrin
- Stereoselective cascade assembling of benzylidenecyanoacetates and 1,3-dimethylbarbituric acid into (1R*,2S*)-1-cyano-5,7-dialkyl-4,6,8-trioxo-2-aryl-5,7-diazaspiro[2.5]octane-1-carboxylates
- Ultrasound mediated synthesis of dihydropyrano[3,2-d][1,3]dioxin-7-carbonitrile derivatives in H2O/EtOH medium
- Simple and efficient approach to synthesis of [1,2,4]triazolo[4,3-b][1,2,4,6]thiatriazine- 1-oxides from N-triazol-3-ylamidines
- Synthesis of 4H-3-aryl-2-cyano-1,4-benzothiazine 1,1-dioxides for antiviral studies
- Design, synthesis and antibacterial evaluation of 2-alkyl- and 2-aryl-3-(phenylamino)quinazolin-4(3H)-one derivatives
- Regio- and stereoselective synthesis of [1,3]thiazolo[3,2-b][1,2,4]triazol-7-ium salts via electrophilic heterocyclization of 3-S-propargylthio-4Н-1,2,4-triazoles and their antimicrobial activity
- Synthesis, spectroscopic characterization, X-ray structure and DFT calculations of Ni(II)bis(3,4 dimethoxybenzoate)bis(nicotinamide) dihydrate
- 13C NMR spectroscopy of heterocycles: 1-phenyl-3-aryl/t-butyl-5-arylpyrazoles
