HETEROCYCLES

Short Paper | Regular issue | Vol. 92, No. 12, 2016, pp. 2235-2243
Received, 30th August, 2016, Accepted, 21st October, 2016, Published online, 11th November, 2016.

■ An Efficient Synthesis of Functionalized Chromeno[4,3-d]pyrazolo[3,4-b]pyridine Derivatives

Wei Lin,* Yongxiang Zheng, Yazhen Wang, and Daqing Shi*

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

Abstract

A series of functionalized chromeno[4,3-d]pyrazolo[3,4-b]pyridine derivatives were synthesized via one-pot three-component reaction of salicylaldehydes, 4-hydroxy-6-methyl-2H-pyran-2-one and pyrazol-5-amines catalyzed by CuSO4 in ethanol. The structures of all synthesized were identified by their IR, 1H NMR, 13C NMR and HRMS analysis, and the structure of compound 4h was confirmed by X-ray diffraction analyses.

Nitrogen-containing heterocyclic compounds are ubiquitous system in natural products and are considered as privileged structures in drug discovery.1 Among the various heteroaromatic compounds, coumarin is one of important heterocyclic compound found in many plants. Compounds bearing the coumarin moiety exhibit antitumor,2 anti-inflammatory,3 anti-coagulant,4 and anti-oxidant5 activities. Some coumarin derivatives have a high fluorescent quantum yield6 and have been widely used as laser dyes, fluorescent probes, solar energy collector, and nonlinear optical dyes.7 Meanwhile, some heteroaryl-condensed coumarin derivatives have been to attract interest because of their broad spectrum of useful biological activities and fluorescent properties.8 Diverse synthetic methodologies available for the construction of coumarin derivatives have been developed, including Pechmann reaction,9 Knoevenagel condensation,10 Wittig reaction,11 C-H activating methods,12 and multi-component reactions.13 However, to the best of our knowledge, only a few literatures14 reported the preparation of chromeno[4,3-d]pyrazolo[3,4-b]pyridine derivatives. As a part of our research on the synthesis of novel functionalized coumarin derivatives,15 herein, we reported the efficient synthesis of novel functionalized chromeno[4,3-d]pyrazolo[3,4-b]pyridine derivatives.
At first the reaction conditions were examined. According to our previously reported synthetic procedure of 3-(1-hydroxy-3-oxobut-1-en-1-yl)-2H-chromen-2-one from the reaction of salicylaldehydes with 4-hydroxy-6-methyl-2H-pyran-2-one,16 salicylaldehydes (1a) was treated with 4-hydroxy-6-methyl- 2H-pyran-2-one (2) in ethanol catalyzed by L-proline (10 mol%) for 1 h to give the desired 3-(1-hydroxy-3-oxobut-1-en-1-yl)-2H-chromen-2-one. Then 3-methyl-1-phenyl-pyrazol-5-amine (3a) was added and the mixture was stirred at different conditions to optimize the reaction conditions. The results are summarized in Table 1. In the absence of catalyst, the desired product 4a was only obtained in 12% yield (Table 1, entry 1). To improve the yield, several catalysts were evaluated, i.e. p-TSA, piperidine, CuSO4·5H2O, CuSO4, CuSO4·2H2O, Cu(OAc)2·H2O and CuI (Table 1, entries 2-8). The results showed that the best catalyst was CuSO4. Subsequently, we further evaluated the effect of solvents. Of all the solvents tested, i.e. ethanol, acetonitrile, DMF, toluene, acetic acid, and THF, ethanol gave the best result (Table 1, entries 5 and 9-13). To optimize the catalyst loading, 5, 10, 15, 20, and 25 mol% of

CuSO4 was tested (Table 1, entries 5 and 14-17). A 20 mol% loading of CuSO4 was sufficient to promote the reaction. The reaction was then conducted at different temperature, such as: 20, 40, 60 °C, and refluxing temperature, to determine the optimum temperature for this transformation. All these experiments were conducted in ethanol catalyzed by CuSO4 (20 mol%) (Table 1, entries 5 and 18-20). So the best temperature for this transformation was at refluxing temperature. Based on all these experiments, the optimum reaction conditions were identified as using ethanol as solvent and 20 mol% CuSO4 as the catalyst at refluxing temperature for several hours.
The optimized reaction conditions were then tested for library construction with substituted salicylaldehydes and pyrazol-5-amines. The results are summarized in Table 2. It was found that there were no remarkable effect of the electronic effect of the substituted groups of salicylaldehydes and pyrazol-5-amines to the yields.

The structures of all the products were identified by their IR, 1H NMR, 13C NMR and HRMS spectra. The structure of compound 4h was further confirmed by X-ray diffraction analysis. The molecular structure of the product 4h is shown in Figure 1.

Based on the references,14b,16 the proposed mechanism for the synthesis of compound 4a is shown in Scheme 1. The intermediate A was formed by the condensation reactions of salicylaldehyde (1a) with 4-hydroxy-6-methyl-2H-pyran-2-one (2) catalyzed by L-proline. The intermediate A then reacted with 3-methyl-1-phenylpyrazol-5-amine (3a) to form imine catalyzed by CuSO4. The desired product 4a was obtained by the Diels-Alder and oxidation in air from the imine.

In conclusion, we have developed a facile and efficient protocol for the construction of functionalized chromeno[4,3-d]pyrazolo[3,4-b]pyridine derivatives via a novel one-pot three-component reaction of salicylaldehydes, 4-hydroxy-6-methyl-2H-pyran-2-one and pyrazol-5-amines catalyzed by CuSO4 in ethanol. This protocol has the advantages of mild reaction conditions, easily accessible starting materials, and wide range of substrates, which makes it a useful and attractive method for the synthesis of the complex chromeno[4,3-d]pyrazolo[3,4-b]pyridines in synthetic and medicinal chemistry. Further expansion of the reaction scope and synthetic applications of this methodology are in progress in our laboratory.

EXPERIMENTAL
IR spectra were recorded on a Varian F-1000 spectrometer in KBr with absorptions in cm-1. 1H NMR (400 MHz or 300 MHz) and 13C NMR (100 MHz or 75 MHz) spectra were recorded on a Varian Inova-300 MHz and Varian Inova-400 MHz in CDCl3 solution. J values are in Hz. Chemical shifts are expressed in ppm downfield from internal standard TMS. High-resolution mass spectra (HRMS) for all the compounds were determined on Bruker MicrOTOF-QII mass spectrometer with ESI resource. X-Ray diffraction analysis was recorded on a Rigaku Mercury CCD/AFC diffractometer.
Starting Materials. All chemicals used in this study were commercially available.
Typical experimental procedure for the synthesis of chromeno[4,3-d]pyrazolo[3,4-b]pyridine derivatives 4. A mixture of salicylaldehydes 1 (1 mmol), 4-hydroxy-6-methyl-2H-pyran-2-one 2 (1 mmol) and L-proline (0.10 mmol) in EtOH (10 mL) was stirred at 80 oC for 1 h. Then pyrazol-5-amines 3 (1 mmol) and CuSO4 (0.20 mmol) was added to the reactor and stirred at 80 oC for 4-48 h. After completion of the reaction confirmed by TLC (eluent acetone/petroleum ether (PE), V/V = 1:3), the reaction mixture was concentrated in vacuo to remove the solvent. The product purified by column chromatography (PE-acetone = 5:1) to afford the pure product 4.
1-Methyl-5-(2-oxopropyl)-3-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4a: white solid; mp 203-205 oC. IR (KBr) 2960, 1726, 1594, 1547, 1507, 1443, 1248, 772 cm-1; 1H NMR (400 MHz, DCl3) δ 8.31 (d, J = 7.6 Hz, 1H, ArH), 8.19 (d, J = 6.8 Hz, 2H, ArH), 7.63 (t, J = 6.8 Hz, 1H, ArH), 7.53 (t, J = 6.8 Hz, 2H, ArH), 7.42-7.35 (m, 3H, ArH), 4.62 (s, 2H, CH2), 2.93 (s, 3H, CH3), 2.43 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.1, 160.3, 159.3, 152.6, 151.6, 143.0, 142.6, 138.5, 133.0, 129.4, 129.2, 127.0, 124.0, 122.3, 117.6, 116.7, 110.8, 109.7, 54.9, 30.6, 19.3; HRMS Calculated for C23H17N3O3Na: [M+Na]+ 406.1168, found: 406.1169.
10-Chloro-1-methyl-5-(2-oxopropyl)-3-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4b: white solid; mp 235-238 oC; IR (KBr) 2959, 1734, 1608, 1278, 1180, 1021, 829, 753, 689 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.37 (s, 1H, ArH), 8.18 (d, J = 8.0 Hz, 2H, ArH), 7.61-7.52 (m, 3H, ArH), 7.38 (d, J = 8.0 Hz, 2H, ArH), 4.63 (s, 2H, CH2), 2.98 (s, 3H, CH3), 2.43 (s, 3H, CH3); 13C NMR (75 MHz, CDCl3) δ 205.0, 159.9, 159.4, 151.7, 151.0, 142.8, 141.4, 138.5, 132.8, 129.6, 129.3, 128.9, 127.2, 122.6, 119.0, 118.0, 110.9, 109.5, 54.9, 30.6, 19.5; HRMS Calculated for C23H16ClN3O3: [M]+ 417.0880, found: 417.0877.
8-Methoxy-1-methyl-5-(2-oxopropyl)-3-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4c : white solid; mp 247-249 oC; IR (KBr) 2936, 1739, 1545, 1506, 1181, 1024, 968, 833, 789, 681 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.19 (d, J = 8.0 Hz, 2H, ArH), 7.87 (d, J = 8.0 Hz, 1H, ArH), 7.53 (t, J = 8.0 Hz, 2H, ArH), 7.36 (t, J = 7.6 Hz, 2H, ArH), 7.20 (d, J = 6.8 Hz, 1H, ArH), 4.63 (s, 2H, CH2), 4.01 (s, 3H, CH3O), 2.93 (s, 3H, CH3), 2.43 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.2, 159.4, 157.2, 148.0, 145.8, 143.4, 138.6, 129.2, 127.0, 125.0, 123.7, 122.5, 120.8, 117.5, 114.7, 110.0, 108.1, 101.9, 56.6, 54.8, 30.6, 19.4; HRMS Calculated for C24H19N3O4: [M]+ 413.1376, found: 413.1380.
9-Methoxy-1-methyl-5-(2-oxopropyl)-3-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4d : white solid; mp 213-216 oC; IR (KBr) 3097, 1721, 1559, 1487, 1389, 1248, 1143, 969, 783, 738, 693 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.24-8.17 (m, 3H, ArH), 7.52 (t, J = 8.0 Hz, 2H, ArH), 7.35 (t, J = 7.6 Hz, 1H, ArH), 6.97 (d, J = 8.8 Hz, 1H, ArH). 6.88 (s, 1H, ArH), 4.59 (s, 2H, CH2), 3.93 (s, 3H, CH3O), 2.91 (s, 3H, CH3), 2.42 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.2, 160.6, 159.5, 154.6, 151.7, 143.0, 138.6, 130.6, 129.2, 128.9, 122.5, 112.2, 109.9, 109.4, 109.3, 101.3, 56.1, 54.9, 30.6, 19.4; HRMS Calculated for C24H18N3O4: [M-H]– 412.1297, found: 412.1348.
10-Chloro-5-(2-oxopropyl)-1,3-diphenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4e : white solid; mp 202-204 oC; IR (KBr) 3048, 1720, 1608, 1540, 1502, 1355, 1312, 1247, 1198, 1162, 1023, 762, 702, 672 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.27 (d, J = 7.6 Hz, 2H, ArH), 7.56-7.50 (m, 7H, ArH), 7.41 (d, J = 6.4 Hz, 2H, ArH), 7.33 (s, 1H, ArH), 7.28 (s, 1H, ArH), 4.69 (s, 2H, CH2), 2.46 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.0, 160.3, 159.7, 151.6, 150.9, 147.3, 141.1, 138.5, 134.0, 132.7, 131.4, 129.8, 129.6, 129.3, 129.1, 128.9, 127.5, 122.9, 118.1, 116.7, 111.3, 107.8, 54.9, 30.6; HRMS Calculated for C28H18ClN3O3: [M]+ 479.1037, found: 479.1065.
8-Methoxy-5-(2-oxopropyl)-1,3-diphenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4f: gray solid; mp 253-256 oC; IR (KBr) 2852, 1734, 1548, 1497, 1250, 1161, 1128, 863, 830, 757, 701 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.28 (d, J = 7.6 Hz, 2H, ArH), 7.57-7.38 (m, 8H, ArH), 6.99 (d, J = 6.8 Hz, 1H, ArH), 6.93 (d, J = 8.0 Hz, 1H, ArH), 6.66 (t, J = 8.0 Hz, 1H, ArH), 4.69 (s, 2H, CH2), 3.96 (s, 3H, CH3O), 2.46 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.2, 160.0, 159.8, 147.6, 147.3, 142.7, 138.7, 134.4, 129.7, 129.3, 129.2, 128.7, 127.3, 123.1, 122.9, 122.7, 116.5, 114.6, 111.4, 108.1, 56.5, 54.9, 29.9; HRMS Calculated for C29H20N3O4: [M-H]- 474.1454, found: 474.1510.
9-Methoxy-5-(2-oxopropyl)-1,3-diphenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4g: gray solid; mp 150-152 oC; IR (KBr) 2919, 1730, 1539, 1488, 1381, 1275, 1156, 1006, 902, 851, 768, 701 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.27 (d, J = 7.6 Hz, 2H, ArH), 7.56-7.39 (m, 8H, ArH), 7.44 (t, J = 8.0 Hz, 1H, ArH), 6.81 (d, J = 8.4 Hz, 1H, ArH), 6.27 (d, J = 8.8 Hz, 1H, ArH), 4.65 (s, 2H, CH2), 3.83 (s, 3H, CH3O), 2.46 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.2, 163.4, 160.6, 159.8, 154.6, 151.8, 147.5, 145.9, 142.7, 138.7, 134.4, 132.8, 129.7, 129.2, 128.9, 127.3, 122.8, 111.3, 109.9, 109.0, 107.5, 100.7, 56.0, 55.0, 30.6; HRMS Calculated for C29H21N3O3: [M]+ 475.1532, found: 475.1521.
10-Chloro-1,3-dimethyl-5-(2-oxopropyl)chromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4h: gray solid; mp 166-168 oC; IR (KBr) 2943, 1718, 1561, 1474, 1385, 1357, 1278, 1248, 1205, 1169, 1096, 970, 778, 675 cm-1; 1H NMR (400 MHz, CDCl3) δ 8.33 (s, 1H, ArH), 7.54 (d, J = 8.4 Hz, 1H, ArH), 7.32 (d, J = 8.4 Hz, 1H, ArH), 4.57 (s, 2H, CH2), 4.12 (s, 3H, CH3), 2.87 (s, 3H, CH3), 2.42 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.0, 160.0, 158.9, 152.0, 150.9, 141.3, 141.2, 132.6, 129.4, 128.6, 118.9, 118.0, 109.8, 107.6, 54.7, 34.1, 30.5, 19.4; HRMS Calculated for C18H14ClN3O3: [M]+ 355.0724, found: 355.0722.
3-Methyl-5-(2-oxopropyl)-1-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4i: cyan solid; mp 237-240 oC; IR (KBr) 2917, 1721, 1589, 1555, 1470, 1383, 1253, 1201, 1166, 851, 827, 687 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.49-7.41 (m, 6H, ArH), 7.33 (t, J = 8.4 Hz, 2H, ArH), 6.71 (t, J = 7.6 Hz, 1H, ArH), 4.66 (s, 2H, CH2), 4.28 (s, 3H, CH3), 2.47 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.3, 160.6, 159.2, 152.6, 152.1, 146.3, 142.5, 134.6, 132.8, 131.5, 129.6, 129.0, 128.8, 123.2, 116.8, 115.8, 110.5, 106.2, 54.8, 34.5, 30.6; HRMS Calculated for C23H17N3O3Na: [M+Na]+ 406.1168, found: 406.1160.
10-Bromo-3-methyl-5-(2-oxopropyl)-1-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4j: yellow solid; mp 218-220 oC; IR (KBr) 2991, 1724, 1546, 1512, 1444, 1306, 1257, 1167, 918, 826, 694 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.53-7.47 (m, 7H, ArH), 7.20 (d, J = 7.6 Hz, 1H, ArH), 4.66 (s, 2H, CH2), 4.28 (s, 3H, CH3), 2.47 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.2, 160.1, 159.3, 152.0, 151.4, 146.3, 141.0, 135.5, 134.3, 129.7, 129.5, 129.1, 118.4, 117.4, 116.2, 110.5, 106.1, 54.8, 34.6, 30.6; HRMS Calculated for C23H16BrN3O3: [M]+ 461.0375, found: 461.0376.
10-Methoxy-3-methyl-5-(2-oxopropyl)-1-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4k: yellow solid; mp 176-178 oC; IR (KBr) 2988, 1722, 1603, 1591, 1556, 1502, 1473, 1444, 1390, 1351, 1201, 1156, 1025, 903, 826, 781, 749, 695 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.55 (d, J = 6.8 Hz, 2H, ArH), 7.49-7.44 (m, 3H, ArH), 7.28 (s, 1H, ArH), 7.03 (d, J = 8.4 Hz, 1H, ArH), 6.90 (s, 1H, ArH), 4.67 (s, 2H, CH2), 4.29 (s, 3H, CH3), 2.90 (s, 3H, CH3O), 2.48 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.3, 160.8, 159.3, 155.0, 152.2, 146.9, 146.0, 142.5, 134.8, 129.7, 129.1, 128.9, 122.3, 118.1, 116.1, 112.7, 110.6, 105.7, 54.8, 54.7, 34.6, 30.6; HRMS Calculated for C24H19N3O4: [M]+ 413.1376, found: 413.1371.
8,10-Dichloro-3-methyl-5-(2-oxopropyl)-1-phenylchromeno[4,3-d]pyrazolo[3,4-b]pyridin-6(3H)-one 4l: yellow solid; mp 176-178 oC; IR (KBr) 3065, 1732, 1559, 1537, 1524, 1450, 1399, 1262, 1012, 788, 701 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.53-7.42 (m, 7H, ArH), 4.67 (s, 2H, CH2), 4.29 (s, 3H, CH3), 2.48 (s, 3H, CH3); 13C NMR: (75 MHz, CDCl3) δ 205.1, 159.4, 152.1, 147.0, 146.2, 143.7, 140.7, 134.1, 133.7, 132.5, 129.9, 129.7, 129.5, 129.1, 128.6, 127.2, 122.5, 117.9, 110.5, 106.2, 102.4, 54.8, 34.7, 30.6; HRMS Calculated for C23H15Cl2N3O3: [M]+ 451.0490, found: 451.0486.

ACKNOWLEDGEMENTS
This work was supported by Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (No. 15KJA150006) and the National Natural Science Foundation of China (No. 21502074).

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