DOI:
10.1039/C5RA14674G
(Communication)
RSC Adv., 2015,
5, 70949-70957
Microwave assisted novel and regioselective functionalization of imidazopyridines with chromene acetals and β-nitrostyrenes†
Received
24th July 2015
, Accepted 12th August 2015
First published on 12th August 2015
Abstract
A facile synthesis of novel functionalized imidazopyridines has been accomplished through the condensation of imidazopyridines with chromene hemiacetals or β-nitro styrenes with high regioselectivity and excellent yields in the presence of a catalytic amount of PTSA or InCl3. In this process, the reaction appears to be very general and suitable for direct functionalization of imidazopyridines. In our present method we successfully achieved regioselectivity.
Introduction
Imidazo[1,2-a]pyridines1 (IPs) are found in many natural products and pharmaceuticals which exhibit a wide range of biological activities like antibacterial,2 antifungal,3 antiviral,4 antitumorous,5 and anti-inflammatory6 properties. Drug formulations7 containing imidazopyridines currently available on the market include alpidem (anxiolytic), zolpidem (hypnotic), and zolimidine (anti-ulcer) (Fig. 1). Owing to their importance in the pharmaceutical and agrochemical fields, the syntheses of these structural units have attracted the attention of chemists. Imidazopyridines are generally synthesized via condensation of 2-aminopyridines with α-haloketones, β-nitro styrenes and aromatic alkynes.8 The biological activities of IPs have proved to be greatly dependent on substituents at the C2 and C3 positions. To introduce further diversity at C3 position, various groups reported metal-catalyzed cross-coupling reactions such as Suzuki,9 Stille,10 Heck,11 Sonogashira,12 Negishitype13 type reactions. However, these reactions require a halogenated precursor, which makes the sequence length and increases waste production. IP is known to be an electron-rich aromatic ring14 that could lead to electrophilic aromatic substitutions at C3. However, the Friedel–Crafts alkylation remains challenging and requires an excess of electrophilic reagent and strong temperature conditions. Many groups synthesized C3-alkylated products via electrophilic formylation and nitration of imidazopyridines followed by multistep conversions to furnish desired product.15 Recently Chaubet et al.16 have reported aza-Friedel–Crafts reaction in the presence of thiamine, HCl. Subsequently, Pericherla et al.17 have reported C3-functionalization of IPs in the presence of Yb(OTf)3 via aza-Friedel–Crafts reaction. There does not appear to be a large effort for C3-alkylation of imidazo[1,2-a]pyridines, which is probably due to the lack of practical synthetic methodologies to access these compounds.
![image file: c5ra14674g-f1.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-f1.gif) |
| Fig. 1 Some biologically active molecules of IPs and 4H-chromenes. | |
In this context a general, flexible and practical approach for C3-functionalization of imidazopyridines is highly desirable.
Chromenes (2H-chromenes and 4H-chromenes) are one of the major classes of naturally occurring compounds.18 During the past decade many groups have focused on their chemistry because of their utility as biologically active agents19 such as β-cell KATP channel openers, DNA-dependent protein kinase(DNA-PK), DNA polymerase β inhibitors, anti-tyrpanosomal agents, anti-bacterial, apoptosis inducers, and their utility as key intermediates in the synthesis of numerous natural products and medicinal reagents.20 Microwave-assisted reactions have found wide applications as an efficient way to accelerate many organic reactions. Microwave radiation is an energy source whose applications in organic synthesis are well established.21 This approach can produce high yields, modifications in the selectivity, lowering of side products, and easier work-up and purification of products.22 Because of the promising biological activity of imidazopyridine and 4H-chromenes, we have envisioned to construct novel hybrid molecules in single compound. To the best of our knowledge there has never been a report of C3-functionalization of imidazopyridine with chromene acetals and β-nitro styrenes. In continuation of our work23 on chromene compounds herein we wish to report C3-functionalization of imidazopyridines with β-nitro styrenes and chromene hemiacetals in presence of acid catalyst under microwave irradiation (Scheme 1).
![image file: c5ra14674g-s1.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-s1.gif) |
| Scheme 1 Synthesis of novel alkyl/4H-chromene substituted imidazopyridines. | |
Results and discussion
Initially we have chosen 2H-chromene hemiacetal as electrophilic substrate. The model reaction of imidazopyridine (2a) (1 mmol) with chromene hemiacetal (1a) (1 mmol) under MW irradiation at 120 °C in water which did not proceed even after 30 minutes. Further we planned the same reaction in presence of acid catalysts like scandium triflate (Sc(OTf)3), indium chloride (InCl3), bismuth chloride (BiCl3), montmorillonite KSF clay, zinc chloride (ZnCl2), aluminium chloride (AlCl3), iodine (I2), ruthenium chloride (RuCl3), cyanuric acid, p-toluenesulfonic acid (PTSA) and camphor sulphonic acid (CSA) and summarized the results in Table 1. Among the screened acid catalysts PTSA gave the best results in terms of yield and time. Next, the effect of solvent was studied and tetrahydrofuran was found to be the best for this transformation.
Table 1 Screening of catalysts for the synthesis of new functionalized imidazopyridinesa
![image file: c5ra14674g-u1.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-u1.gif)
|
S. no |
Catalyst |
Solvent |
Time (min) |
Yieldb (%) |
Reaction conditions: 2H-chromene (1a) (1 mmol), imidazopyridine (1 mmol) (2a) and catalyst stirred in appropriate solvent. Isolated yield of pure product. |
1 |
— |
H2O |
30 |
— |
2 |
Sc(OTf)3 (5 mol%) |
DMF |
10 |
49 |
3 |
InCl3 (5 mol%) |
DMF |
20 |
56 |
4 |
BiCl3 (5 mol%) |
THF |
20 |
28 |
5 |
KSF-clay (>100 mol%) |
THF |
30 |
10 |
6 |
ZnCl2 (10 mol%) |
DMF |
15 |
43 |
7 |
AlCl3 (10 mol%) |
THF |
10 |
52 |
8 |
I2 (10 mol%) |
THF |
25 |
45 |
9 |
RuCl3 (15 mol%) |
DMF |
20 |
15 |
10 |
Cyanuric acid (15 mol%) |
THF |
15 |
29 |
11 |
PTSA (10 mol%) |
THF |
10 |
85 |
12 |
PTSA (10 mol%) |
DMF |
10 |
69 |
13 |
PTSA (5 mol%) |
DCE |
10 |
71 |
14 |
CSA (10 mol%) |
THF |
20 |
35 |
Having established the optimum conditions, the scope of C3-functionalization was investigated (Table 2). The halogen substituted (Br, Cl) 2H-chromenes proceeded smoothly to furnish the required product (4d, 4g) in good yield. Whereas, the unsubstituted chromene gave moderate yield (4a) of product. Similarly chromene having with electron withdrawing groups like NO2 or di-halogen substituents reacted slowly in optimized conditions to give the corresponding product (4j) in low yield. During the reaction it was observed that the ester group at C3 position also influences the reaction. For example the reaction of chromenes possessing t-butyl ester group at C3 position went smoothly and furnished the alkylated IP product (4h) in good yield. Whereas, the chromenes with methyl and ethyl esters at C3 position gave the required product in moderate yield (4b). Chromenes with benzoyl and acetyl groups at C3 position gave the product in comparatively low yield. We observed that there is no effect of the substituent at C2 position on the efficiency of the reaction and yield of the product.
Table 2 Scope of substrates for the synthesis of 4H-chromene substituted imidazopyridinesa,b
![image file: c5ra14674g-u2.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-u2.gif)
|
2H-chromene (1 mmol), imidazopyridine (1 mmol) and PTSA (5 mol%) were added in 1 ml of THF and kept under microwave irradiation for 10–20 minutes. Isolated yield of pure product. |
![image file: c5ra14674g-u3.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-u3.gif) |
We observed that there is no effect of the substituent at C2 position on the efficiency of the reaction and yield of the product. Inspired by these results we intended to apply the reaction of imidazopyridine with other activated alkenes. As a model reaction we performed the reaction of imidazopyridine with β-nitro styrene in presence of PTSA under optimized conditions, but the reaction did not give positive results. Then the same reaction was performed using different acid catalysts like Sc(OTf)3, InCl3, BiCl3, ZnCl2, FeCl3. Among the screened catalysts InCl3 (5 mol%) was turnout as good catalyst for optimum yield in o-xylene (Table 4). It was observed that electronic effect of substituent have considerable influence on the reaction. For example electron donating groups like CH3, Cl and F possessing styrenes proceeded smoothly and gave the desired product (Table 3, 4n, 4o, 4q) in good yield. Whereas, styrenes with electron withdrawing group like NO2 went slowly and furnished the C3 alkylated product in low yield. We have performed the reaction of IP with styrene of nitro ethane, produced as mixture of diastereomers (50
:
50) (4x) in equal ratio.
Table 3 Scope of substrates for the synthesis of novel functionalised imidazopyridinesa,b
![image file: c5ra14674g-u4.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-u4.gif)
|
Imidazopyridine (1 mmol), β-nitro styrene and InCl3 (5 mol%) were added in 1 ml of o-xylene and kept under microwave irradiation for 10–20 minutes. Isolated yield of pure product. |
![image file: c5ra14674g-u5.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-u5.gif) |
Table 4 Screening of catalysts for the synthesis of novel functionalized imidazopyridinesa
![image file: c5ra14674g-u6.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-u6.gif)
|
Entry |
Catalyst |
Solvent |
Time (min) |
Yieldb (%) |
Reaction conditions: β-nitrostyrene (3a) (1 mmol), imidazopyridine (2a) (1 mmol) and catalyst stirred in appropriate solvent. Isolated yield of pure product. |
1 |
Sc (OTf)3 (5 mol%) |
DMF |
10 |
59 |
2 |
BiCl3 (5 mol%) |
THF |
20 |
38 |
3 |
InCl3 (5 mol%) |
o-Xylene |
20 |
85 |
4 |
InCl3 (10 mol%) |
o-Xylene |
20 |
84 |
5 |
FeCl3 (5 mol%) |
Toluene |
20 |
23 |
6 |
ZnCl2 (10 mol%) |
DMF |
15 |
14 |
7 |
PTSA (5 mol%) |
THF |
10 |
5 |
8 |
InCl3 (5 mol%) |
THF |
20 |
63 |
9 |
InCl3 (5 mol%) |
Toluene |
20 |
70 |
10 |
InCl3 (5 mol%) |
CH3CN |
20 |
53 |
11 |
InCl3 (5 mol%) |
1,4-Dioxane |
20 |
59 |
We have attempted the reaction of imidazopyridines with other Michael acceptors like isatin chalcone, cyclohexenone, benzylideneacetophenone, (E)-methyl 4-oxo-4-phenylbut-2-enoate and (E)-4-phenylbut-3-en-2-one. The results were not satisfactory. The reaction was not proceeded under optimized conditions (Scheme 3, see in ESI†).
Plausible mechanism
Initially chromene acetal in presence of acid catalyst (p-toluenesulfonic acid or InCl3) forms more stable oxacarbenium cation, which has two reactive sites (C2 & C4). Imidazopyridine selectively attacks at C4 position (which has less steric hindrance) followed by elimination of hydrogen to give novel functionalized imidazopyridines. On the other hand indium metal activates C3 position of imidazopyridine which undergo reaction with β-nitrostyrene to give C3-functionalized imidazopyridines (Scheme 2).
![image file: c5ra14674g-s2.tif](/image/article/2015/RA/c5ra14674g/c5ra14674g-s2.gif) |
| Scheme 2 A plausible mechanism. | |
Conclusion
We have reported a new strategy for direct regio selective functionalisation imidazopyridines via C–C bond formation with chromene hemiacetals and β-nitrostyrenes in presence of acid catalyst. Operational simplicity, oxidant-free, environmentally benign reaction conditions, and scalability are the attractive features of the present protocol. This work also brings oxocarbenium ions into the realm of viable electrophilic substrates for organic synthesis. We anticipate that this mode of catalysis will find use in related reactions of imidazopyridines with activated alkenes and electrophiles for the synthesis of various functionalized imidazopyridines.
Experimental section
General information
Salicylaldehydes, β-diketones, β-keto esters, amino pyridine, phenacyl bromide, PTSA, InCl3, nitromethane, nitroethane, aldehydes and all solvents were purchased from Sigma Aldrich and Alpha Aesar company and used without further purification as received. All 1H and 13C NMR spectra were recorded in CDCl3 on Avance 300 or Avance 500 spectrometers. Chemical shifts (δ) are reported in parts per million (ppm) relative to residual CHCl3 (1H: δ 7.26 ppm, 13C: δ 77.00 ppm) as an internal reference. Coupling constants (J) are reported in Hertz (Hz). Peak multiplicity is indicated as follows: s—singlet, d—doublet, t—triplet, q—quartet, m—multiplet and dd—doublet of doublet. Melting points were measured on a BUCHI melting point machine. IR spectra were recorded on Thermo Nicolet FT/IR-5700 spectrometer. Mass spectra were recorded using waters mass spectrometer. High resolution mass spectrums (HRMS) were recorded using Applied Bio-Sciences HRMS spectrometer at national center for mass spectroscopy-IICT.
General procedure
Microwave irradiation experiment. All microwave irradiation experiments were carried out in a dedicated CEM-Discover monomode microwave apparatus, operating at a frequency of 2.45 GHz with continuous irradiation power from 0 to 300 W with utilization of the standard absorbance level of 300 W maximum power. A sealed 10 ml glass tube containing 2H-chromene (1a) (1 mmol), imidazopyridine (2a) (1 mmol) and PTSA (5 mol%) in 1 ml of THF kept under microwave irradiation for 10 minutes at 80 °C as mentioned in Table 1. After completion of the reaction (indicated by TLC) removed solvent and the crude product was purified by column chromatography using ethyl acetate/hexane to get pure compounds as white or pale yellow crystals. The isolated compounds were well characterized by IR, 1H NMR, 13C NMR and HRMS.
Characterisation data for the products
Methyl 2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4a). White solid; Mp 118–120 °C; IR: νmax 2926, 1706, 1640, 1583, 1486, 1361, 1213, 1062, 757, 697 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.86 (d, J = 5.4 Hz, 3H), 7.61 (d, J = 9.0 Hz, 1H), 7.36–7.52 (m, 3H), 6.93–7.23 (m, 5H), 6.68 (t, J = 6.8 Hz, 1H), 6.07 (s, 1H), 3.29 (s, 3H), 2.41 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 167.0, 160.2, 149.6, 144.6, 144.1, 135.0, 128.7, 128.6, 128.3, 127.7, 124.9, 124.0, 123.2, 122.2, 120.1, 117.7, 116.2, 112.3, 101.4, 51.2, 31.1, 19.1; m/z (ESI); 397 [M + H]+.
Ethyl 2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4b). White solid; Mp 124–126 °C; IR: νmax 2931, 1705, 1647, 1580, 1474, 1359, 1211, 1056, 759, 699 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.78–7.99 (m, 3H), 7.41–7.49 (m, 1H), 7.45 (t, J = 7.5 Hz, 2H), 7.38 (t, J = 7.3 Hz, 1H), 7.10–7.21 (m, 2H), 6.99–7.06 (m, 2H), 6.98–6.93 (m, 1H), 6.71 (t, J = 6.6 Hz, 1H), 6.08 (s, 1H), 3.88–3.95 (m, 1H), 3.78–3.70 (m, 1H), 2.37 (s, 3H), 0.77 (t, J = 7.1 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 166.6, 159.7, 149.5, 144.5, 143.9, 134.9, 128.7, 128.6, 128.5, 128.3, 127.7, 124.8, 124.0, 123.3, 122.4, 120.3, 117.7, 116.2, 112.3, 101.8, 60.2, 31.0, 19.1, 13.6; m/z (ESI); 411 [M + H]+. HRMS calcd for C26H23O3N2: 411.17032, found: 411.17029.
Ethyl 6-chloro-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4c). White solid; Mp 131–133 °C; IR: νmax 3038, 2969, 2924, 1707, 1633, 1470, 1362, 1302, 1231, 1144, 1065, 809, 704 cm−1; 1H NMR (300 MHz, CDCl3 + DMSO): δ 7.87–7.97 (m, 1H), 7.69–7.80 (m, 2H), 7.63 (d, J = 9.0 Hz, 1H), 7.38.7.59 (m, 3H), 7.13–7.25 (m, 2H), 6.95–7.02 (m, 2H), 6.80 (t, J = 6.3 Hz, 1H), 5.99 (s, 1H), 3.31 (s, 3H), 2.37 (s, 3H); 13C NMR (75 MHz, CDCl3 + DMSO): δ 165.9, 159.2, 147.4, 143.8, 143.6, 143.5, 134.0, 128.7, 128.1, 127.9, 127.6, 127.2, 127.1, 123.7, 122.4, 121.2, 117.1, 116.9, 112.0, 100.5, 50.6, 30.2, 18.4; m/z (ESI); 431 [M + H]+. HRMS calcd for C26H20O3N2Cl: 431.15070, found: 431.1501.
Ethyl 6-chloro-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4d). White solid; Mp 134–136 °C; IR: νmax 3032, 2956, 2922, 1704, 1637, 1469, 1352, 1314, 1225, 1143, 1066, 808, 701 cm−1; 1H NMR (300 MHz, CDCl3 + DMSO): δ 7.90–7.90 (m, 1H), 7.70–7.80 (m, 2H), 7.64 (d, J = 8.8 Hz, 1H), 7.35.7.49 (m, 3H), 7.11–7.21 (m, 2H), 6.98 (d, J = 2.0 Hz, 1H), 6.94 (d, J = 8.7 Hz, 1H), 6.75 (t, J = 6.6 Hz, 1H), 6.00 (s, 1H), 3.87–3.96 (m, 1H), 3.70–3.80 (m, 1H), 2.34 (s, 3H), 0.78 (t, J = 7.1 Hz, 3H); 13C NMR (75 MHz, CDCl3 + DMSO): δ δ 165.7, 158.9, 147.5, 143.9, 143.6, 134.1, 128.9, 128.3, 128.1, 127.8, 127.4, 127.3, 123.8, 122.6, 121.7, 117.3, 117.2, 112.1, 101.0, 59.8, 30.4, 18.5, 13.1; m/z (ESI); 445 [M + H]+.
tert-Butyl 6-chloro-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4e). White solid; Mp 196–198 °C; IR: νmax 3040, 2975, 2928, 1710, 1643, 1482, 1361, 1309, 1229, 1159, 1060, 813, 696 cm−1; 1H NMR (300 MHz, CDCl3): δ 8.08 (d, J = 6.8 Hz, 1H), 7.62–7.75 (m, 3H), 7.32–7.47 (m, 3H), 7.11–7.23 (m, 2H), 6.92–6.99 (m, 2H), 6.78 (t, J = 6.8 Hz, 1H), 5.90 (s, 1H), 2.17 (s, 3H), 1.01 (s, 9H); 13C NMR (75 MHz, CDCl3): δ 165.7, 156.7, 148.1, 144.5, 144.3, 134.5, 129.3, 128.7, 128.3, 127.8, 127.7, 124.2, 123.2, 122.4, 117.8, 112.4, 103.4, 81.0, 31.2, 27.5, 18.5; m/z (ESI); 473 [M + H]+. HRMS calcd for C28H26O3N2Cl: 473.16265, found: 431.16259.
Methyl 6-bromo-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4f). White solid; Mp 130–132 °C; IR: νmax 3028, 1716, 1652, 1477, 1432, 1219, 1072, 741, 694 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.72–7.90 (m, 3H), 7.64 (d, J = 9.1 Hz, 1H), 7.37–7.53 (m, 3H), 7.25–7.32 (m, 1H), 7.13–7.22 (m, 2H), 6.90 (d, J = 8.7 Hz, 1H), 6.73 (t, J = 6.8 Hz, 1H), 6.00 (s, 1H), 3.28 (s, 3H), 2.38 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 166.6, 159.8, 146.6, 144.6, 144.4, 134.7, 131.7, 131.1, 128.6, 128.3, 127.8, 124.2, 123.0, 122.3, 121.7, 117.9, 117.8, 117.2, 112.5, 101.3, 51.3, 30.9, 19.0; m/z (ESI); 475 [M + H]+. HRMS calcd for C25H20O3N2Br: 475.06518, found: 475.06480.
Ethyl 6-bromo-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4g). White solid; Mp 138–140 °C; IR: νmax 3031, 1712, 1648, 1466, 1424, 1208, 1075, 743, 699 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.91–7.99 (m, 1H), 7.72–7.80 (m, 2H), 7.64 (dt, J1 = 9.0 Hz, J2 = 1.0 Hz, 1H), 7.42–7.49 (m, 2H), 7.36–7.42 (m, 1H), 7.27–7.30 (m, 1H), 7.12–7.21 (m, 2H), 6.89 (d, J = 8.7 Hz, 1H), 6.76 (t, J = 6.8 Hz, 1H), 6.01 (s, 1H), 3.86–3.96 (m, 1H), 3.69–3.78 (m, 1H), 0.77 (t, J = 7.1 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 166.2, 159.4, 148.6, 144.5, 144.2, 134.7, 131.7, 131.0, 128.6, 128.4, 127.8, 124.2, 123.1, 122.6, 118.0, 117.9, 117.0, 112.5, 101.8, 60.4, 30.9, 19.0, 13.6; m/z (ESI); 489 [M + H]+. HRMS calcd for C26H22O3N2Br: 489.08083, found: 489.08083.
Tert-butyl 6-bromo-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4h). White solid; Mp 201–203 °C; IR: νmax 3043, 1715, 1646, 1479, 1435, 1223, 1069, 747, 700 cm−1; 1H NMR (300 MHz, CDCl3): δ 8.08 (d, J = 6.0 Hz, 1H), 7.62–7.75 (m, 3H), 7.28–7.49 (m, 4H), 7.20 (t, J = 8.3 Hz, 1H), 7.11 (s, 1H), 6.90 (d, J = 8.7 Hz, 1H), 6.79 (t, J = 8.7 Hz, 1H), 5.90 (s, 1H), 2.16 (s, 3H), 1.00 (s, 9H); 13C NMR (75 MHz, CDCl3): δ 165.7, 156.7, 148.7, 144.5, 144.4, 134.5, 131.7, 130.7, 128.7, 127.9, 124.3, 123.3, 122.9, 118.2, 117.8, 116.8, 112.4, 103.6, 81.1, 31.1, 27.6, 18.6; m/z (ESI); 517 [M + H]+.
1-(6-Bromo-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromen-3-yl)ethanone (4i). White solid; Mp 146–148 °C; IR: νmax 3035, 1719, 1657, 1482, 1445, 1223, 1068, 749, 698 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.93 (d, J = 6.4 Hz, 1H), 7.70–7.77 (m, 2H), 7.65 (d, J = 9.1 Hz, 1H), 7.46–7.51 (m, 2H), 7.42 (t, J = 7.3 Hz, 1H), 7.33 (dd, J1 = 6.6 Hz, J2 = 1.4 Hz, 1H), 7.24–7.26 (m, 1H), 7.17–7.22 (m, 1H), 6.93 (d, J = 8.7 Hz, 1H), 6.75 (t, J = 6.6 Hz, 1H), 5.99 (s, 1H), 2.21 (s, 3H), 1.79 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 198.9, 156.1, 148.8, 144.9, 144.4, 134.3, 131.9, 130.8, 128.7, 128.6, 128.3, 124.8, 123.1, 122.4, 118.2, 117.9, 117.1, 112.8, 110.9, 31.2, 29.4, 19.2; m/z (ESI); 459 [M + H]+. HRMS calcd for C25H20O2N2Br: 459.07027, found: 475.07063.
Ethyl 2-methyl-6-nitro-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4j). White solid; Mp 153–155 °C; IR: νmax 3027, 2934, 1705, 1637, 1585, 1472, 1366, 1224, 1068, 762, 692 cm−1; 1H NMR (300 MHz, CDCl3 + DMSO): δ 8.11–8.21 (m, 1H), 8.06 (dd, J1 = 9.1 Hz, J2 = 2.45 Hz, 1H), 7.87 (d, J = 2.1 Hz, 1H), 7.48–7.70 (m, 3H), 7.34–7.47 (m, 3H), 7.25 (t, J = 8.1 Hz, 1H), 7.10 (d, J = 9.1 Hz, 1H), 6.88 (t, J = 6.8 Hz, 1H), 6.01 (s, 1H), 3.77–4.01 (m, 2H), 2.33 (s, 3H), 0.82 (t, J = 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3 + DMSO): δ 165.3, 158.2, 153.1, 144.4, 143.9, 143.6, 133.9, 128.3, 127.8, 127.5, 124.1, 123.8, 122.5, 121.7, 117.3, 116.8, 112.4, 102.2, 60.1, 30.3, 18.4, 13.2; m/z (ESI); 456 [M + H]+.
Tert-butyl 6,8-dichloro-2-methyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromene-3-carboxylate (4k). White solid; Mp 184–186 °C; IR: νmax 3035, 2954, 2928, 1709, 1645, 1453, 1367, 1306, 1211, 1135, 1058, 803, 703 cm−1; 1H NMR (300 MHz, CDCl3): δ 8.11 (d, J = 6.0 Hz, 1H), 7.94 (d, J = 1.5 Hz, 1H), 7.67 (d, J = 9.1 Hz, 1H), 7.51–7.60 (m, 2H), 7.33–7.46 (m, 3H), 7.15–7.26 (m, 2H), 6.85 (t, J = 6.8 Hz, 1H), 5.82 (s, 1H), 2.19 (s, 3H), 1.02 (s, 9H); 13C NMR (75 MHz, CDCl3): δ 165.7, 156.9, 148.9, 145.9, 144.8, 144.5, 136.8, 134.3, 128.7, 128.3, 128.0, 124.4, 124.0, 123.2, 117.9, 112.7, 104.5, 87.6, 85.8, 81.3, 31.4, 27.6, 18.5; m/z (ESI); 507 [M + H]+.
(6-Bromo-2-phenyl-4-(2-phenylimidazo[1,2-a]pyridin-3-yl)-4H-chromen-3-yl) (phenyl)methanone (4l). White solid; Mp 176–180 °C; IR: νmax 3057, 1665, 1637, 1475, 1312, 1227, 1179, 970, 751, 698 cm−1; 1H NMR (300 MHz, CDCl3): δ 8.22 (d, J = 7.0 Hz, 1H), 7.53–7.63 (m, 3H), 7.30–7.46 (m, 9H), 7.12–7.23 (m, 5H), 7.12–7.22 (m, 3H), 6.95–7.05 (m, 1H), 6.23 (s, 1H); 13C NMR (75 MHz, CDCl3): δ 195.4, 153.3, 149.5, 145.3, 144.9, 136.6, 133.9, 133.0, 132.6, 131.9, 130.9, 129.9, 128.8, 128.7, 128.5, 128.3, 128.1, 128.0, 127.9, 125.9, 125.5, 124.6, 123.3, 122.1, 119.6, 118.4, 117.9, 117.5, 117.1, 112.5, 112.3, 110.3, 108.1, 33.4; m/z (ESI); 583 [M + H]+. HRMS calcd for C35H24O2N2Br: 583.09952, found: 583.10188.
3-(2-Nitro-1-phenylethyl)-2-phenylimidazo[1,2-a]pyridine (4m). Semi solid; IR: νmax 3056, 2932, 2848, 1551, 1530, 1434, 1347, 745, 725 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.72 (d, J = 7.0 Hz, 1H), 7.66 (d, J = 9.0 Hz, 1H), 7.59–7.63 (m, 2H), 7.39–7.46 (m, 3H), 7.27–7.36 (m, 3H), 7.16–7.22 (m, 2H), 6.72 (dt, J1 = 5.9 Hz, J2 = 0.9 Hz, 1H), 5.61 (t, J = 7.6 Hz, 1H), 5.07 (dd, J1 = 7.6, Hz, J2 = 5.6 Hz, 1H), 4.95 (dd, J1 = 7.6 Hz, J2 = 5.6 Hz 1H); 13C NMR (75 MHz, CDCl3): δ 145.3, 145.1, 135.9, 129.4, 129.1, 128.5, 128.4, 128.1, 126.8, 124.8, 123.3, 117.9, 116.6, 112.8, 76.4, 39.1; m/z (ESI); 344 [M + H]+. HRMS calcd for C21H18O2N3: 344.13935, found: 344.13955.
3-(1-(4-Chlorophenyl)-2-nitroethyl)-2-phenylimidazo[1,2-a]pyridine (4n). Semi solid; IR: νmax 3051, 2933, 2847, 1556, 1509, 1434, 1348, 731, 707 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.66–7.71 (m, 2H), 7.55–7.59 (m, 2H), 7.31–7.34 (m, 2H), 7.22–7.26 (m, 1H), 7.11–7.16 (m, 2H), 6.78 (t, J = 7.5 Hz, 1H), 5.57 (t, J = 7.6 Hz, 1H), 5.03 (dd, J1 = 7.6, Hz, J2 = 5.3 Hz, 1H), 4.93 (dd, J1 = 7.3 Hz, J2 = 5.9 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 145.4, 145.2, 134.6, 134.2, 134.0, 129.7, 129.1, 128.7, 128.3, 125.1, 123.1, 118.1, 116.2, 113.1, 76.5, 38.8; m/z (ESI); 378 [M + H]+. HRMS calcd for C21H17O2N3Cl: 378.10038, found: 378.10067.
3-(1-(4-Fluorophenyl)-2-nitroethyl)-2-phenylimidazo[1,2-a]pyridine (4o). Semi solid; IR: νmax 3045, 2928, 2844, 1561, 1524, 1439, 1346, 739, 713 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.67–7.72 (m, 2H), 7.57–7.61 (m, 2H), 7.41–7.49 (m, 3H), 7.22–7.26 (m, 1H), 7.16–7.20 (m, 2H), 7.02–7.08 (m, 2H), 6.78 (dt, J1 = 5.6 Hz, J2 = 1.2 Hz, 1H), 5.57 (t, J = 7.6 Hz, 1H), 5.03 (dd, J1 = 8.1, Hz, J2 = 5.0 Hz, 1H), 4.94 (dd, J1 = 7.2 Hz, J2 = 6.1 Hz 1H); 13C NMR (75 MHz, CDCl3): δ 145.4, 145.2, 134.2, 131.9, 131.8, 129.2, 128.7, 128.6, 124.9, 123.1, 118.2, 116.6, 116.4, 112.9, 76.7, 38.7; m/z (ESI); 362 [M + H]+. HRMS calcd for C21H17O2N3F: 362.12993, found: 362.13026.
3-(2-Nitro-1-(4-nitrophenyl)ethyl)-2-phenylimidazo[1,2-a]pyridine (4p). Semi solid; IR: νmax 3042, 2924, 2853, 1555, 1523, 1442, 1351, 732, 703 cm−1; 1H NMR (300 MHz, CDCl3): δ 8.11–8.16 (m, 1H), 8.07–8.10 (m, 1H), 7.79 (d, J = 7.0 Hz, 1H), 7.69 (d, J = 9.0 Hz, 1H), 7.48–7.55 (m, 4H), 7.40–7.47 (m, 3H), 7.24–7.30 (m, 1H), 6.83 (t, J = 6.9 Hz, 1H), 5.67 (t, J = 7.6 Hz, 1H), 5.13 (dd, J1 = 8.2, Hz, J2 = 5.3 Hz, 1H), 5.02 (dd, J1 = 7.2 Hz, J2 = 6.4 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 148.7, 145.8, 145.2, 138.3, 133.8, 133.2, 130.4, 129.1, 128.7, 128.6, 125.2, 123.2, 122.7, 121.8, 118.2, 115.5, 113.4, 75.9, 38.8; m/z (ESI); 389 [M + H]+. HRMS calcd for C21H17O4N4: 389.12443, found: 389.12451.
3-(2-Nitro-1-(p-tolyl)ethyl)-2-phenylimidazo[1,2-a]pyridine (4q). Semi solid; IR: νmax 3038, 2931, 2865, 1558, 1517, 1438, 1360, 741, 709 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.72 (d, J = 7.0 Hz, 1H), 7.68 (d, J = 9.0 Hz, 1H), 7.60–7.64 (m, 1H), 7.39–7.47 (m, 3H), 7.18–7.24 (m, 2H), 7.11–7.18 (m, 2H), 7.07–7.11 (m, 2H), 6.74 (t, J = 6.9 Hz, 1H), 5.57 (t, J = 7.6 Hz, 1H), 5.04 (dd, J1 = 7.6 Hz, J2 = 5.5 Hz, 1H), 4.94 (dd, J1 = 7.63 Hz, J2 = 5.65 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 145.3, 145.1, 137.9, 134.3, 132.9, 130.1, 129.2, 128.5, 128.4, 126.7, 124.7, 123.3, 118.0, 116.7, 112.7, 76.7, 38.9, 20.9; m/z (ESI); 358 [M + H]+.
3-(1-(Naphthalen-2-yl)-2-nitroethyl)-2-phenylimidazo[1,2-a]pyridine (4r). Semi solid; IR: νmax 3043, 2955, 2923, 2854, 1634, 1550, 1502, 1371, 1236, 923, 780, 730 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.97–8.04 (m, 1H), 7.89–7.94 (m, 1H), 7.81–7.84 (m, 1H), 7.74–7.78 (m, 1H), 7.60–7.69 (m, 3H), 7.51–7.56 (m, 2H), 7.35–7.48 (m, 5H), 7.11–7.22 (m, 1H), 6.70 (dt, J1 = 5.8 Hz, J2 = 1.1 Hz, 1H), 6.19–6.25 (m, 1H), 5.19 (dd, J1 = 9.1 Hz, J2 = 5.13 Hz, 1H), 5.07 (dd, J1 = 6.1 Hz, J2 = 8.1 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 145.5, 145.1, 134.6, 134.3, 130.9, 130.7, 129.5, 129.4, 128.6, 128.5, 127.4, 126.3, 125.3, 125.1, 124.7, 123.2, 121.9, 118.0, 116.3, 112.9, 74.8, 37.1; m/z (ESI); 394 [M + H]+. HRMS calcd for C25H20O2N3: 394.15500, found: 394.15495.
3-(1-(2-Bromophenyl)-2-nitroethyl)-2-phenylimidazo[1,2-a]pyridine (4s). Semi solid; IR: νmax 3034, 2915, 2858, 1553, 1521, 1427, 1336, 730, 715 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.87 (d, J = 7.0 Hz, 1H), 7.67 (d, J = 9.0 Hz, 1H), 7.61–7.65 (m, 1H), 7.43–7.47 (m, 2H), 7.35–7.42 (m, 3H), 7.22–7.26 (m, 1H), 7.13–7.19 (m, 2H), 7.08–7.12 (m, 1H), 6.82–6.87 (m, 1H), 5.71 (dd, J1 = 5.8 Hz, J2 = 3.8 Hz, 1H), 4.97 (dd, J1 = 9.6 Hz, J2 = 4.9 Hz, 1H), 4.84 (dd, J1 = 8.7 Hz, J2 = 5.8 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 145.4, 144.7, 134.8, 134.6, 133.8, 129.8, 129.5, 128.9, 128.5, 128.3, 128.1, 124.9, 124.3, 122.9, 117.8, 115.9, 113.0, 74.2, 39.8; m/z (ESI); 422 [M + H]+.
3-(1-(2-Fluorophenyl)-2-nitroethyl)-2-phenylimidazo[1,2-a]pyridine (3t). Semi solid; IR: νmax 3040, 2927, 2859, 1546, 1511, 1423, 1369, 747, 717 cm−1; 1H NMR (300 MHz, CDCl3 + DMSO): δ 7.76–7.86 (m, 1H), 7.55–7.71 (m, 5H), 7.39–7.54 (m, 3H), 7.23–7.37 (m, 3H), 7.06 (dt, J1 = 5.8 Hz, J2 = 0.9 Hz, 1H), 5.93 (t, J = 7.9 Hz, 1H), 5.24–5.41 (m, 2H); 13C NMR (75 MHz, CDCl3 + DMSO): δ 161.2, 157.9, 144.2, 143.9, 133.5, 129.2, 129.1, 128.3, 127.4, 124.0, 123.8, 122.8, 122.2, 122.0, 116.7, 115.4, 115.1, 114.9, 112.0, 73.9, 32.9; m/z (ESI); 362 [M + H]+.
3-(2-Nitro-1-(thiophen-2-yl)ethyl)-2-phenylimidazo[1,2-a]pyridine (4u). Semi solid; IR: νmax 3049, 2931, 2858, 1540, 1509, 1424, 1355, 747, 715 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.80 (d, J = 6.9 Hz, 1H), 7.68 (d, J = 9.0 Hz, 1H), 7.63–7.66 (m, 2H), 7.39–7.48 (m, 3H), 7.20–7.26 (m, 2H), 6.96–7.00 (m, 1H), 6.88–6.91 (m, 1H), 6.75–6.80 (m, 1H), 5.81 (t, J = 7.6 Hz, 1H), 5.11 (dd, J1 = 7.6 Hz, J2 = 5.5 Hz, 1H), 4.94 (dd, J1 = 7.5 Hz, J2 = 5.6 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 145.3, 139.6, 133.8, 129.0, 128.6, 128.5, 127.4, 125.8, 125.1, 124.9, 123.4, 118.0, 116.2, 112.8, 76.3, 35.6; m/z (ESI); 350 [M + H]+.
3-(1-(4-Methoxyphenyl)-2-nitroethyl)-2-phenylimidazo[1,2-a]pyridine (4v). Semi solid; IR: νmax 3031, 2924, 2853, 1552, 1505, 1431, 1367, 744, 712 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.72 (d, J = 7.0 Hz, 1H), 7.66 (d, J = 9.0 Hz, 1H), 7.60–7.64 (m, 2H), 7.38–7.46 (m, 3H), 7.17–7.22 (m, 1H), 7.10 (d, J = 8.7 Hz, 2H), 6.85 (d, J = 8.7 Hz, 2H), 6.72 (t, J = 6.9 Hz, 1H), 5.54 (t, J = 7.6 Hz, 1H), 5.02 (dd, J1 = 8.1 Hz, J2 = 5.0 Hz, 1H), 4.92 (dd, J1 = 7.5 Hz, J2 = 5.6 Hz, 1H), 3.76 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 159.2, 145.1, 145.0, 134.2, 129.1, 128.5, 128.4, 127.9, 127.7, 124.7, 123.3, 117.9, 116.8, 114.7, 112.7, 76.7, 55.2, 38.6; m/z (ESI); 374 [M + H]+.
3-(1-(Benzo[d][1,3]dioxol-5-yl)-2-nitroethyl)-2-phenylimidazo[1,2-a]pyridine (4w). Semi solid; IR: νmax 3029, 2933, 2857, 1552, 1522, 1440, 1353, 745, 706 cm−1; 1H NMR (300 MHz, CDCl3): δ 7.75 (d, J = 6.9 Hz, 1H), 7.67 (d, J = 9.0 Hz, 1H), 7.59–7.63 (m, 2H), 7.39–7.48 (m, 3H), 7.19–7.24 (m, 1H), 6.73–6.77 (m, 2H), 6.66–6.69 (m, 1H), 6.64 (d, J = 1.7 Hz, 1H), 5.92 (s, 2H), 5.48 (t, J = 7.6 Hz, 1H), 4.98 (dd, J1 = 7.9 Hz, J2 = 5.2 Hz, 1H), 4.89 (dd, J1 = 7.3 Hz, J2 = 5.8 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 148.7, 147.4, 145.0, 134.2, 129.6, 129.2, 128.5, 124.8, 123.3, 119.9, 117.9, 116.6, 112.8, 108.8, 107.4, 101.4, 76.7, 39.1; m/z (ESI); 388 [M + H]+.
3-(1-(4-Methoxyphenyl)-2-nitropropyl)-2-phenylimidazo[1,2-a]pyridine (4x). Semi solid; IR: νmax 3034, 2931, 2865, 1558, 1517, 1438, 1360, 741, 709 cm−1; 1H NMR (300 MHz, CDCl3): δ 8.09 (d, J = 7.0 Hz, 1H), 7.85 (d, J = 7.0 Hz, 1H), 7.72–7.76 (m, 2H), 7.67 (d, J = 7.6 Hz, 1H), 7.56–7.62 (m, 3H), 7.41–7.54 (m, 6H), 7.16–7.23 (m, 4H), 7.09 (d, J = 8.7 Hz, 2H), 6.78–6.85 (m, 5H), 6.73–6.78 (m, 1H), 5.49–5.60 (m, 2H), 5.37 (d, J = 11.1 Hz, 1H), 5.04 (d, J = 10.8 Hz, 1H), 3.76 (s, 3H), 3.75 (s, 3H), 1.55 (d, J = 6.6 Hz, 3H), 1.32 (d, J = 6.7 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 159.1, 159.0, 145.4, 145.1, 144.9, 144.7, 134.9, 134.3, 129.7, 129.2, 129.0, 128.7, 128.5, 128.3, 128.1, 127.7, 124.7, 124.5, 123.4, 123.2, 118.2, 118.1, 117.9, 117.4, 114.6, 114.4, 112.9, 112.5, 84.2, 83.5, 55.2, 55.1, 45.4, 44.1, 19.5, 19.4; m/z (ESI); 388 [M + H]+.
Acknowledgements
The authors thank CSIR, New Delhi, for financial support as part of XII. Five Year Plan Program under the title ORIGIN (CSC-0108) and Dr A. Kamal, outstanding scientist and Head of MCP Division, for his support and encouragement. L. C. R. and N. S. K. thank CSIR for the award of a fellowship.
Notes and references
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Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14674g |
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