A new application of polymer supported, homogeneous and reusable catalyst PEG–SO₃H in the synthesis of coumarin and uracil fused pyrrole derivatives

Abstract

A highly efficient, simple and convenient synthetic protocol for the synthesis of coumarin and uracil fused pyrrole derivatives has been demonstrated. A biodegradable, polymer supported catalyst, PEG–SO₃H, was applied for the two component coupling of 4-aminocoumarin or 6-aminouracil and α,β-unsaturated nitroalkene. The catalyst has efficiently catalyzed the Michael addition and intramolecular cyclisation with the concomitant removal of the nitro group. The catalyst was recycled for five cycles with almost unaltered catalytic activity.

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Introduction

Coumarin and uracil fused heterocycles are one of the most active classes of compounds possessing a wide spectrum of biological activity.¹ Many of the coumarin fused heterocycles show antitumor,² antibacterial,³ antifungal,⁴ anticoagulant,⁵ anti-inflammatory,⁶ and antiviral⁷ activities. A large number of adenosine receptor agonists and antagonists are proved to be highly potent and subtype selective ligands.^8,9 On the other hand heterocyclic compounds containing a pyrrole ring are not only found in many natural products¹⁰ and pharmaceuticals¹¹ but also extensively used in materials science,¹² bioorganic chemistry,¹³ and supramolecular chemistry.¹⁴ Coumarin fused pyrrole derivatives comprise important classes of marine natural products, some of which display remarkable biological and pharmacological properties (Fig. 1). Lamellarins D (A), K (D) and M (B) have been found to be cytotoxic to a wide range of cancer cell lines.¹⁵ Lamellarin D (A) is also a potent inhibitor of human topoisomerase I60, and lamellarin H (C) is a potent inhibitor of both molluscum contagiosum virus topoisomerase and HIV-1 integrase.¹⁶ The astonishing drug activity of these pyrrole based heterocyclic compounds not only attracted many synthetic and medicinal chemists to synthesize this heterocyclic nucleus but also became an active research area of enduring interest.

Fig. 1 Structures of coumarin fused pyrrole core containing bioactive natural products.

A number of methods have been reported for the synthesis of pyrroles. The traditional methods are Knorr,¹⁷ Paal–Knorr,¹⁸and Hantzsch reactions.¹⁹ Although these methods are very helpful for the construction of different types of pyrrole core containing heterocyclic molecules, still these classical reactions have significant drawbacks such as availability of the starting materials, functional group compatibility, regiospecificity, and harsh reaction conditions, which limit their scope. Aside from the traditional methods, the modern approaches for the synthesis of substituted pyrroles are mainly based on the transition-metal catalyzed cyclizations^20,21 and multicomponent coupling reactions.^22,23 The high catalyst loading in some of the above procedures causes significant drawbacks such as higher cost and potential contamination of the products which make these procedures less attractive to the pharmaceutical industry. Consequently, general and efficient strategies for the synthesis of pyrroles from simple and readily accessible precursors are of great value due to the continued importance of the pyrrole core in both biological and chemical fields. There are a handful of reports regarding coumarin and uracil fused pyrrole derivatives.²⁴ Based on their extensive application, it is necessary to further develop an efficient and convenient method to construct such significant heterocyclic compounds.

Recently much attention has been directed towards the use of polymer-supported catalysts due to their low cost, high catalytic activity, easy work-up procedure and recyclability.²⁵ Polyethylene glycol has been used for the synthesis of polymer supported acidic catalysts.²⁶ PEG-6000 (PEG–SO₃H) is an example of a polyethylene glycol supported catalyst that is functionalized by acidic groups and is a mild, non-volatile and non-corrosive organic acid which has been used for the synthesis of several heterocyclic molecules.²⁷ PEG–SO₃H is mostly soluble in many polar solvents and insoluble in a few nonpolar solvents. Hence the problems arising from the use of heterogeneous catalysts, like lower reactivity, extended reaction times and sometimes toxicity, can be removed by taking advantage of such a wide solubility range of PEG–SO₃H. Herein we would like to demonstrate an efficient PEG–SO₃H catalyzed synthetic protocol of pyrrole core containing coumarin and uracil derivatives.

Results and discussion

In view of the potential medicinal importance of such type of coumarin based heterocyclic compounds, we wish to uncover a facile and efficient synthetic protocol for the synthesis of pyrrole core containing coumarin and uracil derivatives by the installation of a two component coupling of nitroalkene and 4-aminocoumarin or 6-aminouracil. Multicomponent coupling reaction (MCR) is a powerful synthetic tool for the synthesis of biologically active compounds.²⁸ At the commencement of our work we tried to synthesize the coumarin fused pyrrole ring via a three component coupling reaction of 4-aminocoumarin (1), benzaldehyde (2) and nitromethane (3) using nitromethane as solvent. Under three component coupling conditions if nitroalkene is produced by the Knoevenagel condensation of benzaldehyde and nitromethane, then it would follow two component Michael reaction with 4-aminocoumarin and this could be the possible tandem synthesis of functionalized pyrroles (Scheme 1). To ascertain this, we have applied a wide variety of Lewis acids such as ZnCl₂, AlCl₃, I₂, FeCl₃, InCl₃, p-toluenesulphonic acid and polymer-supported PEG–SO₃H as catalysts to obtain the corresponding substituted pyrrole rings (Table 1). We found that the three component coupling protocol was not very much efficient for the synthesis of such heterocyclic scaffolds. It was also evident that PEG–SO₃H responded well in comparison to other Lewis acid catalysts.

Scheme 1 Synthesis of pyrrole core containing coumarin derivatives via three component coupling of benzaldehyde, nitromethane and 4-aminocoumarin.

Table 1 Influence of different catalysts on the synthesis of pyrrole core containing coumarin derivatives

Entry	Catalyst	Three component coupling		Two component coupling
		Time/h	Yield^a (%)	Time/h	Yield^a (%)
a Isolated yield of the pure compound.
1	ZnCl2	10	—	10	10
2	AlCl3	10	—	10	20
3	I2	10	—	10	—
4	FeCl3	8	10	4	30
5	InCl3	8	5	6	24
6	p-Toluene sulphonic acid	8	15	6	45
7	PEG–SO3H	8	28	4	81

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Analyzing the experimental results (Table 1), it can be concluded that probably the intermediate α,β-unsaturated nitroalkene was not produced efficiently under the above mentioned reaction conditions. Therefore, we have tried to synthesize the particular heterocyclic scaffold by a two component coupling of 4-aminocoumarin (1) and α,β-unsaturated nitroalkene (5a) (Scheme 2). We have employed the same catalysts as mentioned previously. It was evident that with the use of ZnCl₂ and AlCl₃ the product was formed in poor yields (Table 1, entries 1 and 2). Whereas using FeCl₃, InCl₃ and p-toluenesulphonic acid the product was obtained in moderate yields (Table 1, entries 4–6). In comparison with these Lewis acid catalysts, PEG–SO₃H proved to be the most efficient catalyst which gave higher yield (81%) within 4 h (Table 1, entry 7).

Scheme 2 Synthesis of pyrrole core containing coumarin derivatives via a two component coupling of 4-aminocoumarin and α,β-unsaturated nitroalkenes.

Various solvents such as CH₃OH, dioxan, DMSO, DMF, glycol, nitromethane and CH₃CN were screened for the two component coupling of 4-aminocoumarin (1) and α,β-unsaturated nitroalkene (5) at 80 °C and the results are summarized in Table 2. The catalyst showed moderate catalytic activity in ethanol. The best catalytic activity was observed in methanol compared to other organic solvents such as dioxan, DMSO, DMF, glycol, nitromethane and CH₃CN. This also revealed that 10 mol% of catalyst (with respect to 1 mmol of 4-aminocoumarin) efficiently catalyzed the reaction leading to expected coumarin fused heterocycles in excellent yield.

Table 2 Influence of solvents on the synthesis of pyrrole core containing coumarin derivatives via a two component coupling of 4-aminocoumarin and α,β-unsaturated nitroalkenes

Entry	Catalyst load (mol%)	Solvent	Time^a/h	Yield^b (%)
a Reaction temperature: 80 °C. b Isolated yield of the pure compound.
1	5	CH3OH	4	65
2	10	CH3OH	4	81
3	15	CH3OH	4	81
4	10	C2H5OH	6	50
5	10	Dioxan	8	10
6	10	DMSO	8	—
7	10	DMF	8	—
8	10	Glycol	8	—
9	10	Nitromethane	8	43
10	10	CH3CN	8	15

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With these optimized reaction conditions in hand, we then planned to examine the versatility of the methodology for the preparation of pyrrole core containing coumarin and uracil derivatives, developed by us. The substrate scope of the PEG–SO₃H catalyzed two component coupling of nitroalkenes and 4-aminocoumarin or 6-aminouracil is shown in Table 3 and it was found that nitroolefins bearing an electron-donating group in the benzene ring gave the corresponding pyrroles in good yield compared to electron-withdrawing groups in the benzene ring.

Table 3 PEG–SO₃H catalyzed synthesis of pyrrole core containing coumarin and uracil derivatives

Reaction conditions: 4-aminocoumarin (1.0 mmol) or 6-aminouracil (1.0 mmol), α,β-unsaturated nitroalkenes (1.3 mmol), PEG–SO₃H (10 mol%) and methanol (4 ml), 80 °C.

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On the basis of these results, a probable mechanism for this two component reaction is presented in Scheme 3. In fact this process starts from PEG–SO₃H catalyzed Michael addition of 4-aminocoumarin or 6-aminouracil to α,β-unsaturated nitroalkenes. The nucleophilic attack at the β position of the α,β-unsaturated nitroalkenes is enhanced by PEG–SO₃H may be due to the polarization of the π-electron cloud towards the nitrogroup of α,β-unsaturated nitroalkenes. Then the Michael adduct E is attacked by the nitrogen atom of the amine group, with the formation of the five member ring F. Finally, the elimination of water and nitroxyl molecules²⁹ leads to the formation of the desired pyrroles. The Lewis acid catalyzed activation of Michael reaction and subsequent ring annulations leading to the targeted pyrrole derivatives were confirmed by the isolation of the Michael adduct G formed by the Michael reaction of 6-aminouracil and α,β-unsaturated nitroalkenes.

Scheme 3 Proposed mechanism for the PEG–SO₃H catalyzed formation of pyrrole core containing coumarin and uracil derivatives.

Furthermore, the reaction was scaled up to the 10 mmol scale; excellent results were still obtained in the required time as mentioned in Table 3. Additionally, for large scale synthesis we also investigated the recyclability of PEG–SO₃H for five consecutive cycles with almost the same catalytic activity. After completion of each reaction, methanol was evaporated and H₂O was added to the reaction mixture and was shaken for a few minutes to dissolve PEG–SO₃H. The crude product (insoluble in water) was filtered and it was washed with ethylacetate. The isolated product was recrystallized from DMF–water for further purification. In order to recover the catalyst, H₂O was evaporated under reduced pressure, and the resulting solid was washed with diethyl ether, and dried under reduced pressure.

Conclusions

In summary, we have successfully developed a PEG–SO₃H catalysed facile, efficient and economic procedure for the synthesis of coumarin and uracil fused multisubstituted pyrroles. This general synthetic protocol offers several advantages including short reaction times, readily available starting materials, recyclability of the catalyst and high isolated yields of the products. To the best of our knowledge this is the first report of PEG–SO₃H catalysed synthesis of pyrrole core containing coumarin and uracil derivatives.

Experimental

¹H-NMR and ¹³C-NMR spectral analyses were carried out on Bruker-Advance Digital 300 MHz and 75.5 MHz instruments; tetramethylsilane (TMS) was used as internal standard. Infrared spectra were recorded using KBr pellets in reflection mode on a Perkin Elmer RX-1 FTIR spectrophotometer. Melting points were checked on Köfler Block apparatus. Merck aluminum-blocked silica gel plates coated with silica gel G were used for analytical TLC and monitored under UV light and also by exposure to iodine vapour. Synthetic grade chemicals from Sigma-Aldrich, Spectrochem and E-Merck were used for carrying out the organic reactions. All the solvents used in the reaction were distilled and dried over Na₂SO₄.

Preparation of PEG–SO₃H

At 0 °C, chlorosulfonic acid (10 mmol) was added to a solution of PEG-6000 (1 mmol) in CH₂Cl₂ (10 mL), and the resulting solution was stirred at room temperature overnight. Then, the solution was concentrated under vacuum, and ether was added to it. The resulting precipitate was filtered and washed with ether three times to afford PEG–SO₃H as a gummy solid.

General procedure for the synthesis of pyrrole core containing coumarin and uracil derivatives

A mixture of 4-aminocoumarin (1.0 mmol) or 6-aminouracil (1.0 mmol), α,β-unsaturated nitroalkenes (1.3 mmol) and PEG–SO₃H (10 mol%) was taken in 4 ml methanol. The mixture was stirred at 80 °C for a required period of time (TLC). After completion of each reaction, methanol was evaporated and H₂O was added to the reaction mixture and was shaken for a few minutes to dissolve PEG–SO₃H. The crude product (insoluble in water) was filtered and it was washed with ethylacetate. The isolated product was recrystallized from DMF–water to get the pure product.

Physical characteristics and spectral data of synthesized compounds

3-Phenyl-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4a). Yield: 81% (0.212 g), characteristics: grey amorphous solid, mp: 295 °C (dec), IR (KBr): 3109, 1679, 1574, 1462 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 7.21–7.44 (6H, m), 7.55 (1H, s), 7.73 (2H, d, J = 7.5 Hz), 8.05 (1H, d, J = 7.5 Hz), 12.83 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ 104.6, 113.7, 116.7, 121.3, 122.5, 124.1, 124.5, 126.6, 128.0, 128.6, 128.9, 133.3, 136.5, 151.2, 157.9, 161.9; anal. calcd for C₁₇H₁₁NO₂: C 78.15, H 4.24, N 5.36%. Found: C 78.14, H 4.29, N 5.38%. HRMS calcd for C₁₇H₁₁NO₂ ([M + H]⁺) 262.0869, found: 262.0861.

3-(4-Methoxyphenyl)-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4b). Yield: 85% (0.248 g), characteristics: grey amorphous solid, mp: >300 °C, IR (KBr): 3219, 1685 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 3.74 (3H, s), 6.92 (2H, d, J = 7.8 Hz), 7.32–7.46 (4H, m), 7.66 (2H, d, J = 7.8 Hz), 8.03 (1H, d, J = 7.2 Hz), 12.74 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 55.6, 104.9, 113.9, 114.2, 117.0, 121.7, 122.0, 124.5, 124.6, 126.2, 129.2, 130.1, 136.6, 142.5, 151.6, 158.4, 158.6, 162.3; anal. calcd for C₁₈H₁₃NO₃: C 74.22, H 4.50, N 4.81%. Found: C 74.26, H 4.48, N 4.78%. HRMS calcd for C₁₈H₁₃NO₃ ([M + H]⁺) 292.0974, found: 292.0970.

3-(4-Fluorophenyl)-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4c). Yield: 73% (0.204 g), characteristics: grey amorphous solid, mp: >300 °C, IR (KBr): 3113, 1678 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 7.13–7.37 (5H, m), 7.52 (1H, s), 7.75 (2H, s), 8.02 (1H, d, J = 6.9 Hz), 12.79 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 104.6, 113.8, 114.7, 114.9, 116.7, 121.4, 122.4, 123.4, 124.2, 128.9, 129.8, 130.4, 130.5, 136.5, 151.2, 158.0, 159.7, 162.9; anal. calcd for C₁₇H₁₀FNO₂: C 73.11, H 3.61, N 5.02%. Found: C 73.18, H 3.64, N 5.00%; HRMS calcd for C₁₇H₁₀FNO₂ ([M + H]⁺) 280.0775, found: 280.0778.

3-p-Tolyl-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4d). Yield: 82% (0.226 g), characteristics: yellow amorphous solid, mp: >300 °C, IR (KBr): 3185, 1690 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 2.28 (3H, s), 7.15 (2H, d, J = 7.8 Hz), 7.30–7.45 (3H, m), 7.49 (1H, s), 7.62 (2H, d, J = 7.5 Hz), 8.03 (1H, d, J = 7.5 Hz), 12.76 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 20.8, 104.6, 113.8, 116.7, 121.3, 122.0, 124.1, 124.5, 128.5, 128.8, 130.5, 135.6, 136.3, 151.2, 157.9; anal. calcd for C₁₈H₁₃NO₂: C 78.53, H 4.76, N 5.09%. Found: C 78.59, H 4.73, N 5.05%; HRMS calcd for C₁₈H₁₃NO₂ ([M + H]⁺) 276.1025, found: 276.1019.

3-(3-Nitrophenyl)-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4e). Yield: 79% (0.242 g), characteristics: yellow amorphous solid, mp: 290 °C (dec), IR (KBr): 3117, 1678 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 7.36–7.64 (4H, m), 7.83 (1H, s), 8.05–8.22 (3H, m), 8.69 (1H, s), 13.03 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 113.5, 116.8, 117.8, 121.2, 121.9, 122.7, 123.9, 124.3, 129.5, 134.7, 135.0, 137.1, 147.9, 151.2, 158.0, 161.9; anal. calcd for C₁₇H₁₀N₂O₄: C 66.67, H 3.29, N 9.15%. Found: C 66.63, H 3.30, N 9.19%; HRMS calcd for C₁₇H₁₀N₂O₄ ([M + H]⁺) 307.0720, found: 307.0728.

3-(4-Chlorophenyl)-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4f). Yield: 77% (0.227 g), characteristics: yellow amorphous solid, mp: 296 °C (dec), IR (KBr): 3114, 1684 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 7.31–7.41 (5H, m), 7.60 (1H, s), 7.77 (2H, d, J = 8.1 Hz), 8.03 (1H, d, J = 7.5 Hz), 12.86 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 104.5, 113.6, 116.7, 121.3, 122.8, 122.9, 124.1, 127.9, 129.0, 130.1, 131.1, 132.2, 136.7, 151.2, 157.9; anal. calcd for C₁₇H₁₀ClNO₂: C 69.05, H 3.41, N 4.74%. Found: C 69.02, H 3.48, N 4.77%; HRMS calcd for C₁₇H₁₀ClNO₂ ([M + H]⁺) 296.0479, found: 296.0471.

3-(3-Hydroxy-4-methoxyphenyl)-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4g). Yield: 78% (0.240 g), characteristics: yellow amorphous solid, mp: >300 °C, IR (KBr): 3134, 1657 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 3.75 (3H, s), 6.89 (1H, d, J = 8.1 Hz), 7.11–7.55 (6H, m), 8.03 (1H, d, J = 7.5 Hz), 8.85 (1H, s), 12.71 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 56.2, 104.7, 112.4, 114.2, 116.7, 117.0, 120.0, 121.7, 122.1, 124.5, 125.0, 126.7, 129.2, 136.5, 146.4, 147.2, 151.5, 158.3, 162.3; anal. calcd for C₁₈H₁₃NO₄: C 70.35, H 4.26, N 4.56%. Found: C 70.38, H 4.28, N 4.59%; HRMS calcd for C₁₈H₁₃NO₄ ([M + H]⁺) 308.0924, found: 308.0928.

3-(4-Nitrophenyl)-1H-5-oxa-1-azacyclopenta[a]naphthalene-4-one (4h). Yield: 72% (0.221 g), characteristics: yellow amorphous solid, mp: 290 °C (dec), IR (KBr): 3158, 1680 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 7.30–7.37 (4H, m), 7.73 (1H, s), 8.03–8.15 (4H, m), 12.98 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 104.7, 113.8, 116.8, 118.8, 121.2, 121.4, 121.9, 122.7, 123.9, 124.9, 129.3, 129.5, 134.7, 135.0, 137.1, 147.9, 151.5, 157.7, 161.7; anal. calcd for C₁₇H₁₀N₂O₄: C 66.67, H 3.29, N 9.15%. Found: C 66.63, H 3.30, N 9.19%; HRMS calcd for C₁₇H₁₀N₂O₄ ([M + H]⁺) 307.072, found: 307.0719.

5-Phenyl-1,7-dihydropyrrolo[2,3-d]pyrimidine-2,4-dione (4i). Yield: 71% (0.161 g), characteristics: pink powder, mp: 250 °C (dec), IR (KBr): 3131, 1738, 1663, cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 6.84 (1H, s), 7.15–7.78 (5H, m), 10.46 (1H, s), 11.24 (1H, s), 11.49 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 96.2, 115.2, 121.4, 126.3, 128.2, 128.3, 134.3, 140.9, 151.1, 160.5; anal. calcd for C₁₂H₉N₃O₂: C 63.43, H 3.99, N 18.49%. Found: C 63.41, H 3.96, N 18.51%; HRMS calcd for C₁₂H₉N₃O₂ ([M + H]⁺) 228.0774, found: 228.0771.

5-(4-Methoxyphenyl)-1,7-dihydropyrrolo[2,3-d]pyrimidine-2,4-dione (4j). Yield: 75%, (0.193 g), characteristics: pink powder, mp: 270 °C (dec), IR (KBr): 3135, 1736, 1663, 1601 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 3.70 (3H, s), 6.74 (1H, s), 6.83 (2H, d, J = 8.4 Hz), 7.70 (2H, d, J = 8.4 Hz), 10.42 (1H, s), 11.14 (1H, s), 11.43 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 56.7, 96.1, 113.3, 114.1, 117.8, 120.7, 128.9, 142.6, 150.9, 161.9; anal. calcd for C₁₃H₁₁N₃O₃: C 60.70, H 4.31, N 16.33%. Found: C 60.73, H 4.30, N 16.35%; HRMS calcd for C₁₃H₁₁N₃O₃ ([M + H]⁺) 258.0879, found: 258.0870.

5-(4-Fluorophenyl)-1,7-dihydropyrrolo[2,3-d]pyrimidine-2,4-dione (4k). Yield: 68% (0.167 g), characteristics: pink powder, mp: 281 °C (dec), IR (KBr): 3138, 1734, 1604 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ (ppm) 6.85 (1H, s), 7.09 (2H, d, J = 7.5 Hz), 7.82 (2H, d, J = 7.5 Hz), 10.48 (1H, s), 11.26 (1H, s), 11.50 (1H, s); ¹³C NMR (75 MHz, DMSO-d₆): δ (ppm) 95.7, 114.5, 114.7, 119.8, 129.5, 129.6, 130.4, 140.5, 150.7, 160.1; anal. calcd for C₁₂H₈FN₃O₂: C 58.78, H 3.29, N 17.14%. Found: C 58.80, H 3.27, N 17.18%; HRMS calcd for C₁₂H₈FN₃O₂ ([M + H]⁺) 246.068, found: 246.0678.

Acknowledgements

We gratefully acknowledge the financial support from U.G.C. and Calcutta University. S.P. thanks U.G.C., New Delhi, India, for the grant of Junior Research fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20117h

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