Direct CuO nanoparticle-catalyzed synthesis of poly-substituted furans via oxidative C–H/C–H functionalization in aqueous medium

Abstract

Here, we have reported synthesis of 3,4-dicarbonyl substituted poly-functionalized furan derivatives via direct functionalization of α,β-unsaturated carbonyl compounds through conjugate addition initiated domino reactions using CuO nanoparticles as a reusable catalyst in aqueous ethanol.

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Direct synthesis of polysubstituted furans has attracted considerable attention as these moieties represent the most versatile and important class of five-membered heterocyclic compounds and constitute core entities in many natural products and biologically active drug molecules¹ (Fig. 1). These moieties have also found various biological, agrochemicals, and pharmaceutical applications. In addition, furans are important in organic synthesis as targets and building blocks.² In view of the great importance of furans, the development of clean and straightforward synthetic methods from simple and inexpensive reagents is highly desirable.

Fig. 1 Some furan derived natural products and drugs.

Historically, furans can be synthesized via acid catalyzed Paal–Knorr method via intramolecular cyclization of 1,4-dicarbonyls³ and intermolecular Feist–Benary⁴ methods. Many of the protocols have used acyclic ketone precursors as starting materials and produced furan derivatives via alkyne- or allene-assisted cyclizations.⁵ In the past few decades, synthesis of furan derivatives via transition-metal catalyzed inter/intramolecular condensation have also been carried out extensively.⁶ However, in the context of green chemistry, intermolecular approaches were favored due to the straightforward formation of diverse furan scaffolds from simple building blocks. However, surprisingly the synthesis of dicarbonyl substituted furans has rarely been reported in the literature.⁷ Zhuo et al.^7d have reported for the first time, MnO₂ mediated ZnCl₂ assisted synthesis of 3,4-dicarbonyl furan derivatives in acetic acid medium. However, the Zhuo's method was associated with several limitations such as use of excess amount of ZnCl₂/MnO₂ as catalyst (6 equivalents) which are not reusable, harsh reaction conditions (refluxing in acetic acid at 130 °C), longer reaction time (24 h) and lower isolated yields of products (15–80%). Thus, development of an efficient and greener protocol for the synthesis of 3,4-dicorbonyl furan derivatives is highly appreciated.

The introduction of nanoparticles (NPs) could be an attractive to overcome the above mentioned limitations due to their unique properties of larger and highly reactive surface areas.^8,9 Very recently, metal oxide NPs have been widely applied as catalyst because of their dual Lewis acid and Lewis base nature and redox properties on the surface.¹⁰ Copper oxide (CuO) NPs catalyzed carbon–carbon and carbon–hetero atom bond formations have evolved as useful methods for the synthesis of novel heterocyclic compounds.^11–14 As a part of our continuous effort in exploring catalytic activity of metal oxide NPs in organic transformation,¹⁵ here, we have demonstrated CuO NPs catalyzed synthesis of polysubstituted 3,4-dicarbonyl furan derivatives via oxidative C–H/C–H functionalization reaction between α,β-unsaturated ketones and 1,3-dicarbonyl compounds or β-keto esters in water–ethanol. Here, tertiary butyl hydroperoxide (TBHP) is used as oxidant (Scheme 1).

Scheme 1 Synthesis of 3,4-dicarbonyl functionalized furan derivatives.

Results and discussion

Initially, we have synthesized CuO NPs following previously reported protocol.¹⁶ Briefly, aqueous solution of Cu(NO₃)₂ was condensed under basic conditions (pH was adjusted to 11 by addition appropriate amount of NaOH solution) and stirred for 2 h at 70 °C. The black precipitate appeared which was cooled to room temperature, washed thoroughly with water and ethanol, calcinated at 400 °C for 3 h and analyzed by analytical techniques. The formation of CuO NPs was confirmed by the powder X-ray diffraction (XRD) and transmission electron microscopic (TEM) study (Fig. 2a and b).

Fig. 2 (a) Powder X-ray diffraction pattern of CuO NPs; (b) transmission electron microscopic image of CuO NPs.

The powder XRD pattern indicates the formation of single phase monoclinic CuO NPs with reflection planes at (110), (−111), (111/200), (−202), (020), (202), (−113), (022/−311), (113/220), (311), (004/−222) (card JCPDS 72-0629).¹⁶ The average particle size of CuO NPs was determined to be 10 nm, calculated from full width at half maxima (FWHM) at reflection (111/200) plane using Debye–Scherrer formula. The formation of spherical CuO NPs was indicated from TEM image (Fig. 2b).

Using well characterized CuO NPs, next we have attempted to synthesize less explored 3,4-dicabonyl substituted furan derivative via oxidative reaction of 1,3-diphenyl-prop-2-ene-1-one (1a) and acetylacetone (2b) in ethanol. When a mixture of 1a (1 mmol), 2b (5 mmol) was refluxed in presence of CuO NPs (10 mg), 40% of the desired furan derivative (3a) was obtained after 12 h (entry 1, Table 1) without any oxidizing agent. The addition of tertiary butyl hydroperoxide (TBHP) as oxidizing agent (1 equivalent) significantly improves the yield and shortens the reaction time (3 h) (entry 7, Table 1).

Table 1 Optimization of reaction conditions^a

Entry	Catalyst	Solvent	Oxidant	Time (h)	Yield^b (%)
a Reaction conditions: α,β-unsaturated ketone (0.5 mmol), 1,3-dicarbonyl compound (2.5 mmol), catalyst (10 mg), oxidant (1 equiv.), solvent (2.0 mL), reflux, air.b Yields refer to the isolated products.c Catalyst (15 mg).
1	CuO NPs	EtOH	—	12	40
2	CuFe2O4	EtOH	—	12	17
3	Cu(OAc)2	EtOH	—	12	—
4	ZnO NPs	EtOH	—	12	26
5	CuO NPs	EtOH	K2S2O8	12	49
6	CuO NPs	EtOH	DDQ	12	44
7	CuO NPs	EtOH	TBHP	3	72
8	CuO NPs	EtOH–H2O (1 : 1)	TBHP	3	85
9	CuO NPs	DMSO	TBHP	3	42
10	CuO NPs	DMF	TBHP	3	33
11	CuO NPs	EtOH–H2O (1 : 1)	TBHP	3	86^c

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The TBHP works better than K₂S₂O₈ and DDQ as oxidizing agent (entries 5 and 6, Table 1). CuO NPs showed superior to other catalysts like CuFe₂O₄, Cu(OAc)₂, ZnO NPs tested here (entries 2–4, Table 1). Water–ethanol mixture (1 : 1) was proved to be the best solvent (entry 8, Table 1) over DMF, DMSO etc. as solvent (entries 9 and 10, Table 1). The yield of the product did not increase significantly by increasing the amount of CuO NPs (entry 11, Table 1). Thus, a combination of 1,3-diphenyl-prop-2-ene-1-one (1a), acetylacetone (2b) and TBHP (1 : 5 : 1 molar ratio) in the presence of CuO NPs (10 mg) under refluxing in water–ethanol (1 : 1) for 3 h is considered as optimum reaction conditions (entry 8, Table 1).

Using optimized reaction conditions and following a simple experimental procedure,¹⁷ next, we have explored the scope of this method. It is evident from Table 2 that a number of α,β unsaturated carbonyl compounds and 1,3-dicarbonyl compounds were participated in this C–H/C–H oxidative reaction leading to 3,4-dicarboylsubstiuited furan derivatives in good to excellent yields (75–91%) within 3 h. The reaction conditions are mild enough to tolerate various functional groups such as –Cl, –Br, –NO₂, –OMe (3a–3l) etc. present in substrate. It was noteworthy that electronic effect had little influence on this oxidative coupling. To our delight, benzylidene acetone worked well with acetylacetone, and produced the desired product (3l) in good yield. Moreover, both open chain and cyclic 1,3-dicarbonyl compounds were participated in this reaction without any problem.

Table 2 Substrate scope for the synthesis of 3,4-dicarbonyl furan derivatives^a

a Reaction conditions: α,β-unsaturated ketone (0.5 mmol), 1,3-dicarbonyl compound (2.5 mmol), CuO NPs (10 mg), TBHP (1 equiv.) in EtOH : H₂O (1 : 1; 2.0 mL) at reflux for 3 h under air. Yields refer to those of pure isolated products.

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After successful application of the CuO NPs in synthesizing 3,4-dicarbonyl furans, we have tried to extend the scope of this reaction by replacing 1,3-dicarbonyl compounds with β-keto esters as substrate under the standard reaction conditions. When a mixture of 1,3-diphenyl-prop-2-ene-1-one (1a), ethyl acetoacetate (4a) and TBHP (1 : 5 : 1 molar ratio) was heated at 80 °C in water–ethanol (1 : 1) in presence of CuO NPs (10 mg), good yield of diverse furan derivative (5a) was obtained within practical time period (2 h). In a simple experimental procedure,¹⁷ we observed that different α,β-unsaturated ketones and β-keto esters were participated smoothly in the above methodology under standard conditions. Furthermore, we observed that under the similar reaction conditions the reaction took place in the presence of benzylidene acetone (5f).

All the reactions listed in Tables 2 and 3 were clean and afforded good to excellent yields (70–91%) of the products within practical time period (1.5–3 h). The present method of synthesizing the 3,4-dicarbonyl substituted furans offered several advantages over previously reported Zhuo's method such as (i) better isolated yields of the products, (ii) shorter reaction time, (iii) use of catalytic amount of CuO NPs in place of excess (6 equivalent) catalyst, MnO₂/ZnCl₂, (iv) milder reaction condition of heating at 80 °C in water–ethanol mixture than previous heating at 130 °C in acetic acid, and finally (v) greener alternative protocol. A comparison between the two methodologies has been presented in Table 4.

Table 3 Substrate scope for the synthesis of 3,4-dicarbonyl furan derivatives from α,β-unsaturated ketones and β-keto esters^a

a Reaction conditions: α,β-unsaturated ketone (0.5 mmol), β-keto esters (2.5 mmol), CuO NPs (10 mg), TBHP (1 equiv.) in EtOH : H₂O (1 : 1; 2.0 mL) at reflux for 3 h in open air. Yield of the isolated product.

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Table 4 Comparison of the previous and present method for the synthesis of 3,4-dicarbonyl substituted furans

Sl no.	Catalyst/conditions	Time/yield	Reusability	Ref.
1	MnO2/ZnCl2/AcOH, 130 °C	24 h/15–83%	No	7d
2	CuO NPs (10 mg)/H2O–EtOH, 80 °C	3 h/70–91%	Yes	Present work

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Finally, we have investigated the reusability of the CuO NPs for the synthesis 1-(4-benzoyl-5-(4-methoxyphenyl)-2-methylfuran-3-yl)ethanone (3c, Table 2). After the reaction, the CuO NPs was separated by centrifugation and washed thoroughly with water (2 × 2 mL) and ethanol (2 × 1 mL), dried and reused for subsequent reactions. The CuO NPs was reused up to six times without significant loss of catalytic activity, however, after that yield of the product dropped to 65% (Fig. 3). The decrease in yield could possibly due to the aggregation of NPs during the course of the reactions or loss of surface functional groups.^15h The powder XRD pattern (Fig. S1, ESI-7†) of recycled CuO NPs after 4^th run revels that the monoclinic morphology of the particles was intact however, the particles sizes increased (∼15 nm, calculated from FWHM using Scherrer formula).

Fig. 3 Recyclability of CuO NPs for the synthesis of 1-(4-benzoyl-5-(4-methoxyphenyl)-2-methylfuran-3-yl)ethanone (3c, Table 2).

The reaction of 1,3-diphenyl-prop-2-ene-1-one and acetyl acetone in presence of CuO NPs was suppressed in presence of 2,2,6,6-tetramethylpiperidin-1-yl)oxyl, a radical quencher, which supports the formation of radical intermediate. Thus, following previously reported mechanism^7d and above control experiment, we have given a plausible mechanism for synthesis of furan 3a (Scheme 2) in water–ethanol medium. The feasibility of radical reaction in water was demonstrated by Liu et al.¹⁸

Scheme 2 Plausible mechanism for CuO NPs catalyzed synthesis of 3a.

Conclusions

In conclusion, we have demonstrated CuO NPs catalyzed oxidative C–H/C–H functionalization of 1,3-dicarbonyls or β-keto esters with α,β-unsaturated carbonyl compounds leading to 3,4-dicarbonyl substituted poly-functionalized furan derivatives in aqueous medium. The present protocol offered several advantages such as (i) higher isolated yields (70–91%), (ii) faster reaction time (3 h), (iii) use of catalytic amount (10 mg for 0.5 mmol of 1,3-diketone) of CuO NPs, (iv) milder reaction conditions, (v) use of aqueous ethanol as green solvent and finally, (vi) recyclability of CuO NPs catalyst made the protocol environment benign in nature.

Notes and references

1. (a) J. B. Sperry and D. L. Wright, Curr. Opin. Drug Discovery Dev., 2005, 8, 723; (b) A. Boto and L. Alvarez, Furan and Its Derivatives, in Heterocycles in Natural Product Synthesis, Wiley-VCH Verlag GmbH & Co. KGaA, 2011, pp. 97–152; (c) J. F. Kratochvil, R. H. Burris, M. K. Seikel and J. M. Harkin, Phytochemistry, 1971, 10, 2529; (d) Y. L. Lin, Y. L. Tsai, Y. H. Kuo, Y. H. Liu and M. S. Shiao, J. Nat. Prod., 1999, 62, 1500; (e) T. Krause, M. U. Gerbershagen, M. Fiege, R. Weisshorn and F. Wappler, Anaesthesia, 2004, 59, 364; (f) O. Z. Ye, S. X. Xie, M. Huang, W. J. Huang, J. P. Lu and Z. Q. Ma, J. Am. Chem. Soc., 2004, 126, 13940; (g) H. B. Tan, X. Z. Chen, Z. Liu and D. Z. Wang, Tetrahedron, 2012, 68, 3952.
2. (a) X. L. Hou, H. Y. Cheung, T. Y. Hon, P. L. Kwan, T. H. Lo, S. Y. Tong and H. N. C. Wong, Tetrahedron, 1998, 54, 1955; (b) B. A. Keay, Chem. Soc. Rev., 1999, 28, 209–215; (c) M. Harmata, Adv. Synth. Catal., 2006, 348, 2297; (d) S. F. Kirsch, Org. Biomol. Chem., 2006, 4, 2076; (e) L. Yuhua, S. Feijie, J. Xueshun and L. Yuanhong, Prog. Chem., 2010, 22, 58; (f) R. Yan, J. Huang, J. Luo, P. Wen, G. Huang and Y. Liang, Synlett, 2010, 1071; (g) H. Cao, H.-F. Jiang, H.-W. Huang and J.-W. Zhao, Org. Biomol. Chem., 2011, 9, 7313; (h) W. J. Moran and A. Rodriguez, Org. Prep. Proced. Int., 2012, 44, 103; (i) H. Cao, H. Zhan, J. Wu, H. Zhong, Y. Lin and H. Zhang, Eur. J. Org. Chem., 2012, 2318; (j) W.-B. Liu, C. Chen and Q. Zhang, Synth. Commun., 2013, 43, 951; (k) A. V. Gulevich, A. S. Dudnik, N. Chernyak and V. Gevorgyan, Chem. Rev., 2013, 113, 3084; (l) I. V. Trushkov, M. G. Uchuskin and A. V. Butin, Eur. J. Org. Chem., 2015, 2999.
3. (a) S. Khaghaninejad and M. M. Heravi, Paal–Knorr Reaction in the Synthesis of Heterocyclic Compounds, in Advances in Heterocyclic Chemistry, ed. A. R. Katritzky, Elsevier Academic Press Inc., San Diego, 2014, vol. 111, p. 95; (b) L. Knorr, Ber. Dtsch. Chem. Ges., 1884, 17, 2863; (c) C. Paal, Ber. Dtsch. Chem. Ges., 1884, 17, 2756.
4. (a) F. Feist, Chem. Ber., 1902, 35, 1537; (b) E. Benary, Chem. Ber., 1911, 44, 489.
5. (a) A. V. Kel'in and V. Gevorgyan, J. Org. Chem., 2002, 67, 95; (b) Y. Nishibayashi, M. Yoshikawa, Y. Inada, M. D. Milton, M. Hidai and S. Uemura, Angew. Chem., 2003, 115, 2785; (c) S. Ma and J. Zhang, J. Am. Chem. Soc., 2003, 125, 12386; (d) J. Zhang and H.-G. Schmalz, Angew. Chem., 2006, 118, 6856; (e) R. Sanz, D. Miguel, A. Martinez, J. M. Alvarez-Gutierrez and F. Rodriguez, Org. Lett., 2007, 9, 727; (f) J. Barluenga, L. Riesgo, R. Vicente, L. A. Lopez and M. Tomas, J. Am. Chem. Soc., 2008, 130, 13528; (g) L. K. Sydnes, B. Holmelid, M. Sengee and M. Hanstein, J. Org. Chem., 2009, 74, 3430; (h) C. He, S. Guo, J. Ke, J. Hao, H. Xu, H. Chen and A. Lei, J. Am. Chem. Soc., 2012, 134, 5766; (i) L. Li, M.-N. Zhao, Z.-H. Ren, J. Li and Z.-H. Guan, Synthesis, 2012, 44, 532.
6. (a) A. Sromek, A. V. Kel'in and V. Gevorgyan, Angew. Chem., 2004, 116, 2330 (Angew. Chem., Int. Ed., 2004, 43, 2280); (b) M. Nakamura, L. Ilies, S. Otsubo and E. Nakamura, Angew. Chem., 2006, 118, 958 (Angew. Chem., Int. Ed., 2006, 45, 944); (c) H. Tsuji, C. Mitsui, L. Ilies, Y. Sato and E. Nakamura, J. Am. Chem. Soc., 2007, 129, 11902; (d) A. Sniady, A. Durham, M. S. Morreale, K. A. Wheeler and R. Dembinski, Org. Lett., 2007, 9, 1175; (e) L.-B. Zhao, Z.-H. Guan, Y. Han, Y.-X. Xie, S. He and Y.-M. Liang, J. Org. Chem., 2007, 72, 10276; (f) A. Correa, N. Marion, L. Fensterbank, M. Malacria, S. P. Nolan and L. Cavallo, Angew. Chem., 2008, 120, 730 (Angew. Chem., Int. Ed., 2008, 47, 718); (g) L. Melzig, C. B. Rauhut and P. Knochel, Chem. Commun., 2009, 3536; (h) A. Dudnik, Y. Xia, Y. Li and V. Gevorgyan, J. Am. Chem. Soc., 2010, 132, 7645; (i) Z. Chen, G. Huang, H. Jiang, H. Huang and X. Pan, J. Org. Chem., 2011, 76, 1134; (j) M. R. Kuram, M. Bhanuchandra and A. K. Sahoo, Angew. Chem., Int. Ed., 2013, 52, 4607; (k) U. Sharma, T. Naveen, A. Maji, S. Manna and D. Maiti, Angew. Chem., Int. Ed., 2013, 52, 12669; (l) J. González, J. González, C. Pérez-Calleja, L. A. López and R. Vicente, Angew. Chem., 2013, 125, 5965 (Angew. Chem., Int. Ed., 2013, 52, 5853).
7. (a) H. Cao, H. Jiang, R. Mai, S. Zhu and C. Qi, Adv. Synth. Catal., 2010, 352, 143; (b) H. Cao, H. Jiang, W. Yao and X. Liu, Org. Lett., 2009, 11, 1931; (c) H. Cao, H. Zhan, J. Cen, J. Lin, Y. Lin, Q. Zhu, M. Fu and H. Jiang, Org. Lett., 2013, 15, 1080; (d) Y. Yue, Y. Zhang, W. Song, X. Zhang, J. Liu and K. Zhuo, Adv. Synth. Catal., 2014, 356, 2459.
8. (a) D. Astruc, Nanoparticles and Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, 2008, pp. 1–48; (b) D. Astruc, F. Lu and J. Aranzaes, Angew. Chem., Int. Ed., 2005, 44, 7852; (c) I. Beletskaya and A. Cheprakov, Chem. Rev., 2000, 100, 3009; (d) R. Schlogl and S. B. A. Hamid, Angew. Chem., Int. Ed., 2004, 43, 1628; (e) M. B. Gawande, R. K. Pandey and R. V. Jayaram, Catal. Sci. Technol., 2012, 2, 1113.
9. (a) V. Polshettiwar and R. S. Varma, Green Chem., 2010, 12, 743; (b) M. Gawande, P. Branco and R. S. Varma, Chem. Soc. Rev., 2013, 42, 3371.
10. (a) H. Kung, Transitionmetal oxides: Surface chemistry and catalysis, 1989, vol. 45, p. 1; (b) A. Bell, Science, 2003, 299, 1688; (c) S. Ibrahim, Catal. Rev., 2003, 45, 205.
11. (a) H.-Q. Do and O. Daugulis, J. Am. Chem. Soc., 2011, 133, 13577; (b) X.-R. Qin, B.-Y. Feng, J.-X. Dong, X.-Y. Li, Y. Xue, J.-B. Lan and J.-S. You, J. Org. Chem., 2012, 77, 7677; (c) S. Fan, Z. Chen and X.-G. Zhang, Org. Lett., 2012, 14, 4950.
12. (a) G. Evindar and R. A. Batey, Org. Lett., 2003, 5, 133; (b) J. C. Antilla, J. M. Baskin, T. E. Barder and S. L. Buchwald, J. Org. Chem., 2004, 69, 5578; (c) Y.-X. Xie, S.-F. Pi, J. Wang, D.-L. Yin and J.-H. Li, J. Org. Chem., 2006, 71, 8324; (d) M. Cortes-Salva, C. Garvin and J. C. Antilla, J. Org. Chem., 2011, 76, 1456; (e) O. A. Davis, M. Hughes and J. A. Bull, J. Org. Chem., 2013, 78, 3470.
13. (a) Y. Li, Z.-S. Li, T. Xiong, Q. Zhang and X.-Y. Zhang, Org. Lett., 2012, 14, 3522; (b) P. Saha, T. Ramana, N. Purkaite, M. A. Ali, R. Paul and T. Punniyamurthy, J. Org. Chem., 2009, 74, 8719.
14. (a) E. Sperotto, G. P. M. van Klink, J. G. de Vries and G. van Koten, J. Org. Chem., 2008, 73, 5625; (b) C. Uyeda, Y.-C. Tan, G. C. Fu and J. C. Peters, J. Am. Chem. Soc., 2013, 135, 9548.
15. (a) S. Banerjee and S. Santra, Tetrahedron Lett., 2009, 50, 2037; (b) S. Banerjee and G. Sereda, Tetrahedron Lett., 2009, 50, 6959; (c) S. Banerjee, J. Das, R. Alverez and S. Santra, New J. Chem., 2010, 34, 302; (d) S. Banerjee, V. Balasanthiran, R. Koodali and G. Sereda, Org. Biomol. Chem., 2010, 8, 4316; (e) S. Banerjee, A. Horn, H. Khatri and G. Sereda, Tetrahedron Lett., 2011, 52, 1878; (f) S. Banerjee and A. Saha, New J. Chem., 2013, 37, 4170; (g) S. Payra, A. Saha, G. Sereda and S. Banerjee, Tetrahedron Lett., 2014, 55, 5515; (h) A. Saha, S. Payra and S. Banerjee, Green Chem., 2015, 17, 2859; (i) S. Banerjee, New J. Chem., 2015, 39, 5350; (j) A. Saha, S. Payra and S. Banerjee, RSC Adv., 2015, 5, 101664; (k) S. Payra, A. Saha and S. Banerjee, RSC Adv., 2016, 6, 12402.
16. D. Han, H. Yang, C. Zhu and F. Wang, Powder Technol., 2008, 185, 286.
17. General method for the synthesis of 3,4-dicabonyl substituted furan derivative. Representative procedure for the synthesis of 1-(4-benzoyl-5-(4-methoxyphenyl)-2-methylfuran-3-yl)ethanone (3c, Table 2): a mixture of (E)-3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one (0.5 mmol; 119.14 mg), acetyl acetone (2.5 mmol; 250 mg), TBHP (0.5 equiv.) and CuO nanoparticles (10 mg) was refluxed in ethanol : water (1 : 1, 2 mL) for 3 hours. After completion of the reaction (TLC monitored), the resulting mixture was cooled to room temperature and centrifuged to separate the catalyst. The organic layer was extracted with ethyl acetate (25 mL), washed with brine solution (3 × 5 mL) and dried over anhydrous sodium sulfate. Evaporation of solvent left the crude solid product which was purified by column chromatography on silica gel (ethyl acetate/petroleum ether = 1/9) to provide pure 1-(4-benzoyl-5-(4-methoxyphenyl)-2-methylfuran-3-yl)ethanone (135.4 mg, 81%, 3c; Table 2) as pale yellow solid. The structure of the product was confirmed by spectroscopic (¹H NMR and ¹³C NMR) studies and elemental analysis. ¹H NMR (500 MHz, CDCl₃): δ 8.01 (d, J = 7.5 Hz, 2H), 7.61–7.55 (m, 3H), 7.51 (t, J = 7.5 Hz, 2H), 6.93 (d, J = 8.5, 2H), 3.86 (s, 3H), 2.55 (s, 3H), 2.16 (s, 3H), anal. calc. for C₂₁H₁₈O₄: C, 75.43; H, 5.43%; found: C, 75.40; H, 5.45%. These results are in good agreement with those of reported one.⁷; The similar method was followed for all the furan derivatives listed in Tables 2 and 3 except in case of Table 3 β-ketoesters was used in place of 1,3-diketones.
18. Z. Li, Z. Cui and Z.-Q. Liu, Org. Lett., 2013, 15, 406.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedure, spectral data, copies of ¹H, ¹³C NMR, HRMS and FT-IR spectra, powder XRD of recycled CuO NPs etc. See DOI: 10.1039/c6ra04181g

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