Nano-FGT: a green and sustainable catalyst for the synthesis of spirooxindoles in aqueous medium

Abstract

A glutathione grafted nano-organocatalyst (nano-FGT) was used as an efficient catalyst for the synthesis of spirooxindole derivatives. An aqueous medium, easy separation by an external magnet, efficient catalyst reusability, low catalyst loading, and column chromatography free separation of the product makes the present procedure green, sustainable and economically viable. The TON and TOF of the nanocatalyst reached 850 000 and 56 667 min⁻¹ respectively.

---

Over the years, the use of organocatalysis in organic transformation reactions had been sporadically reported. A vast number of organic transformation reactions have benefitted from such organocatalysts but the synthesis of organocatalysts is difficult, expensive, and gives low TON and TOF. Moreover loss of the catalyst component upon workup is necessarily large and is economically and environmentally not viable.^1,2 For the aforementioned reasons, considerable efforts that address catalyst recycling have come to light. Recently, much research has been attributed to organocatalyst grafted magnetic nanoparticles.³ These functionalized magnetic nanoparticles have emerged as feasible alternatives to conventional bulk catalyst as a readily available, robust and efficient heterogeneous catalyst support.⁴ Magnetically retrievable organocatalyst not only addresses the predicament of recyclability but also result in a far efficient catalytic system for such reactions due to higher surface to volume ratio. Miscellaneous reactions such as Baylis–Hillman,⁵ Diels–Alder,⁶ Michael,⁷ enantioselective C–C, C–N and C–O bond formation⁸ reactions etc. have been catalyzed by such functionalized nanoparticles. Recently, Varma et al. established the homo coupling of organoboronic acid, catalyzed by glutathione functionalized nano-organocatalyst (nano-FGT).^4e,9

Spirooxindole derivatives are privileged indole containing heterocycles of relevant biological and medicinal importance like antitumoral, antifungal and antibacterial activity, inhibitor of muscarinic serotonin receptors and microtubular activity (Fig. 1).^10,11 Because of the tremendous biological and medicinal activities, a growing research towards a much efficient, environmentally benign and cost efficient synthetic methods is still appreciated. Some synthetic methods involve the use of catalyst such as InCl₃,¹² SnCl₄,¹³ NEt₃,¹⁴ NH₄Cl,¹⁵ Sodium stearate,¹⁶ MgO,¹⁷ L-proline,¹⁸ silica-supported organocatalyst¹⁹ and lipase²⁰ etc. are reported. All this reported methods have their own advantages but at the same time they suffer some drawbacks like use of large amount of catalyst,²¹ high temperature,²² long reaction time^14,23 and unreusable catalyst.²⁴ If the catalyst is reusable but separation is difficult.²⁵ So there is enough room for innovation of new synthetic method. Herein, we report nano-FGT as a viable greener catalyst for the synthesis of spirooxidoles under reflux condition in water within a short period of time, offering high yield with high purity of the product without column chromatographic separation. Easy separation and reusability of the catalyst up to eight consecutive runs makes this procedure sustainable.

Fig. 1 Biologically important spirooxindole derivatives.

Results and discussion

Initially, various amino acids such as L-proline, cysteine and glutathione were screened as practicable catalyst for the synthesis of spirooxindole derivative (4aaa) under homogeneous condition in water medium at 80 °C. Among them glutathione showed better catalytic activity for the synthesis of spirooxindole derivatives (Table 1, entry 1, 2 and 3). The recovery and reuse of glutathione is however difficult from the reaction mixture. So, we tried to attach glutathione with magnetic nanoparticles. After literature survey, we followed the procedure for the nano-FGT synthesis outlined by V. Polshettiwar et al.^4g The Fe₃O₄ NPs were dispersed in deionized water and MeOH (3 : 1). The resulting colloidal solution was then sonicated for a period of 15 min. Glutathione dissolved in water and added to this colloidal solution. The resulting solution was sonicated for another 2 h. The magnetic glutathione-functionalized nano-material was then isolated by external magnet, washed with water, MeOH and dried under vacuum at 50–60 °C (Scheme 1). The synthesized post-synthetic surface modified nano-organocatalyst was then characterized using various techniques such as FT-IR, TEM, SEM and EDX.

Table 1 Effect of various catalysts on the reaction^a

Entry	Catalyst	Temperature (°C)	Time (min)	Product	Yield (%)
a Reaction condition: isatin (1a, 1 mmol), malononitrile (2a, 1 mmol), dimedone (3a, 1 mmol) and catalyst in water (3 mL). r.t = room temperature.
1	Glutathione (10 mol%)	80	15	4aaa	96
2	Cysteine (10 mol%)	80	15	4aaa	88
3	L-Proline (10 mol%)	80	15	4aaa	91
4	Without catalyst	r.t	15	4aaa	14
5	Without catalyst	80	15	4aaa	21
6	Fe3O4 NPs (8 mg)	80	15	4aaa	66
7	Nano-FGT (8 mg)	r.t	15	4aaa	57
8	Nano-FGT (8 mg)	40	15	4aaa	69
9	Nano-FGT (8 mg)	60	15	4aaa	85
10	Nano-FGT (8 mg)	80	15	4aaa	94
11	Nano-FGT (8 mg)	100	15	4aaa	94

---

Scheme 1 Synthetic scheme of glutathione@Fe₃O₄ nanoparticles (nano-FGT).

Transmission electron microscope (TEM) was used to analyze the particle size of the prepared nano-FGT. The TEM image is indicative of the fact that the prepared nano-FGT were in the nanometer range. Further, the TEM image shows the uniform distribution of the synthesized nano-FGT with an average size of 10–20 nm (Fig. 2) and also the uniform coating of glutathione over the ferrite NPs (Fig. 2c). Scanning electron microscope (SEM) was also used to examine the surface morphology of the prepared catalyst. From SEM image it is evident that the nano-FGT are spherical in shape (Fig. 3a). Energy Dispersive X-ray (EDX) analysis also indicates the formation of the nano-FGT. The EDX spectra clearly show the presence of Fe, O, C, S and N which is evident for the anchoring of the glutathione to the ferromagnetic Fe₃O₄ NPs (Fig. 4). Infra-red spectroscopy also provides useful information about the anchoring of glutathione to the ferromagnetic Fe₃O₄NPs. The strong absorption peak at 599 cm⁻¹ corresponds to the Fe–O stretching of the Fe₃O₄ NPs. The stretching peaks at 1643 and 1634 cm⁻¹ is characteristic of the acid and amide carbonyls of glutathione respectively. Further, absorption peaks at 3349 and 3166 cm⁻¹ correspond to the –OH and –NH₂ groups. The absence of a strong absorption peak at 2525 cm⁻¹ which is characteristic peak of –SH group manifest for the anchoring of glutathione through the thiol group to the Fe₃O₄ NPs (see ESI, Fig. S1†). The stability of the nano-FGT was also examined by Thermo Gravimetric Analysis (TGA). The degradation at 53 °C in the TGA thermogram corresponds to the solvent molecules trapped. The weight loss at 185 °C is due to the glutathione of nano-FGT. Therefore, the catalyst shows relative stability below 180 °C and can safely be used for organic transformation reactions within this temperature range (Fig. 5).

Fig. 2 TEM image of nano-FGT at (a) lower magnification (b) before use at 10 nm (c) before use at 5 nm (d) after use at 10 nm and (e) SAED of the catalyst before use.

Fig. 3 SEM image of nano-FGT (a) before use and (b) after use.

Fig. 4 EDX spectra of nano-FGT.

Fig. 5 TGA thermogram of nano-FGT.

We initiated our study by taking isatin (1a, 1 mmol), malononitrile (2a, 1 mmol), dimedone (3a, 1 mmol) and nano-FGT in water (3 mL) as the model system (Scheme 2) and the reaction was carried out at room temperature. Interestingly, in the presence of catalyst, the reaction formed the desired product 4aaa in 57% yield in 15 min. To increase the yield of the reaction, we carried out the reaction at higher temperatures ranging from 40–100 °C. It was observed that the yield was enhanced with increasing temperature and the maximum yield of 94% was obtained at 80 °C. Further increase in the temperature did not have any effect in the yield or time of the reaction. The possibility of Fe₃O₄ NPs to catalyse the reaction process was also tested. It was observed that Fe₃O₄ NPs could not efficiently catalyze the reaction, resulting in only moderate yield of the product (66%, Table 1). The structure of the product was then characterized by elemental analysis as well as IR, ¹H and ¹³C NMR and mass spectroscopy.

Scheme 2 Model reaction.

Several sets of reaction were performed in order to determine the optimum amount of catalyst required for the said reaction. The model reaction of isatin (1a, 1 mmol), malononitrile (2a, 1 mmol) and dimedone (3a, 1 mmol) without any catalyst in aqueous medium (3 mL) at 80 °C was carried out. The reaction resulted only in traces of the product 4aaa leaving behind unreacted starting materials even after 15 min of heating. When the same reaction was carried out with varying amount of the catalyst ranging from 2–12 mg, a much promising results were achieved. The yield of the product increased with increasing amount of the catalyst from 2–8 mg. The reaction with 8 mg of the catalyst produced the maximum yield of the product (94%). However further increase in the amount of the catalyst did not lead to any improvement in the yield of the product. Therefore the optimum amount of the catalyst required for the present reaction is 8 mg (Fig. 6). The turn over number (TON) and the turn over frequency (TOF) of the catalyst reached 850 000 and 56 667 min⁻¹ with respect to glutathione.

Fig. 6 Chart showing the loading of catalyst at 80 °C.

To search the best solvent system for the model reaction, we examined various solvents such as toluene, CHCl₃, CH₃CN, EtOH and water. The catalyst shows better activity in acetonitrile (73%) compared with other polar solvents like THF and CHCl₃ but most satisfactory result was achieved in case of EtOH (97%) and water (94%). Toluene was also not effective as a solvent for the present catalytic protocol. Because water and ethanol producing almost same amount of yield so we chose water as the solvent due to environmental awareness, cost effectiveness and easy accessibility (Fig. 7).

Fig. 7 Chart showing the effect of solvent.

After optimization of the reaction parameters, the versatility of the catalyst was tested with various C–H activated compounds like dimedone (3a), pyrazolone (3b), barbituric acid (3c), 1,3-dimethyl barbituric acid (3d), 2-thiobarbituric acid (3e) and 4-hydroxy coumarin (3f) and also 1,2-diketo compounds such as isatin derivatives (1a and 1b) and acenaphthenequinone (1c). In all the cases the reaction proceeded smoothly and produced good to excellent yield of the desired product (Table 2). It was observed that the electronic effect had no influence on the product yield of the reaction. The most important advantage of the present method is the chromatography free separation of the desired product. Only recrystallization from methanol afforded the desired product with excellent purity. The above results are indicative of the fact that the nano-FGT catalyst is highly efficient and can be applied for the synthesis of a diverse library of spirooxindole derivatives (Scheme 3). To outline the industrial applicability of the present methodology, a gram scale reaction was also studied (Scheme 4). Gram scale reaction was performed by taking isatin (1a, 7 mmol), malononitrile (2a, 7 mmol), dimedone (3a, 7 mmol) and catalyst nano-FGT (56 mg) in aqueous medium (20 mL). The resulting reaction mixture was heated at 80 °C with stirring for a period of 15 min. The reaction proceeded smoothly to completion and an isolated yield of 91% was achieved.

Table 2 Synthesis of compounds 4aaa–4cac^a

Entry	1,2-Diketone	Malonates	C–H activated compounds	Product	Yield (%)	Melting point (°C)
a Reaction condition: 1,2-diketone (1a–c, 1 mmol), malonates (2a–c, 1 mmol), enolizable C–H activated compounds (3a–f, 1 mmol) and nano-FGT (8 mg) in aqueous medium (3 mL) at 80 °C for 15 min.
1	1a	2a	3a	4aaa	94	289–292
2	1a	2a	3b	4aab	93	236–237
3	1a	2a	3c	4aac	97	296–298
4	1a	2a	3d	4aad	96	229–231
5	1a	2a	3e	4aae	95	240–241
6	1a	2a	3f	4aaf	91	290–291
7	1a	2b	3a	4aba	95	275–277
8	1a	2b	3b	4abb	96	236–239
9	1a	2c	3a	4aca	92	254–256
10	1b	2a	3a	4baa	90	251–252
11	1b	2a	3b	4bab	92	198–199
12	1b	2b	3a	4bba	94	254–256
13	1c	2a	3a	4caa	95	267–268
14	1c	2a	3b	4cab	93	193–195
15	1c	2a	3c	4cac	91	>300

---

Scheme 3 General scheme for the synthesis of spirooxindole derivatives.

Scheme 4 Synthesis of spirooxindole in gram scale. Reaction condition: isatin (1a, 7 mmol), malononitrile (2a, 7 mmol), dimedone (3a, 7 mmol) and nano-FGT (56 mg) in aqueous medium (20 mL) at 80 °C.

The plausible catalytic cycle for the synthesis of spirooxindole derivatives is given below (Scheme 5). Initially,²⁶ amine functionality of the catalyst forms iminium intermediate 5 with 1,2-diketones (1a–c). This iminium intermediate inturn facilitates the Knoevenagel condensation with malonates (2a–c) and a new intermediate is formed (7). On the other hand nano-FGT helps to enolize the 1,3-diketo compounds (3a–f). These enolized compounds undergo Michael addition with Knoevenagel product 7 and generates another intermediate 8. Finally intramolecular cyclization followed by hydrogen shift furnish the products 4aaa–4cac.

Scheme 5 Plausible mechanistic pathway.

A hot filtration test was performed to measure the leaching of the catalyst nano-FGT. Initially, the solvent water (3 mL) and the catalyst nano-FGT (8 mg) were heated at 80 °C for a period of 15 min. After the time frame, the catalyst was removed by means of simple external magnet. The reactants isatin (1a, 1 mmol), malononitrile (2a, 1 mmol) and dimedone (3a, 1 mmol) were then added to the reaction pot. The reaction mixture was then allowed to proceed for a further period of 15 min. It was observed that after the removal of the catalyst from the reaction only trace amount (21%) of product formed. This trace amount of product formed due to the uncatalyzed pathway (Table 1; entry 5). Thus it confirmed that, there was no significant amount of leaching of nano-FGT taking place.

Reusability is one of the essential parameter of a good and efficient catalyst which makes the procedure economically cheap, industrially profitable and environmentally sustainable. Therefore the reusability of the catalyst was also studied. After the completion of the reaction, the nano-FGT catalyst was separated by means of simple external magnet, washed with MeOH and dried. The nano-FGT was then used for another set of reaction under the same experimental condition. It was interesting to note that the recycled catalyst drove the reaction to completion with almost equal efficiency as the new catalyst. The nano-FGT catalyst could be recycled and reused for eight consecutive runs without any appreciable change in catalytic activity. The reproducibility of the reaction was also accessed. Five model reactions for each set under the same reaction condition were performed. The reproducibility of the reaction was then calculated using the mean data for each set (93.8, 93.8, 93.6, 92.8, 92.8, 92.8, 91.6 and 89.8) and the error bars are reported by means of standard deviation method (Fig. 8).

Fig. 8 Reusability of catalyst.

After completion of eight cycles, we analyzed TEM, and SEM of the reused catalyst. We found that the morphology and shape remained unaltered. The TEM image of the nano-FGT after use (Fig. 2d) revealed that the coating, shape and size almost remained intact even after eight consecutive runs. The SEM image also shows no change in surface morphology of the nano-FGT after eight runs (Fig. 3b).

The compounds 4aaa, 4aab and 4caa were carefully recrystallized from MeOH. The structures of the aforementioned compounds were further confirmed using X-ray crystallography. Fig. 9 shows the ORTEP diagram of the compounds 4aaa, 4aab and 4caa.

Fig. 9 ORTEP diagram of compounds (a) 4aaa (CCDC no. 1031108), (b) 4aab (CCDC no. 1031133) and (c) 4caa (CCDC no. 1032132).

Conclusions

In conclusion, a green, sustainable and an efficient protocol for the synthesis of spirooxindole derivatives in aqueous medium was developed using heterogeneous nano-FGT as nano-organocatalyst. The nano-FGT could easily be separated by means of an external magnet eliminating the process of centrifugation and filtration. The most attractive features of the reaction are very low catalyst loading, water as a solvent, short reaction period, high product yield, easy separation as well as reusability of the catalyst and chromatography free separation of the desired product. So the procedure is cost effective, environmental friendly, green and sustainable. Therefore this process can be useful for industrial and academic purpose.

Experimental

Melting points were determined in open capillaries and are uncorrected. IR spectra were recorded on Spectrum BX FT-IR, Perkin Elmer (ν_max in cm⁻¹) on KBr disks. ¹H NMR and ¹³C NMR (400 MHz and 100 MHz respectively) spectra were recorded on Bruker Avance II-400 spectrometer in CDCl₃ and DMSO-d₆ (chemical shifts in δ with TMS as internal standard). Mass spectra were recorded on Waters ZQ-4000. Transmission Electron Microscope (TEM) was recorded on JEOL JSM 100CX. Scanning electron microscope (SEM) was recorded on JSM-6360 (JEOL). Thermogravimetric analysis (TGA) was recorded on a Perkin Elmer Precisely STA 6000 simultaneous thermal analyzer. CHN were recorded on CHN-OS analyzer (Perkin Elmer 2400, Series II).

X-ray crystallography

The X-ray diffraction data were collected at 293 K with Mo Kα radiation (λ = 0.71073 Å) using Agilent Xcalibur (Eos, Gemini) diffractometer equipped with a graphite monochromator. The software used for data collection CrysAlis PRO (Agilent, 2011), data reduction CrysAlis PRO and cell refinement CrysAlis PRO. The structure were solved by direct methods and refined by full-matrix least-squares calculation using SHELXS-97 (ref. 27) and SHELXL-97.²⁸

Procedure for the synthesis of Fe₃O₄ NPs

A mixture of 3.4 g of ferric nitrate and 3 g of ferrous sulphate was taken in a clean 250 mL round bottom flask. To it 100 mL of deionized water was added and stirred for a period of 15 min, and solution became homogeneous. After that ammonium hydroxide (25%) was then added drop-wise till the pH of the resulting solution was attained 10. During addition of ammonium hydroxide, the formation of black precipitate was observed. The solution was then heated at 50–60 °C for 1 h. After the time mentioned, the magnetic black precipitate was separated, washed with water until the pH became neutral and dried in oven for 5 h.

Procedure for the synthesis of nano-FGT

The Fe₃O₄ NPs (0.5 g) were dispersed in 15 mL of deionized water and 5 mL of MeOH. The resulting colloidal solution was then sonicated for a period of 15 min. Glutathione (0.4 g) was dissolved in 5 mL of deionized water and added to this colloidal solution. The resulting solution was sonicated for 2 h. Then glutathione-functionalized Fe₃O₄ nano-material was then isolated by external magnet, washed with water (3× 5 mL), MeOH (3 × 5 mL) and dried under vacuum at 50–60 °C.

Synthetic procedure of spirooxindole derivatives (4aaa–4cac)

In a clean round bottom flask, 1,2-diketone (1a–c, 1 mmol), malonates (2a–c, 1 mmol), enolizable C–H activated compounds (3a–f, 1 mmol) and nano-FGT (8 mg) in water (3 mL) was heated at 80 °C for the time mentioned in the Table 2. After completion of reaction, the catalyst was separated by simple external magnet, washed with MeOH (3 × 5 mL), dried and reused. The reaction mixture was allowed to cool. Filtration of the solid precipitate followed by washing the residue with hot water (3 × 10 mL) furnished the pure product (4aaa–4cac).

Acknowledgements

We thank the Department of Chemistry, Sophisticated Analytical and Instrumentation Facility (SAIF) of North-Eastern Hill University and IIT-Bombay and UGC for supporting this work under Special Assistance Programme (SAP). We are also thankful to DST for financial support (sanctioned no. SERC/F/0293/2012-13) and DST-Purse. I am also thankful to NEHU-NON NET fellowship for their constant financial assistance.

Notes and references

1. B. H. Lipshutz and S. Ghorai, Org. Lett., 2012, 14, 422.
2. (a) B. M. Trost and C. S. Brindle, Chem. Soc. Rev., 2010, 39, 1600; (b) S. G. Zlotin, A. S. Kucherenko and I. P. Beletskaya, Russ. Chem. Rev., 2009, 78, 737; (c) K. Sakthivel, W. Notz, T. Bui and C. F. Barbas, J. Am. Chem. Soc., 2001, 123, 5260; (d) Z. Tang, Z. H. Yang, X. H. Chen, L. F. Cun, A. Q. Mi, Y. Z. Jiang and L. Z. Gong, J. Am. Chem. Soc., 2005, 127, 9285; (e) S. Samanta, J. Y. Liu, R. Dodda and C. G. Zhao, Org. Lett., 2005, 7, 5321; (f) G. Guillena, M. C. Hita and C. Najera, Tetrahedron: Asymmetry, 2006, 17, 1493.
3. (a) V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara and J.-M. Basset, Chem. Rev., 2011, 111, 3036; (b) D. Wang and D. Astruc, Chem. Rev., 2014, 114, 6949; (c) R. B. N. Baig and R. S. Varma, Green Chem., 2013, 15, 398; (d) R. B. N. Baig and R. S. Varma, Chem. Commun., 2013, 49, 752; (e) R. B. N. Baig, M. N. Nadagouda and R. S. Varma, Coord. Chem. Rev., 2015, 287, 137.
4. (a) A. Hu, G. T. Yee and W. Lin, J. Am. Chem. Soc., 2005, 127, 12486; (b) R. Abu-Reziq, H. Alper, D. Wang and M. L. Post, J. Am. Chem. Soc., 2006, 128, 5279; (c) C. O. Dalaigh, S. A. Corr, Y. Gunko and S. J. A. Connon, Angew. Chem., Int. Ed., 2007, 46, 4329; (d) T. Hara, T. Kaneta, K. Mori, T. Mitsudome, T. Mizugaki, K. Ebitanic and K. Kaneda, Green Chem., 2007, 9, 1246; (e) V. Polshettiwar, B. Baruwati and R. S. Varma, Chem. Commun., 2009, 1837; (f) V. Polshettiwar, B. Baruwati and R. S. Varma, Green Chem., 2009, 11, 127; (g) V. Polshettiwar and R. S. Varma, Tetrahedron, 2010, 66, 1091.
5. (a) G. Masson, C. Housseman and J. Zhu, Angew. Chem., Int. Ed., 2007, 46, 4614; (b) D. Basavaiah, K. V. Rao and R. J. Reddy, Chem. Soc. Rev., 2007, 36, 1581.
6. (a) W. Notz, F. Tanaka and C. F. Barbas, Acc. Chem. Res., 2004, 37, 580; (b) J. Liu, Z. Yang, Z. Wang, F. Wang, X. Chen, X. Liu, X. Feng, Z. Su and C. Hu, J. Am. Chem. Soc., 2008, 130, 5654.
7. (a) Y. Hayashi, H. Gotoh, T. Tamura, H. Yamaguchi, R. Masui and M. Shoji, J. Am. Chem. Soc., 2005, 127, 16028; (b) H. Xie, L. Zu, H. Li, J. Wang and W. Wang, J. Am. Chem. Soc., 2007, 129, 10886.
8. J. Franzen, M. Marigo, D. Fielenbach, T. C. Wabnitz, A. Kjarsgaard and K. A. Jorgensen, J. Am. Chem. Soc., 2005, 127, 18296.
9. R. Luque, B. Baruwati and R. S. Varma, Green Chem., 2010, 12, 1540.
10. (a) W. J. Houlihan, W. A. Remers and R. K. Brown, Indoles: Part I, Wiley, New York, 1992; (b) R. J. Sundberg, The Chemistry of Indoles, Academic, New York, 1996; (c) K. C. Joshi and P. Chand, Pharmazie, 1982, 37, 1; (d) P. R. Sebahar and R. M. Williams, J. Am. Chem. Soc., 2000, 122, 5666; (e) S. Edmondson, S. J. Danishefsky, L. Sepp-Lorenzinol and N. Rosen, J. Am. Chem. Soc., 1999, 121, 2147; (f) T. H. Kang, K. Matsumoto, M. Tokda, Y. Murakami, H. Takayama, M. Kitajima, N. Aimi and H. Watanabe, Eur. J. Pharmacol., 2002, 444, 39.
11. G. Bhaskar, Y. Arun, C. Balachandran, C. Saikumar and P. T. Perumal, Eur. J. Med. Chem., 2012, 51, 79.
12. G. Shanthi, G. Subbulakshmi and P. T. Perumal, Tetrahedron, 2007, 63, 2057.
13. B. Liang, S. Kalidindi and J. A. Porco, Org. Lett., 2010, 12, 572.
14. M. Saeedi, M. M. Heravi, Y. S. Beheshtiha and H. A. Oskooie, Tetrahedron, 2010, 66, 5345.
15. M. Dabiri, M. Bahramnejad and M. Baghbanzadeh, Tetrahedron, 2009, 65, 9443.
16. L.-M. Wang, N. Jiao, J. Qiu, J.-J. Yu, J.-Q. Liu, F.-L. Guo and Y. Liu, Tetrahedron, 2010, 66, 339.
17. B. Karmakar, A. Nayak and J. Banerji, Tetrahedron Lett., 2012, 53, 5004.
18. Y. Li, H. Chen, C. Shi, D. Shi and S. Ji, J. Comb. Chem., 2010, 12, 231.
19. A. Khala-Nezhad, E. S. Shahidzadeh, S. Sarikhani and F. Panahi, J. Mol. Catal. A: Chem., 2013, 379, 1.
20. S.-J. Chai, Y.-F. Lai, J.-C. Xu, H. Zheng, Q. Zhu and P.-F. Zhang, Adv. Synth. Catal., 2011, 353, 371.
21. B. M. Rao, G. N. Reddy, T. V. Reddy, B. L. A. P. Devi, R. B. N. Prasad, J. S. Yadav and B. V. S. Reddy, Tetrahedron Lett., 2013, 54, 2466.
22. J. Deng, L.-P. Mo, F.-Y. Zhao, Z.-H. Zhang and S.-X. Liu, ACS Comb. Sci., 2012, 14, 335.
23. J. Zheng and Y. Li, Mendeleev Commun., 2012, 22, 148.
24. Y. Li, H. Chen, C. Shi, D. Shi and S. Ji, J. Comb. Chem., 2010, 12, 231.
25. (a) A. Dandia, V. Parewa, A. K. Jain and K. S. Rathore, Green Chem., 2011, 13, 2135; (b) A. Hasaninejada, N. Golzara, M. Beyratia, A. Zareb and M. M. Doroodmand, J. Mol. Catal. A: Chem., 2013, 372, 137.
26. (a) Y. Li, H. Chen, C. Shi, D. Shi and S. Ji, J. Comb. Chem., 2010, 12, 231; (b) A. Gupta, R. Jamatia and A. K. Pal, New J. Chem., 2013, 37, 5636.
27. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 1990, 46, 467.
28. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 1031108, 1031133 and ​1032132. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra27552k

---