Synthesis of indeno and acenaphtho cores containing dihydroxy indolone, pyrrole, coumarin and uracil fused heterocyclic motifs under sustainable conditions exploring the catalytic role of the SnO₂ quantum dot

Abstract

A tin oxide (SnO₂) quantum dot (QD) catalyzed approach for the synthesis of indeno and acenaphtho cores containing dihydroxy indolone, pyrrole, coumarin and uracil fused derivatives was achieved via a multicomponent one-pot approach in aqueous medium. A variety of functional groups were compatible with the reaction conditions. This synthesis was established to follow the group-assistant-purification chemistry process, thus avoiding the use of traditional chromatography. An SnO₂ QD was prepared using a simple solvothermal method and characterized using X-ray diffraction and transmission electron spectroscopy images. The easy recovery of the catalyst and high yield of the products make the protocol attractive, sustainable and economic. The catalyst was reused for seven cycles with almost unaltered catalytic activity. The low cost, ease of handling and the simplicity of this catalytic system make the method competitive with other strong mineral acid-catalysed multicomponent protocols.

---

Introduction

A wide variety of homogeneous Brønsted acids (sulfuric acid, p-toluenesulfonic acid, hydrochloric acid and phosphoric acid) have been used for the synthesis of important chemicals, including pharmaceuticals, agrochemicals, and fragrances.^1–3 These acidic catalysts are economic and efficient, but have serious drawbacks associated with product isolation, equipment corrosion, solvent recycling, and reusability of the catalyst. Heterogeneous acid catalysts such as zeolites, transition-metal ions, and strong acid cation exchange resins have also been used sometimes to serve this purpose.^4–6

Catalytic processes in the solution phase are important in many areas of the fine and specialty chemical industries, and the use of solid catalysts means easier catalyst separation and recovery, thus facilitating their reuse. It is widely accepted that a smaller catalyst particle means higher activity.⁷ As a result, both the activity and the stability of a solid catalyst suspended in a liquid media can benefit greatly from the use of small catalyst particles. Nano-catalysts mimic homogeneous (high surface area, easily accessible) as well as heterogeneous (stable, easy to handle) catalyst systems. Thus, nano-catalysts make the system more efficient than conventional heterogeneous catalyst systems. In general, quantum dots (QDs) are nano-materials with a grain size of less than 5 nm which show unique electrical and catalytic properties because of their ultra-fine grain size. The unique properties of QDs occur for two reasons. The main reason can be attributed to the complete depletion of ultra-fine grains by charge carriers. In addition the increase of the surface-to-bulk ratio by decreasing the grain size of the particles plays an important role in the high reactivity of the particles. All these properties render the catalyst cost-effective, making it a promising alternative over the conventional Brønsted and Lewis acid catalysts for industrial applications.

Indeno[1,2-b]indoles are an important class of heterocyclic compound. These molecules consist of small and planar heterocyclic frameworks. Because of the presence of this type of structural feature, indeno[1,2-b]indoles (A and B, Fig. 1) serve as adenosine triphosphate/guanosine triphosphate-competitive inhibitors of protein kinase (CK2) and act as DNA-intercalators and inhibitors of topoisomerase II, with considerable cytotoxic activity towards cancer cells.⁸

Fig. 1 (A) Topoisomerase II inhibitor (B) human protein kinase CK2 inhibitor.

Coumarin and uracil fused heterocycles are one of the most active classes of compounds possessing a wide spectrum of biological activity.⁹ Many of the coumarin fused heterocycles show antitumor,¹⁰ antibacterial,¹¹ antifungal,¹² anticoagulant,¹³ anti-inflammatory,¹⁴ and antiviral¹⁵ activities. A large number of adenosine receptor agonists and antagonists have been proved to be highly potent and subtype selective ligands.^16,17 The astonishing drug activity of these indeno[1,2-b]indoles, coumarin and uracil based heterocyclic compounds has not only attracted many synthetic and medicinal chemists to synthesize this heterocyclic nucleus but has also become an active research area of enduring interest.

Synthesis of tin oxide (SnO₂) nanocrystals have attracted much attention from researchers because of their potential applications based on gas sensing,¹⁸ field-emission,¹⁹ electrochemical,²⁰ photocatalytic,²¹ and photovoltaic properties.^22,23 In continuation of our research program dedicated to the synthesis and application of metal oxide nano-catalysts for design and synthesis of novel heterocyclic systems,²⁴ we have started our investigation with the objective of developing a clean, efficient and straightforward methodology for the synthesis of dihydroxy indeno[1,2-b]indolone, acenaphtho[1,2-b]indolone, coumarin and uracil fused indeno[1,2-b]pyrrole and acenaphtho[1,2-b]pyrrole systems utilizing a non-toxic and environmentally benign catalyst. To the best of our knowledge, only a few references exist concerning their synthesis.²⁵ These reported reactions have several limitations. For example, these procedures involve low yields, high reaction times, multistep approaches together with less substrate scope. Thus, a simple, efficient, and green method to synthesize these highly important heterocyclic cores would be demanding as well as attractive. In this paper, a novel approach is reported for the simple synthesis of dihydroxy indeno[1,2-b]indolone, acenaphtho[1,2-b]indolone, coumarin and uracil fused indeno[1,2-b]pyrrole and acenaphtho[1,2-b]pyrrole containing structural motifs by assembling the basic building blocks and installing monodisperse SnO₂ QDs. Synthesis of monodisperse SnO₂ QDs²⁶ and an exploration of its very high catalytic activity in organic synthesis is demonstrated in this paper.

Results and discussion

The SnO₂ QDs were prepared using a solvothermal method. Monodisperse SnO₂ QDs were characterized using an X-ray diffraction (XRD) study, and images produced using high-resolution transmission electron microscopy (HRTEM) and transmission electron microscopy (TEM).

Fig. 2a shows the XRD patterns of the as-synthesized uncapped SnO₂ QDs. All the diffraction peaks matched well with the standard diffraction data for rutile SnO₂ (JCPDS card no. 41-1445) while no traces of other phases or impurities were found. The gradual widening of the peaks indicates a large decrease in dimensions. The average sizes of the uncapped SnO₂ QDs calculated using the Debye–Scherrer formula for considering the instrumental broadening and strain broadening using the (110) peak of the XRD pattern is 3.9 nm.

Fig. 2 (a) XRD pattern of the SnO₂ QDs (b) XRD pattern of SnO₂ QDs that had been reused seven times.

From the low resolution TEM image of the SnO₂ QDs as displayed in Fig. 3a, the size of the QDs is 4 nm ± 10%. The HRTEM image of the uncapped SnO₂ QDs shown in Fig. 3b indicates 0.32 nm spacing between two adjacent lattice planes of a QD corresponding to the (110) lattice planes of SnO₂.

Fig. 3 (a) Low resolution TEM image and (b) HRTEM images of SnO₂ QDs.

The composition of the as-synthesized SnO₂ QDs was deduced from the energy-dispersive X-ray spectroscopy (EDX) measurements. Fig. 4 clearly indicates that the major peaks in the bulk substrate material are because of the presence of copper (Cu), oxygen (O) and tin (Sn). The Cu peak in the EDX spectra is because of the use of Cu grids for the TEM analysis. The EDX and HRTEM images clearly show that the synthesized particle was pure SnO₂, which was composed of Sn and O only.

Fig. 4 Results from the EDX analysis of the SnO₂ QDs.

These SnO₂ QDs were then explored for use as an heterogeneous catalyst for the synthesis of fully substituted dihydroxy indeno[1,2-b]indolone derivatives applying a three-component reaction (Scheme 1).

Scheme 1 Synthesis of dihydroxy indeno[1,2-b]indolone derivatives.

At the start of the research we focused on a systematic assessment of different catalysts for the model reaction (Scheme 1) for the synthesis of dihydroxy indeno[1,2-b]indolone derivatives. Initially, the one-pot, three-component reaction of aniline (1) (1.0 mmol), dimedone (2) (1.0 mmol) and ninhydrin (3) (1.0 mmol) as the representative substrates for the model reaction was investigated to establish the feasibility of the strategy and optimize the reaction conditions. The results are presented in Table 1. In the preliminary experiment, the reaction shown in Scheme 1 was performed in the absence of any catalyst and employing water as the solvent. It was evident that the reaction proceeded very slowly in the absence of catalyst and the expected product was isolated in a very small quantity after heating the reaction mixture for about 24 h at 70 °C (Table 1, entry 1). A wide array of catalysts including nano metal oxides such as nano-iron(II, III)oxide (Fe₃O₄), silicon dioxide (SiO₂), zinc oxide (ZnO), copper oxide (CuO), aluminium oxide (Al₂O₃) (Table 1, entries 2–6) were employed to improve the yield for the specific synthesis of dihydroxy indeno[1,2-b]indolone derivatives. From the results in Table 1, it was also evident that these nano-sized particles were unable to promote this three-component reaction with comparable yields. After prolonged screening of these heterogeneous Lewis acid catalysts, a stronger Lewis acid catalyst, nano cuprospinel (CuFe₂O₄), was applied. The inefficiency of nano CuFe₂O₄ for the construction of this heterocyclic core made us think about a nano catalyst having a greater acid character. For this we have applied SiO₂–OSO₃H and Al-SBA15, but the results were unsatisfactory. Then we tried using SnO₂ nano particles of different particle size for the multi-component reaction. It was evident that with the decreasing particle size, the catalytic efficiency increases for the SnO₂ nano particles because of the increase of surface area of the nano-catalysts. Table 1 clearly reveals that among the screened SnO₂ nano particles, SnO₂ QD having a particle size of 3.9 nm showed a superior catalytic activity and provided the best yield of the targeted dihydroxy indeno[1,2-b]indolone derivative (4a).

Table 1 Optimization of reaction conditions for the synthesis^a of dihydroxy indeno[1,2-b]indolone derivative (4a)

Entry	Catalyst	Catalyst loading (mol%)	Solvent	Time	Yield^b (%)
a All reactions were carried out with aniline (1 mmol), dimedone (1 mmol), ninhydrin (1 mmol), and the specified catalyst in 5 mL of solvent at 70 °C.b The yield of isolated products.
1	—	—	H2O	24 h	Trace
2	Nano Fe3O4	10	H2O	4 h	5
3	Nano SiO2	10	H2O	4 h	9
4	Nano ZnO	10	H2O	5 h	7
5	Nano CuO	10	H2O	5 h	5
6	Nano Al2O3	10	H2O	5 h	8
7	Nano CuFe2O4	10	H2O	4 h	Trace
8	SiO2–OSO3H	10	H2O	5 h	37
9	Al-SBA15	10	H2O	5 h	18
10	Bulk SnO2	10	H2O	3 h	45
11	SnO2 (20 nm)	10	H2O	3 h	57
12	SnO2 (10 nm)	10	H2O	3 h	62
13	SnO2 QDs	10	H2O	2.0 h	89
14	SnO2 QDs	10	Dioxane	2.0 h	48
15	SnO2 QDs	10	DMSO	2.0 h	63
16	SnO2 QDs	10	CH3CN	2.0 h	45
17	SnO2 QDs	10	Toluene	2.0 h	20
18	SnO2 QDs	10	DMF	2.0 h	59
19	SnO2 QDs	8	H2O	2.0 h	69
20	SnO2 QDs	12	H2O	2.0 h	89

---

Various solvents were also screened to test the efficiency of the catalysts in different reaction media and the results are summarized in Table 1. It was evident that the polar solvents gave a better yield than the nonpolar ones and water was superior to the other solvents. It is worth noting that the quantity of the catalyst plays a vital role in the formation of the desired product. It was found that 10 mol% of SnO₂ QDs was sufficient to give 4a with 89% isolated yield (Table 1, entry 13). The yield remained unaffected when the catalyst loading was increased to 12 mol% (Table 1, entry 20). However, the yield was decreased when the catalyst loading was reduced (Table 1, entry 19).

After standardization of all the reaction parameters, the proposed catalytic system was employed to synthesize a dihydroxy indeno[1,2-b]indolone core in the presence of a wide variety of commercially available amine derivatives. It was evident that aliphatic amine derivatives showed greater reactivity compared to aromatic amine derivatives (Table 2). The application of aromatic amine derivatives with electron-releasing substituents in this three-component protocol showed superior reactivity compared to that of electron-withdrawing substituents or unsubstituted aromatic amine derivatives (Table 2). Furthermore in the presence of a sensitive heterocyclic core containing amine derivatives (Table 2), the reaction proceeded successfully to provide the desired products in high yields (82–86%). The catalytic system was effective for two 1,3-diketo compounds (dimedone and cyclohexane-1,3-dione) in aqueous media under thermal conditions (Table 2).

Table 2 Synthesis of dihydroxy indeno[1,2-b]indolone core via a three-component reaction

---

The optimized reaction conditions were employed thereafter for evaluating the scope of SnO₂ QD catalyzed three-component coupling reactions (Table 3). We have also substituted acenaphthoquinone for ninhydrin to obtain the dihydroxy acenaphtho[1,2-b]indolone core. Unsubstituted aromatic amines and aromatic amines having 4-Me, 4-Cl and 4-Br groups underwent the reaction with two 1,3-diketo compounds (dimedone and cyclohexane-1,3-dione) in aqueous media to give the corresponding dihydroxy acenaphtho[1,2-b]indolone derivatives in high yield (Table 3).

Table 3 Three-component synthesis of a dihydroxy acenaphtho[1,2-b]indolone core

---

On the basis of the experiments and their findings, we proposed a possible mechanism (Scheme 2) for the 3CRs. It is believed that the reaction proceeds in a catalytic cycle which involves synthesis of intermediate I (enaminone), then a Michael addition and finally intramolecular cyclization catalyzed by the SnO₂ QDs as presented in Scheme 2. The first step of the current 3CRs was the formation of intermediate I, through Sn⁴⁺ (active species of the SnO₂ QD catalyst)-promoted condensation of 1,3-diketo compound 1 and amine derivative 2. The strong Lewis acidic Sn⁴⁺ ion of the SnO₂ nanoparticle showed excellent catalytic activity in promoting the condensation reaction for the formation of the intermediate I by enhancing the electrophilicity of carbonyl groups of the 1,3-diketo compound. The difference in reactivity of various aromatic amine derivatives can also be explained by the fact that for the aromatic amine derivatives with electron-releasing substituents the formation of intermediate I is favored compared to that with the electron-withdrawing substituent or unsubstituted aromatic amine derivatives. SnO₂ QDs then facilitated the Michael addition step between intermediate I and the 1,2-diketo compound 3/5 (ninhydrin and acenaphthoquinone) to generate the intermediate II. Finally, the Lewis acidic Sn⁴⁺ interacted with the intermediate II, which in turn facilitated intramolecular electrophilic cyclization with the formation of the five membered ring (P). The SnO₂ QDs catalyzed the activation of the condensation, the Michael reaction and the subsequent ring annulations which led to the desired heterocyclic derivatives which were confirmed by the isolation of intermediate I. The reaction was carried out starting from I (prepared and isolated) with 1,2-diketo compound 3/5 (ninhydrin and acenaphthoquinone) and arrived at the same cyclic product P which further supported the proposed mechanism.

Scheme 2 The catalytic cycle for the formation of the products.

Because of the very high yield of the dihydroxy indeno[1,2-b]indolone derivatives the generality of this novel catalytic method was expanded and so it was decided to synthesize a series of polyfunctionalized dihydroxy indeno[2,1-d]pyrrole derivatives. For this, cyclopentane-1,3-dione was examined to replace cyclohexane-1,3-dione for the formation of intermediate I (Scheme 2). But in practice, the reaction failed to generate the desired dihydroxy indeno[2,1-d]pyrrole derivatives 9 when the SnO₂ QD catalyzed three-component reaction was applied. The reaction was tried with aliphatic, hetero-aromatic and aromatic amines but in all cases, the acyclic intermediate II (Scheme 2) was isolated. This observation clearly indicates that cyclopentan-1,3-dione was not compatible with the uncapped SnO₂ QD catalyzed three-component protocol. This observation also strengthens the idea that the final intramolecular cyclization step is a reversible reaction. As the final step is a reversible one, the dihydroxy indeno[2,1-d]pyrrole derivatives generated from the SnO₂ QDs catalyzed the cyclization of the intermediate 8a–e which then reverted back to an acyclic compound because of the instability of the three linearly stranded, highly strained five member ring containing the dihydroxy indeno[2,1-d]pyrrole core (Table 4).

Table 4 Three-component synthesis using a five membered 1,3-diketo compound

---

Table 5 Two-component synthesis of coumarin and uracil fused dihydroxy indeno[1,2-b]pyrrole and acenaphtho[1,2-b]pyrrole core

---

As molecules containing coumarin and uracil linkages are prevalent in biological and pharmaceutical sciences, coumarin and uracil nuclei were then evaluated as the coupling partners for the SnO₂ QD catalyzed reaction. The proposed mechanism (Scheme 2) showed that the reaction passes through the intermediate I. Like intermediate I, 4-aminocoumarin and 6-aminouracil derivatives have Michael donors and an intramolecular cyclization centre (Fig. 5) which encouraged the development of a two-component protocol using 1,2-diketo compound 3/5 (ninhydrin or acenaphthoquinone) and 4-aminocoumarin or 6-aminouracil.

Fig. 5 Structural similarities among intermediate I, 4-aminocoumarin and 6-aminouracil.

The reaction was initiated by using 4-aminocoumarin and 6-aminouracil derivatives for the synthesis of coumarin and uracil fused dihydroxy indeno[1,2-b]pyrrole and dihydroxy acenaphtho[1,2-b]pyrrole core. The highly acid- and base-sensitive delicate coumarin and uracil moieties underwent the SnO₂ QD catalyzed two-component coupling reaction with a good to excellent yield under the optimized reaction conditions (Table 5).

The various products obtained: 4, 6, 8, 11, were characterized using infra-red spectroscopy, proton-nuclear magnetic resonance spectroscopy (¹H-NMR), ¹³C-NMR and elemental analysis. Finally, the structures of two compounds 4a and 8d were confirmed using single-crystal XRD (Fig. 6 and 7).²⁷

Fig. 6 Oak Ridge Thermal Ellipsoid Plot (ORTEP) diagram of compound 4a (ESI†).

Fig. 7 ORTEP diagram of compound 8d (ESI†).

For applications of heterogeneous catalysts in a practical field, the lifetime of the catalyst and its level of reusability are very important factors. Separation of the catalyst and isolation of the desired product from the reaction mixture is one of the most crucial aspects of organic synthesis. To clarify this issue, a set of experiments were established using the recycled catalyst for the synthesis of dihydroxy indeno[1,2-b]indolone derivative (4a). The reactions were carried out under similar conditions in aqueous media. In the developed protocol, after completion of the reaction, water was removed under reduced pressure from the reaction mixture and was stirred with 5 mL of methanol and then the catalyst was easily removed by filtration, leaving the clear reaction mixture as the filtrate. The recovered catalyst was then washed with 15 mL of methanol and finally dried at 70 °C for 1 h. A new reaction was then performed with fresh solvent and reactants under the same conditions. It was found that the SnO₂ QD catalyst could be used at least seven times without any change (loss) in activity. The XRD patterns (Fig. 2) and the TEM image (Fig. 8) indicated that the crystal structure of the SnO₂ QDs was intact even after seven runs, which not only explained the excellent recycling results, but also reconfirmed the high stability of this catalyst.

Fig. 8 TEM image of the recovered SnO₂ QD catalyst after the seventh run.

Conclusions

In summary, an SnO₂ QD mediated, one-pot, multi-component coupling protocol for the preparation of a wide variety of heterocyclic derivatives has been demonstrated. SnO₂ QDs were prepared by a simple and effective solvothermal method and characterized using XRD, HRTEM and TEM images. The QD catalyst system encompasses a very high surface area and allows rapid and selective chemical transformations with excellent product yield. This is a promising approach from the sustainable and practical chemistry viewpoints. This simple, environmentally benign and convenient methodology extends the scope towards a wide spectrum of novel compounds possessing an important structural subunit of a variety of biologically active molecules.

Experimental section

Preparation of SnO₂ QDs

The uncapped SnO₂ QDs were prepared using a simple solvothermal process. Tin(IV) chloride pentahydrate (SnCl₄·5H₂O; 5.83 mmol) was dissolved in a mixed solvent (18.5 mL methanol and 9.28 mL of ethylenediamine) with constant magnetic stirring for 1 h. Urea (58.3 mmol) in 18.5 mL of deionised H₂O was then added to the reaction mixture. The resultant solution was stirred for 30 min to obtain a white slurry which was then transferred to a 52 mL Teflon lined stainless steel chamber and heat treatment was continued at 90 °C for 8 h. After cooling down to room temperature, the product was centrifuged. The collected product was washed several times with deionised H₂O and ethanol to remove the impurities and the product was then dried overnight in a vacuum.

General procedure for the synthesis of dihydroxy indeno[1,2-b]indolone and dihydroxy acenaphtho[1,2-b]indolone derivatives

A mixture of amine (1.0 mmol), 1,3-dicarbonyl compound (dimedone or cyclohexane-1,3-dione) (1.0 mmol), and SnO₂ QDs (10 mol%) were stirred in 5 mL of water at 70 °C. After 30 min, 1,2-dicarbonyl compound (ninhydrin or acenaphthenequinone) (1.0 mmol) was added. After completion of the reaction (determined using thin-layer chromatography (TLC)), water was removed under reduced pressure from the reaction mixture followed by stirring with 5 mL of methanol (5 min) and then the catalyst was removed by filtration, leaving the clear reaction mixture as the filtrate. Removal of solvent under reduced pressure and purification of the crude product by recrystallization from ethanol provided a pure product.

General procedure for the synthesis of coumarin and uracil fused dihydroxy indeno[1,2-b]pyrrole and acenaphtho[1,2-b]pyrrole derivatives

A mixture of a 6-aminouracil derivative or a 4-aminocoumarin derivative (1.0 mmol), 1,2-dicarbonyl compound (ninhydrin or acenaphthenequinone) (1.0 mmol), and SnO₂ QDs (10 mol%) were stirred in 5 mL of water at 70 °C. After completion of the reaction (determined by TLC), water was removed from the reaction mixture under reduced pressure followed by stirring with 5 mL of methanol (5 min) and then the catalyst was removed by filtration, leaving the clear reaction mixture as the filtrate. Removal of the solvent under reduced pressure and purification of the crude product by recrystallization from ethanol provided a pure product.

Acknowledgements

We gratefully acknowledge the financial support from University Grants Commission (UGC) and the University of Calcutta. K. P. thanks UGC, New Delhi, India, for the grant for a Senior Research Fellowship.

Notes and references

1. Y. R. Leshkov, J. N. Chheda and J. A. Dumesic, Science, 2006, 312, 1933.
2. (a) M. Bicker, J. Hirth and H. Vogel, Green Chem., 2003, 5, 280; (b) Y. R. Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic, Nature, 2007, 447, 982.
3. J. N. Chheda, Y. Roman-Leshkov and J. A. Dumesic, Green Chem., 2007, 9, 342.
4. V. V. Ordomsky, J. van der Schaaf, J. C. Schouten and T. A. Nijhuis, J. Catal., 2012, 287, 68.
5. (a) Z. H. Zhang, B. Liuand and Z. B. Zhao, Carbohydr. Polym., 2012, 88, 891; (b) Y. Yang, C. W. Hu and M. M. Abu-Omar, Green Chem., 2012, 14, 509; (c) B. R. Caes and R. T. Raines, ChemSusChem, 2011, 4, 353; (d) B. Kim, J. Jeong, D. Lee, S. Kim, H. J. Yoon, Y. S. Lee and J. K. Cho, Green Chem., 2011, 13, 1503.
6. X. H. Qi, M. Watanabe, T. M. Aida and R. L. Smith, Green Chem., 2008, 10, 799.
7. W. Teunissen, A. A. Bol and J. W. Geus, Catal. Today, 1999, 48, 329.
8. C. Bal, B. Baldeyrou, F. Moz, A. Lansiaux, P. Colson, L. Kraus-Berthier, S. Léonce, A. Pierré, M.-F. Boussard, A. Rousseau, M. Wierzbicki and C. Bailly, Biochem. Pharmacol., 2004, 68, 1911.
9. (a) A. A. Emmanuel-Giota, K. C. Fylaktakidou, D. J. Hadjipavlou-Litina, K. E. Litinas and D. N. Nicolaides, J. Heterocycl. Chem., 2001, 38, 717; (b) J. Neyts, E. D. Clercq, R. Singha, Y. H. Chang, A. R. Das, S. K. Chakraborty, S. C. Hong, M. H. Hsu and J. R. Hwu, J. Med. Chem., 2009, 52, 1486; (c) T. O. Soine, J. Pharm. Sci., 2006, 53, 231.
10. M. Suzuki, K. Nakagawa-Goto, S. Nakamura, H. Tokuda, S. L. Morris-Natschke, M. Kozuka, H. Nishino and K. H. Lee, Pharm. Biol., 2006, 44, 178.
11. O. Kayser and H. Z. Kolodziej, Z. Naturforsch., C: J. Biosci., 1999, 54, 169.
12. R. C. Sharma and R. K. Parashar, J. Inorg. Biochem., 1988, 32, 163.
13. Y. L. Garazd, E. M. Kornienko, L. N. Maloshtan, M. M. Garazd and V. P. Khilya, Chem. Nat. Prod., 2005, 41, 508.
14. C. A. Kontogiorgis and D. J. Hadjipavlou-Litina, J. Med. Chem., 2005, 48, 6400.
15. J. R. Hwu, R. Singha, S. C. Hong, Y. H. Chang, A. R. Das, I. Vliegen, E. D. Clercq and J. Neyts, Antiviral Res., 2008, 77, 157.
16. (a) P. G. Baraldi, B. Cacciari, S. Moro, G. Spalluto, G. Pastorin, T. Da Ros, K. N. Klotz, K. Varani, S. Gessi and P. A. Borea, J. Med. Chem., 2002, 45, 770; (b) A. Maconi, G. Pastorin, T. Da Ros, G. Spalluto, Z. G. Gao, K. A. Jacobson, P. G. Baraldi, K. Varani, S. Moro and P. A. Borea, J. Med. Chem., 2002, 45, 3579.
17. (a) E. Ongini, A. Monopoli, B. Cacciari and P. G. Baraldi, Farmaco, 2001, 56, 87; (b) K. A. Jacobson, P. Al Ijzerman and J. Linden, Drug Dev. Res., 1999, 47, 45; (c) C. E. Muller, Farmaco, 2001, 56, 77.
18. L. Xiao, H. Shen, R. von Hagen, J. Pan, L. Belkoura and S. Mathur, Chem. Commun., 2010, 46, 6509.
19. T. T. Baby and S. Ramaprabhu, J. Appl. Phys., 2012, 111, 034311.
20. H. Ahn, H. Choi, K. Park, S. Kim and Y. Sung, J. Phys. Chem. B, 2004, 108, 9815.
21. T. Jia, W. Wang, F. Long, Z. Fu, H. Wang and Q. Zhang, J. Phys. Chem. C, 2009, 113, 9071.
22. S. Gubbala, V. Chakrapani, V. Kumar and M. K. Sunkara, Adv. Funct. Mater., 2008, 18, 2411.
23. N. Kudo, Y. Shimazaki, H. Ohkita, M. Ohoka and S. Ito, Sol. Energy Mater. Sol. Cells, 2007, 91, 1243.
24. (a) S. Paul, P. Bhattacharyya and A. R. Das, Tetrahedron Lett., 2011, 52, 4636; (b) P. Bhattacharyya, K. Pradhan, S. Paul and A. R. Das, Tetrahedron Lett., 2012, 53, 4687; (c) K. Pradhan, S. Paul and A. R. Das, Catal. Sci. Technol., 2014, 4, 822.
25. (a) H. J. Hemmerling and G. Reissb, Synthesis, 2009, 985; (b) C. Hundsdörfer, H. J. Hemmerling, C. Götz, F. Totzke, P. Bednarski, M. L. Borgne and J. Jose, Bioorg. Med. Chem., 2012, 20, 2282.
26. S. Paul, K. Pradhan, S. Ghosh, S. K. De and A. R. Das, Tetrahedron, 2014, 70, 6088.
27. ESI.†.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 991059 and 1021234. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra12618a

---