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Journal of Molecular Catalysis A: Chemical 380 (2013) 152– 158

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

j ourna l ho me pa g e: www.elsev ier .com/ locate /molcata

ulfochitosan encapsulated nano-Fe3O4 as an efficient and reusableagnetic catalyst for green synthesis of 2-amino-4H-chromen-4-yl

hosphonates

eza Mohammadi, Mohammad Zaman Kassaee ∗

epartment of Chemistry, Tarbiat Modares University, P.O. Box 14155-4838 Tehran, Iran

r t i c l e i n f o

rticle history:eceived 4 June 2013eceived in revised form6 September 2013ccepted 21 September 2013vailable online 30 September 2013

a b s t r a c t

Highly dispersed chitosan-coated nano Fe3O4 core-shell structures (Fe3O4@CS NPs) are prepared simplythrough in situ co-precipitation of Fe3+ and Fe2+ ions via NH4OH in an aqueous solution of chitosan. Treat-ment of Fe3O4@CS NPs with chlorosulfonic acid leads to the formation of Fe3O4@CS-SO3H NPs whichexerts excellent catalytic activity toward one-pot, three-component synthesis of 2-amino-4H-chromen-4-yl phosphonate derivatives. The core-shell structure and the composition of produced magneticnanocatalyst are analyzed using FT-IR, XRD, TGA, VSM, ICP, SEM, TEM and BET. The results reveal that the

eywords:ulfochitosanagnetic

ecyclable nanocatalystreen synthesis-Amino-4H-chromen-4-yl phosphonate

unique heterogeneous Fe3O4@CS-SO3H NPs appears as an excellent acid catalyst which produces highyields, impressive turnover number (TON) and turnover frequency (TOF) values with good recyclabilitywithout significant loss of the activity. Moreover, the proposed green synthetic method takes advantageof nontoxic reagents in an aqueous media through a simple procedure.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Fabrication of nanoparticles with desired properties throughheir surface modification has attracted growing attention in recentears [1,2]. There is an ever increasing interest in the synthesis,haracterization and surface modification of magnetic nanopar-icles because of their potential applications in biotechnology,iomedicine, environmental, material science and catalysis [3–6].ntriguing feature of these nanoparticles is the possibility to tuneheir properties through a molecular-level design by varying theize of the core and by surface modification with suitable func-ional molecules. Polysaccharides are among the various stabilizinggents used to prevent nanoparticles from aggregating, and rep-esent an attractive choice for preparation of functional materials7–9]. In particular, an increasing attention has recently beenocused on the synthesis of chitosan (CS) coated Fe3O4 NPs [10–13].s an ideal support material, CS has its special set of proper-

ies including availability, safety, non-toxicity, biocompatibility,

iodegradability, low immunogenicity, and antibacterial proper-ies [14–17]. Owing to these fascinating properties, CS has found

wide application in a variety of areas such as biomedicine,

∗ Corresponding author. Tel.: +98 912 1000392; fax: +98 21 88006544.E-mail addresses: kassaeem@modares.ac.ir, drkassaee@yahoo.com

M.Z. Kassaee).

381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.molcata.2013.09.027

pharmaceuticals, metal chelating, food processing, industrial appli-cations, etc. [18–21]. Specifically the amine and hydroxyl groupsin chitosan provide active sites for numerous attractive chemicalmodifications.

Here we employ a green chemistry approach for synthesisof Fe3O4@CS-SO3H NPs, which is applied as an effective, clean,and recoverable magnetic catalyst for the synthesis of importantorganophosphorus compounds including 2-amino-4H-chromen-4-yl phosphonate derivatives. Organophosphorus compounds aresignificant substrates in the study of biochemical processes whichexhibit diverse and interesting biological and biochemical prop-erties [22,23]. They are key antibacterials, enzyme inhibitors,antibiotics, herbicides, fungicides, insecticides, plant growth reg-ulators, antithrombotic agents, agrochemicals, pharmaceuticals,etc. [24–26]. Specifically our target (2-amino-4H-chromenes) areamong important heterocyclics with a number of biological andpharmacological properties. They have also been widely employedas cosmetics, pigments, and potent biodegradable agrochemicals[27–29]. Such wide range of biological activities and pharmaco-logical properties has stimulated interest in new approaches forthe synthesis of a variety of phosphonate derivatives with 2-aminochromenyl rings through multicomponent reactions as the

key steps. A few synthetic methodologies have been developed forthe synthesis of 2-amino-4H-chromen-4-yl phosphonate deriva-tives, using various catalysts and additives [30–34]. Here again,we have made an effort toward development of a new synthetic

R. Mohammadi, M.Z. Kassaee / Journal of Molecular Catalysis A: Chemical 380 (2013) 152– 158 153

cating

pct

2

2F

oto10raw

Scheme 1. Preparation steps for fabri

rotocol involving Fe3O4@CS-SO3H NPs in one-pot, three-omponent reactions of salicylaldehydes, malononitrile andriethyl phosphite in water at room temperature.

. Experimental

.1. Preparation of chitosan-coated magnetic nanoparticles:e3O4@CS NPs

Fe3O4 nanoparticles are prepared by chemical co-precipitationf Fe3+ and Fe2+ ions with molar ratio 2:1, in presence of chi-osan, followed by the hydrothermal treatment. Specifically 1.5 gf chitosan (molecular weight: 100,000–300,000) is dissolved in00 mL of 0.05 M acetic acid solution; to which FeCl3·6H2O (3.51 g,

.013 mol) and FeCl2·4H2O (1.29 g, 0.0065 mol) are added. Theesulting solution is mechanically stirred for 6 h at 80 ◦C under N2tmosphere. Consequently, 6 mL of 25% NH4OH is injected dropise into the reaction mixture with constant stirring. After 30 min,

Fig. 1. FT-IR spectra of CS (a); Fe3O4@CS NPs (b); Fe3O4@CS-SO3H NPs

heterogeneous Fe3O4@CS-SO3H NPs.

the mixture is cooled to room temperature and chitosan coatedover magnetic nanoparticles are separated by an external magnet,first washed with distilled water, then ethanol, and finally driedunder vacuum at room temperature.

2.2. Synthesis of sulfochitosan encapsulated nano iron oxide:Fe3O4@CS-SO3H NPs

500 mg of Fe3O4@CS NPs is dispersed in dry CH2Cl2 (10 mL) inan ultrasonic bath for 20 min, then chlorosulfonic acid (0.8 mL) isadded drop-wise over a period of 10 min, at room temperatureunder N2 atmosphere. Subsequently, the mixture is mechani-cally stirred for 15 min until HCl gas evolution is stopped. Finally,

functionalized magnetic Fe3O4@CS-SO3H NPs is separated by anexternal magnet, washed several times with dry CH2Cl2, until aneutral pH level is achieved, then dried under vacuum at roomtemperature (Scheme 1).

(c); and the recovered Fe3O4@CS-SO3H NPs after seven runs (d).

154 R. Mohammadi, M.Z. Kassaee / Journal of Molecular Catalysis A: Chemical 380 (2013) 152– 158

22

ipstctfwFpl

3

aSy

F(

Fig. 4. VSM curve for Fe3O4@CS-SO3H NPs at room temperature.

Fig. 2. XRD patterns of Fe3O4@CS NPs (a) and Fe3O4@CS-SO3H NPs (b).

.3. General procedure for synthesis of-amino-4H-chromen-4-yl phosphonate derivatives

Fe3O4@CS-SO3H NPs (10 mg) is added to a mixture of sal-cylaldehydes (1 mmol), malononitrile (1 mmol), and triethylhosphite (1 mmol) in water (5 mL), and the reaction mixture istirred at room temperature (Scheme 2). The progress of the reac-ion is monitored by TLC. After the reaction is completed, theatalyst is separated by an external magnet and reused as such forhe next experiment. Consequently the crude product is extractedrom the aqueous phase by EtOAc, and then the organic layer isashed with saturated brine and dried over anhydrous MgSO4.

inally the combined organic layers are evaporated under reducedressure and the resulting crude product is purified by recrystal-

ization from hot ethanol.

. Results and discussion

The heterogeneous catalyst, Fe O @CS-SO H NPs, is fully char-

3 4 3cterized by FT-IR (Fig. 1), XRD (Fig. 2), TGA (Fig. 3), VSM (Fig. 4),EM (Fig. 5), TEM (Fig. 6), BET, ICP-OES, and ion exchange pH anal-ses.

ig. 3. TGA diagram for chitosan (a); Fe3O4@CS NPs (b); and Fe3O4@CS-SO3H NPsc).

Fig. 5. SEM image of Fe3O4@CS-SO3H NPs.

The prepared Fe3O4@CS-SO3H NPs catalyst is placed in anaqueous NaCl solution (1 M, 25 mL), where the pH drops instan-taneously to ≈1.97, indicating ion exchanges between NH SO3Hand OSO H protons and sodium ions. The ion exchange capacity of

3

SO3H groups is about 0.8 mmol/g which is found by back titration.

Fig. 6. TEM image of Fe3O4@CS-SO3H NPs along with the size histogram.

R. Mohammadi, M.Z. Kassaee / Journal of Molecular Catalysis A: Chemical 380 (2013) 152– 158 155

n-4-y

3

NasrPsbm1bttcsgt

t

3

SToss4(sbp

3

eF

TO

Scheme 2. Preparation of 2-amino-4H-chrome

.1. FT-IR analysis

The FT-IR spectra of CS, Fe3O4@CS NPs, and Fe3O4@CS-SO3HPs, as well as the recovered Fe3O4@CS-SO3H NPs after seven runsppear consistent with their respective structures (Fig. 1). The FT-IRpectrum of chitosan shows a broad band at 3427 cm−1 which cor-esponds to the stretching vibrations of N H and O H groups.eaks appearing at 2922 and 2856 cm−1 are characteristic of C Htretching vibrations. The band at 1649 cm−1 is assigned to N Hending vibration and that of 1424 cm−1 to C O stretching of pri-ary alcoholic groups in chitosan. The biosorption bands around

093 and 1027 cm−1 display the stretching vibrations of the C Oonds. For Fe3O4@CS NPs, chitosan absorptions appear in additiono a peak at 574 cm−1 which corresponds to the stretching vibra-ion of Fe O groups; indicating that the magnetic Fe3O4 NPs areoated by chitosan. The FT-IR spectrum of Fe3O4@CS-SO3H NPshows the stretching and out-of-plane bending of acidic O Hroups as two broad bands at 2800–3500 and 877 cm−1, respec-ively. Finally, S O stretching bands of -SO3H in O SO3H andNH SO3H groups appear at 1232 cm−1 and 1059 cm−1, respec-

ively.

.2. X-ray diffraction (XRD) analysis

The structure and phase purity of Fe3O4@CS NPs and Fe3O4@CS-O3H NPs are studied by means of X-ray diffraction analysis (XRD).he sharp peaks in the XRD patterns confirm the good crystallinityf the prepared samples (Fig. 2). The results are in agreement withtandard patterns of inverse cubic spinel magnetite (Fe3O4) crystaltructure, showing six diffraction peaks at 2� about 30.21◦, 35.73◦,3.41◦, 53.73◦, 57.33◦ and 62.85◦; corresponding to (2 2 0), (3 1 1),4 0 0), (4 2 2), (5 1 1), and (4 40 ). No impurity in the XRD patternsuggest formation of pure Fe3O4 nanoparticles. The small and weakroad bands in the range of 25–28◦ indicate the existence of amor-hous sulfonated chitosan.

.3. Thermogravimetric analysis (TGA)

In order to obtain information on the thermal stability, TGAxperiments are carried out by heating CS, Fe3O4@CS NPs, ande3O4@CS-SO3H NPs in air up to 800 ◦C (Fig. 3). Pure chitosan shows

able 1ptimization of the Fe3O4@CS-SO3H NPs catalyzed model reaction for synthesis of 2-ami

Entry Catalyst (mg)

1 Fe3O4@CS-SO3H NPs (10 mg)

2 Fe3O4@CS-SO3H NPs (10 mg)

3 Fe3O4@CS-SO3H NPs (10 mg)

4 Fe3O4@CS-SO3H NPs (10 mg)

5 Fe3O4@CS-SO3H NPs (10 mg)

6 Fe3O4@CS-SO3H NPs (10 mg)

7 Fe3O4@CS-SO3H NPs (10 mg)

8 Fe3O4@CS-SO3H NPs (50 mg)

9 Fe3O4@CS-SO3H NPs (20 mg)

10 –

l phosphonates (4) over Fe3O4@CS-SO3H NPs.

a 5–10% weight loss at 110 ◦C, due to the evaporation of adsorbedmoisture. Another sharp down fall appears at 320 ◦C, which couldbe attributed to the degradation of chitosan chains. The weightloss of Fe3O4@CS NPs is about 38% at 250–350 ◦C, correspondingto the thermal decomposition of chitosan chains over Fe3O4 NPs.As can be seen, decomposition rate of chitosan shell layer is lowerthan pure chitosan chain confirming the success of formation ofchitosan shell over Fe3O4 NPs. This difference in the thermal sta-bility of magnetite-chitosan hybrid is probably due to the narrownanoscopic shell of chitosan attached to the surface of the mag-netic core in the framework of the hybrid structure compared tothe free chitosan with a well-developed polymeric structure [35].Analysis for Fe3O4@CS-SO3H NPs shows two weight losses; thepeak centered at 110 ◦C is due to desorption of water followed by asecond peak at 180 ◦C, corresponding to the decomposition of chi-tosan chains and O SO3H and NH SO3H spacer organic groups,loaded on chitosan. Loss of the thermal stability for Fe3O4@CS-SO3HNPs might be attributed to reduced hydrogen bonding as well as theinterference of molecular organization due to cross-linking [36].The residue weight of Fe3O4@CS-SO3H NPs is 35% indicating theextent of its Fe3O4 NPs content.

3.4. Magnetic properties

The magnetic property of as-synthesized Fe3O4@CS-SO3H NPsis determined by vibrating sample magnetometer (VSM) at roomtemperature. A typical magnetization loop appears with the sat-uration magnetization (Ms) at about 56 emu/g (Fig. 4). There is noremanence and coercivity, suggesting that Fe3O4@CS-SO3H NPs aresuper-paramagnetic. Therefore, the prepared catalyst may easily beseparated with the aid of an external magnet.

Also, the content of Fe3O4 Nps in our catalyst (Fe3O4@CS-SO3HNPs) is measured by inductively coupled plasma optical emissionspectroscopy (ICP-OES). The observed amount of 42% appears con-sistent with our TGA observations.

3.5. Scanning electron microscopy (SEM) and transmission

electron microscopy (TEM)

The size and structure of the Fe3O4@CS-SO3H NPs are estimatedusing SEM (Fig. 5) and TEM (Fig. 6). Uniform dispersed spherical

no-4H-chromen-4-yl phosphonates.

Solvent Time (min) Yield (%)

H2O 30 93EtOH 45 80Toluene 120 40MeCN 90 55CH2Cl2 120 48MeOH 120 37– 60 42H2O 30 94H2O 30 93H2O 360 –

156 R. Mohammadi, M.Z. Kassaee / Journal of Molecular Catalysis A: Chemical 380 (2013) 152– 158

Table 2One-pot aqueous synthesis of 2-amino-4H-chromen-4-yl phosphonates over Fe3O4@CS-SO3H NPs catalyst, at ambient.a

Entry Salicylaldehydes Time (min) Yield (%)b Product Mp (◦C) TONc TOF (h−1)d

4a 30 93 142–143 116.2 232.4

4b 20 96 152–154 120 360

4c 30 92 212–214 115 230

4d 25 95 180–182 118.7 284.9

4e 20 97 178–180 121.2 363.6

4f 30 93 218–220 116.2 232.4

4g 20 97 198–200 121.2 348.6

4h 30 94 174–176 117.5 235

4i 35 88 177–179 110 188.6

a Reaction conditions: Salicylaldehydes (1 mmol), malononitrile (1 mmol), triethyl phosphite (1 mmol), Fe3O4@CS-SO3H NPs (10 mg, 0.8 mol%), water (3 mL).b Isolated yields.c Turnover number (average number of product molecules produced per mole of the catalyst).d Turnover frequency (turnover number per time).

R. Mohammadi, M.Z. Kassaee / Journal of Molecular Catalysis A: Chemical 380 (2013) 152– 158 157

the reaction mixture by an external magnet.

pw

bslw

aoaht(fMlpi

Fig. 7. Separation of the catalyst from

articles are observed, with an average size of 9–15 nm, along witheak agglomerations.

The specific surface area of each nanopowder is determinedy BET. The results show an average surface area of 94 m2 g−1 forynthetic chitosan coated magnetic Fe3O4@CS NPs nanopowder. Itowers to 71 m2 g−1 for magnetic nanopowder Fe3O4@CS-SO3H NPs

hich results from SO3H functionalization of chitosan.Herein, we report the multi-component facile synthesis of 2-

mino-4H-chromen-4-yl phosphonate derivatives, in the presencef catalytic amounts of Fe3O4@CS-SO3H NPs in aqueous media,t room temperature. As a simple model, reaction of salicylalde-yde, malononitrile, and triethyl phosphite, is probed to establishhe feasibility of the strategy and optimize the reaction conditionsScheme 2, Table 1). The model reaction is examined in solventree condition, as well as in H2O, EtOH, toluene, MeCN, CH2Cl2, and

eOH (Table 1, entries 1–7). In order to show the role of the cata-

yst, similar reactions in the absence of the catalyst and also in theresence of different amounts of Fe3O4@CS-SO3H NPs are exam-

ned. In the absence of the catalyst no product is produced and the

Fig. 8. Reusability of Fe3O4@CS-SO3H NPs, as a magnetically recyclable hetero-geneous catalyst ( ); and the yield of the product 2-amino-4H-chromen-4-ylphosphonate for each run ( ).

Scheme 3. A plausible mechanism for Fe3O4@CS-SO3H NPs catalytic synthesis of 2-amino-4H-chromen-4-yl phosphonates.

1 ecular

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58 R. Mohammadi, M.Z. Kassaee / Journal of Mol

se of just 10 mg of Fe3O4@CS-SO3H NPs is sufficient to push theeaction forward. The catalyst amounts higher than 50 mg does notmprove the yield to an appreciable extent (Table 1, entries 8–10).

To investigate the efficiency of our method, a series of differentlyubstituted salicylaldehydes are employed (Scheme 2). The resultslearly demonstrate high tolerance for a wide range of differentlyubstituted salicylaldehydes (Table 2). The reactions are completedithin 20–35 min with excellent yields and high TON and TOF val-es. The resulting products are characterized on the basis of theirT-IR, 1H, 13C and 31P NMR spectra.

In a typical procedure, after completion of the reaction, the mag-etic catalyst is easily and efficiently separated from the producty attaching an external magnet to the reaction vessel, followed byimple decantation of the reaction solution (Fig. 7).

The recovered catalyst is washed with EtOH and dried for 4 ht room temperature to be ready for later run. The percentage ofhe recovery step in most cases is more than 96% (Fig. 8), with-ut changing in the physicochemical properties (Fig. 1d). Moreover,he recovered catalyst is recycled in subsequent runs without anyignificant loss of its activity. For instance, using the same modeleaction, the average isolated yield for seven successive runs is9%, which demonstrates the practical recyclability of the preparedatalyst (Fig. 8).

Our proposed mechanism for the formation of 2-amino-4H-hromen-4-yl phosphonates considers activation of the carbonylroup of a salicylaldehyde (1) by the catalyst (Fe3O4@CS-SO3HPs) leading to formation of 5. Reaction of -NH2 groups on surfacef the catalyst with malononitrile (2) gives 6 through Knoeve-agel condensation (Scheme 3). The catalyst may activate the cyanoroup inviting nucleophilic attack of the OH group which leads toormation of imino coumarin (7) through Pinner reaction. Finallyucleophilic addition of triethyl phosphite (3) to 7 gives interme-iate 8, which is further reacted with water to produce 4.

. Conclusion

Preparation and characterization of sulfochitosan-coated Fe3O4agnetic nanoparticles (Fe3O4@CS-SO3H NPs) is described. Chi-

osan (CS) is used as a catalyst support for its safety, nontoxicity,iocompatibility, and antibacterial properties. Hence Fe3O4@CS-O3H NPs acts as a “green” heterogeneous, highly efficient andecyclable novel catalyst for preparation of 2-amino-4H-chromen--yl phosphonates through one-pot, three-component reactions ofalicylaldehydes, malononitrile, and triethyl phosphite in water at

oom temperature. Mild reaction conditions, green solvents, highields, short reaction time, operational simplicity, practicability,pplicability to various substrates and product purity are amonghe advantages of this protocol.

[

[

Catalysis A: Chemical 380 (2013) 152– 158

Appendix A. Supplementary data

Supplementary material related to this article can befound, in the online version, at http://dx.doi.org/10.1016/j.molcata.2013.09.027.

References

[1] K.M. Ho, P. Li, Langmuir 24 (2008) 1801–1807.[2] D. Qi, J. Lu, C. Deng, X. Zhan, J. Phys. Chem. C 113 (2009) 15854–15861.[3] H.Y. Xie, R. Zhen, B. Wang, Y.J. Feng, P. Chen, J. Hao, J. Phys. Chem. C 114 (2010)

4825–4830.[4] T. Gelbrich, M. Feyen, A.M. Schmidt, Macromolecules 39 (2006) 3469–3472.[5] A.H. Lu, E.L. Salabas, F. Schuth, Angew. Chem. Int. Ed. 46 (2007) 1222–1244.[6] M.Z. Kassaee, H. Masrouri, F. Movahedi, Appl. Catal. A: Gen. 395 (2011)

28–33.[7] G. Feng, D. Hu, L. Yang, Sep. Purif. Technol. 74 (2010) 253–260.[8] G.D. Carlo, A. Curulli, R.G. Toro, C. Bianchini, T.D. Caro, G. Padeletti, D. Zane, G.M.

Ingo, Langmuir 28 (2012) 5471–5479.[9] A. Primo, F. Quignard, Chem. Commun. 46 (2010) 5593–5595.10] K.H. Bae, M. Park, M.J. Do, N. Lee, J.H. Ryu, G.W. Kim, C. Kim, T.G. Park, T. Hyeon,

ACS Nano 6 (2012) 5266–5273.11] X. Zhang, H. Niu, Y. Pan, Y. Shi, Y. Cai, Anal. Chem. 82 (2010) 2363–2371.12] N.A. Frey, S. Peng, K. Cheng, S. Sun, Chem. Soc. Rev. 38 (2009) 2532–2542.13] P. Li, A.M. Zhu, Q.L. Liu, Q.G. Zhang, Ind. Eng. Chem. Res. 47 (2008) 7700–7706.14] V.R. Sinha, A.K. Singla, S. Wadhawan, R. Kaushik, R. Kumria, K. Bansal, S. Dhawan,

Int. J. Pharm. 274 (2004) 1–33.15] E. Guibal, Prog. Polym. Sci. 30 (2005) 71–109.16] M. Bodnar, J.F. Hartmann, J. Borbely, Biomacromolecules 6 (2005) 2521–2527.17] S.H. Lim, S.M. Hudson, Carbohydr. Res. 339 (2004) 313–319.18] A.J. Varma, S.V. Deshpande, J.F. Kennedy, Carbohydr. Polym. 55 (2004)

77–93.19] E.R. Welsh, C.L. Schauer, S.B. Qadri, R.R. Price, Biomacromolecules 3 (2002)

1370–1374.20] M.R. Kasaai, J. Agric. Food Chem. 57 (2009) 1667–1676.21] A. Lertworasirikul, S.I. Tsue, K. Noguchi, K. Okuyama, K. Ogawa, Carbohydr. Res.

338 (2003) 1229–1233.22] M. Klotzi, R.T. Morrison, J. Am. Chem. Soc. 69 (1947) 473.23] A. Peyman, W. Stahl, K. Wagner, D. Ruppert, K.H. Budt, Bioorg. Med. Chem. Lett.

4 (1994) 2601–2604.24] F.R. Atherton, C.H. Hassal, R.W. Lambert, J. Med. Chem. 29 (1986) 29–40.25] J.H. Meyer, P.A. Barlett, J. Am. Chem. Soc. 120 (1998) 4600–4609.26] F. Palacios, C. Alonso, J.M. Santos, Chem. Rev. 105 (2005) 899–931.27] G.P. Ellis, A. Weissberger, E.C. Taylor (Eds.), The Chemistry of Heterocyclic Com-

pounds. Chromenes, Chromanes, and Chromones\, John Wiley, New York, 1977,pp. 11–139 (Chapter 2).

28] E.A. Hafez, M.H. Elnagdi, A.A. Elagamey, F.A.M. El-Taweel, Heterocycles 26(1987) 903–907.

29] R.S. Varma, R. Dahiya, J. Org. Chem. 63 (1998) 8038–8041.30] P. Jayashree, G. Shanthi, P.T. Perumal, Synlett 20 (2009) 917–920.31] S.N. Murthy, B. Madhav, V.P. Reddy, Y.V.D. Nageswar, Tetrahedron Lett. 51

(2010) 3649–3653.32] D.S. Gaikwad, K.A. Undale, T.S. Shaikh, D.M. Pore, C. R. Chim. 14 (2011)

865–868.33] S.R. Kolla, Y.R. Lee, Tetrahedron 68 (2012) 226–237.34] M.A. Kulkarni, V.R. Pandurangi, U.V. Desai, P.P. Wadgaonkar, C. R. Chim. 15

(2012) 745–752.35] V. Belessi, R. Zboril, J. Tucek, M. Mashlan, V. Tzitzios, D. Petridis, Chem. Mater.

20 (2008) 3298–3305.36] A. Salam, J.J. Pawlak, R.A. Venditti, K. El-tahlawy, Biomacromolecules 11 (2010)

1453–1459.