Cite this: RSC Advances, 2013, 3, 2924 CuFe 2 O 4 nanoparticles as a highly efficient and magnetically recoverable catalyst for the synthesis of medicinally privileged spiropyrimidine scaffolds3 Received 10th October 2012, Accepted 17th December 2012 DOI: 10.1039/c2ra22477a www.rsc.org/advances Anshu Dandia,* Anuj K. Jain and Sonam Sharma A highly efficient and green protocol for the synthesis of medicinally important fluorinated spiropyrimidine derivatives involving creation of six new covalent bonds has been developed using a magnetically separable and reusable heterogeneous copper ferrite nanocatalyst under mild reaction conditions. The synthesis of inverse spinel copper ferrite magnetic nanoparticles with average size of 38 nm has been achieved using combined sonochemical and co-precipitation techniques in aqueous medium from readily available inexpensive starting materials without any surfactant or capping agent. The particle size was determined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The magnetic nature of catalyst facilitates its easy removal from the reaction medium and can be reused five times without any significant loss of its catalytic activity. Negligible leaching of Cu and Fe in consecutive cycles makes the catalyst economical and environmentally benign. The structure of final products was established by single crystal X-ray analysis and spectroscopic techniques. 1. Introduction In recent years, nanoscience is an emerging field in the search to exploit diverse technological applications and magnetic nanomaterials are envisaged to have a major impact in many areas, including biotechnology, environmental remediation and especially catalysis. 1 Nanoparticles have materialized as viable alternatives to conventional materials as robust, readily available, large-surface-area, fewer coordination sites, and reactive morphologies, which maximize the reaction rates and minimize consumption of the catalyst. In view of their nano-size, the contact between reactants and catalyst increases dramatically thus mimicking the heterogeneous catalyst. However, the recoverable problem must be addressed before nanocatalytic processes can be scaled-up, due to the fact that nanoparticles, which include nano-scaled metal catalysts and supports, are difficult to separate from the reaction mixture, which can lead to the blocking of filters and valves by the nanoparticle catalyst. Currently, a method used to address this problem is the use of magnetic nanoparticles (MNPs), 2 a route that has attracted wide research interest for its unique physical properties. They possess advantage of being magnetically recoverable, thereby eliminating the requirement for either solvent swelling before or catalyst filtration after completion of the reaction. The strategy of magnetic separation, taking advantage of MNPs, is typically more effective than filtration or centrifugation as it prevents loss of the catalyst. The magnetic separation of MNPs, is simple, economical and promising for industrial applications. The increased interest in organofluorine compounds has led to the development of novel medicinal agents and new strategies in drug discovery and development. The synthesis of fluorine containing complexes or compounds and their derivatives provide unlimited potential for creating novel pharmacologically active lead compounds for use as thera- peutics. 3 The selective introduction of one or more fluorine atoms or trifluormethyl group into specific positions in an organic molecule changes the molecules’ physicochemical properties, including its stability, bioavailability, and lipophi- licity. The above behavior could be explained by the unique physical, chemical, and biological properties of the fluorine atom. 4 The hexahydropyrimidine skeleton is present in a number of alkaloids, eudistomidines H and I, 5 tetraponerines, 6 verbametrine 7 and verbamethine. 8 Hexetidine is a formalde- hyde-releasing antimicrobial agent employed in mouthwashes and numerous products of veterinary and human drugs. 9 Different N-substituted hexahydropyrimidines are synthetic intermediates for spermidine-nitroimidazole drugs for the treatment of A549 lung carcinoma. 10 They form structural Center for Advance Studies, Department of Chemistry, University of Rajasthan, Jaipur-302004, India. E-mail: [email protected]; Fax: +91 141 2609549; Tel: +91 941 4073436 3 Electronic supplementary information (ESI) available. CCDC 897553. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c2ra22477a RSC Advances PAPER 2924 | RSC Adv., 2013, 3, 2924–2934 This journal is ß The Royal Society of Chemistry 2013 Downloaded by University of New Hampshire on 11 March 2013 Published on 19 December 2012 on http://pubs.rsc.org | doi:10.1039/C2RA22477A View Article Online View Journal | View Issue
Transcript
Cite this: RSC Advances, 2013, 3,2924
CuFe2O4 nanoparticles as a highly efficient andmagnetically recoverable catalyst for the synthesis ofmedicinally privileged spiropyrimidine scaffolds3
Received 10th October 2012,Accepted 17th December 2012
DOI: 10.1039/c2ra22477a
www.rsc.org/advances
Anshu Dandia,* Anuj K. Jain and Sonam Sharma
A highly efficient and green protocol for the synthesis of medicinally important fluorinated
spiropyrimidine derivatives involving creation of six new covalent bonds has been developed using a
magnetically separable and reusable heterogeneous copper ferrite nanocatalyst under mild reaction
conditions. The synthesis of inverse spinel copper ferrite magnetic nanoparticles with average size of 38
nm has been achieved using combined sonochemical and co-precipitation techniques in aqueous medium
from readily available inexpensive starting materials without any surfactant or capping agent. The particle
size was determined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and
X-ray diffraction (XRD) analysis. The magnetic nature of catalyst facilitates its easy removal from the
reaction medium and can be reused five times without any significant loss of its catalytic activity.
Negligible leaching of Cu and Fe in consecutive cycles makes the catalyst economical and environmentally
benign. The structure of final products was established by single crystal X-ray analysis and spectroscopic
techniques.
1. Introduction
In recent years, nanoscience is an emerging field in the searchto exploit diverse technological applications and magneticnanomaterials are envisaged to have a major impact in manyareas, including biotechnology, environmental remediationand especially catalysis.1 Nanoparticles have materialized asviable alternatives to conventional materials as robust, readilyavailable, large-surface-area, fewer coordination sites, andreactive morphologies, which maximize the reaction ratesand minimize consumption of the catalyst. In view of theirnano-size, the contact between reactants and catalyst increasesdramatically thus mimicking the heterogeneous catalyst.However, the recoverable problem must be addressed beforenanocatalytic processes can be scaled-up, due to the fact thatnanoparticles, which include nano-scaled metal catalysts andsupports, are difficult to separate from the reaction mixture,which can lead to the blocking of filters and valves by thenanoparticle catalyst. Currently, a method used to address thisproblem is the use of magnetic nanoparticles (MNPs),2 a routethat has attracted wide research interest for its unique physicalproperties. They possess advantage of being magnetically
recoverable, thereby eliminating the requirement for eithersolvent swelling before or catalyst filtration after completion ofthe reaction. The strategy of magnetic separation, takingadvantage of MNPs, is typically more effective than filtration orcentrifugation as it prevents loss of the catalyst. The magneticseparation of MNPs, is simple, economical and promising forindustrial applications.
The increased interest in organofluorine compounds hasled to the development of novel medicinal agents and newstrategies in drug discovery and development. The synthesis offluorine containing complexes or compounds and theirderivatives provide unlimited potential for creating novelpharmacologically active lead compounds for use as thera-peutics.3 The selective introduction of one or more fluorineatoms or trifluormethyl group into specific positions in anorganic molecule changes the molecules’ physicochemicalproperties, including its stability, bioavailability, and lipophi-licity. The above behavior could be explained by the uniquephysical, chemical, and biological properties of the fluorineatom.4
The hexahydropyrimidine skeleton is present in a numberof alkaloids, eudistomidines H and I,5 tetraponerines,6
verbametrine7 and verbamethine.8 Hexetidine is a formalde-hyde-releasing antimicrobial agent employed in mouthwashesand numerous products of veterinary and human drugs.9
Different N-substituted hexahydropyrimidines are syntheticintermediates for spermidine-nitroimidazole drugs for thetreatment of A549 lung carcinoma.10 They form structural
Center for Advance Studies, Department of Chemistry, University of Rajasthan,
Jaipur-302004, India. E-mail: [email protected]; Fax: +91 141 2609549;
Tel: +91 941 4073436
3 Electronic supplementary information (ESI) available. CCDC 897553. For ESIand crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra22477a
RSC Advances
PAPER
2924 | RSC Adv., 2013, 3, 2924–2934 This journal is � The Royal Society of Chemistry 2013
units in trypanothione reductase inhibiting ligands for theregulation of oxidative stress in parasite cells.11 N-(4-amino-butyl) hexahydropyrimidine and N-(3-aminopropyl) hexahy-dropyrimidine are shown to compete with spermidine foruptake by L1210 cells.12 Due to their significant biologicallyactivity, hexahydropyrimidines have received a great deal ofattention in recent years.
Hexahydropyrimidines are classically prepared by conden-sation between substituted propane-1,3-diamines and alde-hydes or ketones.13a–f This method, however, limits the rangeof substitution at 5-position of the hexahydropyrimidines,being restrained by the availability of appropriately functio-nalized 1,3-diamines. There are also a few reports in theliterature describing the synthesis of substituted hexahydro-pyrimidine derivatives by using a,b-unsaturated nitriles, by thereaction of substituted alanine and carbamide14a,b or by thereaction of 1,3-dicarbonyl compounds or cyclic ketones,aromatic amines and formaldehyde.15a,b Thus each of theknown procedure has its own merits; however, further studiesare still necessary for the versatile, simple, ecofriendly andeconomical multicomponent methodology.
Multicomponent reactions allow the creation of severalbonds in a single operation and are attracting increasingattention as one of the most powerful emerging synthetic toolsfor the creation of molecular diversity and complexity.16
Considering the above points and in continuation of ourongoing program in the development of greener and sustain-able processes for heterocyclic synthesis17 and nano-cataly-sis,18,19 we were, thus, intrigued by the possibility of applyingnanotechnology to the design of highly efficient, recyclableand magnetically recoverable CuFe2O4 nanoparticles as aheterogeneous catalyst for the synthesis of highly substitutedspiropyrimidines incorporating pharmacophoric fluorine ortrifluoromethyl group under mild reaction conditions for thefirst time.
2. Results and discussion
The first step entails the synthesis of highly stable copperferrite nanoparticles. The catalyst was prepared by combinedsonochemical and co-precipitation technique in aqueousmedium without using any surfactant or capping agent. Thenanostructure of CuFe2O4 nanoparticles has been wellcharacterized by using X-ray diffraction (XRD), scanningelectron microscopy (SEM) and transmission electron micro-scopy (TEM) technique. The crystallinity and phase purity ofCuFe2O4 nanoparticles were examined by XRD measurements.As shown in Fig. 1, the strong and sharp reflection peaks inXRD patterns of dried precipitate which is mainly composed oftetragonal CuFe2O4 with a good crystallinity (JCPDS card N034-0425).20 The average particle size was calculated to be 38nm using Scherrer formula,
D = 0.94 l/b cosh
where D is the average size of the particles, l is the wavelength ofthe incident X-ray, h is the Bragg angle (in degrees), b is the full-width (in radians) subtended by the half-maximum intensity widthof the powder peak, expressed in units of 2h.
The structural composition and crystallinity of CuFe2O4
nanoparticles was further ascertained by SEM and TEM (Fig. 2and 3). The EDX analysis showed that the distribution of theelements in the product was Cu = 14.28%, Fe = 28.95% and O =56.77%. Thus the iron\copper ratio in the nanocrystals by EDXwas found to be 2.02 which is very much close to the atomicratio in the formula CuFe2O4. The particle size were alsomeasured in SEM micrograph and were found to be in therange of 35–50 nm which is consistent with the particle sizeobtained from XRD analysis.
The FTIR spectra (Fig. S4, ESI3) of the CuFe2O4 nanopar-ticles indicate the presence of two absorption bands at 562cm21 and 480 cm21. These intense absorption bands areattributed to the stretching vibration of Fe3+–O22 in the
Fig. 1 (a) XRD spectrum of native CuFe2O4 catalyst. (b) XRD spectrum of reused CuFe2O4 catalyst after 4th cycle.
This journal is � The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 2924–2934 | 2925
tetrahedral complexes and Cu2+–O22 in the octahedral com-plexes respectively. The positions of these bands confirm theexistence of Cu2+ ions entirely in the octahedral sites and theFe3+ ions in tetrahedral ones.21
The choice of an appropriate reaction medium is of crucialimportance for successful synthesis. Initially, the three-component condensation reaction of cyclohexanone 1a,formaldehyde 2 and 4-fluoroaniline 3a in the presence of 10mol% CuFe2O4 as a simple model substrate was investigatedto establish the feasibility of the strategy and to optimize thereaction conditions (Scheme 1). Different solvents such asethanol, acetonitrile, dimethyl sulfoxide (DMSO), DMF,dichloromethane, tetrahydrofuran and dioxane, were explored.After optimization, we observed that ethanol was the mosteffective solvent for this three-component condensationreaction. The use of ethanol effected not only the condensa-tion reaction of ketone, formaldehyde and aromatic amine ingood yield, but also performed well in the process of magnetic
separation of nanoparticle catalysts, by reducing the viscosityof the reaction mixture and facilitating the congregation ofmagnetic catalyst, when the reaction was complete. Slightlylower yields were obtained when acetonitrile, dimethylsulfoxide (DMSO), and DMF were used as the solvent(Table 1, entries 3, 4 and 5). Dichloromethane, tetrahydrofur-ane and dioxane afforded the products in only low to moderateyields (Table 1, entries 6–8). The corresponding product wasalso obtained in good yield under neat conditions (Table 1,entry 9). However, the mixture was viscous in the absence of asolvent and made the separation of catalyst from productsdifficult magnetically unless an extraction solvent such asether was added. Performing the reaction with a highercatalyst loading (20 mol%) had no significant effect on yield.However, if the amount of the catalyst was reduced to 5 and 1mol%, the product yield was reduced to 62% and 26%respectively.
Fig. 2 (a) SEM image of native CuFe2O4 catalyst. (b) SEM image of reused CuFe2O4 catalyst after 4th cycle.
Fig. 3 (a) TEM image of native CuFe2O4 catalyst. (b) The EDX spectrum of native CuFe2O4 catalyst.
2926 | RSC Adv., 2013, 3, 2924–2934 This journal is � The Royal Society of Chemistry 2013
The CuFe2O4 nanoparticle catalyst plays a crucial role in thesuccess of the reaction. In the absence of CuFe2O4 nanopar-ticle catalyst, the model reaction (Scheme 1) could be carriedout, but the product was obtained in very low yield afterprolonged reaction time (Table 1, entry 1).
With these encouraging results in hand, we turned toexplore the scope of the reaction using different aromaticamines as substrates under the optimized reaction conditions(Table 2). It was observed that the aromatic amines havingelectron donating as well as electron withdrawing groupreacted successfully to furnish spiropyrimidine derivativeswith good yields. In addition, we have also explored thereactivity of cyclic ketones for this transformation. From theresults, it is clear that cyclohexanone showed better reactivitythan 4-methylcyclohexanone and slightly higher yields wereobtained, while, 1,4-dioxaspiro[4.5]decan-8-one is found to beless active than cyclohexanone (Table 2).
Further, the leaching of the metal from the CuFe2O4
nanoparticles was investigated. After completion of thereaction, the supernatant was collected and tested for Fe andCu by inductively coupled plasma-atomic emission spectro-scopy (ICP-AES). The leaching of Fe and Cu in threeconsecutive cycles was found to be ¡0.5 ppm, which is wellbelow the permissible level concerning the toxicity in
humans.22 This study clearly demonstrated that there was nosignificant amount of leaching. It is also observed fromspectral studies that there is no change in the nature of thecatalyst even after four cycles. The powder X-ray diffractionanalysis exhibited identical peaks for both fresh and recoveredCuFe2O4 nanoparticles, which were compared with thosereported in the literature (Fig. 1).20 In addition, the SEManalysis of CuFe2O4 nanoparticles before and after thereaction showed identical shape and size (Fig. 2). Theseexperimental results clearly suggest that there was nosignificant change in the catalytic activity of nano-CuFe2O4
before and after the reaction.The proposed mechanism for the formation of spirohex-
ahydropyrimidine derivatives is shown in Scheme 2. Initiallyan imine is formed due to nucleophilic addition of aromaticamine to formaldehyde and subsequent loss of watermolecule. After the imine formation, in situ generated enolateattacks imine to afford b-amino carbonyl derivative A.Intermediate A reacts further in the same manner to formsubstituted propane-1,3-diamine B. Finally the condensationof the resulting substituted propane-1,3-diamine B withformaldehyde furnishes the desired spirohexahydropyrimi-dines.
Here, oxide (O22) of the metal oxide framework acting as aLewis base and Fe3+ as Lewis acid coordinate with the carbonyloxygen, thus increasing the electrophilicity of the carbonylcarbon and thereby making it possible to carry out the reactionat room temperature in short reaction time.
The reusability of the nano-CuFe2O4 catalyst was examinedand the results are summarized in Table 3. The catalyst wasmagnetically separated from the reaction mixture aftercompletion of the reaction, washed with ethanol, air driedand used directly for further catalytic reactions. No significantloss of catalyst (CuFe2O4) activity was observed up to fourcycles.
All the products were well characterized by IR, 1H, 13C NMR,mass spectra and elemental analysis. The final structure wasconfirmed by single crystal X-ray analysis of 9,11-bis-(4-fluorophenyl)-1,4-dioxa-9,11-diazadispiro [4.1.5.3]pentadecan-13-one (4k) (Fig. 4).23
Scheme 1 Synthesis of spirohexahydropyrimidine 4a.
All the chemicals used were of research grade and were usedwithout further purification. The melting points of allcompounds were determined on a Toshniwal apparatus. Thepurity of compounds was checked on thin layers of silica Gel–G coated glass plates and n-hexane : ethyl acetate (8 : 2) aseluent. IR spectra were recorded on a Shimadzu FT IR–8400Sspectrophotometer using KBr pellets. 1H and 13C NMR spectrawere recorded in CDCl3 using TMS as an internal standard ona Bruker spectrophotometer at 300 and 75 MHz respectively.Mass spectra of representative compounds were recorded onJEOL-SX-102 mass spectrometer at 70 eV. Elemental micro-analyses were carried out on a Carlo-Erba 1108 CHN analyzer.Single crystal X-ray diffraction was performed on a BrukerKappa Apex II instrument.
3.2 Preparation CuFe2O4 nanoparticles
CuFe2O4 nanoparticles were prepared by thermal decomposi-tion of Cu(NO3)2 and Fe(NO3)3 in water in the presence ofsodium hydroxide. Briefly, to a solution of Fe(NO3)3?9H2O
(3.34 g, 8.2 mmol) and Cu(NO3)2?3H2O (1 g, 4.1 mmol) in 75mL of distilled water, 3 g (75 mmol) of NaOH dissolved in 15mL of water was added at room temperature over a period of10 min during which a reddish-black precipitate was formed.Then the reaction mixture was warmed to 90 uC and stirredunder ultrasonic irradiation for two hours. After 2 h, it wascooled to room temperature and the magnetic particles soformed were separated by a magnetic separator. It was thenwashed with water (3 6 30 mL) and catalyst was kept in an airoven for overnight at 80 uC. Then the catalyst was ground in amortar-pestle and kept in a furnace at 700 uC for 5 h (step uptemperature 20 uC min21) and then cooled to room tempera-
Scheme 2 Plausible mechanism for the reaction of 4-fluoroaniline and formaldehyde with cyclohexanone.
Table 3 Reusability of the nano-CuFe2O4 catalyst for the synthesis of 4a
No. of cycles Yield (%)a
1 822 813 804 795 79
a Isolated yields.Fig. 4 Single crystal X-ray structure of 9,11-bis-(4-fluorophenyl)-1,4-dioxa-9,11-diazadispiro[4.1.5.3]pentadecan-13-one (4k).
This journal is � The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 2924–2934 | 2931
ture. 930 mg of magnetic CuFe2O4 particles of size 35–50 nmwere obtained.
3.3 Catalyst characterization
The wide angle X-ray diffraction pattern of the sample wasobtained using Bragg–Brentanno geometry on PANalyticalX9pert pro diffractometer in 2h range of 20–70u with Cu-Karadiation source (l = 1.5406 Å). The X-ray tube was operated at45 kV and 40 mA. TEM measurements of the sample werecarried out using a JEOL transmission electron microscope.Sample for the TEM was prepared by making a cleardispersion of nanoparticles in dimethyl formaldehyde andputting a drop of it on a carbon-coated copper grid. Formationof copper ferrite nanoparticles was first ascertained by electrondispersive X-ray (EDX) analysis combined with scanningelectron microscopy (SEM). SEM was done on a ‘JEOL JSM-6610LV’ Scanning Electron Microscope combined with EDXsystem (INCA Analyzer). For SEM analysis, the sample wasdispersed on the aluminium stub used for sample mounting.The sample was scanned at an accelerating voltage of 20 kV ata working distance of 15 mm. The particle size was measuredat a magnification of 10 kX.
3.4 General procedure for the synthesis of spiropyrimidinederivatives 4(a–o)
To a solution of cyclic ketones (1 mmol), aromatic amines (2mmol), formaldehyde (3.3 mmol, 36% aqueous solution), anda catalytic amount of CuFe2O4 (10 mol%) in ethanol (5 mL)was stirred at room temperature for the stipulated times. Aftercompletion of the reaction monitored by TLC, 10 mL ethanolwas added to the reaction mixture and the catalyst CuFe2O4
was separated magnetically. The reaction mixture was allowedto stand overnight. The solid material was filtered off, washedwith water (2 6 10 mL), dried and recrystallized from ethanolto furnish pure spiropyrimidine derivatives.
After completion of the reaction, 10 mL ethanol was added tothe reaction mixture and the catalyst CuFe2O4 was separatedmagnetically, washed with ethanol and then air dried. Therecovered catalyst was used directly in the next runs and nosubstantial loss of activity was observed up to four cycles.
4. Conclusion
In conclusion, we have developed a novel, green and highlyefficient protocol for the synthesis of fluorine containingspirohexahydropyrimidine derivatives using magneticallyseparable and easily recyclable heterogeneous CuFe2O4 nano-catalyst in ethanol at room temperature. The magnetic natureof this heterogeneous nanocatalyst allows for its easy separa-tion from the reaction mixture by using a simple bar magnet,which is an additional greener attribute of this reaction.Moreover, this method can be considered as an ideal tool forgreen synthesis because (1) rapid assembly of medicinallyprivileged heterocyclic molecules by a three-componentprocess minimizes the generation of waste; (2) the processhas high atom economy and environmentally benign, since
This journal is � The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 2924–2934 | 2933
only water molecules are lost; (3) six covalent bonds aregenerated in a single reaction. Therefore, this one-pot,multicomponent procedure clearly represents an appealingmethodology for the synthesis of highly substituted spiropyr-imidines both in academia and pharmaceutical industries.
Acknowledgements
Financial assistance from the Council for scientific andIndustrial Research New Delhi is gratefully acknowledged.We are thankful to the Bhabha Atomic Research Centre(BARC), Mumbai and Central Drug Research Institute (CDRI),Lucknow, for the spectral and elemental analyses. We are alsothankful to STIC Cochin for single crystal X-ray analysis.
References
1 (a) B. C. Ranu, R. Dey, T. Chatterjee and S. Ahammed,ChemSusChem, 2012, 5, 22; (b) S. B. Kalidindi and B.R. Jagirdar, ChemSusChem, 2012, 5, 65; (c) R. Mout, D.F. Moyano, S. Rana and V. M. Rotello, Chem. Soc. Rev., 2012,41, 2539; (d) A. Fihri, M. Bouhrara, B. Nekoueishahraki, J.-M. Basset and V. Polshettiwar, Chem. Soc. Rev., 2011, 40, 5181;(e) N. Tran and T. J. Webster, J. Mater. Chem., 2010, 20, 8760;(f) V. Polshettiwar and R. S. Varma, Green Chem., 2010, 12,743; (g) V. Polshettiwar and R. S. Varma, Chem. Soc. Rev., 2008,37, 1546; (h) J. Mondal, T. Sen and A. Bhaumik, Dalton Trans.,2012, 41, 6173; (i) T. Sen, I. J. Bruce and T. Mercer, Chem.Commun., 2010, 46, 6807.
2 Recent reviews and references therein: (a) V. Polshettiwar,R. Luque, A. Fihri, H. Zhu, M. Bouhrara and J.-M. Basset,Chem. Rev., 2011, 111, 3036; (b) S. Shylesh, V. Schunemannand W. R. Thiel, Angew. Chem., Int. Ed., 2010, 49, 3428; (c)A.-H. Lu, E. L. Salabas and F. Schuth, Angew. Chem., Int.Ed., 2007, 46, 1222.
3 (a) O. C. Agbaje, O. O. Fadeyi and C. O. Okoro, TetrahedronLett., 2011, 52, 5297; (b) R. Filler and R. Saha, Future Med.Chem., 2009, 1, 777; (c) C. Isanbor and D. O’Hagan, J.Fluorine Chem., 2006, 127, 303; (d) K. L. Kirk, J. FluorineChem., 2006, 127, 1013.
4 (a) L. Hunter, Beilstein J. Org. Chem., 2010, 6, 38; (b)D. O’Hagan, Chem. Soc. Rev., 2008, 37, 308.
5 T. Suzuki, T. Kubota and J. Kobayashi, Bioorg. Med. Chem.Lett., 2011, 21, 4220.
6 B. Renson, P. Merlin, D. Daloze, J. C. Braekman, Y. Roisinand J. M. Pasteels, Can. J. Chem., 1994, 72, 105.
7 K. Drandarov, A. Guggisberg and M. Hesse, Helv. Chim.Acta, 1999, 82, 229.
8 A. Guggisberg, K. Drandarov and M. Hesse, Helv. Chim.Acta, 2000, 83, 3035.
9 P. A. S. Smith and R. N. Loeppky, J. Am. Chem. Soc., 1967,89, 1147.
10 A. Q. Siddiqui, L. Merson-Davies and P. M. Cullis, J. Chem.Soc., Perkin Trans. 1, 1999, 3243.
11 D. Hrvath, J. Med. Chem., 1997, 40, 2412 and the referencestherein.
12 R. J. Bergeron and H. W. Seligsohn, Bioorg. Chem., 1986, 14,345.
13 (a) K. Chantrapromma, J. S. McManis and B. Ganem,Tetrahedron Lett., 1980, 21, 2475; (b) K. Chantraprommaand B. Ganem, Tetrahedron Lett., 1981, 22, 23; (c) I.A. Perillo, M. B. Garcia, J. A. Bisceglia and L. R. Orelli, J.Heterocycl. Chem., 2002, 39, 655; (d) A. R. Katrizky, S.K. Singh and H. Y. He, J. Org. Chem., 2002, 67, 3115; (e)M. Mayr and M. R. Buchmeiser, Macromol. Rapid Commun.,2004, 25, 231; (f) M. Mayr, K. Wurst, K. H. Ongania and M.R. Buchmeiser, Chem.–Eur. J., 2004, 10, 1256.
14 (a) S. Kambe, K. Saito, M. Hirose, A. Sakurai and H.Midorikawa, Synthesis, 1984, 860; (b) Z. J. Beresnevicius,V. Mickevicius, K. Rutkauskas and K. Kantminiene, Polish J.Chem. Technol., 2003, 5, 75.
15 (a) C. Mukhopadhyay, S. Rana and R. J. Butcher,Tetrahedron Lett., 2011, 52, 4153; (b) H.-L. Wei, Z.-Y. Yan,Y.-N. Niu, G.-Q. Li and Y.-M. Liang, J. Org. Chem., 2007, 72,8600.
16 Recent review and references therein: (a) A. Domling,W. Wang and K. Wang, Chem. Rev., 2012, 112, 3083; (b)C. de Graaff, E. Ruijter and R. V. A. Orru, Chem. Soc. Rev.,2012, 41, 3969; (c) S. S. van Berkel, B. G. M. Bogels, M.A. Wijdeven, B. Westermann and F. P. J. T. Rutjes, Eur. J.Org. Chem., 2012, 3543; (d) J. S. Yadav, A. Antony, J. Georgeand B. V. S. Reddy, Curr. Org. Chem., 2010, 14, 414; (e)B. Jiang, F. Shi and S.-J. Tu, Curr. Org. Chem., 2010, 14, 357;(f) J. D. Sunderhaus and S. F. Martin, Chem.–Eur. J., 2009,15, 1300.
17 (a) A. Dandia, A. K. Jain and S. Sharma, Tetrahedron Lett.,2012, 53, 5859; (b) A. Dandia, A. K. Jain and S. Sharma,Tetrahedron Lett., 2012, 53, 5270; (c) A. Dandia, A. K. Laxkarand R. Singh, Tetrahedron Lett., 2012, 53, 3012; (d)A. Dandia, R. Singh, S. Bhaskaran and S. D. Samant,Green Chem., 2011, 13, 1852; (e) A. Dandia, R. Singh andS. Bhaskaran, Ultrason. Sonochem., 2011, 18, 1113; (f)A. Dandia, A. K. Jain and D. S. Bhati, Synth. Commun.,2011, 41, 2905.
18 A. Dandia, V. Parewa, A. K. Jain and K. S. Rathore, GreenChem., 2011, 13, 2135.
19 A. Dandia, V. Parewa and K. S. Rathore, Catal. Commun.,2012, 28, 90.
20 J. E. Tasca, A. Ponzinibbio, G. Diaz, R. D. Bravo, A. Lavatand M. G. Gonzalez, Top. Catal., 2010, 53, 1087.
21 F. Davar and M. S. Niasari, J. Alloys Compd., 2011, 509,2487.
22 (a) C. Poetra, Environ. Health Perspect., 2004, 112, a568; (b)S. Sabhi and J. Kiwi, Water Res., 2001, 35, 1994.
23 The crystal structure (4k: CCDC 897553) has been depositedat the Cambridge Crystallographic Data Center and isavailable on request from the Director, CCDC, 12 UnionRoad, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033;e-mail: [email protected], http://www.ccdc.cam.ac.uk/deposit).
2934 | RSC Adv., 2013, 3, 2924–2934 This journal is � The Royal Society of Chemistry 2013
