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Journal of Chemistry
Volume 2013 (2013), Article ID 151273, 10 pages
http://dx.doi.org/10.1155/2013/151273
Research Article

- -Promoted Solvent-Free Synthesis of Benzoxazoles, Benzimidazoles, and Benzothiazole Derivatives

1Research and Development Division, RA Chem Pharma Limited, Prasanth Nagar, Hyderabad 500072, India
2Department of Chemistry, Acharya Nagarjuna University, Nagarjuna Nagar, Guntur 522510, India

Received 10 June 2012; Revised 17 August 2012; Accepted 21 August 2012

Academic Editor: Antonio Romerosa

Copyright © 2013 K. Ravi Kumar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

An efficient protocol has been developed for the preparation of a library of benzoxazole, benzimidazole, and benzothiazole derivatives from reactions of acyl chlorides with o-substituted aminoaromatics in the presence of catalytic amount of silica-supported sodium hydrogen sulphate under solvent-free conditions. Simple workup procedure, high yield, easy availability, reusability, and use of ecofriendly catalyst are some of the striking features of the present protocol.

1. Introduction

Molecules with benzoxazole, benzimidazole, and benzothiazoles moieties are attractive targets for synthesis since they often exhibit diverse and important biological properties. These heterocycles have shown different pharmacological activities such as antibiotic [1], antifungal [2], antiviral [3], anticancer [4], antimicrobial [5], and anti-Parkinson [6] properties. They have also been used as ligands for asymmetric transformations [7]. Benzimidazole derivatives are a unique and broad spectrum class of antirhino/enteroviral agents such as antiulcerative [8] and antiallergic [9]; they are effective against the human cytomegalovirus [10] and are also efficient selective neuropeptide Y Y1 receptor antagonists [11].

A number of methods are reported for the synthesis of these heterocycles by using different catalysts such as Pd-catalyzed oxidative cyclization [12], ionic liquid-mediated synthesis [13], base-assisted reaction of 1,1-dibromoethanes [14], SiO2-ZnCl2 [15], ZrOCl2·8H2O [16], In(OTf)3 [17], polyethylene-glycol-mediated catalysts [18], and different heteropolyacid catalysts [19], which include condensation of orthoesters [2022], nitriles [23], aldehydes [2427], carboxylic acids [2832], acid chlorides [33], amides [34] and esters [35] with o-substituted aminoaromatics in the presence of different acids and catalysts. Beckmann rearrangement of o-acylphenol oximes [36], photocyclization of phenolic Schiff bases [37], and benzimidazole, synthesis in solvent-free conditions [38] were also used. More recently benzoxazole, benzimidazole, and benzothiozoles were prepared from condensation of aldehydes with o-substituted aminoaromatics in the presence of Indion 190 resin [39]. However, many of these methods suffer from one or more of the drawbacks such as requirement of strong acidic conditions, long reaction times, low yields, tedious workup procedures, requirement of excess amounts of reagents, and use of toxic reagents, catalysts or solvents. Therefore, there is a strong demand for a highly efficient and environmentally benign method for the synthesis of these heterocycles.

In recent years, heterogeneous catalysts [4042] have gained importance in several organic transformations due to their interesting reactivity as well as for economic and environmental reasons. In continuation of our work to develop new methodologies for organic transformations [4346], we observed that silica-supported sodium hydrogen sulphate is highly efficient catalyst for the synthesis of substituted benzoxazole, benzimidazole, and benzothiazole derivatives through the reaction of o-substitued aminoaromatics with different acyl chlorides under solvent-free conditions. The catalyst NaHSO4-SiO2 can easily be prepared [47] from the readily available NaHSO4 and silica gel (230–400 mesh) and these are inexpensive and nontoxic. Besides, as the reaction is heterogeneous in nature, the catalyst can easily be removed by simple filtration (Scheme 1).

151273.sch.001
Scheme 1

2. Results and Discussions

In order to find the optimum reaction conditions for the condensation reaction, preliminary efforts were mainly focused on the evaluation of different solvents. The model reaction has been carried out between o-phenylenediamine and benzoyl chloride in the presence of NaHSO4-SiO2 catalyst under different solvents and at different temperatures, and results are shown in Table 1.

tab1
Table 1: Preparation of 2-phenyl benzimidazole using various solvents and temperauresa.

The effect of solvent, reaction temperature, and time on the reaction was systematically investigated, and the results were summarized in Table 1. The optimized reaction conditions for the reaction were found to be NaHSO4-SiO2 under solvent-free condition for 12 hr at the temperature of 100°C. Thus, we used NaHSO4-SiO2 as a catalyst in the present work. In order to elucidate the role of NaHSO4-SiO2 as catalyst, a controlled reaction was conducted using o-phenylenediamine and benzoyl chloride under solvent-free condition in the absence of catalyst. This resulted in the formation of only 7% of the fused product after 12 hr at 100°C. However, reaction with same substrate using 25%/wt of NaHSO4-SiO2 at 100°C for 12 hr afforded the product in quantitative yield. Lower temperatures required more time for the completion of the reaction and obtained low yields compared to the optimized reaction condition.

As shown in Table 2, different acyl chlorides reacted with different o-substituted aminoaromatics without any significant difference in the reaction time to give the corresponding 2-substituted benzoxazole, benzimidazole, and benzothiazole derivatives in good yield. The method has the ability to tolerate other functional groups such as methoxy, methyl, and halides. The products were synthesized in good to excellent yields and characterized by 1H NMR, LCMS, and physical constant. Physical and spectral data of known compounds are in agreement with those reported in literature [4857].

tab2
Table 2: Synthesis of 2-substituted benzoxazoles, benzimidazoles, and benzothiazolesa.

The reusability of catalyst is important for the large-scale operation and industrial point of view. Therefore, the recovery and reusability of NaHSO4-SiO2 was examined. The catalyst was separated and reused after washing with EtOAc and drying at 100°C. The reusability of catalyst was investigated in the reaction of o-phenylenediamine with benzoyl chloride (Figure 3). The results illustrated in Figure 3 showed that the catalyst can be used four times with consistent yield.

3. Conclusion

In conclusion, NaHSO4-SiO2 was found to be an efficient catalyst for the formation of benzoxazole, benzimidazole, and benzothiazole derivatives. The use of this inexpensive, easily available, and reusable catalyst makes this protocol practical, environment friendly, and economically attractive. The simple workup procedure, high yields of products, and nontoxic nature of the catalyst are other advantages of the present method.

4. Experimental Section

All 1H NMR spectra were recorded on 400 MHz Varian FT-NMR spectrometers. All chemical shifts are given as value with reference to Tetra methyl silane (TMS) as an internal standard. Melting points were taken in open capillaries. The IR spectra were recorded on a PerkinElmer 257 spectrometer using KBr discs. Products were purified by flash chromatography on 100–200 mesh silica gel. The chemicals and solvents were purchased from commercial suppliers either from Aldrich, Spectrochem, and they were used without purification prior to use.

5. FT-IR Spectrum of NaHSO -SiO

The FT-IR spectrum of the catalyst is shown in Figure 1. The catalyst is solid, and its solid-state IR spectrum was recorded using the KBr-disc technique. For silica (SiO2), the major peaks are broad antisymmetric Si-O-Si stretching from 1000–1100 cm−1 and symmetric Si-O-Si stretching near 798 cm−1, and bending modes of Si-O-Si lie around 467 cm−1. The spectrum also shows a broad Si-OH stretching absorption from 3300 to 3500 cm−1.

151273.fig.001
Figure 1: FT-IR spectra of silica-supported sodium hydrogen sulphate.

6. X-Ray Diffraction (XRD) Spectrum of NaHSO -SiO

Powder X-ray diffraction measurement was performed using D8 advance diffractometer. The strongest peaks of XRD pattern correspond to the SiO2 plane with the other peaks indexed as the [22, 23, 32] planes of supported sodium hydrogen sulphate (Figure 2).

151273.fig.002
Figure 2: XRD spectra of silica-supported sodium hydrogen sulphate.
151273.fig.003
Figure 3: Investigation of reusability of NaHSO4-SiO2.

7. General Experimental Procedure

A mixture of 2-amino phenols or o-phenylenediamines (1 mmol) and acyl chloride (1 mmol) were place in a sealed vessel containing NaHSO4-SiO2 (25%/wt) the reaction mixture was stirred at 100°C for 12 hrs. The progress of the reaction was monitored by TLC Hexane: EtOAc (4 : 1) after completion of the reaction, the reaction mixture was cooled and treated by dilution with EtOAc and the catalyst was removed by filtration. Obtained filtrate was evaporated under reduced pressure to get the crude product, which was purified by column chromatography to give 2-substituted benzoxazoles, benzimidazole, and benzothioazole derivatives.

8. Representative Spectral Data

2-Phenyl-1H-benzo [d]Imidazole (Table 2, Entry 1).1H NMR (DMSO-d6): 13.02 (br s, 1H), 8.20 (d, J = 7.6 Hz, 2H), 7.67–7.65 (m, 1H), 7.56–7.49 (m, 4H), 7.22–7.18 (m, 2H); (LC-MS) : 195.08 [M + H]+; IR (KBr, cm−1): 3420, 2920, 2627, 1623, 1410, 1276, 1119, 970, 738. Anal. Calcd. For C13H10N2: C, 80.39; H, 5.19; N, 14.42. Found: C, 80.11; H, 5.01; N, 14.38.

2-Heptyl-1H-benzo [d]Imidazole (Table 2, Entry 9). 1H NMR (DMSO-d6): δ12.11 (br s, 1H), 7.49 (d, J = 8 Hz, 1H), 7.38 (d, J = 6.4 Hz, 1H), 7.09–7.12 (m, 2H), 2.78 (t, J = 7.6 Hz, 2H), 1.77–1.73 (m, 2H), 1.31–1.25 (m, 8H), 0.85 (t, J = 6.4 Hz, 3H); (LC-MS) : 217.21 [M+H]+; IR (KBr, cm−1): 3467, 2926, 2683, 1624, 1451, 1272, 1028, 750. Anal. Calcd. For C14H20N2: C, 77.73; H, 9.32; N, 12.95. Found: C, 77.70; H, 9.28; N, 12.86.

2-Heptyl-5-methyl-1H-Benzo [d]Imidazole (Table 2, Entry 10).1H NMR (DMSO-d6): δ11.98 (br s, 1H), 7.36–7.18 (m, 2H), 6.93-6.89 (m, 1H), 2.74 (t, J = 7.6 Hz, 2H), 2.37 (s, 3H), 1.78–1.70 (m, 2H), 1.30–1.21 (m, 8H), 0.85 (t, J = 6.4 Hz, 3H); (LC-MS) : 231.18 [M+H]+; IR (KBr, cm−1): 2946, 2763, 1861, 1448, 1281, 1030, 803. Anal. Calcd. For C15H22N2: C, 78.21; H, 9.63; N, 12.16. Found: C, 78.19; H, 9.58; N, 12.15.

2-Phenyl Benzo [d]Oxazole (Table 2, Entry 11).1H NMR (CDCl3): δ 8.27–8.24 (m, 2H), 7.79–7.76 (m, 1H), 7.60–7.57 (m, 1H), 7.54–7.51 (m, 3H), 7.38–7.32 (m, 2H); (LC-MS) : 196.20 [M+H]+; IR (KBr, cm−1): 3435, 2921, 1615, 1551, 1240, 743. Anal. Calcd. For C13H9NO: C, 79.98; H, 4.65; N, 7.17. Found: C, 79.86; H, 4.61; N, 7.14; O.

2-Phenyl Benzo [d]Thiazole (Table 2, Entry 24).1H NMR (CDCl3): δ8.11–8.07 (m, 3H), 7.91 (d, J = 8.4 Hz, 1H), 7.51-7.40 (m, 4H), 7.39-7.37 (m, 1H); (LC-MS) : 212.12 [M+H]+; IR (KBr, cm−1): 3063, 2924, 1686, 1477, 1311, 1223, 961, 766, 685. Anal. Calcd. For C13H9NS: C, 73.90; H, 4.29; N, 6.63. Found: C, 73.87; H, 4.27; N, 6.59.

Acknowledgments

We sincerely thank the RA chem. Pharma Ltd for financial support and encouragement. Support from the analytical department is also acknowledged.

References

  1. D. A. Evans, C. E. Sacks, W. A. Kleschick, and T. R. Taber, “Polyether antibiotics synthesis. Total synthesis and absolute configuration of the ionophore A-23187,” Journal of the American Chemical Society, vol. 101, no. 22, pp. 6789–6791, 1979. View at Scopus
  2. M. Yamato, “Study on the development of biological-active compounds after the model of natural products,” Journal of the Pharmaceutical Society of Japan, vol. 112, no. 2, pp. 81–99, 1992.
  3. X. Song, B. S. Vig, P. L. Lorenzi, J. C. Drach, L. B. Townsend, and G. L. Amidon, “Amino acid ester prodrugs of the antiviral agent 2-bromo-5,6-dichloro-1- (β-D-ribofuranosyl)benzimidazole as potential substrates of hPEPT1 transporter,” Journal of Medicinal Chemistry, vol. 48, no. 4, pp. 1274–1277, 2005. View at Publisher · View at Google Scholar · View at Scopus
  4. D. Kumar, M. R. Jacob, M. B. Reynolds, and S. M. Kerwin, “Synthesis and evaluation of anticancer benzoxazoles and benzimidazoles related to UK-1,” Bioorganic and Medicinal Chemistry, vol. 10, no. 12, pp. 3997–4004, 2002. View at Publisher · View at Google Scholar · View at Scopus
  5. I. Yildiz-Oren, I. Yalcin, E. Aki-Sener, and N. Ucarturk, “Synthesis and structure-activity relationships of new antimicrobial active multisubstituted benzazole derivatives,” European Journal of Medicinal Chemistry, vol. 39, no. 3, pp. 291–298, 2004. View at Publisher · View at Google Scholar · View at Scopus
  6. A. Benazzouz, T. Boraud, P. Dubedat, A. Boireau, J.-M. Stutzmann, and C. Gross, “Riluzole prevents MPTP-induced parkinsonism in the rhesus monkey: a pilot study,” European Journal of Pharmacology, vol. 284, no. 3, pp. 299–307, 1995. View at Publisher · View at Google Scholar · View at Scopus
  7. A. Figge, H. J. Altenbach, D. J. Brauer, and P. Tielmann, “Synthesis and resolution of 2-(2-diphenylphosphinyl-naphthalen-1-yl)-1-isopropyl-1H-benzoimidazole; a new atropisomeric P,N-chelating ligand for asymmetric catalysis,” Tetrahedron Asymmetry, vol. 13, no. 2, pp. 137–144, 2002. View at Publisher · View at Google Scholar · View at Scopus
  8. L. J. Scott, C. J. Dunn, G. Mallarkey, and M. Sharpe, “Esomeprazole: a review of its use in the management of acid-related disorders,” Drugs, vol. 62, no. 10, pp. 1503–1538, 2002. View at Scopus
  9. H. Nakano, T. Inoue, N. Kawasaki et al., “Synthesis and biological activities of novel antiallergic agents with 5- lipoxygenase inhibiting action,” Bioorganic and Medicinal Chemistry, vol. 8, no. 2, pp. 373–380, 2000. View at Publisher · View at Google Scholar · View at Scopus
  10. Z. Zhu, B. Lippa, J. C. Drach, and L. B. Townsend, “Design, synthesis, and biological evaluation of tricyclic nucleosides (dimensional probes) as analogues of certain antiviral polyhalogenated benzimidazole ribonucleosides,” Journal of Medicinal Chemistry, vol. 43, no. 12, pp. 2430–2437, 2000. View at Publisher · View at Google Scholar · View at Scopus
  11. H. Zarrinmayeh, A. M. Nunes, P. L. Ornstein et al., “Synthesis and evaluation of a series of novel 2-[(4- chlorophenoxy)methyl]benzimidazoles as selective neuropeptide Y Y1 receptor antagonists,” Journal of Medicinal Chemistry, vol. 41, no. 15, pp. 2709–2719, 1998. View at Publisher · View at Google Scholar · View at Scopus
  12. W. H. Chen and Y. Pang, “Efficient synthesis of 2-(2′-hydroxyphenyl)benzoxazole by palladium(II)-catalyzed oxidative cyclization,” Tetrahedron Letters, vol. 50, no. 48, pp. 6680–6683, 2009. View at Publisher · View at Google Scholar · View at Scopus
  13. A. K. Yadav, M. Kumar, T. Yadav, and R. Jain, “An ionic liquid mediated one-pot synthesis of substituted thiazolidinones and benzimidazoles,” Tetrahedron Letters, vol. 50, no. 35, pp. 5031–5034, 2009. View at Publisher · View at Google Scholar · View at Scopus
  14. W. Shen, T. Kohn, Z. Fu, X. Jiao, S. Lai, and M. Schmitt, “Synthesis of benzimidazoles from 1,1-dibromoethenes,” Tetrahedron Letters, vol. 49, no. 51, pp. 7284–7286, 2008. View at Publisher · View at Google Scholar · View at Scopus
  15. R. G. Jacob, L. G. Dutra, C. S. Radatz, S. R. Mendes, G. Perin, and E. J. Lenardão, “Synthesis of 1,2-disubstitued benzimidazoles using SiO2/ZnCl2,” Tetrahedron Letters, vol. 50, no. 13, pp. 1495–1497, 2009. View at Publisher · View at Google Scholar · View at Scopus
  16. I. Mohammadpoor-Baltork, A. R. Khosropour, and S. F. Hojati, “ZrOCl2·8H2O as an efficient, environmentally friendly and reusable catalyst for synthesis of benzoxazoles, benzothiazoles, benzimidazoles and oxazolo[4,5-b]pyridines under solvent-free conditions,” Catalysis Communications, vol. 8, no. 12, pp. 1865–1870, 2007. View at Publisher · View at Google Scholar · View at Scopus
  17. R. Trivedi, S. K. De, and R. A. Gibbs, “A convenient one-pot synthesis of 2-substituted benzimidazoles,” Journal of Molecular Catalysis A, vol. 245, no. 1-2, pp. 8–11, 2006. View at Publisher · View at Google Scholar · View at Scopus
  18. C. Mukhopadhyay and P. K. Tapaswi, “PEG-mediated catalyst-free expeditious synthesis of 2-substituted benzimidazoles and bis-benzimidazoles under solvent-less conditions,” Tetrahedron Letters, vol. 49, no. 43, pp. 6237–6240, 2008. View at Publisher · View at Google Scholar · View at Scopus
  19. M. M. Heravi, S. Sadjadi, H. A. Oskooie, R. H. Shoar, and F. F. Bamoharram, “Heteropolyacids as heterogeneous and recyclable catalysts for the synthesis of benzimidazoles,” Catalysis Communications, vol. 9, no. 4, pp. 504–507, 2008. View at Publisher · View at Google Scholar · View at Scopus
  20. D. Villemin, M. Hammadi, and B. Martin, “Clay catalysis: condensation of orthoesters with O-substituted aminoaromatics into heterocycles,” Synthetic Communications, vol. 26, no. 15, pp. 2895–2899, 1996. View at Scopus
  21. M. Doise, F. Dennin, D. Blondeau, and H. Sliwa, “Synthesis of novel heterocycles: oxazolo[4,5-b]pyridines and oxazolo[4,5-d]pyrimidines,” Tetrahedron Letters, vol. 31, no. 8, pp. 1155–1156, 1990. View at Publisher · View at Google Scholar · View at Scopus
  22. G. L. Jenkins, A. M. Knevel, and C. S. Davis, “A new synthesis of the benzothiazole and benzoxazole rings,” Journal of Organic Chemistry, vol. 26, no. 1, p. 274, 1961. View at Scopus
  23. D. W. Hein, R. J. Alheim, and J. J. Leavitt, “The use of polyphosphoric acid in the synthesis of 2-aryl- and 2-alkyl-substituted benzimidazoles, benzoxazoles and benzothiazoles,” Journal of the American Chemical Society, vol. 79, no. 2, pp. 427–429, 1957. View at Scopus
  24. P. Salehi, M. Dabiri, M. A. Zolfigol, S. Otokesh, and M. Baghbanzadeh, “Selective synthesis of 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles in water at ambient temperature,” Tetrahedron Letters, vol. 47, no. 15, pp. 2557–2560, 2006. View at Publisher · View at Google Scholar · View at Scopus
  25. N. Parikh, D. Kumar, S. R. Roy, and A. K. Chakraborti, “Surfactant mediated oxygen reuptake in water for green aerobic oxidation: mass-spectrometric determination of discrete intermediates to correlate oxygen uptake with oxidation efficiency,” Chemical Communications, vol. 47, no. 6, pp. 1797–1799, 2011. View at Publisher · View at Google Scholar · View at Scopus
  26. A. K. Chakraborti, S. Rudrawar, K. B. Jadhav, G. Kaur, and S. V. Chankeshwara, “‘on water’ organic synthesis: a highly efficient and clean synthesis of 2-aryl/heteroaryl/styryl benzothiazoles and 2-alkyl/aryl alkyl benzothiazolines,” Green Chemistry, vol. 9, no. 12, pp. 1335–1340, 2007. View at Publisher · View at Google Scholar · View at Scopus
  27. B. Sadeghi and M. G. Nejad, “Silica sulfuric acid: an eco-friendly and reusable catalyst for synthesis of benzimidazole derivatives,” Journal of Chemistry, vol. 2013, Article ID 581465, 5 pages, 2013. View at Publisher · View at Google Scholar
  28. Y. H. So and J. P. Heeschen, “Mechanism of polyphosphoric acid and phosphorus pentoxide-methanesulfonic acid as synthetic reagents for benzoxazole formation,” Journal of Organic Chemistry, vol. 62, no. 11, pp. 3552–3561, 1997. View at Scopus
  29. S. Rudrawar, A. Kondaskar, and A. K. Chakraborti, “An efficient acid- and metal-free one-pot synthesis of benzothiazoles from carboxylic acids,” Synthesis, no. 15, Article ID Z05105SS, pp. 2521–2526, 2005. View at Publisher · View at Google Scholar · View at Scopus
  30. A. K. Chakraborti, C. Selvam, G. Kaur, and S. Bhagat, “An efficient synthesis of benzothiazoles by direct condensation of carboxylic acids with 2-aminothiophenol under microwave irradiation,” Synlett, no. 5, pp. 851–855, 2004. View at Scopus
  31. R. Kumar, C. Selvam, G. Kaur, and A. K. Chakraborti, “Microwave-assisted direct synthesis of 2-substituted benzoxazoles from carboxylic acids under catalyst and solvent-free conditions,” Synlett, no. 9, pp. 1401–1404, 2005. View at Publisher · View at Google Scholar · View at Scopus
  32. D. Kumar, S. Rudrawar, and A. K. Chakraborti, “One-pot synthesis of 2-substituted benzoxazoles directly from carboxylic acids,” Australian Journal of Chemistry, vol. 61, no. 11, pp. 881–887, 2008. View at Publisher · View at Google Scholar · View at Scopus
  33. R. N. Nadaf, S. A. Siddiqui, T. Daniel, R. J. Lahoti, and K. V. Srinivasan, “Room temperature ionic liquid promoted regioselective synthesis of 2-aryl benzimidazoles, benzoxazoles and benzthiazoles under ambient conditions,” Journal of Molecular Catalysis A, vol. 214, no. 1, pp. 155–160, 2004. View at Publisher · View at Google Scholar · View at Scopus
  34. M. Terashima and M. A. Ishii, “A facile synthesis of 2-substituted benzoxazoles,” Synthesis, vol. 1982, pp. 484–485, 1982.
  35. A. K. Chakraborti, S. Rudrawar, G. Kaur, and L. Sharma, “An efficient conversion of phenolic esters to benzothiazoles under mild and virtually neutral conditions,” Synlett, no. 9, pp. 1533–1536, 2004. View at Scopus
  36. B. M. Bhawal, S. P. Mayabhate, A. P. Likhite, and A. R. A. S. Deshmukh, “Use of zeolite catalysts for efficient synthesis of benzoxazoles via Beckmann rearrangement,” Synthetic Communications, vol. 25, no. 21, pp. 3315–3321, 1995. View at Scopus
  37. Y. Chen and D. X. Zeng, “Study on photochromic diarylethene with phenolic schiff base: preparation and photochromism of diarylethene with benzoxazole,” Journal of Organic Chemistry, vol. 69, no. 15, pp. 5037–5040, 2004. View at Publisher · View at Google Scholar · View at Scopus
  38. H. Thakuria and G. Das, “An expeditious one-pot solvent-free synthesis of benzimidazole derivatives,” Arkivoc, vol. 2008, no. 15, pp. 321–328, 2008. View at Scopus
  39. V. S. Padalkar, V. D. Gupta, K. R. Phatangare, V. S. Patil, P. G. Umape, and N. Sekar, “Indion 190 resin: efficient, environmentally friendly, and reusable catalyst for synthesis of benzimidazoles, benzoxazoles, and benzothiazoles,” Green Chemistry Letters and Reviews, vol. 5, no. 2, pp. 139–145, 2012. View at Publisher · View at Google Scholar · View at Scopus
  40. R. G. Jacob, C. S. Radatz, M. B. Rodrigues et al., “Synthesis of 1-H-1,5-benzodiazepines derivatives using SiO2/ZnCl2,” Heteroatom Chemistry, vol. 22, no. 2, pp. 180–185, 2011. View at Publisher · View at Google Scholar · View at Scopus
  41. R. G. Lara, E. L. Borges, E. J. Lenardao, D. Alves, R. G. Jacob, and G. Perin, “Addition of thiols to phenylselenoalkynes using KF/Alumina under solvent-free conditions,” Journal of the Brazilian Chemical Society, vol. 21, pp. 2125–2129, 2010.
  42. R. G. Jacob, M. S. Silva, S. R. Mendes, E. L. Borges, E. J. Lenardao, and G. Perin, “Atom-economic synthesis of functionalized octahydroacridines from citronellal or 3-(phenylthio)-citronellal,” Synthetic Communications, vol. 39, no. 15, pp. 2747–2762, 2009. View at Publisher · View at Google Scholar · View at Scopus
  43. K. R. Kumar, P. V. V. Satyanarayana, and B. S. Reddy, “Simple and efficient method for deprotection of tetrahydropyranyl ethers by using Silica supported sodium hydrogen sulphate,” Chinese Journal of Chemistry, vol. 30, no. 5, pp. 1189–1191, 2012. View at Publisher · View at Google Scholar · View at Scopus
  44. K. Ravi Kumar, P. V. V. Satyanarayana, and B. Srinivasa Reddy, “Simple and efficient method for tetrahydropyranylation of alcohols and phenols by using silica supported sodium hydrogen sulphate as a catalyst,” Asian Journal of Chemistry, vol. 24, no. 9, pp. 3876–3878, 2012. View at Scopus
  45. R. K. Kumar, P. V. V. Satyanarayana, and S. B. Reddy, “NaHSO4-SiO2 promoted synthesis of Benzimidazole derivatives,” Archives of Applied Science Research, vol. 4, no. 3, pp. 1517–1521, 2012.
  46. K. R. Kumar, P. V. V. Satyanarayana, and B. Srinivasa Reddy, “Direct and practical synthesis of 2-arylbenzoxazoles promoted by silica supported sodium hydrogen sulphate,” Der Pharma Chemica, vol. 4, no. 2, pp. 761–766, 2012. View at Scopus
  47. G. W. Breton, “Selective monoacetylation of unsymmetrical diols catalyzed by silica gel-supported sodium hydrogen sulf,” Journal of Organic Chemistry, vol. 62, p. 8952, 1997.
  48. A. J. Blacker, M. M. Farah, M. I. Hall, S. P. Marsden, O. Saidi, and J. M. J. Williams, “Synthesis of benzazoles by hydrogen-transfer catalysis,” Organic Letters, vol. 11, no. 9, pp. 2039–2042, 2009. View at Publisher · View at Google Scholar · View at Scopus
  49. S. B. Sapkal, K. F. Shelke, S. S. Sonar, B. B. Shingate, and M. S. Shingare, “Acidic ionic liquid catalyzed environmentally friendly synthesis of benzimidazole derivatives,” Bulletin of the Catalysis Society of India, pp. 78–83, 2009.
  50. J. Peng, M. Ye, C. Zong et al., “Copper-catalyzed intramolecular C-N bond formation: a straightforward synthesis of benzimidazole derivatives in water,” Journal of Organic Chemistry, vol. 76, no. 2, pp. 716–719, 2011. View at Publisher · View at Google Scholar · View at Scopus
  51. C. S. Cho, D. T. Kim, J. Q. Zhang, S. L. Ho, T. J. Kim, and S. C. Shim, “Tin(II) chloride-mediated synthesis of 2-substituted benzoxazoles,” Journal of Heterocyclic Chemistry, vol. 39, no. 2, pp. 421–423, 2002. View at Scopus
  52. M. M. Guru, M. A. Ali, and T. Punniyamurthy, “Copper-mediated synthesis of substituted 2-aryl-N-benzylbenzimidazoles and 2-arylbenzoxazoles via C-H functionalization/C-N/C-O bond formation,” Journal of Organic Chemistry, vol. 76, no. 13, pp. 5295–5308, 2011. View at Publisher · View at Google Scholar · View at Scopus
  53. J. Bonnamour and C. Bolm, “Iron-catalyzed intramolecular O-arylation: synthesis of 2-aryl benzoxazoles,” Organic Letters, vol. 10, no. 13, pp. 2665–2667, 2008. View at Publisher · View at Google Scholar · View at Scopus
  54. B. Wang, Y. Zhang, P. Li, and L. Wang, “An Efficient and practical synthesis of benzoxazoles from acyl chlorides and 2-aminophenols catalyzed by Lewis acid in(OTf)3 under solvent-free reaction conditions,” Chinese Journal of Chemistry, vol. 28, no. 9, pp. 1697–1703, 2010. View at Publisher · View at Google Scholar · View at Scopus
  55. M. Zhang, S. Zhang, M. Liu, and J. Cheng, “Palladium-catalyzed desulfitative C-arylation of a benzo[d]oxazole C-H bond with arene sulfonyl chlorides,” Chemical Communications, vol. 47, no. 41, pp. 11522–11524, 2011. View at Publisher · View at Google Scholar · View at Scopus
  56. H.-J. Lim, D. Myung, I. Y. C. Lee, and H. J. Myung, “Microwave-assisted synthesis of benzimidazoles, benzoxazoles, and benzothiazoles from resin-bound esters,” Journal of Combinatorial Chemistry, vol. 10, no. 4, pp. 501–503, 2008. View at Publisher · View at Google Scholar · View at Scopus
  57. C. Praveen, A. Nandakumar, P. Dheenkumar, D. Muralidharan, and P. T. Perumal, “Microwave-assisted one-pot synthesis of benzothiazole and benzoxazole libraries as analgesic agents,” Journal of Chemical Sciences, vol. 124, no. 3, pp. 609–624, 2012. View at Scopus