HETEROCYCLES

Paper | Regular issue | Vol. 81, No. 7, 2010, pp. 1651-1659
Received, 20th March, 2010, Accepted, 28th April, 2010, Published online, 28th April, 2010.
DOI: 10.3987/COM-10-11950

■ Microwave-Assisted Selective Synthesis of 2H-Indazoles via Double Sonogashira Coupling of 3,4-Diiodopyrazoles and Bergman–Masamune Cycloaromatization

Hayato Ichikawa,* Haruhiko Ohfune, and Yoshihide Usami

Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1, Izumi-cho, Narashino, Chiba 275-8575, Japan

Abstract

The microwave-assisted double Sonogashira coupling of 3,4-diiodo-1-trityl and 1-phenylpyrazole with terminal acetylene took only three minutes. Dialkynylpyrazoles, the coupling products, were heated at 240 °C in the presence of 1,4-cyclohexadiene to obtain 2H-2-trityl and 2-phenylindazoles, respectively. This synthetic route to 2H-indazole, which was achieved via cyclization to form the 6-membered ring of dialkynylpyrazole, is a novel procedure.

INTRODUCTION
Pyrazoles and indazoles are important molecules in organic synthesis because they serve as the structural framework for large biologically active compounds, such as HIV-1 protease inhibitor,1 COX-2 inhibitor,2 dopamine receptor agonist,3 cisplatin-like metal complexes,4 and others.5 Generally, substituted pyrazoles are synthesized by the condensation of 1,3-dicarbonyl compounds and hydrazine or the 1,3-dipolar cycloaddition reaction of diazomethane with acetylene.6 4-Substituted pyrazoles are synthesized by the condensation of a malonaldehyde derivative and hydrazine; however, the synthesis of 4-substituted pyrazoles requires multiple steps.7 We have recently reported the synthesis of 4-arylprazoles via the Kumada coupling of 4-bromo-1-tritylpyrazole, which can be easily prepared from commercially available pyrazole, and an aryl Grignard reagent.8 Although a great number of cross-coupling reactions of heterocycles using palladium catalysts have been reported, there is little work on the synthesis via coupling of a 4-substituted pyrazole.9 Microwave irradiation, which is frequently used for palladium-catalyzed cross-coupling reactions, is a powerful tool in synthetic chemistry and its ability to shorten reaction times, increase reaction yields, and promote reactions that are otherwise unsuccessful under conventional hood conditions is the property that medicinal chemists are looking for to optimize routine procedures.10
Whereas substituted indazoles are generally prepared as 1H- and 2H-indazole mixtures by the N-alkylation or N-arylation of indazole, only a handful of studies of the syntheses of 2H-indazoles involve the regioselective cyclization of a five-membered ring.11
In this paper, we describe the synthesis of alkynylpyrazoles as invaluable intermediates and the effect of microwave irradiation on the double Sonogashira coupling of 3,4-diiodo-1-tritylpyrazole. We also describe the regioselective synthesis of 2H-indazole via the Bergman-Masamune cycloaromatization of dialkynylpyrazoles, which has not been reported so far (Scheme 1). An ab initio study of the strategy for indazoles has been reported.12

RESULTS AND DISCUSSION
The Sonogashira coupling of 4-iodo-1-tritylpyrazole (1) with propargyl alcohol (2a) in the presence of PdCl2(PPh3)2 and CuI as cocatalyst furnished the corresponding 4-alkynylpyrazole 3 in good yield (Table 1). When 1 mol% each of PdCl2(PPh3)2 and CuI was used for the coupling reaction, a long reaction time was required (entry 3). The reactions of phenyl acetylene (2d) and p-fluorophenyl acetylene (2e) gave coupled products in excellent yields. In contrast, the reaction of p-methoxy phenylacetylene (2f) was slow because of the low acidity of the terminal acetylene (entries 6 – 8).
Then, 3,4-diiodo-1-tritylpyrazole (4) and 2a reacted under same conditions as those specified in Table 1 to produce dialkynylpyrazole (5a) in moderate yield (Table 2). Increasing the catalyst load gave 5 in good yield; however, the reaction time was approximately 5 days under a conventional hood (entries 2 and 3). Since diiodopyrazoles are very inactive than mono-iodopyrazoles for Sonogashira coupling, the reaction condition is drastic changed. To shorten the reaction time for dialkynylation, microwave-assisted Sonogashira coupling was attempted. It is known that the microwave-assisted cross-coupling reaction with palladium complexes reduces the reaction time dramatically.10 In our case, 5 was obtained after only three minutes under microwave conditions in good yields (entries 4 and 7). When the catalyst loading was reduced, the starting material was recovered (entry 5). Microwave irradiation for seven minutes accompanied by detritylation reduced the product yield (entry 6). It is known that the dialkynylpyrazoles are obtained from condensation of (Z)-enediyne compounds and diazomethane, and then cyclization. 13 Our double Sonogashira coupling method is proceeded for very short time and with easier operation than that route.

Silylacetylene 5b and 7, which were prepared by the microwave-assisted double Sonogashira coupling, were used as starting materials for 2H-indazole synthesis, respectively (Scheme 2). The desilylation of 5b and 7 with TBAF proceeded in quantitative yields. Terminal acetylene 8 and 9 were heated at 240 °C for 0.5 h in the presence of an excess amount of 1,4-cyclohexadiene to give Bergman-Masamune cycloaromatization products 10 and 11 in 20% and 40% yields, respectively.
In conclusion, the microwave-assisted double Sonogashira coupling of 3,4-diiodo-1-tritylpyrazole (4) required only three minutes and 2H-indazoles 10 and 11 were obtained from dialkynylpyrazoles (5). This synthetic route to 2H-indazole, which was achieved by cyclization to form a 6-membered ring of dialkynylpyrazole, is a novel procedure. Known synthetic routes to 2H-indazoles involve cyclization to form 5-membered rings. In the future, we will attempt to prepare various dialkynylpirazoles and multi-fused heterocyclic compounds by microwave-assisted tandem radical cyclization of substituted dialkynylpyrazoles.

EXPERIMENTAL
Unless otherwise indicated, all starting materials were obtained from commercial suppliers (Aldrich, Kishida chemical, nacalai tesque, Wako pure chemicals and TCI) and were used without further purification. A microwave-assited Sonogashira coupling was took place by Biotage Initiator as a microwave synthesizer with sealed reaction vessels. IR spectra were obtained with a JEOL FT/IR-680 Plus spectrometer. HRMS was determined with a Hitachi 4000H or a JEOL JMS-700 (2) mass spectrometer. NMR spectra were recorded at 27 °C on Varian UNITY INOVA-500, Gemini-2000, and XL-300 spectrometers in CDCl3 with tetramethylsilane (TMS) as internal standard. Melting points were determined on a Yanagimoto micromelting point apparatus and are uncorrected. Liquid column chromatography was conducted over silica gel (SiliCycle, SilaFlash F60, 40-63 µm). Analytical TLC was performed on precoated Merck aluminum sheets (DC-Alufolien Kieselgel 60 F254), and compounds were detected by spraying an ethanol solution of phosphomolybdic acid, followed by heating.
A representative procedure for the microwave-assisted double Sonogashira coupling (Table 2, entry 4): CuI (38 mg, 0.2 mmol) and PdCl2(PPh3)2 (70 mg, 0.1 mmol) were added to an Et3N (10 mL) and DMF (5 mL) solution of 4 (0.56 g, 1.0 mmol) and propargyl alcohol (0.6 mL, 10.0 mmol) and the mixture was heated by microwave at 130 °C for 3 min. The mixture was poured into saturated aqueous NH4Cl solution and extracted with EtOAc, and the organic layer was washed with 10% HCl solution and dried over MgSO4. The crude product was concentrated under vacuum and purified via silica gel column chromatography (hexane/EtOAc = 1:2) to give 0.34 g (82%) of 5a.
The synthetic procedure for 2-phenyl-2H-indazole (11) (Scheme 2): CuI (38 mg, 0.2 mmol) and PdCl2(PPh3)2 (70 mg, 0.1 mmol) were added to an Et3N (10 mL) and DMF (5 mL) solution of 6 (0.56 g, 1.0 mmol) and trimethylsilylacetylene (1.4 mL, 10.0 mmol), and the mixture was heated by microwave at 130 °C for 3 min. The mixture was poured into saturated aqueous NH4Cl solution and extracted with /EtOAc, and the organic layer was washed with 10% HCl solution and dried over MgSO4. The crude product was concentrated under vacuum and purified by silica gel column chromatography (hexane/EtOAc = 5:1) to give 0.34 g (90%) of 7. 1.0 M THF solution of TBAF (1.92 mL, 1.92 mmol) was added to a solution of 7 (0.24 g 0.71 mmol) in THF (30 mL) and the reaction mixture was stirred for 1 h at room temperature. After concentrating under vacuum, the resultant mixture was poured into brine and extracted with EtOAc. The organic layer was dried over MgSO4 and the crude product was concentrated under vacuum and purified by silica gel column chromatography (hexane/EtOAc = 10:1) to give 0.19 g (99%) of 9. The reaction mixture of 9 (12 mg, 0.06 mmol) in DMF (0.9 mL) and 1,4-cyclohexadiene (0.3 mL, 5 mmol) was sealed in a reaction vessel and heated by microwave at 240 °C for 0.5 h. The reaction mixture was concentrated under vacuum and the crude product was purified by silica gel column chromatography (hexane/EtOAc = 10:1) to give 4.7 mg (40%) of 11.
4-(3-Hydroxyprop-1-ynyl)-1-tritylpyrazole (3a)
White powder; mp 219 °C; 1H-NMR (300 MHz, CDCl3) δ 1.68 (brs, 1H, OH), 4.40 (s, 2H, CH2), 7.10-7.15 (m, 6H, TrH), 7.28-7.33 (m, 9H, TrH), 7.50 (d, J = 0.4 Hz, 1H, pyrazole-H), 7.72 (s, J = 0.4 Hz, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 51.8, 77.2, 79.1, 88.1, 101.0, 127.4, 127.5, 129.6, 135.1, 141.7, 142.1; EI-MS m/z 364 (M+), HRMS m/z 364.1576, Calcd. for C25H20N2O: 364.1576; IR (vmax) (KBr) 3399 (OH), 3055, 3032, 2233 (C≡C), 1596, 1490, 1445, 1372 cm-1.
4-Trimethylsilylethynyl-1-tritylpyrazole (3b)
White powder; mp 144 °C; 1H-NMR (300 MHz, CDCl3) δ 0.20 (s, 9H, CH3Si), 7.09-7.13 (m, 6H, TrH), 7.29-7.31 (m, 9H, TrH), 7.53 (d, J = 0.7 Hz, 1H, pyrazole-H), 7.75 (d, J = 0.7 Hz, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 0.5, 79.1, 95.02, 96.34, 101.8, 127.4, 127.5, 129.7, 135.3, 142.1, 142.6; EI-MS m/z 406 (M+), HRMS m/z 406.1871, Calcd for C27H26N2Si: 406.1865; IR (vmax) (KBr) 3056, 2165 (C≡C), 1596, 1489, 1445, 1398 (Si-CH3), 1352 (Si-CH3)cm-1.
4-Octyn-1-yl-1-tritylpyrazole (3c)
White powder; mp 110 °C; 1H-NMR (300 MHz, CDCl3) δ 0.88 (t, J = 6.9 Hz, 3H, CH3), 1.26-1.44 (m, 6H, CH2), 1.56 (quint, J = 7.3 Hz, 2H, CH2), 2.33 (t, J = 7.1 Hz, 2H, CH2), 7.09-7.15 (m, 6H, TrH), 7.29-7.35 (m, 9H, TrH), 7.44 (d, J = 0.6 Hz, pyrazole-H), 7.68 (d, J = 0.6 Hz, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 14.5, 19.8, 22.9, 29.0, 29.1, 31.7, 71.4, 78.9, 90.8, 102.3, 127.4, 127.4, 129.7, 134.3, 141.6, 142.3; EI-MS m/z 418 (M+), HRMS m/z 418.2400, Calcd for C30H30N2: 418.2409; IR (vmax) (KBr) 3032, 3058, 2928, 2856, 2241(C≡C), 1596, 1486, 1444 cm-1.
4-Phenylethynyl-1-tritylpyrazole (3d)
White powder; mp 212 °C; 1H-NMR (300 MHz, CDCl3); δ 7.14-7.19 (m, 6H, TrH), 7.29-7.34 (m, 12H, Ph and Tr), 7.59 (s, 1H, pyrazole-H), 7.82 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 79.1, 80.8, 90.0, 101.8, 123.0, 127.4, 127.48, 127.53, 127.9, 129.7, 130.8, 134.7, 141.6, 142.1; EI-MS m/z 410 (M+), HRMS m/z 410.1780, Calcd for C30H22N2: 410.1783; IR (vmax) (KBr) 3059, 3032, 2224 (C≡C), 1595, 1489, 1445 cm-1.
4-(4-Fluorophenyl)ethynyl-1-tritylpyrazole (3e)
White powder; mp 220 °C; 1H-NMR (300 MHz, CDCl3) δ 7.00 (dd, J = 8.7, 8.7 Hz, 2H, ArH), 7.12-7.18 (m, 6H, TrH), 7.28-7.35 (m, 9H, TrH), 7.41 (dd, J = 5.4, J = 8.7 Hz, 2H, ArH), 7.59 (s, 1H, pyrazole-H), 7.81 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 79.1, 80.5, 89.0, 101.6, 115.3 (d, 2JC-F = 21.6 Hz), 119.1 (d, 4JC-F = 3.4 Hz), 127.4, 127.5, 129.7, 132.7 (d, 3JC-F = 8.0 Hz), 134.7, 141.6, 142.1, 161.6 (d, 1JC-F = 245.2 Hz); EI-MS m/z 428 (M+), HRMS m/z 428.1692, Calcd for C30H21FN2: 428.1689; IR (vmax) (KBr) 3058, 3033, 2224 (C≡C), 1599, 1503, 1445, 1227 (C-F) cm-1.
4-(4-Methoxyphenylethynyl)-1-tritylpyrazole (3f)
White powder; mp 201 °C; 1H-NMR (300 MHz, CDCl3) δ 3.80 (s, 3H, OCH3), 6.83 (d, J = 8.9 Hz, 2H, ArH), 7.13-7.33 (m, 15H, TrH), 7.37 (d, J = 8.9 Hz, 2H, Ar-H), 7.56 (s, 1H, pyrazole-H), 7.79 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 55.4, 79.0, 79.3, 89.8, 102.0, 113.7, 115.1, 127.4, 127.5, 129.7, 132.3, 134.5, 141.5, 142.2, 158.7; EI-MS m/z 440 (M+), HRMS m/z 440.1891, Calcd for C31H24N2O: 440.1889; IR (vmax) (KBr) 3068, 2022, 1466, 1445, 1251, 1033 cm-1.
3,4-Bis(3-hydroxyprop-1-ynyl)-1-tritylpyrazole (5a)
White powder; mp 203 °C; 1H-NMR (200 MHz, CDCl3) δ 2.10 (br t, J = 5.6 Hz, 1H, CH2OH), 2.24 (brt, J = 5.6 Hz, 1H, CH2OH), 4.45 (d, J = 5.6 Hz, 2H, CH2OH), 4.45 (d, J = 5.6 Hz, CH2OH), 7.07-7.13 (m, 6H, TrH), 7.28-7.34 (m, 9H, TrH), 7.43 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 51.7, 51.8, 76.0, 77.2, 79.8, 90.6, 90.9, 104.9, 127.5, 127.7, 129.6, 135.2, 136.4, 141.6; EI-MS m/z 418 (M+), HRMS m/z 418.1685, Calcd for C28H22N2O2: 418.1681; IR (vmax) (KBr) 3304 (OH), 3056, 3033, 2234 (C≡C), 1596, 1492, 1446 cm-1.
3,4-Bistrimethylsilylethynyl-1-tritylpyrazole (5b)
Yellow powder; mp 196 °C; 1H-NMR (300 MHz, CDCl3) δ 0.22 (s, 9H, TMS), 0.24 (s, 9H, TMS), 7.08-7.11 (m, 6H, TrH), 7.26-7.30 (m, 9H, TrH), 7.43 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ -0.2, -0.1, 79.7, 95.1, 95.5, 98.0, 99.0, 106.5, 127.9, 127.9, 130.1, 135.2, 138.2, 142.2; EI-MS m/z 502 (M+), HRMS m/z 502.2260, Calcd for C32H34N2Si2: 502.2261; IR (vmax) (KBr) 2160, 1249 cm-1.
3,4-Bistrimethylsilylethynyl-1-phenylpyrazole (7)
Yellow powder; 90 °C; 1H-NMR (300 MHz, CDCl3) δ 0.27 (s, 9H, CH3Si), 0.29 (s, 9H, CH3Si), 7.31(t, J = 7.68 Hz, 1H, PhH), 7.44 (dd, J = 7.7 and 7.5 Hz, 2H, PhH), 7.65 (d, J = 7.50 Hz, 2H, PhH), 7.97 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ -0.3, -0.1, 94.3, 94.8, 99.1, 100.0, 109.5, 119.5, 127.3, 129.1, 129.5, 139.0, 139.3; EI-MS m/z 336 (M+), HRMS m/z 336.1480, Calcd for C19H24N2Si2: 336.1478; IR (vmax) (KBr) 2166, 1252 cm-1.
3,4-Diethynyl-1-tritylpyrazole (8)
Yellow powder; 190 °C; 1H-NMR (300 MHz, CDCl3) δ 3.16 (s, 1H, C≡CH), 3.25 (s, 1H, C≡CH), 7.10-7.13 (m, 6H, TrH), 7.29-7.33 (m, 9H, TrH), 7.48 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 73.9, 74.9, 79.9, 80.9, 105.1, 127.9, 128.0, 130.1, 136.0, 136.9, 142.1; EI-MS m/z 358 (M+), HRMS m/z 358.1470, Calcd for C26H18N2: 358.1471; IR (vmax) (KBr) 3307, 3280, 2122 cm-1.
3,4-Diethynyl-1-phenylpyrazole (9)
Yellow liquid; 1H-NMR (300 MHz, CDCl3) δ 3.27 (s, 1H, C≡CH), 3.36 (s, 1H, C≡CH), 7.36 (tt, J = 1.4 and 7.3 Hz, 1H, PhH), 7.47 (dd, J = 7.3 and 8.4, 2H, PhH), 7.68 (dd, J = 1.4, 8.4 Hz, 2H, PhH), 8.04 (s, 1H, pyrazole-H); 13C-NMR (75 MHz, CDCl3) δ 73.1, 74.3, 81.6, 108.1, 119.3, 127.5, 129.4, 130.0, 138.0, 138.8; EI-MS m/z 192 (M+), HRMS m/z 192.0686, Calcd for C13H8N2: 192.0687; IR (vmax) (neat) 3293, 2118 cm-1.
2-Trityl-2H-indazole (10)14
1H-NMR (300 MHz, CDCl3) δ 7.06 (t, 1H, indazole-5), 7.12-7.33 (m, 16H, TrH and indazole-H), 7.59 (d, 1H, J4,5 = 8.4 Hz, indazole-H), 7.74 (d, 1H, J6,7 = 9.0 Hz, indazole-H), 7.90 (s, 1H, indazole-H).
2-Phenyl-2H-indazole (11) 11a
White powder; 1H-NMR (300 MHz, CDCl3) δ 7.11 (t, 1H, J = 7.5Hz, ArH), 7.32 (t, 1H, J = 7.7 Hz, ArH), 7.45 (t, 1H, J = 7.4 Hz, ArH), 7.60 (t, 2H, J = 7.8 Hz, ArH), 7.72 (d, 1H, J = 8.4 Hz, ArH), 7.77 (d, 1H, J = 8.4 Hz, ArH), 8.10 (d, 2H, J = 8.1 Hz, ArH), 9.11 (s, 1H, H-3); EI-MS m/z 194 (M+).

ACKNOWLEDGEMENTS
We are grateful to Ms. Mihoyo Fujitake of this University for MS measurements. This work was partially supported in part by a Grant-in-Aid for "High-Tech Research Center" Project for Private Universities: matching fund subsidy from MEXT (Ministry of Education, Culture, Sports, Science, and Technology), 2002-2009, Japan and Lonza Japan Award in Synthetic Organic Chemistry, Japan.

References

1. (a) G. V. De Lucca, U. T. Kim, J. C. B. Liang, R. M. Klabe, S. Garber, L. T. Bacheler, G. N. Lam, M. R. Wright, K. A. Logue, S. Erickson-V., S. S. Ko, and G. L. Trainor, J. Med. Chem., 1998, 41, 2411; CrossRef (b) M. Patel, L. T. Bacheler, M. M. Rayner, B. C. Cordova, R. M. Klabe, S. Erickson-V., and S. P. Seitz, Bioorg. Med. Chem. Lett., 1998, 8, 823; CrossRef (c) Q. Han, C. H. Chang, R. Li, Y. Ru, P. K. Jadhav, and P. Y. Lam, J. Med. Chem., 1998, 41, 2019; CrossRef (d) P. W. Baures, Org Lett., 1999, 1, 249. CrossRef
2. (a) P. Chavatte, S. Yous, C. Marot, N. Baurin, and D. Lesieur, J. Med. Chem., 2001, 44, 3223; CrossRef (b) J. M. Quintela, C. Peinador, L. Gonzalez, I. Devesa, M. L. Ferr, andiz, M. J. Alcaraz, and R. Riguera, Bioorg. Med. Chem., 2003, 11, 863; CrossRef (c) A. A. Bekhit and T. Abdel-A., Bioorg. Med. Chem., 2004, 12, 1935; CrossRef (d) R. R. Ranatunge, R. A. Earl, D. S. Garvey, D. R. Janero, L. G. Letts, A. M. Martino, M. G. Murty, S. K. Richardson, D. J. Schwalb, D. V. Young, and I. S. Zemtseva, Bioorg. Med. Chem. Lett., 2004, 14, 6049; CrossRef (e) C. Selvam, S. M. Jachak, R. Thilagavathi, and A. Chakraborti, Bioorg. Med. Chem. Lett., 2005, 15, 1793; CrossRef (f) S. Prasanna, E. Manivannan, and S. C. Chaturvedi, Bioorg. Med. Chem. Lett., 2005, 15, 2097; CrossRef (g) K. M. Dawood, H. Abdel-G., E. A. Rageb, M. Ellithey, and H. A. Mohamed, Bioorg. Med. Chem. Lett., 2006, 16, 3672; (h) S. K. Singh, S. Vobbalareddy, S. R. Kalleda, S. R. Casturi, R. Mullangi, R. Ramanujam, K. R. Yeleswarapu, and J. Iqbal, Bioorg. Med. Chem. Lett., 2006, 16, 3921; CrossRef (i) S. M. Sakya, A. Shavnya, H. Cheng, C. Li, B. Rast, J. Li, D. A. Koss, B. H. Jaynes, D. W. Mann, C. F. Petras, S. B. Seibel, M. L. Haven, and M. P. Lynch, Bioorg. Med. Chem. Lett., 2008, 18, 1042. CrossRef
3. (a) N. Matzanke, W. Lowe, S. Perachon, P. Sokoloff, J. C. Schwartz, and H. Stark, Eur. J. Med. Chem., 1999, 34, 791; CrossRef (b) S. Loeber, H. Huebner, and P. Gmeiner, Bioorg. Med. Chem. Lett., 2006, 16, 2955; CrossRef (c) C. Enguehard-G., H. Huebner, A. E. Hakmaoui, H. Allouchi, P. Gmeiner, A. Argiolas, Melis, R. Maria, and A. Gueiffier, J. Med., Chem., 2006, 49, 3938; (d) J. Elsner, F. Boeckler, F. W. Heinemann, H. Hübner, and P. Gmeiner, J. Med. Chem., 2005, 48, 5771; CrossRef (e) Y. Hirokawa, I. Fujiwara, K. Suzuki, H. Harada, T. Yoshikawa, N. Yoshida, and S. Kato, J. Med. Chem., 2003, 46, 702. CrossRef
4. (a) R. Chen, C. S. Liu, H. Zhang, Y. Guo, X. H. Bu, and M. Yang, J. Inorg. Biochem., 2007, 101, 412; CrossRef (b) C. S. Liu, H. Zhang, R. Chen, X. S. Shi, X. H. Bu, and M. Yang, Chem. Pharm. Bull., 2007, 55, 996; CrossRef (c) E. Budzisz, M. Malecka, B. K. Keppler, V. B. Arion, G. Andrijewski, U. Krajewska, and M. Rozalski, Eur. J. Inorg. Chem., 2007, 3728; CrossRef (d) E. Pantoja, A. Gallipoli, S. van Zutphen, S. Komeda, D. Reddy, D. Jaganyi, M. Lutz, D. M. Tooke, A. L. Spek, C. Navarro-R., and J. Reedijk, J. Inorg. Biochem., 2006, 100, 1955; CrossRef (e) C. G. Hartinger, S. Zorbas-S., M. A. Jakupec, B. Kynast, H. Zorbas, and B. K. Keppler, J. Inorg. Biochem., 2006, 100, 891; CrossRef (f) S. Komeda, M. Lutz, A. L. Spek, Y. Yamanaka, T. Sato, M. Chikuma, and J. Reedijk, J. Am. Chem. Soc., 2002, 124, 4738. CrossRef
5. (a) D. L. Ma, T. S. Lai, F. Y. Chan, W. H. Chung, R. Abagyan, Y. C. Leung, and K. Y. Wong, Chem. Med. Chem., 2008, 3, 881; CrossRef (b) J. X. Qiao, D. L. Cheney, R. S. Alexander, A. M. Smallwood, S. R. King, K. He, A. R. Rendina, J. M. Luettgen, M. Rovert, R. R. Wexler, and P. Y. S. Lam, Bioorg. Med. Chem. Lett., 2008, 18, 4118; CrossRef (c) N. Matsumura, K. Kikuchi-U., and T. Nakaki, J. Pharmacol. Exp. Ther., 2008, 325, 357; CrossRef (d) S. Chimichi, M. Boccalini, S. Selleri, C. Costagli, G. Guerrini, and G. Viola, Org. Biomol. Chem., 2008, 6, 739. CrossRef
6. J. Elguero 'Comprehensive Heterocyclic Chemistry,' Vol. 5, ed. by A. R. Katritzky, C. W. Rees, Pergamon, New York, 1984, pp. 167 – 303.
7. B.-R. Tolf, J. Piechaczek, R. Dahlbom, H. Theorell, A. Akeson, and G. Lundquist, Acta Chem. Scand., 1979, B33, 483. CrossRef
8. H. Ichikawa, Y. Ohno, Y. Usami, and M. Arimoto, Heterocycles, 2006, 68, 2247. CrossRef
9. (a) M. Schnürch, R. Flasik, A. F. Khan, M. Spina, M. D. Mihovilovic, and P. Stanetty, Eur. J. Org. Chem., 2006, 3283; CrossRef (b) G. Zeni and R. C. Larcok, Chem. Rev., 2006, 106, 4644. CrossRef
10. K. Olofsson and M. Larhed, 'Microwave Assisted Organic Synthesis' ed. by J. P. Tierney and P. Lidström, Blackwell, Oxford, 2005, pp. 23 – 43. CrossRef
11. (a) D.-Q. Shi, G.- L. Dou, S.-N. Ni, J.-W. Shi, X.-Y. Li, X.-S. Wang, H. Wu, and S.-J. Ji, Synlett, 2007, 2509; CrossRef (b) R. Liu, Y. M. Zhu, L. N. Qin, S. J. Ji, and H. Katayama, Heterocycles, 2007, 71, 1755; CrossRef (c) L. D. Shirtcliff, T. J. R. Weakley, and M. M. Haley, J. Org. Chem., 2004, 69, 6979. CrossRef
12. B. G. Jones and P. M Marner, J. Am. Chem. Soc., 2001, 123, 2134. CrossRef
13. M. Yoshimatsu, M. Kawahigashi, E. Honda, and T. Kataoka, J. Chem. Soc., Perkin Trans. 1, 1997, 695. CrossRef
14. R. M. Claramunt, J. Elguero, and R. Garceran, Heterocycles, 1985, 23, 2895. CrossRef