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K2CO3 catalyzed solvent-free synthesis of molecular tweezers containing chiral unsymmetrical urea unit under microwave irradiation
Chinese Chemical Letters 18 (2007) 1067–1070 www.elsevier.com/locate/cclet
K2CO3 catalyzed solvent-free synthesis of molecular tweezers containing chiral unsymmetrical urea unit under microwave irradiation Zhi Gang Zhao *, Xing Li Liu, Cai E. Zhou College of Chemistry and Environmental Protection Engineering, Southwest University for Nationalities, Chengdu 610041, China Received 23 April 2007
Abstract A facile, rapid and efficient method for the synthesis of molecular tweezers containing chiral unsymmetrical urea unit in solventfree conditions using microwave was reported. # 2007 Zhi Gang Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Molecular tweezers; Chiral unsymmetrical urea; Microwave irradiation; Recognition
Molecular recognition plays a pivotal role in life processes [1]. The study on biomimetic enzyme models is one of the frontiers in bioorganic chemistry. During the past two decades, a number of artificial receptors for the recognition of neutral organic molecules, chiral molecules, and anions have been successfully synthesized [2,3]. Among the various types of artificial receptors synthesized so far, a special class of receptors called molecular tweezers are currently attracting great interest [4–6]. However, nowadays molecular tweezer receptors are usually synthesized by conventional thermal heating, which suffers from the drawbacks such as generation of environmentally harmful byproducts, prolonged reaction time and low isolation yields. Therefore, the development of fast and efficient synthetic methods is of the utmost importance for tweezer receptors. Microwave-assisted organic synthesis has attracted considerable interest and is an important technique in green synthetic chemistry [7–9]. It provides unique chemical process with special attributes, such as ease of manipulation, enhanced reaction rates, clean reaction outcomes and high yield. Organic solvent-free reaction conditions eliminate the toxicity and flammability issues associated with common solvents. Together, solvent-free organic syntheses assisted by microwave irradiation have being regarded as environmentally benign methodologies. In view of the power of microwave-assisted organic synthesis and in continuation of our interest on synthesis of novel molecular tweezer receptors [10–12], we report here our results related to the preparation of chiral unsymmetrical urea molecular tweezer receptors, without solvent, catalyzed by K2CO3 under microwave irradiation, which to our knowledge, was not attempted up to now. The synthetic route is shown in Scheme 1.
* Corresponding author. E-mail address: [email protected] (Z.G. Zhao). 1001-8417/$ – see front matter # 2007 Zhi Gang Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.06.030
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Z.G. Zhao et al. / Chinese Chemical Letters 18 (2007) 1067–1070
Scheme 1.
1. Experimental Melting points were determined on a micro-melting point apparatus and the thermometer was uncorrected. Infrared spectra were obtained on 1700 Perkin-Elmer FTIR using KBr disks. 1H NMR spectra were recorded on a Varian INOVA 400 MHz spectrometer using DMSO-d6 as solvent and TMS as internal standard. Mass spectra were determined on Finnigan LCQDECA instrument. Elemental analysis was performed on a Carlo-Erba-1106 autoanalyzer. Optical rotation was measured on a Wzz-2B polarimeter. 1.2 mmol 1 [13], 0.5 mmol 2 and 2 g K2CO3 were added to a small beaker at room temperature and mixed thoroughly. Then, the mixture was irradiated at 460 W for 7–12 min. The reaction was monitored by TLC until it was completed. After cooling to room temperature, the reaction mixture was extracted with CH2Cl2 (15 mL 3). The extracted liquid was washed with 10% NaHCO3 (15 mL 2), brine (15 mL 2), and finally dried over anhydrous Na2SO4. The solvent was evaporated to give the crude product. The crude product was purified by column chromatography on silica gel H with dichloromethane/ethyl acetate as eluant. 1 Compound 3a: White solid, yield 93%, mp 226–227 8C, ½a20 D 34:2 (c 0.3, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.61 (s, 2H, PhNHCO), 8.12–6.88 (m, 14H, ArH), 6.42 (d, J = 8.4 Hz, 2H, CONH), 4.32–4.24 (m, 4H, OCH2), 4.17–4.10 (m, 2H, NCH), 1.21 (d, J = 6.8 Hz, 6H, CH3). IR (KBr, cm1): 3324, 2960, 1730, 1639, 1597, 1560, 1498. ESI-MS m/z (%): 541 [(M+Na)+, 70]. Anal. calcd. for C28H30N4O6: C 64.85, H 5.83, N 10.80; found: C 64.73, H 5.89, N 10.91. 1 Compound 3b: White solid, yield 90%, mp 208–209 8C, ½a20 D þ 35:1 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.42 (s, 2H, PhNHCO), 8.06–6.87 (m, 14H, ArH), 6.24 (d, J = 9.2 Hz, 2H, CONH), 4.39–4.28 (m, 4H, OCH2), 3.93–3.87 (m, 2H, NCH), 1.99–1.86 (m, 2H, CH), 1.03–0.88 (m, 12H, CH3). IR (KBr, cm1): 3308, 2962, 1729, 1637, 1597, 1558, 1499. ESI-MS m/z (%): 597 [(M+Na)+, 100]. Anal. calcd. for C32H38N4O6: C 66.88, H 6.67, N 9.75; found: C 66.78, H 6.58, N 9.88. 1 Compound 3c: White solid, yield 92%, mp 136–137 8C, ½a20 D 54:5 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.40 (s, 2H, PhNHCO), 8.09–6.87 (m, 14H, ArH), 6.20 (d, J = 8.8 Hz, 2H, CONH), 4.34–4.21 (m, 4H, OCH2), 4.18–4.10 (m, 2H, NCH), 1.75–1.68 (m, 2H, CH), 1.50–1.33 (m, 4H, CH2), 0.95–0.90 (m, 12H, CH3). IR (KBr, cm1): 3336, 2957, 1725, 1649, 1599, 1556, 1493. ESI-MS m/z (%): 625 [(M+Na)+, 100]. Anal. calcd. for C34H42N4O6: C 67.75, H 7.02, N 9.30; found: C 67.85, H 6.87, N 9.19. 1 Compound 3d: White solid, yield 91%, mp 186–187 8C, ½a20 D þ 103:2 (c 0.1, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.74 (s, 2H, PhNHCO), 8.04–7.17 (m, 24H, ArH), 6.88 (d, J = 8.8 Hz, 2H, CONH), 5.27–5.22 (m, 2H, NCH), 4.53 (d, J = 5.6 Hz, 4H, OCH2). IR (KBr, cm1): 3306, 2947, 1721, 1636, 1597, 1557, 1494. ESI-MS m/z (%): 665 [(M+Na)+, 100]. Anal. calcd. for C38H34N4O6: C 71.01, H 5.33, N 8.72; found: C 70.88, H 5.38, N 8.65. 1 Compound 3e: White solid, yield 88%, mp 200–201 8C, ½a20 D 22:4 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.44 (s, 2H, PhNHCO), 8.09–6.84 (m, 24H, ArH), 6.34 (d, J = 8.4 Hz, 2H, CONH), 4.37–4.31 (m, 4H, OCH2),
Z.G. Zhao et al. / Chinese Chemical Letters 18 (2007) 1067–1070
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Table 1 Synthesis of molecular tweezers 3a–3f in the solvent-free condition using microwave irradiation Compound
R
Time (min)
Yield (%)
3a 3b 3c 3d 3e 3f
CH3 CH(CH3)2 CH2CH(CH3)2 Ph CH2Ph CH2CH2SCH3
7 10 10 10 12 8
93 90 92 91 88 97
4.25–4.22 (m, 2H, NCH), 3.00–2.85 (m, 4H, PhCH2). IR (KBr, cm1): 3319, 2939, 1728, 1642, 1598, 1551, 1493. ESIMS m/z (%): 693 [(M+Na)+, 100]. Anal. calcd. for C40H38N4O6: C 71.63, H 5.71, N 8.35; found: C 71.76, H 5.78, N 8.23. 1 Compound 3f: White solid, yield 97%, mp 191–192 8C, ½a20 D 24:4 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.55 (s, 2H, PhNHCO), 8.10–6.88 (m, 14H, ArH), 6.43 (d, J = 9.2 Hz, 2H, CONH), 4.38–4.28 (m, 4H, OCH2), 4.20–4.13 (m, 2H, NCH), 2.64–2.50 (m, 4H, CH2S), 2.07 (s, 6H, SCH3), 1.93–1.76 (m, 4H, CH2). IR (KBr, cm1): 3308, 2911, 1725, 1638, 1597, 1559, 1491. ESI-MS m/z (%): 661 [(M+Na)+, 100]. Anal. calcd. for C32H38N4O6S2: C 60.17, H 6.00, N 8.77; found: C 60.08, H 5.93, N 8.83. 2. Results and discussion In recent years, a practical dimension to the microwave heating protocols has been added by accomplishing reactions on solid supporters under solvent-free conditions. In these reactions, the organic compounds adsorbed on the surface of various mineral supporters adsorb microwave whereas the solid supporters does not adsorb microwave. These reactions offer distinct advantages of selectivity, mild reaction conditions and operational simplicity. In our experiment, the supporters significantly affect the yield of the receptors. We found that only trace amounts of 3f could be detected by TLC analysis when using silica gel H, artificial zeolite, Al2O3 as supporters and irradiating for 8 min, whereas the receptor 3f was obtained in the yield of 97% when using K2CO3 as supporter and irradiating for 8 min. To determine the optimum condition of this reaction, we investigated the effects of microwave irradiation power and time. It was found that the highest yield of compound 3 can be obtained in 460 W for 7–12 min. K2CO3 can be used as supporter and catalyst. The typical results are shown in Table 1. 3. Conclusion In this research, we have developed a fast and convenient method to synthesize molecular tweezer receptors with high yields. These reactions were conducted in the presence of K2CO3, without using solvent, and assisted by microwave irradiation. The salient features of this method include the simple reaction set-ups, high yields of products, and short reaction time. These advantages should render the synthesis of molecular tweezer artificial receptors more efficient and environmental friendly. Acknowledgments We are very grateful to the Natural Science Foundation of Sichuan Province (No. 04JY029-003-8) and Southwest University for Nationalities for the financial support. References [1] [2] [3] [4] [5] [6]
J.M. Lehn, Angew. Chem. Int. Ed. 27 (1) (1988) 89. P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486. X. Guo, F. Liu, G.Y. Lu, Chin. J. Org. Chem. 25 (9) (2005) 1021 (in Chinese). Q.M. Mu, Y. Peng, Z.G. Zhao, et al. Chin. J. Org. Chem. 24 (9) (2004) 1018 (in Chinese). H. Nemoto, T. Kawano, N. Ueji, et al. Tetrahedron Lett. 46 (2005) 551. T. Korenaga, T. Kosaki, Y. Kawauchi, et al. J. Fluorine Chem. 127 (2006) 604.
1070 [7] [8] [9] [10] [11] [12] [13]
Z.G. Zhao et al. / Chinese Chemical Letters 18 (2007) 1067–1070 W. Yin, Y. Ma, J.X. Xu, et al. J. Org. Chem. 71 (11) (2006) 4312. M.L.N. Rao, D.K. Awasthi, D. Banerjee, Tetrahedron Lett. 48 (2007) 431. G.R. Qu, M.W. Geng, S.H. Han, et al. Chin. J. Org. Chem. 27 (4) (2007) 449 (in Chinese). Z.G. Zhao, P.Y. Zhang, Z.X. Yang, et al. Chin. J. Org. Chem. 25 (6) (2005) 679 (in Chinese). Z.G. Zhao, X.L. Liu, Z.X. Yang, et al. Chin. J. Org. Chem. 26 (4) (2006) 523 (in Chinese). Z.G. Zhao, X.L. Liu, S.H. Chen, Chem. Res. Appl. 18 (9) (2006) 1046 (in Chinese). C.E. Zhou, Z.G. Zhao, X.L. Tang, Chin. J. Org. Chem. 27 (4) (2007) 513 (in Chinese).
K2CO3 catalyzed solvent-free synthesis of molecular tweezers containing chiral unsymmetrical urea unit under microwave irradiation Zhi Gang Zhao *, Xing Li Liu, Cai E. Zhou College of Chemistry and Environmental Protection Engineering, Southwest University for Nationalities, Chengdu 610041, China Received 23 April 2007
Abstract A facile, rapid and efficient method for the synthesis of molecular tweezers containing chiral unsymmetrical urea unit in solventfree conditions using microwave was reported. # 2007 Zhi Gang Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Molecular tweezers; Chiral unsymmetrical urea; Microwave irradiation; Recognition
Molecular recognition plays a pivotal role in life processes [1]. The study on biomimetic enzyme models is one of the frontiers in bioorganic chemistry. During the past two decades, a number of artificial receptors for the recognition of neutral organic molecules, chiral molecules, and anions have been successfully synthesized [2,3]. Among the various types of artificial receptors synthesized so far, a special class of receptors called molecular tweezers are currently attracting great interest [4–6]. However, nowadays molecular tweezer receptors are usually synthesized by conventional thermal heating, which suffers from the drawbacks such as generation of environmentally harmful byproducts, prolonged reaction time and low isolation yields. Therefore, the development of fast and efficient synthetic methods is of the utmost importance for tweezer receptors. Microwave-assisted organic synthesis has attracted considerable interest and is an important technique in green synthetic chemistry [7–9]. It provides unique chemical process with special attributes, such as ease of manipulation, enhanced reaction rates, clean reaction outcomes and high yield. Organic solvent-free reaction conditions eliminate the toxicity and flammability issues associated with common solvents. Together, solvent-free organic syntheses assisted by microwave irradiation have being regarded as environmentally benign methodologies. In view of the power of microwave-assisted organic synthesis and in continuation of our interest on synthesis of novel molecular tweezer receptors [10–12], we report here our results related to the preparation of chiral unsymmetrical urea molecular tweezer receptors, without solvent, catalyzed by K2CO3 under microwave irradiation, which to our knowledge, was not attempted up to now. The synthetic route is shown in Scheme 1.
* Corresponding author. E-mail address: [email protected] (Z.G. Zhao). 1001-8417/$ – see front matter # 2007 Zhi Gang Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.06.030
1068
Z.G. Zhao et al. / Chinese Chemical Letters 18 (2007) 1067–1070
Scheme 1.
1. Experimental Melting points were determined on a micro-melting point apparatus and the thermometer was uncorrected. Infrared spectra were obtained on 1700 Perkin-Elmer FTIR using KBr disks. 1H NMR spectra were recorded on a Varian INOVA 400 MHz spectrometer using DMSO-d6 as solvent and TMS as internal standard. Mass spectra were determined on Finnigan LCQDECA instrument. Elemental analysis was performed on a Carlo-Erba-1106 autoanalyzer. Optical rotation was measured on a Wzz-2B polarimeter. 1.2 mmol 1 [13], 0.5 mmol 2 and 2 g K2CO3 were added to a small beaker at room temperature and mixed thoroughly. Then, the mixture was irradiated at 460 W for 7–12 min. The reaction was monitored by TLC until it was completed. After cooling to room temperature, the reaction mixture was extracted with CH2Cl2 (15 mL 3). The extracted liquid was washed with 10% NaHCO3 (15 mL 2), brine (15 mL 2), and finally dried over anhydrous Na2SO4. The solvent was evaporated to give the crude product. The crude product was purified by column chromatography on silica gel H with dichloromethane/ethyl acetate as eluant. 1 Compound 3a: White solid, yield 93%, mp 226–227 8C, ½a20 D 34:2 (c 0.3, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.61 (s, 2H, PhNHCO), 8.12–6.88 (m, 14H, ArH), 6.42 (d, J = 8.4 Hz, 2H, CONH), 4.32–4.24 (m, 4H, OCH2), 4.17–4.10 (m, 2H, NCH), 1.21 (d, J = 6.8 Hz, 6H, CH3). IR (KBr, cm1): 3324, 2960, 1730, 1639, 1597, 1560, 1498. ESI-MS m/z (%): 541 [(M+Na)+, 70]. Anal. calcd. for C28H30N4O6: C 64.85, H 5.83, N 10.80; found: C 64.73, H 5.89, N 10.91. 1 Compound 3b: White solid, yield 90%, mp 208–209 8C, ½a20 D þ 35:1 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.42 (s, 2H, PhNHCO), 8.06–6.87 (m, 14H, ArH), 6.24 (d, J = 9.2 Hz, 2H, CONH), 4.39–4.28 (m, 4H, OCH2), 3.93–3.87 (m, 2H, NCH), 1.99–1.86 (m, 2H, CH), 1.03–0.88 (m, 12H, CH3). IR (KBr, cm1): 3308, 2962, 1729, 1637, 1597, 1558, 1499. ESI-MS m/z (%): 597 [(M+Na)+, 100]. Anal. calcd. for C32H38N4O6: C 66.88, H 6.67, N 9.75; found: C 66.78, H 6.58, N 9.88. 1 Compound 3c: White solid, yield 92%, mp 136–137 8C, ½a20 D 54:5 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.40 (s, 2H, PhNHCO), 8.09–6.87 (m, 14H, ArH), 6.20 (d, J = 8.8 Hz, 2H, CONH), 4.34–4.21 (m, 4H, OCH2), 4.18–4.10 (m, 2H, NCH), 1.75–1.68 (m, 2H, CH), 1.50–1.33 (m, 4H, CH2), 0.95–0.90 (m, 12H, CH3). IR (KBr, cm1): 3336, 2957, 1725, 1649, 1599, 1556, 1493. ESI-MS m/z (%): 625 [(M+Na)+, 100]. Anal. calcd. for C34H42N4O6: C 67.75, H 7.02, N 9.30; found: C 67.85, H 6.87, N 9.19. 1 Compound 3d: White solid, yield 91%, mp 186–187 8C, ½a20 D þ 103:2 (c 0.1, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.74 (s, 2H, PhNHCO), 8.04–7.17 (m, 24H, ArH), 6.88 (d, J = 8.8 Hz, 2H, CONH), 5.27–5.22 (m, 2H, NCH), 4.53 (d, J = 5.6 Hz, 4H, OCH2). IR (KBr, cm1): 3306, 2947, 1721, 1636, 1597, 1557, 1494. ESI-MS m/z (%): 665 [(M+Na)+, 100]. Anal. calcd. for C38H34N4O6: C 71.01, H 5.33, N 8.72; found: C 70.88, H 5.38, N 8.65. 1 Compound 3e: White solid, yield 88%, mp 200–201 8C, ½a20 D 22:4 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.44 (s, 2H, PhNHCO), 8.09–6.84 (m, 24H, ArH), 6.34 (d, J = 8.4 Hz, 2H, CONH), 4.37–4.31 (m, 4H, OCH2),
Z.G. Zhao et al. / Chinese Chemical Letters 18 (2007) 1067–1070
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Table 1 Synthesis of molecular tweezers 3a–3f in the solvent-free condition using microwave irradiation Compound
R
Time (min)
Yield (%)
3a 3b 3c 3d 3e 3f
CH3 CH(CH3)2 CH2CH(CH3)2 Ph CH2Ph CH2CH2SCH3
7 10 10 10 12 8
93 90 92 91 88 97
4.25–4.22 (m, 2H, NCH), 3.00–2.85 (m, 4H, PhCH2). IR (KBr, cm1): 3319, 2939, 1728, 1642, 1598, 1551, 1493. ESIMS m/z (%): 693 [(M+Na)+, 100]. Anal. calcd. for C40H38N4O6: C 71.63, H 5.71, N 8.35; found: C 71.76, H 5.78, N 8.23. 1 Compound 3f: White solid, yield 97%, mp 191–192 8C, ½a20 D 24:4 (c 0.2, DMF). H NMR (DMSO-d6, 400 MHz, d ppm): 8.55 (s, 2H, PhNHCO), 8.10–6.88 (m, 14H, ArH), 6.43 (d, J = 9.2 Hz, 2H, CONH), 4.38–4.28 (m, 4H, OCH2), 4.20–4.13 (m, 2H, NCH), 2.64–2.50 (m, 4H, CH2S), 2.07 (s, 6H, SCH3), 1.93–1.76 (m, 4H, CH2). IR (KBr, cm1): 3308, 2911, 1725, 1638, 1597, 1559, 1491. ESI-MS m/z (%): 661 [(M+Na)+, 100]. Anal. calcd. for C32H38N4O6S2: C 60.17, H 6.00, N 8.77; found: C 60.08, H 5.93, N 8.83. 2. Results and discussion In recent years, a practical dimension to the microwave heating protocols has been added by accomplishing reactions on solid supporters under solvent-free conditions. In these reactions, the organic compounds adsorbed on the surface of various mineral supporters adsorb microwave whereas the solid supporters does not adsorb microwave. These reactions offer distinct advantages of selectivity, mild reaction conditions and operational simplicity. In our experiment, the supporters significantly affect the yield of the receptors. We found that only trace amounts of 3f could be detected by TLC analysis when using silica gel H, artificial zeolite, Al2O3 as supporters and irradiating for 8 min, whereas the receptor 3f was obtained in the yield of 97% when using K2CO3 as supporter and irradiating for 8 min. To determine the optimum condition of this reaction, we investigated the effects of microwave irradiation power and time. It was found that the highest yield of compound 3 can be obtained in 460 W for 7–12 min. K2CO3 can be used as supporter and catalyst. The typical results are shown in Table 1. 3. Conclusion In this research, we have developed a fast and convenient method to synthesize molecular tweezer receptors with high yields. These reactions were conducted in the presence of K2CO3, without using solvent, and assisted by microwave irradiation. The salient features of this method include the simple reaction set-ups, high yields of products, and short reaction time. These advantages should render the synthesis of molecular tweezer artificial receptors more efficient and environmental friendly. Acknowledgments We are very grateful to the Natural Science Foundation of Sichuan Province (No. 04JY029-003-8) and Southwest University for Nationalities for the financial support. References [1] [2] [3] [4] [5] [6]
J.M. Lehn, Angew. Chem. Int. Ed. 27 (1) (1988) 89. P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486. X. Guo, F. Liu, G.Y. Lu, Chin. J. Org. Chem. 25 (9) (2005) 1021 (in Chinese). Q.M. Mu, Y. Peng, Z.G. Zhao, et al. Chin. J. Org. Chem. 24 (9) (2004) 1018 (in Chinese). H. Nemoto, T. Kawano, N. Ueji, et al. Tetrahedron Lett. 46 (2005) 551. T. Korenaga, T. Kosaki, Y. Kawauchi, et al. J. Fluorine Chem. 127 (2006) 604.
1070 [7] [8] [9] [10] [11] [12] [13]
Z.G. Zhao et al. / Chinese Chemical Letters 18 (2007) 1067–1070 W. Yin, Y. Ma, J.X. Xu, et al. J. Org. Chem. 71 (11) (2006) 4312. M.L.N. Rao, D.K. Awasthi, D. Banerjee, Tetrahedron Lett. 48 (2007) 431. G.R. Qu, M.W. Geng, S.H. Han, et al. Chin. J. Org. Chem. 27 (4) (2007) 449 (in Chinese). Z.G. Zhao, P.Y. Zhang, Z.X. Yang, et al. Chin. J. Org. Chem. 25 (6) (2005) 679 (in Chinese). Z.G. Zhao, X.L. Liu, Z.X. Yang, et al. Chin. J. Org. Chem. 26 (4) (2006) 523 (in Chinese). Z.G. Zhao, X.L. Liu, S.H. Chen, Chem. Res. Appl. 18 (9) (2006) 1046 (in Chinese). C.E. Zhou, Z.G. Zhao, X.L. Tang, Chin. J. Org. Chem. 27 (4) (2007) 513 (in Chinese).