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Synlett 2009(6): 929-932  
DOI: 10.1055/s-0028-1088216
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Synthesis of Tetrasubstituted Furans via Sequential Pd(OAc)2/Zn(OTf)2-Catalyzed Oxidation and Cyclization of Aromatic Alkynes with Molecular Oxygen

Azhong Wang, Huanfeng Jiang*, Qiuxiang Xu
State Key Laboratory for Pulp and Paper Engineering, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. of China
Fax: +86(20)87112906; e-Mail: jianghf@scut.edu.cn;
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Publication History

Received 28 November 2008
Publication Date:
16 March 2009 (online)

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Abstract

The development of a new method for the synthesis of tetrasubstituted furans using aromatic alkynes is reported. The strategy involves a tandem process of palladium-catalyzed oxidation and Zn(OTf)2-catalyzed cyclization in the presence of molecular oxygen.

Key words

synthesis - tetrasubstituted furans - aromatic alkynes - palladium-catalyzed - oxidation - cyclization

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General Experimental Procedure
The reaction was carried out in a HF-15 autoclave. Pd(OAc)2 (4.49 mg, 0.02 mmol), Zn(OTf)2 (98.3 mg, 0.3 mmol), MeOH (3 mL), and alkyne (1 mmol) were added into a 15 mL autoclave in sequence. Oxygen was pumped into the autoclave by a cooling pump to reach the desired pressure, then the autoclave was heated by oil bath under magnetic stirring for the desired reaction time. After the reaction finished, the autoclave was allowed to cool to 0 ˚C. Residual O2 was vented, and the surplus was filtrated and condensed under reduced pressure. The product was purified by chromatography on a SiO2 column using light PE-CH2Cl2
as eluent.

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Spectroscopic Data for Tetrasubstituted Furans
2,3,4,5-Tetrakis(4-methoxyphenyl)furan ¹³ Mp 206-208 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 3.78 (s, 12 H), 6.77 (d, J = 6.4 Hz, 4 H), 6.79 (d, J = 7.2 Hz, 4 H), 7.04 (d, J = 8.8 Hz, 4 H), 7.42 (d, J = 9.2 Hz, 4 H) ppm. ¹³C NMR (100 MHz, CDCl3): δ = 125.1, 125.8, 127.1, 127.3, 128.3, 128.5, 130.4, 130.9, 133.1, 147.7 ppm. MS (EI, 70 eV): m/z (%) = 492 (28) [M+], 318 (100), 275 (31), 135 (47).
2,3,4,5-Tetrakis(4-fluorophenyl)furan ¹4 ¹H NMR (400 MHz, CDCl3): δ = 6.93 (m, 8 H), 7.05-7.08 (m, 4 H), 7.41-7.44 (m, 4 H) ppm. ¹³C NMR (100 MHz, CDCl3): δ = 115.6 (t, J = 80 Hz), 123.5, 126.7, 127.6, 127.7, 128.6, 131.9, 147.1, 160.9, 163.4 ppm. MS (EI, 70 eV): m/z (%) = 444 (100) [M+], 321 (73), 123 (69), 95 (55).
2,3,4,5-Tetrakis[4-(trifluoromethyl)phenyl]furan ¹³ Mp 195-197 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 7.24 6 (d, J = 8.0 Hz, 4 H), 7.55-7.57 (m, 12 H) ppm. ¹³C NMR (100 MHz, CDCl3): δ = 119.9, 122.6, 125.1, 125.3, 125.7, 125.9, 126.2, 127.3, 128.0, 129.0, 129.8, 129.9, 130.1, 130.5, 130.8, 133.0, 135.6, 148.0 ppm. MS (EI, 70 eV): m/z (%) = 644 (46) [M+], 471 (13), 173 (100), 145 (33).
2,3,4,5-Tetra- m -tolylfuran ¹5 Mp 100-102 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 2.17 (s, 6 H), 2.27 (s, 6 H), 6.91-7.22 (m, 12 H), 7.62-7.66 (m, 4 H) ppm. ¹³C NMR (100 MHz, CDCl3): δ = 21.3, 29.7, 123.0, 127.0, 127.6, 127.9, 128.1, 129.0, 129.2, 130.1, 130.2, 133.7, 135.3, 136.4, 137.9, 138.14, 144.2 ppm. MS (EI, 70 eV): m/z (%) = 428 (100) [M+], 309 (21), 119 (65), 91 (18).

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Procedure for Cyclization of 1,4-Dione A The reaction was carried out in a HF-15 autoclave. Pd(OAc)2 (4.49 mg, 0.02 mmol), Zn(OTf)2 (98.3 mg, 0.3 mmol), MeOH (3 mL), and 1,4-dione A (388 mg, 1 mmol) were added into a 15 mL autoclave in sequence. Oxygen was pumped into the autoclave by a cooling pump to reach the desired pressure, then the autoclave was heated by oil bath under magnetic stirring for the desired reaction time. After the reaction finished, the autoclave was allowed to cool to
0 ˚C. Oxygen was vented, and the surplus was filtrated and condensed under reduced pressure. The product was purified by chromatography on a SiO2 column using light PE-CH2Cl2 as eluent to give 2a in 72% yield.

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Spectroscopic Data for ( Z )-1,2,3,4-Tetraphenylbut-2-ene-1,4-dione ¹6
Mp 216-217 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 7.14-7.40 (m, 16 H), 7.82-7.84 (m, 4 H) ppm. ¹³C NMR (100 MHz, CDCl3): δ = 128.3, 128.4, 128.5, 128.6, 129.8, 130.0, 133.0, 135.2, 136.3, 144.6, 196.9 ppm. MS (EI, 70 eV): m/z (%) = 388 (12) [M+], 178 (9), 105 (100), 77 (43).