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Synlett 2004(7): 1259-1263  
DOI: 10.1055/s-2004-825604
LETTER
© Georg Thieme Verlag Stuttgart · New York

An Efficient One-Pot Synthesis of Phenol Derivatives by Ring Opening and Rearrangement of Diels-Alder Cycloadducts of Substituted Furans Using Heterogeneous Catalysis and Microwave Irradiation

Andrés Moreno*a, María Victoria Gómeza, Ester Vázqueza, Antonio de la Hoz*a, Angel Díaz-Ortiza, Pilar Prietoa, José Antonio Mayoralb, Elisabet Piresb
a Facultad de Química, Universidad de Castilla-La Mancha, Campus Universitario, 13071 Ciudad Real, Spain
Fax: +34(926)295318; e-Mail: antonio.hoz@uclm.es;
b ICMA-Facultad de Ciencias, Universidad de Zaragoza-CSIC, Pedro Cerbuna 12, 50009 Zaragoza, Spain
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Received 14 November 2003
Publikationsdatum:
19. Mai 2004 (online)

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Abstract

The use of silica-supported Lewis acids as catalysts ­under microwave irradiation promotes regiospecific opening of the 7-oxa bridge of Diels-Alder cycloadducts of furan derivatives and produces polysubstituted phenols in a single step. This rapid and ­efficient procedure permits the synthesis of tri-, tetra- and penta­substituted benzene derivatives by reaction of 2,5-dimethylfuran (1), 2-ethylfuran (2) and 2-methoxyfuran (3) with dienophiles such as dimethyl acetylenedicarboxylate and methyl propiolate.

Key words

furans - Diels-Alder reactions - heterogeneous catalysis - microwaves - phenol derivatives

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Typical Experimental Procedure: A mixture of furan (1-3, 9.0 mmol), an acetylenic dienophile (4 and 5, 1.5 mmol) and silica-supported Lewis acid (0.5 g) was charged to a commercial 25 mL Teflon PTFE vessel. The vessel was closed and irradiated in a Miele Electronic M720 microwave oven at 450 W during 30 min. In all reactions the compounds were isolated by adding 50 mL of CH2Cl2 to the mixture and separating the catalyst by filtration. The solvent was removed from the filtrate under reduced pressure and the crude reaction mixtures were analysed by 1H NMR and 13C NMR spectroscopy in CDCl3. The products were purified by column chromatography on silica gel (hexane-EtOAc 8:1 for compound 9 and 3:1 for product 11) [17] or by distillation under reduced pressure in a Kugelrohr apparatus (for compounds 6, [18] 7, [19] 8 and 10 [20] ). Yields were determined by 1H NMR spectroscopy using CH2Br2 (δ = 4.93 ppm) as an internal standard. It should be remarked that for the reactions in entries 15 and 23 (Table [1] ) the isolated yields show differences of between 3% and 7% less than the calculated yields using CH2Br2 as internal standard, thus demonstrating the accuracy of this method. All compounds were characterised by analytical methods and 1H NMR and 13C NMR spectroscopy, using one- and two-dimensional techniques. The new compounds exhibit NMR spectra consistent with their structures and gave satisfactory molecular weight determinations (mass spectrometry). [21] Catalysts modified with Lewis acids were obtained by treating silica gel with 1 M solutions of ZnCl2, AlEt2Cl or TiCl4 following the previously described method. [12] [13] The silica contained 1.5 mmol of Zn g-1, 1.4 mmol of Al g-1 and 1.2 mmol of Ti g-1, respectively, as determined by plasma emission spectroscopy. For the Si(Zn) catalyst, activation for 2 h at 150 °C under vacuum was necessary before use. Reactions carried out using thermal heating were performed in an oil bath under the same conditions of temperature and time as the microwave reactions. This temperature was determined at the end of a blank reaction using microwave irradiation.

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Program AMPAC, v. 7.0 Semichem, Inc. 2000.

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The physical and spectroscopic data for the new synthetic compounds. Compound 8: yellow oil, bp 225 °C (oven temperature)/0.1 mbar in a Kugelrohr apparatus. MS (EI): m/z = 238 [M+]. 1H NMR (CDCl3): δ = 1.18 (t, J = 7.6 Hz, 3 H, -CH2CH3), 2.50 (q, J = 7.6 Hz, 2 H, -CH2CH3), 3.80 (s, 3 H, 1-COOCH3), 3.84 (s, 3 H, 2-COOCH3), 7.0 (d, J = 8.4 Hz, 1 H, 4-H), 7.34 (d, J = 8.4 Hz, 1 H, 5-H), 10.83 (s, 1 H, -OH). 13C NMR (CDCl3): δ = 17.0 (-CH3), 27.0 (-CH2-), 52.0 (1-OCH3), 53.0 (2-OCH3), 108.2 (2-C), 119.3 (4-C), 132.0 (6-C), 134.0 (1-C), 136.0 (5-C), 159.8 (3-C), 169.3, 169.4 (1-COO, 2-COO). Compound 9: yellow oil. MS (EI): m/z = 180 [M+], 1H NMR (CDCl3): δ = 1.18 (t, J = 7.6 Hz, 3 H, -CH2CH3), 2.88 (q, J = 7.6 Hz, 2 H, -CH2CH3), 3.88 (s, 3 H, 1-COOCH3), 6.40 (s, 1 H, -OH), 6.97 (dd, J = 8.3 Hz, J = 2.7 Hz, 1 H, 3-H), 7.12 (d, J = 8.3 Hz, 1 H, 4-H), 7.38 (d, J = 2.7 Hz, 1 H, 6-H). 13C NMR (CDCl3): δ = 16.0 (-CH3), 27.8 (-CH2-), 52.0 (1-OCH3), 117.9 (6-C), 120.0 (3-C), 130.0 (1-C), 132.0 (4-C), 138.0 (5-C), 153.9 (2-C), 168.2 (1-COO).