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An Efficient Friedel–Crafts/Oxa-Michael/Aromatic Annulation: Rapid Access to Substituted Naphtho[2,1-b]furan, Naphtho[1,2-b]furan, and Benzofuran Derivatives

Authors

  • Dr. Shaik Anwar,

    1. Chemistry Department, National Taiwan Normal University, 88 Sec. 4, TingChow Rd., Taipei 116 (Taiwan), Fax: (+886) 2-29324249
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  • Wan-Yun Huang,

    1. Chemistry Department, National Taiwan Normal University, 88 Sec. 4, TingChow Rd., Taipei 116 (Taiwan), Fax: (+886) 2-29324249
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  • Chih-Hao Chen,

    1. Chemistry Department, National Taiwan Normal University, 88 Sec. 4, TingChow Rd., Taipei 116 (Taiwan), Fax: (+886) 2-29324249
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  • You-Song Cheng,

    1. Chemistry Department, National Taiwan Normal University, 88 Sec. 4, TingChow Rd., Taipei 116 (Taiwan), Fax: (+886) 2-29324249
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  • Prof. Dr. Kwunmin Chen

    Corresponding author
    1. Chemistry Department, National Taiwan Normal University, 88 Sec. 4, TingChow Rd., Taipei 116 (Taiwan), Fax: (+886) 2-29324249
    • Chemistry Department, National Taiwan Normal University, 88 Sec. 4, TingChow Rd., Taipei 116 (Taiwan), Fax: (+886) 2-29324249

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Abstract

Substituted naphthofurans and benzofurans are easily accessible by treatment of naphthols/substituted phenols with nitroallylic acetates through a substitution–elimination process promoted by cesium carbonate. Reactions between naphthols and aromatic/heteroaromatic-substituted nitroallylic acetates gave the desired functionalized naphthofurans in high to excellent chemical yields (14–97 %). On the other hand, treatment of phenol derivatives (i.e., 3-dimethylamino-, 3-methoxy-, and 3,5-dimethoxyphenol) with various nitroallylic acetates afforded the corresponding benzofurans in moderate to good chemical yields (24–91 %). The reaction proceeded through an interesting Friedel–Crafts SN2′ process followed by intramolecular oxa-Michael cyclization and subsequent aromatization. A plot of log (k/kH) against Hammett constants σp showed satisfactory linearity with a positive ρ value, indicating that the initial Friedel–Crafts-type SN2′ process constituted the rate-determining step. This methodology has been applied to the synthesis of various novel C2 and C3 symmetric bis- and trisfurans by using catechol and phloroglucinol as the nucleophilic partners. The reactivity decreased when alkyl-substituted nitroallylic acetate systems were used. This might be related to the decreased electrophilic character of these substrates.

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