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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
Author(s) -
Anwar Shaik,
Huang WanYun,
Chen ChihHao,
Cheng YouSong,
Chen Kwunmin
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201204221
Subject(s) - furan , benzofuran , annulation , friedel–crafts reaction , chemistry , organic chemistry , stereochemistry , catalysis
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 S N 2′ process followed by intramolecular oxa‐Michael cyclization and subsequent aromatization. A plot of log ( k / k H ) against Hammett constants σ p showed satisfactory linearity with a positive ρ value, indicating that the initial Friedel–Crafts‐type S N 2′ process constituted the rate‐determining step. This methodology has been applied to the synthesis of various novel C 2 and C 3 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.