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

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_N2′ 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_N2′ 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.