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.