Naturally occurring dibenzofurans. Part 1. A synthesis of cannabifuran
The synthesis of dibenzofurans from diphenyl ethers by non-phenolic oxidative coupling, u.v. irradiation, and by reaction with palladium(II) acetate has been investigated. 9-Isopropyl-6-methyl-3-pentyldibenzofuran-1-ol (cannabifuran)(35), a minor cannabis constituent, has been synthesized by annulation of 4-isopropyl-7-methylbenzofuran-2-carbaldehyde (28) by Wittig reaction with 2-carboxy-1-ethoxycarbonylethyltriphenylphosphorane and ring closure and functional-group modification of the resultant (E)-4-(4-isopropyl-7-methylbenzofuran-2-yl)-3-methoxycarbonylbut-3-enoic acid (29).