Annelated furans. XIV. Synthetic analogues of usnic acid

Abstract

The cycloaddition of dimethyl acetylenedicarboxylate and methyl propiolate to 2-isopropenyl-3-methylbenzofurans afforded a convenient and versatile route to 9b-methyl-3,9b-dihydrodibenzofurans and 9b- methyl-1,9b-dihydrodibenzofurans, which are synthetic analogues of usnic acid. Selenium dioxide oxidation of dimethyl 4,9b-dimethyl-3,9b- dihydrobenzofuran-1,2-dicarboxylate (5; R¹ = R² = H) has been shown to proceed by oxidative rearrangement and demethylation to give a mixture of dimethyl 1-hydroxy-4,9b-dimethyl-1,9b-dihydrodibenzofuran-1,2- dicarboxylate (9), dimethyl 4-methyldibenzofuran-1,2-dicarboxylate (10), and dimethyl 6-(o-hydroxy-phenyl)-4,6-dimethyl-5-oxocyclohexa- 1,3-diene-1,2-dicarboxylate(8). Evidence concerning the mechanism of this reaction is discussed. Autoxidation of methyl 4,9b-dimethyl-3,9b- dihydrodibenzofuran-1-carboxylate (12) proceeded smoothly to give methyl 4a-hydroperoxy-4,9b-dimethyl-4a,9b-dihydrodibenzofuran-1- carboxylate (17) and methyl 4-methyldibenzofuran-1-carboxylate (16). Reduction of the hydroperoxide (17) with triethyl phosphite gave methyl 6-(o-hydroxyphenyl)-4,6-dimethyl-5-oxocyclohexa-1,3-diene-1-carboxylate (19), which could also be obtained by oxidation of the ester (12) with selenium dioxide. Methyl 4,9b-dimethyl-3-oxo-3,9b-dihydrodibenzofuran- 1-carboxylate (22) and the corresponding 4,4a-epoxide (23) were obtained together with the dibenzofuran (16) and the phenol (19), by oxidation of (12) with chromium trioxide.