Annelated furans

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.