Radical-mediated cyclisation of delta-aryl-beta-dicarbonyl compounds to beta-tetralones [3,4-dihydronaphthalen-2(1H)-ones]

delta-Aryl-beta-dicarbonyl compounds carrying electron-releasing groups in the aromatic ring undergo efficient radical-mediated oxidative cyclisation to beta-tetralones in the presence of four equivalents of manganese(III) acetate in acetic acid. Secondary oxidation invariably results in acetoxylation at the benzylic alpha-position of the initially-formed beta-tetralones. Use of the oxidant cerium(IV) ammonium nitrate in methanol affords the corresponding alpha-methoxylated beta-tetralones. The alpha-acetoxy-alpha-acyl-beta-tetralones, but not their alpha-acetoxy-alpha-alkoxycarbonyl analogues, are aromatised in high yield on treatment with alkaline silica gel, providing an effective synthetic entry to appropriately substituted beta-naphthols. The possible involvement of such radical-mediated intramolecular annulations of delta-aryl beta-diketone intermediates in the biosynthetic formation of the second carbocyclic ring of the naphthalenoid ansamycin antibiotics is discussed in relation to the previously proposed Michael addition mechanism.