Radical-mediated cyclisation of δ-aryl-β-dicarbonyl compounds to β-tetralones [3,4-dihydronaphthalen-2(1H)-ones]
δ-Aryl-β-dicarbonyl compounds carrying electron-releasing groups in the aromatic ring undergo efficient radical-mediated oxidative cyclisation to β-tetralones in the presence of four equivalents of manganese(III) acetate in acetic acid. Secondary oxidation invariably results in acetoxylation at the benzylic α-position of the initially-formed β-tetralones. Use of the oxidant cerium(IV) ammonium nitrate in methanol affords the corresponding α-methoxylated β-tetralones. The α-acetoxy-α-acyl-β-tetralones, but not their α-acetoxy-α-alkoxycarbonyl analogues, are aromatised in high yield on treatment with alkaline silica gel, providing an effective synthetic entry to appropriately substituted β-naphthols. The possible involvement of such radical-mediated intramolecular annulations of δ-aryl β-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.