RADICAL-MEDIATED CYCLIZATION OF DELTA-ARYL-BETA-DICARBONYL COMPOUNDS TO BETA-TETRALONES [3,4-DIHYDRONAPHTHALEN-2(1H)-ONES]

delta-Aryl-beta-dicarbonyl compounds carrying electron-releasing group s in the aromatic ring undergo efficient radical-mediated oxidative cy clisation to beta-tetralones in the presence of four equivalents of ma nganese(III) acetate in acetic acid. Secondary oxidation invariably re sults in acetoxylation at the benzylic alpha-position of the initially -formed beta-tetralones. Use of the oxidant cerium(IV) ammonium nitrat e in methanol affords the corresponding alpha-methoxylated beta-tetral ones. The alpha-acetoxy-alpha-acyl-beta-tetralones, but not their alph a-acetoxy-alpha-alkoxycarbonyl analogues, are aromatised in high yield on treatment with alkaline silica gel, providing an effective synthet ic entry to appropriately substituted beta-naphthols. The possible inv olvement 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 mech anism.