SIMPLE AND SELECTIVE OXIDATION OF 6-ALKYLAZULENE DERIVATIVES

The simple and regioseletive oxidation of 6-alkylazulene derivatives w ith an electron-withdrawing substituent at the 1-position of the azule ne ring is described. Oxidation with molecular oxygen proceeds smoothl y in the presence of a base such as potassium acetate (KOAc) or tetrab utylammonium hydroxide (Bu(4)NOH), and appears to proceed via enolizat ion of the azulene derivatives. Oxidation of 6-isopropylazulene deriva tives gave tertiary alcohols, which were easily converted to primary a lcohols. Oxidation of 6-methylazulene derivatives provided azulene-6-c arboxylate derivatives but dimethyl azulene-1,6-dicarboxylate, which h ad been obtained by the oxidation of methyl 6-methylazulene-1-carboxyl ate, was found to be reduced selectively at the 6-position by sodium b orohydride (NaBH4). In contrast, demethoxycarbonylation of dimethyl az ulene-1,6-dicarboxylate with anhydrous phosphoric acid (100% PA) proce eded selectively at the 1-position to give methyl azulene-6-carboxylat e. Those compounds obtained by the oxidation of 6-alkylazulenes are us eful key intermediates for further modifications of the side chains at the 6-position of azulenes.