Simple and Selective Oxidation of 6-Alkylazulene Derivatives

The simple and regioseletive oxidation of 6-alkylazulene derivatives with an electron-withdrawing substituent at the 1-position of the azulene ring is described. Oxidation with molecular oxygen proceeds smoothly in the presence of a base such as potassium acetate (KOAc) or tetrabutylammonium hydroxide (Bu₄NOH), and appears to proceed via enolization of the azulene derivatives. Oxidation of 6-isopropylazulene derivatives gave tertiary alcohols, which were easily converted to primary alcohols. Oxidation of 6-methylazulene derivatives provided azulene-6-carboxylate derivatives but dimethhyl azulene-1,6-dicarboxylate, which had been obtained by the oxidation of methyl 6-methylazulene-1-carboxylate, was found to be reduced selectively at the 6-position by sodium borohydride (NaBH₄). In contrast, demethoxycarbonylation of dimethyl azulene-1,6-di-carboxylate with anhydrous phosphoric acid (100%PA) proceeded selectively at the 1-position to give methyl azulene-6-carboxylate. Those compounds obtained by the oxidation of 6-alkylazulenes are useful key intermediates for further modifications of the side chains at the 6-position of azulenes.