Synthesis of Unsaturated 1,6-Diketones and 1,6-Diols from Muconic Acid

Cis-,cis-muconic acid (HO₂C–CH═CH–CH═CH–CO₂H) reacts with arenes in Bro̷nsted superacid TfOH, affording the aromatic acylation products, unsaturated E-,E-1,6-diketones [ArOC–CH═CH–CH═CH–COAr]. These diketones are also obtained in Friedel–Crafts acylation of arenes by muconoyl chloride. The reaction in TfOH proceeds through an intermediate formation of muconic acid O,O-diprotonated form [HO(⁺HO═C)–CH═CH–CH═CH–C(═OH)⁺OH], which was investigated experimentally by NMR and theoretically by DFT calculations. Carbonyl reduction of these 1,6-diketones with NaBH₄ results in the formation of the corresponding unsaturated diastereomeric E-,E-1,6-diols [Ar(HO)CH–CH═CH–CH═CH–CH(OH)Ar]. Catalytic hydrogenation of carbon–carbon bonds in 1,6-diketones with Pd or Pt on carbon furnishes the corresponding 1,6-diketones (ArOC–CH₂CH₂CH₂CH₂–COAr). The studied reactions contribute to the development of organic synthesis on the basis of transformations of muconic acid, which is an important compound in industrial chemistry.