Synthesis of Functionalized Aromatic Carboxylic Acids from Biosourced 3-Hydroxy-2-pyrones Through a Base Promoted Domino Reaction

Nowadays, the use of biomass derived synthons as precursor of aromatic products is an important research topic in green chemistry. In particular, Diels–Alder protocol has been applied in the construction of valuable aromatic derivatives such as terephthalic acid and esters using muconic acid and 2-pyrones as diene components. In this context, we now propose the use of 3-hydroxy-2-pyrones, prepared from galactaric acid, as building block for the synthesis of functionalized aromatic carboxylic acids. Our protocol consists in a base promoted domino reaction in which 2-pyrones and electron-poor alkenes lead directly to the aromatic derivatives preserving all the carbon atoms. High yields are obtained by using small amounts of solvent, in neat conditions and in a water biphasic system, typically at 50 °C. The reaction proceeds through a Diels–Alder cycloaddition followed by aromatization reaction with elimination of water. The aromatization represents a key step and a possible reaction mechanism is proposed