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 precursors of aromatic products is an important research topic in green chemistry. In particular, the 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 degrees 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.