Eco-friendly synthesis of fused pyrano[2,3-b]pyrans via ammonium acetate-mediated formal oxa-[3 + 3]cycloaddition of 4H-chromene-3-carbaldehydes and cyclic 1,3-dicarbonyl compounds†
Various substituted polycyclic pyrano[2,3-b]pyrans were synthesized via the condensation of 4H-chromene-3-carbaldehydes and their areno-condensed analogues with hetero- and carbocyclic 1,3-dicarbonyl compounds in acetic acid. Ammonium acetate was used as a green catalyst for the reaction. The process also involves the subsequent Knoevenagel condensation and 6π-electrocyclization of the 1-oxatriene intermediates formed. Fused pyridines were isolated as the products of the conjugated addition of ammonia to 1-oxatriene intermediates while using carbocyclic 1,3-dicarbonyl compounds and increasing the reaction time, indicating the reversibility of the electrocyclization stage. The calculated values of the Gibbs free energies and reaction rate constants for the 1-oxatriene – 2H-pyran equilibrium also testified to the irreversibility of pyrano[2,3-b]pyran formation in the case of using of heterocyclic 1,3-dicarbonyl compounds.