Eco-friendly synthesis of fused pyrano2,3-bpyransviaammonium acetate-mediated formal oxa-3+3cycloaddition of 4H-chromene-3-carbaldehydes and cyclic 1,3-dicarbonyl compounds

摘要

Various substituted polycyclic pyrano2,3-bpyrans were synthesizedviathe 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 6 pi-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 pyrano2,3-bpyran formation in the case of using of heterocyclic 1,3-dicarbonyl compounds.