Facile synthesis and characterization of indene-fused 4-methylcoumarins and an unexpected skeletal rearrangement via Pechmann condensation

Abstract The Pechmann condensation of phenolic fluorenes with ethyl acetoacetate afforded indene-fused 4-methylcoumarins. In addition to the expected indeno[1,2-g]coumarin, a skeletal rearranged compound was obtained as a concomitant product when 9,9-dimethyl-9H-fluorene-2,7-diol was utilized as the substrate. X-ray crystallography was used to identify the structures of the isomers. Photochemical and photophysical studies indicated that compared with the non-rearranged products, the skeletal rearranged isomers have red shifted absorption maxima and much higher fluorescence quantum yields. A mechanism involving two competitive pathways for the simultaneous production of isomers was proposed.