Triflic acid-Mediated Expedient Synthesis of Benzo[a]fluorenes and Fluorescent Benzo[a]fluorenones

Fluorene-based polyaromatic hydrocarbons are renowned compounds for materials applications. Herein, a straightforward route via insitu acetal formation has been presented to access benzo[a]fluorenes by a triflic acid promoted cationic cycloisomerization of enynones in presence of trimethyl orthoformate under metal-free conditions. In the absence of trimethyl orthoformate, the same reaction results in benzo[a]fluorenones. All the synthesized benzo[a]fluorenones are highly fluorescent in solution phase with high Stokes shift while the corresponding benzo[a]fluorenes are not fluorescent.