Synthesis of Polycyclic Heteroaromatic Coumarins via Photoinduced Dehydrogenative Annulation of 4‑Phenyl-3-heteroarylcoumarins

An efficient, oxidant and metal-free synthesis of polycyclic heteroaromatic coumarins was developed. H-Furo­[2′,3′:3,4]­naphtho­[2,1-c]­chromen-4-one (2a–2f), 1H-benzo­furo­[2′,3′:3,4]­naphtho­[2,1-c]­chromen-1-one (2g–2j), and 4H-thieno­[2′,3′:3,4]­naphtho­[2,1-c]­chromen-4-one (2k–2s) derivatives were obtained by the irradiation of 4-phenyl-3-hetero­aryl­coumarin in EtOH–H2O (9:1, v/v) using a high-pressure Hg lamp as the light source, at room temperature and under an Ar atmosphere. Owing to the expansion of the π-conjugation system, 2a–2s showed strong fluorescence emissions in ethanol solution (ΦF = 0.40–0.83).