Synthesis of 6-phenylbenzo[h]quinolines via photoinduced dehydrogenative annulation of (E)-2-phenyl-3-styrylpyridines.

A concise and environmentally friendly protocol was developed for the synthesis of 6-phenylbenzo[h]quinolines. 6-Phenylbenzo[h]quinolines were obtained in good yields via irradiation of (E)-2-phenyl-3-styrylpyridines with a 254 nm UV light (64 W) in EtOH under an argon atmosphere in the presence of TFA. The reaction is a dehydrogenative annulation reaction that proceeds through 6π-electrocyclization, a [1,5]-H shift, 1,3-enamine tautomerization, and elimination of a hydrogen molecule to afford 6-phenylbenzo[h]quinolines. The described protocol not only avoids the usage of a transition metal catalyst and an oxidant but also has the advantages of high atom efficiency and mild reaction conditions.