Bridged isoquinoline derivatives play an important role in various bioactive molecules. The cascade dearomatizative annulation of isoquinolinium salts with bis-nucleophiles is a straightforward strategy to construct bridged isoquinoline skeletons because isoquinolinium ions have two electrophilic sites. However, the reported examples only focused on the synthesis of 1,3-bridged cyclic skeletons. In the previous work, it was reported the first synthesis of 1,4-bridged dihydroisoquinolin-3-ones from isoquinolinium salts and 4-hydroxycoumarins. When cyclic 1,3-diketones were used instead of 4-hydroxycoumarins, isoquinoline-1,3,4(2H)-triones, instead of the expected 1,4-bridged dihydroisoquinolin-3-ones, were unexpectedly yielded. Experimental evidence by high resolution mass spectroscopy supports that the generation of isoquinoline-1,3,4(2H)-triones was initiated via an O-nucleophilic substitution of the cyclic 1,3-diketone, followed by an elimination of the 2-bromo-cyclic 1,3-diketone to give intermediate 4-bromoisoquinolin-3(2H)-one, which subsequently underwent dual hydrolyses and aerobic oxidations. Based on this mechanism, the O-nucleophilic substitution of cyclic 1,3-diketones was successfully inhibited by the addition of a catalytic amount of trifluoromethanesulfonic acid (TfOH). The desired 1,4-bridged dihydroisoquinolin-3-ones were then obtained. This method provides a facile access to 1,4-bridged isoquinoline skeletons under mild reaction conditions (33 examples). The general procedure is as following: under an argon atmosphere, a 5 mL Schlenk flask was charged with isoquinolinium salt 6 (0.2 mmol), phenyliodine(III) diacetate (0.6 mmol), KBr (0.2 mmol), H₂O (3.6 μL), and dry dichloroethane (2 mL). The mixture was continually stirred at room temperature until 6 was consumed as indicated by thin-layer chromatography (TLC). TfOH (5 μL) and cyclic 1,3-diketone 7 (0.4 mmol) were sequentially added. The reaction mixture was heated at 50 ℃ in the oil bath until the intermediate was consumed as indicated by TLC, then cooled to room temperature, diluted with water (5 mL), and extracted with ethyl acetate (5 mL×3). The combined organic layer was washed with brine, dried over Na₂SO₄, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (petroleum ether/EtOAc as the eluent) to give the 1,4-bridged product 8.

Key words: isoquinolinium salt, cyclic 1,3-diketone, isoquinolineone, bridged heterocycle, annulation, oxidation