Visible-Light-Induced [2+2+1] Dearomative Cascade Cyclization of Indole/Furan Alkynes to Synthesize Sulfonyl Polycycles

Herein, we report a visible-light-induced [2+2+1] dearomative cascade cyclization of indole/furan alkynes with NaHSO3, providing an array of diverse highly strained sulfonyl polycycles. Compared to our previous work, this method does not require the use of additional sacrificial oxidants and have wider reaction scope. Preliminary mechanistic studies suggest that the indole moiety initiated the reaction, which proceeded via single-electron oxidation pathway. An alkenyl radical formed by intramolecular addition capture SO2, and the resulting specie is cyclized to furnish the final product. Also, DFT calculations disclosed that the groups on the indole ring or at the side chain probably affect the single electron distribution at the reaction site of IM-RC-I, and the distribution may be a decisive factor of spirocyclization.