Photo-induced iron-catalyzed [2 + 2 + ] radical cascade for the synthesis of alkylated/spirocyclic polycyclic quinolinones.

A novel photo-induced [2 + 2 + ] radical cascade synthesis strategy of alkyl/spirocyclic polycyclic quinolinones has been developed, using 1,7-enynes and alkyl carboxylic acids as substrates. The reaction was conducted in the presence of inexpensive iron salt as the catalyst, proceeding through a photo-induced LMCT-mediated decarboxylation process of carboxylic acids to generate primary, secondary and tertiary alkyl radicals, which were then added to the 1,7-enyne substrates, initiating the radical cascade cyclization. Gram scale amplification experiments were also conducted using a continuous-flow photocatalytic device.