Visible-Light-Induced Radical Cascade [4 + 2]/[4 + 2] Cycloaddition of Underexplored N -Acryloyl Indoles To Access Dihydropyrido[1,2- a ]-indolones

We report a visible-light-mediated radical cascade [4 + 2]/[4 + 2] cycloaddition of simple N-acryloyl indoles and N-hydroxyphthalimide esters, which provides a streamlined route to structurally complex dihydropyrido[1,2-a]-indolones. The reaction features a sequence of four consecutive radical additions involving two N-acryloyl indole molecules, which forges four new C-C bonds and induces dearomatization of one indole ring. The proposed mechanism is supported by observations of [4 + 2] cycloaddition byproducts and additional control experiments. The synthetic utility of this method is demonstrated by the scale-up reaction and downstream derivatizations.