Photocatalyzed de novo synthesis of fused tetracyclic skeletons via stepwise formal [3 + 2]/[4 or 5 + 2] cycloadditions

We successfully developed an unprecedented visible light-promoted synthesis route to fused tetracyclic skeletons with rich sp³ carbons through stepwise radical cycloadditions and further cyclization of the final radical intermediates with adjacent aromatic rings. This convenient and sustainable protocol features mild reaction conditions, excellent H₂O and air tolerance, good functional group compatibility and wide substrate adaptability. Moreover, based on direct excitation, radical-trapping, radical clock, and EPR experiments, a possible stepwise formal [3 + 2]/[4 or 5 + 2] cycloaddition process was proposed.