Visible-Light-Induced Radical Cascade Cyclization: Synthesis of the ABCD Ring Cores of Camptothecins

A new strategy for constructing indolizino­[1,2-b]­quinolin-9­(11H)-ones (ring cores of camptothecins)
from readily available isocyanoarenes and N-(alkyl-2-yn-1-yl)­pyridin-2­(1H)-ones has been developed through a visible-light-induced
radical cascade cyclization process. The reaction proceeds under mild
conditions with fair to excellent yields. The easy introduction of
substituents for both reactants and the broad functional group tolerance
of the reaction make it a straightforward route to the cores of the
marketed camptothecins and their derivatives.