Palladium-Promoted Cascade Reactions of Isonitriles and 6-Iodo-N-propargylpyridones: Synthesis of Mappicines, Camptothecins, and Homocamptothecins

Ambient-temperature reactions of electron-rich aryl isonitriles with substituted 6-iodo-N-propargylpyridones in the presence of silver carbonate and palladium acetate produce 11H-indolizino[1,2-b]quinolin-9-ones in good to excellent yield. Experimental evidence suggests that the process occurs though organopalladium rather than radical intermediates. It is applied to synthesis analogues of mappicine and camptothecin, including the silatecans DB-67 and DB-91 (homo-DB-67).