The marcfortine alkaloids are complex indolic secondary metabolites isolated from various Penicillium sp. and are potent anthelmintic agents. Herein, we report the first total synthesis of marcfortine B (I). The key features of our synthesis are the use of the carboxylative TMM-cycloaddn. to construct the central spirocyclic cyclopentane core and an intramol. Michael-addn. and radical cyclization to access its strained bicyclo[2.2.2]diazaoctane ring system.