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. The key features of our synthesis are the use of the carboxylative TMM-cycloaddition to construct the central spirocyclic cyclopentane core and an intramolecular Michael-addition and radical cyclization to access its strained bicyclo[2.2.2]diazaoctane ring system.