The synthesis of vitamin B 12 in four steps from an artificial green secocorrinoid is presented. The stereospecific reconstitution of the B-ring of the cobalamin involves a quantitative and rapid ligand-centered radical ring closure reaction leading first to a new B 12 derivative with antivitamin activity that is subsequently converted to the natural product. Chemoselectivity in the one-electron reduction of the macrocycle was achieved by introducing inorganic cyanide as an axially coordinating protecting group of the otherwise reduction-sensitive Co III -ion. The integrity of structure and function of the reconstituted natural product was unequivocally proven by single crystal structural analysis and a microbiological assay using Lactobacillus leichmannii .