Inorganic Cyanide as Protecting Group in the Stereospecific Reconstitution of Vitamin B12 from an Artificial Green Secocorrinoid.

The synthesis of vitamin B12 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 B12 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.