Synthesis of a “Base‐On” Yellow Corrinoid From Vitamin B 12

Xanthocorrinoid s constitute a largely unexplored class of yellow corrin derivatives characterized by an interrupted corrin π‐system. Typically, air‐stable and biologically inactive, they often arise as minor side products in microbiological and chemical transformations. Here, we report the semi‐synthesis and characterization of (5R,6S)‐5,6‐dihydroxyaquacobalamin 3 + , derived from vitamin B 12 . The compound was prepared from the corresponding free‐base ligand 4 using Co(II) and dioxygen from air. This “base‐on” cobalamin derivative contains an additional diol functionality at C5 and C6 of the corrin macrocycle relative to aquacobalamin and displays a characteristic visible absorption maximum at 459 nm. Its constitution was established by ESI‐MS and extensive NMR spectroscopic characterization, with diagnostic ROESY correlations between the dimethylbenzimidazole moiety and the corrin framework providing additional support for the “base‐on” structure. The resulting octahedral, low‐spin d 6 Co(III) complex represents a structurally unusual “base‐on” member of the xanthocorrinoid family.