Carbon Dioxide Radical Anion by Photoinduced Equilibration between Formate Salts and [11C, 13C, 14C]CO2 : Application to Carbon Isotope Radiolabeling

The need for carbon-labeled radiotracers is increasingly higher in drug discovery and development (carbon-14, -, t1/2 = 5730 years) as well as in PET, for in vivo molecular imaging applications (carbon-11, +, t1/2 = 20.4 min). However, the structural diversity of radiotracers is still systematically driven by the narrow available labeled sources and methodologies. In this context, the emergence of carbon dioxide radical anion chemistry might set forth potential unexplored opportunities. Based on a dynamic isotopic equilibration between formate salts and [13C, 14C, 11C]CO2, C-labeled radical anion CO2●- could be accessed under extremely mild conditions within seconds. This methodology was successfully applied to hydro-carboxylation and bis-carboxylation reactions in late-stage carbon isotope labeling of pharmaceutically relevant compounds. The relevance of the method in applied radiochemistry was showcased by the whole-body PET biodistribution profile of [11C]oxaprozin in mouse.