Rapid one-pot radiosynthesis of [carbonyl-¹¹C]formamides from primary amines and [¹¹C]CO₂

Background: Formamides are common motifs of biologically-active compounds (e.g. formylated peptides) and are frequently employed as intermediates to yield a number of other functional groups. A rapid, simple and reliable route to [carbonyl-¹¹C]formamides would enable access to this important class of compounds as in vivo PET imaging agents. Results: A novel radiolabelling strategy for the synthesis of carbon-11 radiolabelled formamides ([¹¹C]formamides) is presented. The reaction proceeded with the conversion of a primary amine to the corresponding [¹¹C]isocyanate using cyclotron-produced [¹¹C]CO₂, a phosphazene base (2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine, BEMP) and phosphoryl chloride (POCl₃). The [¹¹C]isocyanate was subsequently reduced to [¹¹C]formamide using sodium borohydride (NaBH₄). [¹¹C]Benzyl formamide was obtained with a radiochemical yield (RCY) of 80% in 15 min from end of cyclotron target bombardment and with an activity yield of 12%. This novel method was applied to the radiolabeling of aromatic and aliphatic formamides and the chemotactic amino acid [¹¹C]formyl methionine (RCY = 48%). Conclusions: This study demonstrates the feasibility of ¹¹C-formylation of primary amines with the primary synthon [¹¹C]CO₂. The reactivity is proportional to the nucleophilicity of the precursor amine. This novel method can be used for the production of biomolecules containing a radiolabelled formyl group.