Broad Scope and High-Yield Access to Unsymmetrical Acyclic [¹¹ C]Ureas for Biomedical Imaging from [¹¹ C]Carbonyl...

Effective methods are needed for labelling acyclic ureas with carbon-11 (t_1/2 =20.4 min) as potential radiotracers for biomedical imaging with positron emission tomography (PET). Herein, we describe the rapid and high-yield syntheses of unsymmetrical acyclic [¹¹ C]ureas under mild conditions (room temperature and within 7 min) using no-carrier-added [¹¹ C]carbonyl difluoride with aliphatic and aryl amines. This methodology is compatible with diverse functionality (e. g., hydroxy, carboxyl, amino, amido, or pyridyl) in the substrate amines. The labelling process proceeds through putative [¹¹ C]carbamoyl fluorides and for primary amines through isolable [¹¹ C]isocyanate intermediates. Unsymmetrical [¹¹ C]ureas are produced with negligible amounts of unwanted symmetrical [¹¹ C]urea byproducts. Moreover, the overall labelling method tolerates trace water and the generally moderate to excellent yields show good reproducibility. [¹¹ C]Carbonyl difluoride shows exceptional promise for application to the synthesis of acyclic [¹¹ C]ureas as new radiotracers for biomedical imaging with PET.