Synthesis of C-Labelled Ureas by Palladium(II)-Mediated Oxidative Carbonylation.

Positron emission tomography is an imaging technique with applications in clinical settings as well as in basic research for the study of biological processes. A PET tracer, a biologically active molecule where a positron-emitting radioisotope such as carbon-11 has been incorporated, is used for the studies. Development of robust methods for incorporation of the radioisotope is therefore of the utmost importance. The urea functional group is present in many biologically active compounds and is thus an attractive target for incorporation of carbon-11 in the form of [C]carbon monoxide. Starting with amines and [C]carbon monoxide, both symmetrical and unsymmetrical C-labelled ureas were synthesised via a palladium(II)-mediated oxidative carbonylation and obtained in decay-corrected radiochemical yields up to 65%. The added advantage of using [C]carbon monoxide was shown by the molar activity obtained for an inhibitor of soluble epoxide hydrolase (247 GBq/μmol-319 GBq/μmol). DFT calculations were found to support a reaction mechanism proceeding through an C-labelled isocyanate intermediate.