Synthesis of highly pure 14C-labelled DL-allantoin and 13C NMR analysis of labelling integrity

A number of synthetic approaches are assessed to prepare allantoin labelled with C-14 given certain requirements and technical limitations. A method that fulfils these criteria is described to achieve the synthesis of highly pure C-14-labelled allantoin with the label introduced to the ureido carbonyl group in the final step by reaction of 5-chlorohydantoin with [C-14]urea. The chosen method favours high purity at the expense of radiochemical yield, which is achieved at a level of 8%. The integrity of the label is then investigated by performing an NMR analysis of C-13-labelled allantoin synthesized by the same method. The C-13 NMR spectrum confirms partial scrambling of the label to the C-2 position by, equilibration of the product via a putative bicyclic intermediate, which had been suggested by other workers. The C-14-labelled allantoin synthesized by this method is therefore assigned as DL-[(H2NCO)-C-14/C-14-2]allantoin. This study also includes the first full characterization of a side product, 5-hydroxy-5-methoxyhydantoin, obtained by the reaction of a 5-hydroxyhydantoin intermediate with the methanol solvent.