Direct conversion of radiolabelled methyl iodide to formaldehyde offers many new opportunities for preparing 11C-containing molecules
The ability to incorporate radiolabelled atoms into small molecules is important for their use in, for example, in vivo drug studies by positron emission tomography. However, methods of introducing such labels are limited by the possible sources of labelled atoms. The short half-life of the labels — which is desirable to avoid excessive exposure of patients to radiation — also means that the synthetic steps that introduce the label have to be carried out at the medical facility in a specialized lab. In the case of carbon-11 labels, the source is usually labelled methyl iodide and this limits the chemistry that can be performed within the requisite short time. Formaldehyde (methanal) has long been regarded as a promising labelling reagent because of the wide range of chemistry into which it can be incorporated, but preparing it has been problematic.
Now, Jacob Hooker from Brookhaven National Laboratory, and collaborators from the Johannes Gutenberg University in Mainz, Germany, have reported1 a simple conversion of labelled methyl iodide into labelled formaldehyde in a single rapid and high-yielding step. Hooker and colleagues developed oxidation chemistry to achieve this and, after settling on trialkylamine N-oxides as the ideal oxidizing agents, they optimized the conditions to achieve a high yield in just two minutes. This will allow easy incorporation of labelled carbon atoms in molecules of interest without significant loss of activity.
The preparation of labelled methyl iodide is already possible in many labelling laboratories, so this simple conversion process can easily be adopted without significant expense.
Hooker, J. M., Schönberger, M., Schieferstein, H. & Fowler, J. S. A simple, rapid method for the preparation of [11C]formaldehyde. Angew. Chem. Int. Ed. 10.1002/anie.200800991 (2008).
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Davey, S. Tagging formaldehyde. Nature Chem (2008). https://doi.org/10.1038/nchem.37