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Synthesis

Synthesis of fluorinated pyridine offers new avenue for PET imaging

Direct nucleophilic fluorination of pyridine N-oxide leads to a tracer for imaging brain disorders

by Stephen K. Ritter
August 29, 2016 | APPEARED IN VOLUME 94, ISSUE 34

Developing new positron emission tomography (PET) tracers for biomedical imaging is one of the hottest areas of fluorine chemistry, and one of the most challenging. For example, using nucleophilic fluorination to develop meta-substituted pyridines that can be used as tracers is difficult because the aromatic ring is too electron-rich. Postdoctoral researcher Pedro Brugarolas of the University of Chicago and his colleagues have overcome this challenge by starting with an N-oxide instead of a pyridine, enabling development of a new PET tracer for brain imaging. The electron-withdrawing nature of the N-oxide group alters the ring electron density enough to enable nucleophilic substitution of a meta halogen with a fluoride quickly at room temperature, Brugarolas explained. After fluorination, the chemists reduce the N-oxide group via palladium-catalyzed hydrogenation to generate the pyridine (Chem. Commun. 2016, DOI: 10.1039/c6cc02362b). The end product, 3-fluoro-4-aminopyridine, is an analog of the multiple sclerosis drug 4-aminopyridine, which is a potassium ion channel blocker that helps improve neuronal function in people with MS. Working with University of Chicago neuroscientist Brian Popko, Brugarolas is testing the new tracer by imaging brain lesions noninvasively in rodents.

For the longer, original version of this story, reported from the ACS national meeting in Philadelphia, click here.
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