Platinum Metals Rev., 2001, 45, (1), 33
Palladium Cross-Coupling Reaction for Brain Tracers
Brain imaging via positron emission tomography (PET) using the short-lived 11C positron emitter (half-life ∼ 20 minutes) requires a fast chemical reaction to incorporate the small amounts of 11C into a carrier (tracer) which delivers it to the tissue under examination.
Researchers in Japan and Sweden have now developed a new rapid, efficient Stille methylation of arylstannanes which produces a structure system that can deliver sufficient quantities of 11C to prostaglandin receptors (IP2) in the brain (M. Suzuki, H. Doi, K. Kato, M. Björkman, B. Långström, Y. Watanabe and R. Noyori, Tetrahedron, 2000, 56, (42), 8263-8273). The Stille methylation involves a palladium-promoted cross-coupling reaction of methyl iodide and tributyltin derivatives of tolylisocarbacyclins (TICs); the TIC binds to the IP2 receptors. In one methylation, which is a novel stepwise operation, an initial methylpalladium complex is produced and is then mixed with other materials for the cross-coupling. The Pd promoter is a Pd(0) complex generated from Pd2(dba)3 and P(o -CH3C6H4)3 in the presence of CuCl and K2CO3. The synthesis is highly reproducible, and with 11C incorporated, results in a [11C]-labelled PET tracer with radioactivity of several GBq, which can be injected intravenously.