Introduction positron emission tomography

Considerable interest has been focused on the efficient and rapid synthesis of 2-deoxy-2-[ F]fluoro-D-gIucose, a popular imaging agent for positron-emission tomography (see Section III, 1). However, introduction of a fluorine atom at C-2 by nucleophilic displacement is generally not easy on account of the weak nucleophilic character of the fluoride ion. One possible... 

Because of the very short half-life of fluorine-18 the synthetic steps for introduction of the isotope into organic substrates must be kept simple and fast. The F-labeled diagnostics are prepared immediately before use. Their main field of application is medical diagnostics and elucidation of metabolic pathways by positron emission tomography (PET) [84] (Figure 4.22). 

Among the neutron-deficient isotopes of Tc, " Tc seems to have suitable nuclear properties for application in positron emission tomography (PET) (Browne and Firestone 1986). Considering the relatively new lipophilic Tc-labeled flow tracers used in cardiac perfusion imaging (2-methoxyisobutyl isocyanide [MlBl], teboroxime, tetrofosmin) it would be reasonable to replace Tc with " Tc in order to carry out quantitative investigations of biodistribution and clearance with PET. With this replacement, the pharmacokinetics of useful Tc compounds could be studied in a number of cases, which would enhance the introduction of new Tc-labeled pharmaceuticals into clinical practice. The labeling and quality control methods are well known and partly subject of this book. Our task is to review the different methods for the production of Tc and also the separation technique from irradiated targets. 

Additional examples include the electrophilic arylations of sodium arenesulflnates [866], potassium car-bonotrithioates [867] and benzazoles [868] using diaryliodonium salts in ionic liquids and the arylations of anilines [869], sodium tetraphenylborate [870] and vinyUndiums [871]. Particularly important are the reactions of diaryliodonium salts with fluoride anion as nucleophile. This reaction is widely applied for introduction of the radioactive F isotope into different organic substrates to obtain labeled agents for positron emission tomography. This topic is covered separately in Chapter 7 due to its importance and wide usage. 

Aryltriazenes can also be decomposed by hydrogen fluoride in organic solution after extraction from their aqueous mother phase. In this case, hydrogen fluoride can be used in small excess but the nature of the solvent is crucial for example, tetrahydrofuran gives complex mixtures, dichloromethane promotes radical reactions (dimerizations, reductions) and acetic acid favors triazene decomposition before fluorination. Aromatic and haloaromatic compounds seem to be the best solvents.283 Such a technique, especially suited for the rapid introduction of an 18F atom, has been employed to produce [ 8F]haloperidol (3), the specific receptors of which have been localized in the brain by positron emission transaxial tomography.298... 

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