Nucleophilic substitution of nitro groups by [F]fluoride in methoxy-substituted ortho-nitrobenzaldehydes—A systematic study |
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Authors: | Bin Shen,Gerald Reischl,Hans-Jü rgen Machulla |
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Affiliation: | a Radiopharmacy, PET Centre, Eberhard Karls University Tübingen, Germany b Institute of Organic Chemistry, Eberhard Karls University Tübingen, Germany |
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Abstract: | As model reactions for the introduction of [18F]fluorine into aromatic amino acids, the replacement of NO2 by [18F]fluoride ion in mono- to tetra-methoxy-substituted ortho-nitrobenzaldehydes was systematically investigated. Unexpectedly, the highly methoxylated precursors 2,3,4-trimethoxy-6-nitrobenzaldehyde and 2,3,4,5-tetramethoxy-6-nitrobenzaldehyde showed high maximum radiochemical yields (82% and 48% respectively). When the electrophilicity of the leaving group substituted carbon atom is expressed by its 13C NMR chemical shift a good correlation with the reaction rate at the beginning of the reaction (first min) was found (R2 = 0.89), whereas the maximum radiochemical yields correlated much poorer with this electrophilicity parameter. This may be caused by side reactions becoming influencial in the further reaction course. As possible side reactions the demethylation of methoxy groups and intramolecular redox reactions could be detected by HPLC/MS. |
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Keywords: | Nucleophilic substitution [18F]Aromatic amino acid [18F]Fluoride ion Isotopic labelling PET |
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