Here we report the synthesis of 3β-(p-trimethylsilylphenyl)tropane-2β-carboxylic acid methyl ester, a new, easily prepared precursor for the synthesis of radioiodinated versions of RTI-55. In contrast to the 3β-(p-trimethylstannylated) precursor for radioiodination, the aryl silane is prepared directly by the 1,4 addition of p-trimethylsilylphenylmagnesium iodide to anhydroecgonine methyl ester in 49% yield (Scheme 1). Radioiododesilylation proceeded smoothly (approx. 85% radioincorporation after 15min at room temperature) using 1 μg of the novel precursor in trifluoroacetic acid and chloramine-T as oxidant. Isolated radiochemical yields for 125I and 123I radiolabelings were 72 and 48%, respectively, with a calculated specific activity for [123I]RTI-55 of > 2000 mCi/μmol.
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