Combining Bifunctional Chelator with (3+2)-Cycloaddition Approaches: Synthesis of Dual-Function Technetium Complexes

A new concept for the synthesis of dual-functionalized technetium (Tc) compounds is presented, on the basis of the reactivity of fac-{(TcO3)-O-VII}(+) complexes. The concept combines the classical bifunctional chelator (BFC) approach with the new ligand centered labeling strategy of fac-{TcO3)(+) complexes with alkenes ((3 + 2)-cycloaddition approach). To evidence this concept, fac-{(TcO3)-Tc-99}(+) model complexes containing functionalized 1,4,7-triazacyclononane (tacn) derivatives N-benzyl-2-(1,4,7-triazonan-1-yl)acetamide (tacn-ba) and 2,2',2 ''-(1,4,7-triazonane-1,4,7-triyl]triacetic acid (nota center dot 3H) were synthesized and characterized. Whereas [(TcO3)-Tc-99(tacn-ba)](+) [2](+) can be synthesized following a established oxidation procedure starting from the Tc-V complex [(TcO)-Tc-99(glyc)(tacn-ba)](+) [1](+), a new synthetic pathway for the synthesis of [(TcO3)-Tc-99(nota)](2-) [5](2-) had to be developed, starting from [Tc-99(nota center dot 3H)(CO)(3)](+) [4](+) and using sodium perborate tetrahydrate (NaBO3 center dot 4H(2)O) as oxidizing reagent. While [(TcO3)-Tc-99(nota)](2-) [5](2-) is a very attractive candidate for the development of trisubstituted novel multifunctional radioprobes, (3 + 2)-cycloaddition reactions of [(TcO3)-Tc-99(tacn-ba)](+) [2](+) with 4-vinylbenzenesulfonate (styrene-SO3-) demonstrated the suitability of monosubstituted tacn derivatives for the new mixed BFC-(3 + 2)-cycloaddition approach. Kinetic studies of this reaction lead to the conclusion that the alteration of the electronic structure of the nitrogen donors by, e.g., alkylation can be used to tune the rate of the (3 + 2)-cycloaddition.