A [two-step/one week] synthesis of C-functionalized homocyclens and cyclams. Application to the preparation of conjugable BCAs without chelating properties alteration
A versatile and efficient bisaminal template approach for the synthesis of cyclam and aneN4 (homocyclen) bifunctional chelating agents (BCAs) bearing a hydroxyethyl function as a C-appended group is presented. The synthesis is rapid (two steps/one week) and does not require fastidious protection–deprotection steps or chromatographic purification. Another reactional route and alternative work-up give access to their oxo-cyclam and oxo-homocyclen analogues. The procedure consists of the cyclization of the preorganized tetraamine 1,4,8,11-tetraazaundecane, in its cis-butanedione-bisaminal form, with an α,β-unsaturated lactone to provide the tetracyclic oxo-intermediate whose bisaminal bridge can be easily removed and/or its amide function reduced under mild conditions. Furthermore, the synthetic route was successfully applied to the synthesis of teta and trita BCAs analogues starting from the linear tetraamine 1,4,7,10-tetraazadecane. Additionally, the appended alcohol functions of various cyclam and homocyclen based ligands were converted into their ethylamine functions following a very convenient procedure. Finally, preliminary analytical and complexation studies highlight that the supplementary hydroxyethyl C-appended chain has only a low impact on the acid–base behaviour and copper(II) or zinc(II) coordination properties of the macrocycle.