Synthesis of a versatile building block combining cyclen-derivative DO3A with a polyamine via a rigid spacer

The five-step synthesis of a polydentate building block combining a cyclen-based macrocycle (DO3A) with N-(2-aminoethyl)propane-1,3-diamine, which are linked through the xylylen moiety as a rigid C-spacer is described. These two molecular parts were coupled by subsequent bromine atom substitution in 1,4-bis(bromomethyl)benzene. First, N-(2-aminoethyl)propane-1,3-diamine was protected by phthaloyl moieties and then it was reacted with 1,4-bis(bromomethyl)benzene to form (2-phthalimidoethyl)(3-phthalimido-prop-1-yl)(4-bromomethylbenzyl)amine (2). This compound underwent a substitution reaction with DO3A in the form of its tert-butyl esters leading to the intermediate 1-{4-[(2-phthalimidoethyl)(3-phthalimidoprop-1-yl)aminomethyl]phenylmethyl}-4,7,10-tris(t-butoxy-carbonylmethyl)-1,4,7,10-tetraazacyclododecane (3). The phthaloyl as well as the t-butyl protecting groups were removed in the next two reaction steps to form the final product 1-{4-[(2-aminoethyl)(3-aminoprop-1-yl)aminomethyl]phenylmethyl}-4,7,10-tris(carboxy-methyl)-1,4,7,10-tetraazacyclododecane (5). The intermediates 1-4 as well as the final product 5 were characterized by elemental analysis, mass spectrometry, and multinuclear (1H and 13C) and two-dimensional NMR spectroscopy. The final product 5 could serve as a potential building block in subsequent syntheses of binuclear complexes of lanthanides and/or transition metals.