Versatile Synthesis of MRI Contrast Agents Based on Carbon-Substituted Triazacycloalkanes

3-(4-Iodophenoxy)-tris(2-nitrobenzenesulfonyl)-1,4,7-triazacyclodecane (5a) was synthesized by a Mitsunobu reaction between pernosylated diethylenetriamine (4a) and 1,3-bis(trityloxy)propan-2-ol (2). 5a served as a key intermediate in the preparation of MRI contrast agents allowing biomolecule labeling both in solution and on solid phase.