Gadolinium DOTA Chelates Featuring Alkyne Groups Directly Grafted on the Tetraaza Macrocyclic Ring: Synthesis, Relaxation Properties, “Click” Reaction, and High-Relaxivity Micelles

Abstract

This paper reports on the synthesis and relaxivity properties of tetraacetic DOTA-type chelating agents featuring one or two alkyne groups directly grafted on the tetraaza macrocyclic ring and available for "click" reactions with azide-bearing substrates. The racemic DOTAma ligand bearing one alkyne group was obtained by a bisaminal template route. The same approach was used to prepare ligand DOTAda substituted by two alkyne groups located on two adjacent carbon atoms. The S,S enantiomer of DOTAda was also prepared by a "crab-like" condensation. This ligand is the first example of a DOTA derivative featuring two reactive functions adjacent to each other on the macrocyclic ring. A triacetic monoalkyne ligand (DO3ma) was also synthesized for comparison purposes. NMR studies indicate that the Yb(III) chelates of DOTAma and DOTAda adopt two conformations in solutions in which the tetraaza ring is rigidified. The hydration state of the Eu(III) chelates was determined by luminescence spectroscopy, and the water exchange time of the Gd(III) complexes was measured by (17)O NMR. Ring substitution accelerates the water exchange. These data were used to interpret nuclear magnetic relaxation dispersion curves of the Gd(III) chelates. Two long aliphatic chains have been added to DOTAda by a "click" procedure to form the (C18)(2)DOTAda ligand. The corresponding Gd(III) complex forms micelles of unusually high relaxivity presumably because of the close proximity of the aliphatic chains on the macrocyclic ring that ensures a rigid double anchoring into the micelles.