Scheme 2.

General synthetic scheme of the dimeric compounds 27 and 30. The monomeric unit was obtained by attaching either 3-(tritylthio)propionic acid or TrtS-Hx-OH in solid phase to 16 to obtain 24 or 28, respectively, after cleavage from the resin. Reaction of 24 with 1,2,3,5-dichloro-tetrazine provided the dimerized compound 25, which was then clicked to the DOTA-TCO chelator (26) to form the final compound 27. Similarly, the reaction of 28 with the tetrazine yielded the dimerized compound 29 and the final compound 30 after reacting with 26. The synthesis of 24, 25, and 27 are detailed in the Supplementary Information.