Catalyst-free thia-Diels-Alder click reaction for the labeling of peptides for imaging applications

A thia-Diels-Alder reaction between a phosphonodithioester and a highly reactive exocyclic diene for the chemoselective labeling of peptides under catalyst-free mild conditions is described. An efficient method to introduce the dithioester function into a model tripeptide was first developed and then applied to two bioactive peptides, PSMA and apelin-13. The tripeptide-dithioester was used to optimize the cycloaddition with dienes bearing fluorine-18 or a fluorophore. Then, the cycloaddition of the PSMA-dithioester with the ¹⁸F-diene led to a new PSMA-radiotracer, which was used for in vivo Positron Emission Tomography (PET) imaging. In the same way, a fluorescent apelin-13 probe was obtained from the apelin-dithioester and a squaraine-linked diene before beingused for in cellulo optical imaging.