Description Application 2,5-Dioxo-1-pyrrolidinyl 5-[4-(1,2,4,5-tetrazin-3-yl)benzylamino]-5-oxopentanoate may be used in the synthesis of Tz-PEG11-1,4,7-triazacyclononane-N,N,N-triacetate, a precursor for Al[18F]-NOTA-labeled tetrazine radio ligand useful in 18F-based pretargeted PET imaging system. Succinimidyl ester functionalized tetrazine for inverse electron demand Diels-Alder cycloaddition reactions. Succinimidyl ester will react with amines for small molecule, biomolecule or surface modification. Tetrazine will react with strained alkenes such as transcyclooctene, norbornene and cyclopropene to yield a stable covalent linkage. Tetrazines have proven useful in bioorthogonal reactions for many biological imaging and bioconjugation applications. Packaging 1, 5, 25 mg in amber glass bottle