4.2.1. Radiosynthesis of [18F]6

In the one-step method, precursor 6, dissolved in 500 μL of anhydrous acetonitrile, was added into the dried K[18F]F/K2.2.2 and incubated for 2 min (25 ◦C). The reaction mixture was diluted with an additional 500 μL of anhydrous acetonitrile for the radio-TLC and radio-HPLC analysis.

In the two-step method, after evaporation of the solvent, SiFA (**5**) in anhydrous ACN (0.5 mL) was added into the reaction vial containing K[18F]F/K2.2.2 and incubated at room temperature for 2 min. Tetrazine oxyamine (**3**) in 0.3 M anilinium acetate buffer pH 4.6 (200 μL) and ACN (50 μL) was added into the reaction mixture, and the reaction was further incubated at room temperature, for 15 min (RCY 74%, radio-TLC), and purified by preparative HPLC, providing 385 MBq of [18F]**6** at 115 min from end-of-bombardment (EOB, 14.8 ± 1.6% DCY) and 5 GBq/μmol at end-of-bombardment (EOS). The purified (RCP 98.8 <sup>±</sup> 1.3%, *<sup>n</sup>* <sup>=</sup> 16) product [18F]**<sup>6</sup>** was formulated (10% ethanol, 0.5% Solutol HS 15 in 0.01 M PBS), sterile filtered (0.22 μm), and used as such for injections.
