Scheme 1. Synthesis of the tri-functional platform 3. Reagents and conditions: (a) N-Boc-l-lysine, i-Pr2NEt, DMF, 23 °C, 16 h, 78%; (b) 4-(6-methyl-1,2,4,5-tetrazin-3-yl)benzyl amine, HATU, i-Pr2NEt, DMF, 23 °C, 36 h, 71%; (c) 4 M HCl/dioxane, 23 °C, 6 h, 40%; (d) DOTA-NHS, Et3N, DMF, 23 °C, 16 h, 49%. HATU = 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate; DOTA-NHS = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono-N-hydroxysuccinimide ester.
