Crude 13 was purified via reverse-phase automated chromatography (76%). Boc-removal was performed at 10 mM in a 9:1 DCM:TFA solution, the crude was purified by RP HPLC (water/CH3CN gradient with of 0.1% HCOOH) and PM68 was isolated in 92% yield as the double formate salt. With a similar approach, the long-linker dimers PM70 and PM74 (Figure ) were prepared as detailed in the Sup Info section.