To a solution of 20 (1 eq.) and the linker (1.1 eq.) in dry DMF (~ 14 mL per mmol), DBU (1.1 eq.) was added at 0 °C under a nitrogen atmosphere. The reaction mixture was stirred at r.t. for 4 h and monitored by LC-MS (acidic method). The reaction was quenched with a 5% v/v aqueous solution of citric acid and the solvent was evaporated under high vacuum. The crude was purified by HPLC using a gradient from 5% to 90% v/v acetonitrile with 0.01% v/v aqueous solution of formic acid over 15 min to yield the desired compound.