The dicarboxylic acid linker (1 eq.) and NHS (1.1 eq.) were dissolved in dry DCM (~10 mL per mmol). DCC (1.2 eq.) was added and the reaction was left to stir overnight. The dicyclohexylurea was filtered off, the solution was evaporated and the residue dissolved in dry DMF. Compound 3 (0.5 eq.) and DIPEA (3 eq.) were added. The reaction mixture was left to stir at r.t. for 2 h, quenched with ice, dried under high vacuum and purified by HPLC using a gradient from 10% to 80% v/v acetonitrile with 0.01% v/v aqueous solution of formic acid over 15 min to yield the desired compound.