To a solution of the carboxylic compound (1 eq.) in DMF dry (~ 100 mL per mmol), COMU (1 eq.), compound 3 (1.1 eq.) and DIPEA (3 eq.) were added. The reaction mixture was left to stir for 1 h and monitored by LC-MS (acidic method). Then, ice was added to quench the reaction, the volatiles were evaporated under reduced pressure and the residue 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.