(1) In a polypropylene syringe with a polyethylene frit and cannula (MULTISYNTHECH GmbH), 2-chlorotritylchloride polystyrene resin (125 mg, 200 μmol, 1.60 mmol/g loading density, 100-200 mesh, 1.00 equiv.) was swollen for at least 30 minutes at 21 °C in dichloromethane (DCM) using an orbital shaker (IKA Labortechnik, 160-200 rpm). The solvent was removed, and for the coupling of the first amino acid Fmoc-L-Phenylalanine (310 mg, 800 μmol, 4.00 equiv.), dissolved in 2.00 mL of N-methylpyrrolidone (NMP) diisopropylethylamine (DIPEA, 139 μL, 103 mg, 800 μmol, 4.00 equiv.), was added to the resin. The reaction mixture was shaken overnight at 21 °C. After removing the solvent, it was washed with 2.00 mL each of dimethylformamide (DMF), methanol, DCM, and DMF once more.(2) To cleave the Fmoc-protecting group the loaded resin was treated with 2.50 mL of a 20% piperidine solution in DMF and shaken for 5 minutes at 21 °C. The solution was removed, and the process was repeated twice. Subsequently, the resin was washed with 2.00 mL each of DMF, methanol, DCM, and DMF once more.(3) For the acetylation the loaded resin was treated with 1.60 mL of a 1.00 M solution of bromoacetic acid (222 mg, 1.60 mmol, 8.00 equiv.) in DMF, with diisopropylcarbodiimide (DIC, 250 μL, 202 mg, 1.60 mmol, 8.00 equiv.) and incubated for 30 minutes at 21 °C. The reaction mixture was removed, and the resin was washed with 2.00 mL each of DMF, methanol, DCM, and DMF once more.(4) For the substitution p-chlorobenzylamine (195 μL, 1.60 mmol, 8.00 equiv.), dissolved in 2.00 mL of DMF was added to the resin, and incubated for 1 hour at 21 °C. The solution was removed, and the resin was washed with 2.00 mL each of DMF, methanol, DCM, and DMF once more.Acetylation and substitution with benzylamine (174 μL, 171 mg, 1.60 mmol, 8.00 equiv.), p-chlorobenzylamine (195 μL, 227 mg, 1.60 mmol, 8.00 equiv.), and N-Boc-diaminobutane (310 mg, 1.60 mmol, 8.00 equiv.) were repeated. (5) Subsequently, Fmoc-L-Phenylalanine (310 mg, 800 μmol, 4.00 equiv.) was coupled by dissolving it in 2.00 mL of NMP along with 1-hydroxybenzotriazole (HOBt, 123 mg, 800 μmol, 4.00 equiv.) and treating the solution with DIC (125 μL, 101 mg, 800 μmol, 4.00 equiv.). The reaction mixture was then added to the resin and agitated for 4 hours at 21 °C. After the coupling solution was removed, the resin was washed with 2.00 mL of DMF, methanol, DCM, and DMF once more.(6) For the cleavage of the resin, the resin was washed three times with 2.00 mL of DCM, and then treated with 2.50 mL of a 33% solution of hexafluoroisopropanol (HFIP) in DCM for two rounds of 1 h at 21 °C. The cleavage solution was collected, and the resin was washed with DCM (5×). The solvent was removed by a counter-flow of air, the residue was dissolved in acetonitrile and water, and subsequently lyophilized.
