In a fritted syringe, commercial 2-chlorotrityl chloride resin (125 mg, 200 µmol, 1.00 equiv.) was swollen in 2 mL of methylene chloride by shaking at 21 °C for at least 30 min. The solvent was removed by filtration. (2S)-2-(9H-fluoren-9-ylmethoxycarbonylamino)-3-phenyl-propionic acid (310 mg, 800 μmol, 4.00 equiv) and N-ethyl-N-propan-2-ylpropan-2-amine (103 mg, 800 μmol, 4.00 equiv) were dissolved in 2 mL of 1-methylpyrrolidin-2-one and added to the resin. After shaking for at least 4 h, the solvent was removed and the resin was washed with dimethylformamide, methanol and methylene chloride. Deprotection of the Fmoc-protecting group was carried out using a solution of 20% piperidine in dimethylformamide. 3 mL of this mixture were added to the resin and shaken for at least 5 min. The deprotection cocktail was removed and the procedure was repeated twice. The resin was washed with dimethylformamide, methanol and methylene chloride. Subsequently, N,N'-di(propan-2-yl)methanediimine (202 mg, 1.60 mmol, 8.00 equiv) was added to an 1 M solution of 2-bromoacetic acid (222 mg, 1.60 mmol, 8.00 equiv) dimethylformamide. The mixture was immediately added to the resin and shaken for 30 min. The solvent was removed and the resin was washed with dimethylformamide, methanol and methylene chloride. Allylamine (91.4 mg, 1.60 mmol, 8.00 equiv) was dissolved in 2 mL of dimethylformamide, added to the resin and shaken for at least 1 h. The solvent was removed and the resin was washed with dimethylformamide, methanol and methylene chloride. (2R)-1-(9H-fluoren-9-ylmethoxycarbonyl)proline (270 mg, 2.00 mL, 800 μmol, 0.400M, 4.00 equiv), 1-hydroxybenzotriazole;hydrate (123 mg, 800 μmol, 4.00 equiv) and N,N'-di(propan-2-yl)methanediimine (101 mg, 125 μL, 800 μmol, 4.00 equiv) were dissolved in 2 mL of 1-methylpyrrolidin-2-one, added to the resin and shaken for at least 4 h. The resin was washed with dimethylformamide, methanol and methylene chloride and deprotected as described before. Acetylation and substitution with cyclohexylamine (159 mg, 1.60 mmol, 8.00 equiv) were repeated. The last washing step after substitution was followed by washing with methylene chloride. 2.5 mL of a solution of 33% 1,1,1,3,3,3-hexafluoropropan-2-ol in methylene chloride were added. The resin was incubated for 16 h. The solvent was collected and the resin was flushed with methylene chloride. The solvent was removed under air stream resulting in the crude linear precursor N-allyl-N-cyclohexylglycyl-D-prolylglycyl-L-phenylalanine (97.5 mg, 196 μmol, 98% yield). The linear tetramer was cyclized without further purification (see: https://www.chemotion-repository.net/pid/12190).
