Synthesis of α/β/γ-peptides

All of the α/β/γ-peptides described here were synthesized on Nova PEG rink amide resin (25–50 μmol scale) by microwave-assisted solid-phase reactions using a CEM MARS microwave reactor.^30,31,38 NovaPEG Rink Amide resin was swelled in DMF for 30 minutes before sequential amino acid coupling reactions. For the α- and β-amino acids coupling reactions, 4 equiv of Fmoc-protected α-or β-amino acids, 3.95 equiv of HBTU, 8 equiv of diisopropylethylamine (DIEA), and 4 equiv of HOBT (0.1 M) were dissolved in DMF (1 mL DMF per 25 μmol resin scale) in a separate vial 1–2 minutes prior to the coupling reaction to pre-activate the amino acids. The amino acid solution was added to resin in a fritted syringe, which was then subjected to microwave irradiation: 2 minutes ramp to 70 °C, 4 minutes or 12 minutes hold at 70 °C for α- or β-amino acids, respectively. For acyclic γ-amino acid coupling reactions, the conditions used for β-amino acid coupling were employed, but DIEA was added just before the start of microwave irradiation in order to avoid the cyclization side-reaction of the γ-amino acid. Cyclic γ-amino acid coupling reactions were performed using 4 equiv of Fmoc-amino acid, 4 equiv of EDCI, 8 equiv of DIEA and 4 equiv of HOAt (0.1 M at final) in DMF (1 mL DMF per 25 μmol resin). The amino acid solution was added to the resin and allowed to nutate for 14 hours at room temperature. Fmoc deprotection reactions were carried out using 20 % (v/v) piperidine in DMF under microwave irradiation (2 min ramp to 80 °C, 2 min hold at 80 °C). The resin was washed with 3–5 resin volumes of DMF after each coupling and Fmoc deprotection reactions. The N-terminal amino group of the final residue was acetylated by stirring the resin in 8:2:1 (e.g., 1.6 mL:0.4 mL:0.2 mL for 25–50 μmol resin scale) DMF:DIEA:acetic anhydride solution for 10 minutes at room temperature.