Compound 5a'

View in PubChem | MDL Molfile | Chemdraw file

Compound data: 1H NMR

Compound data: 13C NMR

Compound data: 19F NMR

Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.

To a solution of fluoroserine S3 (806 mg, 4.05 mmol, see Supplementary information) in THF/H₂O (1:1, 0.1 M) was added triethylamine (564 µL, 4.05 mmol) and the mixture was sonicated until homogeneous. The mixture was added to a solution of oxazolone S2 (2.00 g, 3.37 mmol, see Supplementary information) in THF (34 mL). The reaction was stirred at rt for 15 h. The mixture was acidified with 10% KHSO₄ (aq), extracted with 3x with EtOAc, and the organic layer was washed with brine. The organic layer was dried over anhydrous Na₂SO₄ and concentrated under reduced pressure to afford the desired carboxylic acid which was used without further purification. To a round bottom flask equipped with a stir bar was added the carboxylic acid (3.37 mmol) and EtOAc (6.7 mL). To this solution was added acetic anhydride (3.18 mL, 33.7 mmol) and sodium acetate (552 mg, 6.74 mmol) and the mixture was stirred at rt for 26 h. Sat. NaHCO₃ (aq) was added to quench excess acetic anhydride and acetic acid until the reaction mixture stopped bubbling. The mixture was extracted 3x with EtOAc and the combined organic layer was washed with brine and dried over anhydrous Na₂SO₄. The solution was filtered and solvent was evaporated under reduced pressure. The oil was dissolved in a minimal amount of DCM and hexanes was added to precipitate the oxazolone. The solvent was decanted and the solid was washed 2x with hexanes to afford the desired oxazolone which was used in the next step without further purification. To a round bottom flask equipped with a stir bar under N₂ was added the resulting oxazolone (3.37 mmol) and anhydrous DCM (34 mL). The reaction mixture was cooled to 0 °C and TFA (5.20 mL, 67.4 mmol) was slowly added. The reaction slowly warmed to rt and stirred for 2 h. The DCM was concentrated under reduced pressure and to the mixture was added toluene to form a TFA azeotrope, which was subsequently concentrated under reduced pressure. The crude reaction mixture was further dried on the high vacuum and subsequently triturated with Et₂O to afford TFA·amine salt and was immediately used in the next reaction without further purification. The TFA·amine salt (3.37 mmol) was dissolved in anhydrous DCM (67 mL, 0.05 M) and to this solution was added anhydrous Et₃N (0.94 mL, 6.74 mmol) and DMAP (41 mg, 0.34 mmol). The reaction mixture was stirred at rt for 3 h and subsequently concentrated under reduced pressure. The crude reaction mixture was then purified via column chromatography (eluting with 25% DCM/Acetone) to afford cyclic pentapeptide 5a’ as a light yellow solid (235 mg, 12% yield over 4 steps). ¹H NMR (499 MHz, DMSO, 373 K, d1=25 s) δ 9.65 (s, 1H), 9.40 (s, 1H), 9.35 (s, 1H), 9.22 (s, 1H), 8.09 (s, 1H), 7.71 – 7.53 (m, 7H), 7.49 – 7.31 (m, 10H), 7.27 (s, 1H), 7.25 – 7.15 (m, 4H), 7.09 (s, 1H), 4.00 (d, J = 5.4 Hz, 2H). ¹³C NMR (151 MHz, DMSO, 373 K, d1=25 s) δ 169.1, 165.3, 165.3, 164.8, 163.9, 161.6 (d, J = 247.5 Hz), 133.8, 133.5, 133.3, 131.2 (d, J = 8.3 Hz), 130.5 (d, J = 3.1 Hz), 129.5, 129.4, 129.4, 129.3, 129.2, 129.1, 128.7, 128.5, 128.4, 128.4, 128.4, 128.3, 127.99, 127.97, 127.8, 127.4, 127.4, 114.7 (d, J = 21.6 Hz), 43.4. ¹⁹F NMR (376 MHz, DMSO, 298 K) δ -112.20. ¹⁹F NMR (376 MHz, DMSO, 373 K) δ -112.34. IR (ATR): 3050, 1633, 1601, 1509, 1233, 1203 cm^-1. HRMS (ESI-TOF) m/z calc’d for C₃₈H₃₀FN₅O₅Na [M+Na]⁺: 678.2129, found: 678.2134.