N-{[4-((2R,3R)-1-(4-Fluorophenyl)-3-{[2-(4-fluorophenyl)-2-oxoethyl]thio}-4-oxoazetidin-2-yl)phenoxy]acetyl}-D-valine (13.3 mg, 0.023 mmol), tert-butyl L-methioninate hydrochloride (7.4 mg, 0.031 mmol) and N-methylmorpholine (10 μl, 0.091 mmol) were dissolved in 1 ml. After 5 minutes, TBTU (8.9 mg, 0.028 mmol) was added and the resulting suspension was stirred overnight. Additional tert-butyl L-methioninate hydrochloride (2.1 mg, 0087 mmol), N-methylmorpholine (5 μl, 45 μmol) and TBTU (2.1 mg, 6.54 μmol) were added and the mixture was stirred for 2 h. The formation of the ester was confirmed. MHz 768.1 (M−H) and 770.0 (M+H). The yellow suspension was purified on silica gel (1 g) and eluted with EtOAc:DCM (15:85). The pure fractions were concentrated and formic acid (1.5 ml) was added. The solution was stirred at 50° C. overnight. The solvent was removed under reduced pressure. Toluene was added and removed under reduced pressure. The residue was dissolved in methanol (1 ml) and sodium borohydride (9.9 mg, 0.26 mmol) was added. The resulting reaction mixture was stirred for 10 minutes. Ammonium acetate (18.9 mg) was added and the solvent was removed under reduced pressure. The residue was purified with preparative HPLC on a C8 column. A gradient from 20% to 40% MeCN in 0.1 M ammonium acetate buffer was used as eluent. After lyophilisation, the title compound was obtained as a white solid was obtained (4.6 mg, 28%). 1H-NMR (400 MHz, DMSO-d6): 0.75 (d, 3H), 0.79 (d, 3H), 1.79-1.97 (m, 3H), 1.99 (s, 3H), 2.36-2.44 (m, 2H), 2.86-2.92 (m, 2H), 2.24-4.35 (m, 3H), 4.58 (d, 2H), 4.67-4.76 (m, 1H), 5.03 (d, 0.5H), 5.5 (d, 0.5H), 5.63 (t, 1H), 6.95 (d, 2H), 7.05-7.16 (m, 4H), 7.18-7.24 (m, 2H), 7.30-7.38 (m, 2H), 7.82 (d, 1H), 7.37 (d, 1H). M/z: 714.0 (M−1) and 716.1 (M+H).
