To a stirred solution of N-{[4-((2R,3R)-1-(4-fluorophenyl)-3-{[2-(4-fluorophenyl)-2-oxoethyl]thio}-4-oxoazetidin-2-yl)phenoxy]acetyl}glycine, (20.3 mg, 0.038 mmol) in DMF (2 ml) was added N-methylmorpholine (15 μl, 0.14 mmol). TBTU (14.8 mg, 0.046 mmol) was added the mixture was stirred at 30° C. for 1.5 hours. O-benzyl-L-serine (8.8 mg, 0.045 mmol) was added and the mixture was stirred overnight. The formation of the ketone of the title compound was confirmed. M/z: 718.04 (M+1) and 716.02 (M−1). Methanol (3 ml) and sodium borohydride (10.3 mg, 0.27 mmol) were added and the mixture was stirred for 1.5 hours. Ammonium acetate (20 mg) was added and the methanol was removed under reduced pressure. The remaining DMF-solution was purified by preparative HPLC on a C8 column, UV 240/260 nm. A gradient from 20 to 45% MeCN in 0.1M NH4OAc buffer was used as eluent. The pure fractions were collected and the MeCN was removed under reduced pressure. The remaining water solution was acidified to pH 1 with HCl (1M) and extracted with DCM. The combined organic phases were concentrated and the residue was dissolved in MeCN and water. After lyophilisation, the title compound was obtained as a white solid (2.3 mg, 8.5%).
