Pentafluorophenyl trifluoroacetate (2.2 ml, 12.9 mmol) was added dropwise with stirring to a solution of 5-(S)-azidomethyl-3-[4′-carboxy-3′-fluorophenyl]oxazolidine-2-one (3.0 g, 10.7 mmol; prepared as described in Example 3) and pyridine (3.5 ml, 42.8 mmol) in DMF (10 ml). The mixture was stirred at r.t. for 2.5 h, and ethyl acetate with 3% aq. citric acid added. Organic layer was washed twice with 3% citric acid, brine, and dried (Na2SO4). The crude material was washed with ether-hexanes to afford a white crystalline product (4.3 g, 90%). HPLC Rt=7.1. MS (m/z): [M+H]+=447.
