A suspension of 51 (12.49 g, 34.90 mmol) in methylene chloride (CH2Cl2, 150 mL) was treated with triethylamine (Et3N, 7.07 g, 9.7 mL, 70 mmol, 2.0 equiv) at 25° C., and the resulting mixture was cooled to 0–5° C. before being treated dropwise with methanesulfonyl chloride (4.80 g, 3.24 mL, 41.9 mmol, 1.2 equiv) at 0–5° C. The resulting reaction mixture was subsequently stirred at 0–5° C. for 2 h. When TLC and HPLC showed the reaction was complete, the reaction mixture was treated with H2O (100 mL) at 0–5° C. The mixture was then concentrated in vacuo to remove most of the CH2Cl2, and the resulting slurry was treated with H2O (150 mL). The mixture was stirred at room temperature for 10 min before being cooled to 0–5° C. for 30 min. The solid precipitates were collected by filtration, washed with H2O (2×100 mL) and 20% EtOAc/hexane (2×50 mL), and dried in vacuo. The crude desired methanesulfonic acid 4′-[5-(acetylamino-methyl)-2-oxo-oxazolidin-3-yl]-2′-fluoro-biphenyl-4-ylmethyl ester 52 (11.84 g, 78% yield) was obtained as off-white solids, which by TLC and HPLC was found to be essentially pure and was directly used in the subsequent reaction without further purification. LCMS (ESI) m/e 437 (M+H)+.
