A slurry of (S)-3-[[(phenylmethoxy)carbonyl]amino]-2-azetidinone (0.78 g, 3.54 mmole) in dry dichloromethane was warmed briefly to 50° C. and then cooled to -5° C. Triethylamine (0.354 g, 3.54 mmole) and phosgene (5.7 ml of 12.5% phosgene/toluene) were added and the reaction was stirred at -5° C. for 2 hours. The reaction was concentrated to a residue and fresh dichloromethane was added. After the solution was cooled to -50° C., triethylamine (0.714 g, 7.08 mmole) and azetidine-2-carboxylic acid, benzyl ester hydrochloride (0.8 g, 3.54 mmole) were added. The reaction was stirred at -5° C. for 45 minutes. The reaction mixture was poured into 10% monobasic potassium phosphate and the dichloromethane layer was separated and washed with water and brine. After drying over sodium sulfate the solvent evaporated. The crude oil was flash chromatographed on silica (LPS-1) eluting with ethyl/acetate:hexane (1:1). Fractions containing pure compound were concentrated to give 0.622 g of the title compound.
