To 664 mg (4.0 mmol) of benzyl glycolate was added 5.8 ml of 12.5% phosgene (in toluene) at 0° C. To this solution at 0° C. was added pyridine (0.372 ml, 4.6 mmol) dropwise. The reaction was allowed to stir at room temperature for 2 hours and was then filtered under argon. Most of the volatiles were removed under vacuum without external heating, and the residual chloroformate was dissolved in methylene chloride (8 ml). Approximately half of this solution was added to (3S-trans)-3-[[(t-butyloxy)carbonyl]amino]-4-methyl-2-azetidinone (440 mg, 2.0 mmol), and the resulting solution was cooled to -20° C. Triethylamine (0.293 ml, 2.1 mmol) was then added dropwise. After 2 hours at -20° C., the remaining chloroformate was added to the azetidinone reaction mixture followed by 0.293 ml of triethylamine. After 3 hours at -20° C., the product was extracted from aqueous KH2PO4 with three portions of ethyl acetate. The combined organic layers were dried over sodium sulfate, and the volatiles were removed. The residue was subjected to column chromatography on silica gel (eluting with 35% ethyl acetate-hexane) to yield 488 mg of the title compound.
