A solution of 5.5 g (18.8 mmole) of methyl cis-3-amino-1-(2,4-dimethoxybenzyl)-4-oxoazetidine-2-carboxylate in 100 ml of dry toluene is cooled to -78°; 2.5 ml (18.8 mmole) of triethylamine is added followed by rapid addition of 35 ml (42 mmole) of a 12% solution of phosgene in benzene. The mixture is stirred 15 min at -78°, 3 hr at -45° (acetonitrile-dry ice), then warmed to room temperature and concentrated to half volume in vacuum. To the resulting solution is added 50 ml of t-butanol and the mixture is stirred at room temperature overnight. The solvents are removed in vacuum, the residue is diluted with ethyl acetate and filtered. The filtrate is transferred to a separatory funnel and washed with 5% NaHCO3, 5% HCl and brine; dried over magnesium sulfate and evaporated to dryness. Recrystallization of the crude, crystalline product affords 3.8 g (52%) of the title compound. Recrystallization from ether gives an analytical sample.
