30 ul of boron trifluoride etherate were added to a suspension of 216 mg (1.15 mmols) of (1R, 5S)-3-phenyl-4-oxa-2,6-diazabicyclo[3.2.0]hept-2-en-7-one and 400 mg (2.3 mmols) of freshly prepared tert.-butyl 4-hydroxyacetoacetate in 4 ml of anhydrous tetrahydrofuran at room temperature. After a short time a clear, light yellow solution formed, which was stirred for 0.5 hour at room temperature. It was then poured into dilute NaHCO3 solution. The mixture was extracted with methylene chloride, washed with water and dried over magnesium sulphate. After the solvent had been evaporated off in vacuo and the residue had been chromatographed on 60 g of silica gel (toluene:ethyl acetate 1:3), 128 mg (31%) of tert.-butyl 4-[3(R)-benzoylamino-2-azetidinon-4(R)-yloxy]acetoacetate were obtained as colorless crystals, melting point: 142° C., RF : 0.36 (toluene:ethyl acetate 1:4), [α]D20 =32.24° (0.955 percent in CHCl3).
