Chlorotrimethylsilane → benzyl (2R)-4-oxoazetidine-2-carboxylate

In 485 ml of dry ether was dissolved 45 g (143.8 m mol) of D-aspartic acid dibenzyl ester (42) and after cooling the solution to 0° C. under an argon atmosphere, 20 ml (143.8 m mol) of triethylamine was added dropwise to the solution. Then, 15.6 g (143.9 m mol) of trimethylsilyl chloride was further added dropwise to the mixture at the same temperature as above and the resultant mixture was stirred for one hour. Precipitates thus formed were filtered off under an argon atmosphere and the filtrate was cooled to 0° C. to -5° C. and then 134.8×1.01 m mol of an ether solution of t-butyl magnesium chloride was added dropwise to the mixture with stirring. After further stirring the mixture for 2 hours at 0° C. and then for 3 hours at room temperature, the mixture was cooled to 0° C., 100 ml of 2N HCl (saturated with NH4Cl) was added to the mixture and after stirring the mixture for 30 minutes, 100 ml of a saturated aqueous ammonium chloride solution was added to the mixture. The ether layer thus formed was separated and the aqueous layer was extracted twice each time with 200 ml of ethyl acetate. The ether layer was combined with the ethyl acetate extract, and after washing the mixture with 300 ml of a saturated aqueous ammonium chloride solution and drying with anhydrous magnesium sulfate, ether and ethyl acetate were distilled off under reduced pressure. To the residue was added 10 ml of ethyl acetate to form crystals, which were collected by filtration to provide 13.7 g of (R)-4-benzyloxycarbonyl-2-azetidinone (43). In addition, the mother liquor was concentrated and purified by silica gel column chromatography (eluant:ethyl acetate-n-hexane (2:1)) to provide 5.1 g of the desired product. M.p. 136°-138° C.