5.3 g of 2-acetylthio-1,3-bis(p-nitrobenzyloxycarbonylamino)propane in a mixture of 161 ml of methanol and 40 ml of tetrahydrofuran were cooled to -20° C. To the solution were added 9.95 ml of a methanolic solution of sodium methoxide containing 1 mM of sodium methoxide per 1 ml of methanol. The mixture was stirred at -10° to -15° C. for 30 minutes, after which it was mixed with 0.961 ml of carbon disulphide and stirred at -20° C. for 15 minutes. Then 20 ml of a methanolic solution containing 3.59 g of 4-acetoxy-3-[1-(R)-t-butyldimethylsilyloxyethyl)-2-azetidinone was added to the above mixture and stirred at -20° C. for 1 hour. The mixture was cooled to -50° C. and then poured into a mixture of 20 ml of acetic acid and 200 ml of water. The mixture was extracted with ethyl acetate. The extract was washed with a saturated aqueous solution of sodium chloride and dried over anhydrous sodium sulphate. The solvent was distilled off and the residue was subjected to medium pressure column chromatography througn silica gel, eluted with 3:1-2:1 by volume mixtures of benzene and ethyl acetate, to give 6.1 g of the desired compound.
