313 μl of a 0.75 M solution of potassium tert-butoxide in tert-butanol are added at -30° C. to a solution of 107 mg (0.23 mmol) of p-nitrobenzyl 5-azido-2-(4-chloro-2-oxo-1-azetidinyl)-4,4-dimethyl-3-oxopentanoate in 4.5 ml of dry tetrahydrofuran and the solution is stirred at -30° C. during the course of 30 minutes. The reaction mixture is then diluted with 20 ml of ethyl acetate, and the mixture is washed with 10 ml of water and 10 ml of NaCl solution. The aqueous phases are extracted using 10 ml of ethyl acetate and the combined organic phases are dried over MgSO4, filtered and then evaporated in vacuo. In this way, 105 mg of a yellow residue are obtained which is crystallized from methylene chloride/ diisopropyl ether. Yield 61 mg of melting point 81°-82° C. IR spectrum in CH2 --Cl2 : 2950, 2860, 2110, 1810, 1720, 1590, 1530, 1370, 1320, 1085, 1020 cm-1.
