To obtain an intermediate compound containing p-nitrobenzyl ester- protecting group useful for synthesizing thienamycin and other carbapenem antibiotics.

This carbapenem synthetic intermediate is shown by formula I (R4 is H or a hydroxyl-protecting group) such as (3R,4R)-3-{(1R)1[(2,4,6-tri-tert- butylphenoxy)dimethylsilyloxy]ethyl}-4-[3-(4nitrobenzyloxy)carbonyl-2-oxo-3- diazopropyl]azetidin-2-one. The compound of formula I is obtained by reacting a compound of formula II (L is an eliminable group; R4' is a hydroxyl-protecting group) with an enol silyl ether of formula III (R1, R2 and R3 are each irrelevantly a 1-4C alkyl) in the presence of a Lewis acid (e.g. ZnCl2) in an inert organic solvent (e.g. methylene chloride).