PURPOSE: To prepare the titled compound useful as an intermediate of thienamycin, etc., in high stereo-selectivity, by reacting 3-(substituted)-alkyl-4-acyloxyazetidin-2-one with an α-diazoacetoacetic acid ester in the presence of a base.

CONSTITUTION: The objective compound of formula II (R2 is protecting group) is prepared by reacting the comound of formula I (R1 is substituted or non-substituted lower alkyl; R8 is acyl) with an α-diazoacetoacetic acid ester in the presence of a base such as lithium hexamethyl disilazide. The trans-type compound of formula II having the same steric confugration as natural thienamycin, can be prepared by this process from the compound of formula I wherein the configuration of the 3- and the 4-positions may be cis or trans. The compound of formula II can be converted easily, in high yield, to the compound having 1-carba-2-penam skeleton, from which the thienamycin of formula III (R3 is protecting group) useful as an antibacterial agent, etc. can be synthesized in high stereo-selectivity, in a short step.