NEW MATERIAL:A compound shown by formula I [R1 is organic group reductively eliminable under acidic condition; R2 and R5 are H or carboxyl- protecting group; R3 is alkyl, OH, methoxy or acetoxy; R4 is ethyl, i-propyl or CH3CR6(OR7) (R6 is H or methyl; R7 is OH-protecting group)].

EXAMPLE: 2(S)-Benzyloxycarbonylamino-5-keto-6-t-butyloxycarbonyl-octanoic acid benzyl ester.

USE: An intermediate for producing carbapenam or carbapenem such as β-lactam antibiotics, etc., having excellent antibacterial action.

PREPARATION: A lithium enolate of a carboxylic acid ester shown by formula III is added to a pyrrolidonecarboxylic acid ester shown by formula II containing protected amino group and carboxyl group, which is subjected to ring formation to give a compound shown by formula I.