NEW MATERIAL:A compound shown by the formula I (R1 and R2 are lower alkyl, etc.; R3 is H, lower alkyl, etc.; R4 is H or trimethylsilyl; R5 is H, alkali metal, etc.).

EXAMPLE: 1,1-Dimethyl-2-hydroxy-1-carbapenam-3-carboxylic acid benzyl ester.

USE: An intermediate for synthesizing a thienamycin type antibiotic. Also having antibacterial activiey by itself.

PROCESS: 4-(1,1-Substituted-2-propenyl) azetidine-2-one is reacted with an acetic ester, and oxidized to give an aldehyde derivative shown by the formula II. This compound is reacted with a Grignard reagent, to give a compound shown by the formula III, which is subjected to intramolecular aldol reaction in the presence of a base, and, if necessary, the ester part is removed or converted, to give a compound shown by the formula I.