A chiral intermediate bearing the 1-β-methyl moiety (14a) for 1-β-methylcarbapenem antibiotics (1) was prepared by stereoselective aldol-type reaction between 4-acetoxyazetidinone 3a and the dianion (or corresponding bissilyl enol ether) of a 3-oxopentanoate 12 or 13 in the presence of a metal salt. A major side reaction has also been elucidated.