We synthesized useful intermediates 5 and 6 for 1β- and 1α-methylcarbapenems from 4-carboxy-3-[(R)-1-hydroxyethyl]-2-azetidinone 4-as a starting material by using stereoselective hydrogenation and hydroboration, respectively. A practical synthetic route from 4 to the (3S, 4S)-4-[(R)-1-carboxyethyl]-3-[(R)-1-hydroxyethyl]-2-azetidinone derivative 1, a useful intermediate for the synthesis of 1β-metylcarbapenem antibiotics, was established.