Synthesis of J-111, 347 (1), a new 1β-methylcarbapenem with broad-spectrum antibacterial activity including that against methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa, was achieved via diastereoselective preparationo f a side-chain thiol 3 from an optically active (R)-3, 4-dihydroxybutanal 4.