An efficient and practical method of synthesizing 1β-methylcarbapenem, S-4661, was developed.(4R)-4-[(1S)-1-Methylallyl]-2-azetidinone 2, prepared from commercially available acetoxy-azetidinone 3, was converted to the β-keto ester 6 in four steps. The iodonium ylide 5 was prepared from 6, then cyclized to obtain the bicyclic β-keto ester 4. Finally, 4 was processed to the target product, S-4661, by conventional procedures.
