A Simple, highly diastereoselective synthesis of the key 1 beta-methylcarbapenem intermediate 1 has been accomplished via a palladium catalyzed deallyloxycarbonylation of diallyl malonate derivative 4a which was readily prepared from (3S,4R)-4-acetoxy-3-[(R)-1-(t-butyldimethylsilyloxy)ethyl]-2-azetidimone 2 in two steps.