Optically pure 4-acetoxy-3-[1-(t-butyldimethylsilyloxy)ethyl]-2- azetidinone(), which is a highly versatile intermediate for the synthesis of thienamycin() and other biologically active [β-lactam analogs, was synthesized from 4-carboxy-3-[1-(t-butyldimethylsilyloxy)ethyl]-2-azetidinone() by Kolbe-type electrolysis.