4-Acetoxy-2-azetidinone was synthesized from 4-carboxy-2-azetidinone by Kolbe-type electrolysis. Optically pure 4-acetoxy-3-[1-(t-butyldimethylsiloxy)ethyl]-2-azetidinone, which is an important intermediate for the synthesis of thienamycin, and (+)-PS-5 were synthesized by use of this method.