there is disclosed an azetidinone compound of the formula : wherein r is hydrogen , or a hydroxy protecting group , r 1 and r 2 are each independently alkyl of 1 - 15 carbon atoms , benzyl or cyclized together with the carbon atom to which they are attached to form a 5 or 6 - membered cyclic hydrocarbon or a heterocyclic radical having one or two hetero ring atoms , said hetero ring atoms being selected from the group consisting of o and s ; r 3 is lower alkyl or — coo r 4 is phenyl , or phenyl substituted with halogen , lower alkoxy or nitro which is useful as a synthetic intermediate to the 1 ′ β - methylcarbapenem - type antibacterial agent .