A versatile stereocontrolled total synthesis of thienamycin starting from L-aspartic acid is reported. Stereocontrol is achieved by potassium tri-<latex>$sec$</latex>-butylborohydride reduction of a thermodynamically formed 3<latex>$\alpha$</latex>-acetylazetidinone intermediate. The key [3. 2. 0] bicyclic ring system is prepared by a metal catalyzed carbene insertion reaction.