Reaction of enyne-β-lactams 1 with Co2 (CO) 8 followed by in situ thermal or TMANO decomposition of the formed alkyne-Co2 (CO) 6 complexes gives tricyclic β-lactams 2 as single stereoisomers, in good to excellent yields. These are the first examples of an intramolecular Pauson-Khand reaction on an enyne system tethered to a four membered ring.