The synthesis of 4/5/6polycyclicβ-lactams by enyne metathesis and Diels−Alder reactions is described. Compared to the synthesis of 4/6/6 and 4/7/6 polycyclicβ-lactams previously reported by our laboratory, formation of the strained 4/5/6 compounds requires alternative reaction conditions. Indeed, the synthesis of the 4/5 bicyclic diene 12 was more difficult than those of the 4/6 (13) and 4/7 (14)