An efficient gold-catalyzed cascade cyclization of alkenyl diynes involving alkyne oxidation, carbene-alkyne metathesis and cyclopropanation has been developed, furnishing a series of tetracyclic γ-lactams bearing one quaternary carbon center and one tertiary carbon center in moderate to good yields with excellent diastereoselectivities. In addition, the proposed mechanistic rationale for this oxidative cyclization is well supported by theoretical calculations.