A highly efficient and mild palladium‐catalyzed, one‐pot, four‐step cascadeannulation has been developed to afford functionalized β‐ and γ‐lactones in moderate to good yields with high regio‐ and diastereoselectivities in ionic liquids. The employment of ionic liquids under mild reactionconditions makes this transformation green and practical. Especially, this reaction provided a novel and convenient