W(OTf)6‐Catalyzed Synthesis of γ‐Lactones by Ring Contraction of Macrolides or Ring Closing of Terminal Hydroxyfatty Acids in Ionic Liquid

γ‐Lactones are an important class of fine chemical products and are widely used in perfumes, medicines, pesticides, dyes, and other fields. Herein, a new method for γ‐lactones preparation based on ring contraction was developed. Starting from macrolides, W(OTf)6 was used to catalyze the ring‐opening polymerization then depolymerization. The depolymerization step was not a common ring‐closing process, and the carbon number of the ring was reduced one by one by rearrangement to form the most thermodynamically stable five‐membered ring compounds. γ‐Caprolactone (180 °C for 10 h) was obtained in a yield of 94 % when [EMIM]OTf was used as the solvent, and the yield of isolated product was up to 85 %. The interaction of various components and the reaction mechanism were studied by FTIR spectroscopy and 1H NMR spectroscopy, respectively. Furthermore, γ‐lactones could be produced when the substrate was extended to terminal hydroxyfatty acids. Unexpectedly, the catalyst was poisoned by 1 equivalent of H2O added during the process and thus the yield decreased greatly. The reaction is green and simple, and proceeds in one pot with high atom economy (100 % for macrolides and with water as the only byproduct for terminal hydroxyfatty acid), which provides a promising approach to synthesizing γ‐lactones. Fatty acids slim down: A new method for γ‐lactone preparation based on ring contraction has been developed. Starting from macrolides, W(OTf)6 was used to catalyze ring‐opening polymerization then depolymerization to produce the stable five‐membered γ‐lactones in the ionic liquid [EMIM]OTf. γ‐Lactones were also produced when the substrate was extended to terminal hydroxyfatty acids.