Phosphine-Catalyzed [4 + 2] Annulation of γ‑Substituent Allenoates: Facile Access to Functionalized Spirocyclic Skeletons

The first phosphine-catalyzed [4 + 2] annulation of γ-substituted allenoates with 2-arylidene-1<i>H</i>-indene-1,3(2<i>H</i>)-diones is disclosed. In the reaction, the γ-substituted allenoate serves as a new type of 1,4-dipolar synthon; this broadens the application of γ-substituted allenoates. This method also offers a powerful approach to the construction of highly substituted spiro[4.5]dec-6-ene skeletons in excellent yields, and with complete regioselectivity and high diastereoselectivity