Phosphine-Catalyzed Chemo- and Diastereoselective [2 + 2 + 2] and [3 + 2] Annulations of γ‑Methyl Allenoates with Doubly Activated Olefins: Syntheses of Highly Substituted Cyclohexanes and Cyclopentenes

Phosphine-catalyzed chemoselective [2 + 2 + 2] and [3 + 2] annulations of γ-methyl allenoates with doubly activated olefins have been developed, which afford highly substituted cyclohexanes bearing five continuous stereogenic centers and cyclopentenes bearing three continuous stereogenic centers, respectively, in generally high yields with excellent diastereoselectivity. The [2 + 2 + 2] annulation represents an unprecedented reactivity pattern of γ-methyl allenoates with activated CC bonds to access six-membered carbocycles. In addition, the study herein also evidences that the acidic protic additives such as benzoic acid can exert influence on chemoselectivity of phosphine-catalyzed annulation reactions involving allenoates.