Rh-Catalyzed Cycloaddition Cascade of Allenynes and Maleimides: A Powerful Strategy for Constructing Complex Pentacyclic Structures with a Bicyclo[2.2.2]octene Core

Herein, we report that 1,5- and 1,6-allenynes react with two equivalents of maleimide to afford pentacyclic frameworks featuring a bicyclo[2.2.2]octene core in a fully diastereoselective fashion. DFT calculations and deuterium-labeling studies reveal an unconventional mechanism initiated by a noncanonical [2 + 2 + 2] cycloaddition that, through intramolecular hydrogen shifts, diverges from the classical pathway to generate a conjugated diene. A subsequent thermal Diels-Alder reaction with a second maleimide completes the cascade, unveiling a distinct reactivity mode.