Beyond Wittig Olefination: Phosphorus Ylide as a Ring-Expansion Reagent for Dibenzocycloheptanone Synthesis.

We report an unprecedented transformation between phenanthrene-9,10-diones and phosphorus ylide, where methylenetriphenylphosphane exhibits dual reactivity patterns, enabling both olefination and ring expansion to construct seven-membered carbocycles in moderate to good yields. This process proceeds through initial Wittig olefination followed by an unusual alkenyl group 1,2-migration pathway from either an oxaphosphetane or betaine intermediate. Mechanistic studies, including deuterium-labeling experiments and isolation of key intermediates, support a pathway involving conformationally controlled ring expansion.