Strain-Induced Benzannulation of Oxaquadricyclanes Promoted by a Polyoxometalate Photocatalyst.

Given the significance of strain-release reactions, various spring-loaded molecules have emerged as important building blocks in diverse fields of chemical science. Despite progress in this area, oxaquadricyclanes remain an understudied class of molecules. In the present report, the strain energy of in situ generated oxaquadricyclane was utilized for benzannulation reaction, which was photocatalyzed by a polyoxometalate (POM)-based photocatalyst ({Mo₇O24@γ-FeO(OH)}), in which the Anderson-type polyoxomolybdate [Mo₇O₂₄]⁶⁻ anions enhance the stability of photoactive γ-FeO(OH) nanocore. The band alignment of this POM-based complete inorganic semiconductor enables the harvesting of photons from visible light and provides superior thermal and oxidative stability while addressing the cost and performance limitations of conventional organo-photocatalysts. Using this novel photocatalyst, we successfully synthesized benzindene and dihydrophenanthrene scaffolds from dienyl sulfoxonium ylides and electron-deficient alkynes through unusual photochemical rearrangements. These benzannulated compounds are notable for their structural similarity to bioactive aromatic steroids and demonstrate their potential as lead candidates for therapeutic development. Through systematic optimization and mechanistic studies, we highlight the critical role of the POM catalyst, the challenges of controlling strain energy-driven reactivity, and the selective rearrangement of oxaquadricyclane scaffolds.