A Tandem Photocycloaddition—Ring Expansion Strategy for the Synthesis of Fused [5.3.0] Triketone

A tandem synthetic sequence involving photo-induced intramolecular [2+2] cycloaddition followed by acid-promoted ring expansion was developed to access the novel bicyclic triketone framework. The process begins with the UV (254 nm) irradiation of cyclic vinylogous ester, affording a highly strained cyclobutane-fused diketone in an 86% yield. This unique intermediate feature is of a fused four- and six-membered ring system with spatially compressed carbonyl groups. Upon acidic hydrolysis in aqueous MeCN, the strained system undergoes retro-aldol ring expansion, delivering [5.3.0] bicyclic triketones bearing a seven- and five-membered fused ring with three strategically oriented carbonyl units in a 75% yield. Structural elucidation was performed using NMR spectroscopy, UV-Vis, HRMS, and single-crystal X-ray crystallography. The method highlights a concise route for constructing a fused bicyclic triketone of relevance to synthetic and medicinal chemistry.