Energy-Transfer-Enabled Skeletal Rearrangement of Cyclic Ketones into Strained Cyclopropane-Fused Medium Rings.

Cyclopropane-fused medium rings (CFMR) are attractive structural motifs that combine high strain with unique three dimensionality, however their efficient and modular synthesis remains a formidable challenge. Herein, we present a general photocatalytic skeletal editing strategy that enables a direct topological leap-a single-step reorganization that simultaneously alters both ring size and fusion topology-from readily available five- to eight-membered cyclic ketones into diverse seven- to ten-membered cyclopropane-fused medium rings. This transformation proceeds through a novel energy-transfer-induced, diradical-mediated 1,4-carbonyl migration, orchestrating a "ring expansion-collapse" cascade to forge the strained bicyclic frameworks straightforward, which is supported by DFT calculations. This strategy features broad substrate scope, excellent functional-group compatibility, and high efficiency, enabling the late-stage diversification of complex molecules and exploration of CFMR chemical space that was previously inaccessible. Moreover, integration of this strategy with further skeletal modification enables rapid construction of versatile [n.3.0] bicycles.