Modular Construction of Cyclic and Linear Quaternary Carbon Centers through Visible‐Light‐Induced Triplet Nitrene Transfer Reaction

All‐carbon quaternary centers represent a fundamental structural unit found in a wide range of functional molecules. Within the versatile synthetic toolbox, the semipinacol rearrangement (SPR) stands as one of the most powerful tools for assembling this sterically congested motif. In this context, ‌radical‐mediated SPR pathways‌, especially those entailing alkyl migrations, remain underexplored from a strategic standpoint. Herein, we report a visible light‐promoted formal SPR of tertiary allyl alcohols using in situ‐generated triplet nitrene species as the initiator under redox‐neutral conditions. This method demonstrates broad substrate scope, enabling efficient construction of all‐carbon quaternary centers across various steric environments, including rigid cyclic/spiro frameworks and flexible acyclic systems. Mechanistic investigations reveal substrate‐dependent pathways: alkenyl cyclobutanols undergo triplet nitrene addition followed by concurrent ring‐expansion and hydrogen atom transfer (HAT) to form cyclic ketones, while nonactivated allyl alcohols proceed through aziridination/ring‐opening/SPR sequences to produce linear β‐amino ketones. Preliminary biological assessments of these α‐quaternary β‐aminoketone derivatives revealed that compound 4a exhibited potent anti‐DENV‐2 activity, with an EC 50 value of 0.95 ± 0.02 µM against viral replication.