Sequential 1,4-/1,2-Addition of Lithium(trimethylsilyl)diazomethane onto Cyclic Enones to Induce C−C Fragmentation and N−Li Insertion

α,β‐Unsaturated ketones generally undergo addition reactions with nucleophiles with a preference for either 1,2‐ or 1,4‐addition, but rarely both. However, the right combination of reagents allows for consecutive 1,4‐ and 1,2‐additions to occur: Cyclic α,β‐unsaturated ketones undergo double additions with lithium(trimethylsilyl)diazomethane, effectively generating various molecular frameworks with complexity and diversity. Owing to the sequential generation of several intermediates of multifaceted reactivity, including diazoalkane derivatives and alkylidene carbenes, it is possible to induce novel Grob‐type C−C fragmentations, alkylidene carbene mediated Li−N insertions, and dipolar cycloadditions by controlling the reaction parameters. The sequential 1,4‐/1,2‐addition of lithium(trimethylsilyl)diazomethane to cyclic α,β‐unsaturated ketones induces various subsequent transformations, such as Grob‐type C−C fragmentation or alkylidene carbene mediated Li−N insertion, depending on the ring size and the position of the double bond. Various nitrogen heterocycles were thus obtained in good yields.