Nitrosoallene-Mediated endo-Cyclizations for the Synthesis of (Hetero)cyclic α‑Substituted exo-Unsaturated Oximes

Nitrosoallene-mediated endo-dig cyclization reactions producing (hetero)­cyclic exo-unsaturated oximes (enoximes) are described. The intramolecular 1,4-type addition to in situ generated nitrosoallenes afforded α-substituted cyclic enoximes with exo-methylene units, which are the favored conformation for further cyclizations. The strong electron-withdrawing ability of the nitroso group facilitated the construction of five-to-seven-membered ring systems via C–O, C–N, C–S, and C–C bond formations, including a quaternary carbon center, at low temperatures.