Synthesis of Spirocyclic 5,6‐Dihydro‐4H‐1,2‐oxazines via [4+2] Cycloaddition of Methylideneimidazolones With Nitroso Alkenes Generated Under Diffusion Mixing Conditions

A simple and effective method for the generation of nitroso alkenes from α‐halo oximes using the technique of diffusion mixing with a volatile reagent (DIPEA) had been proposed. The formed nitroso alkenes add regioselectively to 5‐methylidenehydantoins with the formation of spiro‐5,6‐dihydro‐4H‐1,2‐oxazine‐hydantoins; the similar reaction with 5‐methylidene‐2‐thio‐ or 2‐selenohydantoin as dienophiles resulted in the same spiro‐dihydroisoxazine‐imidazolone with desulfurization or deselenation of the substrates.