Synthesis of 5-Bromo-6-methylene-1,3-oxazinan-2-ones via Bromoheterocyclization of Allenyl Carbamates under Metal-Free Conditions

Allenyl carbamates have emerged as highly versatile building blocks in heterocyclic chemistry, serving as powerful precursors to structurally diverse nitrogen- and oxygen-containing rings. These substrates offer an efficient platform to access functionalized 1,3-oxazinan-2-ones, which represent privileged scaffolds widely found in bioactive natural products and modern pharmaceuticals. Herein, we report a novel, metal-free synthetic methodology for the efficient preparation of bromine-functionalized 1,3-oxazinan-2-ones via the electrophilic cyclization of these allenyl derivatives. This approach leverages the high reactivity and inherent selectivity of the allenic system to systematically target the oxazinanone core. While traditional syntheses of these complex heterocycles heavily rely on expensive and scarce transition-metal catalysts, this protocol successfully utilizes -bromosuccinimide (NBS) as a dual-purpose reagent, serving simultaneously as the bromine source and the electrophilic promoter. Operating under mild reaction conditions, this method completely avoids the necessity of metal purification steps. A major advantage of this protocol is its complete regioselectivity, affording a single isomer instead of the exo/endo mixtures typically produced via metal catalysis. Additionally, the reaction displays broad versatility, tolerating both electron-donating and electron-withdrawing groups on the aromatic ring. Consequently, this protocol offers an exceptionally simple, efficient, and environmentally friendly alternative for accessing highly functionalized heterocycles, providing a valuable, high-yielding strategy for modern organic synthesis and drug discovery.