Synthesis of gem -dibromo 1,3-oxazines by NBS-mediated electrophilic cyclization of propargylic amides.

We present a highly efficient and practical method for synthesizing gem -dibromo 1,3-oxazines through 6- endo-dig cyclization of propargylic amides, using N -bromosuccinimide (NBS) as an electrophilic source. The metal-free reaction can be conducted under mild conditions with good functional group compatibility, delivering excellent yields of the desired products. Mechanistic studies suggest that the reaction proceeds via a double electrophilic attack by NBS on the propargylic amide substrate.