Nucleophilic Halogenation of Cyclic Nitronates: A General Access to 3‑Halo-1,2-Oxazines

In this article, comprehensive studies
on the nucleophilic chlorination
and bromination of readily available six-membered cyclic nitronates
(1,2-oxazine-N-oxides) are reported. Under optimized
conditions (POCl3 or (COBr)2 with Hünig’s
base), 3-halo-substituted 1,2-oxazines, which are difficult to access
by other routes, were obtained in good to high yields. The latter
were shown to be convenient precursors to other 3-substituted 1,2-oxazine
derivatives using Lewis/Brønsted acid-assisted substitution of
the halide atom for C-, S-, and N-nucleophiles.