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.

The review summarizes recent advances in the synthesis of C2‐halogenated nitrogen heterocycles via nucleophilic halogenation of corresponding N‐oxides as well as application of this methodology in the synthesis of pharmaceuticals. Both scientific and patent literature is reviewed mostly over the period of the last ten years. The review contains a practical guide (typical procedures and infographic based on extensive literature analysis), which can help practitioners in choosing optimal reagents/conditions for nucleophilic halogenation of N‐heterocycles of different types. The bibliography contains 199 references.

Reaction of six-membered cyclic nitronates with disubstituted ketenes affords hitherto unknown saturated oxazolo­[3,2-b]­[1,2]­oxazines possessing up to four contiguous stereogenic centers. The process involves a tandem of [3+2]-cycloaddition across the CO bond of ketene, followed by a spontaneous [1,3]-rearrangement of transient vinylidene-substituted bicyclic nitrosoacetals. DFT calculations of the mechanism suggest that the [1,3]-O,C-shift proceeds through a recyclization of a biradical intermediate formed by an unusually mild homolytic cleavage of the N–O bond. The resulting products can be utilized as precursors of other fused 1,2-oxazines derivatives, in particular 1,2-oxazino-1,2,4-triazin-3-ones.