Azirine‐2 <i>H</i> ‐Carbaldehydes as a Versatile Building Block: Ring Expansion into Structurally Diverse <i>N‐</i> and <i>O</i> ‐ Heterocycles

Abstract Azirine‐2 H ‐carbaldehydes have garnered significant attention as a powerful bifunctional building block for the construction of structurally diverse nitrogen‐ and oxygen‐ containing heterocyclic frameworks. The presence of a highly strained three‐membered framework, a nucleophilic nitrogen atom, and an electrophilic formyl moiety imparts significant reactivity to this building block, enabling more than 50 transformations that lead to over 10 classes of heterocycles. These features facilitate an array of regioselective and chemoselective annulation reactions, primarily through ring‐expansion pathways. This work presents an overview of all the major contributions towards the synthesis of various N ‐ and O ‐heterocycles—including oxazoles, isoxazoles, pyrazines, pyrimidines, oxadiazines, etc. using azirine‐2 H ‐aldehydes as a template. Special emphasis has been given to reaction mechanisms so that this effort catalyzes the further advancement in this field. The strategic utilization of azirine‐2 H ‐carbaldehydes not only offers streamlined access to complex heterocyclic motifs but also holds great promise for applications in medicinal chemistry, agrochemicals, and material science.