Skeletal editing involves making specific point‐changes to the core of a molecule through the selective insertion, deletion or exchange of atoms. It thus represents a potentially powerful strategy for the step‐economic modification of complex substrates and is a perfect complement to methods such as C−H functionalization that target the molecular periphery. Given their ubiquity in biologically active compounds, the ability to perform skeletal editing on – and therefore interconvert between – aromatic heterocycles is especially valuable. This review summarizes both recent and key historical examples of skeletal editing as applied to interconversion of aromatic rings; we anticipate that it will serve to highlight not only the innovative and enabling nature of current skeletal editing methods, but also the tremendous opportunities that still exist in the field.

We demonstrate that arylchlorodiazirines serve as photo‐activated halocarbene precursors for the selective one‐carbon ring expansion of N‐substituted pyrroles and indoles to the corresponding pyridinium and quinolinium salts. Preliminary investigations indicate that the same strategy also enables the conversion of N‐substituted pyrazoles to pyrimidinium salts. The N‐substituent of the substrate plays an essential role in: (1) increasing substrate scope by preventing product degradation, (2) enhancing yields by suppressing co‐product inhibition, and (3) activating the azinium products towards subsequent synthetic manipulations. This latter point is illustrated by subjecting the quinolinium salts to four complementary partial reductions, which provide concise access to ring‐expanded products with different degrees of increased C(sp3) character. Thermal analysis of the diazirines by differential scanning calorimetry (DSC) provides detailed insight into their energetic properties, and highlights the safety benefits of photolyzing—rather than thermolyzing—these reagents.

We demonstrate that arylchlorodiazirines serve as photo-activated halocarbene precursors for the selective one-carbon ring expansion of N-substituted pyrroles and indoles to the corresponding pyridinium and quinolinium salts. Preliminary investigations indicate that the same strategy also enables the conversion of N-substituted pyrazoles to pyrimidinium salts. The Nsubstituent of the substrate plays an essential role in: (1) increasing substrate scope by preventing product degradation, (2) enhancing yields by suppressing co-product inhibition, and (3) activating the azinium products towards subsequent synthetic manipulations. This latter point is illustrated by subjecting the quinolinium salts to four complementary partial reductions, which provide concise access to ring-expanded products with different degrees of increased C(sp 3 ) character. Thermal analysis of the diazirines by differential scanning calorimetry (DSC) provides detailed insight into their energetic properties, and highlights the safety benefits of photolyzing-rather than thermolyzing-these reagents.