A single atom can determine whether a heterocycle is easy to build, difficult to modify, or inaccessible through conventional synthetic routes. In a new study published in Nature, researchers report a strategy that converts isoxazoles into pyrroles by replacing the oxygen atom in the isoxazole ring with carbon. The one-pot transformation offers a fundamentally different route to pyrroles, a class of nitrogen-containing rings found throughout medicinal chemistry, natural products and bioactive molecules. Rather than assembling the pyrrole framework from separate fragments, the method edits an existing ring skeleton, preserving much of the substrate’s architecture while changing its elemental identity.
Isoxazoles and pyrroles are deceptively similar. Both are five-membered aromatic heterocycles, and both contain nitrogen. The critical distinction is the atom adjacent to nitrogen: an isoxazole contains oxygen, whereas a pyrrole contains carbon in that position. This apparently modest difference produces major consequences for reactivity and synthesis. Isoxazoles possess an electronically organized ring system that can be accessed through several well-established bond-forming disconnections. Pyrroles, by contrast, often require carefully coordinated multistep procedures to establish the correct carbon–carbon and carbon–nitrogen connectivity, especially when the desired substitution pattern is unusual.
The researchers’ approach belongs to the rapidly developing field of skeletal editing, in which the framework of an existing molecule is reorganized rather than simply decorated with new substituents. Traditional retrosynthesis usually breaks a target molecule into fragments and then proposes reactions to reconnect them. Skeletal editing takes a different view: a functional group or ring can serve as a molecular platform that is transformed into another scaffold. In this case, the isoxazole is not treated as a final structure but as a programmable precursor to a pyrrole. The O-to-C replacement therefore creates a retrosynthetic pathway that conventional pyrrole synthesis does not normally provide.
Central to the reaction is an N-propargylic enaminone, an intermediate that links the two heterocyclic systems. Enaminones contain an amino-substituted alkene conjugated to a carbonyl group, giving them a combination of nucleophilic and electrophilic properties. The propargylic substituent introduces an alkyne-containing carbon framework that can participate in the reorganization required to construct the pyrrole ring. According to the study, formation and subsequent transformation of this intermediate allows the original isoxazole connectivity to be redirected toward the carbon-based architecture of a pyrrole.
The process is conducted as a one-pot sequence, meaning that the intermediate does not need to be isolated before the next stage. This feature is important both practically and conceptually. Isolating unstable or highly reactive intermediates can reduce overall yield, require additional purification and complicate scale-up. By linking the steps directly, the researchers create a continuous pathway from the isoxazole starting material to the pyrrole product. The strategy also demonstrates how a reaction sequence can exploit temporary changes in electronic structure before restoring aromaticity in the final heterocycle.
The discovery was not entirely straightforward. During their investigations, the researchers observed unexpected reactivity in the enaminone intermediates. Instead of behaving uniformly, these compounds could follow different reaction outcomes depending on their conformational characteristics. Molecular conformation—the three-dimensional arrangement adopted by a molecule through rotation around single bonds—can determine which atoms are positioned favorably for bond formation, cyclization or rearrangement. In systems containing competing reactive sites, even small conformational preferences may decide whether the desired skeletal edit occurs or whether an alternative pathway dominates.
To understand and predict this behavior, the team developed a computational model focused on the conformational features controlling reaction outcomes. Such a model can be valuable because enaminone reactivity is not dictated only by obvious electronic effects. Steric interactions, torsional preferences and the relative orientation of the alkyne, carbonyl and nitrogen-containing portions of the intermediate can all influence the transition state—the high-energy molecular arrangement through which a reaction proceeds. By connecting calculated conformational properties with experimental results, the researchers sought to turn an initially surprising observation into a predictive element of the synthetic method.
That predictive capability is particularly relevant for regioselectivity. A molecule may contain several positions at which new bonds could theoretically form, but only one arrangement may produce the desired pyrrole substitution pattern. Regioselective control is often one of the greatest challenges in heterocycle synthesis because small changes in the starting material can redirect a reaction toward constitutional isomers, compounds with the same atoms but different connectivity. The reported approach links substrate structure, enaminone conformation and reaction outcome, providing a framework for anticipating which pyrrole is likely to emerge from a given isoxazole.
The significance of the work extends beyond the preparation of a single class of compounds. Pyrroles appear in numerous pharmaceutical candidates and biologically active molecules, but their synthesis can become increasingly difficult as more substituents are added or as specific positions on the ring must be controlled. An isoxazole-to-pyrrole conversion could allow chemists to begin with an isoxazole scaffold that is easier to prepare and then apply the skeletal edit at a later stage. This could expand the range of pyrrole architectures available for drug discovery, where libraries of structurally varied molecules are routinely needed to explore biological activity.
The study also illustrates why skeletal editing has become an influential idea in modern organic chemistry. Replacing one atom within an established ring challenges the assumption that complex molecules must be built through the same disconnections used in textbook synthesis. In the reported case, oxygen is removed from the isoxazole framework and the ring is reorganized so that carbon occupies its place, producing a pyrrole in a single connected sequence. By combining unexpected enaminone chemistry with computational analysis of molecular shape, Bracken, Lawrie, Romita and colleagues have created a route that joins two heterocyclic worlds. The result is not simply a new reaction, but a change in how chemists can plan the synthesis of elusive pyrroles: instead of constructing the ring from scratch, they can edit a related structure that is already within reach.
Subject of Research: O-to-C skeletal editing of isoxazoles to synthesize pyrroles.
Article Title: Synthesis of pyrroles from isoxazoles by an O-to-C skeletal edit.
Article References: Bracken, A.J., Lawrie, A.P., Romita, I.F. et al. Synthesis of pyrroles from isoxazoles by an O-to-C skeletal edit. Nature (2026). https://doi.org/10.1038/s41586-026-10933-6
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41586-026-10933-6
Keywords: skeletal editing, isoxazoles, pyrroles, heterocyclic chemistry, enaminones, O-to-C atom replacement, organic synthesis, regioselectivity, computational chemistry, medicinal chemistry
