Researchers develop mild skeletal editing method for medicinal chemistry

Researchers at Ritsumeikan University report a halogen-guided method for transforming accessible hydroxycoumarins into coumaranones, valuable heterocyclic structures in medicinal chemistry. Inspired by halogenation-driven bond cleavage in natural product biosynthesis, the method deletes a carbonyl group through simultaneous C–C and C–O bond cleavage under room temperature without the need for transition metals. The approach offers a practical strategy for molecular scaffold editing and potentially streamlining synthesis of biologically relevant compounds.

Restructuring a molecule without rebuilding it from scratch is an increasingly important goal in modern organic chemistry. Skeletal editing is an approach that helps chemists explore new chemical structures and simplify the synthesis of molecules with potential pharmaceutical applications. But in case of functional groups such as esters, skeletal editing remains difficult because their carbon–carbon and carbon–oxygen bonds are resistant to cleavage under mild conditions.

Now, a research team led by Professor Toshifumi Dohi of Ritsumeikan University, along with Mr. Yusuke Yoto also of Ritsumeikan University, and Dr. Hideyasu China of Doshisha Women's College of Liberal Arts, developed a unique solution inspired by nature. Their study, made available online in the JACS Au journal on July 26, 2026, demonstrates that introducing chlorine into hydroxycoumarins can trigger a sequence of bond-cleavage and bond-forming events that removes a carbonyl group and reconstructs the molecule as a coumaranone.

"We aimed to develop a new way of editing molecular skeletons for esters, one that could break difficult bonds under mild conditions and immediately reconstruct the molecule into a useful framework," says Prof. Dohi.

The idea started from the team's interest in a "cut-to-fuse" strategy. In this concept, halogenation first "cuts" bonds in a cyclic compound, generating a reactive chain, before a subsequent intramolecular reaction "fuses" the chain into a new heterocyclic structure. The researchers projected that a similar process might enable carbonyl deletion-the net removal of a carbonyl unit from hydroxycoumarins.

The initial experiments produced an unexpected result. The team had been investigating fluorine-induced carbon–carbon bond cleavage, but fluorination caused the hydroxycoumarin to fragment into separate products.

"Chlorine changed the reaction pathway completely," says Prof. Dohi. Treating a hydroxycoumarin with N-chlorosuccinimide (NCS) led to formation of a chlorinated intermediate that underwent decarbonylative reconstruction, ultimately producing a coumaranone rather than fragmenting the molecule.

The researchers then optimized the reaction and found that the transformation could proceed at room temperature in near-neutral conditions, without transition-metal catalysis. Under the optimized conditions, hydroxycoumarin was treated with NCS, water, and sodium acetate in ethyl acetate, followed by potassium phosphate. The method produced the model coumaranone in more than 99% yield. According to the researchers, this represents the mildest nonenzymatic conditions reported to date for simultaneous cleavage of the C–C and C–O bonds involved in this type of carbonyl deletion.

This reaction also proved broadly applicable. Hydroxycoumarins containing methoxy, halogen, azide, phenol, carboxylic acid, and boron-containing functionalities were tolerated, as were substrates bearing substituted aromatic rings, naphthalene, pyridine, thiophene, furan, and aliphatic groups. Several products were obtained in good to excellent yields, demonstrating that the method can accommodate considerable structural diversity. A related cyclic β-keto ester also underwent reconstruction, showing that the chemistry is not limited to a single substrate class.

Mechanistic experiments highlighted the importance of selective chlorination. When the chlorinating reagent was omitted, the starting material was recovered unchanged. Stepwise experiments showed that chlorination occurred first, followed by decarboxylation and intramolecular cyclization. The team also demonstrated the method's practical potential. On a gram scale, the model reaction produced the desired coumaranone in 91% yield. The resulting scaffold could then be further modified, including conversion to a benzofuran, introduction of a quaternary carbon center, and transition-metal-catalyzed coupling reactions. In this case, a coumaranone bearing a boron pinacol ester was useful as being directly applicable for palladium-catalyzed coupling without isolation.

Taking inspiration from halogenation-driven transformations found in natural product biosynthesis, researchers developed a new way to rethink carbonyl deletion and molecular scaffold construction. Their "cut-to-fuse" strategy provides an efficient route from hydrocoumarins to coumaranones while avoiding the harsh conditions usually required for ester bond cleavage. This study proves to be useful for future approaches in medicinal chemistry, where streamlined molecular editing is increasingly valuable for rapidly generating structurally diverse compounds.