Light-driven reaction overrides a 60-year-old rule of synthetic chemistry

A new light-driven method overrides this classic rule, allowing researchers to selectively build two different types of these double-ring structures from a single class of starting material (Nat. Chem. 2026, DOI: 10.1038/s41557-026-02238-y).

“[This rule] has been one of the greater dependable and predictive ones in organic chemistry to nearly 6 decades,” says Robert S. Paton, a chemist at Colorado State University and co-corresponding author of the study. Using computational modeling to understand the interaction’s underlying mechanism, the team discovered that tuning the starting material’s electronic characteristics makes this “long-standing constraint negotiable rather than absolute,” he says.

To kick off the interaction, the researchers relied on an open-chain precursor: an amide-tethered aza-1,5-diene featuring an embedded nitrogen backbone. When an iridium-based photocatalyst absorbs blue light-emitting diode (LED) light, it passes that energy to the molecule, triggering a standard photochemical ring closure.

The path the interaction takes from there depends heavily on what the chemists attached to that nitrogen. When the nitrogen holds an electron-rich group, such as a benzyl unit, the interaction behaves exactly as expected, dutifully obeying the rule of five. The diene's two double bonds initially connect to form a classic five-membered ring intermediate, which then zips shut to yield a standard bridged bicyclo[2.1.1]hexane scaffold—one of the highly prized 3D architectures.

The researchers initially expected that tweaking this nitrogen substituent would affect only this interaction’s yield, says Varinder K. Aggarwal, a chemist at the University of Bristol and co-corresponding author. Instead, Aggarwal says, they “were blown away to find that equipping it with an electron-withdrawing group [such as an acetyl group] completely changed the product’s identity.”

This simple electronic switch alters the energy hurdles that the molecule faces, lowering the barrier to the historically disfavored pathway. Rather than hitting a chemical dead end, the molecule successfully completes a strained ring closure to yield a fused bicyclo[2.2.0]hexane scaffold, an ultratight architecture previously considered nearly impossible to build from these simple chain precursors.

“[This] is a powerful light-driven strategy” that succeeds “by harnessing substrate electronics to steer competing interaction pathways,” says Dhevalapally B. Ramachary, an organic chemist at the University of Hyderabad who was not involved in the work. “The work transforms a challenging photochemical process into a versatile platform to constructing architecturally complex, highly strained, yet valuable molecules.”

The researchers demonstrated this advantage by using the new method to synthesize a 3D analog of HT-0712, an experimental memory-enhancing drug, successfully swapping a flat aromatic ring to the tightly wound [2.2.0] scaffold. This new architecture features a β-lactam-like core and built-in hydrogen-bonding capacity—features that can affect characteristics such as solubility and how drug molecules interact with biological targets.