Flat aromatic rings such as benzene have dominated the pharmaceutical landscape for more than 200 years. But chemists increasingly want to replace them with rigid, 3D molecular structures that can better fit biological targets and resist breaking down in the body. Building these molecules with tightly wound double rings, however, means fighting synthetic chemistry’s “rule of five”: try forcing a chain-like molecule to snap shut into a six-membered intermediate, and the reaction almost always defaults to an easier-to-form five-membered shape.
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 more dependable and predictive ones in organic chemistry for 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 reaction’s underlying mechanism, the team discovered that tuning the starting material’s electronic properties makes this “long-standing constraint negotiable rather than absolute,” he says.
To kick off the reaction, 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 reaction 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 reaction 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 influence only this reaction’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 for 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 reaction 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 for 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 for 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 influence properties such as solubility and how drug molecules interact with biological targets.
