Programmable regiodivergent light-driven cyclization of acyclic 1,5-dienes unlocks rigid bicyclic architectures.

Controlling regioselectivity in radical cyclizations remains a major challenge in synthetic chemistry, hindering the efficient assembly of strained, rigid bicyclic architectures from simple acyclic precursors. Here we show that a visible-light-mediated intramolecular [2 + 2] photocycloaddition of aza-1,5-dienes can successfully overcome the classical 'rule-of-five' selectivity governing radical cyclizations. By tuning the electronic properties of an easily removable amide N-substituent, we reprogram the initial energy-transfer-driven cyclization event, diverging from the usually kinetically favoured 5-exo-trig to the 6-endo-trig pathway. As a result, closely related substrates can be selectively directed to generate either bridged bicyclo[2.1.1] or vastly underexplored fused bicyclo[2.2.0] architectures, both of which offer rich downstream derivatization potential. Furthermore, an extensive computational study of this transformation revealed how the intricate interplay of electronic effects controls reaction regioselectivity. Overall, this work establishes a programmable, light-driven cyclization strategy that enables precise and selective construction of distinct bicyclic frameworks within a unified reaction manifold.