Scheme 5. Proposed Orbital Mechanism for the Stepwise Cyclization of 7c along the Triplet Pathway.

(i) Departure of a leaving group (L) and either photochemical excitation (hν) or triplet sensitization (T*) of a cyclopropyl precursor leads to conrotatory ring opening to form reactant 7a in its 1 ³B state; (ii) Bergman-like interrupted [2 + 2] cycloaddition of the alkyne termini of 7a forms the cyclic diradical 7c in its X³B₂ state; (iii) further intramolecular radical elimination to form bicyclic 7d in its X³A₁ state is spin-forbidden.