[6π] Photocyclization to cis-Hexahydrocarbazol-4-ones: Substrate Modification, Mechanism, and Scope

Upon irradiation at lambda = 366 nm, tertiary N-alkoxycarbonyl-N-aryl-beta-enaminones furnished exclusively the trans-hexahydrocarbazol-4-ones by a conrotatory [6 pi] photocyclization but epimerized on silica to cis-hexahydrocarbazol-4-ones (14 examples, 44-98% yield). The acceptor substitution on the nitrogen atom enhanced the stability of the cyclized products compared to N-alkyl-N-aryl-beta-enaminones reported previously. The mechanism of the [6 pi] photocyclization was investigated by quenching experiments, deuterium-labeling experiments, and DFT calculations, suggesting a triplet pathway for the conrotatory ring closure followed by a suprafacial [1,4] hydrogen migration.