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

Upon irradiation at λ = 366
nm, tertiary N-alkoxycarbonyl-N-aryl-β-enaminones
furnished
exclusively the trans-hexahydrocarbazol-4-ones by
a conrotatory [6π] 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-β-enaminones reported previously. The mechanism
of the [6π] 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.

在λ = 366纳米的照射下，三级的N-烷氧羰基-N-芳基-β-内酰胺酮经[6π]光环化反应，仅生成对位的六氢碳唑-4-酮，但在硅胶上发生异构化生成顺式的六氢碳唑-4-酮（共14例，产率44%-98%）。相较于先前报道的N-烷基-N-芳基-β-内酰胺酮，氮原子上的受体取代增强了环化产物的稳定性。通过淬灭实验、氘标记实验和DFT计算对[6π]光环化反应机制进行了研究，表明其环闭过程遵循一个三重态路径，随后发生超面对位的[1,4]氢迁移。