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

Area of Science:

• Organic Chemistry
• Photochemistry
• Synthetic Methodology

Background:

• β-enaminones are versatile synthetic intermediates.
• Photocyclization reactions offer unique pathways for ring formation.
• Understanding reaction mechanisms is crucial for synthetic development.

Purpose of the Study:

• To investigate the photocyclization of tertiary N-alkoxycarbonyl-N-aryl-β-enaminones.
• To explore the stereochemical outcome and stability of the cyclized products.
• To elucidate the mechanism of the [6π] photocyclization reaction.

Main Methods:

• Photochemical irradiation at λ = 366 nm.
• Synthesis of 14 examples of β-enaminones.
• Stereochemical analysis and epimerization studies on silica.
• Mechanistic investigations using quenching, deuterium-labeling, and DFT calculations.

Main Results:

• Exclusive formation of trans-hexahydrocarbazol-4-ones via conrotatory [6π] photocyclization.
• Epimerization to cis-hexahydrocarbazol-4-ones on silica.
• Enhanced stability of N-alkoxycarbonyl substituted products.
• Evidence for a triplet pathway and subsequent [1,4] hydrogen migration.

Conclusions:

• Tertiary N-alkoxycarbonyl-N-aryl-β-enaminones are efficient precursors for hexahydrocarbazol-4-ones.
• The reaction proceeds via a conrotatory [6π] photocyclization mechanism.
• The study provides insights into the stereochemical control and mechanistic details of this transformation.