Stereoselective Visible‐Light Catalyzed Cyclization of Bis(enones): A Viable Approach to the Synthesis of Enantiomerically Enriched Cyclopentane Rings

Photoredox catalytic cyclization of aryl enones in the presence of visible light, promoted either by metals or organic dyes, represent a valuable strategy for the synthesis of cycloalkanes. The development of a stereoselective version of such transformation, in the presence of the metal‐free catalyst Eosin Y was studied, with the aim to realize an efficient protocol for the in‐flow synthesis of enantiomerically enriched functionalized cyclopentane rings, taking advantage of the flow reactors technology. The use of a chiral auxiliary on the bisenone to be cyclized offers a straightforward and convenient option to exert a stereocontrol on the light‐driven cyclization. By exploiting Evans’ oxazolidinones, the stereoselective light‐driven cyclization affords, after the removal of the chiral auxiliary, a functionalized 1,2‐ trans cyclopentane ring in up to 83/17 enantiomeric ratio. When the reaction was performed in continuo, in a homemade coil photoreactor, high yields were observed. The cyclization was also successfully realized in a 3D‐printed mesoreactor, without any change in the diastereoseletctivity of the process.