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

Abstract 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.