Stereoselective Synthesis of Cyclohepta[b]indoles by Visible‐Light‐Induced [2+2]‐Cycloaddition/retro‐Mannich‐type Reactions

A novel method for the concise synthesis of cyclohepta[b]indoles in high yields was developed. The method involves a visible‐light‐induced, photocatalyzed [2+2]‐cycloaddition/ retro‐Mannich‐type reaction of enaminones. Experimental and computational studies suggested that the reaction is a photoredox process initiated by single‐electron oxidation of an enaminone moiety, which undergoes subsequent cyclobutane formation and rapidly fragmentation in a radical‐cation state to form cyclohepta[b]indoles. A visible‐light‐induced photoredox [2+2]‐cycloaddition/retro‐Mannich‐type reaction was developed for the concise synthesis of cyclohepta[b]indoles, which is initiated by a novel process of single‐electron transfer from the enaminone moiety.