Stereoselective Synthesis of Cyclohepta[b]indoles via 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.