New Reactions of 3‐Vinyl‐ and 3‐(2‐Propenyl)Indoles with N‐Phenylmaleimide: [4 + 2] Cycloaddition, ene reaction, and dimerization

Depending on the substitution pattern of the aminobutadiene subunit in the selected 3‐vinylindoles 1, 3, 6, and 9, stereospecific [4 + 2] cycloadditions (‘endo’‐preference) and dimerizations take place on reaction with N‐phenylmaleimide. In the reaction of 9 with N‐phenylmaleimide in the absence of a Lewis‐acid catalyst, a competing ene reaction occurs in addition to the Diels‐Alder reaction.