Vinylindenes and some heteroanalogues in the Diels-Alder reaction. VIII. 1-Benzyl-3-vinylindole as a diene

1-Benzyl-3-vinylindole gives stable adducts resulting from [2+4] cycloadditions with 1,4-benzo-quinone, 1,4-naphthoquinone, N- phenylmaleimide and maleic anhydride. Vicinal 1H n.m.r. coupling constants for protons on the cyclohexene rings are used to show that the first two adducts probably have half-chair conformations, while the last have boat conformations. Non-equivalence of the benzyl methylene protons of the benzoquinone adduct has been shown to display a surprising temperature dependence. The adduct of the vinylindole and ethenetetracarbonitrile is shown to be a cyclobutane resulting from a [2+2] cycloaddition.