Biradicals/Zwitterions from Enallene-Isonitriles. Formal [4 + 1] Cycloadditions Leading to 11H-Indeno[1,2-b]quinoline and Related Compounds

1,3-Prototropic rearrangement of the benzannulated enyne-isonitriles to the corresponding enallene-isonitriles followed by cycloaromatization generated the putative quinoline biradicals/zwitterions. A subsequent intramolecular radical−radical coupling or electrophilic aromatic substitution then gave the formal [4 + 1] cycloaddition adducts leading to 11H-indeno[1,2-b]quinoline and related compounds.