Synthesis of novel heteroaromatic compounds structurally related to natural products via biradical cycloaromatizations of enyne-carbodiimides, enyne-ketenimines, and enallene-isonitriles.

Thermolysis of the benzannulated enyne-carbodiimides provides easy access to a variety of pyrido[1′,2′:1,2]pyrimido[4,5- b]indoles and related heteroaromatic compounds. Methylation of some of these new compounds with methyl iodide occurs exclusively at the site of the indolo nitrogen. The simplicity of the reaction sequence makes the process especially attractive for the synthesis of novel heteroaromatic compounds as potential DNA-intercalating agents. The formation of pyrimido[6 ′,1′:2,3]pyrimido[4,5-b]indoles was unexpected and likely involved a novel [2+2] cycloaddition pathway of the benzannulated enyne-carbodiimides. Similarly, several novel heteroaromatics structurally related to ellipticine alkaloids are synthesized by thermolysis of the pyridannulated enyne-carbodiimides. Two competing reaction pathways involving the C2–C6 cyclization and the C2–C7 cyclization are observed upon themolysis of the benzannulated enyne-ketenimines with the nitrogen at the terminal position of the conjugated system. These enyne ketenimines were generated in situ by dehydration of the corresponding amides. The nature of the substituent at the acetylenic terminus plays a major role in determining the course of cyclization reaction. A new synthetic pathway involving transformation of the benzannulated enyne-isonitriles to 11H-indeno[1,2-b]quinoline and related compounds is established. The reaction presumably proceeds through an initial 1,3-protrotropic rearrangement to form the corresponding benzannulated enallene-isonitriles followed by a facile cycloaromatization reaction to generate the putative quinoline biradicals/zwitterions. A subsequent intramolecular radical-radical coupling or electrophilic aromatic substitution then furnishes the formal [4+1] cycloaddition adducts leading to 11H-indeno[1,2- b]quinoline and related compounds