C=C-CONJUGATED CARBODIIMIDES AS 2-AZADIENES IN INTRAMOLECULAR [4 + 2] CYCLOADDITIONS - ONE-POT PREPARATION OF QUINOLINE, ALPHA-CARBOLINE, AND QUININDOLINE DERIVATIVES

Iminophosphoranes 2 derived from o-aminostyrenes react with aryl isocyanates to give the corresponding carbodiimides 13 which by thermal treatment at 160-degrees-C undergo 6-pi-electrocyclization to give quinoline derivatives 14. However, the reaction with styryl isocyanates leads to alpha-carbolines 19 through the intermediate carbodiimides 15 which undergo a tandem intramolecular hetero-Diels-Alder cycloaddition/aromatization process to give 19. Similarly, related alpha-carbolines 20-22 can be obtained from the reaction of iminophosphoranes derived from ortho-substituted anilines containing an unsaturated side chain with styryl isocyanates. Iminophosphorane 6a, derived from o-butadienylaniline, and related 10 and 12 react with aryl isocyanates under the same reaction conditions to give quinindoline derivatives 25-27, respectively. Finally, iminophosphoranes 2 and 6 by reaction with ketenes lead directly to quinolines 32 and benzo[b]carbazoles 33, respectively.