2-Imino-2H-1-benzopyrans as Versatile Synthons in Heterocyclic Synthesis: Studies on Novel Rearrangements of 3-Substituted 2-Imino-2H-1- benzopyrans Under Action of N-Nucleophiles

A new chemoselective approach to synthesis of a series of 3-substituted coumarin derivatives and 6- substituted benzo[4,5]imidazo[1,2-a]quinolines was developed. It was based on novel rearrangements of different 3-substituted 2-imino-2H-1-benzopyrans 1 (2-imino-2H-1-benzopyran-3-carboxamide and 2-imino-3-(1Hbenzoimidazol- 2-yl)-2H-1-benzopyran) under action of N-nucleophiles. By using this methodology, two series of heterocyclic systems were synthesized, namely: i) containing coumarin (2-oxo-2H-1-benzopyran) moiety substituted at C-3 position with different heterocycles such as 4H-1,2,4-triazole (7 and 10), 1,3,4-oxadiazole (8), 1,3,4-thiadiazole (9), 5-oxo-1,4- dihydro-benzo[e][1,2,4]triazepine (12), 1,1-dioxo-2H-benzo[e][1,2,4]thiadiazine (13), 4-oxo-3H-quinazoline (17), 4-oxo-5,6,7,8-tetrahydro-3H-benzo[4,5]thieno[2,3-d]pyrimidine (19), 1H-benzoimidazole (20), benzooxazole (21), benzothiazole (22), and ii) comprising benzo[4,5]imidazo[1,2-a]quinoline backbone substituted at C-6 position with 1Hbenzoimidazole (23) and benzooxazole (24) units. Possible mechanisms of the revealed rearrangements were discussed and this approach is considered to be a new and efficient alternative route to a variety of heterocyclic compounds.