An Environmentally Benign Cascade Reaction of 1,1-Enediamines (EDAMs) for Site-Selective Synthesis of Highly Functionalized 2,10-Dihydro-1H-imidazo[1',2':1,6]pyrido[2,3-b]indoles and Pyrroles.

A novel protocol for the synthesis of pyrido[2,3-b]indoles (α-carbolines, 3) from (E)-3-(2-oxo-2-phenylethylidene) indolin-2-one derivatives 1 and 1,1-enediamine (EDAM) 2a via an unexpected cascade reaction in ethanol was developed. Pyrido[2,3-b]indole derivatives 4 were obtained by the same reaction, albeit by stirring the mixture for a longer period of time (about 48 h). As a result, two kinds of functionalized α-carbolines 3 and 4 were synthesized by the facile reaction of the (E)-3-(2-oxo-2-phenylethylidene)indolin-2-one derivatives and 2-(nitromethylene)imidazolidine under basic conditions (Cs2CO3) in ethanol. In addition, a diverse array of EDAM substrates (2b-2k) were tested in this reaction to afford the expected target compounds 5. This protocol is suitable for the combinatorial and parallel syntheses of natural-like products, including highly functionalized α-carbolines and pyrroles, especially 2-oxoindolin-3-yl pyrroles. This approach features several advantages, such as being a simple and practical operation (requiring only filtration and washing without column chromatography), furnishing excellent yields (72-98%), and producing diverse libraries of target compounds with potential biological activities.