Novel convenient 2-step synthesis of pyrido[1,2-a]indoles from pyrylium salts and o-bromoanilines

A novel convenient 2-step synthesis of substituted pyrido[1,2-a]indoles is developed starting from easily available pyrylium tetrafluoroborates and ortho-bromoanilines. A conversion of the pyrylium tetrafluoroborates to pyridinium ones followed by their palladium catalyzed intramolecular cyclization allows the formation of 24 examples of N-fused heterocycles. A one-pot two-stage cyclization procedure was developed. The utility of the methodology was demonstrated with the synthesis of new pyrido[1,2-a]indoles bearing different alkyl, aryl, chlorine, fluorine and methoxy substituents.

Synthesis of two nitrogen-containing polyaromatic compounds through gold catalysis/DBU-promoted cyclizations

This work reports an efficient synthesis of novel benzo[7,8]indolizino[2,3,4,5-ija]quinazoline derivatives between 2-(2-ethynylaryl)acetonitriles 1 and anthranils 2. The synthetic approach involves the initial formation of 7-formylindole intermediates that can be implemented by DBU to activate a novel indole-nitrile-aldehyde cyclization.

Chemoselective Divergent Transformations of N-(Propargyl)indole-2-carbonitriles with Nitrogen Nucleophiles: Alkyne Hydroamination or Domino Cyclizations

Interactions of N-(propargyl)indole-2-carbonitriles with nitrogen nucleophiles were studied. It was found that lithium hexamethyldisilazane (LiHMDS)-promoted reactions give mixtures of two product types, originating from an initial attack onto carbon-carbon or carbon-nitrogen triple bonds. Performing the reaction at reduced temperature and in the presence of catalytic amounts of LiHMDS delivered alkyne hydroamination products exclusively. On the contrary, the one-pot reaction of N-(propargyl)indole-2-carbonitriles with methanol and LiHMDS on heating, followed by the addition of a nitrogen nucleophile, allowed a selective domino cyclization sequence toward 1-aminopyrazino[1,2-a]indoles. Anilines and nitrogen heterocycles could be employed as N-nucleophiles to obtain products of both types. Moreover, an alternative one-pot route toward a third product type has been developed. When N-(propargyl)indole-2-carbonitrile was first combined with aniline and LiHMDS at reduced temperature, further heating of the in situ generated hydroamination product led to the intramolecular cyclization into 1-imino-2-phenylpyrazino[1,2-a]indoles. Thus, chemodivergent transformations of the same starting material into three compound classes were investigated. The possible reaction routes were studied, and N-(allenyl)indole-2-carbonitrile was identified as the key intermediate. Acyclic and cyclic products exhibit fluorescence emission in the blue to green range.

Cascade synthesis of indolizines and pyrrolo[1,2-a]pyrazines from 2-formyl-1-propargylpyrroles

A straightforward synthesis of indolizines and pyrrolo[1,2-a]pyrazines was performed through a cascade condensation/cyclization/aromatization reaction of substituted 2-formyl-N-propargylpyrroles with active methylene compounds such as nitromethane, alkyl malonates, methyl cyanoacetate and malononitrile. Under basic conditions, the reaction proceeded satisfactorily to provide the corresponding 6,7-disubstituted indolizines. The condensation of the pyrrolic analogues with ammonium acetate gave rise to pyrrolo[1,2-a]pyrazines in high yields. N-Allenyl-2-formylpyrroles behaved as more reactive substrates than 2-formyl-N-propargylpyrroles, furnishing the expected indolizines in higher yields. Hence, an allenyl-containing intermediate was probably generated as the reactive species in the reaction mechanism of some N-propargyl pyrroles prior to the cyclization reaction.

Synthesis of New Highly Functionalized 1H-Indole-2-carbonitriles via Cross-Coupling Reactions

An approach for the preparation of polysubstituted indole-2-carbonitriles through a cross-coupling reaction of compounds 1-(but-2-ynyl)-1H-indole-2-carbonitriles and 1-benzyl-3-iodo-1H-indole-2-carbonitriles is described. The reactivity of indole derivatives with iodine at position 3 was studied using cross-coupling reactions. The Sonogashira, Suzuki–Miyaura, Stille and Heck cross-couplings afforded a variety of di-, tri- and tetra-substituted indole-2-carbonitriles.

Site-Selective Electrochemical C-H Cyanation of Indoles

An electrochemical approach for the site-selective C-H cyanation of indoles employing readily available TMSCN as cyano source has been developed. The electrosynthesis relies on the tris(4-bromophenyl)amine as a redox catalyst, which achieves better yield and regioselectivity. A variety of C2- and C3-cyanated indoles were obtained in satisfactory yields. The reactions are conducted in a simple undivided cell at room temperature and obviate the need for transition-metal reagent and chemical oxidant.

Recent advances in the synthesis of pyrido[1,2-a]indoles

This review deals with the different accesses leading to the pyrido[1,2-a]indole nucleus in the last 20 years.