Novel intramolecular cyclization of N-alkynyl heterocycles containing proximate nucleophiles

Ionic Liquids: Advances and Applications in Phase Transfer Catalysis

Ionic liquids are a family of liquids that are composed entirely of ions and usually have melting points lower than 100 °C. Extensive research, along with the ever-growing interest of the scientific community, allowed for the development of a multitude of ionic liquids with low melting points. Such compounds are considered neoteric materials as well as ideal, custom-made solvents for a variety of different chemical transformations. In this regard, the importance of phase transfer catalysis is evident in a diversity of substrates and reactions. The use of phase transfer catalysts allows the reaction to proceed, facilitating the transfer of otherwise insoluble reactants to the desired phase. Recent scientific advances led to the emergence of ionic liquids, which are excellent candidates as phase transfer catalysts. The inherent fine-tuning capability of these molecules, along with the potential of phase transfer catalytic reactions, epitomize the sustainable aspect of this field of research. Herein, a cohesive report of such applications will be presented, including the period from the last decade of the 20th century up to date.

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 and Biological Activities of Pyrazino[1,2-a]indole and Pyrazino[1,2-a]indol-1-one Derivatives

This review concerns the synthesis and biological activities of pyrazino[1,2-a]indoles and pyrazino[1,2-a]indol-1-ones reported since 1997 and the discovery of biological activity of pyrazinoindole derivatives. In the first part, we first presented the synthetic routes that have been reported from a methodological point of view to access the pyrazinoindole unit according to cyclization reactions using or not using metal catalysts. Then, syntheses and neuropsychiatric, auto-immune, anti-infectious and anti-cancer properties of pyrazinoindoles were detailed. In the second part, we first reported the main accesses to pyrazinoindol-1-one substrates according to Michael reactions, metal-catalyzed and metal-free cyclization reactions. The syntheses and anti-cancer, anti-infectious, anti-allergenic and neuropsychiatric properties of pyrazinoindolones were next described and discussed.

A convenient one-pot approach to the synthesis of novel pyrazino[1,2-a]indoles fused to heterocyclic systems and evaluation of their biological activity as acetylcholinesterase inhibitors

Pyrazino[1,2-a]indoles fused with various heterocycles, such as oxazolidine, oxazinane, imidazolidine, hexahydropyrimidine and benzimidazole, were synthesized transition metal-free by domino reactions which involved the condensation of 1-(2-bromoethyl)-3-chloro-1H-indole-2-carbaldehydes 28–31 with various nucleophilic amines, resulting in the formation of two new interesting fused heterocycles. The anticholinesterase, antioxidant and antibacterial activities of the compounds were evaluated. Acetylcholinesterase (AChE) inhibitory activities were tested by Ellman’s assay, antioxidant activities were detected using the 2,2-azinobis[3-ethylbenzthiazoline]-6-sulfonic acid (ABTS•+) free-radical scavenging method and antibacterial activities were determined by agar diffusion tests. The oxazolo-pyrazino[1,2-a]indoles (8, 10), the oxazino-pyrazino[1,2-a]indoles (16, 18, 19), the pyrimido-pyrazino[1,2-a]indole (22), and the benzoimidazo-pyrazino[1,2-a]indole (27) possessed the highest inhibitory activity against AChE with IC50 values in the range 20–40 μg mL−1. The oxazolo-pyrazino[1,2-a]indoles (8, 9), the imidazo-pyrazino[1,2-a]indoles (12, 13), and the benzoimidazo-pyrazino[1,2-a]indole (24) revealed the highest antioxidant values with IC50 values less than 300 μg mL−1. However, the oxazolo-pyrazino[1,2-a]indole (11) and imidazo-pyrazino[1,2-a]indoles (12, 13) exhibited weak to moderate bioactivities against all tested Gram-positive bacteria, namely Staphylococcus aureus, Bacillus subtilis and Bacillus cereus.

A room-temperature protocol to access isoquinolines through Ag(i) catalysed annulation of o-(1-alkynyl)arylaldehydes and ketones with NH4OAc: elaboration to berberine and palmatine

A silver catalysed protocol for the synthesis of a wide range of isoquinolines from o-(1-alkynyl)arylaldehydes has been developed under mild conditions and elaborated to the synthesis of berberine and palmatine.

TiCl4/t-BuNH2-Promoted Hydroamination/Annulation of δ-Keto-acetylenes: Synthesis of Novel Pyrrolo[1,2-a]indol-2-carbaldehydes

[reaction: see text] An original TiCl(4)/t-BuNH(2)-mediated hydroamination/annulation domino reaction of delta-keto-acetylenes is described. The synthesis of pyrrolo[1,2-a]indole-2-carbaldehydes, starting from 2-carbonyl-1-propargyl-1H-indoles runs under mild reaction conditions. A conceivable mechanism is also discussed. TiCl(4) has proved to be an effective multiactivity reagent: catalyst/Lewis acid/water scavenger. Some unpublished 2-carbonyl-1-propargyl-1H-indoles are prepared by means of Suzuki- and Negishi-type reactions.

Palladium-Assisted Multicomponent Synthesis of 2-Aryl-4-aminoquinolines and 2-Aryl-4-amino[1,8]naphthyridines

A palladium-mediated multicomponent domino reaction leading to 2-aryl-4-amino-quinolines and 2-aryl-4-amino[1,8]naphthyridines is reported. The scope of the reaction was examined using carbon monoxide, two 2-ethynyl-arylamines, four aryl iodides, and 10 primary amines as substrates. The selection of the appropriate catalytic system was achieved testing several palladium/phosphine systems and overrides previously reported drawbacks associated with the use of primary amines in related reactions. Moreover several features concerning the role of both palladium [(0) and (II)] and phosphines are reported.

Intramolecular Cyclization of δ-Iminoacetylenes: A New Entry to Pyrazino[1,2-a]indoles

[reaction: see text] The synthesis of the pyrazino[1,2-a]indole nucleus was achieved by intramolecular cyclization of several 2-carbonyl-1-propargylindoles in the presence of ammonia. The reaction conditions were optimized using microwave heating and a pool of catalysts. Cyclization of 1-alkynylindole-2-carbaldehydes was easily accomplished under standard heating conditions, whereas microwave heating contributed to reduced reaction times and improved overall yields. Moreover, a fine-tuning of the microwave irradiation time made possible the selective synthesis of both pyrazino[1,2-a]indole isomers. TiCl4 proved the catalytic system of choice to achieve pyrazinoindoles in satisfactory yields starting from 1-alkynyl-2-acetylindoles and 1-alkynyl-2-benzoylindole derivatives. Also in these cases, microwave heating contributed to faster reactions and improved yields. The uncatalyzed versus catalyzed reaction mechanism is discussed.