Multicomponent Tether Catalysis Synthesis of Highly Functionalized 4-(Pyridin-2-ylmethyl)-2-aminopyrroles via Cascade Reaction Is Accompanied by Decarboxylation

Iridium/Acid Dual-Catalyzed Enantioselective Aza-ene-type Allylic Alkylation of Nitro Ketene Aminals with Racemic Allylic Alcohols

The enantioselective allylic alkylation of nitro ketene aminals with racemic allylic alcohols was realized by iridium/acid dual catalysis. An allyl group was installed on the α-position of nitro ketene aminals in a branched-selective manner in high efficiency with excellent enantioselectivities (93-99% ee). The protocol was applied to the late-stage modification of neonicotinoid insecticides, which directly furnished a novel neonicotinoid analogue with good insecticidal activity against Aphis craccivora (LC50 = 6.40 mg/L). On the basis of the control experiment, an aza-ene-type allylic alkylation reaction mechanism was proposed.

Substrate-Controlled Three-Component Synthesis of Diverse Fused Heterocycles

A chemoselective strategy toward a variety of fused heterocyclic scaffolds relying on a three-component condensation of heterocyclic ketene aminals (HKAs) or corresponding thioaminals with aryl glyoxals and cyclic 1,3-dicarbonyl compounds has been developed and explored. Depending on the applied combination of substrates, the strategy can be tuned to provide straightforward access to imidazo[1,2-a]quinoline, pyrrolo[1,2-a]imidazole, and pyrrolo[2,1-b]thiazole frameworks.

An Environmentally Benign Multicomponent Cascade Reaction of 3-Formylchromones, 2-Naphthols, and Heterocyclic Ketal Aminals: Site-Selective Synthesis of Functionalized Morphan Derivatives

A novel protocol has been developed for the preparation of highly functionalized 2-azabicyclo[3.3.1]nonane (morphan) derivatives by the interesting three-component cascade reaction of 3-formylchromones, 2-naphthol, and heterocyclic ketal aminals (HKAs) in the ionic liquid [BMIM]PF₆ promoted by the organic base Et₃N. A complex cascade reaction is required, which includes a 1,2-addition, two Michael reactions, two tautomerizations, and an N-alkylation accompanied by a ring-opening reaction and involving the cleavage of one C-O bond and the formation of four bonds (one C-N bond, one C-O bond, and two C-C bonds). As a result, functionalized morphans (5 and 6) bearing naphthalene-structured skeletons were prepared by simple heating of a mixture of 3-formylchromones, 2-naphthols, and HKAs in the environmentally friendly ionic liquid [BMIM]PF₆. This protocol can be used in the synthesis of various morphans and is suitable for combinatorial and parallel syntheses of natural-like morphan derivatives. This approach has several advantages such as the use of an environmentally friendly solvent, simple and practical operation (multicomponent one-pot reaction), and satisfactory yields (65-88%).

Nanomagnetic macrocyclic Schiff-base-Mn(ii) complex: an efficient heterogeneous catalyst for click approach synthesis of novel β-substitued-1,2,3-triazoles

In the present work, a novel symmetrical 15-membered macrocyclic Schiff base complex of manganese was prepared using the reaction of the synthetic 2,6-diacetylpyridine functionalized Fe₃O₄ MNPs with 2,2-(piperazine-1,4-diyl)dianiline and Mn(ii) bromide salt via a template approach. The resulting [Fe₃O₄@PAM-Schiff-base-Mn][ClO₄] heterogenized complex was characterized using FT-IR, XRD, BET, TGA, EDX, Xray-mapping, SEM, TEM and VSM analysis. To demonstrate proof of concept, Huisgen 1,3-dipolar cycloaddition synthesis of 1,2,3-triazoles was selected to evaluate the activity and reusability of the catalyst. The ethanol as a green solvent proved to be an excellent reaction medium for this synthesis. Yields of up to 100% were obtained in some cases. Significantly, as demonstrated, [Fe₃O₄@PAM-Schiff-base-Mn][ClO₄] catalyst was recycled for 8 cycles without losing catalytic activity under the optimized reaction conditions. The hot filtration and ICP-OES tests ratified that there was no leaching of metal during the catalytic reaction, indicating the heterogeneous manner of the catalyst.

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.

Recent Advancements in Pyrrole Synthesis

This review article features selected examples on the synthesis of functionalized pyrroles that were reported between 2014 and 2019. Pyrrole is an important nitrogen-containing aromatic heterocycle that can be found in numerous compounds of biological and material significance. Given its vast importance, pyrrole continues to be an attractive target for the development of new synthetic reactions. The contents of this article are organized by the starting materials, which can be broadly classified into four different types: substrates bearing π-systems, substrates bearing carbonyl and other polar groups, and substrates bearing heterocyclic motifs. Brief discussions on plausible reaction mechanisms for most transformations are also presented.

Selectfluor-triggered fluorination/cyclization of o-hydroxyarylenaminones: A facile access to 3-fluoro-chromones

A fast and efficient Selectfluor-triggered fluorination/cyclization reaction of o-hydroxyarylenaminones has been successfully developed. The reaction successfully provides an expedient method for the synthesis of 3-fluoro-chromones promoted by potassium carbonate, which shows readily available starting materials and is easy to operate. In addition, a plausible mechanism of this tandem cyclization reaction was proposed where 4 H-chromen-4-one, 2-(dimethylamino)-3,3-difluorochroman-4-one, and 3,3-difluoro-2-hydroxychroman-4-one were not found to be the reactive intermediates. Moreover, these novel compounds have been obtained in moderate to good yields, and their structures have been confirmed by 1H NMR, 13C NMR, and high-resolution mass spectrometry.

Cascade reaction of isatins with nitro-substituted enamines: highly selective synthesis of functionalized (Z)-3-(1-(arylamino)-2-oxoarylidene)indolin-2-ones

A novel protocol for the construction of functionalized (Z)-3-(1(-arylamino)-2-oxoarylidene)indolin-2-ones (AOIDOs) from isatins 1 with nitro-substituted enamines 2via an unprecedented cascade reaction catalyzed by sulfamic acid is developed.

Multicomponent Cascade Reaction by Metal-Free Aerobic Oxidation for Synthesis of Highly Functionalized 2-Amino-4-coumarinyl-5-arylpyrroles

A novel approach has been constructed for the synthesis of two types of 2-amino-4-coumarinyl-5-arylpyrroles (ACAPs, 5 and 6) through a cascade reaction and a metal-free catalyzed aerobic oxidation reaction of arylglyoxal monohydrates 1, 1,1-enediamines (EDAMs) 2 and 3, and 4-hydroxy-2H-chromen-2-ones 4 via multicomponent reactions to produce the target compounds with good to excellent yields. Specifically, hydroxyl-substituted 2-amino-4-coumarinyl-5-arylpyrroles, that is, 2-amino-4-coumarinyl-5-aryl-6-hydroxylpyrroles (ACAHPs) 6, were obtained by metal-free aerobic oxidation in 1,4-dioxane at simple reflux for approximately 10 h. As a result, ACAHPs 6 have been produced without metal catalysts or traditional oxidizing agents. This method represents a route to obtain the novel ACAPs in an environmentally friendly, concise, rapid, and practical manner with potential biological activity of the product.