Trisubstituted Pyrroles Research Articles

Catalysts-Controlled Roles Exchange between 2,3-Diaryl-2H-azirines and Acetone: Chemodivergent Synthesis of Pyrroles and 3-Oxazolines.

We report two practical and step-economical methodologies for the chemodivergent synthesis of tri-substituted pyrroles and 3-oxazolines from the domino reactions of 2H-azirines and acetone. For instance, acetone served as a nucleophile to react with 2H-azirines under the basic conditions to furnish pyrroles. Upon changing the catalyst to TfOH, acetone served as an electrophile to synthesize 3-oxazolines. Moreover, the products could be synthesized on a gram scale, and the possible catalytic cycles were proposed.

Substituent-Controllable Cascade Regioselective Annulation of β-Enaminones with N-Sulfonyl Triazoles for Modular Access to Imidazoles and Pyrroles.

A controllable synthesis of trisubstituted imidazoles and pyrroles has been developed through rhodium(II)-catalyzed regioselective annulation of N-sulfonyl-1,2,3-trizaoles with β-enaminones. The imidazole ring was formed through a 1,1-insertion of the N-H bond to α-imino rhodium carbene, followed by a subsequent intramolecular 1,4-conjugate addition. This occurred when the α-carbon atom of the amino group was bearing a methyl group. Additionally, the pyrrole ring was constructed by utilizing a phenyl substituent and undergoing intramolecular nucleophilic addition. The mild conditions, good tolerance towards functional groups, gram-scale synthesis capability, and ability to undergo valuable transformations of the products qualify this unique protocol as an efficient tool for the synthesis of N-heterocycles.

Regioselective competitive synthesis of 3,5-bis(het) aryl pyrrole-2-carboxylates/carbonitriles vs. β-enaminones from β-thioxoketones

An easily adaptable protocol for the synthesis of 3,5-bis(het)aryl-2-carboxylate/nitrile pyrroles and the related 2,3,5-tri(het)aryl-pyrroles with complementary regioselectivity from the corresponding 1,3-bis(het)aryl-1,3-monothio diketones or β-enaminone precursors has been reported. Initially, regioselective base catalyzed condensation of glycine ethyl/methyl esters or aminoacetonitrile with 1,3-bis(het)aryl-1,3-monothio diketones give β-enaminones, which then in-situ undergoes cyclization to form 3,5-bis(het)aryl-2-carboxylate/nitrile pyrroles. The synthesis of 2,3,5 tri substituted pyrroles with full control over 3rd & 5th position on pyrrole ring is the noteworthy feature of this protocol. Although the product yields are moderate to good, the method offers a facile regioselective entry to 2,3,5 trisubstituted pyrroles without the aid of transition metal.

Facile fabrication of porous magnetic covalent organic frameworks as robust platform for multicomponent reaction

AbstractThe design of cheap yet efficient nanoporous magnetic catalysts for the environmentally benign process's widespread application is an extremely attractive, challenging chemical research field. A novel porous magnetic covalent organic framework was prepared by the condensation reaction of melamine and terephthaladehyde on the surface of 3,4‐dihydroxybenzaldehyde coated magnetic Fe3O4 nanoparticles COF@Fe3O4 under hydrothermal conditions for the first time. The high surface area magnetic COF could exhibit superior catalytic activity for sustainable synthesis of trisubstituted and tetrasubstituted imidazoles and pyrroles in good to excellent yields in PEG as solvent under environmentally friendly, ambient conditions and making the overall process economical, efficient, and green. The retrievable catalyst in PEG is general and applicable to a broad substrate scope and functional group compatibility. The structure and morphology of the COF@Fe3O4 were characterized by FTIR, XRD, EDX, and SEM spectroscopy. The COF@Fe3O4 magnetic catalyst was recovered by an external magnet and used for several cycles without significant catalytic activity loss.

Mild and Efficient Copper-Catalyzed Synthesis of Trisubstituted Pyrroles

AbstractA sustainable and time economic approach has been developed for the synthesis of polysubstituted pyrroles using copper iodide as a catalyst. The reaction proceeded through imine formation followed by cyclization with alkyne-Cu intermediate, which was supported by control experiments studies. The newly formed substituted pyrroles were obtained in excellent yields with high regioselectivity under mild conditions.

Ru(II)-Catalyzed, Cu(II)-mediated carbene migratory insertion in the synthesis of trisubstituted pyrroles from isoxazoles.

A convenient, "one-pot" synthesis of trisubstituted pyrroles via a Ru(ii)-catalyzed, Cu(ii)-mediated reaction of substituted isoxazoles with sulfonylhydrazones has been developed. A series of highly functionalized pyrroles are obtained via a synergistic formation of new C-C and C-N bonds. Mechanistic investigations were carried out to propose the plausible pathway. This protocol provides a facile and expeditious approach for the synthesis of various heterocyclic compounds bearing the pyrrole skeleton.

Ruthenium(II)‐ and Copper(II)‐Mediated Synthesis of Trisubstituted Pyrroles from Isoxazoles and Acrylate Esters

AbstractAn efficient ruthenium(II)‐ and copper(II)‐mediated, “one‐pot” synthesis of pyrroles has been reported. The reaction most probably proceeds via a reductive cleavage of isoxazoles. This protocol results in the synergistic formation of new C−C, C−N bonds, which leads to a simple and an efficient method for the synthesis of highly functionalized pyrroles from isoxazoles and acrylate esters.

Vinyl sulfone-modified carbohydrates: Michael acceptors and 2π partners for the synthesis of functionalized sugars and enantiomerically pure carbocycles and heterocycles.

Increasing demands for molecules with skeletal complexity, including those of stereochemical diversity, require new synthetic strategies. Carbohydrates have been used extensively as chiral building blocks for the synthesis of various complex molecules. On the other hand, the vinyl sulfone group has been identified as a unique functional group, which acts either as a Michael acceptor or a 2π partner in cycloaddition reactions. A combination of the high reactivity of the vinyl sulfone group and the in-built chiralities of carbohydrates has the potential to function as a powerful tool to generate a wide variety of enantiomerically pure reactive intermediates. Since CS bond formation in carbohydrates is easily achieved with regioselectivity, further synthetic manipulations of these thiosugars has led to the generation of a wide range of vinyl sulfone-modified furanosyl, pyranosyl, acyclic, and bicyclic carbohydrates. Several approaches have been studied to standardize the preparative methods for accessing vinyl sulfone-modified carbohydrates at least on a gram scale. Reactions of these modified carbohydrates with appropriate reagents afford a large number of new chemical entities primarily via (i) Michael addition reactions, (ii) desulfostannylation, (iii) Michael-initiated ring-closure reactions, and (iv) cycloaddition reactions. A wide range of desulfonylating reagents in the context of sensitive molecules such as carbohydrates have also been extensively studied. Applications of these strategies have led to the synthesis of (a) amino sugars and branched-chain sugars, (b) C-glycosides, (c) enantiomerically pure cyclopropanes, five- and six-membered carbocycles, (d) saturated oxa-, aza-, and thio-monocyclic heterocycles, (e) bi-and tricyclic saturated oxa and aza heterocycles, (f) enantiomerically pure and trisubstituted pyrroles, (g) 1,5-disubstituted 1,2,3-triazolylated carbohydrates and the corresponding triazole-linked di- and trisaccharides, (h) divinyl sulfone-modified carbohydrates and densely functionalized S,S-dioxothiomorpholines, and (i) modified nucleosides. Details of reaction conditions were incorporated as much as possible and mechanistic discussions were included wherever necessary.

Selective synthesis of trisubstituted pyrroles through the reactions of alkynyl Fischer carbene complexes with oxazolones.

An efficient and simple synthesis of novel trisubstituted 1H-pyrroles 4a-qvia 1,3-dipolar cycloaddition of Δ3-trifluoromethyloxazolones 2a-d with both chromium and tungsten alkynyl Fischer carbene complexes (1a-h) is described. An unexpected and unreported -CF3 group elimination process was observed in the pyrrole structure. Our experimental and theoretical data suggested that the metal fragment may be responsible for this phenomenon. The dipolar cycloaddition proceeded efficiently to produce a single regioisomer, which was unambiguously established through NMR and single-crystal X-ray diffraction studies. Nevertheless, the reaction of alkynyl carbenes bearing an α,β,γ,δ-unsaturated moiety with excess oxazolone 2a produced a polycyclic compound 6 speculatively formed through a cascade reaction involving 1,6-, 1,4- and 1,2-nucleophilic addition steps.

Visible Light-Induced, Metal-Free Denitrative [3+2] Cycloaddition for Trisubstituted Pyrrole Synthesis.

A metal-free, regioselective synthesis of trisubstituted pyrroles has been developed through a formal [3+2] cycloaddition reaction between 2H-azirines and nitroalkenes under visible light/photoredox-catalyzed conditions. The reaction proceeds through 2H-azaallenyl radical addition on β-nitrostyrenes in a Michael fashion followed by a base-mediated denitration reaction. The directive group influence of the nitro group controls the regiochemistry of the reaction.

Catalyst Control in Positional-Selective C-H Alkenylation of Isoxazoles and a Ruthenium-Mediated Assembly of Trisubstituted Pyrroles.

High levels of catalyst control are demonstrated in determining the positional selectivity in C-H alkenylation of isoxazoles. A cationic rhodium-mediated, strong-directing group promotes C( sp2)-H activation at the proximal aryl ring whereas, the palladium-mediated electrophilic metallation leads to the C( sp2)-H activation at the distal position of the directing group. Synthetic elaboration of this C-H alkenylation product via ruthenium and copper co-catalysis leads to an efficient method for the assembly of densely substituted pyrroles.

Visible-Light-Induced C (sp3)–H Functionalization of Tosylhydrazones: Synthesis of Polysubstituted Pyrroles under Metal-Free Conditions

Iodine catalyzed C (sp3)-H functionalization of tosylhydrazones with β-enamino esters under visible light irradiation for the synthesis of trisubstituted pyrroles has been described. The present method is also applicable to α- substituted tosylhydrazones to yield the tetra-substituted pyrroles.

Copper-Promoted Regioselective Synthesis of Polysubstituted Pyrroles from Aldehydes, Amines, and Nitroalkenes via 1,2-Phenyl/Alkyl Migration.

The facile copper-catalyzed synthesis of polysubstituted pyrroles from aldehydes, amines, and β-nitroalkenes is reported. Remarkably, the use of α-methyl-substituted aldehydes provides efficient access to a series of tetra- and pentasubstituted pyrroles via an overwhelming 1,2-phenyl/alkyl migration. The present methodology is also accessible to non α-substituted aldehydes, yielding the corresponding trisubstituted pyrroles. On the contrary, the use of ketones, in place of aldehydes, does not promote the organic transformation, signifying the necessity of α-substituted aldehydes. The reaction proceeds under mild catalytic conditions with low catalyst loading (0.3-1 mol %), a broad scope, very good functional-group tolerance, and high yields and can be easily scaled up to more than 3 mmol of product, thus highlighting a useful synthetic application of the present catalytic protocol. Based on formal kinetic studies, a possible radical pathway is proposed that involves the formation of an allylic nitrogen radical intermediate, which in turn reacts with the nitroalkene to yield the desired pyrrole framework via a radical 1,2-phenyl or alkyl migration.

ChemInform Abstract: Synthesis of Substituted Pyrroles via Copper‐Catalyzed Cyclization of Ethyl Allenoates with Activated Isocyanides.

A new method for the synthesis of di- and trisubstituted pyrroles via copper-catalyzed cyclization of ethyl allenoates with activated isocyanides has been developed. In contrast to related annulation reactions previously reported, this new process features a skeletal rearrangement in which the aryl sulfonyl moiety, which functions as the electron-withdrawing group in the α-carbon of the isocyanide, was found to migrate to the γ-carbon of the starting allenoate in the final product for the first time.

On the behavior of bis(sulfonyl)nitrobutadienes towards primary amines: a convenient access to 1-alkyl-2-aryl-4-(phenylsulfonyl)pyrroles

The conjugated bis(sulfonyl)nitrobutadienes 1 undergo, with primary amines, competitive MeSO2 replacement [vinylic substitution at C(1)] versus aza-Michael addition to the nitroethenyl C(3)–C(4) double bond. The latter pathway eventually leads to the trisubstituted pyrroles 2 and conditions have been optimized in order to maximize the yield of such polyfunctionalized heterocycles. Interestingly, in trifluoroethanol tetrasubstituted pyrroles 7 are also formed, thanks to a final aromatization via oxidation by ‘endogenous’ nitrite.

Synthesis of Substituted Pyrroles via Copper‐Catalyzed Cyclization of Ethyl Allenoates with Activated Isocyanides

A new method for the synthesis of di- and trisubstituted pyrroles via copper-catalyzed cyclization of ethyl allenoates with activated isocyanides has been developed. In contrast to related annulation reactions previously reported, this new process features a skeletal rearrangement in which the aryl sulfonyl moiety, which functions as the electron-withdrawing group in the α-carbon of the isocyanide, was found to migrate to the γ-carbon of the starting allenoate in the final product for the first time.

Green multicomponent reaction for synthesis of trisubstituted pyrroles in ionic liquid [bmim]BF4

2,4,5-Trisubstituted pyrrole derivatives were efficiently synthesized by one-pot condensation of 1,3-diones, α-bromoacetophenones, and ammonium acetate in ionic liquid [bmim]BF4. The new synthetic method offers multisubstituted pyrroles with the advantages of mild reaction conditions, operational simplicity, higher yield, and environmental friendliness.

Synthesis of diverse acyclic precursors to pyrroles for studies of prebiotic routes to tetrapyrrole macrocycles

Some 50 ketones, β-diketones, β-ketoesters and α-aminoketones have been prepared for studies of the formation of trisubstituted pyrroles equipped for self-condensation leading to tetrapyrrole macrocycles.

Catalytic formal [2+2+1] synthesis of pyrroles from alkynes and diazenes via Ti(II)/Ti(IV) redox catalysis.

Pyrroles are structurally important heterocycles. However, the synthesis of polysubstituted pyrroles is often challenging. Here, we report a multicomponent, Ti-catalysed formal [2+2+1] reaction of alkynes and diazenes for the oxidative synthesis of penta- and trisubstituted pyrroles: a nitrenoid analogue to classical Pauson-Khand-type syntheses of cyclopentenones. Given the scarcity of early transition-metal redox catalysis, preliminary mechanistic studies are presented. Initial stoichiometric and kinetic studies indicate that the mechanism of this reaction proceeds through a formally Ti(II)/Ti(IV) redox catalytic cycle, in which an azatitanacyclobutene intermediate, resulting from [2+2] alkyne + Ti imido coupling, undergoes a second alkyne insertion followed by reductive elimination to yield pyrrole and a Ti(II) species. The key component for catalytic turnover is the reoxidation of the Ti(II) species to a Ti(IV) imido via the disproportionation of an η(2)-diazene-Ti(II) complex.

ChemInform Abstract: Deep Eutectic Solvent: A Simple, Environmentally Benign Reaction Media for Regioselective Synthesis of 2,3,4‐Trisubstituted 1H‐Pyrroles.

AbstractA variety of trisubstituted pyrroles are obtained by reaction of isocyanoacetate (I) with various α,β‐unsaturated ketones using inexpensive, non‐toxic, recyclable, and environmentally‐benign choline hydroxide as efficient base as well as reaction media.