Synthesis of 3-(1,2-dioxoethyl)- and 2,3-dicarbonyl-containing pyrroles

Synthesis of Azirinyl Ethynyl Ketones and Their Use in the Preparation C(O)C≡CR-Substituted NH-Pyrroles/NH-Imidazoles and Azole-Azine/Azole-Azole Heterocyclic Hybrids

Azirinyl-substituted conjugated ynones were first synthesized by the reaction of lithiated acetylenes with 2H-azirine-2-carbonyl chlorides. Azirinyl ethynyl ketones containing only a C(O)C≡CR substituent at position 2 of the azirine ring are good precursors for obtaining α-C(O)C≡CR substituted NH-pyrroles and 5-[C(O)C≡CR]-substituted NH-imidazoles, reacting selectively only at the azirine moiety with 1,3-diketones and arylamidines, respectively. Various N-heterocyclic hybrids (pyrrole-pyrimidine, imidazole-pyrimidine, pyrrole-isoxazole, imidazole-isoxazole, pyrimidine-pyrrole-thiophene, pyrrole-pyrimidine-thiophene, imidazole-pyrimidine-thiophene, isoxazole-pyrrole-thiophene) have been prepared based on orthogonal or domino reactions of an azirine and C(O)C≡CR moieties of azirinyl ethynyl ketones.

Triethylamine-Promoted Oxidative Cyclodimerization of 2H-Azirine-2-carboxylates to Pyrimidine-4,6-dicarboxylates: Experimental and DFT Study

An unprecedented oxidative cyclodimerization reaction of 2H-azirine-2-carboxylates to pyrimidine-4,6-dicarboxylates under heating with triethylamine in air is described. In this reaction, one azirine molecule undergoes formal cleavage across the C-C bond and another across the C=N bond. According to the experimental study and DFT calculations, the key steps of the reaction mechanism include nucleophilic addition of N,N-diethylhydroxylamine to an azirine to form an (aminooxy)aziridine, generation of an azomethine ylide, and its 1,3-dipolar cycloaddition to the second azirine molecule. The crucial condition for the synthesis of pyrimidines is generation of N,N-diethylhydroxylamine in the reaction mixture in a very low concentration, which is ensured by the slow oxidation of triethylamine with air oxygen. Addition of a radical initiator accelerated the reaction and resulted in higher yields of the pyrimidines. Under these conditions, the scope of the pyrimidine formation was elucidated, and a series of pyrimidines was synthesized.

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.

1-(2H-Azirine-2-carbonyl)benzotriazoles: building blocks for the synthesis of pyrrole-containing heterocycles

A one-pot method was developed for the preparation of 2H-azirine-2-carbonylbenzotriazoles, formed by the reaction of benzotriazole with 2H-azirine-2-carbonyl chlorides, which were generated by the Fe(ii)-catalyzed isomerization of 5-chloroisoxazoles. The Co(ii)-catalyzed reaction of 2H-azirine-2-carbonylbenzotriazoles with 1,3-diketones provides 2-((benzotriazol-1-yl)carbonyl)pyrroles in moderate to good yields. Base-promoted annulations of 2-((benzotriazol-1-yl)carbonyl)pyrroles with aldehydes, ketones, isocyanates and isothiocyanates afford various substituted pyrrolo[1,2-c]oxazole and 1H-pyrrolo[1,2-c]imidazole derivatives in moderate to high yields. The 6-acyl group of these adducts can be removed by triflic acid, giving further new pyrrolo-fused O- and N-heterocycles, such as 6-unsubstituted pyrrolo[1,2-c]oxazol-1(3H)-one and 1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione, while the 6-acetyl substituent of 1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione, when treated with POCl3/pyridine, is transformed into the 6-ethynyl substituent.

Selective Cu-Catalyzed Intramolecular Annulation of 3-Aryl/Heteryl-2-(diazoacetyl)-1H-pyrroles: Synthesis of Benzo/Furo/Thieno[e]-Fused 1H-Indol-7-oles and Their Transformations

The Co(III)-catalyzed reaction of 1,3-dicarbonyl compounds with 2-(diazoacetyl)-2H-azirines, prepared by a simplified procedure from 2H-azirin-2-carbonyl chlorides, led in high yields to the formation of 2-(diazoacetyl)pyrroles, while leaving the diazoacetyl function intact. The intramolecular aromatic substitution reaction of 2-(diazoacetyl)pyrroles, catalyzed by Cu(OTf)₂, provided selectively previously unknown benzo[e]- and hetero[e]-fused indol-7-oles in good yields. Formylation of benzo[e]indol-4-ol led selectively to the 5-formyl derivative, which is a good precursor for an unusual salen ligand and its Ni-complex. Triflates prepared from benzo[e]indol-4-oles gave various 4-substituted benzo[e]indoles carrying aryl, 2-thienyl, 2-pyridyl, and alkynyl groups, in excellent yields using cross-coupling reactions. 4-(2-Pyridyl)benzo[e]indoles, upon treatment with BF₃·Et₂O/Et₃N, afforded a new type of fluorescent boron complexes with large Stokes shifts.

2-Diazoacetyl-2 H-azirines: Source of a Variety of 2 H-Azirine Building Blocks with Orthogonal and Domino Reactivity

A synthesis of 2-diazoacetyl-2 H-azirines was developed starting from 2 H-azirine-2-carbonyl chlorides, generated by Fe(II)-catalyzed isomerization of 5-chloroisoxazoles. 2-Diazoacetyl-2 H-azirines easily undergo reactions characteristic of α-diazo ketones with preservation of the azirine ring. Reactions with hydrohalogenic, carboxylic, and p-toluenesulfonic acids provide novel 1-(3-aryl-2 H-azirin-2-yl)-2-halo- and 2-(R-oxy)ethan-1-ones in good yields. The synthesized 2 H-azirines can offer many possibilities for chemical manipulation in heterocyclic synthesis, due to the presence of highly reactive azirine and the exocyclic C(O)-CHN₂ or C(O)-CH₂X functionalities, which can show orthogonal or domino reactivity. The synthetic usefulness of the developed building blocks was demonstrated by the preparation of new types of heterocyclic dyads (azirine-oxazole, azirine-pyrazoline, azirine-thiazole, azirine-oxirane, pyrrole-oxazole) as well as an azirine chalcone analogue, 2-azidoacetyl-2 H-azirine, and 2-diazoacetylaziridine derivatives.

A straightforward sequence to alkyl 1H-pyrrole-2,5-dicarboxylates starting from acylhydrazono esters and alkyl 2-aroyl-1-chlorocyclopropanecarboxylates

A domino reaction between alkyl 2-aroyl-1-chlorocyclopropanecarboxylates 1 and acylhydrazono esters 2 proceeded readily in the presence of Cs₂CO₃ to afford 2,3,5-trifunctionalized pyrroles in good to excellent yields.

A new heterocyclic skeleton with highly tunable absorption/emission wavelength via H-bonding

A new heterocyclic system, pyrido[2,1-a]pyrrolo[3,2-c]isoquinoline, showing stimuli responsive fluorescence, was synthesized via Pd-catalyzed intramolecular cyclization of 1-[1-benzyl-2-(2-bromophenyl)-1H-pyrrol-3-yl]pyridin-1-ium bromides.