SYNTHESIS OF PYRROLE AND SUBSTITUTED PYRROLES (REVIEW) p_451... · 2019-01-29 · Diana Tzankova, Stanislava Vladimirova, Lily Peikova, Maya Georgieva 451 SYNTHESIS OF PYRROLE AND

Diana Tzankova, Stanislava Vladimirova, Lily Peikova, Maya Georgieva

451

SYNTHESIS OF PYRROLE AND SUBSTITUTED PYRROLES (REVIEW)

Diana Tzankova1, Stanislava Vladimirova2, Lily Peikova1, Maya Georgieva1

ABSTRACT

Pyrrole is widely known as a biologically active scaffold which possesses a diverse nature of activities. The combination of different pharmacophores in a pyrrole ring system has led to the formation of more active compounds. Pyrrole containing analogs are considered as a potential source of biologically active compounds that contains a significant set of advantageous properties and can be found in many natural products.

The present review highlights the synthetic methods of representatives of nitrogen heterocycles such as pyrrole, sub-stituted pyrroles and other related compounds. The aim of this review is to indicate and summarise the different methods for the synthesis of nitrogen containing heterocycles from the group of pyrrole and pyrrole related structures.

Keywords: pyrrole, synthesis, pharmacological activity.

Received 14 September 2017Accepted 20 December 2017

Journal of Chemical Technology and Metallurgy, 53, 3, 2018, 451-464

1 Department of Pharmaceutical Chemistry, Faculty of Farmacy Medical University, 2 Dunav str., 1000 Sofia, Bulgaria2 Department of Organic Synthesis and Fuels University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected]

INTRODUCTION

Pyrrole is a five membered heterocyclic compound, corresponding to the C4H4NH general formula [1]. It is a colorless volatile liquid, unstable in the presence of air, where it easily darkens. Thus a preliminary distillation before use is necessary [2].

Pyrrole is included in the group of aromatic com-pounds, and its hydrogenated is difficult. The Diels-Alder reactions or usual olefin reactions are not char-acteristic for this ring. Due to the fact, that it can easy polymerize, most of the electrophilic reaction, used in benzene chemistry, are not applicable to pyrroles. On the other hand, the substituted pyrrole derivatives have been included in various transformations [3].

REACTION OF PYRROLE WITH ELECTROPHILESIn general pyrroles most commonly react with elec-

trophiles at α-position, due to the highest stability of the obtained intermediate protonation (Scheme 1).

From the group of electrophilic addition, pyrroles are easily nitrated, halogenated and sulfonated, where normally polyhalogenated derivatives are obtained, but monohalo-genation may also occur (Scheme 2) [6]. When the pyrrole nitrogen is silated, a halogenation on 3rd position is also possible, thus this is considered as a useful procedure for functionalization of the less active 3rd position [4].

Origin of pyrroleIn 1834 F.F. Runge has detected pyrrole for the first

time as a constituent of coal tar [5]. Later in 1857 it has

Scheme 1. Mehanism of the reaction of pyrrole with electrophiles.

Journal of Chemical Technology and Metallurgy, 53, 3, 2018

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been isolated for the first time from bone pyrolysate. Its name comes from the Greekpyrrhos - based on the reaction used for its detection [6]. Pyrrole itself is not naturally occurring, but many of its derivatives are found in a variety of cofactors and natural products. Pyrroles are components of more complex macrocycles, including vitamin B12, bile pigments like bilirubin and biliverdin, and the porphyrins of heme, chlorophyll, chlorins, bac-teriochlorins, and porphyrinogens [7, 8]. Pyrrole is a constituent of tobacco smoke and not as an ingredient [9].

Pharmacological activity of pyrrole and its derivativesPyrrole and its derivatives play an important role in

pharmaceutical and natural chemistry. Commonly they are widely used as an intermediate in the synthesis of pharma-

ceuticals, medicines, agrochemicals, dyes, photographic chemicals, perfumes and other organic compounds. For example, chlorophyll, heme are derivatives which are made by four pyrrole ring formation of porphyrin ring system (Fig. 1). In addition they are used as catalysts for polymeri-zation process, corrosion inhibitors, preservatives, solvents for resins and terpenes, standard in a chromatographic analysis and they are also used in organic synthesis in the pharmaceutical industry. It is an important constituent in the structure of a number of pharmaceutical products and new active agents with variety of pharmacological effects like: atorvastatine - antihyperlipidemic, aloracetam for treat-ment of Alczheimer’ disease, elopiprazole - antipsychotic, lorpiprazole - anxiolytic, tolmetin - anti-inflammatory activity (Fig. 2) [10].

Scheme 2. Electrophilic addition of pyrroles. Fig. 1. Heme b.

Fig. 2. Structure of drugs, containing pyrrole cycle.


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CLASSICAL APPROACHES FOR PYRROLE SYNTHESIS

Industrial preparationIn industry pyrrole is produced by treatment of furan

with ammonia in the presence of solid acid catalysts, like SiO2 and Al2O3 (Scheme 3) [6].

Pyrrole can also be obtained by catalytic dehydro-genation of pyrrolidine (Scheme 4).

Paal-Knorr pyrrole synthesisThe most prominent and applied method for synthe-

sis of pyrroles, furans and thiophenes, and their deriva-tives is the well known Paal-Knorr synthesis, based on a reaction of a 1,4-dicarbonyl compound with ammonia or a primary amine to form pyrrole and substituted pyrrole, respectively (Scheme 5) [11, 12].

Mechanism of Paal-Knorr pyrrole synthesisIn 1991 V. Amarnath et al. [13] suggest the mecha-

nism of Paal-Knorr reaction based on the attack of the amine to the protonated carbonyl, forming hemiaminal. Further the amine attacks the other carbonyl and forms 2,5-dihydroxytetrahydropyrrole derivative, which is further dehydrated to form the corresponding substituted pyrrole [14]. The proposed mechanism is presented on Scheme 6. The reaction is typically run under protic or Lewis acidic conditions, with a primary amine. The us-age of ammonium hydroxide or ammonium acetate (as reported by Paal) gives the N-unsubstituted pyrrole [14].

SYNTHESIS OF 1,4-DIKETONESA number of methods used for synthesis of the nec-

essary for cyclisation 1,4-diketones has been reported:One-pot method for synthesis of γ-diketones or

γ-keto esters by conjugated addition of primary nitroal-kans to α,β-unsaturated ketones or esters (Scheme 7) [15].

Pd-catalyzed addition [16] and cross-coupling [17] for obtaining a 1,4-diketones in good yield, as presented on Scheme 8 and Scheme 9, respectively:

Scheme 3. Industrial production of pyrrole.

Scheme 4. Preparation of pyrrole by catalytic dehydro-genation of pyrrolidine.

Scheme 5. Reaction of Paal-Knorr.

Scheme 6. Mechanism of Paal-Knorr reaction.

Scheme 7. Obtaining 1,4-diketones by one-pot synthesis.


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The last method enables convenient preparion of functionalized 1,4-diketones and allows obtaining a stereoselective products [17].

Various 1,4-diketones have been synthesized in moderate to good yields through a Michael addition type reaction between aroyl chlorides and chalcones in the presence of samarium metal in N,N-dimethylformamide as solvent (Scheme 10) [18].

The merger of photoredox catalysis and primary amine catalysis enables a direct construction of all-carbon quaternary stereocenters via α-photoalkylation of β-ketocarbonyls with high efficacy and enantioselectivi-ties (Scheme 11) [19].

MODIFIED APPROACHES FOR PAAL-KNORR PYRROLE SYNTHESISModifications in Paal-Knorr pyrrole synthesis

The classical Paal-Knorr approach has been cur-rently modified and optimized by a number of changes in the initial reagents and/or reaction conditions as presented on Scheme 12.

A high increase of yields and an improvement of reaction rates have been accomplished by inclusion of bismute nitrate (A) [20], organic-inorganic hybrid (B) [21], silica-supported bismuth(III) chloride (BiCl3/SiO2) (G) [22] and metal triflates as a possibility to run the reaction in a solvent free media (E) [23]. In addition

Scheme 8. Obtaining 1,4-diketones by Pd-catalyzed addition.

Scheme 9. Obtaining 1,4-diketones by cross-coupling.

Scheme 10. Obtaining 1,4-diketones by Michael addition type reaction.

Scheme 11. Obtaining 1,4-diketones by α-photoalkylation of β-ketocarbonyls.


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one-pot ecofriendly catalyst has been also suggested by Rahmatpour et al. (F) [24]. Currently the green chem-istry is highly recommended for synthesis of new and available products.

In the last years a number of modifications in Paal-Knorr pyrrole synthesis have been additionally per-formed and reported. Wang et al. have reported synthesis of substituted pyrroles using ionic liquids as solvent. The reaction is characterized with an avoidance of using toxic catalysts and simplicity in the isolation procedure (D) [25] as well as an introduction of water as a reaction media as reported by Dilek et al. (C) [26].

Handy and associates report a method consisting in application of inexpensive, non-toxic and recyclable deep

eutectic solvents (the combination of either urea or glyc-erol with choline chloride) as effective solvents/catalysts for Paal-Knorr reactions to form pyrroles of furans. The reaction conditions are quite mild and do not require ad-ditional Bronsted or Lewis acid catalyst (H) [27].

Zhang et al. have described the synthesis of N-substituted pyrroles in good to excellent yields from various substituted 1,4-diketones with primary amines catalyzed by MgI2 etherate (Scheme 13) [28].

Veitch et al. has developed method for synthesis of pyrrole analogues by cyclocondensation of 1,4-dicarbo-nyl compounds with magnesium nitride as a source of ammonia (Scheme 14) [29]. Phan et al. have synthesized pyrrole analogues from benzyl amine with 2,5-hexan-

Scheme 12. Modifications in Paal-Knorr pyrrole synthesis.

Scheme 13. Synthesis of N-substituted pyrroles from 1,4-diketones with primary amines catalyzed by MgI2 etherate.

Scheme 14. Synthesis of pyrrole analogues by cyclocondensation of 1,4-dicarbonyl compounds with magnesium nitride.


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edione using efficient heterogeneous catalyst - a highly porous metal-organic framework (IRMOF-3) as shown below (Scheme 15) [30]:

On the other hand, currently number of different methods has also been developed for synthesis of pyr-role and its derivatives.

Other methods for pyrrole synthesisAn operationally simple and economical condensa-

tion of 2,5-dimethoxytetrahydrofuran with various pri-mary aromatic amides in the presence of one equivalent of

thionyl chloride [31] or amines and sulfonamides in water in the presence of catalytic amount of iron(III) chloride [32] led to formation of N-substituted pyrroles under mild reaction conditions in good yields (Scheme 16).

An interesting approach for polysubstituted pyr-role synthesis is through reaction of 1-sulfonyl-1,2,3-triazoles with allenes in the presence of a nickel(0) catalyst as described in Miura et al. [33] (Scheme 17A). This compound has been considered also from other authors as an initial component for synthesis of mono-, di- and tri-substituted pyrroles by rhodium (II)-catalyzed

Scheme 15. Synthesis of pyrrole analogues from benzyl amine with 2,5-hexanedione.

Scheme 16. Synthesis of N-substituted pyrroles under mild reaction conditions.

Scheme 17A. Synthesis of polysubstituted pyrrole through reaction of 1-sulfonyl-1,2,3-triazoles with allenes.

Scheme 17B. Synthesis of mono-, di- and tri-substituted pyrroles by rhodium (II)-catalyzed cycloaddition with isoxazoles.


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cycloaddition with isoxazoles (Scheme 17B) [34] or transannulation with vinyl ether (Scheme 17C) [35] or with alkenyl alkyl ethers (Scheme 17D) [36].

A highly efficient reaction of 1,4-dihalo-1,3dienes

has been described for formation of pyrroles and het-eroaryl pyrroles under Cu-catalysis (Scheme 18) [37] and copper catalyzed double alkenylation of amides (Scheme 19) [38].

Scheme 17C. Synthesis of mono-, di- and tri-substituted pyrroles by transannulation with vinyl ether.

Scheme 17D. Synthesis of mono-, di- and tri-substituted pyrroles by transannulation with alkenyl alkyl ethers.

Scheme 18. Synthesis of pyrroles by Cu-catalysis reaction of 1,4-dihalo-1,3 dienes.

Scheme 19. Synthesis of pyrroles by copper catalyzed double alkenylation of amides.

Scheme 20. Synthesis of substituted NH pyrroles by vinyl azides annulation with esters and/or aldehydes.


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New approaches for obtaining substituted NH pyrroles have been developed based on vinyl azides annulation with esters and/or aldehydes, as presented on Scheme 20, ensuring good yields obtained under mild, neutral and very simple con-ditions [39 - 43] (Scheme 21A, Scheme 21B and Scheme 21C).

Various polysubstituted pyrroles are easily acces-sible from acetylenes and alkynes interacting with the corresponding N-containing substances in one-step (Scheme 22A) [44] and in highly regioselective manner (Scheme 22B) [45], (Scheme 23) [46].

Scheme 21C. Synthesis of substituted NH pyrroles under mild and simple conditions.

Scheme 21A. Synthesis of substituted NH pyrroles under mild and simple conditions.

Scheme 21B. Synthesis of substituted NH pyrroles under mild and simple conditions.

Scheme 22A. One-step synthesis of polysubstituted pyrroles.

Scheme 22B. Synthesis of polysubstituted pyrroles by regioselective manner.

Scheme 23. Synthesis of polysubstituted pyrroles by regioselective manner.


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Recently other authors have obtained substituted pyrroles in good yields, using gold-catalyzed reactions of 2H-azirines with ynamides [47], and cyclization of α-amino ketones with alkynes [48], (Scheme 24A and Scheme 24B).

Other preparation of pyrrole derivatives has been reported, consisting of condensation of propargyl amines with ethyl vinyl ether under microwave irradiation in good yields (Scheme 25) [49].

Another microwave-promoted formation of 2-acylpyrroles in good yields is iminyl radical cycliza-tions, terminated by trapping with TEMPO, affording functionalized adducts without using toxic and hazard-ous reagents and using alkynes as radical acceptors (Scheme 26) [50].

Currently Zheng et al. and Gao et al. have synthe-sized polysubstituted pyrroles by formation of C-C and C-N bonds from N-homo allylicamines with arylboronic acids and phenyl acetaldehydes with primary amines as shown in Scheme 27A and Scheme 27B [51, 52].

Another method is synthesis of pyrrole derivatives by an one-pot hetero-Diels-Alder/ring contraction cascade affords N-arylpyrroles from 1,3-dienylboronic esters with nitrosoarenes in excellent yields (Scheme 28) [53].

Recently Bayat et al. have described new fused het-erocyclic derivatives of pyrrole containing acetonitrile or cyanoacetonitrile moiety at 3-position by an one-pot multicomponent reaction in good yields, Scheme 29 [54].

Scheme 24A. Synthesis of pyrroles by gold-catalyzed reactions of 2H-azirines with ynamides.

Scheme 24B. Synthesis of pyrroles by cyclization of α-amino ketones with alkynes.

Scheme 25. Synthesis of pyrrole derivatives by condensation of propargyl amines with ethyl vinyl ether under microwave irradiation.

Scheme 26. Microwave-promoted formation of 2-acylpyrroles by iminyl radical cyclizations.


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Bunrit et al. have described a new method for syn-thesis of 2-substituted pyrroles in overall good yields with only water and ethene as side-products using Pd, Ru and Fe catalyst, (Scheme 30), [55].

SYNTHESIS OF SUBSTITUTED PYRROLESSubstituted pyrroles in excellent yields have for-

mulated by highly regioselective N-substitution of pyr-role with alkyl halides, sulfonyl chlorides, and benzoyl

Scheme 27B. Synthesis of polysubstituted pyrroles from phenyl acetaldehydes with primary amines.

Scheme 27A. Synthesis of polysubstituted pyrroles from N-homo allylicamines with arylboronic acids.

Scheme 28. Synthesis of N-arylpyrroles from 1,3-dienylboronic esters with nitrosoarenes.

Scheme 29. Synthesis of heterocyclic derivatives of pyrrole by one-pot multicomponent reaction.

Scheme 30. Synthesis of 2-substituted pyrroles using Pd, Ru and Fe catalyst.


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chloridein ionic liquids [Bmim][PF6] or [Bmim][BF4] as shown in Scheme 31 [56].

1-Vinylpyrroles have synthesized by the N,N-dimethyl¬formamide/oxalyl chloride reagent system to give the corresponding 1-vinylpyrrole-2-carbaldehydes in good yields in short reaction times (Scheme 32) [57].

2-alkyl-1H-indoles and 2-substituted or 2,3-disub-stituted 5-alkyl-1H-pyrroles have been synthesized in good yields by the 1H-indoles and electron-deficient 1H-pyrroles, palladium/norbornene-cocatalyzed regi-oselective alkylation with primary alkyl bromides at the C-H bond adjacent to the NH group (Scheme 33) [58].

The transient acid iodide intermediates undergo nucleophilic attack from a variety of relatively weak nucleophiles - including Friedel-Crafts acylation of N-methylpyrroles N-acylation of sulfonamides, and acylation reactions of hindered phenol derivatives (Scheme 34) [59].

CONCLUSIONSThe current review presents the synthetic methods for

obtaining pyrrole, substituted pyrroles and other related compounds in high to excellent yields under mild, neutral and very simple reaction conditions and/or presence of different catalysts like microwave-promoted irradiation, introduction of a number of metals and metal complexes as a reaction catalysist like Pd, Ru, Li, Fe, etc. A green chemistry reaction conditions and one-pot synthetic approaches are also discussed, based on application of media-free reactions and usage of enviromentally friendly recyclable catalysts.

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SYNTHESIS OF PYRROLE AND SUBSTITUTED PYRROLES (REVIEW) p_451... · 2019-01-29 · Diana Tzankova, Stanislava Vladimirova, Lily Peikova, Maya Georgieva 451 SYNTHESIS OF PYRROLE AND