Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

540

Review Article Use of Ionic Liquids as Neoteric Solvents in the Synthesis of Fused Heterocycles Reshma J. Nevagi1, Santosh N. Dighe2, Satish N. Dighe3, Pratip K. Chaskar4, Kumar V. Srinivasan3, and Kishor S. Jain3 1

2 3

4

Department of Pharmaceutical Chemistry, SMBT College of Pharmacy, Nandi Hills, Dhamangaon, Igatpuri, Nashik, Maharashtra, India Department of Chemistry, Sir Parshurambhau College, Pune, Maharashtra, India Department of Pharmaceutical Chemistry, Sinhgad College of Pharmacy, Vadgaon (Bk.), Pune, Maharashtra, India Department of Pharmaceutical Chemistry, Vivekanand Education Society’s College of Pharmacy, Chembur, Mumbai, Maharashtra, India

Medicinal chemistry has been benefited by combinatorial chemistry and high-throughput parallel synthesis. Ionic liquids reduce the materials and energy intensity of chemical processes and products, minimize or eliminate the dispersion of harmful chemicals in the environment, maximize the use of renewable resources and extend the durability and recyclability of products. It is possible to tune the physical and chemical properties by varying the nature of the cations and anions. Ionic liquids can be easily recovered, cleaned up, and reused repeatedly. Keywords: Fused heterocycles / Ionic liquids / Neoteric solvents / Reaction media cum promoters Received: January 14, 2014; Revised: March 23, 2014; Accepted: April 4, 2014 DOI 10.1002/ardp.201400018

Introduction Hundreds of tons of hazardous waste are released to the air, water, and land by industry every hour of every day. The chemical industry is the biggest source of such waste. The drive toward clean technology in the chemical industry with an increasing emphasis on the reduction of waste at source will require a high level of innovation and new technology. Solvents constitute a major factor in deciding the efficacy of an environmental friendly technology. The ideal solvent should have a very low volatility, it should be chemically and physically stable, recyclable, reusable, and eventually easy to handle. In addition, solvents that allow more selective and rapid transformations will have a significant impact. It is in this context that ionic liquids have come up as a novel class of neoteric solvents in recent times [1, 2].

Correspondence: Pratip K. Chaskar, Department of Pharmaceutical Chemistry, Vivekanand Education Society’s College of Pharmacy, Chembur (E), Mumbai, Maharashtra 400074, India E-mail: [email protected] Fax: þ91 9881381552

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ionic liquids are classified into two categories: (i) Binary ionic liquids – salts where equilibrium is involved, e.g., mixture of aluminum(III) chloride and 1,3-dialkylimidazolium chlorides contain several different ionic species and their melting point and properties depend upon the mole fractions of the aluminum(III) chloride and 1,3-dialkylimidazolium chloride present. (ii) Simple salts – made of single anion and cation, e.g., [EtNH3][NO3] is a simple salt and these show simple melting behavior. Various developments have taken place in the generation of ionic liquids. The first generation ionic liquids contain a mixture of metal halide and dialkylimidazolium chloride. The second generation ionic liquids consist of simple cation and anion, e.g., ethyl ammonium nitrate ([EtNH3]þ[NO3]
) and dialkylimidazolium ionic liquids [bmim]Br. The third generation ionic liquids consist of chiral ionic liquids made from either chiral cations or anions, monoalkyl imidazolium ionic liquids and task specific ionic liquids (TSILs). Solvent polarity has often a strong influence on the outcome of reactions. Different investigations of solute solvent interactions in ionic

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

Ionic Liquids as Solvents in Fused Heterocycles Synthesis

liquids using solvatochromic dyes have been reported [3]. The data indicate that polarities of 1,3-dialkylimidazolium salts based on the PF6, BF4, CF3SO3, and NTf2 anions can be compared to that of short chain primary alcohol with a little lower polarity for the NTf2 anion. The ionic liquid nucleophilicity is dependent on the anions and is much lower than that of polar solvents, which makes ionic liquids highly polar but yet non-coordinating, which is a unique property. Numerous literature material is available, which shows the importance of ionic liquids, e.g., a number of excellent books [1] and recent general reviews [2] as well as those covering specific topics such as catalysis (including biocatalysts) in ionic liquids [4], synthesis of organometallic complexes in ionic liquids [5], biphasic systems and supported ionic liquids [6], solvent properties [7], ionic liquids with fluorine containing anions [8], analytical applications of ionic liquids [9], chiral ionic liquids [10], electrochemistry in ionic liquids [11], and physical properties of ionic liquids are available [12]. In addition, a number of special issues [13] have appeared covering a range of topics including ionic liquids as green solvents [14], physical and thermodynamic data [15] and organometallic chemistry in ionic liquids [16]. Syntheses of ionic liquids by using various methods have also been reported [17].

NH2 R 5

Y

X 2

NH2

H +

1

[bmim]PF6 / [bmim]BF4 R

80°C

TMSCN 3

R

N

r.t.

7

HN R'

6

O Ph

N H

8

Na2CO3, PhI(OH)OTs

N

N Ph

N

BPyBF4

10

9

H

Scheme 3. IL: 80–98%, 2–3 h; conventional: 25–30%, 12–72 h.

tion of an aldehyde 1, 2-aminopyridine 5 and isocyanide 6 in 1-butyl-3-methylimidazolium bromide ([bmim]Br) at room temperature (Scheme 2). Xie [21] also reported the synthesis of imidazopyridines 10 by a one-pot procedure through the treatment of ketones 9 with HTIB and 2-aminopyrimidine 8 successively in BPyBF4 (Scheme 3). Srinivasan and Siddiqui [22] also reported the condensation of di(pyridin-2-yl)methanone 11, aldehydes 1 and ammonium acetate in 1-butylimidazolium tetraflouroborate ([Hbim]BF4)], which afforded substituted imidazo[1,5-a]pyridines 12 in excellent yields in the absence of any added catalyst (Scheme 4).

Dihydrobenzimidazo[2,1-b]quinazolinones Bridgehead nitrogen heterocycles with benzazole skeleton have potential biological properties [23, 24]. Shaabani et al. [25] reported the synthesis of dihydrobenzimidazo[2,1-b]quinazolinone 16 which involved the heating of a mixture of 2-aminobenzimidazole 13, cyclic-b-diketone 14, and orthoester 15 without using any catalyst in the ionic liquid [bmim]Br to give a family of 3,4-dihydrobenzimidazo[2,1-b]quinazolin-1(2H)-ones 16 in moderate yields (Scheme 5). They have examined the best solvents like CHCl3, methyl acetate, ethanol, and water that only gave trace yields of the products. However, among the various ionic liquids, 1-butyl-3methylimidazolium bromide ([bbim]Br) gave the best results for this condensation reaction. NH2

R

R

N Y

X

N 4

X,Y = CH, N R = H, Me, Br, 4-ClC6H4, 4-MeC6H4, 4-pyridyl, 2-pyridyl, 4-MeOC6H4, 4-O2NC6H4

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

H

N R

1

+

NH2

N

Imidazo[1,2-a]pyridines have emerged as versatile biologically active compounds spanning applications in anti-inflammatory [18a, 18b] and antibacterial agents [18c], inhibitors of gastric acids secretion [18d], calcium channel blockers [18e], and antiulcer based therapies [18f]. The classical synthesis of imidazo[1,2-a]azines involves the condensation of a-haloketones with 2-aminoazines [19]. Shaabani et al. [20] report the facile synthesis of 3-aminoimidazo[1,2-a]pyridines 4 by a one-pot three-component condensation of an aldehyde 1, 2-aminoazine 2, and trimethylsilylcyanide 3 with stirring under heating conditions at 80°C in various solvents and ionic liquids. The results showed that the efficiency and the yield of the reaction in [bmim]Br was higher than those obtained in other molecular solvents, such as MeOH, EtOH, CH2Cl2, and toluene and other ionic liquids like [bmim]PF6 and [bmim]BF4 (Scheme 1). Shaabani et al. [20] also reported the synthesis of 3-aminoimidazo[1,2-a]pyridines 7 via the three component condensaO

[bmim]Br

Scheme 2. IL: 70–99%, 3 h; conventional: 25%, 12–72 h.

Imidazoazines

N

R

R' N C

Fused heterocycles containing imidazole ring

R

N

O

541

Scheme 1. IL: 1–2 h, 60–92%; conventional: 6–8 h, 15–18%. www.archpharm.com

R. J. Nevagi et al.

542

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

O

N

+

N

N

NH4OAc

CHO R

R

[Hbim]BF4 100°C

N

N 12 R = H, 4-OMe, 2-Me, 2-OH, 3-OMe-4-OH, 3-NO2, 2,6-t-Bu-4-OH, 2-Cl, 3-OH, 4-OH 11

1

Imidazo[2,1-a]isoquinolines Imidazo[2,1-a]isoquinolines are of interest due to their antiinflammatory potential [26, 27], antirhinoviral [28], longacting local anesthetic properties [29, 30], and as antiulcer agents [31]. Hou et al. [32] used ionic liquid for the synthesis of imidazo[2,1-a]isoquinolines 19 by stirring a mixture of atosyloxyacetophenone 17, 1-aminoisoquinoline 18, and Na2CO3 in BPyBF4 (Scheme 6). Simple stirring of a mixture of a-tosyloxyketones 17, 1aminoisoquinoline 18, and sodium carbonate in BPyBF4 at room temperature for about 1 h gave the desired imidazo[2,1a]isoquinolines 19 in very good yields. When the reaction was conducted in the classical molecular solvents, such as acetonitrile, the preparation of 2-phenylimidazo[2,1-a]isoquinoline 19 needs refluxing for 5 h to obtain significantly good yields.

Imidazo[2,1-b]-1,3,4-thiadiazoles Imidazo [33–35] and thiadiazole [36–38] moieties when fused together lead to the imidazothiadiazoles resulting in enhanced biological activity. Imidazo[2,1-b]-1,3,4-thiadiazoles are well known for analgesic [39], ulcerogenic [39], antipyretic [39], antifungal [40], and antileishmanial [41] activities. Kidwai and Rastogi [42] reported the first synthesis of imidazothiadiazole 22 in an ionic liquid by carrying out the O

N NH2

N H

13

O

120°C

X + R' 15

14

OR OR OR

O

R' [bbim]Br

N N

X

N 16

R' = H, Ph, CH2CH2CH3 CH2CH2CH2CH3

X = CH2, C(Me)2 R = Me, Et

Scheme 5. IL: 58–69%, 30–50 min; conventional: 10–58%, 30 min.

O OTs

R

Na2CO3 +

N

R 17

18

BPyBF4

NH2 R = H, Ph

N

N

19

Scheme 6. IL: 1 h, 72–82%; conventional: 5 h, 52–55%.

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

R R

Scheme 4. IL: 70–99%, 3 h; conventional: 25%, 12–72 h.

reaction of different phenacyl bromides 21 with thiadiazoles 20 in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6) in good to very good yields (Scheme 7). Ionic liquids give the best yields in short reaction times as compared to molecular solvents wherein reaction times took three to four times more with moderate yields of the product.

Fused heterocycles containing pyran ring Benzopyran 4H-Benzopyran derivatives have wide range of activities such as antiallergic and anticarcinogen properties [43], antihypertensive [44] and anti-ischemic [45] behavior. Jiang et al. [46] reported a convenient method to synthesize 4H-benzopyran derivatives 25 by reaction of aldehyde 1, dimedone 23, and malononitrile 25 in room temperature ionic liquids without any catalyst (Scheme 8). The yields of the desired product in the ionic liquids containing hexafluorophosphate are higher than those in the ionic liquids containing tetrafluoroborate.

Pyranobenzopyrans Pyrano[3,2-c]benzopyrans have broad spectrum of biological activities [47, 48]. Shaabani et al. [49] reported a simple and efficient method for the synthesis of pyran annulated heterocyclic systems 28 by a three component condensation of an aldehyde 1, an alkyl nitrile 27, and an a-hydroxy or an a-amino activated C–H acid such as 4-hydroxycoumarin 26 in the presence of 1,1,3,3N,N,N0 ,N0 -tetramethylguanidinium trifluoroacetate (TMGT) as an ionic liquid, which does not require any other reagent or organic solvent (Scheme 9). Yadav et al. [50] reported a novel, convenient, and economical protocol for the generation of pyrano[3,2-c]benzopyran 31 by stirring a mixture of o-hydroxybenzaldehyde 29, 5-methyl-4-hexen-1-ol 30, trimethylorthoformate, and [bmim]BF4 at room temperature (Scheme 10). Yadav et al. [51] treated o-hydroxybenzaldehyde 29 and amines 33 with 3,4-dihydropyran 32 in presence of 10 mol% Bi(OTf)3 in [bmim]PF6 which afforded the cis-fused pyranobenzopyran 34 (Scheme 11). Similarly, they also have reported the synthesis of furanobenzopyranes. www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

S

R

O

NH2 +

[bmim]PF6 R

S

R

base

BrH2C

N N

Ionic Liquids as Solvents in Fused Heterocycles Synthesis

N R

N N 22

21

20

543

R = H, Cl, Me, Ph, n-C7H15, n-C6H13, 4-CH3OC6H4

Scheme 7. IL: 1–4 h, 60–80%; conventional: 8– 10 h, 25–45%.

Synthesis of furanobenzopyranes 36 was carried out by treatment of o-hydroxybenzaldehyde 29 and amines 33 with 2,3-dihydrofuran 35 in the presence of 10 mol% Bi(OTf)3 in [bmim]PF6 at ambient temperature affording the cis-fused furanobenzopyran 36. The reaction proceeded efficiently at room temperature with high diastereoselectivity (Scheme 12).

use of ionic liquids for synthesis of pyranopyran 38 by heating a mixture of malononitrile 24, aldehyde 1, hydroxycoumarins 37, and TMGT or TBSAB with stirring at 100°C for 60 min (Scheme 13).

Pyrano[2,3-d]pyrans Pyrano[2,3-d]pyrans have attracted much attention owing to their biological activities [52]. Shabbani et al. [49] reported the O

O CN O

+

23

CN

R CN

[bmim]PF6 O

24

NH2

25

RCHO 1

Scheme 8. IL: 1–4 h, 72–97%; conventional: 7–8 h, 28–55%.

H 1 R R

+

O

O

X = H, NO2, OMe R = CN, COOEt

27

b-Carbolinequinoxalinones

R

O

26

CN

R

O

TMGT

Pyrano[2,3-d]pyrimidine derivatives have diverse pharmacological activities such as antitumor, cardiotonic, hepatoprotective, antibronchitic, and antifungal activity [53–56]. Yu and Wang [57] reported the use of room-temperature ionic liquids as solvents for the synthesis of pyrano[2,3-d]pyrimidine derivatives 41 by condensation reaction between arylmethylidene-malononitrile 40 and barbituric acid 39 (Scheme 14). [EMim]BF4 and [BMim]BF4 afforded the best results. Shaabani et al. [49] also reported the synthesis of pyranopyrimidines 43 by heating a mixture of malononitrile 27, aldehyde 1, and 1,3-dimethylbarbituric acid 42 in presence of TMGT or TBAB with stirring at 100°C for 60 min (Scheme 15).

Fused heterocycles containing b-carboline ring

NH2

OH

O

Pyrano[2,3-d]pyrimidines

b-Carbolines exhibit a wide spectrum of biological activities such as anti-HIV agents, antihypertensives, and ligands for a number of protein receptors [58]. Tseng et al. [59] reported the synthesis of tetrahydro-b-carbolinequinoxalinones in an ionic

O

28

Scheme 9. IL: 64–75%, 1–2 h; conventional: 28–55%, 7–8 h. CHO

R OH

O

HO

CHO

OH

[bmim]BF4

+

29

R

r.t.

31

O

30

29

Bi(OTf)3 O

[bmim]PF6

35

+

O

O

HN

R NH2 33

36

R

Scheme 12. IL: 92%, 1.5–2.5 h; conventional: 28–55%, 7–8 h.

Scheme 10. IL: 75–98%, 1–2 h; conventional: 35–46%, 5–6 h. O

OH

O

OH R CHO 29

+

R NH2 33

O

Bi(OTf)3 32

[bmim]PF6

O

R

H H3C

34

HN

O 37

O +

R CN

CN 24

TMGT X= H, NO2, OCH3

CN

O H H3C

R O

O

38

R

Scheme 11. IL: 1.2–3 h, 72–92%; conventional: 5–8 h, 58–76%.

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

H

NH2

1

Scheme 13. IL: 25–50 min, 71–77%; conventional: 120–180 min, 28–55%. www.archpharm.com

R. J. Nevagi et al.

544

O H O

O

R

N

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

+ O

[EMim]BF4 / [BMim]BF4

N

N H 39

N

90°C

H

R

O

N N H

O

40

O

CN O

NH2

49

41

N H

O

O

[bmim][BF4]

R +

OHC

50

O

51

52

N R

Scheme 14. IL: 1–3 h, 85–95%; conventional: 6–8 h, 46–84%. Scheme 17. IL: 5–8 h, 90–96%; conventional: 15–18 h, 25–65%.

H

O

R 1 +

CN R

Me

Me

O TMGT / TBAB

N O

27

al [64], antiproliferative [65], antimalarial [66], and anticancer [67]. Therefore, synthesis of quinolines has attracted much attention in organic synthesis. Wang et al. [68] reported the synthesis of indeno[1,2-b]quinolinone 52 by the reaction of arylaldehyde 51, 3-arylamino-5,5-dimethylcyclohex-2-enone 49 and 1,3-indenedione 50 in [bmim][BF4] (Scheme 17).

O

Me N

O

O

42

NH2

O

N

CN

N Me

R 43

X = H, NO2, OMe R = CN, COOEt

Scheme 15. IL: 60 min, 73–74%; conventional: 120–180 min, 28– 55%.

Pyrroloquinolines and pyrroloisoquinolines Pyrroloquinolines has been found in marine sponges and ascidians [69]. These include makaluvamines [70], damirones [71], batzellines [72], and discorhabdines [73]. Ma et al. [74] reacted heteroarenecarbonyl chloride 54 and a terminal alkyne 53 under the reaction conditions of Pd/ Cu-catalyzed Sonogashira coupling in an ionic liquid such as [bmim][PF6] at room temperature, and after 3 h, a quinolium bromide 55 or isoquinolium bromide 57 was added to furnish, after 12 h of stirring at 70°C, pyrroloquinoline 56 or pyrroloisoquinoline 58 (Scheme 18).

liquid. Scheme 16 outlines the total synthesis of fused tetrahydro-b-carbolinequinoxalinones in ionic liquids. In the first step, the Pictet–Spengler reaction was utilized to construct tetrahydro-b-carbolines 45. Under the optimized condition in the ionic liquid, reaction required only a short time to complete. In the second step, 45 was used for the subsequent step of nucleophilic aromatic substitution reaction to give quinoxalinones 47. The last step commonly involves the preparation of quinoxalinones 48 by direct coupling of 2-nitrofluorobenzene with tetrahydro-b-carbolines to afford arylamines in both [bdmim][Tf2N] and [bdmim][PFBuSO3] at 70°C with respectable yields.

Fused polycyclic quinolines Quinolines exhibit a wide spectrum of activities such as antiplasmodial [60], intrinsic [61], cytotoxic [62], functional [63], antibacterial [64], antimalarial [65], antiproliferative [66], and anticancer [75] activities. Various authors reacted o-aminosubstituted aromatic carbonyls 59 and ketones/diketones/ketoesters 60 affording polyheterocycles 61 in excellent yields [76] (Scheme 19) in 1-butylimidazolium tetraflouroborate ([Hbim]BF4).

Fused heterocycles containing quinoline ring Indeno[1,2-b]quinolinones Quinoline containing compounds exhibit a wide spectrum of pharmacological activities, such as antiplasmodial [60], intrinsic [61], cytotoxic [62], functional [63], antibacteriCOOMe NH2 N H

NH

70°C

N H

45

46

[bdmim][Tf2N] / 70°C [bdmim][PFBuSO3]

O NH

N H

F

44

N

NO2

COOMe

[bdmim][Tf2N] / [bdmim][PFBuSO3]

COOMe NO2 N

[bdmim][Tf2N] / [bdmim][PFBuSO3] 70°C

48

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N H

47

Scheme 16. IL: 1–4 h, 69–93%; conventional: 10–12 h, 34–55%. www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

O N

R

[bmim][PF6] Pd(PPh3)2Cl2, Cul, Et3N

N O

R

R

R

R

O

56

Ionic Liquids as Solvents in Fused Heterocycles Synthesis

CH 53 +

55

57

[bmim][PF6] Pd(PPh3)2Cl2, Cul, Et3N

O

R 54

O N

Cl

Fused heterocycles containing xanthene ring Dibenzo[a.j]xanthenes Benzoxanthenes have a wide range of properties such as antibacterial [77], antiviral [78], and anti-inflammatory [79] activities. Kantevari et al. [80] used tetra-n-butylammonium bromide for the synthesis of 1,4-(4-nitrophenyl)-1,4H-dibenzo[a.j]xanthenes 68 (Scheme 20) by the condensation of 4nitrobenzaldehyde 66 with b-naphthol 67 in different sets of conditions. The best results were obtained when the condensation of b-naphthol and 4-nitrobenzaldehyde was catalyzed by 10 mol% of TBAB at 125°C for 60 min which afforded 1,4-(4-nitrophenyl)-1(4H)-dibenzo[a.j]xanthene 68. 14-Aryl-14H-dibenzo[a,j]xanthenes 69 were synthesized from a mixture of b-naphthol 67, aldehyde 1 and DSIMHS was stirred and heated in an oil-bath for 3–8 min in excellent yields as compared to conventional methods [81–83] (Scheme 21).

R

O

R

Scheme 18. IL: 3–12 h, 32–47%; conventional: 7–8 h, 28–35%.

Hydroxanthene-1,8-diones Pfitzinger [84] reported the synthesis of 9-aryl-3,4,5,6,7,9hexahydroxanthene-1,8-dione 70 by using task specific ionic liquid, which acts both as a catalyst and solvent (Scheme 22). Variety of substituted aromatic aldehydes 51 was subjected to the condensation reaction with dimedone 23. Dabiri et al. [85] have also reported the method for the synthesis of 1,8-dioxo-octahydroxanthene derivatives 70 in ionic liquids (Scheme 23). Various authors also reported the synthesis of 1,8dioxooctahydroxanthene 70 by using ionic liquid but under ultrasound irradiation at room temperature [81, 86]. For that purpose, the authors has treated the various substituted benzaldehydes 51 with 5,5-dimethyl-1,3-cyclohexanedione 23 in an ionic liquid under ultrasound irradiation (Scheme 24).

Xanthene-11-ones

+

[Hbim]BF4

Y

NH2

60

X

R

X 61

N

OH 2

O

Y

O

O 23

51

CHO

Catalyst

O

67

70

Scheme 22. IL: 25–40 min, 84–93%; conventional: 60–120 min, 18–56%.

CHO

O

TBAB 66

RTILs

+

O

NO2

O

O

CHO

100°C

Scheme 19. IL: 3–4 h, 93–98%; conventional: 7–8 h, 53–58%.

+

58

R

R

O R

59

O

N

Some authors also heated dimedone 23, aldehyde 1 and b-naphthol 67 in the presence of DSIMHS to give 12-aryl-

O R

545

+

68 NO2

23

O

O

TBAB 80°C

O 70

51

Scheme 20. IL: 1–11 h, 40–97%; conventional: 15–17 h, 28–45%. Scheme 23. IL: 3 h, 90%; conventional: 6–9 h, 23–43%.

O

CHO OH +

2 67

O R

DSIMHS

+

H 1

69

O

Scheme 21. IL: 3–8 min, 85–94%; conventional: 10–15 h, 18–25%.

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

O

O

TBAB

R

2

MeOH

O 51

O 23

70

Scheme 24. IL: 30–90 min, 75–95%; conventional: 6 h, 28–46%. www.archpharm.com

546

R. J. Nevagi et al.

O

O

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

OH +

+ 67

23

R

DSIMHS H O 71

1

tetrahydrobenzo[a]xanthene-11-ones 71 in an oil-bath for 20– 40 min in excellent yields [81] (Scheme 25).

Fused heterocycles containing acridine ring 1,8-Dioxodecahydroacridines A primary amine 33 was added to the mixture of an aromatic aldehyde 51 and 5,5-dimethyl-1,3-cyclohexanedione 23 in [Hmim]TFA to give 1,8-dioxodecahydroacridine derivatives 72 [87] (Scheme 26).

Octahydroacridines Treatment of citronellal 73 with aniline 74 in 1-butyl-3methylimidazolium tetrafluoroborate ([bmim]BF4) at room temperature over a period of 1 h resulted in the formation of 3,9,9-trimethyl-1,2,3,4,4a,9,9a,10-octahydroacridine 75 in 95% yield [88] (Scheme 27).

Miscellaneous Pyrazolopyridines Pyrazolopyridine derivatives have been found to be of interest for their various applications such as good vasodilators, hypotensive, hypoglycemic, anti-inflammatory, analgesic, and antipyretic agents [77]. Zhang et al. [78] reported a novel procedure for the preparation of pyrazolo[3,4-b]pyridines 77 through a three-component reaction of aromatic aldehydes 51, 5-amino-3-methyl-1-phenylpyrazole 76, and malononitrile 24 in 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]) at 80°C (Scheme 28). Ar CHO 51

O

O

Ar

72

N R

O

2

+

All the aldehydes gave the product pyrazolo[3,4-b]pyridines in very good to excellent yields. The process was changed by first adding aromatic aldehyde 51 and malanonitrile 78 to [bmim][BF4]. The mixture was stirred at 80°C. Then, 76 was added to the reaction system. The product obtained was 79 (Scheme 29). In the presence of 20 mol% FeCl3  6H2O, 5-cyano-3-methyl1,4-diphenyl-7H-pyrazolo-[3,4-b]pyridin-6-one 79 was obtained with high yields.

Indoles The one-pot Fischer indole synthesis of phenylhydrazine hydrochloride 80 (Scheme 30) with cyclohexanone 81 was conducted so as to produce the desired indoles 82 [89].

Benzimidazoles, benzoxazoles, and benzthiazoles 2-Arylbenzimidazoles, benzoxazoles, and benzthiazoles have received considerable attention in diverse areas of chemistry [90–93]. Srinivasan et al. [94] have used two sets of new ionic liquids based on N,N-di-n-butylimidazolium (bbim) and N-butylimidazolium (Hbim) salts with varying basicity of the anions along with two nonimidazolium ionic liquids such as ethyl ammonium nitrate and n-butylpyridinium tetrafluoroborate for the typical reaction of benzoyl chloride 83 with 1,2-phenylenediamine 84 under ambient conditions in the absence of any added catalyst to afford 2-phenylbenzimidazole 85 (Scheme 31). The ionic liquids, [Hbim]BF4 and [bbim]BF4, afforded the best results for this reaction. Consequently, these two ionic liquids were used to generate a variety of benzoxazoles, 2-arylbenzimidazoles, or benzthiazoles 88 by the reaction of 1,2phenylenediamines, 2-aminophenols, or 2-aminothiophenol 86 with alkyl chlorides 87, respectively, as shown in Scheme 32.

Coumarins

O

51

23

Scheme 26. IL: 30–90 min, 75–95%; conventional: 6 h, 28–46%.

+

H2N 74

Coumarins and their derivatives are widely used as additives in foods, perfumes, cosmetics, pharmaceuticals [95], and in the preparation of insecticides, optical brighteners [96], and disperse fluorescent and laser dyes [97]. Potdar et al. [98] CHO

[bmim]BF4

73

Scheme 25. IL: 20–40 min, 84–93%; conventional: 10–15 h, 14–22%.

[Hmim]TFA

R NH2

CHO

O

R

O

N H

+ 75

Scheme 27. IL: 60 min, 95%; conventional: 6–8 h, 28–64%.

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

51

R

Me N

N Ph

NH2 76

+

CN [bmim][BF4] CN 24

80°C

Me

CN N

N Ph

N NH2 77

Scheme 28. IL: 10–12 h, 70–93%; conventional: 18–20 h, 42–45%. www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

Ionic Liquids as Solvents in Fused Heterocycles Synthesis

Ph

CHO

H3C

CN +

+

COOEt

H3 C

[bmim][BF4] N

78

51

N Ph

NH2

CN N

80°C

76

N Ph

synthesized coumarin in Lewis acidic chloroaluminate ionic liquid and neutral ionic liquids, Gu and coworkers reported Pechmann reaction in no-chloroaluminate acidic ionic liquids [99]. Dong et al. [100] reported the synthesis of coumarins using a few greener halogen-free TSILs with phosphate or octylsulfate anions. Resorcinols 89 are reacted with the diketones 90 to give coumarin derivatives 91 (Scheme 33). Hence, [TMPSA][HSO4] should be the best catalyst for the Pechmann condensation among all the TSILs.

Isatin Isatins are synthetically versatile substrates that display diverse pharmacological properties [101, 102]. Pinto et al. [103] reported preparation of isatin-3-oximes derivatives 93 by use of ionic liquids as a green media to support different

H2O, IL

+ NHNH2HCl

N H

O 79

Scheme 29. IL: 10–11 h, 84–96%; conventional: 15– 18 h, 28–55%.

Lewis and Bronsted acids and to promote direct formation of isatin-3-oximes derivatives from substituted isonitrosoacetanilides 92 (Scheme 34).

Furo[2,3-d]pyrimidines The furo[2,3-d]pyrimidine derivatives act as sedatives, antihistamines, diuretics, muscle relaxants, and antiulcer agents. Shaabani et al. [104] developed the synthesis of furo[2,3-d] pyrimidine-2,4-(1H,3H)-diones 94 via the three-component condensation of N,N-dimethylbarbituric acid 42, aldehyde 1, and an alkyl or aryl isocyanide 6 in 1-butyl-3-methylimidazolium bromide ([bmim]Br) as the solvent as well as promoted at room temperature (Scheme 35).

Flavones Flavones are important naturally occurring organic compounds possessing wide range of biological activities [105] used in the treatment of various diseases [106]. Pawar et al. [107] reported the synthesis of flavones 96 from diones

O

80

N H

81

82 NOH

Scheme 30. IL: 15–20 min, 86–95%; conventional: 6 h, 38–45%.

Cl

H2N

O

O

H2N

83

85

Scheme 31. IL: 10–25 min, 80–96%; conventional: 30–90 min, 75– 85%. O

NH2 + XH

Cl

[Hbim]BF4 / [bbim]BF4 R

R

O

+ 89

O

Me

[TMPSA][HSO4] OEt

90

N

HO

N H

O

+

91

Me

Scheme 33. IL: 15–120 min, 47–94%; conventional: 300–360 min, 25–45%.

R

O N

1

42

O N

[bmim]Br

O

Me

N C 6

94

N H

R

Scheme 35. IL: 10–20 min, 55–90%; conventional: 120–150 min, 15–30%.

OH R

R

C2H5NH3 NO2 MW

H2O, 80-100°C

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

93

Me H

R

R

O

O

Me

88

X = NH, O, S

Scheme 32. IL: 40–90 min, 70–93%; conventional: 180–240 h, 56– 68%.

OH

N

O

X

87

86

N

R

135°C

O Me

R

O

R

Scheme 34. IL: 72–96%, 90 min; conventional: 15–41%, 120– 180 min.

N 84

NOH

[Hbim]BF4

92

H N

Bbim / Hbim

Lewis acid / Bronsted acid

R N H

HO

547

O

O

95

R

O R O

96

Scheme 36. IL: 22–50 sec, 80–90%; conventional: 30–120 min, 50–60%. www.archpharm.com

R. J. Nevagi et al.

548

X

O

OH + NH2

R

SR +

[Emim] [CF3SO3] / [Bmim][BF4] H

O

r.t.

O

N H 101

85°C

100

95 promoted by the ionic liquid, ethyl ammonium nitrate as medium and catalyst, under microwave irradiation, in excellent yield in short reaction time (Scheme 36).

1,3-Benzoxazine 1,3-Benzoxazines are a series of potent nonsteroidal progesterone receptor agonists [108–110] and have many other applications such as carbonaceous electrode, plant growth regulating, and antistress activities [111–113]. Kitazume et al. [114] introduced the use of ionic liquids in the synthesis of benzoxazines 98 from the reaction of aldehydes 1 with 2aminobenzyl alcohols 97 as shown in Scheme 37.

Benzothiazines 1,4-Benzothiazines are a set of biologically active heterocyclic molecules [115]. Tandon et al. [116] reported the synthesis of benzothiazines in ionic liquids. Interestingly, the alkylsulfanylamines 99 with the dibromo derivatives 100 were converted to benzothiazines 101 in shorter time and in higher yields compared with those of larger alkyl groups (Scheme 38).

103

N H

R 2 R CH3

H3C R

Scheme 37. IL: 30 min, 89–99%; conventional: 30– 120 min, 50–60%.

Benzodiazepines are having pharmacological activities like anticonvulsant, antianxiety, and hypnotic agents [117, 118]. They also have uses as dyes for acrylic fibers [119] and as antiinflammatory agents [120]. Also, they are key intermediates for the preparation of other fused ring compounds such as triazolo- [121], oxazino- [122], oxadiazolo-, or furanobenzodiazepines [123]. Srinivasan et al. [124] reported for the first time the synthesis of 1,5-benzodiazepine with ionic liquids. The reaction of the OPD 84 with ketones 102, 81 was performed without incurring any loss in yield of the benzodiazepine 103, 104 and in relatively short reaction times using 1,3-n-dibutylimidazolium bromide ([bbim]Br), at ambient temperature in the absence of any added catalyst (Scheme 39). Du et al. [125] also reported synthesis of 1,5-benzodiazepines 106 by condensation of o-phenylenediamine 84 with ketones 105 using catalytic amount of [Bpy]HSO4 and [i-BQu]HSO4 under mild conditions (Scheme 40). The reaction of o-phenylenediamine with chalcones at 80°C in the presence of [Bpy]HSO4 was performed. The reaction was accomplished when the reaction was carried out using o-phenylenediamine.

O

Scheme 38. IL: 30 min, 76–83%; conventional: 30–120 min, 45– 65%.

N

R

Benzodiazepines

S

([bmim]Br

R= Me, Et, Bu, Pr, -(CH2)3-, -(CH2)2-, -(CH2)2-O-

R

N H

98

Br

Br

99

X

1

97

NH2

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

Conclusions This review deals with the use of ionic liquids in the synthesis of a variety of five-membered fused heterocycles like imidazoles, benzimidazoles, isatins, benzoxazoles, and benz-

O O

102

NH2

R

28°C, [bbim]Br

84

2

NH2

81

N

R

28°C, [bbim]Br

104

N H

R = Me, Ph, CH(Me)2

Scheme 39. IL: 87–96%, 50 min; conventional: no reaction. R

O NH2 R

R

+

[Bpy]HSO4

H N

NH2 105

84

N 106 R

ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Scheme 40. IL: 3 h, 43–88%; conventional: 5–6 h, 50–60%. www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2014, 347, 540–551

thiazoles. The review also deals with the use of ionic liquids in the synthesis of variety of six- and seven-membered fused heterocycles like pyrans, quinolines, pyrazolopyridines, xanthenes, coumarins, carbolines, 1,3-benzoxazine, benzothiazines, flavones, furo[2,3-d]pyrimidines, and benzodiazepines. Deliberate attempts were made to compare the use of ionic liquids with those of the conventional conditions. The conditions for ionic liquids proved to be faster in terms of reaction times, dramatically decrease the reaction times and improve the product yields and purity as compared to the conventional conditions. The authors have declared no conflict of interest.

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