Recent developments on nano-ZnO catalyzed synthesis of bioactive heterocycles

Abstract

Last decade has seen tremendous applications of nano-ZnO as a mild, cheap, efficient, commercially available, environmentally benign, non-toxic, reusable, heterogeneous catalyst for the various organic transformations. The present review summarizes the applications of nano-ZnO as an efficient heterogeneous catalyst for the synthesis of diverse biologically relevant heterocycles reported so far.

Graphical abstract

Introduction

Heterocycles are the core structural motif in the majority of organic compounds, known so far [ 1 , 2 ]. Heterocyclic moieties are very common in naturally occurring compounds and are important because of their significant biological efficacies that include anticancer [ 3 ], cytotoxic [ 4 ], anti-malarial [ 5 ], anti-microbial [ 6 ], anti-inflammatory [ 7 ], anti-oxidant [ 8 ] and many more [ 9 , 10 ]. Figure  1 represents a glimpse of marketed drugs containing heterocycles as the core structural unit [ 11 , 12 , 13 , 14 , 15 – 16 ].

Fig. 1

Some of the marketed drugs containing heterocycles

It is well-established that along with other favorable conditions catalyst plays a crucial role for synthesis of heterocycles [ 17 , 18 ]. Worldwide scientists are always trying to modify the catalyst to increase the efficiency of the reaction and to reduce their toxicity level as well. Screening of suitable catalyst is the key to success among the other significant parameters during chemical synthesis. Recently, various nano catalysts have gained much attention due to their greater surface area per unit mass [ 19 ]. In recent past, among the other nano catalysts, metal oxides such as ZnO, CuO, SiO 2 , CeO 2 , Fe 3 O 4 , CaO, In 2 O 3 ZrO 2 , etc. in nano form, have drawn considerable attention as efficient, environmentally sustainable, heterogeneous catalysts and have found immense applications in various organic transformations that include C-H funtionalizaion [ 20 ], synthesis of 2-aminobenzimidazoles, 2-aminobenzothiazoles, benzoxazoles [ 21 ], tetrahydrobenzofurans [ 22 ], α -aminophosphonate [ 23 ], bis -2,3-dihydroquinazolin-4(1 H )-ones [ 24 ], 1,4-substituted 1,2,3-triazoles [ 25 ], xanthenes [ 26 ], pyrano[2,3- d ]pyrimidines, 4 H -chromenes, and dihydropyrano[3,2- c ]chromenes [ 27 ], substituted pyridines [ 28 ], quinoxalin-2-amine [ 29 ] and many more [ 30 , 31 ].

Among these, ZnO-nanoparticles were used in various areas, such as optoelectronics [ 32 , 33 ], ferromagnetism [ 34 ], piezoelectric transducers [ 35 ], solar cell [ 36 ], gas sensors [ 37 ], etc. [ 38 ]. They also possess antibacterial [ 39 ] and antioxidant efficacies [ 40 ]. Various ZnO nanostructures, such as nanopar-ticles, nanorods, nanowires, nanobelts, nanotubes, nanobridges and nanonails, nanowalls, nanohelixes and polyhedral cages, have been synthesized and well characterized in recent years [ 41 , 42 – 43 ]. ZnO-nanoparticles can easily be synthesized from zinc acetate using either sol–gel [ 44 ] or precipitation method [ 45 ]. In many occasions, it was also synthesized in biogenic pathway using various plant extracts [ 46 ]. Thereby, past decade has seen tremendous applications of various morphologies of nano-ZnO as catalysts in different organic name reactions that include Mannich reaction [ 47 ], knoevenagel condensation [ 48 , 49 ] and in various organic transformations, such as the synthesis of antiplatelet drug (Clopidogrel) [ 50 ], phosphonomalonates [ 51 ], ferrocenylphosphonates [ 52 ], 3-indolyl-3-hydroxy oxindoles [ 53 ], β-acetamido ketones/esters [ 54 ], O -acylation of alcohol [ 55 ], enamination of 1,3-dicarbonyls [ 56 ] and β-amino carbonyl compounds [ 57 ], etc.

The favorable physical and chemical properties like mild, low toxicity, low corrosion, large surface area, high pores volume, reusability, low cost, environmental sustainability and commercial availability make this Lewis acidic heterogeneous nano catalyst superior than others.

The present review focuses on the nano-ZnO catalyzed synthesis of diverse biologically relevant heterocycles and when possible to compare its catalytic efficiency with the rest of the congeners reported so far. The research groups working with this fascinating nano catalyst will surely be attracted by this review.

The following sections describe the catalytic applicability of nano-ZnO for the synthesis of biologically relevant heterocycles.

Nano-ZnO catalyzed synthesis of N-heterocycles

Synthesis of polysubstituted pyrroles

Pyrroles are very common in naturally occurring porphyrins, alkaloids and co-enzymes possessing various pharmacological efficacies [ 58 , 59 , 60 – 61 ]. Some marketed drugs like tallimustine, atorvastatin (Lipitor) contains pyrrol skeleton [ 62 , 63 ]. Sabbaghan et al. (Table  1 ) [ 64 ] employed ZnO-nanorod as an efficient, reusable catalyst for the synthesis of polysubstituted pyrroles ( 4 ) via a one-pot three-component reaction between primary amines ( 1 ), dialkyl acetylenedicarboxylates ( 2,2a ) and phenacyl bromide ( 3 ) under solvent free conditions at 50 °C. The catalyst was reused for the successive runs without significant loss of its activity. ZnO nanorod was found to be much more efficient than the commercial ZnO and even better that the ZnO-nanoparticles or sheets.

Table 1

ZnO nanoparticles catalyzed synthesis of polysubstituted pyrroles

Entry

R

R1

R2

Time (min)

Yield (%)

Entry

R

R1

R2

Time (min)

Yield (%)

1

CH3

CH3

H

45

90

6

4-CH3C6H4

H

C2H5

60

86

2

CH3

C2H5

H

45

88

7

4-ClC6H4

H

C2H5

60

82

3

C6H5

CH3

H

50

90

8

C5H11

H

C2H5

45

86

4

C6H5

C2H5

H

50

94

9

CH3

OCH3

C2H5

60

75

5

CH3

CH3

Cl

60

78

10

C5H11

H

CH3

45

92

Synthesis of imidazoles

Imidazole and its derivatives possess a wide-range of biological activities that include anti-inflammatory [ 65 ], anti-tumor [ 66 ], anti-fungal [ 67 ] efficacies. Some marketed drugs like omeprazole consist of modified imidazole as a core unit [ 68 ]. Nikoofar et al. [ 69 ] synthesized ZnO-nanorods. After characterized it by X-ray diffraction (XRD), IR, and scanning electron microscopy (SEM) techniques, they successfully employed this ZnO-nanorods as an efficient, mild, reusable catalyst for the synthesis of 2,4,5-triaryl-1 H -imidazolesin ( 8 ) via one-pot, three-component reactions of various aldehydes ( 5 ), benzils ( 6 ) and ammonium acetate ( 7 ) in water under reflux conditions (Table  2 ).

Table 2

ZnO nanoparticles catalyzed synthesis of 2,4,5-triaryl-1 H -imidazoles

Entry

R

R1

Time (h)

Yield (%)

Entry

R

R1

Time (h)

Yield (%)

1

C6H5

H

2.5

80

14

1-Naphthyl

H

3.45

78

2

4-ClC6H4

H

1.45

83

15

2-furyl

H

3.45

79

3

4-BrC6H4

H

1.45

86

16

Cinnamyl

H

3.5

80

4

4-N(CH3)2C6H4

H

3.5

75

17

C6H5

Cl

2.5

82

5

4-CH3C6H4

H

3.45

81

18

4-ClC6H4

Cl

2.25

87

6

4-NO2C6H4

H

1.15

90

19

4-OCH3C6H4

Cl

3

85

7

3-ClC6H4

H

2

90

20

4-CH3C6H4

Cl

3

84

8

3-OCH3C6H4

H

2.25

81

21

4-N(CH3)2C6H4

Cl

3.25

86

9

2-NO2C6H4

H

1.45

78

22

C6H5

CH3

3.5

87

10

2-OHC6H4

H

3.25

78

23

4-ClC6H4

CH3

2.45

83

11

2-OCH3C6H4

H

3.45

70

24

4-OCH3C6H4

CH3

4

82

12

3,5-(OCH3)2C6H3

H

4

78

25

4-OCH3C6H4

OCH3

4

80

13

2-OH-6-NO2-C6H3

H

3.15

80

Synthesis of benzimidazoles

In 2012, Alinezhad et al. (Table  3 ) [ 70 ] reported the synthesis of benzimidazoles ( 11 ) from the reaction of o -phenylenediamines ( 9 ) and formic acid ( 10 ) in the presence of nano-ZnO as catalyst under solvent-free conditions at 70 °C. Very recently, in 2017, Paul et al. [ 71 ] have synthesized ZnO-nanoparticles under biogenic pathway from the seeds extract of Parkia roxburghii. Using these ZnO-nanoparticles as catalyst they have synthesized a series of 2-substituted-benzimidazoles ( 11a ) from the reaction of o -phenylenediamine ( 9 ) and various aldehydes ( 5 ) under ultrasonic irradiation in ethanol at room temperature (Table  4 ).

Table 3

ZnO nanoparticles catalyzed synthesis of benzimidazoles

Entry

R

Time (min)

Yield (%)

Entry

R

Time (min)

Yield (%)

1

H

6

97

4

COOH

120

90

2

CH3

30

94

5

COC6H5

150

92

3

OCH3

240

98

Table 4

ZnO nanoparticles catalyzed synthesis of 2-substituted-benzimidazoles

Entry

R

Yield (%)

Entry

R

Yield (%)

1

H

93

6

3-OH-C6H4

93

2

C6H5

97

7

4-OCH3-C6H4

92

3

4-OH-C6H4

95

8

4-Cl-C6H4

92

4

2-OH-C6H4

91

9

4-NO2-C6H4

94

5

4-CH3-C6H4

96

Synthesis of imidazo-fused polyheterocycles

Nano-crystalline ZnO catalyzed simple, efficient, environmentally benign protocol was developed by Swami et al. (Scheme  1 ) [ 72 ] for the synthesis of biologically promising pyrazole coupled imidazo[1,2- a ]pyridine derivatives ( 15 ) via a one-pot three component reaction between various alkyl-4-formyl-1-phenyl-1 H -pyrazole-3-carboxylates ( 12 ), 2-aminopyridines ( 13 ) and isocyanides ( 14 ) in ethanol at 70 °C.

Scheme 1

ZnO nanoparticles catalyzed synthesis of imidazo-fused polyheterocycles

Synthesis of polyhydroquinoline

A simple, efficient one-pot, four-component condensation of aldehydes ( 5 ), dimedone ( 16 ), alkyl acetoacetate ( 17 / 17a ) and ammonium acetate ( 7 ) was achieved by Kassaee et al. (Scheme  2 ) [ 73 ] to synthesize 2-methyl-hexahydroquinoline derivatives ( 19 ) using ZnO nano-particles as catalyst under solvent-free conditions at room temperature. Changing alkyl acetoacetate ( 17 / 17a ) by malononitrile ( 18 ) of the four component reaction they also synthesized a series of 2-amino-hexahydroquinolines ( 20 ) under the same optimized reaction conditions. After completion of the reaction, the catalyst was recovered and reused four times without any significant loss in catalytic activity.

Scheme 2

ZnO nanoparticles catalyzed synthesis of polyhydroquinoline

Synthesis of 1,4-diaryl dihydropyridine derivatives

ZnO-nanoparticles were found to be an efficient, reusable catalyst for the synthesis of 1,4-diaryl dihydropyridines ( 21 ) from the reaction between aldehydes ( 5 ), alkyl acetoacetate ( 17 / 17a ) and substituted anilines ( 20 ) (Table  5 ) [ 74 ].

Table 5

ZnO nanoparticles catalyzed synthesis of 1,4-diaryl dihydropyridine derivatives

Entry

R

R1

R2

Time (min)

Yield (%)

Entry

R

R1

R2

Time (min)

Yield (%)

1

H

C2H5

H

120

82

6

H

C2H5

3-CH3

135

85

2

H

C2H5

4-Br

90

92

7

H

C2H5

4-OCH3

100

87

3

H

C2H5

4-Cl

80

90

8

Cl

C2H5

4-OCH3

120

89

4

H

C2H5

4-CH3

110

88

9

Cl

CH3

4-CH3

105

88

5

H

C2H5

3-Cl

120

86

10

Cl

CH3

3-Br

109

87

Synthesis of substituted 2,4,6-triaryl pyridines

Shafiee et al. (Table  6 ) [ 75 ] demonstrated a simple, convenient, ZnO nanopowder catalyzed condensation between benzaldehydes ( 5 ), acetophenones ( 22 ) and ammonium acetate ( 7 ) for the synthesis of substituted 2,4,6-triaryl pyridines ( 23 ) in good yields under solvent-free condition at 120 °C.

Table 6

ZnO nanoparticles catalyzed synthesis of substituted 2,4,6-triaryl pyridines

Entry

R

R1

Time (min)

Yield (%)

Entry

R

R1

Time (min)

Yield (%)

1

H

4-Br

135

86

7

4-Cl

4-Cl

135

83

2

4-Cl

4-Br

120

88

8

H

4-OCH3

45

75

3

4-F

4-Br

100

91

9

4-CH3

4-OCH3

30

82

4

4-OCH3

4-Br

90

95

10

4-Cl

4-OCH3

45

78

5

H

4-Cl

105

87

11

4-NO2

4-OCH3

20

83

6

4-CH3

4-Cl

150

85

12

H

4-OH

30

86

Synthesis of quinoxalines

Sadeghi et al. [ 76 ] described ZnO nanoparticles as an efficient and reusable catalyst for the synthesis of a series of quinoxaline derivatives ( 24 ) via the condensation between various 1,2-diketones ( 6 ) and 1,2-diamines ( 9 ) under solvent-free condition at room temperature (Scheme  3 ).

Scheme 3

ZnO nanoparticles catalyzed synthesis of quinoxalinein

Synthesis of dihydropyrimidinones

Dihydropyrimidinones ( 26 ) possess significant biological efficacies that include antiviral, antibacterial, antihypertensive and antitumor activity [ 77 ]. In 1893, Biginelli [ 78 ] first reported the synthesis of dihydropyrimidinones ( 26 ) with only 20–50% yields. In 2015, Hassanpour et al. [ 79 ] successfully employed ZnO nanoparticles as an efficient catalyst for the synthesis of dihydropyrimidinones ( 26 ) in good yields via a one-pot three component reaction between various aldehydes ( 5 ), alkyl acetoacetate ( 17a / 17b ) and urea ( 25 ) or thiourea ( 25a ) in water at 50 °C. ZnO nanoparicles was found to be much more efficient than the commercial ZnO (Scheme  4 ). After completion of the reaction, the catalyst was recovered and reused successfully without any significant loss in catalytic activity.

Scheme 4

ZnO nanoparticles catalyzed synthesis of dihydropyrimidinones

Synthesis of hexahydropyrido[2,3-d]pyrimidines

Abdolmohammadi [ 80 ] synthesized a series of hexahydropyrido[2,3- d ]pyrimidine derivatives ( 29 ) via the cyclocondensation reaction between arylmethylidenepyruvic acids ( 27 ) and 6-aminouracils ( 28 , 28a ) in the presence of catalytic amount of ZnO nanoparticles under solvent-free condition at 70 °C. After completion of the reaction, ZnO nanoparticles were recovered and recycled three times without any apparent loss in catalytic activity (Table  7 ).

Table 7

ZnO nanoparticles catalyzed synthesis of hexahydropyrido[2,3- d ]pyrimidines

Entry

R

R1

Yield (%)

Entry

R

R1

Yield (%)

1

Cl

H

93

4

Cl

CH3

98

2

OCH3

H

94

5

OCH3

CH3

93

3

CH3

H

91

6

CH3

CH3

96

Synthesis of 1H-pyrazolo[1,2-b]phthalazine-5,10-diones and pyrazolo[1,2-a] [1, 2, 4] triazole-1,3-diones

Azarifar et al. [ 81 ] explored the catalytic activity of ZnO nano-particles for the synthesis of 1 H -pyrazolo[1,2- b ]phthalazine-5,10-diones ( 32 ) and pyrazolo[1,2- a ] [ 1 , 2 , 4 ] triazole-1,3-dione derivatives ( 33 ) via a three-component coupling reaction between aromatic aldehydes ( 5 ), malononitrile ( 18 ), and phthalhydrazides ( 30 ) or 4-arylurazoles ( 31 ), respectively, under solvent-free condition at 80–110 °C. High product yields, short reaction times, non-toxicity, easy work-up and reusability of the catalyst are some of the merits of this developed protocol (Scheme  5 ).

Scheme 5

ZnO nanoparticles catalyzed synthesis of 1 H -pyrazolo[1,2- b ]phthalazine-5,10-diones and pyrazolo[1,2- a ] [ 1 , 2 , 4 ] triazole-1,3-diones

Synthesis of imidazo[1,2-a]quinoline

Imidazo[1,2- a ]quinolines possess immense biological efficacies that include antiallergic [ 82 ], anxiolytic [ 83 ] activity. Recently, in 2014, a simple, efficient and ultrasound-assisted convenient protocol was developed by Abaszadeh et al. [ 84 ] for the synthesis of a series of imidazo[1,2- a ]quinoline derivatives ( 35 ) via a one-pot three-component reaction between cyclic enaminoketones ( 34 ), aromatic aldehydes ( 5 ) and malononitrile ( 18 ) in the presence of catalytic amount of ZnO nanoparticles in EtOH at 80 °C (Table  8 ).

Table 8

ZnO nanoparticles catalyzed synthesis of imidazo[1,2- a ]quinoline

Entry

R

R1

Time (min)

Yield (%)

Entry

R

R1

Time (min)

Yield (%)

1

C6H5

CH3

45

90

4

C6H5

H

48

89

2

4-CH3C6H4

CH3

48

88

5

4-CH3C6H4

H

55

88

3

4-ClC6H4

CH3

42

92

6

4-ClC6H4

H

45

90

Synthesis of imidazo[1,2-a]pyridine/pyrimidine derivatives

Imidazo[1,2- a ]pyridine/pyrimidines have gained significant attention from the pharmaceutical industry because of their promising biological efficacies that include antibacterial, antifungal, antiviral and anti-inflammatory activities [ 85 , 86 ]. Many marketed drugs such as alpidem (anxiolytic), zolpidem (hypnotic), and zolimidine (antiulcer) contains imidazo[1,2- a ]pyridine as the core structural unit [ 87 , 88 ]. Sadjadi et al. [ 89 ] successfully employed ZnO-nanorods as an efficient, cost effective catalyst for the rapid synthesis of imidazo[1,2- a ]pyrimidines ( 38 ) and imidazo[1,2- a ]pyridines ( 39 ) via the one-pot three-component condensation between benzaldehydes ( 5 ), trimethylsilylcyanide ( 37 ) and pyrimidin-2-amines ( 36 ) or pyridin-2-amines (36a ), respectively, in ethanol under the influence of ultrasonic irradiation at room temperature. After completion of reaction they were able to recover ZnO nanoparticles and reused it up to third run without any significant loss in its catalytic activity (Table  9 ).

Table 9

ZnO nanoparticles catalyzed synthesis of imidazo[1,2- a ]pyridine/pyrimidines

Entry

R

R1

Time (min)

Yield (%) of 38

Entry

R

R1

Time (min)

Yield (%) of 39

1

4-NO2C6H4

H

7

90

1

C6H5

Br

10

85

2

4-OCH3C6H4

H

12

83

2

3-NO2C6H4

CH3

8

88

3

4-ClC6H4

CH3

7

90

4

4-CH3C6H4

CH3

12

85

Synthesis of 5-substituted 1H-tetrazoles

A simple, mild and eco-friendly method was developed for the synthesis of 5-substituted 1 H -tetrazoles ( 42 ) via the cycloaddition reaction of various nitriles ( 40 ) and sodium azide ( 41 ) using ZnO nanoparticles as a heterogeneous, reusable catalyst in DMF as solvent at 120–130 °C (Table  10 ) [ 90 ].

Table 10

ZnO nanoparticles catalyzed synthesis of 5-substituted 1 H -tetrazoles

Entry

R

Time (min)

Yield (%)

Entry

R

Time (min)

Yield (%)

1

C6H5

14

72

5

4-CHOC6H4

14

69

2

4-ClC6H4

14

74

6

4-ClC6H4CH2

24

71

3

2-ClC6H4

14

70

7

2-pyridyl

6

79

4

2-CNC6H4

24

81

8

pyrazine-2-yl

5

82

Synthesis of benzo[b][1,5]diazepines

Benzodiazepine represents core structural motif in many marketed drugs such as olanzapine and clozapine (schizophrenia treatment) [ 91 ], clobazam (anxiolytic agents) [ 92 ], etc. Ghasemzadeh et al. (Table  11 ) [ 93 ] reported a mild, simple and convenient approach for the efficient synthesis of a series of biologically promising benzo[ b ][1,5]diazepines ( 44 ) via one-pot three-component reactions of aromatic diamines ( 9 ), various isocyanides ( 14 ) and Meldrum’s acid ( 43 ) in the presence of a catalytic amount of ZnO nanoparticles in dichloromethane at room temperature. For this transformation, ZnO nanoparicles were found to be much more efficient than the commercial ZnO. After completion of the reaction, they were able to recover ZnO nanoparticles and reused six times without apparent loss in catalytic activity.

Table 11

ZnO nanoparticles catalyzed synthesis of benzo[ b ][1,5]diazepines

Entry

R1

R2

Time (h)

Yield (%)

Entry

R1

R2

Time (h)

Yield (%)

1

H

cyclohexyl

3.5

93

6

CH3

cyclohexyl

3

95

2

H

C(CH3)3

3

95

7

CH3

C(CH3)3

3

96

3

H

CH2C6H5

3.5

91

8

CH3

CH2C6H5

3.5

94

4

H

CH2(CH2)3CH3

3.5

92

9

CH3

CH2(CH2)3CH3

3.5

95

5

H

4-OCH3C6H4

4

91

10

CH3

4-OCH3C6H4

3.5

93

Nano-ZnO catalyzed synthesis of O-heterocycles

Synthesis of furan derivatives

Furans are very common in naturally occurring bioactive heterocycles. This important structural motif has gained considerable attention because of its significant biological efficacies. Many marketed drugs such as rubrolide, sarcophine, benfurodil hemisuccinate [ 94 , 95 ] contain furan skeleton. 5 H -Furan-2-one derivatives exhibit many pharmacological and biological activities including antifungal, antibacterial, anti-oxidants, anti-inflammatory, anti-microbial and anti-cancer agents [ 96 , 97 , 98 , 99 – 100 ]. Benzo[ b ]furan containing heterocycles possesses immense pharmaceutical efficacies that include antifungal [ 101 ], antitumor [ 102 ] activity. Tekale et al. [ 103 ] synthesized a series of biologically promising 3,4,5-trisubstituted furan-2(5 H )-one derivatives ( 45 ) by the one-pot three-component condensation between various aromatic aldehydes ( 5 ), dimethylacetylenedicarboxylate ( 2a ) and substituted anilines ( 20 ) using nano-ZnO as an efficient, reusable, heterogeneous catalyst in aqueous ethanol at 90 °C (Table  12 ). After completion of the reaction, ZnO nanoparticles were recovered and reused several times without apparent loss in its catalytic efficacy. Safaei-Ghomi et al. [ 104 ] demonstrated a ZnO-nanoparticles catalyzed simple and efficient method for the convenient synthesis of 2,3-disubstituted benzo[ b ]furans ( 49 ) via a one-pot three-component coupling reaction between substituted salisaldehydes ( 46 ), secondary amines ( 47 ) and phenylacetylene ( 48 ) in aqueous ethanol under reflux conditions. ZnO nanoparicles was found to be more efficient than the other bulk metal oxides such as MgO, CuO, Fe 2 O 3 etc (Table  13 ).

Table 12

ZnO nanoparticles catalyzed synthesis of 3,4,5-trisubstituted furan-2(5 H )-ones

Entry

R

R1

Yield (%)

Entry

Q

R1

Yield (%)

1

H

H

94

7

3-OCH3

H

85

2

H

4-CH3

95

8

4-CH3

H

84

3

4-OCH3

H

88

9

H

4-CH(CH3)2

88

4

H

4-F

84

10

H

2-F

84

5

4-Cl

H

89

11

2,4-diCl

2-F

85

6

2-Cl

H

87

12

2,4-(OCH3)2

H

83

Table 13

ZnO nanoparticles catalyzed synthesis of benzo[ b ]furans

Entry

X

R1 and R2

Time (min)

Yield (%)

Entry

X

R1 and R2

Time (min)

Yield (%)

1

H

Morpholine

90

92

7

Cl

Morpholine

60

92

2

H

Piperidine

90

90

8

Cl

Piperidine

65

94

3

H

Dibenzyl

110

80

9

Cl

Dibenzyl

80

85

4

Br

Morpholine

70

94

10

NO2

Morpholine

55

96

5

Br

Piperidine

75

94

11

NO2

Piperidine

55

94

6

Br

Dibenzyl

90

85

12

NO2

Dibenzyl

65

88

Synthesis of pyran derivatives

Pyrans and pyran-annulated heterocyclic scaffolds possess a broad spectrum of significant biological activities that include anticancer, cytotoxic, anti-HIV, anti-inflammatory, antimalarial, antimicrobial activity [ 105 , 106 – 107 ]. Das and his group reported a simple, efficient, environmentally benign protocol for the synthesis of 2-amino-3-cyano-4 H -pyran derivatives via a one-pot three-component coupling reaction between a series of aromatic aldehydes ( 5 ), malononitrile ( 18 ) and 1,3-dicarbonyl compounds ( 17 , 50 ) using nano-ZnO as the catalyst in water at room temperature [ 108 ] (Scheme  6 ). Under the same optimized condition they also synthesized 2-amino-3-cyano-4 H -chromenes ( 52 ) in good yields from the three-component reaction between aldehydes ( 5 ), malononitrile ( 18 ) and dimedone ( 16 ) (Scheme  6 ).

Scheme 6

ZnO nanoparticles catalyzed synthesis of 4 H -pyran derivatives

Very recently, in 2017, Zavar [ 109 ] has also synthesized a series of 2-amino-3-cyano-4 H -chromenes ( 52 ) within just 10 min using nano-ZnO as catalyst in ethanol under reflux conditions (Table  14 ).

Table 14

Nano-ZnO catalyzed synthesis of 2-amino-3-cyano-4 H -chromenes

Entry

R

Yield (%)

Entry

R

Yield (%)

1

C6H5

90

6

2-NO2C6H4

85

2

4-N(CH3)2C6H4

80

7

3-NO2C6H4

92

3

4-OCH3C6H4

78

8

2,4-diClC6H4

80

4

2-OCH3C6H4

75

9

4-ClC6H4

80

5

3,4-(OCH3)2C6H3

80

10

4-BrC6H4

95

Synthesis of xanthenes

Xanthenes, in particular, 1,8-dioxo-octahydroxanthene moieties, have received significant attention due to their potent pharmacological efficacies such as antimicrobial, anticancer and enzyme inhibitory activity [ 110 , 111 – 112 ]. Recently, Safaei-Ghomi et al. have synthesized a variety of structurally diverse xanthene derivatives using ZnO-nanoparticles as catalyst. In 2013, they developed a simple protocol for the efficient synthesis of 1,8-dioxooctahydroxanthene derivatives ( 53 ) via a one-pot pseudo three-component condensation between various aldehydes ( 5 ) and dimedone ( 16 ) in the presence of catalytic amount of ZnO-nanoparticles at 90 °C under solvent-free conditions [ 113 ] (Scheme  7 ). The optimized reaction conditions also worked satisfactorily in synthesizing a variety of N -aryl-1,8-dioxodecahydroacridine derivatives ( 54 ) in one-pot when the reaction was carried out in presence of aromatic amines ( 20 ) with excellent yield of 70–91% within just 5–25 min (Scheme  7 ). After completion of reaction, nano-ZnO was successfully recovered and recycled for five successive runs with little loss in the catalytic activity.

Scheme 7

ZnO nanoparticles catalyzed synthesis of 1,8-dioxo-decahydroacridines and 1,8-dioxooctahydro-xanthenes

To explore the catalytic efficiency of this fascinating catalyst, the same group has also employed nano-ZnO as catalyst for the synthesis of tetrahydrobenzo[ a ]xanthen-11-ones ( 56 ) via one-pot three-component reactions of aldehydes ( 5 ), 2-naphthol ( 55 ) and dimedone ( 16 ) under solvent-free condition at 120 °C [ 114 ] (Table  15 ). By changing 4-hydroxycoumarin ( 57 ) instead of dimedone ( 16 ) in the above-mentioned reaction they were also able to synthesize a range of 7-alkyl-6 H ,7 H -naphtho[1′,2′:5,6]pyrano[3,2- c ]chromen-6-one derivatives ( 58 ) using the same nano-ZnO as catalyst under solvent-free condition at 110 °C [ 115 ] (Table  16 ). Short reaction times, high yields, easy workup procedure, wide substrate tolerance, small catalyst loading, reusability of the catalyst and solvent-free conditions are some of the salient features of these developed protocols.

Table 15

ZnO nanoparticles catalyzed synthesis of tetrahydrobenzo[ a ]xanthen-11-ones

Entry

R

Time (min)

Yield (%)

Entry

R

Time (min)

Yield (%)

1

C6H5

16

88

9

4-CHOC6H4

18

91

2

3-NO2C6H4

15

90

10

2-S(CH3)C6H4

22

87

3

4-NO2C6H4

10

95

11

4-CH(CH3)2C6H4

20

86

4

4-ClC6H4

12

93

12

4-OCH3C6H4

22

88

5

2,4-diClC6H3

20

92

13

2-OCH3C6H4

30

85

6

4-BrC6H4

15

90

14

4-OHC6H4

25

90

7

4-FC6H4

15

92

15

3-CH3C6H4

22

85

8

4-CNC6H4

15

90

16

4-CH3C6H4

18

90

Table 16

ZnO nanoparticles catalyzed synthesis of pyrano[3,2- c ]chromen derivatives

Entry

R

Time (min)

Yield (%)

Entry

R

Time (min)

Yield (%)

1

C6H5

50

70

7

2,4-diClC6H3

45

85

2

4-ClC6H4

40

91

8

4-BrC6H4

40

89

3

4-FC6H4

40

92

9

4-OCH3C6H4

50

89

4

4-CH3C6H4

60

85

10

4-OHC6H4

50

90

5

4-NO2C6H4

40

93

11

3,4-diClC6H3

60

86

6

3-NO2C6H4

60

70

12

2,5-(OCH3)2C6H3

60

81

Synthesis of coumarins

Coumarins are very common in the naturally occurring heterocycles like warfarin, phenprocoumon, coumatetralyl, carbochromen, bromadialone, etc. Heterocycles containing this important structural motif exhibit a wide range of pharmaceutical activities that include antibacterial, anti-HIV, antiviral, anticoagulant, antioxidant and anticancer activities [ 116 , 117 , 118 , 119 – 120 ]. Goswami [ 121 ] synthesized a wide range of 4-substituted coumarins ( 60 ) by the reaction of a wide range of structurally diverse phenols ( 59 ) and ethyl acetoacetate ( 17 ) or ethyl benzoyl acetate ( 17c ) in the presence of nanocrystalline ZnO as catalyst and pyridine dicarboxylic acid as co-catalyst in acetonitrile under reflux conditions (Table  17 ).

Table 17

ZnO nanoparticles catalyzed synthesis of coumarins

Entry

R

Time (h)

Yield (%) of 60

Entry

R

Time (h)

Yield (%) of 60a

1

H

3

90

1

3-OH

4

88

2

3-OH

3

93

2

3,5-diOH

4

85

3

3,5-diOH

3

89

3

3-OCH3

4

87

4

3-OCH3

3

93

4

3,5-diCH3

4

85

5

2-CH3-3-OH

3.5

86

6

3-NH2

4

89

7

4-NO2

4

92

8

4-Cl

4

73

Kumar et al. [ 122 ] achieved the synthesis of various 3-substituted coumarins ( 60b ) by the nano-ZnO catalyzed reactions between salisaldehydes ( 46 ) and various 1,3-dicarbonyl compounds ( 17 , 18a , 61 ) under microwave irradiation at 120 °C (Table  18 ). Under the same optimized reaction conditions they also synthesized benzo[ f ]chromen-3-ones ( 63 ) by using 2-hydroxy naphthaldehyde ( 62 ) instead of salisaldehyde ( 46 ) (Table  18 ). After completion of reaction, nano-ZnO was successfully recovered and recycled for several runs with consistent catalytic activity. During optimization, ZnO nanoparicles were found to be much more efficient than the commercially available bulk ZnO. Das and his group employed nanocrystalline ZnO as an efficient, reusable catalyst for the one-pot green synthesis of a series of benzylamino coumarin derivatives ( 64 ) from the reactions of various aromatic aldehydes ( 5 ), 4-hydroxycoumarin ( 57 ) and cyclic secondary amines ( 47 ) in water at room temperature [ 123 ] (Scheme  8 ).

Table 18

ZnO nanoparticles catalyzed synthesis of coumarins

Entry

R

R1

Time (min)

Yield (%) of 60b

1

H

COOC2H5

5

92

2

H

COCH3

6

90

3

3,4-(OH)2

COOC2H5

4

86

4

3-OCH3

COOC2H5

5

92

5

3-OCH3

COCH3

5

90

6

4-OCH3

COOC2H5

6

92

7

4-OCH3

COCH3

5

93

8

3-N(C2H5)2

COOC2H5

5

87

9

3-N(C2H5)2

COCH3

7

88

10

3-N(C2H5)2

CN

7

90

11

3-OH

COOC2H5

5

62

12

4-OH

COOC2H5

7

85

13

5-Cl

COCH3

9

92

14

5-Br

COCH3

6

95

15

5-NO2

COCH3

8

93

16

5-OH

COCH3

6

93

Scheme 8

ZnO nanoparticles catalyzed synthesis of benzylamino coumarin derivatives

Synthesis of highly functionalized 4H-chromenes

Das and his group developed another nano-ZnO catalyzed protocol for the efficient synthesis of dihydropyrano[2,3- c ]chromenes ( 66 ) via a one-pot, three-component coupling reaction between aromatic aldehydes ( 5 ), malononitrile ( 18 ) and 3-hydroxycoumarin ( 65 ) in water at 70 °C [ 124 ] (Table  19 ). Later on, the same group also synthesized a series of structurally diverse 4 H -chromene derivatives ( 69 ) by the reactions of salisaldehydes ( 46 ), active methylene compounds ( 16 , 16a , 18 , 67a ) and C-H activated nucleophiles ( 28 , 55 , 57 , 68a , 68b , 68c , 68d ) employing the same nano-ZnO as catalyst in aqueous medium at 55 °C [ 125 ] (Scheme  9 ). During optimization it was found that the catalytic activity of nano-ZnO is superior to the other nano metal oxides such as nano-Al 2 O 3 , nano-MgO tested for these reactions. Nano-ZnO was recovered easily and recycled six times without significant loss in catalytic activity.

Table 19

ZnO nanoparticles catalyzed synthesis of dihydropyrano[2,3- c ]chromenes

Entry

R

Time (h)

Yield (%)

Entry

R

Time (h)

Yield (%)

1

C6H5

3

87

6

2-naphthyl

2.5

89

2

4-NO2C6H4

2.5

91

7

4-FC6H4

2

90

3

4-OCH3C6H4

3

78

8

4-CNC6H4

2.5

90

4

4-BrC6H4

2.5

89

9

2-thienyl

3

75

5

4-CH3C6H3

3

83

10

4-CHOC6H4

4

65

Scheme 9

ZnO nanoparticles catalyzed synthesis of densely functionalized 4 H -chromenes

Nano-ZnO catalyzed synthesis of N- as well as O-heterocycles

Synthesis of 6-amino-5-cyano-pyrano[2,3-c]pyrazoles

Pyrano[2,3- c ]pyrazole and its derivatives possess significant biological efficacies that include anti-inflammatory, molluscicidal, insecticidal, antitumor and anticancer activity [ 126 , 127 ]. Tekale et al. [ 128 ] developed a simple, convenient and practical method for the efficient synthesis of 6-amino-5-cyano-pyrano[2,3- c ]pyrazoles ( 71 ) via a four-component reaction of ethyl acetoacetate ( 17 ), hydrazine hydrate ( 70 ), malononitrile ( 18 ) and various aromatic aldehydes ( 5 ) using nano-ZnO as a recyclable heterogeneous catalyst in aqueous medium at 70 °C (Table  20 ). In the same year, from the same batch of reactions, Sachdeva et al. [ 129 ] replaced malononirile ( 18 ) by ethylcyano acetate ( 18a ) and synthesized ethyl 6-amino-pyrano[2,3- c ]pyrazoles-5-carboxylate derivatives ( 71a ) in good yields using the same nano-ZnO as a reusable catalyst in aqueous medium at room temperature (Table  21 ).

Table 20

ZnO nanoparticles catalyzed synthesis of 6-amino-5-cyano-pyrano[2,3-c]pyrazoles

Entry

R

Time (min)

Yield (%)

Entry

R

Time (min)

Yield (%)

1

C6H5

60

94

8

4-BrC6H4

65

85

2

4-ClC6H4

60

90

9

4-NO2C6H4

90

87

3

4-N(CH3)2C6H4

70

86

10

4-OCH3C6H4

70

90

4

4-S(CH3)C6H4

70

88

11

4-NO2C6H4

90

87

5

4-OH-C6H4

90

82

12

2-furyl

80

86

6

2-ClC6H4

80

89

13

4-OH-3-OCH3C6H3

80

91

7

4-CH3C6H4

70

90

Table 21

ZnO nanoparticles catalyzed synthesis of ethyl 6-amino-pyrano[2,3- c ]pyrazoles-5-carboxylate derivatives

Entry

R

Time (min)

Yield (%)

Entry

R

Time (min)

Yield (%)

1

3,4-(OCH3)2C6H3

60

90

6

3-CH3-2-furyl

60

86

2

3-OCH3C6H4

55

85

7

2-thienyl

55

87

3

3,4,5-(OCH3)3C6H2

55

86

8

3-pyridyl

60

85

4

4-ClC6H4

60

87

9

2-OH-C6H4

60

87

5

4-OCH3C6H4

55

89

10

3-OH-4-OCH3-C6H3

60

85

Synthesis of pyrazole based pyrido[2,3-d]pyrimidine-diones

Heravi et al. [ 130 ] reported the efficient synthesis of a series of pyrazolo-[4′,3′:5,6]pyrido[2,3- d ]pyrimidine-dione derivatives ( 72 ) via a one-pot five-component condensation between hydrazine hydrate ( 70 ), ethyl acetoacetate ( 17 ), 1,3-dimethyl barbituric acid ( 67a ), aromatic aldehydes ( 5 ) and ammonium acetate ( 7 ) in the presence of nano-ZnO in water under reflux conditions (Table  22 ). Use of nano-ZnO as catalyst offers several advantages such as operational simplicity, wide range of substrate tolerance, easy work-up and high yields of products.

Table 22

ZnO nanoparticles catalyzed synthesis of pyrido[2,3- d ]pyrimidine-diones

Entry

R

Time (h)

Yield (%)

Entry

R

Time (h)

Yield (%)

1

C6H5

4

91

6

4-NO2C6H4

4

91

2

4-FC6H4

3.5

90

7

3-NO2C6H4

4.2

89

3

4-ClC6H4

3.5

91

8

4-OHC6H4

4.5

85

4

4-BrC6H4

3.8

90

9

4-N(CH3)2C6H4

4.5

85

5

2,4-diClC6H3

4

90

Synthesis of structurally diverse pyridine derivatives

Siddiqui et al. [ 131 ] demonstrated the catalytic efficiency of ZnO-nanoparticles for the efficient synthesis of a series of novel pyridine derivatives ( 76 , 77 , 78 , 79 , 80 ) via a sequential three-component reaction of β-enaminones ( 73 ), ammonium acetate ( 7 ) and various active methylene compounds ( 16 , 17 , 17a , 43 , 57 , 67a , 67aa , 74 ) under solvent-free conditions at 70 °C. Nano-ZnO was recovered easily and recycled six times without significant loss in catalytic activity (Scheme  10 ).

Scheme 10

ZnO nanoparticles catalyzed synthesis of structurally diverse pyridine derivatives

Synthesis of 1,3-oxazoline-2-thione derivatives

Haerizade et al. [ 132 ] synthesized a series of functionalized 2-thioxo-2,3-dihydro-1,3-oxazoles ( 83 ) from one-pot three-component reactions of ammonium thiocyanate ( 81 ), various acid chlorides ( 82 ) and phenacyl bromides ( 3 ) in the presence of a catalytic amount of nano-ZnO and N -methylimidazole as co-catalyst under solvent-free conditions at room temperature (Table  23 ).

Table 23

ZnO nanoparticles catalyzed synthesis of 1,3-oxazoline-2-thione derivatives

Entry

Ar

R

Yield (%)

Entry

Ar

R

Yield (%)

1

4-BrC6H4

4-NO2

95

4

4-OCH3C6H4

4-Br

85

2

4-NO2C6H4

4-Br

75

5

4-BrC6H4

4-OCH3

83

3

4-NO2C6H4

4-CH3

70

Supported nano-ZnO catalyzed heterocycles synthesis

Synthesis of thieno[2,3-c]pyridine derivatives

Sangshetti et al. [ 133 ] employed combined nano zinc oxide and titanium dioxide [nano (ZnO–TiO 2 )] as an efficient catalytic system for the synthesis of a series of novel 4,5,6,7-tetrahydro-6-((5-substituted-1,3,4-oxadiazol-2-yl)methyl)thieno[2,3- c ]pyridines ( 85 ) from the reactions of 2-(4,5-dihydrothieno[2,3- c ]pyridin-6(7 H )-yl)acetohydrazide ( 84 ) and various substituted aldehydes ( 5 ) in ethanol under reflux conditions (Table  24 ). Catalytic activity of nano (ZnO–TiO 2 ) was found to be superior to the individual effect of nano ZnO or nano TiO 2 . All the synthesized compounds were screened for their antimicrobial activities. Among them compounds 85f , 85k , 85l have promising antibacterial as well as antifungal efficacies whereas compound 85j possess promising antibacterial activity.

Table 24

ZnO nanoparticles catalyzed synthesis of thieno[2,3- c ]pyridine derivatives

Entry

R

Product

Time (min)

Yield (%)

1

4-ClC6H4

85a

12

96

2

C6H5

85b

12

91

3

4-OCH3C6H4

85c

13

95

4

3,4-(OH)2C6H3

85d

14

91

5

2-ClC6H4

85e

12

95

6

2,6-(Cl)2C6H3

85f

12

91

7

2,4-(OCH3)2C6H3

85g

14

94

8

4-OH-C6H4

85h

13

94

9

C4H4N

85i

15

95

10

C4H3S

85j

14

94

11

4-FC6H4

85k

13

94

12

2,4-diClC6H3

85l

12

92

13

C5H4N

85m

15

95

Synthesis of pyrano[3,2-b]chromene

Ziraka et al. [ 134 ] prepared bismuth oxide supported nano zinc dioxide [nano Bi 2 O 3 -ZnO] by sol–gel method and characterized it by FT-IR, XRD, SEM, TEM and energy-dispersive X-ray analysis (EDX) analysis. They employed this newly synthesize catalytic system for the synthesis of pyrano[3,2- b ]chromenes ( 87 ) via one-pot three-component reactions of various aromatic aldehydes ( 5 ), kojic acid ( 86 ) and dimethone ( 16 ) under solvent-free condition at 100 °C (Table  25 ). During optimization it was found that the catalytic activity of combined nano (ZnO–TiO 2 ) is superior to the individual effect of nano ZnO or nano Bi 2 O 3 .

Table 25

ZnO nanoparticles catalyzed synthesis of pyrano[3,2- b ]chromene

Entry

R

Yield (%)

Entry

R

Yield (%)

1

C6H5

80

6

4-NO2C6H4

81

2

4-ClC6H4

82

7

2,4-diClC6H3

78

3

2-ClC6H4

79

8

4-OCH3C6H4

75

4

3-NO2C6H4

84

9

4-CH3C6H4

77

Synthesis of 1,2,3-triazoles

Albadi et al. [ 135 ] synthesized CuO supported nano-ZnO [nano-CuO–ZnO] by a co-precipitation method and well characterized it by XRD, SEM, TEM and EDS analysis. Using this newly prepared efficient catalytic system they synthesized a series of 1,2,3-triazoles ( 89 ) via one-pot three-component reactions of various benzyl halides ( 88 ) phenylacetylenes ( 48 ) and sodium azide ( 41 ) in water under reflux condition (Table  26 ). During optimization combined nano (CuO–ZnO) showed better catalytic efficacy than the individual nano-ZnO or nano-CuO. After completion of reaction, the catalyst was recovered easily and recycled for several runs without loss in catalytic activity.

Table 26

ZnO nanoparticles catalyzed synthesis of 1,2,3-triazoles

Entry

R

X

R1

Time (min)

Yield (%)

1

H

Br

H

20

92

2

3-CH3

Br

H

20

91

3

4-OCH3

Br

H

15

92

4

2,4-diCl

Br

H

35

90

5

4-NO2

Br

H

35

91

6

H

Br

3-NH2

30

89

7

4-OCH3

Br

3-NH2

20

90

8

H

Cl

H

30

92

9

2,4-diCl

Cl

H

18

89

Conclusions

The present review describes the recent developments on the synthesis of biologically promising heterocycles using nano zinc oxide as a mild, cheap, non-toxic, efficient, reusable, Lewis acidic heterogeneous catalyst. For many organic transformations, catalytic efficacy of nano ZnO was found to be superior to the commercially available bulk ZnO. After completion of reaction, in many occasions nano-ZnO was successfully recovered and reused further for the several runs without significant loss in catalytic activity. This review will enrich the readers about the developments of ZnO nanoparticles catalyzed synthesis of various heterocycles reported so far. Therefore, the present review will surely attract attention of the organic methodologists working with this fascinating catalyst worldwide.

Acknowledgements

The author is grateful to Dr. Sudhir Kartha, Chancellor, Indus International University, Una, Himachal Pradesh, India for his wholehearted support throughout and the Kartha Education Society, Mumbai, India for the financial help. Special thanks are due to Mr. Pankaj Choudhary, Dr. Samik Mukherjee and Dr. Sudipta Som for their able assistance to search the literature.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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