Greener Synthesis of some Bioactive 2,5-Disubstituted-1,3,4-Oxadiazoles

 

Parin K. Vora, Dr. Rakesh R. Somani^*, Madhuri H. Jain, Sonali S. Parab, Pallavi M. Patil

Department of Pharmaceutical Chemistry, Vivekanand Education Society’s College of Pharmacy, Hashu Advani Memorial Complex, Behind Collectors Colony, Chembur, Mumbai-74, India.

 

ABSTRACT:

In the present paper, a series of substituted 1,3,4-oxadiazoles have been synthesized by three different methods i.e. conventional heating, microwave irradiation method 1 and microwave irradiation method 2. Conventional heating, microwave irradiation method 1 and microwave irradiation method 2 were compared in terms percent yield, reaction time, percent atom economy and environment quotient (eco-friendly). The results showed that microwave irradiation method 2 was remarkably greener and ecofriendly than microwave irradiation method 1 and conventional heating. Derivatives of 1,3,4-oxadiazoles were characterized on the basis of IR, ¹H NMR and Mass spectral data and the structures of the synthesized compounds were confirmed. All compounds were screened for their in vitro antibacterial, antitubercular and antifungal activity. However the results indicated that the synthesized compounds have less activity with reference to their respective standards.

 

 

 

1. INTRODUCTION:

Drug discovery involves validation of new molecular entities (NME’s) to target various diseases. To synthesize these new molecular entities appropriate methods of synthesis is needed. However most of the methods of synthesis are inconvenient and remains a challenge due to several reasons such as production of waste, use of hazardous chemicals, , higher reaction time, less purity, less product yield etc^1,2. Thus in order to reduce the environmental impact there is a need for chemical processes including monitoring analysis, catalysts, synthetic procedures and reaction conditions. Thus to achieve these goals a sustainable and alternate route of synthesis was developed, known as Green Chemistry^3,4.

 

 

The main aim of green chemistry is to design most efficient processes and make use of chemicals that reduces time of the reactions, achieve high % yields with higher purity of products, use of safer and better chemicals and to reduce or eliminate the use and generation of hazardous substances and thus in order to achieve these goals, green chemistry approach is being developed based on nonconventional and innovative synthetic procedures including reactions carried out in water⁵, by ultrasound⁶, solvent free reactions⁷, in ionic liquids⁸, by microwaves⁹ and using solid backbone like clays k-10¹⁰. Thus green synthesis reduces reaction time, increases % yield and is eco-friendly. The procedures which are described for 2,5-disubstituted-1,3,4-oxadiazoles are not environmentally friendly, less % yields with longer reaction time. Thus the present work aims to synthesize 2,5-disubstituted-1,3,4-oxadiazole analogues using conventional heating, microwave irradiation method 1 and microwave irradiation method 2. Thus it describes the comparison of all three methods for efficient synthesis of target molecules. Furthermore these compounds were evaluated for antibacterial, antifungal and antitubercular activities to check their biological potential.

 

2. EXPERIMENTAL SECTION:

2.1. MATERIAL AND METHODS:

Solvents were purchased from Loba chemicals, India. Sulphuric acid, glacial acetic acid, potassium permanganate, hydrazine hydrate (99%), liquid bromine (99%), sodium acetate, aromatic acids like benzoic acid and nicotinic acid were purchased from Pallav chemicals, India. Aromatic aldehydes were purchased from Rajesh chemicals, India. Montmorillonite k-10 clay was purchased from Sigma-Aldrich, India. Melting points were determined on a "Veego" VMP-I apparatus by open capillary method. Microwave synthesis was carried out at power level ranging from 1 to H at 85 to 850 watts in Catalyst Microwave oven - CATA-2R,. Purity of compounds was checked on silica G 60 F₂₅₄ plates and visualized in UV chamber. The IR spectra were recorded on Shimadzu IR Infinity in the 400-4000 cm^-1 range by placing sample directly on probe. ¹H NMR were recorded on Bruker Avance II (400 MHz) spectrometer using trimethylsilane (TMS) as internal reference standard in DMSO or CDCl₃ as solvent. High-resolution mass spectra were obtained with a LC-MS (Q-TOF Micromass) LC (Waters). ¹H NMR and Mass spectra were recorded at Sophisticated Analytical Instrument Facility (SAIF), Punjab, India. Biological activities were done at Central Drug Research Institute, Lucknow, India.

 

2.2. General experimental procedures:

2.2.1.  Synthesis of esters (3,4)

Conventional - Mixture of aromatic acid (0.1 mol) in ethanol (1 mol) with catalytic quantity of conc. H₂SO₄ was refluxed for 4-5 hrs. The reaction mixture was cooled and was added in chilled water (50-100 mL). Excess of acid was neutralized with saturated NaHCO₃ solution. The contents were transferred to separating funnel and was extracted with diethyl ether. The ethereal layers were collected and evaporated to obtain pure ester (Scheme 1).

 

Microwave method 1 -

Solution of aromatic acid (0.1 mol) in ethanol (1 mol) with catalytic amount of conc. H₂SO₄ was irradiated under microwave oven for 60 mins at power level 2, 170 watts. The reaction mixture was cooled and was added in chilled water (50-100 mL). Excess of acid was neutralized with saturated NaHCO₃ solution. The contents were transferred to separating funnel and was extracted with diethyl ether. The ethereal layers were collected and evaporated to obtain pure ester (Scheme 2).

 

 

2.2.2.  Synthesis of hydrazides (5,6)

Conventional - Mixture of ester (0.1 mol) and hydrazine hydrate 99% (0.2 mol) in ethanol was refluxed for 4.25-5.25 hrs. Excess of ethanol was evaporated to obtain crude product and was recrystallized from 60% methanol (Scheme 1).

 

Microwave method 1 -

Mixture of ester (0.1 mol) and hydrazine hydrate 99% (0.2 mol) in ethanol was irradiated under microwave oven for 45-50 min at power level 3, 225 watts. Excess of ethanol was evaporated to obtain crude product and was recrystallized from 60% methanol (Scheme 2).

 

Scheme 1 (Conventional Method)

 

Microwave method 2 -

Mixture of aromatic acid (0.1 mol) and hydrazine hydrate (0.2 mol) was irradiated under microwave oven for 40-47 mins at power level 2, 170 watts. Solution was cooled to obtain crude product and was recrystallized from 60% methanol (Scheme 3).

 

 

2.2.3.  Synthesis of schiff bases (7-16)

Conventional -

Mixture of hydrazide (0.1 mol) and aromatic aldehyde (0.1 mol) in ethanol with few drops of glacial acetic acid (pH 4-5) was refluxed for 7-9 hrs. Excess of ethanol was evaporated to obtain crude product. Further the crude product was washed with water, dried and recrystallized from ethanol (Scheme 1).

 

Microwave method 1 -

Mixture of hydrazide (0.1 mol) and aromatic aldehyde (0.1 mol) in ethanol with few drops of glacial acetic acid (pH 4-5) was irradiated under microwave oven for 38-55 mins at power level 3, 225 watts. Excess of ethanol was evaporated to obtain crude product. Further the crude product was washed with water, dried and recrystallized from ethanol (Scheme 2).

 

 

Scheme 2 (Microwave Irradiation Method 1)

 

 

Microwave method 2 -

Mixture of hydrazide (0.1 mol) and aromatic aldehyde (0.1 mol) in ethanol with 3-5 drops of conc. H₂SO₄ (pH 4-5) was irradiated under microwave oven for 5-12 mins at power level 3, 225 watts. Excess of ethanol was evaporated to obtain crude product. Further the crude product was washed with water, dried and recrystallized from ethanol (Scheme 3).

 

 

Synthetic Scheme 3 (Microwave Irradiation Method 2)

 

2.2.4.  Synthesis of 2,5-disubstituted-1,3,4-oxadiazoles (17-26)

Conventional -

Mixture of schiff base (1 mol) and sodium acetate (2 mol) was dissolved in glacial acetic acid (5-10 mL) by stirring on magnetic stirrer. Solution of liquid bromine 99% (1.6 mol) in glacial acetic acid (5-10 mL) was added dropwise to the above mixture with stirring. The solution was left for stirring for 15-33.5 hrs (until the colour of bromine completely disappears). The solution was poured on crushed ice and solid separated was filtered, dried and recrystallized from 50-60% ethanol (Scheme 1).

 

 

Microwave method 1 -

Mixture of schiff base (1 mol) and potassium permanganate (1.6 mol) in mixture of acetone: water (20:5) was irradiated under microwave oven for 12-25 mins at power level 2, 170 watts. Acetone was evaporated and aqueous layer was extracted with chloroform. Chloroform layer was separated and evaporated to obtain crude product and was recrystallized from 50-60% ethanol (Scheme 2).

 

Microwave method 2 -

Mixture of schiff base (1 mol), potassium permanganate (1.6 mol) and activated montmorillonite k-10 clay (20-25% by weight of total reactants) were mixed properly and were irradiated under microwave oven for 11-26 mins at power level 2, 170 watts. Mixture was then washed with water: chloroform mixture. Chloroform was separated and evaporated to obtain crude product and was recrystallized from 50-60% ethanol (Scheme 3).

 

The characterization data of the compounds are tabulated in table no. 1.

 

 

 

Table No.1 - Characterization data of compounds

Compound Code
	Ar1	Ar2	Melting point (°C)
17		Phenyl	138
18		2-flurophenyl	120-124
19		3-flurophenyl	128-130
20		4-flurophenyl	150-152
21		4-chlorophenyl	160
22		4-bromophenyl	164-168
23		3-chlorophenyl	100-102
24		2-chlorophenyl	96-98
25		4-chlorophenyl	150-156
26		3-chlorophenyl	118-122

 

 

 

2.3 Physicochemical and spectral characteristics of the synthesized compounds:

2.3.1.    2,5-diphenyl-1,3,4-oxadiazole (17)                                                                                                   

C₁₄H₁₀N₂O; IR (cm^-1): 3061.03 - CH stretch, 1550.77 - C=N stretch, 1068.56 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, CDCl₃): δ 7.42-7.49 (m, 6H), 8.05-8.07 (m, 4H).

 

2.3.2.   2-(2-flurophenyl)-5-phenyl-1,3,4-oxadiazole (18)                                                                          

C₁₄H₉FN₂O; IR (cm^-1) : 3062.96 - CH stretch, 1587.42 - C=N stretch, 1064.71 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, CDCl₃): δ 7.27-7.36 (m, 2H), 7.53-7.6 (m, 4H), 8.16-8.2 (m, 3H).

 

2.3.3.   2-(3-flurophenyl)-5-phenyl-1,3,4-oxadiazole (19)                                                                          

C₁₄H₉FN₂O; IR (cm^-1) : 3087.2 - CH stretch, 1547.94 - C=N stretch, 1068.61 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, CDCl₃): δ 7.25-7.3 (m, 1H), 7.51-7.59 (m, 4H), 7.84-7.87 (d, 1H), 7.95-7.97 (d, 1H), 8.14-8.17 (m, 2H).

 

2.3.4.   2-(4-flurophenyl)-5-phenyl-1,3,4-oxadiazole (20)                                                                          

C₁₄H₉FN₂O; IR (cm^-1) : 3066.95 - CH stretch, 1549.87 - C=N stretch, 1070.54, 1148.66 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, CDCl₃): δ 7.22-7.28 (m, 2H), 7.53-7.58 (m, 3H), 8.13-8.18 (m, 4H).

 

2.3.5. 2-(4-chlorophenyl)-5-phenyl-1,3,4-oxadiazole (21)                                                                         

C₁₄H₉ClN₂O; IR (cm^-1) : 3061.03 - CH stretch, 1550.77 - C=N stretch, 1074.35, 1087.85 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, CDCl₃): δ 7.49-7.58 (m, 5H), 8.06-8.08 (d, 2H), 8.11-8.14 (d, 2H).

 

2.3.6. 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (22)                                                                         

C₁₄H₉BrN₂O; IR (cm^-1) : 3061.03 - CH stretch, 1552.77 - C=N stretch, 1072.42 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, DMSO): δ 7.60-7.67 (m, 3H), 7.78-7.8 (d, 2H), 8.05-8.07 (d, 2H), 8.12-8.13 (d, 2H).

 

2.3.7. 2-(3-chlorophenyl)-5-phenyl-1,3,4-oxadiazole (23)                                                                         

C₁₄H₉ClN₂O; IR (cm^-1) : 3068.88 - CH stretch, 1549.87 - C=N stretch, 1086.93 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, CDCl₃): δ 7.41-7.49 (m, 2H), 7.5-7.58 (m, 4H), 8.09-8.12 (d, 1H), 8.14-8.16 (d, 2H).

 

2.3.8. 2-(2-chlorophenyl)-5-phenyl-1,3,4-oxadiazole (24)                                                                         

C₁₄H₉ClN₂O; IR (cm^-1) : 3061.03 - CH stretch, 1550.77 - C=N stretch, 1074.35, 1087.85 - C-O-C stretch (oxadiazole); HR-MS (ESI) (m/z) (M+1) for C₁₄H₉ClN₂O was found 257.12846727 (100%) and 259.13492437 (35%).

 

2.3.9. 3-(5-(4-chlorophenyl)-1,3,4-oxadiazol-2-yl)pyridine (25)                                                                             

C₁₃H₈ClN₃O; IR (cm^-1) : 3086.11 - CH stretch, 1598.99 - C=N stretch, 1074.35, 1089.78 - C-O-C stretch (oxadiazole); ¹H NMR (400 MHz, CDCl₃): δ 7.51-7.56 (d, 3H), 8.1-8.12 (d, 2H), 8.46-8.48 (d, 1H), 8.85 (s, 1H), 9.39 (s, 1H); HR-MS (ESI) (m/z) (M+1) for C₁₃H₈ClN₃O was found 258.12904218 (100%) and 260.13611165 (30%).

 

2.3.10. 3-(5-(3-chlorophenyl)-1,3,4-oxadiazol-2-yl)pyridine (26)                                                                            

C₁₃H₈ClN₃O; IR (cm^-1) : 3068.88 - CH stretch, 1549.87 - C=N stretch, 1086.93 - C-O-C stretch (oxadiazole); HR-MS (ESI) (m/z) (M+1) for C₁₃H₈ClN₃O was found 258.13703841 (100%) and 260.14841279 (30%).

 

3. RESULTS AND DISCUSSION:

3.1.  Comparison of all 3 methods to synthesize 1,3,4-oxadiazoles:

All 3 methods are compared in terms of % yield, reaction time, atom economy and eco-friendly.

 

3.1.1.  % Yield -

The % yield for hydrazide and schiff base by conventional and microwave  method 1 was less as compared to microwave 2 while % yield for oxadiazoles by Microwave 1 was greater than microwave 2 and conventional method (Table No. 2).

 

3.1.2.  Reaction time -

The reaction time for hydrazides and schiff bases synthesis by microwave 2 was less than microwave 1 and conventional method. The reaction time for oxadiazoles synthesis by microwave 2 and microwave 1 was almost same while conventional method required much longer time. Thus overall microwave method 2 was faster than microwave 1 and conventional method (Table No. 3 and Table No. 4).

 

Table No. 2 - Comparison of % yield of all 3 methods

Compound Code		% Yield
		Conventional	Micro wave 1	Micro wave 2
5	Synthesis of Hydrazides	64.8	72.06	84.32
6		61.84	70.81	83.77
7	Synthesis of Schiff bases	74.16	79.33	81.46
8		71.63	79.49	80.34
9		71.9	78.93	80.62
10		70.79	79.21	80.1
11		68.42	77.63	79.21
12		69.73	76.23	78.48
13		71.58	78.27	82.2
14		70.79	78.95	79.74
15		69.13	76.52	77.31
16		69.92	75.73	77.04
17	Synthesis of Oxadiazoles	31.31	69.7	60.61
18		29.3	65.7	57.6
19		31.82	68.7	59.6
20		32.53	71.2	61.62
21		30.3	70.71	63.64
22		33.4	71.72	64.14
23		28.59	64.14	55.56
24		28.18	62.73	54.34
25		26.87	62.63	53.94
26		26.06	60.61	52.53

 

 

 

Table No. 3 - Comparison of reaction time for hydrazide synthesis by all 3 methods

Compound Code	Synthesis of Hydrazides		Reaction time
			Conventional (hrs)	Microwave 1(mins)	Microwave 2 (mins)
5		Acid to ester	4-5	60	-
		Ester to hydrazide	4.25-5.25	45	-
		Acid to hydrazide	-	-	40-45
		Total	8.25-10.25	105	40-45
6		Acid to ester	4-5	60	-
		Ester to hydrazide	5.25	50	-
		Acid to hydrazide	-	-	45-47
		Total	9.25-10.25	110	45-47

 

Table No. 4 - Comparison of reaction time for schiff base and oxadiazole synthesis by all 3 methods

Compound Code		Reaction time
		Conventional (hrs)	Microwave 1 (mins)	Microwave 2 (mins)
7	Synthesis of Schiff bases	8-9	50-55	10-12
8		7-8	45	8-9
9		7.5-8	40-45	7-9
10		7-7.5	40-43	7
11		7-7.5	40-42	7
12		7	38-40	5-6
13		7.25-8	42-46	7-8
14		7-8	43-45	8
15		8-8.5	47-50	8-11
16		8.5	50	10-11
17	Synthesis of Oxadiazoles	15-16	12-14	11-13
18		32-32.5	23	22-23
19		30.5-31	20-23	22-24
20		24-25	20	19-21
21		20-21	15-17	14-16
22		16-16.5	13-16	12-15
23		28-29	20-21	21
24		32.5-33.5	24-25	24-26
25		21.5-22	17-19	18
26		30-30.5	21-22	20-23

3.1.3. % Atom Economy -

The concept of atom economy is a method of expressing how efficiently a particular reaction makes use of the reactant atoms. This is defined as the percentage of the ratio of molecular weight of the target molecule to the sum total of molecular weights of all the substances produced in the equation for the reaction involved.

 

% Atom economy for compound 17 by conventional method (Scheme 4) is shown in table no. 5.

 

Scheme 4: Synthesis of compound 17 by conventional method

 

 

Table No. 5 - Calculation of % Atom Economy of compound 17 (Conventional method)

Sr. No.	Mol. formula of reactants	Mol. wt. of reactants	No. of moles used	No. of moles × Mol. wt. of reactant (M)	Atoms utilized	Mol. wt. of products
1.	C14H12N2O	224	1	224	14C, 10H, 2N,O	222
2.	Liq. Br2	160	1.6	256	-	-
3.	CH3COONa	82	2	164	-	-
Total	16C, 15H, 2N, 3O, 2Br, 1Na	466	4.6	644	14C, 10H, 2N,O	222
% Atom Economy = (Mol. wt. of desired product/ M) × 100 = (222/ 644) × 100 = 34.47%

 

 

 

% Atom economy for compound 17 by microwave method 1 (Scheme 5) is given in table no. 6.

 

 

Scheme 5: Synthesis of compound 17 by microwave  method 1

 

 

 

 

Table No. 6 - Calculation of % Atom Economy of compound 17 (Microwave method 1)

 

% Atom economy for compound 17 by microwave method 2 (Scheme 6) is given in table no. 6

 

 

Scheme 6: Synthesis of compound 17 by microwave  method 2

Calculations and % atom economy is same as microwave method 1.

 

The % atom economy of compound 17 by microwave 1 and microwave 2 methods (46.56%) is greater than conventional method (34.47%).

 

Similarly atom economy of other  compounds was calculated and represented in the Table No. 7.

 

 

 

Table No. 7 - Comparison of % atom economy of oxadiazoles by all 3 methods

 

 

 

3.1.4.  Environmental quotient (eco-friendly) - In conventional and microwave  method 1, solvent (ethanol) and conc. H₂SO₄ were used while microwave 2 is a solventless method. Also conc. H₂SO₄  (non-greener component) was not used in microwave 2. In schiff base synthesis, glacial acetic acid was used as a catalyst (pH adjustment) in conventional and microwave 1 while in microwave 2 conc. H₂SO₄ was used. Even though conc. H₂SO₄ is a non-greener component, it is considered as a greener component only for schiff base synthesis as it increased % yield, decreased reaction time and quantity of conc. H₂SO₄ used was less as compared to glacial acetic acid. In conventional method for oxadiazoles synthesis, liquid bromine was used which is hazardous and required precaution while handling. Thus use of liquid bromine is not considered as eco-friendly. On the other side in microwave 1 and microwave 2, potassium permanganate was used which is less toxic and easy to handle than liquid bromine but in microwave 1 acetone: water mixture was used as solvent and acetone is carcinogenic (hazardous) while in microwave 2 montmorillonite K-10 clay was used as a solid backbone which eco-friendly and it is recyclable. Thus microwave method 2 is eco-friendly than conventional and microwave method 1.

 

3.2. Antibacterial, antifungal and antitubercular activity:

For antibacterial activity Gram positive strain of Staphylococcus aureus and Gram negative strain of Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa were used. For antifungal activity Aspergillus niger and Candida albicans were used. For antitubercular activity Mycobacterium tuberculosis H37Ra strain was used. Results of anti-bacterial, anti-fungal and antitubercular activity are mentioned in table no. 8 and 9 respectively. For anti-bacterial activity standard used was Levofloxacin, for anti-fungal activity Fluconazole was used and for antitubercular Rifampicin was used. The results indicated that all synthesized compounds have less antibacterial, antifungal  and antitubercular activity with reference to their respective standards.

 

 

 

 

Table No. 8 - Antibacterial activity

 

Table No. 9 - Antifungal activity and antitubercular activity

 

 

 

4. CONCLUSION:

Microwave irradiation method 2 was remarkably greener than microwave irradiation method 1 and conventional heating. All the compounds were screened for their in vitro antibacterial, antifungal and antitubercular activity and results shown that they have less antibacterial, antifungal  and antitubercular activity with reference to their respective standards.

 

5. ACKNOWLEDGEMENT:

We would like to acknowledge CDRI, Lucknow for anti-bacterial, anti-fungal and antitubercular activity. We are grateful to Sophisticated Analytical Instrument Facility (SAIF), Punjab for ¹H NMR and Mass spectral data.

 

6. CONFLICT OF INTEREST:

The authors confirm that this article’s content has no conflicts of interest.

 

7. REFERENCES:

1.         Gujral SS et al. A Focus and Review on the Advancement of Green Chemistry. Indo Global Journal of Pharmaceutical Sciences. 2 (4); 2012:  397-408.

2.         Somani RR and Bhanushali UV. Synthesis and Evaluation of Antiinflammatory, Analgesic and Ulcerogenic Potential of NSAIDs Bearing 1,3,4-Oxadiazole Scaffold. India Journal of Pharmaceutical Sciences. 73 (6); 2011: 634-640.                                                                                          

3.         Linthorst JA. An overview: origins and development of green chemistry. Foundations of Chemistry. 12 (1); 2009: 55-68.                                                        

4.         Anastas PT and Warner JC. Green Chemistry: Theory and practice Eds. Oxford University Press: Oxford, UK. 1998.                                                                                                                                                                                                                                         

5.         Katritzky AR et al. Reactions in High-Temperature Aqueous Media. Chemical Reviews. 101 (4); 2001: 837-892.                                                                                                                                     

6.         Puri S et al. Applications of Ultrasound in Organic Synthesis - A Green Approach. Current Organic Chemistry. 17 (16); 2013: 1790-1828.

7.         Tanaka K and Toda F. Solvent-free organic synthesis. Chemical Reviews. 100 (3); 2000: 1025-1047.                                                                

8.         Earle MJ and Seddon KR. Ionic liquids. Green solvents for the future. Pure and Applied Chemistry. 72 (7); 2000: 1391-1398.                                            

9.         Nuchter M et al. Microwave assisted synthesis - a critical technology overview. Green Chemistry. 6 (3); 2004: 128-141.                                                                                        

10.       Habibi D and Marvi O. Montmorillonite KSF and montmorillonite K-10 clays as efficient catalysts for the solventless synthesis of bismaleimides and bisphthalimides using microwave irradiation. ARKIVOC. 2006 (13); 2006: 8-15.

 

 

 

 

Accepted on 05.07.2016        © RJPT All right reserved

Research J. Pharm. and Tech 2016; 9(9):1433-1440.