Document Type : Original Article

Authors

1 Department of Chemistry, Rajarshi Shahu Mahavidyalaya, (Autonomous) Latur (M. S.), India

2 Department of Chemistry, Rajarshi Shahu Mahavidyalaya, (Autonomous) Latur (M. S.), India.

Abstract

ABSTRACT
A new series of Schiff bases derived from DHA (Dehydroacetic acid) and aromatic primary amines were synthesized and characterized by elemental and spectral (Electronic, IR and 1HNMR) analysis. The signals of 1HNMR spectrum and characteristic peaks in IR spectra are used for the molecular structure elucidation of synthesized Schiff bases. The antimicrobial activities of synthesized compounds have been screened in vitro against Escherichia coli, Staphylococcus aureus, Bacillus subtilis bacterial species, Candida albicans and Aspergillus niger fungal organism and found to exhibit strong antifungal activity than antibacterial activity. Among the studied DHA Schiff base of 2-amino-4,6-dimethylpyrimidine depicted discerning antifungal and antibacterial activities.

Graphical Abstract

Synthesis, Spectral and Biological Studies of DHA Schiff Bases

Keywords

Main Subjects

 

Introduction

S

*Corresponding Author:Dhananjay Gangadhar Palke (dhananjaypalke@gmail.com)

chiff bases are most widely used chelating agents in co-ordination chemistry [1-3]. Antiviral agents are very effective against mouse hepatitis virus (MHV) [4, 5]. Schiff bases and their complexes have broad range of application synthesized from the condensation of an amine scaffold with carbonyl compounds. The Schiff bases and their compounds exhibit a broad range of biological activities as antiproliferative, including antifungal, antibacterial, antimalarial, anti-inflammatory, antiviral, and antipyretic properties. Many Schiff bases are metal complexes and exhibit catalytic property, which play a significant role in the organic transformation to increase the yield of the product as well chemoselectivity of the reaction.[6]. Research studies have been done on synthesis of various Schiff bases derivatives from the reaction of dehydroacetic acid (DHA) and aliphatic/aromatic primary amines, hydrazides, and thiosemicarbazides [7-8].Spectral studies of Schiff bases containing heterocyclic ring are comparatively minor[9-10].

Based on what stated above and to pursue our research on the synthesis of novel DHA- Schiff and their biological properties, herein we have reported the synthesis, characterisation and biological activity of novel DHA- Schiff derivativres. 

Experimental

DHA was purchased from E-Merck Germany, 4-amino phenol, 2-aminopyridine, 2-amino-6-nitrobenzothiazole, 2-amino-5-chlorobenzophenone and 2-amino-4,6-dimethylpyrimidine were obtained from Avra and Acros Organics chemicals. The solvents were dried and distilled before use as per reported procedure [11]. Elemental (C, H and N) analysis was carried out on Perkin Elmer CHN Analyser (2400). The electronic spectra were recorded on Shimadzu UV–VIS Spectrophotometer UV1700, in the range of 200–500 nm at our research center. The IR spectra of Schiff’s bases were recorded on Perkin Elmer (1430) FTIR spectrophotometer in the range 4000 to 666 cm-1 by KBr pellet method and on alpha Brucker at our research centre RajarshiShahuMahavidyalaya (Autonomous) Latur. 1HNMR spectra were recorded on Brucker FT 300/400/500 MHz NMR spectrophotometer in CDCl3 solvent using TMS as reference. The biological activities (antibacterial and antifungal) of synthesized Schiff bases were tested against Escherichia coli (E.coli) (ATCC2331), Staphylococcus aureus (S.aureus) (NCIM-2079) and Bacillus subtilis (B.subtilis) (NCIM-2063) as bacterial strains and Candida albicans (C.albicans) (MTCC-227) and Aspergillus niger (A.niger) (NCIM-545) as fungal strains as per the procedure [12, 14] from our Biotechnology Research Centre.

Synthesis of schiffbases

DHA Schiff bases were synthesized by the addition of ethanolic solution of individual primary amine (0.05 mol) into hot ethanolic solution of DHA (Dehydroacetic acid) (0.05 mol) as per the procedure [13, 14] [Figure 1].


Figure 1. Synthesis of DHA Schiff bases


Entries

R

Entries

R

L1

L4

L2

L5

L3


Results and Discussion

Elemental analysis

All the synthesized DHA Schiff bases are yellow, orange to brown coloured solids, stable to air and non-hygroscopic. They are insoluble in water and soluble in hot ethanol. Their physical characteristics and elemental analysis data are summarized in Table 1.


Table1. Elemental analysis

Entries

Compound

Formula

Colour

F.Wt.

M.P.

0C

Found/(Calculated) %

C

H

N

L1

C14H13N

Gray

259

120

63.92(64.86)

5.12(5.01)

5.34(5.40)

L2

C13H12N2

Dark Orange

244

90

64.01(63.93)

04.64(4.92)

11.41 (11.47)

L3

C15H11N3O5S

Orange

345

158

51.91(52.17)

3.24(3.18)

11.35(12.17) S 7.87(9.27

L4

C21H16N

Yellow

379.5

98

64.54(66.40)

4.25(4.21)

3.72(3.68) Cl-8.55(9,35)

L5

C14H13N3

Brown

271

85

62.25(61.99)

4.30(4.80)

14.69(15.50)

 


Spectral characterization

Electronic spectra

All the synthesized DHA Schiff bases exhibited weak absorption bands λmax in the region 45871 to 42918 cm-1, which was assigned to conjugated azomethine π→π* transition appear for >C=N.

The absorption band λmax in the region 40000 - 37453 cm-1 appeared due to π→π*and n→π* transitions of >C=N in a heterocyclic ring of amine moiety i.e pyridine at 38785 cm-1, thiazole 38934 cm-1 and Pyrimidine at 38979 cm-1. Benzene ring also showed absorption at 40000 cm-1 [15].

All the DHA Schiff bases showed one more absorption band λmax in the region 33167 to 31655 cm-1 assigned to lactone carbonyl group >C=O, auxochrome –OH attached to homoannular conjugated diene, which was closer to calculated value 31948 cm-1 [16].

Conjugated Lactone displayed absorption in the region of 50000-41666 cm-1. The extended conjugation and auxochrome produce a bathochromic shift [15].

FTIR analysis

The absorption peak pattern in IR spectra exhibited complex nature due to various vibrational modes. However, with few objectives, only characteristic peaks which were specific to all Schiff bases related to enolic O-H, aromatic C=C, azomethine >C=N, aryl azomethine, lactone carbonyl C=O and enolic C-O/C=O of Schiff bases were taken into consideration for characterization.

In all the synthesized DHA Schiff bases the characteristic O-H stretching frequencies were observed as broad weak band at 3454 to 3368 cm-1 due to strong intermolecular hydrogen bonding between enolic O-H and N of azomethine group.

Azomethine >C=N stretching frequency is dependent on its substituent and mostly causes resonance interaction and H-bonding. In the present work azomethine (>C=N-) depicted strong absorption stretching vibration band in the region 1644-1634 cm-1.

The peak in the region 1708 to 1685 cm-1 appeared due to>C=O lactone carbonyl stretching vibrations; peak in the region 1388 to 1333 cm-1appeared due toaryl/aliphatic (C-N) stretching vibrations and the peak in the region 1256 to 1214 cm-1appeared due to (C-O) enolic stretching vibrations.[15,16,17] IRCharacteristic IR frequencies are summarized in Table 2.


Table 2.Characteristic IR frequencies (cm-1) of Schiff bases

Compounds

(O—H)

(>C=O)

(>C=N)

(>C=C<)

(C--N)

(C--O)

L1C14H13NO4

3427

1689

1646

1571

1358

1256

L2C13H12N2O3

3419

1688

1644

1588

1333

1214

L3C15H11N3O5S

3454

1691

1643

1564

1376

1234

L4C21H16NO4Cl

3368

1690

1641

1563

1382

1261

L5C14H13N3O3

3417

1708

1634

1590

1353

1216


 


1HNMR Spectra

1HNMR spectra of all the compounds were recorded in CDCl3 at room temperature. The following signals, chemical shift value δ (ppm) relative to TMS as internal standard were observed:

Singlet signal at δ value 2.1—2.28 for the methyl group at C6, singlet signal at δ value 14.72 —16.00 of enolic O-H group which is highly deshielded, singlet signal at δ value 5.00—5.94 belonging to H atom at C5 and singlet signal at δ value 2.57—2.75 for the methyl group attached to azomethine C atom of DHA moiety and different signals of amine moiety.[17]

L1 =1HNMR (500 MHz, CDCl3, δ, ppm : 2.26 (3H, s, C6–CH3), 15.88 (1H, s, O –H), 5.92 (1H, s, C5–H), 2.58 (3H, s, N=C-CH3,H bonded to ‘C’ azomethine) for DHA moiety,7.4 (2H, d, Ar–H), 7.6 (2H, d, Ar–H), 9.6 (1H, s, Broad) for phenol moiety.

L2 = 1HNMR (400 MHz CDCl3, δ, ppm: 2.2 (3H, s, C6–CH3), 16.00 (1H, s, O –H), 5.8(1H, s, C5–H), 2.6(3H, s, N=C-CH, H bonded to ‘C’ azomethine) for DHA moiety, 7.7-8.5 (4H, m, Ar)  for pyridine  moiety.

L3 = 1HNMR (400 MHz CDCl3, δ, ppm: 2.1(3H, s, C6 –CH3), 15.85(1H, s, O–H), 5.9(1H, s, C5 –H), 2.6(3H, s, N=C-CH3, H bonded to ‘C’ azomethine) for DHA moiety, 7.7 (1H, m, C5-H), 6.9-7.2 (2H, d, C7& C8-H),  for benzothiazole moiety.

L4= 1HNMR (300 MHz CDCl3, δ, ppm: 2.2 (3H, s, C6 –CH3), 15.54 (1H, s, O–H), 5.75(1H, s, C5–H), 2.7(3H, s, N=C-CH3,H bonded to ‘C’ azomethine) for DHA moiety, 2.3(3H, s, ring –CH3), 7.4 -7.6 (5H, s, Ar.)  7.6 -7.8 (3H, m, Ar.) for benzophenone moiety.

L5=1HNMR (300 MHz CDCl3, δ, ppm: 2.11 (3H, s, C6–CH3), 14.82(1H, s, O–H), 5.7(1H, s, C5–H), 2.75(3H, s, N=C-CH3, H bonded to ‘C’ azomethine) for DHA moiety, 2.34 (6H, s,  two CH3 to Ar), 7.6 (1H, s,), for pyrimidine moiety.

Screening for bioactivity

In vitro antibacterial and antifungal activities of the compounds were screened by considering zone of inhibition of growth. The synthesized DHA Schiff bases were screened with their different concentrations with standard antibiotics such as streptomycin (1 mg/mL) and griseofulvin (1 mg/mL).

The DHA Schiff bases have shown harmonious antibacterial and antifungal action. All the DHA Schiff bases were found to be biologically active against E.coli with maximum zone of inhibition 21 mm by L5. L1, L2 and L3 showed moderate zone of inhibition 17 to 18 mm and L4 has showed minimum zone of inhibition 15 mm againstE.coli.  L5 showed 16.4 mm, maximum zone of inhibition against S. aureus.  L2 and L4, were found to be moderate in growth inhibition while L3 showed low activity with maximum zone of inhibition 9.00 mm against the same species. All the compounds were found to be moderate in growth with 11.3 mm zone of inhibition by L2 while Schiff base L4 low activity with 4.5 mm zone against B. subtilis. All the DHA Schiff bases have shown harmonious antifungal action.  All compounds   showed about 13 to 16 mm zone of inhibition and L5 depicted maximum zone of inhibition 19 mm against A. niger. All compounds showed about 13 to 16 mm zone of inhibition, while L2 showed 21 mm maximum zone of inhibition, along with the compound L1 low zone of inhibition with 11.3 mm against human opportunistic pathogen, C. albicans.  The bioactivity data is summarized in Table 3.


Table 3.Summarized bioactivity data

Sr. No

Compound

Antibacterial (mm) Zone of Inhibition

Antifungal (mm) Zone of Inhibition

B1

B2

B3

F1

F2

1

L1

17.0

10.0

10.0

13.0

11.3

2

L2

18.0

12.7

11.3

16.5

21.0

3

L3

17.5

09.0

08.2

16.0

13.0

4

L4

15.5

10.3

04.5

13.4

17.0

5

L5

21.0

16.4

09.0

19.0

16.0

6

Streptomycin

28.5

19.5

16.0

00.0

00.0

7

Griseofulvin

00.0

00.0

00.0

22.0

25.0


B1 - E. coli, B2 -S. aureus, B3- B. subtilis, F1-C. albicans, F2- A. niger, NA-Not applicable


 

Figure 2. Graphical presentation of bioactivity


Conclusion

All the Schiff bases are yellow, orange to brown coloured solids, stable to air and non-hygroscopic. The composition of all synthesized Schiff bases was confirmed by elemental analysis and their structures were determined by IR and 1HNMR spectroscopic techniques.All the synthesized Schiff bases were found to possess strong antifungal activity than antibacterial activity. The compound L5 the Schiff Base of 2-amino-4,6-dimethylpyrimidine exhibited the stronger inhibitor of growthas compared to other Schiff bases. Least growth inhibitory activity was shown by compound L4the Schiff Bases of 2-amino-5-chlorobenzophenone.

Acknoledgement

The authors express sincere thanks to Principal RajarshiShahuMahavidyalaya, (Autonomous) Latur. Director Department of Chemistry, S. R. T. M. University, Nanded. Head, Department of Chemistry and Analytical Chemistry and All the staff of Department of Chemistry and Analytical Chemistry RajarshiShahuMahavidyalaya, (Autonomous) Latur. The authors are also thankful to A. A. Yadav IQAC Coordinator, S. H. Kathawate Savitribai Phule Pune University, Pune, Manisha Dhotre, S.S. Kulkarni Department of Biotechnology, RajarshiShahuMahavidyalaya, (Autonomous) Latur.

Orcid:

Shubham Bhagwatrao Biradar:

https://orcid.org/0000-0003-4270-4738

 DnyaneshwarVithalNarte: https://orcid.org/0000-0001-5458-1945

Rushikesh Pradip Kale:

 https://orcid.org/0000-0002-7133-4820

KalimoddinInayatsabMomin                             : https://orcid.org/0000-0001-6297-6509

MarotiSayabuSudewad:

https://orcid.org/0000-0001-5303-7841

Kundan Chandramani Tayade:

https://orcid.org/0000-0001-5492-2121

Dhananjay Gangadhar Palke:

https://orcid.org/0000-0002-3096-7269

 

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