16046
Synthesis of Novel Β-Lactams of Benzothiazole as Antibacterial Agents
Zeid Hassan Abood*, Zainab Salam Merzah, and Husham Attallah Suhail Chemistry Department, College of Science, University of Kerbala, Kerbala-Iraq
Corresponding author‟s E-mail: zainab.s @uokerbala.edu.iq ABSTRACT
Sodium nitrites and sulfuric acids were used to diazotize the amino function of 2- aminobenzothiazole [I]. To produce azo derivative consisting of aldehyde groups, the process of making diazonium salt was directly completed together with 2-hydroxybenzaldehyde dissolved in sodium hydroxide solution. Several substituted anilines such as (4-nitroaniline, 3- nitroaniline, 2-nitroaniline, 4-chloroaniline, 2-chloroaniline, 2, 4-dichloroaniline, and 4- bromoaniline) were used to treat the compound, alongside the use of the technique of microwave irradiation in absolute ethanol so that the imine derivatives were given, correspondingly. The use of the technique of microwave irradiation in dimethylformamide, in the company of cyclization reaction of imines with α-chloroacetyl chloride separately presented new β-lactams of benzoltiazole [IV] a-g. Escherichia coli (Gram-negative) and staphylococcus aurous (Gram-positive) were used to complete the antibacterial activity of β-lactam compounds. The results of the study showed that improved activities to gentamycin against Gram-positive bacteria were displayed by the entire azetidin-2-one compounds. Alternatively, the azetidine-2-one compounds [IV]band [IV] c demonstrated larger activities against Gram- negative bacteria in comparison with that of the control drug.
Keywords: β-lactams; Benzothiazoles; Schiff bases; Azo; Antibacterial potency.
INTRODUCTION
Heterocyclic compounds are regarded as available and obtainable compounds in nature.
Heterocyclic compounds are also considered substantial compounds as they are featured with various physiological activities related to this category of chemical materials(Mathur, 2019).
Benzothiazole is a heterocyclic compound known for the diverse biological activities and fragile base, and the continuous scientific importance and interest among scholars and chemists(Venugopala et al., 2019). Benzolhiazoles are extensively used in medical and bioorganic areas of chemistry with various scientific uses and applications in new drug inventions and discoveries(Amnerkar et al., 2015). Several biological activities, namely: anti- diabetic(Chhabra et al., 2016), anthelmintic (Patil et al., 2015), anticancer (Osmaniye et al., 2018), and antimicrobial (Keri et al., 2015)activities.
Developing antibiotics is still one of the most noteworthy scientific improvements in contemporary medicine (Drawz & Bonomo, 2010), (Zaffiri et al., 2012), (Qin et al., 2014), (Bell & MacLean, 2018). Limitless lives are saved by antibiotics making them a constant backbone of treatment and cure from microbial infections such as bacteria (Adediran & Pratt, 2008), (Solensky, 2012),(Lingzhi et al., 2018),(Lima et al., 2020). Since the 1940s, penicillin is one of the antibiotics featured with the availability of azetidin-2-one (β-lactam) structure in the pharmaceutical chemistry arena (Kumar et al., 2013),(Solensky, 2014),(Veiga & Paiva, 2018).
The β-lactam (2-azetidinone) ring, one of the highly praised and recently investigated
heterocycles, speaks of a tale full of coincidence, curiosity, and gravity covering the fields of medicine, biology, and chemistry(Tahlan & Jensen, 2013). The β-lactam (2-azetidinone) ring is famous in the scientific field as it significantly affects the health area at the world level(Pitts &
Lectka, 2014). Its fame is traced back to the penicillin discovery by Sir Alexander Fleming and its capability to destroy pathogenic bacteria in 1928 (Holikatti et al., 2014) alongside Dorothy Crowfoot-Hodgkin‟s use of X-ray crystallography to chemically confirm and validate its structure in 1945 (Banik, 2017).
In 1907, H. Staudinger makes efforts to firstly synthesize the chemistry of azetidin-2-one regarded as a 4-member cyclic amide (Troisi et al., 2009), (Ansari & Lal, 2009). Azetidinones consist of more common types such as β- lactam rings, namely: azetidin-2-ones, and less common types such as azetidin-3-ones (Putra, 2016). This variety in types is an outcome of the fact that they do not normally exist in nature (Haneishi et al., 2014), (Geesala et al., 2016), and (Patel & Bhasin, 2016). B-lactams‟ non-antibacterial properties together with cholesterol- lowering effects (Jones, 2014), antifungal (McFarland et al., 1995), anticancer (Khdur &
Zimam, 2018), and antiviral (Twamley et al., 2020) have recently been of a great interest among chemical researchers. Along with their medical uses as antibacterial agents, b-lactams are used as strongly suitable synthons to prepare numerous compounds of positive biological impact (e.g., side chain of taxol) (Kidwai et al., 2000).
Experimental General
Sigma Aldrich, Fluka, and Merck were used to provide all the required chemical materials.
Silica gel 60 F254 plates with iodine vapor as an improver were used to conduct the analytical TLC. With the use of the melting point device of the Electro-thermal Stuart SMP 30 capillary, it was possible to measure several melting points that were found uncorrected points. Infrared spectra were noted as potassium bromide discs on SHIMADZU FTIR–8400S Infrared Spectrophotometer. 1H NMR spectra were collected on INOVA 500 MHz Varian, USA NMR spectrometer in DMSO-d6 as solvent and TMS as an internal standard at the University of Tehran, Iran. (CHNS) Analyses were also deduced with Perkin Elmer 300A at the University of Tehran in Iran.
METHODS
(E)-5-(benzo[d]thiazol-2-yldiazenyl)-2-hydroxy benzaldehyde [II] [31]
The preparation process began with cooling a solution of 2-aminobenzothiazole [I] (8.1 g, 0.054 mol) in H2SO4 (15 mL) to 0°C. Along with continuous stirring, one drop at a time of a cold solution of (NaNO3) (3.726 g, 0.054 mol) dissolved in (H2O) (20 mL) was added as required. With the completion of the needed addition, the new reaction mixture was left in the ice-chest for 1h. Then, one drop at a time of an ice-cold solution of diazonium bisulfate was added to the cold solution of 2-hydroxy benzaldehyde (6.588 g, 0.054 mol) dissolved in (44 mL) of (10% w/v) sodium hydroxide with a continuous shake process. Having the more alkaline solution of phenol derivative led to having a more darkened dark dye. Once again, with the completion of the addition, the new reaction mixture was strongly stirred. After separating a solid out and standing it at room temperature for a period of 30 minutes, it was properly filtered off and completely washed with distilled water. Then, it was essential to collate and recrystallize the precipitated substance from ethanol to produce [II] a dark brown solid,
16048 namely: yield (7.9 g, 52%), m.p. 141-143 ºC.
General procedures to prepare imines [III] a-g
To prepare the imines [III] a-g, it was essential to place the aldehyde derivative [II] (0.283 g, 1 mmol), suitable aromatic amines (1 mmol), and absolute ethanol (1 mL) in a pot. In a local microwave oven, the reaction mixture was irradiated at (300W) for (40 min). The end of the entire reaction was shown in TLC (n-hexane: EtOAc, 1:2), where ethanol was used to recrystallize crude yields.
General procedures to prepare β-lactams [IV] a-g
To prepare the imines [III] a-g, it was essential to place the aldehyde derivative [II] (0.283 g, 1 mmol), suitable aromatic amines (1 mmol), and absolute ethanol (1 mL) in pot. In a local microwave oven, the reaction mixture was irradiated at (300W) for (40 min). The end of the entire reactions was shown in TLC (n-hexane: EtOAc, 1:2), where ethanol was used to recrystallize crude yields.
Scheme 1: Synthesis of β-lactams, Reagents and conditions (i) Conc. H2SO4, NaNO2, 0 oC; (ii) 2-hydroxybenzaldehyde, NaOH 10% , 5oC; (iii) Ar-NH2, EtOH, MW (300W), (40 min); (iv) α- chloroacetyl chloride, DMF, MW (300W), (150 min).
Preliminary antibacterial assay
The agar diffusion technique with the use of representative Gram (+) and Gram (–) bacteria on tryptic soy agar media assisted to determine the antibacterial activities of the newly synthesized azetidin-2-one [IV] a-g. Staphylococcus aurous (Gram- (+) and Escherichia coli (Gram-(-) were the main test microorganisms to assess the possible antibacterial activity of the anew synthesized azetidine-2-one. To prepare the required test solutions of 20 mg/mL concentration, the compounds were dissolved in dimethyl sulfoxide. Table (2) showed the use of a Amoxicillin-clavulanate s a reference and the presentation of activities as zones of inhibition for each compound.
RESULTS AND DISCUSSION
Sodium nitrite and sulfuric acid were used to diazotize 2-aminobenzothiazole [I] to produce the analogs diazonium salt reacted with 2-hydroxybenzaldehyde dissolved in sodium hydroxide solution to provide azoaldehyde derivative [II]. The microwave irradiation in absolute ethanol was used to condense Aldehyde group in compound [II] with (4-nitroaniline, 3-nitroaniline, 2- nitroaniline, 4-chloroaniline, 2-chloroaniline, 2, 4-dichloroaniline, and 4-bromoaniline) to
produce seven imine derivatives [III] a-g correspondingly, being the required platforms for this work (Scheme 1). The β-lactam derivatives of benzothiazole [IV] a-g, respectively in medium- good yields were produced by cyclizing imines [III] a-g with α-chloroacetyl chloride with the use of microwave irradiation in dimethylformamide as shown in (Table 1). The synthesized target compounds‟ chemical structures were extracted from IR, 1H NMR spectral means, and (CHNS) elemental analysis and were in consistent with the suggested structures.
IR spectrum of azoaldehyde compound [II] showed the sharp bands‟ disappearance at 3402 cm-
1 and 3273 cm-1 for (NH2)str, sharp band‟s absence belonging to (NH2) bending at 1643 cm-1, band‟s appearance at 3277 cm-1 assigned to (O-H)str, the strong band at 1654 cm-1 due to (C=O)str, the weak band at 1435 cm-1 due to azo group (N=N)str, and the benzothiazolic (C=N)str appeared as a weak band at 1587 cm-1. IR spectra of imines [III]a-g showed the strong band‟s disappearance at 1654 cm-1 for aldehydic (C=O)str, the doublet band‟s disappearance for (NH2)str in the starting amines at the general range (3400-3250) cm-1, and the disappearance of a band at the range (1595-1620) cm-1 assigned to imine function (C=N)str. IR spectra of azetidinone compounds (4a-g) provided an obvious evidence that the reactions successfully occurred through the appearance of two bands for (C=O) stretching of β-lactam ring; the first at the range (1710-1676 cm-1) while the second at the scope (1678-1643 cm-1)due to the effect of the field between chlorine atom and carbonyl group oxygen atom, as the position of chlorine atom may be up or down of plane of the azetidine ring and thus the π-bond characteristic in carbonyl group (C=O) was changed. Besides, the spectra showed the benzothiazolic (C=N) str at the range (1606-1587 cm-1), while (C=N) str of Schiff bases disappeared. Other bands were listed in the table (3-2).
The β-lactam compounds‟ structures of [IV] a-g were confirmed by their 1H NMR spectra (500 MHz, DMSO-d6) indicating a singlet peak for (CH-N) proton of lactam ring at δ 2.72 ppm. The peak of (CH-Cl) proton for lactam ring appeared at δ 2.88 ppm. The signals of (Ar-H) protons were around δ 6.91–8.69 ppm, where the quick exchange with acidic impurities in (DMSO) solvent, the (O-H) proton signal did not appear except in compounds [IV] c and [IV] e around δ 10 ppm.
Antibacterial activities
The agar diffusion technique with the use of representative standard strains of Gram (+) and Gram (–) bacteria on tryptic soy agar media was used to assess the antibacterial activities of the anew synthesized azetidine-2-one [IV]a-g, as itemized in Table 2. Importantly, it was essential to use Dimethylsulfoxide as a solvent for the test compounds.
More importantly, the entire azetidin-2-one compounds indicated a better-quality activity than the control drug against Gram-positive bacteria, Harfard Community compounds [IV]band [IV]c showed a more improved activity Amoxicillin-clavulanate against Gram-negative bacteria.
Table 1: Some physical properties of compounds [III] a-g and [IV] a-g
Product Physical state Rf [III]) n-hexane/ EtOAc, 1:2) Rf [IV]) n-hexane/ EtOAc, 1:1)
Mp (oC) Yield (%)
[III]a Roan hard 0.71 175-177 66
[III]b Dark roan hard 0.69 145-147 55
[III]c Roan hard 0.70 137-139 60
[III]d Roan hard 0.89 162-164 67
16050
[III]e Roan hard 0.80 151-153 70
[III]f Dark roan hard 0.79 125-127 60
[III]g Roan hard 0.81 171-173 72
[IV]a Roan hard 0.70 209-211 70
[IV]b Dark roan hard 0.64 179-181 61
[IV]c Dark roan hard 0.71 217-219 69
[IV]d Dark roan hard 0.90 169-171 63
[IV]e Roan hard 0.90 189-191 73
[IV]f Roan hard 0.72 219-221 80
[IV]g Dark roan hard 0.93 179-181 71
Table 2: The antibacterial activities of compounds [V]a-g and Amoxicillin-clavulanate as control drug
Product Staphylococcus aurous (Gram- positive)
Escherichia coli (Gram-negative)
[IV]a 0 15
[IV]b 30 19
[IV]c 28 20
[IV]d 30 10
[IV]e 0 0
[IV]f 0 23
[IV]g 0 21
DMSO 0 0
Amoxicillin- clavulanate
15 15
Table 3: FT-IR data of compounds [III] a-g and [IV] a-g in cm-1
FT-IR bands
Com.
No.
νC=O, lactam, field
effect
νC=C, benzene δo.o.p.C-
H benzene νs.NO2 νN=N
νC-H, benzene and νC-H,
lactam, overlapped
νas.NO2 and νC=C,
benzene, overlapped
νC=N, imine and νC=N, benzothiazole,
overlapped
νC-H,
benzene νO-H
[III]a 752 1303 1444 1494 1595 3064 3362 and
3219
[III]b 1479 752 1348 1433 1527 1614 3072 3350
[III]c 1479 756 1346 1433 1525 1620 3063 3473 , 3381
[III]d 1529 and 1485 752 1446 1614 3057 3257
[III]e 1529 754 1444 1616 3057 3200
[III]f 1523 and 1471 758 1446 1614 3063 3282
[III]g 1575, 1533 and
1479 744 1450 1612 3061 3458
[IV]a 1685 and
1649 1469 756 1307 1539 1606 3063 3412
[IV]b 1676 and
1645 1465 756 1309 1435 1531 1600 3061 3346
[IV]c
1689 and
1649 1518 and 1464 750 1311 1431 1546 1599 3018 3018
[IV]d
1710 and
1678 1525 and 1489 756 1435 1593 3059 3423
[IV]e 1705 and
1643 1525 and 1475 752 1591 3053 3416
[IV]f 1693 and
1656 1525 and 1471 748 1417 2978 br 1600 3387
[IV]g 1685 1529 and 1475 756 1442 1587 3064 3390
16052 Table 4: (CHNS) Elemental analysis of compounds [IV]a-g
Com. No. Calculated % Found %
C H N S C H N S
[IV]a 57.25 3.40 13.91 6.37 57.62 3.37 13.53 6.6 9 [IV]b 57.25 3.40 13.91 6.37 56.97 3.66 13.53 6.76 [IV]c 60.75 3.82 11.81 6.76 60.37 3.45 11.42 7.16 [IV]d 61.47 4.13 11.47 6.56 61.08 3.75 11.09 6.17 [IV]e 53.64 3.19 10.43 5.97 54.03 3.04 10.03 6.31 [IV]f 58.48 3.48 11.37 6.50 58.10 3.12 10.97 6.88 [IV]g 54.66 3.06 10.62 6.08 55.03 2.82 11.01 6.39
CONCLUSIONS
In a nutshell, the entire synthesized azetidin-2-one diones indicated a better-quality effect against positive bacteria as part of them (compounds [IV] b and [IV]showed an enhanced activity against negative bacteria more than that of control drug.
Acknowledgements
Endless gratitude to the central laboratory staff at the University of Tehran in Iran for their substantial and continuous assistance in 1H NMR and the target compounds‟ elemental analysis.
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