Cytotoxicity and Antibacterial Evaluation of O-Alkylated/Acylated Quinazolin-4-one Schiff Bases

Article abstract of DOI:10.1002/cbdv.202100096

A series of quinazolin-4-one Schiff bases were synthesized and tested in vitro for their cytotoxicity against two cancerous cell lines (MCF-7, Caco-2) and a human embryonic cell line (HEK-293) including their antibacterial evaluation against two Gram-positive and four Gram-negative bacterial strains. Most of the quinazoline-Schiff bases exhibited potent cytotoxicity against Caco-2. 3-[(Z)-({4-[(But-2-yn-1-yl)oxy]phenyl}methylidene)amino]-2-methylquinazolin-4(3H)-one (6f) with the O-butyne functional group displayed three-fold higher cytotoxic activity (IC₅₀=376.8 μM) as compared to 5-fluorouracil (5-FU; IC₅₀=1086.1 μM). However, all compounds were found to be toxic to HEK-293, except for 3-[(Z)-({4-[(2,4-difluorophenyl)methoxy]phenyl}methylidene)amino]-2-methylquinazolin-4(3H)-one (6h) that showed ～three-fold lower toxicity and higher selectivity index than 5-FU. Structure–activity relationship (SAR) analysis revealed that O-alkylation generally increased the anticancer activity and selectivity of quinazoline-4-one Schiff bases toward Caco-2 cells. The fluorinated Schiff-base generally exhibited even more significant cytotoxic activity compared to their chlorine analogs. Surprisingly, none of the quinazoline-4-one Schiff bases displayed encouraging antibacterial activity against the bacterial strains investigated. Most of the compounds were predicted to show compliance with the Lipinski parameters and ADMET profiles, indicating their drug-like properties.

Full text of DOI:10.1002/cbdv.202100096

DOI: 10.1002/cbdv.202100096
FULL PAPER
Cytotoxicity and Antibacterial Evaluation of O-Alkylated/Acylated
Quinazolin-4-one Schiff Bases
Neha Manhas,a Parvesh Singh,*a Chunderika Mocktar,b Moganavelli Singh,c and Neil Koorbanally*a
a
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa,
e-mail: Koorbanally@ukzn.ac.za; singhp4@ukzn.ac.za
Discipline of Pharmaceutical Sciences, School of Health Sciences, University of KwaZulu-Natal,
b
Private Bag X54001, Durban 4000, South Africa
Non-Viral Gene Delivery Laboratory, Discipline of Biochemistry, School of Life Sciences,
c
University of KwaZulu-Natal, Durban 4000, South Africa
A series of quinazolin-4-one Schiff bases were synthesized and tested in vitro for their cytotoxicity against two
cancerous cell lines (MCF-7, Caco-2) and a human embryonic cell line (HEK-293) including their antibacterial
evaluation against two Gram-positive and four Gram-negative bacterial strains. Most of the quinazoline-Schiff
bases exhibited potent cytotoxicity against Caco-2. 3-[(Z)-({4-[(But-2-yn-1-yl)oxy]phenyl}methylidene)amino]-2-
methylquinazolin-4(3H)-one (6f) with the O-butyne functional group displayed three-fold higher cytotoxic
activity (IC50 =376.8 μM) as compared to 5-fluorouracil (5-FU; IC50 =1086.1 μM). However, all compounds were
found to be toxic to HEK-293, except for 3-[(Z)-({4-[(2,4-difluorophenyl)methoxy]phenyl}methylidene)amino]-2-
methylquinazolin-4(3H)-one (6h) that showed ~three-fold lower toxicity and higher selectivity index than 5-FU.
Structure–activity relationship (SAR) analysis revealed that O-alkylation generally increased the anticancer
activity and selectivity of quinazoline-4-one Schiff bases toward Caco-2 cells. The fluorinated Schiff-base
generally exhibited even more significant cytotoxic activity compared to their chlorine analogs. Surprisingly,
none of the quinazoline-4-one Schiff bases displayed encouraging antibacterial activity against the bacterial
strains investigated. Most of the compounds were predicted to show compliance with the Lipinski parameters
and ADMET profiles, indicating their drug-like properties.
Keywords: quinazoline-4-one Schiff-bases, anticancer activity, breast cancer, colon cancer, human embryonic
cells, ADMET, Lipinski.
cancer globally, and that it was responsible for
8.8 million deaths in 2015; thus, globally, the second
Introduction
Over the years, cancer has been one of the leading leading cause of death.[2] In terms of cost, this has had
causes of death globally, with startling figures and a serious impact on global economies. In the US alone,
statistics published annually by various research the total cancer related medical costs in 2013 and
agencies. According to the International Agency for 2014 was US $74.8 and $87.8 billion, respectively,
Research on Cancer (IARC), cancer is the major cause while an approximate amount of US $1.16 trillion was
of morbidity and mortality with approximately 14 mil- estimated in 2010.[3–5] These Figures are expected to
lion new cases and 8 million cancer related deaths in rise by 70% in just two decades, with the greatest
2012.[1] The World Health Organization (WHO) also impact felt by low income countries due to insufficient
indicated that nearly 1 out of 6 deaths is caused by services catering for cancer patients.
Amongst the five major forms of cancer in women
(breast, colorectum, lung, cervix, and stomach), breast
cancer is the most commonly diagnosed cancer in
women and accounts for 25% of the total cases of
Supporting information for this article is available on the
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Chem. Biodiversity 2021, 18, e2100096
cancer and the highest cancer related mortality rates metabolic stability.[21] Derivatives of quinazolines have
in women. Breast cancer accounts for 34% of deaths also demonstrated good activity against the breast
in Europe and North America.[1] Although this rate is cancer cell lines – MCF-7.[14,22,23] Additionally, quinazo-
declining in developed countries such as the UK and line analogs and their Schiff bases showing their
the US, the situation remains aggressive in developing potential as antibacterial agents have also been
nations.[6] Closely related to breast cancer in terms of documented in literature.[24,25]
epidemiology is colorectal cancer. It occurs in both
Considering the increasing occurrence of MDR
men and women and remains the second leading tumors, and in our quest to synthesize new biologically
cause of cancer death in both sexes combined.[4] It is important heterocyclic scaffolds, this study is focused on
the third most diagnosed form of cancer and respon- the synthesis of new quinazolin-4-one Schiff bases and
sible for 10% of global cancer cases.[7]
their in vitro antiproliferative activity in breast cancer
Chemotherapy (the use of chemical agents with (MCF-7) and colon cancer (Caco-2) cell lines. Finally, the
pharmacological importance) – remains the treatment antibacterial screening of these compounds against two
of choice in combating cancer. Heterocyclic scaffolds Gram-positive and four Gram-negative strains was also
can be considered the soul of cancer chemotherapy undertaken under in vitro conditions.
due to their ubiquity in almost all available anticancer
agents. Food and Drug Administration (FDA) survey of
2017 drug approvals in oncology, showed that all Results and Discussion
drugs approved contain a heterocyclic skeleton.[8]
Chemistry
Quinazolines and its derivatives are an important
class of heterocycles in cancer research. Heterocycles The aim of this work was to synthesize a new series of
has continued to receive significant attention by both Schiff-bases of quinazoline-4-one. Although the most
synthetic organic and medicinal chemists over the common and established route for the synthesis of
years. For example, the commercially available anti- libraries of quinazoline-4-one Schiff bases is to condense
cancer drugs; Gefitinib – a reversible epidermal the amino group with various aldehydes, we only used
growth factor receptor (EGFR) inhibitor and Afatinib – para hydroxybenzaldehyde and formed ethers with the
a dual EGFR-ErbB2 inhibitor, both contain a quinazo- hydroxy group. As such, the aromatic imine is seen to be
line unit in their matrix. The epidermal growth factor a linker between the quinazoline-4-one and the alkyl
receptor (EGFR) is highly expressed in the breast group forming the ether. To the best of our knowledge,
cancer cell lines, MCF-7. It is thus commonly used in this has not been done previously.
anti-breast cancer research. To this effect, quinazolines
The quinazoline-4-ones were prepared by refluxing
have been applied with significant success.[9–13] They a mixture of anthranilic acid 1 and acetic anhydride
have also found application as inhibitors of vascular (2), to form the intermediate 3, followed by the careful
endothelial growth factor (VEGF) receptors, responsi- addition of hydrazine hydrate, forming the 3-amino-2-
ble for angiogenesis with a promising IC50 of 48 nM methylquinazolin-4-one (4). The Schiff base precursor
reported.[14] In addition, they were studied for their 5 was subsequently synthesized by the condensation
cytotoxicity against colorectal cancer using the Caco- of the 3-aminoquinazoline-4-one 4 and 4-hydroxyben-
2 cell line,[15] where they induced an acute cytotoxic zaldehyde in a yield of 75%. No purification was
effect against the cell lines at 149.2 and 74.6 μM.
necessary for the quinazoline-4-one Schiff base 5. The
In the era of multidrug resistant (MDR) tumors, subsequent base promoted reaction of 5 with different
where single target therapy is ineffective in combating alkyl/acyl halides in the presence of a mild inorganic
the malignant process, molecular hybridization – the base K2CO3, afforded quinazoline-4-one Schiff bases 6
combination of two or more pharmacophoric units to in excellent yields (88–95%) (Scheme 1).
create a new hybrid with improved capability of being
The structures of the synthesized compounds were
recognized by multiple receptors – is becoming unambiguously assigned on the basis of NMR, IR and
popular in drug design for anticancer research.[16] The HR-MS data. With the help of 2D (HSQC, HMBC and
versatility of quinazolines is expressed in this regard, COSY) NMR techniques, the full structure elucidation
and new molecular hybrids with important pharmaco- of all compounds was accomplished. For example, the
phores such as triazole[17,18] and isatin[19,20] improved 1H spectrum of compound 6f, 3-[(Z)-({4-[(but-2-yn-1-yl)
cytotoxicity against MCF-7 breast cancer cell lines.
oxy]phenyl}methylidene)amino]-2-methylquinazolin-
Schiff bases play a vital role in drug design and 4(3H)-one, showed a characteristic quartet (δ 4.73, J=
development due to their chelating ability and high 2.3) for H-11 and a highly shielded triplet (δ 1.87, J=
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Scheme 1. Synthetic scheme for the preparation of Schiff bases of quinazoline-4(3H)-ones.
2.3) for H-14 due long range coupling (5J) between ces, respectively. The iminic carbon (C-10) resonated at
CH2-11 and CH3-14. The aromatic region showed two δ 166.3.
doublets (J=8.8) at δ 7.07 and δ 7.85 attributed to
Both C-4 and C-8a showed HMBCs to H-5, and
H3’/H5’ and H2’/H6’, respectively, a double doublet at these two resonances were distinguished based on a
δ 8.28 (J=8.0, 1.2, H-5) and a doublet at δ 7.66 (J=7.8, HMBC from H-7 to C-8a. In a similar manner, C-4a was
H-8). In addition, two triplets of doublets at δ 7.45 (J= distinguished from C-1’ by a HMBC to H-6. The two
8.0, 1.0, H-6) and δ 7.72 (J=8.2, 1.4, H-7) were also methyl groups were assigned based on HMBCs from
present. The most deshielded singlet resonance at H-2a to C-2 and H-14 to C-12. The HMBC between the
δ 8.87 was due to the iminic proton (H-10).
iminic proton (H-10) and C-2’/6’, allowed C-2’/6’ and C-
The 13C spectrum of 6f displayed several character- 3’/5’ to be differentiated (Figure 1).
istic resonances. The acetylene moiety attached to the
aromatic ring was characterized by the methylene
resonance at δ 56.5 (C-11), two acetylene resonances at Biological Evaluation
δ 73.3 (C-13) and 84.5 (C-12), and a highly shielded
methyl resonance at δ 3.6 (C-14). The methyl group
Cytotoxicity Studies
attached to the quinazoline-4-one was seen at δ 22.8 (C- All the synthesized compounds were tested in vitro for
2a). The para substituted aromatic ring contained two their cytotoxicity against two cancer cell lines, namely
intense resonances at δ 115.3 (C-3’/C-5’) and 130.6 (C-2’/
C-6’), while C-5 to C-8 on the quinazoline-4-one ring
were present at δ 127.2, 126.3, 134.2 and 126.8,
respectively. Four deshielded singlet aromatic resonances
attributed to the oxygenated aromatic resonance C-4’,
the carbonyl resonance C-4, and two aromatic C–N
resonances, C-2 and C-8a were present at δ 161.3, 158.7,
154.0 and 146.5, respectively, while the two more
shielded resonances at δ 121.5 and 125.7 were attributed
to C-4a and C-1’, two aromatic singlet carbon resonan-
Figure 1. Characteristic HMBC data observed in compound 6f.
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Chem. Biodiversity 2021, 18, e2100096
human breast cancer (MCF-7), human colon adenocar- benzoylation did result in more active compounds
cinoma (Caco-2) as well as normal human embryonic than 5-FU. Amongst these, 6i bearing a 2,6-difluor-
cells (HEK-293), using the well-established [3-(4,5- obenzyl moiety exhibited the highest activity with an
dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide IC50 value of 423.8 μM, whereas 6l with a 2,6-
(MTT) based cell viability assay. 5-Fluorouracil (5-FU) dichlorobenzoyl moiety emerged as the second most
was used as the reference drug. The samples were active (IC50 =457.5 μM) compound against the Caco-2
dissolved in DMSO to generate different dilutions for cell lines in this category of compounds.
the assay. The results are summarized in Table 1.
The second potent compound of the series was 6h,
Most quinazoline-4-one Schiff bases displayed with a 2,4-diflourobenzyl group, which showed almost
cytotoxicity against the colon cancer cell lines (Caco- 3-fold lower toxicity than 5-FU against the normal cell
2). Structure–activity relationship (SAR) analysis re- lines. Incidentally, the activity of this compound
vealed that the nature of the different moieties against the Caco-2 cell lines was almost 1.5-fold lower
introduced to the basic framework, after O-substitu- than 5-FU. However, the relatively better selectivity
tion, influenced the cytotoxic effect of the Schiff bases index (SI) (2.3, Table 1) of 6h outweigh this 1.5 fold
against Caco-2 cells by 1–2 fold. Compound 6f with decrease in activity. Some important SAR points from
an O-butyne group exhibited the highest activity these results are: (i) O-alkylation generally increases
(IC50 =376 μM) amongst the tested compounds, com- the cytotoxic activity of quinazoline-4-one Schiff bases
parable to 5-FU, but was 8 fold more toxic than 5-FU against Caco-2 cells; (ii) fluorine-containing Schiff
against the HEK293 cell lines. The attachment of the bases are generally more potent than their chlorine
same functionality through its secondary carbon (6e), analogs; and (iii) the quinazoline-4-one ring connected
dramatically reduced (~three-fold) the cytotoxicity, to the phenyl ring via an iminic bond is a satisfactory
indicating that a methyl group in the terminal position pharmacophore for cytotoxicity against the Caco-2 cell
(as in 6f) was more desirable for cytotoxic activity.
lines.
For breast cancer activity, only four compounds, 6a
Compounds with O-butyl (6b), O-hexyl (6c), and O-
bromopentyl (6d) substitution also displayed good (with an ethyl group), 6f (but-2-ynyl group), 6j (2,4-
cytotoxicity against Caco-2, with 6b having 2.5 fold dichlorobenzyl group), and 6k (2,4-difluorobenzoyl
higher activity than 5-FU, but with greater cytotoxicity. group) of the series showed encouraging cytotoxic
In contrast, the presence of an ethyl functionality effects against the MCF-7 cancer cell lines, with IC50
dramatically reduced (~seven-fold) the activity of values ranging from 326.0 to 1509 μM. All Compounds,
compound 6a with respect to 6f, the most active except 6f, displayed superior activity as compared to
compound of the series. However, O-benzylation or O- 5-FU (1057.6 μM). All compounds, however, were
found to be more cytotoxic (between 98 and
185.4 μM) than 5-FU (1352.3 μM) against the
Table 1. In vitro cytotoxic analysis of Schiff bases 6a–6m
HEK293 cell lines.
against two cancer cell lines (Caco-2 and MCF-7) including
human embryonic cells (HEK293).
Antibacterial Activity
Compound IC50 (μM)
Selectivity
Index (Caco-2)
HEK293
Caco-2 MCF-7
The synthesized compounds 6a–6m were also
screened to determine their antibacterial potential
against two Gram-positive strains viz. Staphylococcus
aureus and MRSA and four Gram-negative strains viz.
Pseudomonas aeruginosa, Escherichia coli (E. coli),
Klebsiella pneumonia and Salmonella typhimurium. The
disc diffusion method was used for screening these
compounds for antimicrobial activity under in vitro
conditions. All compounds showed no activity against
two bacterial strains, K. pneumonia and P. aeruginosa
at a concentration of 10 μgmLÀ 1 (not shown in
Table 2). The Schiff base (6f) bearing a butyne moiety
exhibited the highest activity against E. coli with a
zone of inhibition (ZI) value of 12 mm, while three
compounds, 6d, 6e and 6k showed moderate anti-
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
5-FU
185.4
126.5
200.7
154.6
155.9
169.2
126.8
3859.8
96.0
2686.3
420.9
633.4
507.7
1048.8
538.1
NA
NA
NA
NA
0.07
0.3
0.3
0.3
0.1
0.4
0.2
2.3
0.2
–
0.2
0.2
0.2
1.2
376.8 1509.0
507.5
1679.2
423.8
NA
473.5
457.5
707.1
NA
NA
NA
326.0
779.1
NA
98.2
117.8
113.5
135.8
1352.3
NA
1086.1 1057.6
NA=not active (cell viability <30%).
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Table 2. In vitro antibacterial activity of the synthesized Schiff
bases.
No.
S. aureus
MRSA
E. coli
S. typhimurium
6b
6d
6e
6f
6g
6k
6m
–
–
–
–
–
–
–
–
–
7�3.1
10�1.5
–
9�2.1
–
–
–
12�1.7
–
11�0.8
–
–
–
–
7�2.6
7�2.4
–
–
7�1.8
* Only active compounds are included.
Figure 2. 2D plot between atom-type partition coefficient
(ALogP98) and polar surface area (PSA) in Å2 for all compounds,
obtained using DS. The area encompassed by the ellipse is a
prediction of good absorption without any ADMET violation.
bacterial activity against MRSA, with zones of inhib-
ition of between 9–11 mm (Table 2). Since, none of
the compounds displayed any promising activity (zone
of inhibition >15 mm), no further screening was
undertaken.
indicator for hydrogen forming capacity of a molecule
and log P (n-octanol-water coefficient) values, respec-
tively. According to the ADMET model, the drug with
optimum membrane permeability should have PSA<
ADMET Properties and Lipinski’s Rule of Five
The absorption, distribution, metabolism, excretion, 140 Å2 and AlogP98<5. Following the AlogP98 crite-
and toxicity (ADMET) properties of a molecule within a ria, all compounds, except 6j, 6l and 6m, displayed
living organism is an in silico prediction that allows for acceptable hydrophobicity (<5, Table 3), and satisfied
the screening of compounds in the early stage to the 95% and 99% confidence ellipse for both BBB and
avoid the synthesis of molecules, which have the human intestinal absorption. All compounds also
potential to be toxic, or metabolized to an inactive showed acceptable PSA (52.2 and 69.5) and undefined
form in the body. It also serves as a guide for further values for BBB (from À 0.18 to À 6.8) with the exception
structural manipulations to improve the molecule’s of compound 6j showing a high probability of BBB
pharmacokinetics and toxicity.
penetration.
The aqueous solubility plays an important role in
The pharmacokinetic parameters of our com-
pounds were predicted using six predetermined the bioavailability of a drug. Accordingly, the two
ADMET models embedded in the Discovery Studio compounds (6b and 6f) found to be most active
(DS) 4.0 Client (Accelrys, San Diego, CA, USA),[26] and against the Caco-2 cancer cell lines showed a better
are listed in Table 3. A bi-plot (Figure 2) obtained solubility profile in comparison to the other com-
between the 2D polar surface area (ADMET_PSA_2D) pounds. None of the compounds were predicted to be
and
the
atom-type
partition
coefficient toxic to the liver, based on hepatotoxicity predictions.
(ADMET_AlogP98) comprises 99% and 95% confi- Moreover, compound 6j showed no cytochrome P450
dence space for the prediction of the membrane 2D6 enzyme inhibition, suggesting its optimal metab-
permeability of the compounds, and corresponds to olism in Phase-1, whereas the false predictions were
the Blood Brain Barrier (BBB) and Intestinal Absorp- achieved for the remaining compounds of the series.
tion models.[27] The 95% confidence ellipse refers to a
In order to determine if the synthesized com-
region of chemical space where �95% drug is pounds show compliance to the Lipinski’s ‘rule of
absorbed, whereas the 99% confidence ellipse repre- five’,[28] their other descriptors were predicted viz.
sents a region where a substantial amount of the hydrogen bond donors (HBD) and hydrogen bond
drug is absorbed though the membrane.
acceptors (HBA), in addition to their predetermined
Considering that the plasma membrane is capable topological polar surface area (TPSA) and AlogP
of forming both hydrophilic and hydrophobic inter- values. The knowledge of these parameters is again
actions, the optimal drug should have a balance very useful to check the probability of hit molecules
between its hydrophilic and lipophilic character, and to be developed further as drug candidates. As
can be determined by its polar surface area (PSA), an mentioned previously, a TPSA greater than 140 has
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Chem. Biodiversity 2021, 18, e2100096
been associated with the poor bioavailability of drug
molecules. Similarly, a HBD greater than five, HBA
greater than 10, molecular weight greater than 500
and ClogP greater than five indicates poor absorp-
tion/permeation of a drug candidate.
All computations were done employing the ‘Filter
by Lipinski and Veber Rules’ module in DS program.
The results are included in Table 3. The analysis of
computed descriptors denotes that all compounds
showed compliance with the Lipinski’s rule of five,
with the exception of compounds 6j, 6l and 6m
showing violation of ClogP (LogP). These results
clearly indicate that the majority of compounds have
the potential to be developed as drugs with optimal
pharmacokinetic properties.
Conclusions
A series of quinazolin-4-one Schiff bases 6a–6m were
prepared by a simple base-promoted O-alkylation/
acylation
reaction
of
3-{[(4-hydroxyphenyl)
methylidene]amino}-2-methylquinazolin-4(3H)-one
(5) with a variety of substituted alkyl and acyl halides.
All reactions yielded the final products without
complicated purification procedures in excellent
yields. Full 1H- and 13C-NMR assignments of all
synthesized compounds was documented using 2D
NMR techniques. The in vitro cytotoxicity screening of
these compounds allowed us to identify several
potent compounds against the Caco-2 cancer cell
lines, where 3-[(Z)-({4-[(but-2-yn-1-yl)oxy]phenyl}
methylidene)amino]-2-methylquinazolin-4(3H)-one
(6f) showed 3-fold more potent activity than 5-FU.
Compound 6h was found to be less toxic than 5-FU
against HEK-293 cells. Generally, O-alkylation and
introduction of fluorine bearing moieties resulted in
more cytotoxic compounds. None of these com-
pounds, however, exhibited good antibacterial activ-
ity in their preliminary screening. Hopefully, their
coupling with other pharmacophoric units would
synergize their antibacterial activity and decrease
toxicity against normal cells at the same time. Finally,
most of the compounds showed compliance with
Lipinski and ADMET parameters in silico.
Experimental Section
General
All reagent grade chemical reagents used in this study
were purchased from Sigma-Aldrich through Capital
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Chem. Biodiversity 2021, 18, e2100096
Lab, South Africa. Organic solvents were distilled
before use. Kieselgel 60 F254 plates purchased from
Merck were used to perform thin-layer chromatogra-
General Preparation for the Schiff Bases of Quinazoline-
4(3H)-one (6a–6m)
phy. Melting points of all synthesized compounds Activated K2CO3 (0.02 mmol) was added in small
were determined on a sealed capillary tube in an proportions over a period of 5 min at room temper-
°
Electrothermal IA9100 melting point apparatus, and ature (25 C) to a solution of 3-{[(4-hydroxyphenyl)
are uncorrected. IR spectra were recorded on a methylidene]amino}-2-methylquinazolin-4(3H)-one (5;
PerkinElmer Spectrum 100 FT-IR spectrometer. NMR 0.01 mmol) in dimethylformamide (DMF). The reaction
analysis was carried out in deuterated chloroform mixture was allowed to stir for another 5 min before
using a Bruker Avance 400-MHz NMR spectrometer addition of alkyl or acyl halides (0.01 mmol) to the
(9.4 T; Bruker, Germany), using tetramethylsilane (TMS) reaction mixture at the same temperature, and
as an internal standard. Chemical shifts of the monitored by TLC. Upon completion, the reaction
deuterated solvent were 7.24 (1H) and 77.0 (13C), mixture was poured in crushed ice and left to stand
relative to TMS. NMR data was analyzed using Bruker for 30 min. The solid compound obtained was filtered
TopSpin 3.1 (2008) software. Chemical shift values using a Buchner funnel, washed with hexane (20 mL),
were expressed as ppm downfield from TMS and J and recrystallized from absolute ethanol.
values (coupling constants) are given in Hz. High-
resolution mass spectral analysis of all compounds was
All the spectroscopic data of 6a–6m can be found
performed on a Waters Micromax LCT Premier TOF-MS in the Supporting Information.
instrument using ~1 ppm concentration of sample.
Antibacterial Screening (Disc Diffusion Method)
Synthetic Methods
Antibacterial screening was undertaken against four
Gram-negative strains of bacteria viz. Pseudomonas
aeruginosa (ATCC 27853), Escherichia coli (ATCC
Preparation of 3-Amino-2-Methyl-3H-Quinazolin-4-one (4)
A mixture of 2-aminobenzoic acid (1 mmol) and acetic 25922), Klebsiella pneumonia (ATCC 31488) and Salmo-
°
anhydride (2 mmol) was refluxed at 140 C for 3 h. nella typhimurium ATCC 14026, and two Gram-positive
Completion of the reaction and formation of oxazine 3 strains viz. Staphylococcus aureus (ATCC 25923), and
was monitored by thin layer chromatography (TLC). methicillin resistant Staphylococcus aureus (MRSA)
The reported method for the preparation of 4[29] (ATCC BAA-1683).
involves the isolation of the latter, followed by
refluxing with hydrazine hydrate (2 mmol) for 2–3 h.
All Schiff bases (6a–6m) were initially subjected to
Oxazine 3 was generated in situ, excess acetic anhy- preliminary antibacterial screening using the disc
dride evaporated and then reacted with hydrazine diffusion assay. The bacterial strains were grown
°
hydrate at 5 C (exothermic reaction), using hexane as overnight in nutrient broth (Biolab, South Africa) at
°
a solvent. The purpose of hexane was to facilitate the 37 C and adjusted to a 0.5 McFarland standard.
precipitation of compound after complete addition of Mueller-Hinton Agar (MHA) plates were prepared by
hydrazine hydrate. The solid (fluffy buff powder) dissolving agar (38 g) in water (1L), and pouring it into
obtained was filtered and washed with water (2× sterile petri dishes, allowing them to cool and set at
10 mL) and then, with hexane (20 mL).
room temperature. The different bacterial strains were
inoculated onto the agar plates by streaking a swab
dipped into the bacterial broth over the surface of the
agar. A 5μL aliquot of the synthesized compounds
(1 mg in 1 mL DMSO) was spotted onto 12mm anti-
Preparation
of
3-{[(4-Hydroxyphenyl)methylidene]
amino}-2-Methylquinazolin-4(3H)-one (5)
A mixture of 4 (0.01 mmol) and p-hydroxybenzaldehye biotic discs and placed in the inoculated MHA plates,
°
(0.01 mmol) was refluxed in absolute ethanol (30 mL) which were then incubated for 24h at 37 C.
in the presence of 3–4 drops of glacial acetic acid for
a period of 16 h and monitored by TLC. Upon
completion, the solvent was removed under vacuum.
The solid obtained was filtered and recrystallized using
Acknowledgement
absolute ethanol.
The authors would like to thank the Center for High
Performance Computing (CHPC) based in Cape Town
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Chem. Biodiversity 2021, 18, e2100096
docking study of novel quinazoline derivatives as PARP-1
for access to computational resources and programs.
N.K. thanks the National Research Foundation South
Africa for a Competitive Grant for Rated Researchers
(Grant No. 118534) and Incentive Funding for Rated
Researchers (Grant No. 114817).
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the computer simulations, analyzed the data, and
assisted with editing the manuscript. C. M. assisted
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cytotoxicity screening. N. K. supervised the entire
project, edited the manuscript, and checked the data
for scientific correctness.
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Received February 6, 2021
Accepted March 15, 2021
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