Diazabenzo[a]phenoxazone sulphonamides: synthesis, in-silico and in-vitro antimicrobial studies

Article abstract of DOI:10.1007/s00044-018-2251-4

The syntheses of new sulphonamide derivatives of 8,10-diazabenzo[a]phenoxazones are reported. The condensation of 4,5-diamino-6-hydroxy-2-mercaptopyrimidine and 2,3-dichloro-1,4-naphthoquinone in a basic medium gave the key functional intermediate, 11-amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenoxazin-5-one. The conversion of the later compound to its sulphonamide derivatives was achieved via nickel catalyzed cross-coupling Buchwald-Hartwig protocol. Reaction between 11-amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenoxazin-5-one and various aryl sulphonamides and sulphonyl chlorides furnished eight new mono sulphonamide substituted diazaphenoxazone compounds. Subsequent coupling of mono sulphonamide substituted diazaphenoxazone compounds 5a-d with four different arylsulphonyl chlorides under similar reaction conditions gave the disubstituted derivatives 8a-d. The products were isolated in 74 – 88% yields and characterized by means of Uv-visible, FT-IR, ¹H-NMR, ¹³C-NMR, and Mass spectroscopy. The synthesized compounds were screened for antimicrobial activity against bacterial strains: Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Salmonella typhi and Klebsiella pneumonia, and fungal strains, Aspergillus niger, and Candida albican, using agar-well diffusion method. The activities of the compounds were compared with that of colymycin, which is a strong antibacterial, and antifungal drug, Most of the compounds showed appreciable antimicrobial activities comparable with the activity of colymycin. The in silico study revealed that all the synthesized compounds showed significant binding affinity for both intact and mutated DNA gyrase. Compounds 8a and 5b showed the highest binding affinities of -12.31 and -13.30 kcal/mol for intact and mutated DNA gyrase respectively.

Full text of DOI:10.1007/s00044-018-2251-4

MEDICINAL
CHEMISTRY
RESEARCH
Medicinal Chemistry Research
https://doi.org/10.1007/s00044-018-2251-4
ORIGINAL RESEARCH
Diazabenzo[a]phenoxazone sulphonamides: synthesis, in-silico and
in-vitro antimicrobial studies
1 ●
2 ●
3 ●
1 ●
Mercy A. Ezeokonkwo
Cosmas C. Eze Sunday N. Okafor Efeturi A. Onoabedje
4 ●
2
Evelyn U. Godwin-Nwakwasi Fidelia N. Ibeanu
Received: 13 June 2018 / Accepted: 18 September 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
The syntheses of new sulphonamide derivatives of 8,10-diazabenzo[a]phenoxazones are reported. The condensation of 4,5-
diamino-6-hydroxy-2-mercaptopyrimidine and 2,3-dichloro-1,4-naphthoquinone in a basic medium gave the key functional
intermediate, 11-amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenoxazin-5-one. The conversion of the later compound to
its sulphonamide derivatives was achieved via nickel catalyzed cross-coupling Buchwald-Hartwig protocol. Reaction
between 11-amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenoxazin-5-one and various aryl sulphonamides and sulpho-
nyl chlorides furnished eight new mono sulphonamide substituted diazaphenoxazone compounds. Subsequent coupling of
mono sulphonamide substituted diazaphenoxazone compounds 5a-d with four different arylsulphonyl chlorides under
similar reaction conditions gave the disubstituted derivatives 8a-d. The products were isolated in 74 – 88% yields and
1
13
characterized by means of Uv-visible, FT-IR, H-NMR, C-NMR, and Mass spectroscopy. The synthesized compounds
were screened for antimicrobial activity against bacterial strains: Bacillus subtilis, Staphylococcus aureus, Escherichia coli,
Salmonella typhi and Klebsiella pneumonia, and fungal strains, Aspergillus niger, and Candida albican, using agar-well
diffusion method. The activities of the compounds were compared with that of colymycin, which is a strong antibacterial,
and antifungal drug, Most of the compounds showed appreciable antimicrobial activities comparable with the activity of
colymycin. The in silico study revealed that all the synthesized compounds showed significant binding affinity for both intact
and mutated DNA gyrase. Compounds 8a and 5b showed the highest binding affinities of -12.31 and -13.30 kcal/mol for
intact and mutated DNA gyrase respectively.
●
●
●
●
Keywords Sulphonamide Diazaphenoxazone Antimicrobial activity Nickel catalysis DNA gyrase
Introduction
against existing clinical drugs (Aarestrup 2009; Threlfall
et al. 2009). This necessitates a purposeful search for novel
chemical moieties that can effectively handle these diseases.
Bacterial DNA gyrase (Fig. 1) has been established as a
drug target for the development of new antibiotics (Hooper
The field of Medicinal Chemistry has continued to witness
unabated interest for novel chemotherapeutic drugs because
of the increasing resistance of bacterial and fungal microbes
1998 and Maxwell 1997). It is a type II DNA topoisomerase
that catalyses changes in the topology of DNA (Bates and
Maxwell 2005; Wang 2009). Essentially, this enzyme is
very crucial for almost all cellular processes, including
replication, recombination and transcription in DNA.
Phenoxazines have been discovered to inhibit DNA
topoisomerase. The phenoxazine compound represented as A
in Fig. 2 has been shown to exhibit a low micro molar inhi-
bitory activity (IC50 = 0.8–2.0 μM) against Escherichia coli
topoisomerase I (Yu et al. 2017). Compounds B & C in Fig. 2
have been reported as antibacterial agents (Rádl and Zikán
1989, Chu and Maleczka 1987). In fact, the pharmacological
activities of phenoxazines are well known and numerous.
*
Mercy A. Ezeokonkwo
mercy.ezeokonkwo@unn.edu.ng
1
2
3
4
Department of Pure & Industrial Chemistry, University of Nigeria,
Nsukka, Enugu State 410001, Nigeria
Natural Science Unit, School of General studies, University of
Nigeria, Nsukka, Enugu State 410001, Nigeria
Department of Pharmaceutical and Medicinal Chemistry,
University of Nigeria, Nsukka, Enugu State 410001, Nigeria
Department of Chemistry, Gregory University, Uturu, Abia State,
Nigeria

Medicinal Chemistry Research
They are used as antitumor (Shimamoto et al. 2001, Harton
et al. 1993), antituberculosis (Boothroyd and Clark 1952), and
antibacterial (Chu Daniel 1986) agents among others. A water
particularly because of their low cost, low toxicity and
excellent activity against common bacterial diseases (Ozbek
et al. 2007). Many sulphonamide derivatives have been
reported as carbonic anhydrase inhibitor (Zimmerman et al.
2004), anticancer (Scozzafava et al. 2003), anti- inflam-
matory agents (Weber et al. 2004), antibacterial (Eshghi
et al. 2011), anti- HIV (Miller et al. 2002), antimalarial
(Alqasoumi et al. 2010), antitumor (Chen et al. 2010), and
antiviral agents (Rostom 2006). Some of the sulphonamide
drugs currently available in the market include Bosentan
(Ueda et al. 2011) (an anti hypertensive agent), Amprenavir
(Shen et al. 2010 (an antiviral HIV protease inhibitor),
Sildenafil (a phosphodiasterase-5 inhibitor) (Setter et al.
2005), Glibenclamide (Marble 1971) (an antidiabetic drug),
Glimepriride (Marble 1971) (an antidiabetic nonantibiotic)
and Torasemide ((Lopez et al. 2009)(the diuretic drug).
The wide range of medicinal and industrial applications of
phenoxazine and sulphonamide compounds have led to con-
tinued synthesis of phenoxazine and sulphonamide deriva-
tives (Ugwu et al. 2014; Onoabedje et al. 2016; Ibeanu et al.
2018). Therefore, we described the synthesis and the in vitro
antimicrobial screening of diazabenzo[a]phenozaxine deriva-
tives incorporating sulphonamide and phenoxazine moieties.
The synthesized compounds were subjected to in-silico stu-
dies in order to evaluate their drug-likeness. Their molecular
docking using DNA gyrase, was also carried out to determine
their binding affinities with the receptors. We also went fur-
ther to mutate the DNA gyrase with a view to determine how
effective the compounds can tackle resistant strains.
soluble
2-amino-4,4α,7-dimethyl-3H-phenoxazine
was
reported to possess antiproliferative, immunosuppressive,
antibacterial and antiviral effects (Onoabedje et al. 2016).
Natural products containing phenoxazine core such as acti-
nomycin D, an antibiotic produced by Streptomyces, and
others exhibit anticancer, antiviral and antifugal properties
(Barness et al. 2015). More recently, Wang and his group
isolated actinomycins A and B bearing 5H-oxazolo[4,5-b]
phenoxazine chromophore from marine derived Streptomyces
species. Actinomycins A showed potent cytotoxic activities
against human cancer cell lines in the nanomolar range and
moderate antibacterial activities against methicillin-resistant
Staphylococcus aureus and vancomycin-resistant Enterococci
strains (Wang et al. 2017).
On the other hand the sulphonamides have several
applications especially as drugs. Sulphonamide drugs act by
a competitive inhibition of p-aminobenzoic acid (PABA)
use in the synthesis of dihydrofolate, which bacteria need to
make DNA (Alovero et al. 1998). The sulphonamide drugs
are synthetic antimicrobial agents that have broad spectrum
of use with respect to Gram-positive and Gram-negative
microorganisms (Vardayan and Hruby 2006). They are one
of the most widely used antibacterial drugs in the world,
EXPERIMENTAL SECTION
General
The reagents used were of analytical grades and were pro-
ducts of Sigma-Aldrich chemicals. Melting points were
determined using electrothermal melting point apparatus in
open capillaries and were uncorrected. Ultraviolet and
visible spectra were recorded on UV-25500 PC spectro-
photometer using matched 1 cm quart cell; absorption
maxima were given in nanometers (nm), while the figures in
parenthesis were the log of molar absorptivity coefficient
Fig. 1 3D ribbon representation of DNA gyrase with the co-
crystallized ligand (1-Ethyl-3-[8-methyl-5-(2-methylpyridin-4-yl) iso-
quinolin-3-yl]urea)
A
F
B
C
Fig. 2 Chemical structures of
phenoxazine derivatives that are
DNA topoisomerase inhibitors
O
N
O
O
N
O
NH2
O
N
O
F
OH
OH
F
OH
F
N
N
O
O
O
H N
2
H N
2
NH2
NH
2
NH2

Medicinal Chemistry Research
1
(
ɛ). The IR spectra were recorded on 8400 S FTIR spec-
aromatic), 849 (C-Cl). H-NMR (DMSO-d ) δ: 7.89–7.52
6
1
13
13
trometer using KBr discs. The H-NMR and C-NMR
were determined using varian NMR 400 MHZ spectrometer
at Old Core Laboratory, Indian Institute of Technology,
Pradesh, India; chemical shifts reported in (δ-ppm) scale.
The antimicrobial screening was done at the Faculty of
Pharmaceutical Sciences, University of Nigeria, Nsukka.
All the products were purified through repeated recrys-
tallization using suitable solvent(s). Bis(triphenylpho-
sphine)nickel(II)chloride was synthesized according to
literature procedure (Venanzi 1958). The physicochemical
properties used for the evaluation of the drug-likeness of the
synthesized compounds were calculated using the molecular
descriptors available in the online open access molinspira-
tion (www.molinspiration.com). The molecular descriptors
calculated include: molecular weight (MW), partition
coefficient (log P), hydrogen bond acceptor (HBA),
hydrogen bond donor (HBD), topological polar surface area
(4H, m, Ar-H), 6.84 (2H, s, NH ), 3.42 (1H, s, SH). C-
NMR (DMSO-d ) δ: 186.42 (CO), 134.36, 134.13, 132.29,
131.31, 130.21, 127.43, 127.02, 126.37, 126.17, 124.70 and
124.39 (aromatic carbons). MS: in m/z [rel.%]: 315.00 [m
- NH , 27%], 313.07 [m - O, 48%], 301.13 [m - SH,
100%], 295.00 [m - Cl, 33%].
2
6
+
+
+
2
+
General procedure for the synthesis of
monosubstituted derivatives (5a-d and 7a-d)
To a mixture of bis(triphenylphosphine) nickel (II) chloride
(3.00 g, 5 mmol) and triphenylphosphine (1.85 g, 10 mmol)
was added t-butanol (6 ml) and distilled water (3 ml) with
continuous stirring for 10 min at room temperature under
nitrogen atmosphere. The resultant mixture was heated at
80 °C in an oil bath for 1.5 min. To the resultant mixture
was added a mixture of 11-amino-6-chloro-9-mercapto-
8,10-diazabenzo[a]phenoxazin-5-one 8 (2.50 g, 8 mmol),
K CO (1.38 g, 10 mmol) and the aryl sulphonamides or
sulphonyl chlorides (1.80 g) in the solvents (t-butanol and
distilled water in the ratio of 2:1). The mixture was refluxed
at 100–110 °C under nitrogen atmosphere for 1 h with
continuous stirring. The mixture was allowed to cool, fil-
tered via suction, and washed with distilled water. The
crude product was recrystallized from equal mixture of
acetone and diethyl ether to give derivatives 11a-d and 12a-
d respectively, in good to excellent yields.
(
TPSA) and number of rotatable bond (NoRB). We
retrieved the crystal structure of DNA gyrase (PDB codes:
MMN) containing the co-crystallized inhibitor from the
2
3
5
online protein data bank repository (https://www.rcsb.org/
structure/5mmn). Further preparations of the protein and the
ligands were done using the discovery studio. These pre-
parations included deleting of multiple chains, water of
crystallization from the protein and energy minimization of
the structures. The molecular docking and the visualization
of the protein-ligand complex interaction were done using
Autodock/Autodock vina and discovery studio respectively.
11-Amino-6-(phenylsulphonamido)-9-mercapto-8,10-
Synthesis of 11-amino-6-chloro-9-mercapto-8,10-
diazabenzo[a]phenoxazin-5-one (3)
diazabenzo[a]phenoxazin-5-one (5a)
11-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
Into a 250 ml three-necked round bottom flask equipped
with magnetic stirrer bar and a reflux condenser was added
azin-5-one and benzene sulphonamide (1.80 g, 11 mmol)
were converted to the title compound 5a as a dark brown
crystalline solid. Yield 2.66 g (78 %), mp. 206–208 °C. UV-
Vis (MeOH) (nm): 330 (2.0791), 446 (2.000), 639 (1.7781).
4
,5-diamino-6-hydroxy-2-mercaptopyrimidine
1 (2.50 g,
1
6 mmol) and anhydrous sodium carbonate (2.50 g, 23
−
1
mmol). A mixture of benzene (120 ml) and DMF (15 ml)
was added and the mixture stirred for 45 min while heating
on a water bath at 75–80 °C. 2,3-Dichloro-1,4-naphthoqui-
none 2 (3.60 g, 15 mmol) was then added and the resulting
mixture was refluxed with continuous stirring for 5 h in a
water bath at 75–80 °C. Benzene was distilled off and the
slurry poured into a beaker containing distilled water (100
ml) and stirred to dissolve the inorganic materials. The
mixture was allowed to cool overnight, filtered and the
residue recrystallized from equal mixture of acetone and
diethyl ether to give an intense reddish-brown crystalline
solid of compound 3. Yield 4.25 g, (81%), m.p. 195–197 °
C. Uv-Vis (MeOH) (nm): 330 (2.5797), 450 (2.3424), 630
IR (KBr) (cm ): 3505, 3369 (NH), 3128 (CH aromatic),
2556 (SH), 1774 (C=O), 1619 (C=N), 1447 (C=C aro-
1
matic), 1277, 1134 (SO two bands). H-NMR (DMSO-d )
2
6
δ: 7.68 (1H, s, NH), 7.66 (1H, d, Ar-H), 7.65 (1H, d, Ar-H),
7.63 (m, 1H, Ar-H), 7.55–7.46 (m, 4H, Ar-H), 7.24 (s, 2H,
1
3
NH ), 1.58 (s, 1H, SH). C-NMR (DMSO-d ) δ: 134.92,
2
6
133.92, 133.74, 132.23, 132.14, 132.05, 128.80, 128.66,
128.61, 128.54, 119.11 (aromatic carbons).
11-Amino-6-(4-methylphenylsulphonamido)-9-mercapto-
8,10-diazabenzo[a]phenoxazin-5-one (5b)
11-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
azin-5-one and p-toluene sulphonamide (1.0 g, 10 mmol)
were converted to the title compound 5b furnished as a
−
1
(
1.7781), 752 (1.6021). IR (KBr) (cm ): 3510 (NH), 3140
(
CH aromatic), 2507 (SH), 1723 (C=O), 1623 (C=N, C=C

Medicinal Chemistry Research
brownish crystalline solid. Yield 3.01 g (87%), mp. 212–
11-(Phenylsulphonamido)-6-chloro-9-mercapto-8,10-
2
6
14 °C. UV-Vis (MeOH) (nm): 330 (2.1271), 440 (1.9031),
51 (1.4771). IR (KBr) (cm ): 3533, 3336, (NH), 3099,
diazabenzo[a]phenoxazin-5-one (7a)
−
1
(
1
(
CH aromatic), 2509 (SH), 1700 (C=O), 1628, (C=N),
475 (C=C), 1293, 1125 (SO2 two bands). H-NMR
11-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
azin-5-one and benzene sulphonylchloride (1.80 g, 10
mmol) were converted to the title compound 7a as a dark
reddish crystalline solid. Yield 2.70 g (76%), mp. 210–212 °
C. UV-Vis (MeOH) (nm): 330 (2.5797), 550 (2.5563), 692
1
DMSO-d ) δ: 7.67 (1H, s, NH), 7.65 (1H, d, Ar-H), 7.62
6
(
1H, d, Ar-H), 7.53–7.44 (4H, m, Ar-H), 7.24 (2H, s, NH ),
2
13
2
.41 (1H, s, SH), 1.63 (s, 3H, Ar-CH3). C-NMR (DMSO-
−
1
d6) δ: 177.14 (CO), 139.72, 132.23, 132.12, 132.04, 128.65,
28.52, 126.37, 116.50 (aromatic carbons), 35.02 (CH ).
(1.9031). IR (KBr) (cm ): 3361, 3242 (NH), 3127 (CH
1
aromatic), 2489 (SH), 1653 (C=O, C=N), 1472 (C=C
3
+
+
1
MS: in m/z [rel.%]: 295.13 [m - PhCH NHSO , 100%],
2
PhCH NHSO NH , 4%], 155.00 [13%], 152.33 [2%].
aromatic), 1231, 1102 (SO two bands), 868 (C-Cl). H-
3
2
2
+
+
+
67.13[m - PhCH NHSO
SH, 9%], 261.27 [m
-
NMR (DMSO-d ) δ: 8.79 (1H, s, NH), 7.59 (1H, d, Ar-H),
3
2
6
+
7.57 (1H, d, Ar-H), 7.54 (1H, m, Ar-H), 7.51–7.26 (4H, m,
3
2
2
1
3
Ar-H), 3.45 (1H, s, SH). C-NMR (DMSO-d ) δ: 173.72
6
1
8
1-Amino-6-(4-chlorophenylsulphonamido)-9-mercapto-
,10-diazabenzo[a]phenoxazin-5-one (5c)
(CO), 153.72, 132.59, 132.05, 131.95, 129.35, 129.23,
128.95, 128.18, 127.74 (aromatic carbons).
1
1-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
11-(4-Methylphenylsulphonamido)-6-chloro-9-mercapto-
8,10-diazabenzo[a]phenoxazin-5-one (7b)
azin-5-one and 4-chlorobenzene sulphonamide (1.80 g, 9
mmol) were converted to the title compound 5c obtained as
a deep brown crystalline solid. Yield 3.05 g, (83%), mp.
11-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
azin-5-one and p-toluenesulphonylchloride (1.80 g, 9
mmol) were coupled to obtain the title compound 7b
furnished as a brownish crystalline solid. Yield 3.15 g
(86%), mp. 218–220 °C. UV-Vis (MeOH) (nm): 330
(2.2041), 460 (1.7781), 600 (1.3010), 692 (1.4771). IR
2
16–218 °C. UV-Vis (MeOH) (nm): 330 (2.3010), 448
−
1
(
1.9030), 648 (1.6020). IR (KBr) (cm ): 3507, 3349 (NH),
3
119 (aromatic CH), 2573 (SH), 1734 (C=O), 1574
(
C=N), 1498 (C=C aromatic), 1231, 1128 (SO two
2
1
bands), 841 (C-Cl). H-NMR (DMSO-d ) δ: 7.77 (1H, s,
NH), 7.56 (1H, d, Ar-H), 7.50 (1H, d, Ar-H), 7.34 (4H, m,
Ar-H), 7.18 (2H, s, NH ), 3.29 (1H, s, SH). C-NMR
6
−
1
(KBr) (cm ): 3251 (NH), 3115 (CH aromatic), 2491 (SH),
13
1639 (C=O), 1510 (C=N), 1438 (C=C aromatic), 1123,
2
1
(
DMSO-d ) δ: 195.80 (CO), 154.09, 137.17, 133.82,
1076 (SO two bands), 897 (C-Cl). H-NMR (DMSO-d ) δ:
6
2
6
1
1
-
33.69, 132.58, 132.37, 132.04, 131.95, 129.52, 129.33,
7.85 (1H, s, NH), 7.50 (4H, m, Ar-H), 7.12 (4H, m, Ar-H),
3.32 (1H, s, SH), 2.20 (3H, s, Ar-CH3). C-NMR (DMSO-
d6) δ: 133.65, 132.57, 132.01, 131.94, 129.31, 129.22
(aromatic carbons), 38.97 (CH3).
+
13
29.25 (aromatic carbons). MS: in m/z [rel.%]: 297.07 [m
PhClNHSO , 12%], 297.33 [m - PhClNHSO2+ NH2,
+
+
2
+
+
3
7%], 245.00 [m - PhClNHSO2 SH, 11%], 152.93 [6%].
1
8
1-Amino-6-(4-nitrophenylsulphonamido)-9-mercapto-
,10-diazabenzo[a]phenoxazin-5-one (5d)
11-(4-Chlorophenylsulphonamido)-6-chloro-9-mercapto-
8,10-diazabenzo[a]phenoxazin-5-one (7c)
1
1-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
11-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
azin-5-one and 4-chlorobenzenesulphonylchloride (1.80 g,
8 mmol) were converted to the title compound 7c was
furnished as a greyish crystalline solid. Yield 3.29 g (86%),
mp. 219–221 °C. UV-Vis (MeOH) (nm): 330 (2.3873), 453
azin-5-one and 4-nitrobenzene sulphonamide (1.80 g, 9
mmol) were converted to the title compound 5d furnished
as a pale brownish crystalline solid. Yield 3.11 g, (83%),
mp. 222–224 °C. UV-Vis (MeOH) (nm): 330 (2.5797), 651
−
1
−1
(
1.6232). IR (KBr) (cm ): 3567, 3337 (NH), 3070 (CH
(2.1461), 690 (1.7781). IR (KBr) (cm ): 3334 (NH), 3193
aromatic), 2586 (SH), 1739 (C=O), 1568 (C=N), 1407
(CH aromatic), 2331 (SH), 1667 (C=O), 1545 (C=N, C=C
1
(
C=C aromatic), 1277 (N=O), 1159, 1078 (SO two
aromatic), 1220, 1137 (SO two bands), 879 (C-Cl). H-
2
2
1
bands). H-NMR (DMSO-d ) δ: 8.20 (1H, s, NH), 7.57
NMR (DMSO-d ) δ: 7.69 (1H, s, NH), 7.49 (4H, m, Ar-H),
6
6
1
3
(
1H, d, Ar-H), 7.51 (1H, d, Ar-H), 7.28–7.11 (4H, m, Ar-
3.28 (1H, s, SH). C-NMR (DMSO-d + CDCl ) δ:
6
3
13
H), 5.77 (2H, s, NH ), 3.29 (1H, s, SH). C-NMR (DMSO-
132.48, 131.99, 131.93, 129.16, 129.07 (aromatic carbons).
2
+
+
d6) δ: 191.59 (CO), 152.93, 133.71, 132.70, 132.55, 132.32,
MS: in m/z [rel.%]: 315.13 [m - PhClNHSO , 10%],
2
301.13 [m+ - PhClNHSO2+ O, 100%], 297.33 [m
PhClNHSO2+ Cl, 67%], 175.00 [21%], 153.27 [2%].
+
-
1
32.01, 131.92, 129.31, 129.21 (aromatic carbons). MS: in
+
+
m/z [rel.%]: 291.83 [m - PhNO NHSO , 10%], 277.53
2
2
+
+
[
m
- PhNO NHSO2 NH , 4%], 187.87 [5%], 156.33
2 2
[
4%].

Medicinal Chemistry Research
1
8
1-(4-Nitrophenylsulphonamido)-6-chloro-9-mercapto-
,10-diazabenzo[a]phenoxazin-5-one (7d)
(DMSO:d ) δ (ppm): 7.60 (1H, s, NH), 7.58 (1H, d, Ar-H),
6
7.57 (1H, d, Ar-H), 7.52 (1H, m, Ar-H), 7.51 (4H, m, Ar-
1
3
H), 3.38 (1H, s, SH). C-NMR (DMSO) δ (ppm): 136.66
(imine carbon), 134.97–127.63 (aromatic carbons).
1
1-Amino-6-chloro-9-mercapto-8,10-diazabenzo[a]phenox-
azin-5-one reacted with 4-nitrobenzenesulphonylchloride
1.80 g, 8 mmol) to afford the title compound 7d as a pale
brownish crystalline solid. Yield 3.28 g (84 %), mp. 226–
(
Synthesis of 6,11-bis(4-methylphenylsulfonamido)-9-
mercapto-8,10-diazabenzo[a]phenoxazin-5-one 8b
2
7
28 °C. UV-Vis (MeOH) (nm): 330 (2.5797), 504 (1.9031),
00 (1.6021). IR (KBr) (cm ): 3393 (NH), 3155 (CH
−
1
Compound 8b was obtained as a brownish crystalline solid
aromatic), 2453 (SH), 1639 (C=O, C=N), 1469 (aromatic
when
11-amino-6-(4-methylphenylsulfonamido)-9-mer-
C=C), 1338 (N=O), 1165, 1012 (SO two bands), 838 (C-
capto-8,10-diazabenzo[a]phenoxazin-5-one 5b (2.50 g, 5
mmol) reacted with p-toluenesulfonylchloride (1.80 g, 9
mmol). Yield 2.71 g (85%), m.p. 264–268 °C. Uv-Vis
(MeOH) λmax: 330 (2.5185), 460 (2.6628), 600 (2.7782)
nm. FT-IR (KBr);Vmax: 3261 (N-H stretch), 3190, 3110 (Ar
C-H stretch), 2498 (S-H stretch), 1660 (C=O, C=N), 1585,
2
1
Cl). H-NMR (DMSO-d ) δ: 8.18 (1H, s, NH), 7.84 (1H, d,
6
Ar-H), 7.55 (1H, d, Ar-H), 7.51–7.11 (4H, m, Ar-H), 3.31
1
3
(
1H, s, SH). C-NMR (DMSO-d ) δ: 133.68, 132.57,
6
1
32.01, 131.94, 129.31, 129.21, 125.22 (aromatic carbons).
+
+
MS: in m/z [rel.%]: 316.93 [m - PhNO NHSO , 7%],
2
2
+
+
+
−1
3
01.13 [m - PhNO NHSO O, 100%], 297.33 [m
-
1420 (Ar-C=C), 1024 (S=O), 719 cm (para substitution).
2
2
+
1
PhNO NHSO Cl, 35%], 185.93 [16%], 156.20 [3%].
H-NMR (DMSO:d ) δ (ppm): 7.68 (1H, s, NH), 7.66 (1H, d,
2
2
6
Ar-H), 7.59 (1H, d, Ar-H), 7.57 (4H, m, Ar-H), 3.35 (1H, s,
1
3
General procedure for synthesis of disubstituted
derivatives (8a–d)
SH), 2.03 (3H, s, CH3). C-NMR (DMSO) δ (ppm): 142.40
(C=O), 141.93 (imine carbon), 133.70–126.13 (aromatic
carbons), 39.06 (CH3).
To a mixture of bis(triphenylphosphine) nickel (II) chloride
3.00 g, 5 mmol) and triphenylphosphine (1.85 g, 10 mmol)
was added t-butanol (6 ml) and distilled water (3 ml) with
continuous stirring for 10 min at room temperature under
nitrogen atmosphere. The resultant mixture was heated at
(
Synthesis of 6,11-bis(4-chlorophenylsulfonamido)-9-
mercapto-8,10-diazabenzo[a]phenoxazin-5-one 8c
Compound 8c was furnished as a dark reddish crystalline
8
0 °C in an oil bath for 1.5 min, after which 11-amino-6-
solid
ophenylsulfonamido)-9-mercapto-8,10-diazabenzo[a]phe-
noxazin-5-one 5c (2.50 g, 5 mmol), with
on
coupling
11-amino-6-(4-chlor-
substituted-9-mercapto-8,10-diazabenzo[a]phenoxazin-5-
one 8a-d (2.50 g), K CO (1.38 g, 10 mmol) and aryl sul-
4-
2
3
fonylchlorides (1.80 g) were added to it with the help of
solvents (t-butanol and distilled water in the ratio of 2:1).
The mixture was refluxed at 100–110 °C under nitrogen
atmosphere for 1 h with continuous stirring. At the end of
the refluxing period, the mixture was allowed to cool, fil-
tered via suction, and washed with distilled water. The
crude product was recrystallized from equal mixture of
acetone and diethyl ether to give derivatives 8a-d in good to
excellent yields.
chlorobenzenesulfonylchloride (1.80 g, 8 mmol). Yield
2.89 g (88%), m.p. 255–257 °C. Uv-Vis(MeOH) λmax: 330
(2.5155), 520 (2.7160) nm. FT-IR(KBr);Vmax: 3221 (N-H
stretch), 3069 (Ar C-H stretch), 2501, 2413 (S-H stretch),
−
1
1715 cm (C=O, C=N), 1582, 1456 (Ar-C=C), 1096
(S=O), 827, 678 cm− (C-Cl stretch, para substitution). H-
1
1
NMR (DMSO:d ) δ (ppm): 7.78 (1H, s, NH), 7.76 (1H, d, Ar-
6
H), 7.59 (1H, d, Ar-H), 7.57 (4H, m, Ar-H), 3.31 (1H, s,
1
3
SH). C-NMR (DMSO) δ (ppm): 167.34 (C=O), 158.96
imine carbon), 145.77–110.04 (aromatic carbons).
(
Synthesis of 6,11-bis(phenylsulfonamido)-9-mercapto-8,10-
diazabenzo[a]phenoxazin-5-one 8a
Synthesis of 6,11-bis(4-nitrophenylsulfonamido)-9-
mercapto-8,10-diazabenzo[a]phenoxazin-5-one 8d
Compound 8a was obtained as a reddish-yellow crystalline
solid when 11-amino-6-(phenylsulfonamido)-9-mercapto-
Compound 8d was obtained as a brownish crystalline solid
when 11-amino-6-(4-nitrophenylsulfonamido)-9-mercapto-
8,10-diazabenzo[a]phenoxazin-5-one 5d (2.50 g, 5 mmol)
was coupled with 4-nitrobenzenesulfonylchloride (1.80 g, 8
mmol). Yield 2.69 g (81%), m.p. 274–276 °C. Uv-Vis
(MeOH) λmax: 330 (2.5185), 680 (2.8325) nm. FT-IR(KBr);
Vmax: 3395 (N-H stretch), 3157 (Ar C-H stretch), 2592,
2507 (S-H stretch), 1642 (C=O, C=N), 1563, 1469 (Ar-
8
,10-diazabenzo[a]phenoxazin-5-one 5a (2.50 g, 5 mmol)
was coupled with benzenesulfonylchloride (1.0 g, 10
mmol). Yield 2.43 g (74%), m.p. 230–232 °C. UV-Vis
(
MeOH) λmax: 330 (2.5185), 640 (2.6628), 700 (2.8451)
nm. FT-IR(KBr);Vmax: 3293 (N-H stretch), 3043 (Ar C-H
stretch), 2481, 2418 (S-H stretch), 1661 (C=O, C=N),
−
1
1
1
591, 1421 (Ar-C=C), 1267, 1040 cm (S=O). H-NMR

Medicinal Chemistry Research
C=C), 1330 (N=O), 1250 (S=O), 693 cm−1 (para sub-
In silico studies
1
stitution). H-NMR (DMSO:d + CDCl ) δ (ppm): 7.49 (1H,
6
3
s, NH), 3.26 (1H, s, SH). 13C-NMR (DMSO + CDCl ) δ
The physicochemical properties used for the evaluation of
the drug-likeness of the synthesized compounds were cal-
culated using the molecular descriptors available in the
online open access molinspiration (www.molinspiration.
com). The molecular descriptors calculated include: mole-
cular weight (MW), partition coefficient (log P), hydrogen
bond acceptor (HBA), hydrogen bond donor (HBD), topo-
logical polar surface area (TPSA) and number of rotatable
bond (NoRB). We retrieved the crystal structure of DNA
gyrase (PDB codes: 5MMN) containing the co-crystallized
inhibitor from the online protein data bank repository
(https://www.rcsb.org/structure/5mmn). The chemical
structures of the synthesized compounds were drawn using
ChemSketch. Further preparations of the protein and the
ligands were done using the discovery studio. These pre-
parations included deleting of multiple chains, water of
crystallization from the protein and energy minimization of
the structures. The molecular docking and the visualization
of the protein-ligand complex interaction were done using
Autodock/Autodock vina and discovery studio respectively.
3
(ppm): 133.38 (imine carbon), 132.25–128.90 (aromatic
carbons).
Antimicrobial Activity Assays
The general sensitivity testing and MICs of the sulphona-
mides were investigated against Bacillus subtilis, Staphy-
lococcus aurues, Escherichia coli, Salmonella typhi,
klebsiella pneumoniae, Aspergillus niger and Candida
albican which are associated with skin, urinary and chronic
obstructive pulmonary infections, as well as gastrointestinal
tract damage and renal failure.
Sensitivity Test
Agar-well diffusion technique as described (Okorie 2005)
was used to determine the antimicrobial sensitivity test of
the phenoxazine intermediate and that of the sulphonamide
derivatives. Sensitivity test agar plates were seeded with
0
.1 ml of culture of each microorganism into its corre-
sponding Petri-dish. The plates were allowed to set after
which cups were made in each sector drawn on the backside
of the bottom plate using marker. Using a pipette, each cup
was filled with six drops of their corresponding anti-
microbial agents in appropriate solvent at a concentration of
RESULTS AND DISCUSSION
Chemistry
5
0 mg/ml. The plates were incubated at 37 °C for 24 h for
Anhydrous base catalyzed cross-coupling of 4,5-diamino-6-
hydroxy-2-mercaptopyrimidine 1 and 2,3-dichloro-1,4-
naphthoquinone 2 under refluxing condition for 5 h afforded
the key intermediate, 11-amino-6-chloro-9-mercapto-8,10-
diazabenzo[a]phenoxazin-5-one 3 (Scheme 1) (Okafor
bacteria and 48 h for fungi. The zones of inhibition pro-
duced after the period of incubation was measured.
The procedure was repeated for the reference drug
colomycin.
1986). The UV-visible absorption bands of compound 3
Minimum inhibitory concentration (MIC)
gave 630 and 752 nm due to the presence of extensive pi-
bond conjugation. The IR spectra of compound 3 revealed
−
1
Agar-well diffusion method was used to determine the
minimum inhibitory concentration (MIC) of the synthesized
compounds (Adeniyi and Odelola 1996). Serial dilutions of
the compounds were prepared from 50 mg/ml solution of
the sulphonamide derivatives to give 50, 25, 12.5, 6.25 and
the presence of N-H stretching vibrations at 3510 cm due
−
1
to NH , while the weak band at 2507 cm depicted the
presence of S-H. There is also the presence of strong band at
1723 cm , assigned to C=O, while the bands at 1623 and
1492 cm− are assigned to C=N and C=C aromatic
respectively. The proton nuclear magnetic resonance of
compound 3 furnished aromatic protons at δ 7.89–7.52,
2
−
1
1
3
.125 mg/ml. After dilution, the test solutions were added
into their corresponding cups previously made in the molten
agar, starting from the lowest concentration (3.125 mg/ml).
This was followed by incubation at the appropriate incu-
bation temperature and time. The resultant inhibition zones
of diameter (IZD) were measured and the values subtracted
from the diameter of the borer (8 mm) to give the inhibition
1
3
NH - at δ 6.84 and HS- at 3.42. The C-NMR provided
2
carbonyl carbon at 186.42, in addition to other carbon
peaks. Compound 3 was converted into 6-aryl sulphona-
midodiazaphenoxazinones 5a-d via reactions with aryl
sulphonamides 4a-d.
Compounds 4a-b were obtained by reactions of aryl
sulphonyl chlorides with ammonium hydroxide solution
(Ugwu et al. 2014). In this reaction, the aryl sulphonyl
chlorides were stirred in aqueous ammonium hydroxide for
2
zone diameter (IZD). The graph of IZD against log of
concentration was plotted for each plate and the anti-
logarithm of the intercept on x-axis gave the MIC. The
procedure was also repeated for colomycin.

Medicinal Chemistry Research
5
3
6
min followed by addition of water with stirring for further
mins. Thereafter, the entire reaction mixture was heated at
0 C for 5 min, and allowed to cool with chilling. The
stretching) and the absence of absorption bands at about
480–400 cm− (S-S stretch) in derivatives 5a-d means that
there was no coupling between the mercapto group in
compound 3 and the chloride (Cl) of the aryl sulphonyl
chlorides (Scheme 2).
Furthermore, compound 3 reacted with 4-substituted aryl
sulphonyl chlorides 6a-d to furnish compounds 7a-d in
excellent isolated yields after recrystalisation from mixture
of acetone and diethyl ether.
In another reaction, 4-substitued aryl sulphonyl chloride
6a-d reacted with 11-amino-6-(sulphonamido)-9-mercapto-
8,10-diazabenzo[a]phenoxazin-5-ones 5a-d under nickel-
phosphine catalytic system to furnish 6,11-bis(phe-
nylsulfonamido)-9-mercapto-8,10-diazabenzo[a]phenox-
azin-5-ones 8a-b in high yields (Scheme 3). The structures
1
o
desired products were obtained after filtration and recrys-
tallization from aqueous ethanol. Compounds 5a-d were
highly coloured and obtained in high yields. Their structures
were confirmed by combination of spectroscopic and ana-
lytical data. IR spectra of compound 5a-d showed N-H
stretching vibrations at two bands in the range 3503–3567
cm (assigned to NH ), and 3369–3337 cm (assigned to
NH of NHSO ). The appearance of the second N-H vibra-
tion at lower bands together with that of S=O vibrations at
1
−
1
−1
2
2
−
1
277–1012 cm supports the formation of sulphonamide
group in the derivatives. Furthermore, the continued pre-
sence of absorption bands at 2489–2453 cm
−
1
(S-H
2
NH2
3
O
NH
2
12
1
NH2
NH2
O
S
O
NiCl (PPh )
2
3
, PPh3
10
11
Cl
N
4
NH2
OH
Na SO
N
O
3
N
2
3
N
N
5
9
+
+ Cl
t-BuOH-H O
O
C H -DMF
2
HS
N
8
O
6
6
6
HS
N
HS
N
O
o
7
80 C , 5h
o
NH
O
100 - 110 C
Cl
1
R
a-d
1 h
O S O
2
4
5a-d
Ar R
5
a
5b
5c
Cl-
5d
R
H
CH3-
NO2-
Scheme 1 The synthesis of 6-substituted aryl sulphonamide diazabenzo[a]phenoxazinones 5a-d
R
Scheme 2 Synthesis of 11-
arylsulphonamidodiazabenzo[a]
phenoxazinones 7a-d
Ar
Cl
O S O NiCl (PPh ) , PPh
3
O S O
NH2
2
2
3
3
4
N
O
1NH 12
1
N
1
+
N
t-BuOH-H O
10
2
N
HS
N
O
5
9
o
1
00 - 110 C
Cl
HS
N
O 7
7a-d
O
R
1 h
6
3
8
Cl
6a-d
7
a
7b
CH3-
7c
Cl-
7d
R
H
NO2-
R
Scheme 3 Synthesis of 6,11-
bisarylsulphonamidodiazabenzo
Ar
Cl
O S O NiCl (PPh ) , PPh
3
O S O
NH2
2
[
a]phenoxazinones 8a-d
2
3
3
4
N
1NH 12
1
N
1
+
N
t-BuOH-H O
10
2
N
HS
N
O
O
5
9
o
1
00 - 110 C
NH
O S O
Ar R
HS
N
O7
8a-d
O
R
1 h
6
5a-d
8
NH
6a-d
O S O
Ar R
8
a
8b
8c
Cl-
8d
R
H
CH3-
NO2-

Medicinal Chemistry Research
Table 1 Inhibition zone
diameter (IZD) (mm) and MIC
Bacteria
Fungi
(
mg/ml) in bracket ()
Compd No
B.subtilis
S.aureus
E.coli
S.typhi
K.pneumoniae
A.niger
C.albican
3
14(0.53)
10(0.83)
12(0.80)
--
--
--
--
5
5
5
5
7
7
7
7
8
8
8
8
a
b
c
--
--
--
--
--
--
--
--
--
--
--
--
--
--
12(0.90)
--
14(0.50)
--
--
--
--
--
--
d
a
b
c
--
--
--
--
--
18(0.26)
19(0.24)
--
20(0.20)
18(0.26)
--
22(0.12)
12(0.86)
16(0.28)
--
--
16(0.28)
22(0.09)
14(0.36)
--
21(0.15)
--
--
--
--
--
d
a
b
c
--
--
--
--
--
--
--
22(0.10)
20
12(0.78)
15
26(0.05)
15(0.30)
19(0.25)
13(0.81)
22(0.10)
–
–
–
–
–
10(0.92)
--
8(1.10)
--
–
–
–
–
14(0.83)
–
d
--
–
--
–
Ref. Drug
23(0.09)
18(0.27)
19(0.24)
15(0.32)
21(0.14)
12(0.89)
17(0.29)
Reference drug = Colomycin
Table 2 Free binding energy of compounds with intact and mutated
DNA gyrase
the concentration of 50 mg/ml in agar media following the
method described (Okorie 2005). Colymycin was used as
reference drug for antibacterial and antifungal drugs. The
activity of the compounds was measured in form of inhi-
bition zone diameter (IZD) around the wells as well as
minimum inhibitory concentration (MIC) and the results are
presented in Table 1. Generally while some derivatives
showed significant antimicrobial activity, others lost their
activity completely on derivatization. Compounds 3, 5c, 7a,
Intact DNA gyrase
Mutated DNA gyrase
Comp
ΔG (kcal/mol)
ΔG (kcal/mol)
3
−11.25
−11.46
−11.53
−11.43
−11.90
−10.94
−11.01
−11.76
−12.09
−12.31
−11.98
−11.62
−11.12
−10.31
−11.59
−12.47
−13.30
−12.02
−12.67
−11.49
−12.56
−12.37
−12.43
−11.57
−11.13
−10.31
−11.69
−12.07
5
5
5
5
7
7
7
7
8
8
8
8
a
b
c
d
a
b
c
7b, 8a, 8b and 8c showed some degree of activity while
compounds 5a, 5b, 5d, 7c, 7d, 8d showed no activity
against any of the organisms tested. In particular com-
pounds 7a, 7b and 8a have broad spectrum of activity
against Gram negative and Gram-positive bacteria. Speci-
fically, both bacteria and fungi are susceptible to compound
d
a
b
c
8a like the colomycin. Interestingly, compounds 7b and 8a
have marked activity more than the standard drug at a very
low dose against S. Typhi and E. coli respectively. Com-
pounds 7b and 8a are therefore potential drugs for the
treatment of infectious diseases like typhoid fever, food
poisoning, gastroenteritis and enteric fever.
d
Native ligand
Native ligand = 1-Ethyl-3-[8-methyl-5-(2-methylpyridin-4-yl) isoqui-
nolin-3-yl]urea
In silico studies
were confirmed by combination of spectroscopic and ana-
lytical data.
Lipinski’s rule of five (ro5) was applied to adjudge the
drug-likeness of the compounds. The ro5 implies that for a
molecule to possess drug-likeness, the MW ≤ 500, HBD ≤ 5,
HBA ≤ 10 and Log P ≤ 5 must be obeyed. If the molecule
fails in more than one type of parameters, the molecule will
probably have challenge in oral bioavailability. In Table 2,
it can be seen that only 8c failed in more than one whereas
others passed. TPSA and NoRB are also very useful in
assessing how useful a molecule can be as a drug. TPSA ≤
Antimicrobial activity
The antimicrobial screening of the azaphenoxazone and its
derivatives against Bacillus subtilis, Staphylococcus aureus,
Escherichis coli, Salmonella typhi, Klebsiella pneumoniae,
Candida albican and Aspergillus niger were carried out at

Medicinal Chemistry Research
Table 3 Physicochemical properties of the synthesized compounds
Comp
MW
logP(o/w) NoRB HBD HBA TPSA
3
330.755 1.863
433.448 1.96
465.514 2.816
467.893 2.919
478.445 2.262
470.917 3.782
484.944 4.08
505.362 4.374
515.914 3.717
591.649 3.939
619.703 4.535
660.539 5.123
681.643 3.809
0
3
3
3
4
3
3
3
4
6
6
6
8
5
2
2
2
2
2
1
1
1
1
2
2
2
2
2
4
6
6
6
6
6
6
6
6
8
8
8
8
3
121.16
136.63
175.43
136.63
182.45
149.41
149.41
149.41
195.23
195.58
195.58
195.58
287.22
66.91
5
5
5
5
7
7
7
7
8
8
8
8
a
b
c
d
a
b
c
d
a
b
c
d
Native ligand 320.396 2.931
Fig. 3 Ligplot + of co-crystallized ligand (1-Ethyl-3-[8-methyl-5-(2-
methylpyridin-4-yl) isoquinolin-3-yl]urea) in the binding cavity of
intact DNA gyrase
94, ARG 76, ILE 78, PRO 79 and GLY 77) in the intact
DNA gyrase surrounding the co-crystallized ligand. H-
bonding interactions between the ASP 76 and two different
atoms of the co-crystallized ligand, in addition to other
interactions were clearly shown. These were obviously
absent in the mutated DNA gyrase. The ASP 76 was absent
in the mutated gyrase as it has been replaced by ALA 76.
There were observed, two pi-H interactions between the 6-
membered ring of the ligand and the ILE 78 in the mutated
enzyme.
Gross et al., in their extensive work on the active-site
residues of Escherichia coli DNA gyrase observed that
substitution of GLU 42 and ASP 73 with ALA leads to
microbial resistance to some conventional antibiotics (Gross
et al. 2003). The synthesized compounds were docked into
the binding cavity of the DNA gyrase and the free binding
energy was calculated. The DNA gyrase were mutated and
the compounds were docked into the mutated target and the
results are shown in Table 3.
The results as shown in Table 3 revealed that the syn-
thesized compounds interacted significantly both with the
intact and mutated DNA gyrase. There was no significant
difference in the binding affinity of all the synthesized
compounds. However, their binding energies are lower
Fig. 4 Ligplot + of co-crystallized ligand (1-Ethyl-3-[8-methyl-5-(2-
methylpyridin-4-yl) isoquinolin-3-yl]urea) in the binding cavity of
mutated DNA gyrase
2
1
40 Å and NoRB ≤ 10 would have a high probability of
good oral bioavailability in rats (Veber et al. 2002).
(high binding affinities) than the reference (native ligand)
Molecular docking
for the intact drug target. Compounds 8a and 5b have been
used to show the binding interaction of the synthesized
compounds with the both intact and mutated DNA gyrase
respectively. The 2D interaction of 8a with the intact gyrase
has been shown in Fig. 5 while Fig. 6 shows the 2D
interaction of compound 5b with mutated DNA gyrase.
Moreover, the compounds also showed significant
binding affinity with the mutated DNA gyrase. The 6-
substitued aryl sulphonamide diazabenzo[a]phenoxazones
Mutation of DNA gyrase
The ligplot + of the intact and the mutated DNA gyrase are
shown in Figs. 3 and 4 respectively. The mutation was done
by replacing the GLU 42 and ASP 73 with ALA. Figure 4
shows clearly 12 active amino acid residues (THR 165,
VAL 43, ALA 47, VAL 71, ASP 73, ASN 46, GLU 50, ILE

Medicinal Chemistry Research
Fig. 5 2D interaction of compound 8a with intact DNA gyrase
Fig. 7 Chemical interactions of 8a with intact DNA gyrase
one of the aromatic rings and GLY77. One of the O-atoms
entered into hydrogen bonding interaction with ARG76.
The details of the chemical interactions of 8a with DNA
gyrase, including the distance of interaction (Å) are shown
in Fig. 7. These various chemical interactions can explain
the strong binding affinity of 8a and its significant inhibi-
tory effect on the protein. All the five 6-membered rings of
compound 5b were involved in the interactions between the
compound and the mutated DNA gyrase (Fig. 6). The
interactions were mainly pi-H interactions. The 6-
membered ring interacted with the CB of ASN 46, CG1
of ILE 78, CD1 of ILE 78, CD of PRO 79 and CG1 of ILE
94. Though 5b showed no antimicrobial activity in vitro, it
showed a promising binding affinity for the mutated DNA
gyrase and may be a useful lead in the development of
drugs to tackle multi drug resistant bacterial strains.
Fig. 6 2D interaction of compound 5b with mutated DNA gyrase
(
5a-d) showed a better binding affinity with the mutated
DNA gyrase than the rest of the compounds. Compound 5b
had the highest binding affinity as depicted by its lowest
binding energy (−13.30 kcal/mol). It can be deduced that
these 6-substitued aryl sulphonamide diazabenzo[a]phe-
noxazones may be more effective in the treatment of
infections caused by resistant strains.
CONCLUSION
Newly synthesised aryl sulphonamidobenzo[a]phenozax-
inones presented significant anti-microbial comparable to
colymycin. In particular, 11-(4-methylphenylsulphona-
mido)-6-chloro-9-mercapto-8,10-diazabenzo[a]phenoxazin-
5-one 7b and 6,11-bis(phenylsulfonamido)-9-mercapto-
8,10-diazabenzo[a]phenoxazin-5-one 8a elicited broad
spectra of activity against Gram positive and Gram negative
bacteria. All the synthesized compounds have strong bind-
ing affinity with DNA gyrase; and therefore, can act as
DNA gyrase inhibitors. They also have significant binding
affinity for the mutated DNA gyrase. Therefore, they can be
A closer analysis of the binding of 8a with intact 5MMN
(
Figs. 5 and 7) showed various chemical interaction
between the compound and the receptor. The amino acid
residues: VAL43, ILE78, ILE94, PRO79, VAL120 and
VAL167 showed pi-alkyl interactions with the aromatic
rings. There were also amide-pi sacked interactions between

Medicinal Chemistry Research
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