Synthesis and anti-bacterial activity of novel dihydrochromeno[8,7-e][1,3] oxazine-2(8H)-thiones

Article abstract of DOI:10.1080/17415993.2013.769543

A novel series of sulfur-containing dihydrochromeno[8,7-e][1,3]oxazine- 2(8H)-thiones has been synthesized through an eco-friendly Mannich-type condensation cyclization reaction of 7-hydroxy-4-methyl-2-thiocoumarin or 6-chloro-7-hydroxy-4-methyl-2-thiocoumarin with formaldehyde and primary amines in water at 80-90°C for 2 h. All the synthesized compounds were screened for their in vitro anti-bacterial efficacy against two Gram-positive and three Gram-negative bacterial strains by using the disc diffusion method. The compound (8c) was found to be most potent with the zone of inhibition of 16 and 15 mm against Staphylococcus aureus ATCC 2937 and Klebsiella pneumoniae ATCC 31488, respectively.

Full text of DOI:10.1080/17415993.2013.769543

This article was downloaded by: [Florida International University]
On: 20 December 2014, At: 16:51
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Sulfur Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gsrp20
Synthesis and anti-bacterial activity
of novel dihydrochromeno[8,7-e]
[1,3]oxazine-2(8H)-thiones
Bijoy P. Mathewa, Nisha Aggarwalb, Rajesh Kumarb & Mahendra
Natha
a Department of Chemistry, University of Delhi, Delhi 110 007,
India
b Division of Agricultural Chemicals, IARI, Pusa Campus, New Delhi
110 012, India
Published online: 20 Mar 2013.
Click for updates
To cite this article: Bijoy P. Mathew, Nisha Aggarwal, Rajesh Kumar & Mahendra Nath (2014)
Synthesis and anti-bacterial activity of novel dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thiones,
Journal of Sulfur Chemistry, 35:1, 31-41, DOI: 10.1080/17415993.2013.769543
To link to this article: http://dx.doi.org/10.1080/17415993.2013.769543
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Sulfur Chemistry, 2014
Vol. 35, No. 1, 31–41, http://dx.doi.org/10.1080/17415993.2013.769543
Synthesis and anti-bacterial activity of novel
dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thiones
Bijoy P. Mathewa, Nisha Aggarwalb, Rajesh Kumarb and Mahendra Natha*
aDepartment of Chemistry, University of Delhi, Delhi 110 007, India; bDivision of Agricultural Chemicals,
IARI, Pusa Campus, New Delhi 110 012, India
(Received 20 December 2011; final version received 20 January 2013)
A novel series of sulfur-containing dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thiones has been synthe-
sized through an eco-friendly Mannich-type condensation cyclization reaction of 7-hydroxy-4-methyl-2-
thiocoumarin or 6-chloro-7-hydroxy-4-methyl-2-thiocoumarin with formaldehyde and primary amines in
water at 80–90◦C for 2 h. All the synthesized compounds were screened for their in vitro anti-bacterial
efficacy against two Gram-positive and three Gram-negative bacterial strains by using the disc diffusion
method. The compound (8c) was found to be most potent with the zone of inhibition of 16 and 15 mm
against Staphylococcus aureus ATCC 2937 and Klebsiella pneumoniae ATCC 31488, respectively.
R
N
R
N
O
O
S
O
O
S
Cl
CH3
CH3
7a–7g
8a–8g
Keywords: thiocoumarins; dihydro[1,3]oxazines; Lawesson’s reagent; Mannich reaction; anti-bacterial
activity
1. Introduction
Heterocycles containing thioxo group are known to exhibit promising biological activities (1–3).
Among these, 2-thiocoumarin analogues have shown a wide range of pharmaceutical properties
such as apoptogenic effect (4), anti-coagulant (5), anticancer (6) and anti-HIV activity (7). In
addition, they have also been found to inhibit TNF-α induced ICAM-1 expression on human
umbilical vein endothelial cells and microsomal lipid peroxidation (8). On the other hand, dihy-
dro[1,3]oxazines have been reported as promising antimicrobial agents in the recent past (9–13).
Recently, the benzoxazines having thioxo group have demonstrated enhanced antimycobacte-
rial activity (14). Considering all these facts, two heterocyclic entities, i.e. 2-thiocoumarin and
dihydro[1,3]oxazine have been coupled in a single molecular scaffold to prepare a series of
novel dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione derivatives as anti-bacterial agents. In
*Corresponding author. Email: mnath@chemistry.du.ac.in
© 2013 Taylor & Francis

32 B.P. Mathew et al.
continuation of our previous work on the development of antimicrobial agents (13), we report
herein an eco-friendly one-pot synthesis and in vitro anti-bacterial studies of a series of novel
4-methyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione derivatives.
2. Results and discussion
2.1. Chemistry
Forsynthesizingthetargetmolecules, 7-acetoxy-4-methylcoumarin(1)and7-acetoxy-6-chloro-4-
methylcoumarin (2) were synthesized as precursors through Pechmann condensation of resorcinol
or 4-chlororesorcinol with ethyl acetoacetate under acidic conditions followed by acetylation
using acetic anhydride (15–20). Further, these compounds were successfully converted to the
corresponding 2-thiocoumarins (3 and 4) by the treatment with Lawesson’s reagent (LR) in
refluxing toluene (21). Finally, the desired starting materials, 7-hydroxy-4-methyl-2-thiocoumarin
(5) and 6-chloro-7-hydroxy-4-methyl-2-thiocoumarin (6) were obtained after the deacetylation
of acetoxy-thiocoumarins 3 and 4, respectively, using triethyl amine in methanol at ambient
temperature (22) (Scheme 1).
AcO
X
O
S
AcO
X
O
O
HO
X
O
S
Et N, MeOH
LR, toluene
Reflux, 3 h
3
25°C, 24 h
1, X = H
2, X = Cl
3, X = H, Yield = 65%
4, X = Cl, Yield = 60%
5, X = H, Yield = 75%
6, X = Cl, Yield = 70%
Scheme 1. Synthetic route to 2-thiocoumarins.
In the context of our efforts to synthesize various dihydro[1,3]oxazines (23), the target tri-
cyclic dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione derivatives (7a–7g and 8a–8g) were
synthesized in good yields (69–80%) using an environmentally benign protocol involving one-pot
Mannich-type condensation reaction of 7-hydroxy-4-methyl-2-thiocoumarin (5) or 6-chloro-7-
hydroxy-4-methyl-2-thiocoumarin(6)withformaldehydeandprimaryaminesinwaterat80–90◦C
for 2 h (Scheme 2).
R
N
HO
X
O
S
O
X
O
S
RNH2,HCHO, water
80–90 , 2 h
°
C
X = H
X = Cl
7a–7g; X = H
8a–8g; X = Cl
5;
6;
Yield = 69–80%
Scheme 2. Synthesis of dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thiones.
The plausible mechanistic pathway for the formation of dihydrochromeno[8,7-e][1,3]oxazine-
2(8H)-thiones (7 and 8) is given in Scheme 3. On the basis of our hypothesis, together with
the literature report (24), the reaction is believed to proceed via the formation of an imine

Journal of Sulfur Chemistry 33
intermediate (9) from the reaction of amine with formaldehyde. In the next step, imine (9) under-
goesanelectrophilicattackon2-thiocoumarins(5and 6)atC-8positionviaMannich-typereaction
to form an amino phenol intermediate, 10 and 11, respectively. Further, these aminophenols react
with a second molecule of formaldehyde to form an oxazine ring and afford desired products 7
and 8, respectively.
R
N
H
HCHO
O
X
O
S
O
X
O
S
H
RN=CH2
H
RNH2
9
5; X = H
6; X = Cl
R
R
N
HN
HCHO
O
X
O
S
O
O
S
H
X
7; X = H
8; X = Cl
10; X = H
11; X = Cl
Scheme 3. A plausible mechanism for the formation of products 7 and 8.
Though, there was a possibility to obtain two regioisomers from the reaction of 7-hydroxy-
4-methyl-2-thiocoumarin (5) with formaldehyde and primary amines, surprisingly a single
regioisomer of products (7a–7g) has been isolated in 69–80% yields possibly due to the dif-
ference in the reactivity of C-6 and C-8 positions of precursor (5) as reported in the literature for
the similar coumarin system (24).
The structures of dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione derivatives (7a–7g and
1
8a–8g) were confirmed on the basis of Infrared (IR), H NMR, 13C NMR, mass and elemental
analysis data. For example, the IR spectrum of 4-methyl-9-phenyl-9,10-dihydrochromeno[8,7-e]
[1,3]oxazine-2(8H)-thione (7d) confirmed the absence of hydroxyl group which was present in the
starting compound 5 at 3311 cm−1 and also showed the appearance of new characteristic absorp-
tions at 1253 and 1049 cm−1 corresponding to asymmetric and symmetric C O C stretching,
− −
respectively, indicating the formation of dihydro-1,3-oxazine ring.
1
To confirm the structure of isolated regioisomer (7d), H NMR spectra of both 7-hydroxy-
4-methyl-2-thiocoumarin (5) and 4-methyl-9-phenyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-
2(8H)-thione (7d) have been compared in Figure 1. The proton NMR spectrum of starting
compound 5 showed three singlets at δ 2.36, 7.06 and 10.86 ppm corresponding to methyl,
olefinic (H3) and the hydroxy protons, respectively. The protons on the C-5 and C-8 positions
were appeared as doublet at δ 7.72 ppm (J = 8.7 Hz) and 6.89 ppm (J = 2.0 Hz) due to the
ortho and meta coupling with C-6 proton, respectively. The remaining C-6 proton appeared
slightly downfield than the C-8 proton as double-doublet at δ 6.92 ppm (J1, J2 = 8.7, 2.3 Hz).
In contrast, the proton NMR of product 7d showed two singlets at δ 2.27 and 7.03 ppm
corresponding to methyl and olefinic (H3) protons, respectively. However, a singlet corre-
sponding to OH group in compound 5 was completely disappeared due to the formation
of oxazine ring in product 7d. The appearance of two characteristic singlets at δ 4.88 and

34 B.P. Mathew et al.
Figure 1. 1H NMR spectra of (a) 7-hydroxy-4-methyl-2-thiocoumarin (5) and (b) 4-methyl-9-phenyl-9,
10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione (7d) in DMSO-d6 and CDCl3, respectively.
5.41 ppm due to two methylene groups further confirmed the presence of the oxazine moi-
ety in the compound 7d. In addition, two doublets appeared at δ 6.80 and 7.37 ppm with
the J-value 8.8 Hz were assigned to C-6 and C-5 protons, respectively. Furthermore, the
1
absence of C-8 proton in the H NMR spectrum of product 7d supports the assigned struc-
ture and ruled out the formation of other regioisomer involving C-6 position in the oxazine
ring.Themassspectrumof4-methyl-9-phenyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-
thione (7d) displayed the molecular ion peak (M + H)+ at m/z = 310 which further supports the
suggested structure.

Journal of Sulfur Chemistry 35
Table 1. The in vitro anti-bacterial activity of dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione derivatives
(7a–7g and 8a–8g) at 250 μg/ml concentration.
Zone of inhibition in mm
Compd. no.
R
Sa
Sm
Ec
Pa
Kp
7a
7b
7c
7d
7e
7f
7g
8a
8b
8c
8d
8e
8f
8g
4-FC6H4
2-ClC6H4
3-ClC6H4
C6H5
CH2C6H5
2-(CH3)C6H4
3-(CH3)C6H4
4-FC6H4
2-ClC6H4
3-ClC6H4
C6H5
–
8
12
–
–
–
–
–
12
16
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
16
–
–
–
–
–
–
–
–
–
–
–
–
–
–
15
–
–
–
–
–
–
–
–
–
–
–
–
–
–
13
8
11
10
–
–
–
–
–
14
15
–
CH2C6H5
2-(CH3)C6H4
3-(CH3)C6H4
–
–
–
–
18
–
–
–
19
Streptomycin
Note: Sa, Staphylococcus aureus ATCC 2937; Sm, Staphylococcus mutans ATCC 25175; Ec, Escherichia coli ATCC 8739; Pa,
Pseudomonas aeruginosa ATCC 9027; Kp, Klebsiella pneumoniae ATCC 31488; –, no zone of inhibition.
2.2. Anti-bacterial evaluation
The in vitro anti-bacterial activities of compounds (7a–7g and 8a–8g) and standard drug (strepto-
mycin) were carried out against five bacterial strains, viz. Staphylococcus aureus ATCC 2937
(Sa), Staphylococcus mutans ATCC 25175 (Sm), Escherichia coli ATCC 8739 (Ec), Pseu-
domonas aeruginosa ATCC 9027 (Pa), Klebsiella pneumoniae ATCC 31488 (Kp) and results
are summarized in Table 1. Out of 14 newly synthesized compounds, only 5 compounds
(7a–7c and 8b-8c) were found active against Sa and/or Kp at 250 μg/ml concentration. Though
none of the compounds has shown better anti-bacterial efficacy than the standard drug, strepto-
mycin, dihydrochromeno[8,7-e][1,3]oxazine-2(8H)thiones (7c and 8c) bearing 3-chlorophenyl
substituent at position 9 were found to be significantly active with the average zone inhibi-
tion of 12–16 and 10–15 mm against Sa and Kp, respectively, at 250 μg/ml concentration. In
addition, dihydrochromeno[8,7-e][1,3]oxazine-2(8H)thiones (7b and 8b) having 2-chlorophenyl
substituent at position 9 exhibited moderate activity with the average zone inhibition of 8–12 and
11–14 mm against Sa and Kp, respectively, at 250 μg/ml concentration. In contrast, compound 7a
with 4-fluorophenyl group at position 9 has shown poor activity (average zone inhibition of 8 mm)
specifically against a Gram-negative bacterial strain Kp. Surprisingly, all the target compounds
were found inactive against bacterial strains S. mutans, E. coli and P. aeruginosa.
3. Conclusions
Inconclusion, various4-methyl-9-substituted-9,10-dihydrochromeno[8,7-e][1,3]-oxazine-2(8H)-
thione (7a–7g) and 6-chloro-4-methyl-9-substituted-9,10-dihydrochromeno[8,7-e][1,3]oxazine-
2(8H)-thione (8a–8g) derivatives have been successfully synthesized using a green methodology
and evaluated for their in vitro anti-bacterial activity. Compound 8c has shown a significant zone
of inhibition of 16 and 15 mm at 250 μg/ml concentration against S. aureus and K. pneumoniae.
In a nutshell, these results are interesting and could be useful to medicinal chemists in order to
develop novel anti-bacterial agents.

36 B.P. Mathew et al.
4. Experimental
The chemicals and solvents were used as received without further purification. Thin-layer chro-
matography was performed on pre-coated Merck silica 60 F254 aluminum plates and spots were
detected under UV light (254 nm). All the synthesized compounds were purified by column
chromatography using a silica gel (60-120 mesh, Ranbaxy). Melting points were taken in open
capillaries and are uncorrected. Mass spectra were recorded on JEOL-AccuTOF JMS-Y100LC
Mass spectrometer having a direct analysis in real time (DART) source. IR spectra were recorded
1
on Perkin–Elmer IR spectrometer and absorption maxima (νmax) are given in cm−1. The H
and 13C NMR spectra were recorded in DMSO-d6 or CDCl3 on a Jeol 400 MHz spectrome-
ter by using TMS as the internal standard. Elemental analysis was performed on Elementar
Analysensysteme GmbH VarioEL V3.00. Anhydrous sodium sulfate was used as a drying agent.
7-Acetoxy-4-methyl-2-thiocoumarin (3) was prepared according to the literature procedure (21).
4.1. Synthesis of 7-acetoxy-6-chloro-4-methyl-2-thiocoumarin (4)
A mixture of 7-acetoxy-6-chloro-4-methylcoumarin 2 (3.7 mmol), LR (4 mmol) and anhydrous
toluene (10 ml) was refluxed for 3 h. After the completion of the reaction, the solvent was evapo-
rated under reduced pressure. The residue obtained was then subjected to column chromatography
on a silica gel using 50% ethyl acetate in hexane as eluent.Yellow solid; yield 60%; mp > 200◦C;
1
IR (nujol) ν (cm−1): 1766, 1608, 1539, 1455, 1378, 1296 (C S), 1205, 1153, 1079, 908; H
=
=
NMR (CDCl3, 400 MHz): 2.34 (s, 3H, CH3), 2.41 (s, 3H, COCH3), 7.15 (s, 1H, CH), 7.30
(s, 1H, ArH), 7.69 (s, 1H, ArH).
4.2. Synthesis of 7-hydroxy-4-methyl-2-thiocoumarin (5) and
6-chloro-7-hydroxy-4-methyl-2-thiocoumarin (6)
A
mixture of 7-acetoxy-4-methyl-2-thiocoumarin 3 or 7-acetoxy-6-chloro-4-methyl-2-
thiocoumarin 4 (3 mmol), triethyl amine (2.5 mmol) and methanol (10 ml) was stirred at room
temperature for 24 h and then the solvent was evaporated under reduced pressure. The residue
was subjected to column chromatography on a silica gel using 60% ethyl acetate in hexane as an
eluent to afford compounds 5 and 6 as yellow solids in 75% and 70% yields, respectively. The
structure of 7-hydroxy-4-methyl-2-thiocoumarin 5 was confirmed on the basis of spectroscopic
data as reported in the literature (21). Physical and spectral data for 6-chloro-7-hydroxy-4-methyl-
2-thiocoumarin 6: mp > 200◦C; IR (film) ν (cm−1): 3358 (OH), 1594, 1552, 1497, 1438, 1289
1
=
=
(C S), 1213, 1095, 910; H NMR (DMSO-d6, 400 MHz): 2.33 (s, 3H, CH3), 7.04 (s, 1H, CH),
−
7.08 (s, 1H, ArH), 7.86 (s, 1H, ArH), 11.69 (br, 1H, OH).
4.3. General procedure for the synthesis 4-methyl-9-substituted-9,
10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thiones (7a–7g) and 6-chloro-4-methyl-
9-substituted-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thiones (8a–8g)
To a mixture of 7-hydroxy-4-methyl-2-thiocoumarin (5) or 6-chloro-7-hydroxy-4-methyl-2-
thiocoumarin (6) (0.568 mmol) and primary amine (0.568 mmol) in water (1 ml), formalin (37%,
w/v, 1.136 mmol) was added. The reaction mixture was stirred at 80–90◦C for 2 h. After the
completion of the reaction, the product was extracted with ethyl acetate (10 ml, two times). The
organic layers were combined and washed with brine solution (10 ml, two times) followed by water
(25 ml). The organic layer was dried over sodium sulfate and evaporated under reduced pressure.
The crude product was purified by column chromatography on a silica gel using 20–30% ethyl
acetate in heptane as eluent to afford the desired product.

Journal of Sulfur Chemistry 37
4.3.1. 9-(4-Fluorophenyl)-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione
(7a)
Yellow solid; yield 80%; mp 145.1◦C; IR (film) ν (cm−1): 2918, 1625, 1591, 1557, 1508, 1400,
=
− −
− −
1290 (C S), 1252 (C O C asymmetric stretching), 1154, 1104, 1088, 1048 (C O C sym-
1
− −
metric stretching), 985, 961 (N C O stretching), 899, 828; H NMR (400 MHz, CDCl3): δ 2.30
−
−
−
−
−
(s, 3H, CH3), 4.84 (s, 2H, N CH2 Ar), 5.38 (s, 2H, N CH2 O), 6.84 (d, 1H, J = 8.7 Hz,
=
ArH), 6.93 (d, 1H, J = 8.7 Hz, ArH), 6.96 (d, 1H, J = 8.7 Hz, ArH), 7.05 (s, 1H, CH), 7.11 (dd,
2H, J1 = 8.7, J2 = 4.7 Hz, ArH), 7.41 (d, 1H, J = 8.7 Hz, ArH); 13C NMR (100 MHz, CDCl3):
− −
− −
δ 18.0 (CH3), 46.9 (N C Ar), 80.7 (O C N), 108.2, 115.1, 115.8, 116.0, 120.7, 120.8, 123.6,
126.6, 144.2, 145.3, 153.8, 157.8, 196.8; DART–MS: m/z 328.13 (M + H)+; Anal. Calcd for
C18H14FNO2S: C, 66.14; H, 4.31; N, 4.28; S, 9.79. Found: C, 66.38; H, 4.54; N, 4.53; S, 9.82.
4.3.2. 9-(2-Chlorophenyl)-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione
(7b)
Yellow solid; yield 75%; mp 173.9◦C; IR (film) ν (cm−1): 2925, 2855, 1625, 1594, 1557, 1503,
−
=
− −
1483, 1437, 1401, 1383 (C N), 1291 (C S), 1252 (C O C asymmetric stretching), 1216,
− − − −
1199, 1090, 1045 (C O C symmetric stretching), 985, 960 (N C O stretching), 922, 901,
1
−
−
−
861, 813; H NMR (400 MHz, CDCl3): δ 2.33 (s, 3H, CH3), 4.82 (s, 2H, N CH2 Ar), 5.37
−
−
(s, 2H, N CH2 O), 6.89 (d, 1H, J = 8.8 Hz, ArH), 7.03 (dd, 1H, J1 = 7.3, J2 = 1.5 Hz, ArH),
=
7.07 (s, 1H, CH), 7.12 (dd, 1H, J1 = 8.0, J2 = 1.5 Hz, ArH), 7.24–7.25 (m, 1H, ArH), 7.41
(d, 1H, J = 8.0 Hz, ArH), 7.45 (d, 1H, J = 8.8 Hz, ArH); 13C NMR (100 MHz, CDCl3): δ 18.0
− −
− −
(CH3), 46.2 (N C Ar), 81.2 (O C N), 108.3, 114.9, 115.2, 122.6, 123.5, 125.5, 126.6, 127.6,
128.9, 130.7, 145.1, 145.4, 153.8, 157.5, 196.6; DART–MS: m/z 344.01 (M+H)+ and 346.01
[(M + H)+ + 2] peaks in the ratio of 3:1; Anal. Calcd for C18H14ClNO2S: C, 62.88; H, 4.10; N,
4.07; S, 9.33. Found: C, 62.98; H, 3.74; N, 4.22; S, 9.45.
4.3.3. 9-(3-Chlorophenyl)-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione
(7c)
Yellow solid; yield 69%; mp 144.9◦C; IR (film) ν (cm−1): 2923, 2853, 1625, 1595, 1560, 1559,
=
− −
− −
1483, 1292 (C S), 1252 (C O C asymmetric stretching), 1154, 1111, 1089, 1049 (C O C
1
− −
symmetric stretching), 989, 962 (N C O stretching), 906, 858, 814; H NMR (400 MHz,
−
−
−
−
−
CDCl3): δ 2.32 (s, 3H, CH3), 4.89 (s, 2H, N CH2 Ar), 5.41 (s, 2H, N CH2 O), 6.85 (d, 1H,
J = 8.8 Hz,ArH), 6.93 (dd, 1H, J1 = 8.1, J2 = 1.5 Hz,ArH), 7.02 (dd, 1H, J1 = 8.8, J2 = 2.2 Hz,
=
ArH), 7.07 (s, 1H, CH), 7.14–7.15 (m, 1H, ArH), 7.19 (t, 1H, J = 8.1 Hz, ArH), 7.42 (d, 1H,
J = 8.8 Hz, ArH); DART–MS: m/z 344.01 (M + H)+ and 346.00 [(M + H)+ + 2] peaks in the
ratio of 3:1; Anal. Calcd for C18H14ClNO2S: C, 62.88; H, 4.10; N, 4.07; S, 9.33. Found: C, 62.77;
H, 4.22; N, 4.12; S, 9.54.
4.3.4. 4-Methyl-9-phenyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione (7d)
Yellow solid; yield 77%; mp 150◦C; IR (film) ν (cm−1): 2924, 2854, 1625, 1592, 1557, 1497,
=
− −
1438, 1400, 1381, 1291 (C S), 1253 (C O C asymmetric stretching), 1218, 1196, 1154, 1107,
− − − −
1089, 1049 (C O C symmetric stretching), 1030, 983, 960 (N C O stretching), 904, 814;
1
−
−
−
H NMR (400 MHz, CDCl3): δ 2.27 (s, 3H, CH3), 4.88 (s, 2H, N CH2 Ar), 5.41 (s, 2H,
−
−
=
N CH2 O), 6.80 (d, 1H, J = 8.8 Hz, ArH), 6.93 (t, 1H, J = 7.3 Hz, ArH), 7.03 (s, 1H, CH),
7.12 (d, 2H, J = 8.0 Hz, ArH), 7.23 (d, 2H, J = 7.3 Hz, ArH), 7.37 (d, 1H, J = 8.8 Hz, ArH); 13
C

38 B.P. Mathew et al.
− −
− −
NMR (100 MHz, CDCl3): δ 18.0 (CH3), 46.4 (N C Ar), 80.1 (O C N), 108.5, 115.1, 117.1,
118.7, 122.2, 123.5, 126.6, 129.3, 145.3, 147.7, 153.9, 157.9, 196.8; DART–MS: m/z 310.22
(M + H)+; Anal. Calcd for C18H15NO2S: C, 69.88; H, 4.89; N, 4.53; S, 10.36. Found: C, 70.11;
H, 4.55; N, 4.59; S, 10.56.
4.3.5. 9-Benzyl-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-thione (7e)
Yellow solid; yield 72%; mp 164.4◦C; IR (film) ν (cm−1): 2921, 2852, 1624, 1591, 1554,
=
− −
1500, 1437, 1400, 1290 (C S), 1250 (C O C asymmetric stretching), 1153, 1132, 1093, 1058
1
− −
(C O C symmetric stretching), 1023, 999, 867, 813; H NMR (400 MHz, CDCl3): δ 2.34 (s, 3H,
−
−
−
−
−
−
CH3), 3.90 (s, 2H, CH2), 4.32 (s, 2H, N CH2 Ar), 4.92 (s, 2H, N CH2 O), 6.87 (d, 1H,
J = 8.8 Hz,ArH), 7.06 (s, 1H, CH), 7.28–7.36 (m, 5H,ArH), 7.45 (d, 1H, J = 8.8 Hz,ArH); 13C
=
− −
−
− −
NMR (100 MHz, CDCl3): δ 18.0 (CH3), 45.6 (N C Ar), 55.9 (C Ph), 82.5 (N C O), 107.8,
114.8, 115.0, 123.3, 126.5, 127.7, 128.6, 128.9, 137.3, 145.3, 154.8, 158.0, 196.9; DART–MS:
m/z 324.06 (M + H)+; Anal. Calcd for C19H17NO2S: C, 70.56; H, 5.30; N, 4.33; S, 9.91. Found:
C, 70.15; H, 4.98; N, 4.54; S, 9.66.
4.3.6. 4-Methyl-9-(2-methylphenyl)-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-
thione (7f)
Viscous liquid; yield 70%; IR (film) ν (cm−1): 2924, 2853, 1625, 1592, 1556, 1494, 1436, 1401,
−
=
− −
1383 (C N), 1290 (C S), 1251 (C O C asymmetric stretching), 1222, 1204, 1188, 1098, 1087,
1
− −
− −
1046 (C O C symmetric stretching), 1009, 985, 961 (N C O stretching), 889, 814; H NMR
−
−
−
−
(400 MHz, CDCl3): δ 2.32 (s, 3H, CH3), 2.43 (s, 3H, CH3), 4.68 (s, 2H, N CH2 Ar), 5.29
−
−
=
(s, 2H, N CH2 O), 6.90 (d, 1H, J = 8.8 Hz, ArH), 7.02–7.04 (m, 2H, ArH), 7.05 (s, 1H, CH),
7.19–7.22 (m, 2H, ArH), 7.45 (d, 1H, J = 8.79 Hz, ArH); 13C NMR (100 MHz, CDCl3): δ 18.0
− −
− −
(CH3), 18.4 (CH3), 46.3 (N C Ar), 81.9 (N C O), 108.4, 115.0, 115.1, 121.5, 123.3, 124.8,
126.5, 126.6, 131.3, 132.8, 145.3, 147.7, 153.9, 157.9, 196.8; DART–MS: m/z 324.06 (M + H)+.
4.3.7. 4-Methyl-9-(3-methylphenyl)-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-
thione (7g)
Yellow solid; yield 71%; mp 160.9◦C; IR (film) ν (cm−1): 2924, 2855, 1625, 1592, 1557, 1494,
−
=
− −
1438, 1400, 1381 (C N), 1292 (C S), 1252 (C O C asymmetric stretching), 1178, 1154,
− − − −
1107, 1090, 1051 (C O C symmetric stretching), 1014, 1000, 962 (N C O stretching), 922,
1
−
−
901, 859, 814; H NMR (400 MHz, CDCl3): δ 2.31 (s, 3H, CH3), 2.32 (s, 3H, CH3), 4.90
−
−
−
−
(s, 2H, N CH2 Ar), 5.43 (s, 2H, N CH2 O), 6.79 (d, 1H, J = 7.3 Hz, ArH), 6.84 (d, 1H,
=
J = 8.8 Hz, ArH), 6.95–6.98 (m, 2H, ArH), 7.06 (s, 1H, CH), 7.13–7.17 (m, 1H, ArH), 7.40 (d,
1H, J = 8.8 Hz, ArH); 13C NMR (100 MHz, CDCl3): δ 18.0 (CH3), 21.6 (CH3), 46.4 (N C Ar),
− −
− −
79.9 (N C O), 108.5, 114.9, 115.1, 115.5, 119.3, 122.9, 123.4, 126.4, 129.1, 139.2, 145.3,
147.7, 153.8, 157.9, 196.7; DART–MS: m/z 324.06 (M + H)+; Anal. Calcd for C19H17NO2S: C,
70.56; H, 5.30; N, 4.33; S, 9.91. Found: C, 70.45; H, 5.22; N, 4.65; S, 9.69.
4.3.8. 6-Chloro-9-(4-fluorophenyl)-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]-
oxazine-2(8H)-thione (8a)
Yellow solid; yield 78%; mp 147.8◦C; IR (film) ν (cm−1): 2925, 2855, 1618, 1590, 1553, 1508,
−
=
− −
1489, 1438, 1402, 1376 (C N), 1288 (C S), 1263 (C O C asymmetric stretching), 1233,
− − − −
1158, 1094, 1060 (C O C symmetric stretching), 985, 965 (N C O stretching), 905, 881,

Journal of Sulfur Chemistry 39
1
−
−
−
829; H NMR (400 MHz, CDCl3): δ 2.30 (s, 3H, CH3), 4.86 (s, 2H, N CH2 Ar), 5.50 (s, 2H,
−
−
=
N CH2 O), 6.96 (d, 1H, J = 8.8 Hz, ArH), 6.98 (d, 1H, J = 8.0 Hz, ArH), 7.07 (s, 1H, CH),
7.12 (dd, 2H, J1 = 8.8, J2 = 4.4 Hz,ArH), 7.52 (s, 1H,ArH); 13C NMR (100 MHz, CDCl3): δ 17.9
− −
− −
(CH3), 47.0 (N C Ar), 81.6 (N C O), 109.6, 115.0, 115.8, 116.1, 119.5, 120.8, 120.9. 123.4,
127.4, 143.8, 151.9, 153.0, 196.2; DART–MS: m/z 362.09 (M + H)+ and 364.09 [(M + H)+ + 2]
peaks in the ratio of 3:1; Anal. Calcd for C18H13ClFNO2S: C, 59.75; H, 3.62; N, 3.87; S, 8.86.
Found: C, 59.76; H, 3.65; N, 3.84; S, 8.80.
4.3.9. 6-Chloro-9-(2-chlorophenyl)-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]-
oxazine-2(8H)-thione (8b)
Yellow solid; yield 72%; mp 173.9◦C; IR (film) ν (cm−1): 2925, 2855, 1621, 1593, 1557, 1480,
−
=
− −
1442, 1404, 1377 (C N), 1289 (C S), 1261 (C O C asymmetric stretching), 1214, 1169,
− − − −
1095, 1066, 1049 (C O C symmetric stretching), 987, 963 (N C O stretching), 912, 884,
1
−
−
−
822; H NMR (400 MHz, CDCl3): δ 2.31 (s, 3H, CH3), 4.83 (s, 2H, N CH2 Ar), 5.48 (s, 2H,
−
−
=
N CH2 O), 7.04–7.08 (m, 1H, ArH), 7.07 (s, 1H, CH), 7.13–7.17 (m, 1H, ArH), 7.24 (dd, 1H,
J1 = 8.0, J2 = 1.5 Hz, ArH), 7.41 (dd, 1H, J1 = 7.3, J2 = 1.5 Hz, ArH), 7.54 (s, 1H, ArH); 13
C
− −
− −
NMR (100 MHz, CDCl3): δ 17.9 (CH3), 46.3 (N C Ar), 82.2 (N C O), 109.8, 115.2, 119.5,
122.7, 123.3, 125.8, 127.5, 127.7, 129.1, 130.8, 143.6, 145.1, 152.02, 152.9, 196.2; DART–MS:
m/z 377.97 (M+H)+, 379.97 [(M+H)+ + 2] and 381.97 [(M+H)+ + 4] peaks in the ratio of 9:6:1;
Anal. Calcd for C18H13Cl2NO2S: C, 57.15; H, 3.46; N, 3.70; S, 8.48. Found: C, 57.23; H, 3.63;
N, 3.74; S, 8.70.
4.3.10. 6-Chloro-9-(3-chlorophenyl)-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]-
oxazine-2(8H)-thione (8c)
Viscous liquid; yield 72%; IR (film) ν (cm−1): 2925, 2855, 1618, 1595, 1483, 1401, 1289
=
− −
− −
(C S), 1261 (C O C asymmetric stretching), 1095, 1045 (C O C symmetric stretching),
1
− −
−
965 (N C O stretch), 889; H NMR (400 MHz, CDCl3): δ 2.30 (s, 3H, CH3), 4.91 (s, 2H,
−
−
−
−
N CH2 Ar), 5.52 (s, 2H, N CH2 O), 6.96 (d, 1H, J = 8.8 Hz, ArH), 7.04 (dd, 1H, J1 = 8.0,
=
J2 = 2.2 Hz, ArH), 7.08 (s, 1H, CH), 7.15–7.16 (m, 1H, ArH), 7.21 (t, 1H, J = 8.0 Hz, ArH),
7.52 (s, 1H,ArH); 13C NMR (100 MHz, CDCl3): δ 17.9 (CH3), 46.4 (N C Ar), 80.2 (N C O),
109.6, 115.1, 116.6, 118.7, 119.6, 122.4, 123.4, 127.4, 128.0, 130.4, 143.6, 148.5, 151.9, 152.9,
196.1; DART–MS: m/z 377.97 (M + H)+, 379.97 [(M + H)+ + 2] and 381.97 [(M + H)+ + 4]
peaks in the ratio of 9:6:1; Anal. Calcd for C18H13Cl2NO2S: C, 57.15; H, 3.46; N, 3.70; S, 8.48.
Found: C, 57.23; H, 3.63; N, 3.74; S, 8.70.
− −
− −
4.3.11. 6-Chloro-4-methyl-9-phenyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-
thione (8d)
Yellow solid; yield 77%; mp 144.2◦C; IR (film) ν (cm−1): 2924, 2344, 1618, 1594, 1552, 1497,
=
− −
− −
1438, 1402, 1289 (C S), 1263 (C O C asymmetric stretching), 1095, 1060 (C O C sym-
1
− −
metric stretching), 964 (N C O stretching), 881; H NMR (400 MHz, CDCl3): δ 2.30 (s, 3H,
−
−
−
−
−
CH3), 4.93 (s, 2H, N CH2 Ar), 5.56 (s, 2H, N CH2 O), 6.99 (t, 1H, J = 7.3 Hz, ArH), 7.07
(s, 1H, CH), 7.17 (d, 2H, J = 8.0 Hz, ArH), 7.27–7.31 (m, 2H, ArH), 7.50 (s, 1H, ArH); 13C
=
− −
− −
NMR (100 MHz, CDCl3): δ 17.9 (CH3), 46.5 (N C Ar), 80.9 (N C O), 109.9, 115.0, 118.8,
119.5, 122.6, 123.3, 127.3, 129.4, 143.8, 147.3, 152.0, 153.2, 196.3; DART–MS: m/z 344.01
(M + H)+ and 346.01 [(M + H)+ + 2] peaks in the ratio of 3:1; Anal. Calcd for C18H14ClNO2S:
C, 62.88; H, 4.10; N, 4.07; S, 9.33. Found: C, 62.56; H, 3.98; N, 4.32; S, 9.43.

40 B.P. Mathew et al.
4.3.12. 9-Benzyl-6-chloro-4-methyl-9,10-dihydrochromeno[8,7-e][1,3]oxazine-2(8H)-
thione (8e)
Yellow solid; yield 76%; mp 164.4◦C; IR (film) ν (cm−1): 2925, 2854, 1590, 1560, 1401, 1287
=
− −
− −
(C S), 1260 (C O C asymmetric stretching), 1235, 1194, 1137, 1097, 1070 (C O C sym-
1
−
metric stretching), 1024, 862; H NMR (400 MHz, CDCl3): δ 2.32 (s, 3H, CH3), 3.91 (s, 2H,
−
−
−
−
−
=
CH2), 4.32 (s, 2H, N CH2 Ar), 5.06 (s, 2H, N CH2 O), 7.07 (s, 1H, CH), 7.29–7.36 (m,
5H, ArH), 7.54 (s, 1H, ArH); 13C NMR (100 MHz, CDCl3): δ 17.9 (CH3), 45.3 (N C Ar), 55.9
− −
−
− −
(C Ph), 83.8 (N C O), 109.1, 114.9, 119.2, 123.0, 127.2, 127.7, 128.6, 128.7, 136.9, 143.8,
152.9, 153.2, 196.4; DART–MS: m/z 358.02 (M + H)+ and 360.02 [(M + H)+ + 2] peaks in the
ratio of 3:1; Anal. Calcd for C19H16ClNO2S: C, 63.77; H, 4.51; N, 3.91; S, 8.96. Found: C, 63.63;
H, 4.63; N, 3.74; S, 8.70.
4.3.13. 6-Chloro-4-methyl-9-(2-methylphenyl)-9,10-dihydrochromeno[8,7-e][1,3]oxazine-
2(8H)-thione (8f)
Yellow solid; yield 74%; mp 143.8◦C; IR (film) ν (cm−1): 2925, 2854, 1591, 1468, 1288 (C S),
=
1
− −
− −
1264 (C O C asymmetric stretching), 1097, 1067 (C O C symmetric stretching), 876; H
−
−
−
−
NMR (400 MHz, CDCl3): δ 2.31 (s, 3H, CH3), 2.43 (s, 3H, CH3), 4.68 (s, 2H, N CH2 Ar),
−
−
=
5.42 (s, 2H, N CH2 O), 7.05 (s, 1H, CH), 7.06–7.08 (m, 2H, ArH), 7.18 (dd, 1H, J1 = 7.3,
J2 = 2.2 Hz, ArH), 7.22 (dd, 1H, J1 = 6.6 Hz, J2 = 2.2 Hz, ArH), 7.54 (s, 1H, ArH); DART–MS:
m/z 358.13 (M + H)+ and 360.13 [(M + H)+ + 2] peaks in the ratio of 3:1; Anal. Calcd for
C19H16ClNO2S: C, 63.77; H, 4.51; N, 3.91; S, 8.96. Found: C, 63.43; H, 4.43; N, 3.74; S, 8.72.
4.3.14. 6-Chloro-4-methyl-9-(3-methylphenyl)-9,10-dihydrochromeno[8,7-e][1,3]oxazine-
2(8H)-thione (8g)
Yellow solid; yield 73%; mp 139.1◦C; IR (film) ν (cm−1): 2925, 2854, 1591, 1463, 1376 (C N),
−
=
− −
− −
1288 (C S), 1262 (C O C asymmetric stretching), 1180, 1095, 1069 (C O C symmetric
1
−
−
stretching), 877; H NMR (400 MHz, CDCl3): δ 2.30 (s, 3H, CH3), 2.33 (s, 3H, CH3), 4.92
−
−
−
−
(s, 2H, N CH2 Ar), 5.55 (s, 2H, N CH2 O), 6.81 (d, 1H, J = 7.3 Hz, ArH), 6.97 (d, 2H,
J = 8.8 Hz, ArH), 7.07 (s, 1H, CH), 7.15–7.18 (m, 1H, ArH), 7.50 (s, 1H, ArH); 13C NMR
=
− −
− −
(100 MHz, CDCl3): δ 17.9 (CH3), 21.6 (CH3), 46.6 (N C Ar), 80.9 (N C O), 110.0, 114.9,
115.7, 119.6, 123.3, 123.4, 127.4, 128.1, 129.2, 139.4, 143.8, 147.4, 152.1, 153.2, 196.4; DART–
MS: m/z 358.1 (M + H)+ and 360.1 [(M + H)+ + 2] peaks in the ratio of 3:1; Anal. Calcd for
C19H16ClNO2S: C, 63.77; H, 4.51; N, 3.91; S, 8.96. Found: C, 63.46; H, 4.43; N, 3.76; S, 8.62.
4.4. In vitro anti-bacterial assay
All the newly synthesized compounds were screened for their in vitro anti-bacterial efficacy
against five bacterial strains, viz. Sa, Sm, Ec, Pa and Kp using disc diffusion method (25, 26) at
250 μg/ml concentration. Stock cultures were maintained at 4◦C on slopes of nutrient agar (NA).
Active cultures for experiments were prepared by transferring a loop full of cells from the stock
cultures to test tubes of nutrient broth that were incubated without agitation for 24 h at 37◦C.
The cultures were diluted with fresh nutrient broth to achieve optical densities corresponding to
2 × 106 colony forming units (CFUs/mL) for bacteria. The in vitro antimicrobial activity was
screened by using NA obtained from Himedia (Delhi, India). The NA plates were prepared by
pouring 25 ml of molten media into sterile petriplates. The plates were allowed to solidify for
3–4 h. On the surface of the media, microbial suspension was spread with the help of sterilized

Journal of Sulfur Chemistry 41
L-shaped loop. All the synthesized compounds (250 μg/ml) were loaded on 6-mm sterile disc.
The loaded disc was placed on the surface of medium and the compound was allowed to diffuse
for 5 min and the plates were then kept for incubation at 37◦C for 24 h. DMSO was used as a
solvent for all the compounds and also as a control. After 24 h, inhibition zones formed around
the disc were measured with transparent ruler in millimeter. These studies were performed in
triplicate. Streptomycin was used as a standard drug at 10 μg/ml concentration.
Acknowledgements
We thank the University of Delhi for financial support. B.P.M. is thankful to CSIR, New Delhi, India, for awarding the
Senior Research Fellowship (SRF).
References
(1) Vengurlekar, S.; Sharma, R.; Trivedi, P. Lett. Drug Des. Discov. 2012, 9, 549–555.
(2) Petrlíková, E.;Waisser, K.; Doležal, R.; Holý, P.; Gregor, J.; Kunes, J.; Kaustova, J. Chem. Papers 2011, 65, 352–366.
(3) Jayanthi, P.; Lalitha, P. Pharma Sci. Monitor 2011, 2, 210–215.
(4) Goel, A.; Prasad, A.K.; Parmar, V.S.; Ghosh, B.; Saini, N. Chemico-Biological Interact. 2009, 179, 363–374.
(5) Park, W.S.; Jeong, J.C.; Kim, J.C.; Jang, B.S. Repub. Korean Kongkae Taeho Kongbo KR 2000 33231, 2000; Chem.
Abstr. 2002, 136, 200101x.
(6) Shalaby, N.M.M.; Elgamal, M.H.A. Egypt. J. Chem. 2001, 44, 201–214.
(7) Jie, W.; Zhen, Y.; Reza, F.; Qiang, Z. US Patent 20040180950, 2004; Chem. Abstr. 2004, 139, 395816b.
(8) Kumar, S.; Singh, B.K.; Kalra, N.; Kumar, V.; Kumar, A.; Prasad, A.K.; Raj, H.G.; Parmar, V.S.; Ghosh, B. Bioorg.
Med. Chem. 2005, 13, 1605–1613.
(9) Clauson-Kaas, N.; Denss, R.; Ostermayer, F.; Renk, E. (J.R. Geigy A.G.); Swiss Appl. 1,339,175 (Cl. A 61k, C01d);
Chem. Abstr. 1964, 60, 4158d.
(10) Clauson-Kaas, N.; Denss, R.; Ostermayer, F.; Renk, E. (J.R. Geigy A.G.); Swiss Appl.; Chem. Abstr. 1967, 66,
115718v.
(11) Chylinska, J.B.; Janowiec, M.; Urbanski, T. Br. J. Pharmac. 1971, 43, 649–657.
(12) Sapkal, S.B.; Shelke, K.F.; Shingate, B.B.; Shingare, M.S. J. Korean Chem. Soc. 2010, 54, 437–442.
(13) Mathew, B.P.; Kumar, A.; Sharma, S.; Shukla, P.K.; Nath, M. Eur. J. Med. Chem. 2010, 45, 1502–1507.
(14) Waisser, K.; Petrlíková, E.; Peøina, M.; Klimešová,V.; Kuneš, J.; Palát Jr., K.; Kaustová, J.; Dahse, H.-M., Möllmann,
U. Eur. J. Med. Chem. 2010, 45, 2719–2725.
(15) Negoro, N.; Sasaki, S.; Mikami, S.; Ito, M.; Suzuki, M.; Tsujihata, Y.; Ito, R.; Harada, A.; Takeuchi, K.; Suzuki, N.;
Miyazaki, J.; Santou, T.; Odani, T.; Kanzaki, N.; Funami, M.; Tanaka, T.; Kogame, A.; Matsunaga, S.; Yasuma, T.;
Momose, Y. ACS Med. Chem. Lett. 2010, 1, 290–294.
(16) Raju, B.C.; Tiwari, A.K.; Kumar, J.A.; Ali, A.Z.; Agawane, S.B.; Saidachary, G.; Madhusudana, K. Bioorg. Med.
Chem. 2010, 18, 358–365.
(17) Madhav, J.V.; Kuarm, B.S.; Someshwar, P.; Rajitha, B.; Reddy, Y.R.T.; Crooks, P.A. J. Chem. Res. 2008, 2008,
232–234.
(18) Reddy,Y.T.; Sonar, V.N.; Crooks, P.A.; Dasari, P.K.; Reddy, P.N.; Rajitha, B. Synth. Commun. 2008, 38, 2082–2088.
(19) Zagotto, G.; Gia, O.; Baccichetti, F.; Uriarte, E.; Palumbo, M. Photochem. Photobiol. 1993, 58, 486–491.
(20) Chilvers, K.F.; Perry, J.D.; James, A.L.; Reed, R.H. J. Appl. Microbiol. 2001, 91, 1118–1130.
(21) Gadre, J.N.; Audi, A.A.; Karambelkar, N.P. Ind. J. Chem. Section B: Org. Chem. Med. Chem. 1996, 35B(1), 60–62.
(22) Furuta, T.; Takeuchi, H.; Isozaki, M.; Takahashi,Y.; Kanehara, M.; Sugimoto, M.; Watanabe, T.; Noguchi, K.; Dore,
T.M.; Kurahashi, T.; Iwamura M.; Tsien R.Y. ChemBioChem 2004, 5, 1119–1128.
(23) Mathew, B.P.; Nath, M. J. Heterocycl. Chem. 2009, 46, 1003–1006.
(24) Chen, X.; Zhou, Y.; Qu, L.; Liu, X.; Yuan, J.; Zhao, Y. Synthetic Commun. 2010, 40, 1992–1997.
(25) Weinstein, M.J.; Wagman, G.H. Antibiotics, Isolation, Separation and Purification; Elsevier Scientific Publishing
Company: Amsterdam, 1978.
(26) Barry, A.L.; Thornsbery, C. In Manual of Clinical Microbiology; Lennette, E.H, Balows, A., Truant, J.P, Eds., 3rd
ed.; American Society for Microbiology: Washington, DC, 1980; pp 463–474.