Synthesis and antitubercular activity evaluation of 4-furano-coumarins and 3-furano-chromones

Article abstract of DOI:10.1080/00397911.2018.1480041

The synthesis of novel 4-furano-coumarins and 3-furano-chromones has been achieved using silver-mediated oxidative C–H/C–H functionalization of 4-ethynylcoumarin/3-ethynylchromone with ethyl acetoacetate in 80%–90% and 82%–90% yields, respectively. The st

Full text of DOI:10.1080/00397911.2018.1480041

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Synthesis and antitubercular activity evaluation of
4-furano-coumarins and 3-furano-chromones
Ankita Singh, Devla Bimal, Rajesh Kumar, Vipin K. Maikhuri, M. Thirumal,
Nihar Nalini Senapati & Ashok K. Prasad
To cite this article: Ankita Singh, Devla Bimal, Rajesh Kumar, Vipin K. Maikhuri, M. Thirumal,
Nihar Nalini Senapati & Ashok K. Prasad (2018) Synthesis and antitubercular activity evaluation of
4
1
-furano-coumarins and 3-furano-chromones, Synthetic Communications, 48:18, 2339-2346, DOI:
0.1080/00397911.2018.1480041
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https://doi.org/10.1080/00397911.2018.1480041
Synthesis and antitubercular activity evaluation of
4-furano-coumarins and 3-furano-chromones
a
a
a
a
a
Ankita Singh , Devla Bimal , Rajesh Kumar , Vipin K. Maikhuri , M. Thirumal ,
b
a
Nihar Nalini Senapati , and Ashok K. Prasad
a
b
Bioorganic Laboratory, Department of Chemistry, University of Delhi, Delhi, India; Department of
Anaesthesia, PGIMER, Dr RML Hospital, New Delhi, India
ABSTRACT
ARTICLE HISTORY
The synthesis of novel 4-furano-coumarins and 3-furano-chromones
has been achieved using silver-mediated oxidative C–H/C–H function-
alization of 4-ethynylcoumarin/3-ethynylchromone with ethyl acetoa-
cetate in 80%–90% and 82%–90% yields, respectively. The structure
of the synthesized compounds was established on the basis of their
spectral data analysis and the structure of one of the 4-furano-
coumarins has been further confirmed on the basis of its X-ray crys-
tallographic study. All eight synthesized 4-furano-coumarins and
Received 23 January 2018
KEYWORDS
3
-Furano-chromone; 4-fura-
no-coumarin; antitubercular
activity; C–H/C–H
functionalization
3
-furano-chromones exhibited moderate antitubercular activity against
sensitive reference strain H37Rv of Mycobacterium tuberculosis.
GRAPHICAL ABSTRACT
Mycobacteriumtuberculosis
MIC = 10-12 µg/mL
Introduction
The furan ring is present in several important natural products, such as furanoflavo-
[
1]
[2]
[3,4]
[5]
noids, furanolactones, furanocoumarins,
furanochromones, etc. These com-
[
6]
[7]
pounds are known for their potential anti-inflammatory, nucleic acid targeting, and
most importantly antimicrobial activities.
[
8–11]
The broad spectrum biologic activity of
coumarin and chromone pharmacophores is due to derivatization of its basic skeleton
[12]
either at benzene ring or at pyrone moiety or at both positions. Tandon et al.
and
CONTACT Ashok K. Prasad
of Delhi, Delhi 110007, India; Nihar Nalini Senapati
ashokenzyme@gmail.com
Department of Chemistry, Bioorganic Laboratory, University
Department of Anaesthesia, PGIMER, New Delhi 110 001, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2018 Taylor & Francis

2
340
A. SINGH ET AL.
Figure
1. Few
naturally
occurring/synthesized
furan-fused
coumarin/chromones
and
2-furanochromone.
[13]
Srivastava et al.
reported that acylated coumarin derivatives possess good antitubercu-
lar activity against H37Rv-sensitive reference strain as well as multidrug-resistant clinical
isolate 591 of Mycobacterium tuberculosis by their action on the cell wall of the M.
tuberculosis. They discovered that acylated coumarins target mycobacterial InhA and
[
14]
DNA gyrase bacterial enzymes. Zhou et al.
reported some benzo-furan derivatives
with antitubercular activity which inhibited mycobacterium protein tyrosine phosphatase
B in signaling pathway. There are several naturally occurring/synthetic furan-fused cou-
marins and chromones, but furano-substituted coumarins and chromones are rarely
[
3–5,15]
known (Fig. 1).
Furthermore, World Health Organizations (WHO) report 2016
indicates that tuberculosis has become a global menace and there has been an abrupt
[
16]
increase in multidrug-resistant and extremely drug-resistant tuberculosis.
Various
efforts are ongoing for the development of novel antitubercular agents against both sen-
[
12–17]
sitive and resistant strains of tuberculosis.
We herein report the synthesis of eight novel 4-(3 -ethoxycarbonyl-2 -methylfuran-5 -
0
0
0
0
0
0
yl)-coumarins and 3-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-chromones, and study of
their antitubercular activity against sensitive reference strain H37Rv of M. tuberculosis.
The preliminary screening results revealed that the synthesized compounds have moder-
ate antitubercular activity.
Results and discussion
4
-Ethynylcoumarin (2a), 7-methoxy-4-ethynylcoumarin (2b), 7-isopropoxy-4-ethynyl-
coumarin (2c), and 7,8-dimethoxy-4-ethynylcoumarin (2d) were synthesized from corre-
sponding 4-hydroxycoumarins 1a–1d in three steps, that is, by triflation of the
4-hydroxyl group in the precursor coumarin using triflic anhydride and triethylamine in
dichloromethane (DCM) followed by Sonogashira reaction on the triflated coumarins
with trimethylsilylacetylene to obtain 4-trimethylsilylethynylcoumarins, which in turn
were desilylated to give 4-ethynylcoumarins 2a–2d in 60%–64% overall yields
[
18]
(Scheme 1).
4-Hydroxycoumarin (1a) was procured from Sigma-Aldrich Chemicals

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Scheme 1. Synthesis of 4-furano-coumarins 3a–3d.
Pvt. Limited, India and other three hydroxycoumarins 1b–1d were synthesized following
[
19]
procedure as reported in the literature.
The synthesis of 4-furanocoumarins, that is, 4-(3 -ethoxycarbonyl-2 -methylfuran-5 -
0
0
0
0
0
0
yl)-coumarin (3a), 4-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-7-methoxycoumarin (3b),
0
0
0
0
4
-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-7-isopropoxycoumarin (3c), and 4-(3 -ethoxy-
0
0
carbonyl-2 -methylfuran-5 -yl)-7,8-dimethoxycoumarin (3d) was achieved by silver-
mediated selective oxidative C–H/C–H functionalization
[
20]
of 4-ethynylcoumarins
2a–2d with ethyl acetoacetate in the presence of silver carbonate and potassium acetate
in dimethylformamide (DMF) in 80%–90% yields (Scheme 1). The synthesis of the tar-
geted furano-coumarins starts with the C–C bond formation between terminal acetylene
3
carbon (sp hybridized) and methylene carbon (sp hybridized) of the 1,3-dicarbonyl
compound, that is, ethyl acetoacetate through silver-mediated oxidative C–H/C–H func-
tionalization reaction. The coupled product finally cyclizes selectively with the keto-car-
bonyl group of ethyl acetoacetate moiety to afford the final product 3a–3d.
The silver-mediated oxidative C–H/C–H functionalization reaction has also been car-
ried out on 3-ethynylchromone using ethyl acetoacetate to synthesize novel 3-furano-
chromones. Thus, the reaction of 3-ethynylchromone (7a), 3-ethynyl-6-methoxychro-
mone (7b), 3-ethynyl-7-methoxychromone (7c), and 3-ethynyl-7-ethoxychromone (7d)
with ethyl acetoacetate in the presence of silver carbonate and potassium acetate in
0
0
0
DMF led to the formation of 3-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)chromone (8a),
0
0
0
0
3
-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-6-methoxychromone (8b), 3-(3 -ethoxycar-
0
0
0
0
bonyl-2 -methylfuran-5 -yl)-7-methoxychromone (8c), and 3-(3 -ethoxycarbonyl-2 -meth-
ylfuran-5 -yl)-7-ethoxychromone (8d) in 82%–90% yields (Scheme 2). The synthesis of
0
3
-ethynylchromones 7a–7d, in turn, was carried out from the corresponding 2-hydrox-
[
21]
yacetophenones in four steps. Thus, the reaction of hydroxyacetophenones 4a–4d
with DMF-dimethylacetal (DMA) in DMF led to the formation of 3-(dimethylamino)-1-
[
22]
(
2-hydroxyphenyl)prop-2-en-1-ones 5a–5d
in 89%–92% yield, which on reaction with
[
23]
iodine in methanol afforded 3-iodochromones 6a–6d in 72%–78% yield.
Sonogashira
reaction on 3-iodochromones 6a–6d with trimethylsilylacetylene followed by desilylation
of the resulted 3-trimethylsilylethynylchromones led to the formation of 3-ethynylchro-
[
24]
mones 7a–7d in 65%–72% overall yields (Scheme 2).

2
342
A. SINGH ET AL.
Scheme 2. Synthesis of 3-furano-chromones 8a–8d.
0
0
0
Figure 2. ORTEP diagram of 4-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-coumarin (3a).
The structures of all the synthesized compounds, that is 2a–2d, 3a–3d, 5a–5d, 6a–6d,
7
(
a–7d, and 8a–8d were unambiguously established on the basis of their spectral data
1
13
H- & C-NMR spectra, IR spectra, and HRMS) analysis. The structure of known
[18]
[18]
[18]
[22]
[23]
[24]
compounds 2a,
2b,
2d,
5a–5d,
6a–6c,
and 7a–7c
were further con-
firmed on the basis of comparison of their physical and spectral data with those
reported in the literature. The single crystal X-ray diffraction analysis has been per-
0
0
formed on one of the 4-furano-coumarins, that is, 4-(3 -ethoxycarbonyl-2 -methylfuran-
5
0
-yl)-coumarin (3a) and its detailed crystallographic data have been deposited in the
Cambridge Crystallographic Data Centre with CCDC no. 1542202 (Fig. 2).
Antitubercular activity
0
0
0
All the synthesized 4-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-coumarins 3a–3d and 3-
(
0
0
0
3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-chromones 8a–8d were evaluated for antituber-
cular activity against sensitive strain H37Rv of M. tuberculosis by measuring their

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Table 1. MIC of compounds 3a–3d and 8a–8d against sensitive reference strain H37Rv of M. tuberculosis.
MIC against
M. tuberculosis
H37Rv (mg/mL)
MIC against
M. tuberculosis
H37Rv (mg/mL)
S. No.
4-Furano-coumarins
S. No.
3-Furano-chromones
1
2
3
4
3a
3b
3c
10
12
10
5
6
7
8
8a
8b
8c
8d
10
12
10
3d
12
10
First-line drugs
Isoniazid
Rifampicin
0.03
0.015
Ethambutol
Streptomycin
0.25–2
2
capability to inhibit the growth of the virulent strain obtained from the Department of
Microbiology, V. P. Chest Institute, University of Delhi, India. The minimum inhibitory
concentrations (MIC) of the test compounds and the first-line drugs, isoniazid, rifampi-
cin, ethambutol, and streptomycin taken as reference standards, were determined using
[12,25]
the micro-plate alamar blue assay (MABA).
The preliminary in vitro screening
results of these compounds against M. tuberculosis-sensitive reference strain H37Rv
showed moderate antitubercular activity with MICs ranging between 10 and 12 mg/mL
(Table 1).
The cell wall of M. tuberculosis plays an important role in pathogenesis and lipopro-
teins (mycolic acid being the most prominent lipid) are major constituent of cell enve-
lope. InhA, the enoyl acyl carrier protein reductase from M. tuberculosis, is one of the
key enzymes involved in the mycobacterial fatty acid elongation cycle. Inhibition of
InhA disrupts the biosynthesis of the mycolic acids which is detrimental to the life of
bacteria. The cell wall attacking nature of coumarins and chromones has been well
established and it is believed that compounds 3a–3d and 8a–8d also act in a
[
12]
same way.
Conclusion
Four each of 4-furano-coumarins and 3-furano-chromones have been successfully syn-
thesized using silver-mediated oxidative C–H/C–H functionalization of sp-hybridized
3
carbon of acetylene moiety in coumarins/chromones and sp -hybridized carbon in ethyl
acetoacetate. The synthesized novel furano-chromones and furano-coumarins were eval-
uated as antitubercular agents against sensitive reference strain H37Rv of M. tubercu-
losis. These compounds were found to be moderate antitubercular agents exhibiting
MICs ranging between 10 and 12 mg/mL.
Experimental
0
0
0
General procedure for the synthesis of 4-(3 -ethoxycarbonyl-2 -methylfuran-5 -
yl)-coumarin (3a)
To a stirred solution of 4-ethynylcoumarin (2a; 0.59 mmol) in DMF (10 mL) were added
ethyl acetoacetate (1.76 mmol), silver carbonate (1.17 mmol), and potassium acetate
ꢀ
(
1.17 mmol) and the reaction was refluxed at 80 C for 10–12 h. On completion, the
compound was extracted from the reaction mixture using ethyl acetate (3 ꢁ 50 mL). The
combined ethyl acetate layer was washed with saturated NaCl solution, dried over

2
344
A. SINGH ET AL.
anhydrous Na SO , and then concentrated to dryness under vacuum. The crude product
2
4
thus obtained was purified over silica gel column using ethyl acetate–petroleum ether as
an eluent to get pure compounds 3a as solids in 90% yield, respectively. It was obtained
ꢀ
ꢂ1
as yellow solid in 90% yield; M.Pt. ¼ 119–121 C. IR (cm , film): 2926, 2335, 1720,
1
1
608, 1242, 1087. H NMR (400 MHz, CDCl ): d 8.03–8.05 (1H, m), 7.52–7.57 (1H, m),
3
7
.31–7.73 (2H, m), 7.28 (1H, s), 6.65 (1H, s), 4.32 (2H, q, J ¼ 7.2 Hz), 2.69 (3H, s), 1.36
1
3
(
1
3H, t, J ¼ 7.6 Hz). C NMR (100.6 MHz, CDCl ): 163, 161, 160, 154, 145, 141, 131,
3
þ
25, 124, 117, 116, 116, 115, 60, 14. HRMS (ESI) m/z calcd. for C H O [M þ H]
17
15 5
2
99.0914, found 299.0928.
0
0
0
General procedure for the synthesis of 3-(3 -ethoxycarbonyl-2 -methylfuran-5 -yl)-
chromone (8a)
To the stirred solution of 3-ethynylchromone (7a; 0.59 mmol) in DMF (10 mL) were
added ethyl acetoacetate (1.76 mmol), silver carbonate (1.98 mmol), and potassium acet-
ꢀ
ate (1.98 mmol) and the reaction was refluxed at 80 C for 9–12 h. On completion, the
compound was extracted from the reaction mixture using ethyl acetate (3 x 50 mL). The
combined ethyl acetate layer was washed with saturated NaCl solution, dried over
anhydrous Na SO , and then concentrated to dryness under vacuum. The crude product
2
4
thus obtained was purified by column chromatography over silica gel using ethyl aceta-
te–petroleum ether as eluent to afford compound 8a as solid in 87% yield, respectively.
ꢀ
The compound 8a was obtained as light yellow solid in 87% yield. M.Pt. ¼ 146–148 C.
ꢂ1
1
IR (cm , film): 2980, 1718, 1668, 1612, 1463, 1274, 1244, 1145, 761. H NMR
(
7
(
1
400 MHz, CDCl ): d 8.82 (1H, s), 8.25 (1H, d, J ¼ 7.6 Hz), 7.66 (1H, t, J ¼ 7.2 Hz),
3
.39 (1H, d, J ¼ 8.4 Hz), 7.32 (1H, t, J ¼ 7.6), 7.24 (1H, s), 4.34 (2H, q, J ¼ 7.2 Hz), 2.67
13
3H, s), 1.38 (3H, t, J ¼ 6.8 Hz). C NMR (100.6 MHz, CDCl ): 176, 166, 157, 155, 147,
3
34, 130, 126, 124, 121, 120, 119, 117, 61, 22, 14. HRMS (ESI) m/z calcd. for C H O
17 15 5
þ
[M þ H] 299.0914, found 299.0927.
Acknowledgments
We are thankful to CIF-USIC University of Delhi for providing crystallographic data and NMR
spectral recording facility.
Funding
We are grateful to the University of Delhi for providing financial support under DU-DST Purse
Grant and under scheme to strengthen research and development. A.S. thanks DRDO for the
Junior Research Fellowship. D.B. and R.K. thank CSIR for Senior Research Fellowship.
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