Design, synthesis and antitubercular evaluation of novel 2-substituted-3H-benzofuro benzofurans via palladium-copper catalysed Sonagashira coupling reaction

Article abstract of DOI:10.1016/j.bmcl.2013.07.048

A series of novel natural product like 2-substiuted-3H-benzofurobenzofurans designed by molecular hybridization were synthesized in very good yields. The key reactions involved in the synthesis are iodination of 2-dibenzofuranol using iodine monochloride followed by palladium-copper catalyzed Sonagashira-coupling of 1-iododibenzofuran-2-ol with various alkyl and aryl acetylenes. Among the all 10 new compounds screened for in vitro anti-mycobacterial activity against Mycobacterium tuberculosis H37Rv, 2-(4-methoxy-2-methyl phenyl)-3H-benzofuro[3, 2-e]benzofuran (7c) was found to be most active with MIC 3.12 μg/mL and has shown lower cytotoxicity with good therapeutic index.

Full text of DOI:10.1016/j.bmcl.2013.07.048

Accepted Manuscript
Design, Synthesis and antitubercular evaluation of novel 2-substituted-3H-ben‐
zofuro benzofurans via palladium-copper catalysed Sonagashira coupling reac‐
tion
Thirumal Yempala, Jonnalagadda Padma Sridevi, Perumal Yogeeswari,
Darmarajan Sriram, Srinivas Kantevari
PII:
S0960-894X(13)00893-7
DOI:
http://dx.doi.org/10.1016/j.bmcl.2013.07.048
BMCL 20713
Reference:
To appear in:
Bioorganic & Medicinal Chemistry Letters
Received Date:
Revised Date:
Accepted Date:
8 June 2013
19 July 2013
23 July 2013
Please cite this article as: Yempala, T., Sridevi, J.P., Yogeeswari, P., Sriram, D., Kantevari, S., Design, Synthesis
and antitubercular evaluation of novel 2-substituted-3H-benzofuro benzofurans via palladium-copper catalysed
Sonagashira coupling reaction, Bioorganic & Medicinal Chemistry Letters (2013), doi: http://dx.doi.org/10.1016/
j.bmcl.2013.07.048
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Design, synthesis and antitubercular evaluation of
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Thirumal Yempala, Jonnalagadda Padma Sridevi , Perumal Yogeeswari, Darmarajan Sriram and Srinivas
Kantevari*
R
O
O
O
O
O
O
MIC= 5.0 µg/mL
MIC= 32µg/mL
R=(4-OMe,2-Me)Ph; MIC=3.12µg/mL

Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com
Design, Synthesis and antitubercular evaluation of novel 2-substituted-3H-benzofuro
benzofurans via palladium-copper catalysed Sonagashira coupling reaction
Thirumal Yempala,a Jonnalagadda Padma Sridevi,b Perumal Yogeeswari,b Darmarajan Sriram,b and
Srinivas Kantevari*,a
aOrganic Chemistry Division-II (CPC Division), CSIR-Indian Institute of Chemical Technology, Hyderabad-500 007, INDIA.
bMedicinal Chemistry and Antimycobacterial Research Laboratory, Pharmacy Group, Birla Institute of Technology & Science-Pilani, Hyderabad Campus,
Jawahar Nagar, Hyderabad-500078 INDIA
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Accepted
Available online
A series of novel natural product like 2-substiuted-3H-benzofurobenzofurans designed by
molecular hybridization were synthesized in very good yields. The key reactions involved in
the synthesis are iodination of 2-dibenzofuranol using iodine monochloride followed by
palladium-copper catalyzed Sonagashira-coupling of 1-iododibenzofuran-2-ol with various
alkyl and aryl acetylenes. Among the all 10 new compounds screened for in vitro anti-
mycobacterial activity against Mycobacterium tuberculosis H37Rv, 2-(4-methoxy-2-methyl
phenyl)-3H-benzofuro[3,2-e]benzofuran (7c) was found to be most active with MIC 3.12 µg/
mL and has shown lower cytotoxicity with good therapeutic index.
Keywords:
Dibenzofuran
Benzofuran
2009 Elsevier Ltd. All rights reserved.
Sonagashira reaction
Antimycobacterial activity
Molecular hybridization
Tuberculosis (TB) is an ancient, highly contagious, chronic
infectious disease caused mainly by the pathogen
Mycobacterium tuberculosis (MTB).1 It is the world second
leading cause in mortalities due to infections claiming 1.4
million deaths in 2011 which include 350,000 deaths from TB
associated HIV-positive people2 and also dwells hidden in as
many as two billion people. Additionally, the resurgence of its
new virulent forms like multi drug resistant (MDR-TB) and
extremely drug resistant (XDR-TB) has become a major threat
to human kind.3 The worsening situation has prompted the
world health organization (WHO) to declare tuberculosis a
global public health crisis.4 All the above facts also necessitates
the re-engineering and repositioning of old natural and
synthetic bioactives for the development of fast acting new
antitubercular drugs with novel mechanism of action to achieve
effective TB control.5
also manifests a number of important and therapeutically
useful biological activities such as antibacterial, antidepressant,
and antitubercular agents9.
OCH3
CH3
CH3
OH
OH
O
O
1
2
O
OHC
O
O
OH
O
OH
OCH3
HO
O
O
O
3
O
Usnic acid
OH Paucinervin C
O
O
H3CO
OH
OR
OH HO
Cl
MeO
OH
Natural products derived from plants or microbes play a
major role in drug discovery as a source of original bioactive
structures and offer models for rational drug design.6 In
antitubercular agents, natural products 1-3 (figure 1) with
benzofuran architecture exhibited broad range of activity
profile.7 The lichen secondary metabolite Usnic acid derived
from dibenzofuran has shown to display an interesting
antitubercular activity,8a but its weak potency did not permit
its further development as an antimycobacterial drug. More
recently isolated Lucidafuran8b, Ruscodibenzofuran8c and
AB0022A8d also shown promising antimicrobial activity.
Synthetic heterocycles derived from dibenzofuran (figure 1)
OMe
O
O
O
Cl
Cl
R=H;
Lucidafuran
Ruscodibenzofuran
R=CH : Eriobofuran
AB0022A
3
O
O
O
N
O
N
O
O
6
4, Benzofurobenzopyran
5
Figure1: Representative bioactive analogues of natural and
synthetic benzofurans and dibenzofurans

coupling reaction. Screening all new compounds for in vitro
activity against Mycobacterium tuberculosis H37Rv (MTB)
O
resulted
(2-(4-methoxy-2-methylphenyl)-3H-benzofuro[3,2-
R
e]benzofuran (10c) as most active antitubercular agents with
MIC of 3.12 µg/ mL and has shown lower cytotoxicity and
therapeutic index (>10) against HEK-293 cells.
R
O
Molecular
Hybridization
O
As a starting point for the preparation of new analogues,
dibenzo[b,d]furan-2-ol (8), was chosen as substrate and
prepared from 2-acetyldibenzofuran by following the literature
described procedure.11c Longer reaction time (more than 3
days) required to achieve ~80% yield of product 8 prompted us
to revisit the reaction and the conditions adopted. After series
attempts, varying reaction conditions and substrates, 8 was
successfully prepared with excellent yields (~ 92%) in shorter
reaction time (14 h) using dibenzo[b,d]furan-2-carboxaldehyde
as substrate, m-chloroperbenzoic acid as an oxidant under
Bayer-Villager reaction conditions followed by hydrolysis.12
Having dibenzo[b,d]furan-2-ol (8) in hand, it was further
subjected to iodination using Iodine monochloride (ICl) in
acetic acid to give corresponding regioselective 1-iododibenzo
[b,d]furan-2-ol (9) in 85% yield (Scheme-1). Compound 9 was
O
O
New Hybrids
O
Figure 2: Design strategy for new benzofurobenzofuran-hybrids.
CH3
H3CO
H3C
H3CO
7e
CH3
7d
7a
7c
7b
F
F3CO
1
H3C
S
fully characterized by H and 13C NMR, IR and mass (EI-MS)
F
spectral data.13 Further to synthesize the desired analogues
10a-j, compound 9 was reacted with acetylenes 7a-j (figure 3)
using palladium-copper catalyzed Sonagashira coupling
conditions. For example, compound 9 was refluxed with
phenyl acetylene 7a in DMF and triethyl amine in the presence
of catalytic amounts of CuI and PdCl2(PPh3)2 to give 2-
(phenyl)-3H-benzofuro[3,2-e]benzofuran 10a in moderate
yield (48%) along with the formation of phenyl acetylene
dimer. Formation of phenyl acetylene dimer is proved by
isolation and identification with spectral data and is in
agreement with the literature.14 To generalize the reaction, 9
were reacted with series of acetylenes 7a-j in presence of
palladium and copper catalyst under optimized conditions to
give 10a-j in 48-73% yields (Scheme-1). All the reactions
proceeded reasonably well and the products 10a-j was fully
characterized by 1H and 13CNMR, IR and mass (EI-MS)
spectral data.15
7g
7h
7i
7j
7f
Figure 3: Acetylenes 7a-j used in the present study.
Benzofurobenzopyran 4 (MIC 5.0 µg/mL) and derived
analogues 5 and 6 (MIC 2.5 µg/mL) were described to show
good in vitro antitubercular activity but were found to be more
cytotoxic .10 In this context, we envisaged that designing newer
heterocycles through molecular hybridization of bioactive
benzofuran and dibenzofuran derivatives in one molecular
frame (figure 2) could result pharmacologically relevant
natural product like newer analogues as potential candidates
for biological evaluation. Continuing our work11 on synthesis
of dibenzofuran derived antitubercular agents, we herein report
an efficient synthesis and antitubercular evaluation of natural
product like 2-substituted-3H-benzofurobenzofurans 10a-j in
very good yields via palladium-copper catalyzed Sonagashira
I
CHO
a
OH
OH
R
c
b
O
O
O
O
O
1
0
a-
j
9
8
_____________________________
_____________________________________________________
OCH3
CH3
OCH3
CH3
O
O
CH3
O
O
O
O
O
O
O
O
10a
10b
10c
10d
10e
OCF3
F
S
F
O
O
O
O
O
O
O
O
O
O
10g
10f
10h
10i
10j
Reaction conditions:
(a) (i) mCPBA, DCM, 12h (ii) aq.KOH, MeOH,2h (b) ICl, AcOH, HCl, 24 h (c) Acetylenes 7a-j,
PdCl2(PPh3)2, CuI, Et3N, DMF, reflux, 12h.
Scheme-1: Synthesis of 2-substituted-3H-benzofurobenzofurans 10a-j.

Table 1: Physical data of benzofurobenzofurans 10a-j
10c is most active antitubercular agent with therapeutic index
>15 compared to the other evaluated compounds. The results
described here demonstrate the potential utility of
benzofurobenzofurans as antitubercular agents and data
presented will help global efforts for identification of new
chemical entities as potent drug like antimycobacterial agents.
Entry Alkynes Product Yield
(%)a
m.p.
(oC)
LogP/
CLogPb
48
51
73
54
56
63
57
50
60
62
120-121 4.04/6.74
95-96 4.53/7.24
1
2
7a
7b
7c
7d
7e
7f
10a
10b
10c
10d
10e
10f
10g
10h
10i
119-121 4.40/6.86
152-153 4.53/7.24
137-139 3.92/6.66
3
4
5
88-90
syrup
syrup
5.78/8.83
4.80/7.79
5.57/7.77
6
7
7g
7h
7i
8
149-150 4.36/7.03
9
Figure 4: Antitubercular evaluation of new analogues 10a-j
against M. tuberculosis H37RV.
185-186 3.97/6.39
10
7j
10j
a Isolated yields; b Calculated using Chembiodraw Ultra 12.0.
The antimycobacterial activity of the synthesized 2-
substituted-3H-benzofurobenzofurans 10a-j has been screened
against M. tuberculosis H37Rv (MTB) by agar dilution
method16 for the determination of minimum inhibitory
concentration (MIC) in triplicates. The MIC is defined as the
minimum concentration of compound required to completely
inhibit the bacterial growth. The MIC values of 10a-j along
with the standard drugs for comparison are furnished in figure
4. All ten new compounds 10a-j screened have showed in vitro
activity against MTB with MICs ranging from 3.12–50.0 µg/
mL. One compound 10c inhibited MTB with MIC of 3.12
µg/mL which is equipotent to standard drug Ethambutol.When
compared to pyrazinamide (MIC 50.8 µg/mL), five compounds
10a-d & 10f-g were found to be more potent, though all the
compounds were less potent than other anti-TB drug isoniazid
(0.1 µg/mL). LogP/ CLogP data of new derivatives 10a-j
reveal that the synthetic analogues are more liphophilic in
nature compared to standard drugs; Isoniazid, Ethambutol and
pyrazinamide. Structure-activity correlations of the new
compounds 10a-j with respect to their antitubercular activity
reveals that most of the benzofurobenzofuran derivatives
(except 10e) bearing electron donating substituent on phenyl
ring are found to be more potent than those having electron
withdrawing group on phenyl ring and hetero atom group as
substituent.
Figure 5: Percentage inhibition of dibenzofuran analogues
10a-j against HEK-293Tcells at 50µg/ mL concentration.
Acknowledgments
Authors (TY and SK) are thankful to Dr. Lakshmi Kantham,
Director and Dr. V. J. Rao, Head, Crop Protection Chemicals
Division, IICT, Hyderabad for their continuous support.
Financial assistance from MLP0002 and net work projects
ORIGIN projects. TY (SRF) is thankful to CSIR for
fellowship.
References and notes
All the compounds evaluated for anti-TB activity were also
tested for in vitro cytotoxicity17 against HEK-293Tcells at
50µg/ mL concentration by (4,5-dimethylthiazol-2-yl)-2,5-
diphenyl tetrazolium bromide (MTT) assay. Percentage growth
of cells was reported in figure 5. The most promising anti-TB
compound 10c showed 32% inhibition at 50µg/mL with
therapeutic index of >15. Comparatively 10a, 10d and 10g
were also found to be less cytotoxic at 50 µg/ mL with
favourable therapeutic index.
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Mudassar, P. Asian J. Biol. Sci. 2011, 4, 101-108.
In conclusion we have designed and synthesized a novel
series of 2-substituted-3H-benzofurobenzofurans 10a-j through
molecular hybridization of natural and synthetic benzofuran
and dibenzofurans. These new analogues were prepared by
palladium-copper catalyzed Sonogashira-coupling of 1-iodo
dibenzofuran-2-ol with various alkyl and aryl acetylenes 7a-j
in moderate to good yields and were fully characterized by
spectral data. Screening all these new derivatives against M.
tuberculosis H37Rv (MTB) and cytotoxicity revealed that the

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126.1, 124.5, 120.7, 120.6, 113.2, 110.6, 110.6, 74.8. IR (KBr) 3262,
1852, 1620, 1571, 1450, 1418, 1385, 1210, 1051, 808, 747cm-1. MS-EI
Calcd for C12H7O2 310, found: [M]+=310(100%).
14. Ohtaka, A.; Teratani, T.; Fujii, R.; Ikeshita, K.; Kawashima, T,;
Tatsumi, K.;Shimomura, O.; Nomura, R. J Org Chem., 2011, 76, 4052-
4060.
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15. General procedure for the synthesis of compounds 10a-j: Compound
9 (0.5 mmol), acetylene 7a-j (0.5 mmol), triethyl amine (2 mmol), CuI
(5 mol %), PdCl2(PPh3)2(5 mol %) were dissolved in dry DMF (5 mL)
and refluxed for overnight under N2 atmosphere. After completion the
reaction mixture was cooled to RT, diluted with water and extracted
with ethyl acetate (2x15 mL), the combined organic layers were washed
with brine solution and dried over anhydrous Na2SO4. Evapouration of
solvent followed by column chromatography over silica gel gave
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A.G.; Siqueira, V.L.; Lonardoni, M.V.; Cardoso, R.F.; Cortez, D.A.
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products 10a-j.
2-(phenyl)-3H-benzofuro[3,2-e]benzofuran 10a: 1H NMR (500 MHz,
CDCl3) δ 8.06(d, J=7.0Hz, 1H), 7.94(d, J=8.0Hz, 2H), 7.59(dd, J=3.0 &
8.0Hz, 2H), 7.50-7.42(m, 5H), 7.40-7.32(m, 2H).13CNMR (75MHz,
CDCl3) δ 157.2, 156.4, 152.8, 151.4, 130.3, 128.8, 127.0, 126.4, 124.9,
124.3, 122.6, 121.1, 120.5, 115.9, 111.7, 109.8, 107.6, 101.9, 99.9. IR
(KBr) 2923, 1615, 1419, 1383, 1198, 1017, 790, 746cm-1. MS-EI Calcd
for C20H12O2 284, found: [M]+=284(100%).2-(3-methylphenyl)-3H-
benzofuro[3,2-]benzofuran 10b: 1H NMR (300 MHz, CDCl3)
δ
8. (a) Honda, N.K.; Pavan, F.R.; Coelho, R.G.; de Andrade Leite, S.R.;
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Leite, C.Q. Phytomedicine, 2010 , 17, 328-332. (b) Chen, J.J.; Luo,
Y.T.; Liao, C.H.; Chen, I.S.; Liaw, C.C. Chem Biodivers. 2009, 6, 774-
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05.022.
8.05(d, J=7.5Hz, 1H), 7.81(d, J=8.1Hz, 2H), 7.63-7.52(m, 2H), 7.49-
7.33(m, 4H), 7.29-7.22(m, 2H), 2.43(s, 3H). 13C NMR (75 MHz,
CDCl3) δ 157.6, 156.5, 152.8, 151.3, 138.6, 129.5, 127.7, 126.8, 126.3,
125.0, 124.5, 122.6, 121.1, 111.7, 109.8, 107.4, 99.3, 96.2, 21.5. IR
(KBr) 2920, 2852, 1613, 1501, 1422, 1381, 1198, 1001, 787, 743cm-1.
MS-EI Calcd for C21H14O2 298, found: :[M]+=298(100%). 2-(4-
methoxy-2-methylphenyl)-3H-benzofuro[3,2-e]benzofuran 10c: 1H
NMR (300 MHz, CDCl3) δ 8.05(d, J=7.5Hz, 1H), 7.82(d, J=8.3Hz, 1H),
7.58(t, J=8.3Hz, 2H), 7.50-7.31(m, 3H), 7.18(s, 1H), 6.83(d, J=7.5Hz,
2H), 3.86(s, 3H), 2.65(s, 3H). 13C NMR (75 MHz, CDCl3) δ 159.7,
157.4, 156.4, 152.7, 150.8, 137.5, 134.2, 129.7, 126.3, 124.4, 122.5,
121.1, 116.6, 115.7, 115.1, 111.6, 111.5, 109.6, 107.0, 102.3, 55.2, 22.1.
IR (KBr) 2923, 2853, 1618, 1497, 1435, 1274, 1142, 967, 853, 743cm-1.
MS-EI Calcd for C22H16O3 328, found: [M]+= 328 (100%), 313(35%).
2-(4-methylphenyl)-3H-benzofuro[3,2-e]benzo furan 10d: 1H NMR
(300 MHz, CDCl3) δ 7.97(s, 1H), 7.94(d, J=7.5Hz, 1H), 7.72-7.62(m,
2H), 7.56-7.26(m, 5H), 7.16(d, J=7.5Hz, 1H), 7.08(s, 1H), 2.46(s, 3H).
13C NMR (75 MHz, CDCl3) δ 157.5, 156.9, 151.9, 151.5, 138.4, 132.9,
130.0, 129.5, 128.7, 128.2, 126.9, 125.5, 122.4, 122.1, 120.3, 111.4,
101.9, 101.6, 21.5. IR (KBr) 2922, 2852. 1595. 1450. 1364. 1155. 929.
853. 785cm-1. MS-EI Calcd for C21H14O2 298, found:[M]+=298(100%).
2-(4-methoxyphenyl)-3H-benzofuro[3,2-e]benzofuran 10e: 1H NMR
(300 MHz, CDCl3) δ 8.04(d, J=6.7Hz, 1H), 7.85(d, J=8.3Hz, 2H),
7.56(t, J=7.5 & 8.3Hz, 2H), 7.50-7.34(m, 3H), 7.29(s, 1H), 6.97(d,
J=9.0Hz, 2H), 3.87(s, 3H). 13C NMR (75 MHz, CDCl3) δ 160.1, 157.5,
156.4, 151.2, 126.4, 126.3, 124.3, 123.1, 122.6, 121.1, 114.2, 111.6,
109.7, 107.0, 98.4, 96.1, 55.3. IR (KBr) 2925, 1609, 1503, 1461, 1248,
1178, 1019, 833, 795cm-1. MS-EI Calcd for C21H14O3 314,
9. Izumizono, Y.; Arevalo, S.; Koseki, Y.; Kuroki, M.; Aoki, S. Eur J Med
Chem. 2011, 46, 1849-1856.
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Jackson, M.; Brosch, R.; Janin, Y. L.; Daffe, M.; Tillequin, F.; Michel,
S. Eur. J. Med. Chem. 2010, 45, 5833-5847. (c) Prado, S.; Janin, Y. L.;
Saint- Joanis, B.; Brodin, P.; Michel, S.; Koch, M.; Cole, S. T.;
Tillequin, F.; Bost, P. E. Bioorg. Med. Chem. 2007, 15, 2177-2186.
11. (a) Yempala, T.; Sriram,D.; Yogeeswari, P.; Kantevari, S.; Bioorg.
Med. Chem. Lett., 2012, 22, 7426-7430. (b) Patpi, S. R.; Pulipati, L.;
Yogeeswari, P.; Sriram, D.; Jain, N.; Sridhar, B.; Murthy, R.; Devi
T.A.; Kalivendi, S.V.; Kantevari, S.; J. Med. Chem., 2012, 55, 3911.
(b) Kantevari, S.; Yempala,T.; Surineni, G.; Sridhar, B.; Yogeeswari,
P.; Sriram. D.; Eur. J. Med. Chem., 2011, 46, 4827. (c) Kantevari, S.;
Yempala, T.; Yogeeswari, P.; Sriram, D.; Sridhar.B.; Bioorg.Med.
Chem. Lett., 2011, 21, 4316. (d) Kantevari, S. Patpi, S.R.; Sridhar, B.;
Yogeeswari,P.; Sriram, D.; Bioorg. Med. Chem. Lett. 2011, 21, 1214.
(e) Kantevari, S.; Patpi, S.R.; Addla, D.; Putapatri, S.R.; Sridhar, B.;
Yogeeswari, P.; Sriram, D. ACS Comb. Sci. 2011, 13, 427.
found:[M]+=314(100%)
benzofuro[3,2-e]benzofuran 10f: 1H NMR (400 MHz, CDCl3)
&
299 (52%). 2-(4-pentylphenyl)-3H-
12. Preparation of dibenzo[b,d]furan-2-ol
8 : To a solution of
dibenzo[b,d]furan-2-carbaldehyde 7 (2.5 g, 12.7 mmol) in CH2Cl2 (35
mL) at 0oC, m-CPBA (2.63 g, 15.3 mmol) was added slowly at 0oC .
After stirring at room temperature for 12 h, the reaction mixture was
quenched with ferrous sulphate, washed with water (20 mL) and
saturated sodium bicarbonate (20 mL), the organic layer was separated,
dried over anhydrous Na2SO4 and concentrated under reduced pressure.
The crude residue was dissolved in methanol (35 mL), 7 mL of
10%KOH was added and stirred at RT for 2 h. The reaction mixture was
quenched with 2N HCl (15 mL) and solvent was removed under
vacuum. The residue thus obtained was treated with water, extracted
with chloroform (2 x 30 mL), combined organic layers was dried over
anhydrous Na2SO4 and concentrated under vacuum. Purification of
crude residue over silica gel column chromatography yielded
dibenzofuran-2-ol 8 (2.1 g, 92%) as pale yellow solid. The product
obtained was fully characterized by spectra data and compared with
authentic sample. m.p 126 °C ( Lit11 m.p 125-128°C).
δ
8.05(d, J=6.8Hz, 1H), 7.82(d, J=8.5Hz, 2H), 7.62-7.55(m, 2H), 7.47-
7.34(m, 4H), 7.30-7.23(m, 2H), 2.65(t, J=7.6Hz, 2H), 1.67(q, J=7.6Hz,
2H), 1.41-1.32(m, 4H), 0.93(t, J=6.8Hz, 3H).
13C NMR (75
MHz,CDCl3) δ 157.6, 156.3, 152.7, 151.2, 143.8, 128.8, 127.7, 126.3,
124.9, 124.3, 122.6, 121.0, 120.2, 115.7, 111.6, 109.7, 107.3, 101.8,
100.9, 99.2, 35.7, 31.4, 31.0, 22.5, 14.0. IR (KBr) 2922, 2848, 1629,
1414, 1339, 1199, 1153, 922, 856, 789cm-1. MS-EI Calcd for C25H22O2
354, found:[M]+=354(78%)
&
297(100%)
.
2-(hexyl)-3H-
benzofuro[3,2-e]benzofuran 10g: 1H NMR (300 MHz, CDCl3) δ
7.91(d, J=7.5Hz, 1H), 7.86(s, 1H), 7.58-7.46(m, 2H), 7.38(t, J=8.3Hz,
1H), 7.32-7.24(m, 1H), 6.43(s, 1H), 2.80(t, J=7.5Hz, 2H), 1.78(q,
J=7.5Hz, 2H), 1.47-1.29(m, 6H), 0.919(t, J=6.7Hz, 3H). 13C NMR (75
MHz, CDCl3) δ 161.4, 156.8, 152.7, 151.3, 126.5, 124.9, 122.3, 121.1,
120.1, 111.4, 102.3, 101.6, 101.4, 96.1, 31.6, 28.9, 28.7, 27.6, 22.6,
14.1. IR (neat) 2926, 2855, 1723, 1459, 1215, 1153, 1107, 863, 747cm-1.
MS-EI Calcd for C20H20O2 292, found:[M]+=292(60%), 221(100%). 2-
(4-trifluoromethoxyphenyl)-3H-benzofuro[3,2-e]benzofuran 10h: 1H
NMR (500 MHz, CDCl3) δ 8.05(d, J=7.6Hz, 1H), 7.96-7.90(m, 2H),
7.63-7.56(m, 2H), 7.50-7.44(m, 2H), 7.40-7.34(m, 2H), 7.20-7.14(m,
2H). 13C NMR (75 MHz, CDCl3) δ 158.6, 156.4, 133.0, 132.6, 132.0,
127.6, 126.6, 126.3, 124.3, 122.7, 122.5, 121.3, 121.0, 120.9, 111.6,
111.0, 109.3, 108.1, 100.6. IR(neat) 2922, 2852, 1503, 1419, 1260,
1212, 1165, 1017, 795, 748cm-1. MS-EI Calcd for C21H11FO3 368,
13. Preparation of 1-iododibenzo[b,d]furan-2-ol 9 : To a solution of
dibenzo[b,d]furan-2-ol 8 (3.1 g, 16.8 mmol) in acetic acid (20 mL) was
added a solution of iodine monochloride (3.23 g, 16.8 mmol) in conc.
HCl (4 mL) and acetic acid (7 mL) and stirred at room temperature for
24 h. After completion of reaction, water (100 mL) was poured into the
reaction mixture, solid precipitate was filtered, washed twice with water
(20 mL) and dried under vacuum. Recrystalization of the solid from
methanol gave pure 1-iodo dibenzo[b,d]furan-2-ol 9 (4.4 g, 85%) as
light brown solid. m.p 128 °C; 1H NMR (300 MHz, CDCl3) δ 8.60(d,
J=7.7Hz, 1H), 7.56-7.31(m, 4H), 7.11 (d, J=8.8Hz, 1H), 5.32(br, s, 1H).
13C NMR (75 MHz, CDCl3+ DMSO ) δ 155.9, 151.8, 149.2, 126.7,
found:[M]+=368(100%).
2-(2,4-difluorophenyl)-3H-benzofuro[3,2-
e]benzofuran 10i: 1H NMR (300 MHz, CDCl3) δ 8.14-8.00(m, 1H),
7.98-7.91(m, 1H), 7.69-7.23(m, 5H), 7.09-6.79(m, 3H). 13C NMR (75

MHz, CDCl3) δ 157.0, 156.5, 153.2, 150.8, 129.2, 128.0, 127.1, 126.6,
124.6, 122.4, 121.3, 120.4, 111.5, 109.7, 108.2, 106.3, 102.4, 101.8,
96.1. IR (neat) 2924, 2853, 1619, 1498, 1275, 1103, 1031, 968, 853,
743cm-1. MS-EI Calcd for C20H10O2F2 320, found: [M]+=320(100%). 2-
dextrose and catalase; Difco) Growth Supplement adjusted to 1 mg/mL
(wet weight) in Tween 80 (0.05%) saline diluted to 10−2 to give a
concentration of ~ 107 cfu/mL. A 5 µL amount of bacterial suspension
was spotted into 7H11 agar tubes containing 10-fold serial dilutions of
drugs per mL. The tubes were incubated at 37 °C, and final readings
were recorded after 28 days. This method is similar to that
recommended by the National Committee for Clinical Laboratory
Standards for the determination of MIC in triplicate.
1
(3-thienyl)-3H-benzofuro[3,2-e]benzofuran 10j: H NMR (300 MHz,
CDCl3) δ 8.08(d, J=6.7Hz, 1H), 8.00-7.96(m, 1H), 7.80(dd, J=2.2 &
11.3Hz, 1H), 7.68-7.33(m, 6H), 6.94(s, 1H). 13C NMR (75 MHz,
CDCl3) δ 156.4, 154.1, 132.1, 126.8, 126.6, 126.4, 125.0, 124.3, 122.6,
121.7, 121.1, 111.6, 111.4, 109.7, 107.4, 101.4, 99.8. IR (KBr) 2922,
2852, 1605, 1449, 1427, 1288, 1224, 1200, 1154, 1042, 942, 853,
781cm-1. MS-EI Calcd for C18H10O2S 290, found:[M]+=290(100%).
16. Antitubercular evaluation assay: Two-fold serial dilutions (50.0,
25.0, 12.5, 6.25, 3.13, 1.56, 0.78 and 0.4 µg/mL) of each test
compounds 10a-j and drugs were prepared and incorporated into
Middlebrook 7H11 agar medium with OADC Growth Supplement.
Inoculum of M. tuberculosis H37Rv ATCC 27294 was prepared from
fresh Middlebrook 7H11 agar slants with OADC (oleic acid, albumin,
17. Evaluation of Cytotoxicity: Some compounds were further examined
for toxicity in a HEK cell line at the concentration of 50 µg/mL. After
72 h of exposure, viability was assessed on the basis of cellular
conversion of MTT into a formazan product using the Promega Cell
Titer 96 non-radioactive cell proliferation assay.
Supplementary Material:
1
Copies of H, 13C NMR and mass spectra of all the new compounds 9
& 10a-j can be obtained free of charge from the internet.

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Design, synthesis and
antitubercular evaluation of
novel 2-substituted-3H-benzofurobenzofurans via
palladium-copper catalysed Sonagashira-coupling
reaction
Thirumal Yempala, Jonnalagadda Padma Sridevi , Perumal Yogeeswari, Darmarajan Sriram and Srinivas
Kantevari*
R
O
O
O
O
O
O
MIC= 5.0 µg/mL
MIC= 32µg/mL
R=(4-OMe,2-Me)Ph; MIC=3.12µg/mL