Antioxidant and antibacterial evaluation of synthetic furomollugin and its diverse analogs

Article abstract of DOI:10.1007/s00044-014-0929-9

Diverse furomollugin (3) and its analogs (11-22) were synthesized in high yields via ceric ammonium nitrate-catalyzed formal [3 + 2] cycloaddition as a key step. The in vitro antioxidant activities of synthesized compounds were determined by analyzing rad

Full text of DOI:10.1007/s00044-014-0929-9

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res
DOI 10.1007/s00044-014-0929-9
ORIGINAL RESEARCH
Antioxidant and antibacterial evaluation of synthetic furomollugin
and its diverse analogs
•
Likai Xia Akber Idhayadhulla Yong Rok Lee
•
•
•
Sung Hong Kim Young-Jung Wee
Received: 5 December 2013 / Accepted: 24 January 2014
ꢀ Springer Science+Business Media New York 2014
Abstract Diverse furomollugin (3) and its analogs (11–
22) were synthesized in high yields via ceric ammonium
nitrate-catalyzed formal [3 ? 2] cycloaddition as a key step.
The in vitro antioxidant activities of synthesized compounds
were determined by analyzing radical scavenging activities
for 1,1-diphenyl-2-picrylhydrazyl (DPPH), nitric oxide, and
hydrogen peroxide assays. The results showed that the syn-
thesized furomollugin analogs had effective antioxidant
power. Dihydronaphthofurans with 2-alkyoxy or 2-aryl
group were the most potent radical scavengers in DPPH
assay. Moreover, the antibacterial activities of those com-
pounds were also evaluated and the highly active com-
pounds were selected for further determination of minimal
inhibitory concentrations (MICs). Compound 19 (MIC =
2 lg/mL) was found to be highly active against the gram-
negative bacteria Escherichia coli (KCTC-1924) than
the Ampicillin standard (MIC = 4 lg/mL). Compound
22 (MIC = 0.5 lg/mL) inhibited gram-positive bacteria
Staphylococcus aureus (KCTC-1916) growth as effectively
as ampicillin (MIC = 0.5 lg/mL).
Introduction
Dihydronaphthofuran and naphthofuran derivatives have
been shown to possess a range of important biological and
pharmacological properties (Ho et al., 1996; Srivastava
et al., 2006; Le et al., 2009; Matthews et al., 2007 Lumb
et al., 2008; Sastry et al., 2010; Chandrashekhar et al.,
2011; Shashikala et al., 2011; Kwiecien et al., 2012).
Among these, compounds 1–3 were isolated from natural
sources (Schildknecht and Straub 1976; Inoue et al., 1984;
Wu et al., 1991; Otani et al., 2000; Jang et al., 2012).
(-)-Nocardione A (1) was isolated from the fermentation
broth of Nocardia sp. TP-A0248 (Otani et al., 2000), which
inhibited the activity of Cdc25B, PTP1B, and FAP-1 pro-
tein tyrosine phosphatases at a concentration of 10 lM
(Fig. 1). It also showed potent antifungal and cytotoxic
activities (Otani et al., 2000). Tanshinone I (2) was isolated
from the roots of S. miltiorrhiza and tested for its in vitro
fatty acid synthase inhibitory activity. It inhibited the
enzyme activity with an IC50 value of 12.0 lM (Jang et al.,
2012). Mollugin and furomollugin derivatives are biolog-
ically interesting compounds isolated from Galium species
(Inoue et al., 1984). Both mollugin and furomollugin (3)
were also isolated from the Rubiaceae family and have a
broad diverse bioactivity (Claessens et al., 2006; Haboni-
mana et al., 2006; Sastry et al., 2010). Especially, fur-
omollugin (3) showed cytotoxic activity in human colon
carcinoma cells (HT-29) and strongly suppressed HBsAg
secretion by human hepatoma Hep3B cells (Schildknecht
and Straub 1976; Inoue et al., 1984; Velmurugan et al.,
2012). In addition, it has been reported to possess inhibi-
tory activity against DNA topoisomerases I and II (Son
et al., 2008). Several synthetic compounds containing a
naphthofuran scaffold have been reported to have various
biological activities, such as antifungal (Qian et al., 1994),
Keywords CAN ꢀ Formal [3 ? 2] cycloaddition ꢀ
Antioxidant ꢀ Antibacterial ꢀ Furomollugin
L. Xia ꢀ A. Idhayadhulla ꢀ Y. R. Lee (&)
School of Chemical Engineering, Yeungnam University,
Gyeongsan 712-749, Republic of Korea
e-mail: yrlee@yu.ac.kr
S. H. Kim
Analysis Research Division, Daegu Center, Korea Basic Science
Institute, Taegu 702-701, Republic of Korea
Y.-J. Wee
Department of Food Science and Technology, Yeungnam
University, Gyeongsan 712-749, Republic of Korea
123

Med Chem Res
OH
O
OH
O
from the commercially available 1,4-dihydroxy-2-naph-
thoic acid (Sigma-Aldrich, 97 %, 8). The selective meth-
ylation of 8 with MeI in the presence of 2.0 equivalents
NaHCO3 in N,N-dimethylformamide (DMF) at room tem-
perature for 3 h gave methyl 1,4-dihydroxynaphthalene-2-
carboxylate (9) in 95 % yield (Jacobs et al., 2009). Sil-
ver(I) oxide-mediated reaction of 9 afforded methyl 1,4-
dioxo-1,4-dihydronaphthalene-2-carboxylate (10) in 98 %
yield (Brimble et al., 2004). Reactions of 10 with vinyl
ethers, such as n-propyl vinyl ether, n-butyl vinyl ether, and
ethyl vinyl ether in the presence of 5 mol% of CAN in
acetonitrile at room temperature for 20 min provided the
products 11–13 in 96, 93, and 95 % yields. However,
treatment of 10 with 2-methoxypropene gave compounds
14 (30 %) and 15 (68 %), whereas treatment with ethyl
1-propenyl ether, as a 30:70 mixture of cis and trans-iso-
mers, provided 16 and 17 in 42 and 44 % yields, respec-
tively. The use of 1-vinyl-2-pyrrolidinone resulted in
product 18 in 85 % yield, and a-methylstyrene or
4-methoxystyrene produced 19 and 20 in 94 and 84 %
yield, respectively. These compounds were easily sepa-
rated by column chromatography and identified by spec-
troscopic analyses. Compound 19 showed a strong IR
absorption peak at 1,665 cm-1 due to an ester group on the
O
O
CO2Me
O
O
O
1
2
3
Furomollugin ( )
(-)-Nocardione A ( )
Tanshinone I ( )
OMe
OMe
MeO
NO2
O
OH
H
MeO
O
H
O
HO
O
MeO
O
4
7
MeO
5
6
Fig. 1 Naturally occurring and synthetic dihydronaphthofurans and
naphthofurans
antibacterial (Einhorn et al., 1984), antiviral (Giovanninetti
et al., 1974), b-adrenolytic (Gaggi et al., 1982), antitumor
(Hranjec et al., 2003; Mahadevan et al., 2001; Mahadevan
´
and Vaidya, 2003), and anthelmintic effects (Ronto et al.,
1992; Debnath et al., 1993). Synthetic pterocarpan analog
(4) showed significant growth inhibitory properties on cells
free of HIV-infection (IC50 [ 15 lM) (Engler et al., 1996).
7-Methoxy-2-nitronaphtho[2,1-b]furan (5) was reported to
be one of the strongest mutagens of mammalian cells (Nair
et al., 2002). Another synthetic compound 6 showed sig-
nificant anticancer activity against the human cancer cell
lines, COLO 320DM (Colon, human), Caco-2 (Colon,
human), and WRL68 (Liver, human) at concentrations of
0.7, 0.65, and 0.50 lg/mL, respectively (Srivastava et al.,
2006). Moreover, the a-naphthol derivative, 2,2,4-tri-
methyl-2,3-dihydronaphtho[1,2-b]furan-5-ol (7) was known
as one of the most active phenolic antioxidants (Barclay
et al., 1993, 1995, 1997). Therefore, in the present study,
furomollugin (3) was chosen as an active structural basis for
molecular modification. We have been interested in ceric
ammonium nitrate (CAN)-mediated or -catalyzed reactions
between 1,3-dicarbonyls or naphthalene-1,4-dione, and ole-
fins, for the preparation of heterocycles (Lee et al., 1998,
2000, 2002, 2003; Lee and Kim, 2001; Xia and Lee, 2013).
Using CAN-catalyzed formal [3 ? 2] cycloaddition as a key
step, we synthesized diverse furomollugin derivatives and
evaluated them for antioxidant and antibacterial activities.
1
aromatic ring. In H-NMR spectrum of 19, a characteristic
methylene peak on the dihydrofuran ring exhibited at d
3.64 ppm and two singlet peaks showed at d 3.82 and
1.76 ppm due to a methoxy and a methyl group, respec-
tively. Reaction of 10 with 3,4-dihydro-2H-pyran in the
presence of 5 mol% of CAN for 0.5 h gave 21 in 95 %
yield, but reaction in the presence of two equivalents of
CAN provided unexpectedly compound 22 in 85 % yield.
The formation of naphthofuran 22 was confirmed by its 1H-
NMR spectrum due to the presence of three methylene
peaks on the pyranyl ring at d 4.40 (dd, J = 5.4, 5.1 Hz,
2H), 2.81 (dd, J = 5.4, 5.1 Hz, 2H), and 2.08–2.01 (m, 2H)
ppm. Finally, the elimination of OEt from 13 in the pre-
sence of 10 mol% p-TsOH in toluene under reflux for 3 h
afforded furomollugin (3) in 92 % yield.
Biological activity
Antioxidant activity
The synthesized furomollugin (3) and its analogs (11–22)
were evaluated for direct scavenging activity against a
variety of reactive oxygen and nitrogen species, such as 1,1-
diphenyl-2-picrylhyrazyl (DPPH) (Cefarelli et al., 2006;
Burits and Bucar, 2000), nitric oxide (NO) (Marcocci et al.,
1994), and hydrogen peroxide (H2O2) (Ruch et al., 1989).
Free radical scavenging activities are expressed as percent
inhibitions, and the results are presented in Tables 1, 2 and 3.
All synthesized compounds showed moderate to good
Results and discussion
Chemistry
Furomollugin (3) and its analogs (11–22) were synthesized
as shown in Scheme 1 (Xia and Lee 2013). The structures
1
of the synthesized compounds were confirmed by IR, H
NMR, 13C NMR, and HRMS. Our synthesis commences
123

Med Chem Res
OH
O
OH
O
O
OH
O
O
e
OMe
OMe
OMe
OH
O
3
O
+
13
OH
O
12
OH
O
OMe
OEt
On-Bu
OMe
OMe
O
O
OEt
11
On-Bu
c
O
OMe
O
On-Pr
15
c
OMe
c
14
On-Pr
c
OH
O
OH
O
O
O
OH
O
O
H
OH
O
a
OH
OMe
b
OMe
OEt
OMe
OMe
+
H
(cis:trans=30:70)
8
9
10
OH
OH
O
c
O
H
O
17
O
16
H
OEt
OEt
OH
d
N
OMe
c
c
O
c
c
OH
O
O
N
O
22
OH
O
OMe
OH
O
OH
O
OMe
O
OMe
OMe
H
OMe
O
O
H
18
21
O
O
20
19
O
OMe
Scheme 1 Synthesis of furomollugin (3) and its analogs (11–22).
Reagents and conditions: a Mel, (1.0 equiv), NaHCO3, (2.0 equiv),
DMF, RT, 3 h, 95 %. b Ag2O, (3.3 equiv), MgSO4, (2.0 equiv), Et2O,
RT, 16 h. c Olefin (2.0 equiv), catalyst CAN (0.5 equiv), acetonitrile,
RT, 20 min, (11–21, 30–96 %). d 3,4-Dihydro-2H-pyran (2.0 equiv),
CAN (2.0 equiv), acetonitrile, RT, 30 min, (22, 85 %). e p-Toluene-
sulfonic acid (PTSA, 0.1 equiv), toluene, reflux, 7 h, (3, 92 %)
scavenging activities in DPPH, NO, and H2O2 assays.
Compounds 11–14, 16, 18, and 20–21, showed high DPPH
free radical scavenging activities (67–85 %, at 100 lg/mL),
whereas compounds 15, 17, 19, and 22 showed low DPPH
reducing activities (37–56 %, at 100 lg/mL). Compounds
16 and 18 have potent NO scavenging activities (56 and
66 % at 100 lg/mL, respectively), but these were lower than
that of Trolox (88 % at 100 lg/mL). All other compounds
Table 1 The in vitro DPPH radical scavenging activity of furomollugin (3) and its analogs (11–22)
Compounds
Concentration (lg/mL)a
IC50 (lg/mL)
25
50
75
100
3
24.2 0.36
59.2 0.56
67.2 1.36
61.1 0.15
78.8 1.08
31.2 1.41
61.1 0.15
39.5 0.96
51.1 1.69
35.3 0.56
61.2 0.98
51.1 1.39
24.1 1.69
72.9 1.27
30.3 1.89
61.0 1.46
69.2 1.98
64.2 1.36
80.6 0.16
34.1 0.19
66.1 0.19
42.1 1.62
56.3 0.26
39.8 1.16
65.2 1.77
57.4 1.95
31.3 1.06
80.1 1.41
35.2 0.22
66.2 1.77
71.2 1.47
66.2 0.26
82.8 2.16
36.0 1.59
72.4 0.66
45.1 0.15
62.3 1.09
49.2 1.41
70.2 1.49
61.2 1.16
36.2 1.05
82.8 0.36
41.1 1.54
68.1 1.01
73.0 1.16
71.9 1.12
84.9 1.65
37.0 1.65
80.8 1.36
55.8 0.97
69.7 1.20
53.7 1.47
76.9 1.06
67.5 0.65
39.9 0.45
84.1 0.55
[100
11
12
13
14
15
16
17
18
19
20
21
22
Trolox
a
19.2 0.16
18.2 0.06
20.8 0.23
13.4 0.33
[100
17.2 0.15
84.7 0.22
21.2 0.26
87.5 0.29
16.2 0.31
22.7 0.65
[100
15.0 0.23
Value were the means of three replicates SD
123

Med Chem Res
Table 2 The in vitro nitric oxide (NO) scavenging activity of furomollugin (3) and its analogs (11–22)
Compounds
Concentration (lg/mL)a
IC50 (lg/mL)
25
50
75
100
3
21.2 1.64
26.8 1.10
10.3 1.49
34.2 0.98
NA
27.0 1.00
30.3 0.18
14.6 0.11
39.6 0.44
06.0 1.19
NA
30.2 0.74
36.2 1.15
20.3 0.18
42.7 0.24
10.3 0.18
5.0 0.15
50.6 1.11
1.1 1.19
57.6 0.79
NA
32.7 0.95
41.2 0.21
26.6 0.65
48.9 1.30
15.2 1.56
7.5 1.45
55.8 1.20
5.6 0.12
66.1 1.56
5.3 1.14
28.6 0.65
26.6 0.85
14.1 1.74
87.5 1.47
[100
11
12
13
14
15
16
17
18
19
20
21
22
Trolox
a
[100
[100
[100
[100
NA
[100
44.9 1.98
NA
47.9 1.60
NA
50.7 0.19
[100
42.4 0.68
NA
55.4 0.70
NA
39.4 0.11
[100
15.3 0.36
11.3 0.89
NA
20.1 1.06
15.7 1.26
5.4 1.29
82.2 0.27
24.5 0.19
21.8 1.44
10.7 0.55
84.7 0.57
[100
[100
[100
12.0 1.32
80.1 0.61
Value were the means of three replicates SD, NA (Not active)
Table 3 The in vitro hydrogen peroxide (H2O2) scavenging activity of furomollugin (3) and its analogs (11–22)
Compounds
Concentration (lg/mL)a
IC50 (lg/mL)
25
50
75
100
3
30.3 1.12
12.7 0.25
NA
36.2 0.49
18.6 1.48
NA
40.7 0.29
21.7 1.55
1.3 0.21
1.59
45.3 0.12
27.4 0.25
4.1 1.26
3.5 0.24
46.6 0.65
2.4 0.48
0.1 0.65
1.1 0.03
3.5 0.65
70.3 1.56
13.3 0.26
3.5 0.65
54.8 0.36
74.2 0.55
[100
11
12
13
14
15
16
17
18
19
20
21
22
Trolox
[100
[100
NA
NA
[100
33.7 1.69
NA
38.54 0.69
NA
41.3 1.29
NA
[100
[100
NA
NA
NA
[100
NA
NA
NA
[100
NA
NA
NA
[100
58.3 1.09
0.3 0.29
NA
61.2 1.54
8.9 1.47
NA
68.3 0.12
10.6 0.94
1.6 0.24
42.7 1.69
70.3 0.36
18.2 0.26
[100
[100
26.2 0.44
65.2 1.27
36.8 0.36
68.1 1.41
91.2 1.84
16.0 1.23
NA not active
a
Value were the means of three replicates SD
were weak NO scavengers (5–49 %, at 100 lg/mL). In the
H2O2 assay, compound 19 was found to possess significant
free radical scavenging activity (70 % at 100 lg/mL) as
compared with Trolox (74 %, at 100 lg/mL), whereas the
other compounds 3, 11, 14, and 22 showed moderate inhib-
itory activity (27–55 %, at 100 lg/mL). The remaining
compounds were very weak H2O2 scavengers. It is worth
noting that comparable free radical scavenging activities
were observed for all compounds tested at a concentration of
25 lg/mL. Importantly, compound 14 showed excellent
antioxidant activity with an IC50 value of 13.4 lg/mL in
DPPH assay, whereas the IC50 value of the Trolox standard
was 15.0 lg/mL. Compound 18 showed high nitric oxide
(NO) scavenging activity with an IC50 value of 39.4 lg/mL,
whereas the IC50 valueoftheTroloxstandardwas12.0 lg/mL.
In hydrogen peroxide (H2O2) assay, Compound 19 exhibited
123

Med Chem Res
potent scavenging activity with an IC50 value of 18.2 lg/mL,
whereas the IC50 value of the Trolox standard was 16.0 lg/
mL.
Table 4 Antibacterial activities of furomollugin (3) and its analogs
(11–22)
Compounds
Diameter of growth inhibition zone (mm)
The present study also revealed that the synthesized fur-
omollugin (3) and its analogs (11–22) were more effective in
theDPPHassaythanNOandH2O2 assays. InDPPH assay,the
5-hydroxy and naphthofuran moiety on the furomollugin
analogs have significant influence on their radical scavenging
activity. A 5-hydroxy group on a furomollugin skeleton is
essential to exhibit the antioxidant activity. The synthesized
furomollugin analogs bearing 2-alkyoxy group or 2-aryl, such
as compounds 14 and 20, have more effective antioxidant
powers which are similar to the well-known phenolic anti-
oxidant 7. The results could be explained by the strong elec-
tron-donating inductive effect onthe 2,3-dihydronaphtho[1,2-
b]furans which will stabilize the phenolic radicals (Barclay
et al., 1995). On the other hand, the order of inhibitory activity
of 14 [ 13 > 21 [ 19, based on their IC50 values, is also
largely explainable in terms of stereoelectronic constraints,
and thus reflects their intrinsic antioxidant properties (Burton
et al., 1985). Interestingly, cis-16 exhibited much higher
antioxidant activity than trans-17 isomer for steric reasons
(Burton et al., 1985). Finally, naphtho[1,2-b]furans 3, 15, and
22 showed very poor radical scavenging activities because of
the combination of electron inductive effects and stereoelec-
tronic constraints as mentioned above.
Escherichia coli
(Gram-negative)
Staphylococcus aureus
(Gram-positive)
Control: DMSO
0.0
17
13
8
0.0
23
11
10
13
12
15
15
11
8
St: Ampicillin
3
11
12
13
14
15
16
17
18
19
20
21
22
NA
10
11
11
13
13
14
18
14
15
13
NA
12
10
20
23
NA not active
compound 19 was found to be highly active (18 mm zone
of inhibition) using Ampicillin as a positive control
(17 mm zone of inhibition), whereas 3, 14–18, and 20–22
exhibited moderate activity (11–15 mm zone of inhibition).
Compounds 11 and 13 showed weak inhibitory activities,
and compound 12 exhibited no activity. Against S. aureus
at a concentration of 100 lg/mL, compounds 21 (20 mm
zone of inhibition) and 22 (23 mm zone of inhibition) were
equally active as Ampicillin (23 mm zone of inhibition),
whereas compounds 3, 11–16, 19, and 20 displayed mod-
erate activity (10–15 mm zone of inhibition), and com-
pounds 17 and 18 exhibited weak or no activity,
respectively, against S. aureus.
Antibacterial evaluation
Antibacterial activities of furomollugin (3) and its analogs
11–22 were screened using the diffusion disk method. The
experiments were performed using test bacterial organisms
belonging to the gram-negative and gram-positive groups,
namely, Escherichia coli (E. coli, KCTC-1924) and Staph-
ylococcus aureus (S. aureus, KCTC-1916), respectively.
Escherichia coli, a typical gram-negative bacterium, can
be grown easily in a laboratory setting and has been
intensively investigated for over 60 years. E. coli is high
sensitivity for antibiotics. Thus, it is most widely studied
prokaryotic model organism, and it is an important species
in the fields of biotechnology and microbiology. On the
other hand, S. aureus, a common gram-positive bacterium,
which is a member of the Firmicutes, and is frequently
found in the human respiratory tract and on the skin.
Moreover, both E. coli and S. aureus are commercially
available from the Korean Collection for Type Cultures
(KCTC) in the Korea Research Institute of Bioscience and
Biotechnology (KRIBB). For all the reasons mentioned
above, we focused on E. coli (KCTC-1924) and S. aureus
(KCTC-1916) in our studies.
The highly active compounds (14, 15, 19, 21, and 22)
were selected for further determination of minimal inhib-
itory concentrations (MICs), as shown in Table 5. The
results were in accordance with the results obtained in the
primary screening. It would be concluded that compound
19 (MIC = 2 lg/mL) was more active against E. coli than
Ampicillin standard (MIC = 4 lg/mL). In addition, com-
pound 22 (MIC = 0.5 lg/mL) showed equipotential
activity against S. aureus with respect to the standard drug
Ampicillin (MIC = 0.5 lg/mL).
Conclusions
In summary, we described the synthesis of diverse fur-
omollugin (3) and its analogs 11–22, and screened for their
antioxidant and antibacterial activities. Those compounds
Zone of inhibition was determined for 3 and 11–22, and
the results are summarized in Table 4 (Bauer et al., 1966).
Against E. coli at a concentration of 100 lg/mL,
123

Med Chem Res
olefin (2.0 mmol) in MeCN (5.0 mL) was added cerium
(IV) ammonium nitrate (CAN, 5.0 mol%) at room tem-
perature. The reaction was monitored by TLC. After all
starting material had been consumed, water (10 mL) was
added, and the solution was extracted with ethyl acetate
(10 mL 9 3). Evaporation of the solvent and purification
by silica gel column chromatography using hexane–ethyl
acetate (10:1) as eluant gave the desired products.
Table 5 The minimal inhibitory concentrations (MIC, lg/mL) of
selected compounds
Compound no.
Minimal inhibitory concentration (MIC, lg/mL)
E. coli
S. aureus
14
ND
ND
2
64
64
ND
2
15
19
21
64
ND
4
22
0.5
0.5
Methyl 5-hydroxy-2-propoxy-2,3-dihydronaphtho[1,2-
Ampicillin
b]furan-4-carboxylate (11)
ND not determined
1
Yellow liquid; yield 290 mg, 96 %; H NMR (300 MHz,
CDCl3) d 11.82 (s, 1H), 8.37 (d, J = 8.4 Hz, 1H), 7.92 (d,
J = 8.4 Hz, 1H), 7.61 (dd, J = 8.4, 7.2 Hz, 1H), 7.49 (dd,
J = 8.4, 7.2 Hz, 1H), 5.85–5.82 (m, 1H), 3.94 (s, 3H),
3.64–3.52 (m, 3H), 3.45–3.38 (m, 1H), 1.76–1.55 (m, 2H),
0.94 (dd, J = 7.5, 7.2 Hz, 3H); 13C NMR (75 MHz,
CDCl3) d 171.51, 156.18, 145.63, 129.05, 125.38, 124.39,
124.30, 121.06, 115.06, 106.02, 103.03, 100.76, 70.17,
52.03, 40.18, 22.84, 10.66; IR (neat): 3,066, 2,961, 2,877,
1,663, 1,448, 1,351, 1,235, 1,162, 1,104, 1,016, 897,
768 cm-1; HRMS (EI?): m/z: calcd for C17H18O5:
302.1154, Found 302.1151.
were more effective in the DPPH assay than NO and H2O2
assays. Importantly, compounds 14 and 20 showed excel-
lent DPPH scavenging activities, which were almost as
efficient as the Trolox standard. Comparing with
the Trolox standard, compounds 16 and 18 showed decent
radical scavenging activities in NO assay, and compound
19 showed potent radical scavenging activity in H2O2
assay. Most of compounds were active in vitro evaluation
of their antibacterial activities. Importantly, compound 19
was more active against E. coli bacterial strain than the
standard drug Ampicillin. Compound 22 was highly active
against the S. aureus bacterial strain at a level of Ampi-
cillin. Our results suggest that these substances represent
interesting lead compounds for the development of novel
antioxidant and antibacterial agents.
Methyl 2-butoxy-5-hydroxy-2,3-dihydronaphtho[1,2-
b]furan-4-carboxylate (12)
1
Yellow liquid; yield 294 mg, 93 %; H NMR (300 MHz,
CDCl3) d 11.78 (s, 1H), 8.34 (d, J = 8.4 Hz, 1H), 7.89 (d,
J = 8.4 Hz, 1H), 7.58 (dd, J = 8.1, 7.2 Hz, 1H), 7.46 (dd,
J = 8.1, 7.2 Hz, 1H), 5.80 (dd, J = 6.6, 2.4 Hz, 1H), 3.91
(s, 3H), 3.62–3.50 (m, 3H), 3.41–3.34 (m, 1H), 1.61–1.49
(m, 2H), 1.39–1.32 (m, 2H), 0.94–0.87 (m, 3H); 13C NMR
(75 MHz, CDCl3) d 171.52, 156.19, 145.64, 129.05,
125.38, 124.31, 124.26, 121.06, 115.06, 106.02, 103.04,
100.76, 68.25, 52.03, 40.18, 31.61, 19.38, 13.78; IR (neat):
3,423, 3,065, 2,954, 2,871, 1,663, 1,599, 1,447, 1,350,
1,235, 1,162, 1,091, 1,018, 896, 767, 646 cm-1; HRMS
(EI?): m/z: calcd for C18H20O5: 316.1311, Found 316.1312
Experimental
All chemicals were purchased from the Sigma-Aldrich,
Fluka, or Tokyo Chemical Industry, and used without further
purification. Solvents were dried and distilled prior to use.
All synthetic experiments were carried out in a N2 atmo-
sphere. Merck precoated silica gel plates (Art. 5554) with a
fluorescent indicator were used for analytical TLC. Flash
column chromatography was performed using silica gel
7734 (Merck). The 1H and 13C NMR spectra were recorded
on a Bruker Model ARX (300 and 75 MHz, resp.) spec-
trometer using TMS as internal standard in deuterated
chloroform or dimethylsulfoxide. The IR spectra were
recorded on a BioRad FTS 3000 spectrophotometer. The
HRMS were carried out at the Korea Basic Science Institute.
Methyl 2-ethoxy-5-hydroxy-2,3-dihydronaphtho[1,2-
b]furan-4-carboxylate (13)
Yellow solid; yield 274 mg, 95 %; mp 81–82 ꢁC; 1H NMR
(300 MHz, CDCl3) d 11.75 (s, 1H), 8.34 (d, J = 8.4 Hz,
1H), 7.88 (d, J = 8.4 Hz, 1H), 7.56 (dd, J = 8.4, 8.1 Hz,
1H), 7.45 (dd, J = 8.4, 8.1 Hz, 1H), 5.78 (d, J = 6.6 Hz,
1H), 4.05–3.94 (m, 1H), 3.88 (s, 3H), 3.72–3.62 (m, 1H),
3.48 (dd, J = 17.7, 6.3 Hz, 1H), 3.33 (dd, J = 18.0,
2.4 Hz, 1H), 1.24 (t, J = 7.2 Hz, 3H); 13C NMR (75 MHz,
CDCl3) d 171.37, 156.11, 145.55, 128.94, 125.27, 124.37,
Synthesis
General procedure for the preparation
of dihydronaphtho[1,2-b]furan derivatives 11–21
To a solution of methyl 1,4-dihydroxynaphthalene-2-car-
boxylate (9, 218 mg, 1.0 mmol) and the corresponding
123

Med Chem Res
trans-Methyl 2-ethoxy-5-hydroxy-3-methyl-2,3-
124.25, 124.18, 121.00, 115.00, 105.82, 102.95, 63.82,
51.87, 40.13, 15.09; IR (KBr): 3,062, 2,974, 1,661, 1,599,
1,446, 1,349, 1,236, 1,162, 1,100, 1,017, 946, 896, 767,
646 cm-1; HRMS (EI?): m/z: calcd for C16H16O5:
288.0998, Found 288.0998.
dihydronaphtho[1,2-b]furan-4-carboxylate (17)
1
Yellow solid; yield: 133 mg, 44 %; mp 112–113 ꢁC; H
NMR (300 MHz, CDCl3) d 11.98 (s, 1H), 8.38 (d,
J = 8.4 Hz, 1H), 7.93 (d, J = 8.1 Hz, 1H), 7.60 (dd,
J = 8.4, 8.1 Hz, 1H), 7.48 (dd, J = 8.4, 8.1 Hz, 1H), 5.40
(brs, 1H), 4.03–3.98 (m, 1H), 3.97 (s, 3H), 3.73–3.65 (m,
Methyl 5-hydroxy-2-methoxy-2-methyl-2,3-
dihydronaphtho[1,2-b]furan-4-carboxylate (14)
2H), 1.28 (d, J = 6.9 Hz, 3H), 1.22 (t, J = 7.2 Hz, 3H); 13
C
NMR (75 MHz, CDCl3) d 171.45, 156.90, 145.23, 129.20,
125.57, 124.79, 124.77, 124.48, 121.17, 121.09, 111.95,
102.65, 63.85, 52.01, 46.46, 18.38, 15.18; IR (KBr): 2,970,
1,657, 1,444, 1,355, 1,234, 1,101, 910, 766 cm-1; HRMS
(EI?): m/z: calcd for C17H18O5: 302.1154, Found 302.1155.
Yellow solid; yield 86 mg, 30 %; mp 160–161 ꢁC; 1H
NMR (300 MHz, CDCl3) d 11.80 (s, 1H), 8.36 (d,
J = 8.4 Hz, 1H), 7.92 (d, J = 8.4 Hz, 1H), 7.60 (dd,
J = 8.1, 7.2 Hz, 1H), 7.48 (dd, J = 8.1, 7.2 Hz, 1H), 3.96
(s, 3H), 3.61 (d, JAB = 18.0 Hz, 1H), 3.44 (d,
JAB = 18.0 Hz, 1H), 3.32 (s, 3H), 1.75 (s, 3H); 13C NMR
(75 MHz, CDCl3) d 175.55, 156.17, 145.93, 129.15,
125.52, 124.54, 124.36, 123.94, 121.16, 115.32, 111.39,
102.87, 52.07, 49.66, 43.79, 24.63; IR (KBr): 3,449, 2,946,
1,656, 1,598, 1,445, 1,381, 1,344, 1,281, 1,236, 1,160,
1,019, 860, 777, 731 cm-1; HRMS (EI?): m/z: calcd for
C16H16O5: 288.0998, Found 288.0994.
Methyl 5-hydroxy-2-(2-oxopyrrolidin-1-yl)-2,3-
dihydronaphtho[1,2-b]furan-4-carboxylate (18)
Yellow solid; yield: 278 mg, 85 %; mp 173–174 ꢁC. IR
(KBr): 3,055, 2,984, 1,694, 1,448, 1,235, 1,163, 1,094, 1,016,
890, 773, 652 cm-1. 1H-NMR (300 MHz, CDCl3) d 11.81 (s,
1H), 8.36 (d, J = 8.4 Hz, 1H), 7.87 (d, J = 8.1 Hz, 1H), 7.60
(dd, J = 8.1,6.9 Hz,1H),7.49(dd,J = 8.4,6.9 Hz,1H), 6.77
(dd, J = 9.3, 4.5 Hz, 1H), 3.98 (s, 3H), 3.77 (dd, J = 18.0,
9.6 Hz, 1H), 3.38 (dd, J = 17.7, 4.2 Hz, 1H), 3.27–3.20 (m,
2H), 2.48–2.42 (m, 2H), 2.04–1.94 (m, 2H). 13C-NMR
(75 MHz, CDCl3) d 175.42, 171.32, 156.38, 146.19, 129.38,
125.83, 124.57, 124.31, 123.51, 121.22, 113.86, 102.46,
83.46, 52.24, 41.17, 36.16, 31.17, 17.71. HRMS (EI?): m/z:
calcd for C18H17NO5: 327.1107, Found 327.1103.
Methyl 5-hydroxy-2-methylnaphtho[1,2-b]furan-4-
carboxylate (15)
White solid; yield 174 mg, 68 %; mp 135–136 ꢁC; 1H
NMR (300 MHz, CDCl3) d 12.19 (s, 1H), 8.41 (d,
J = 8.1 Hz, 1H), 8.11 (d, J = 8.1 Hz, 1H), 7.65 (dd,
J = 8.1, 7.2 Hz, 1H), 7.45 (dd, J = 8.1, 7.2 Hz, 1H), 6.77
(s, 1H), 4.04 (s, 3H), 2.52 (s, 3H); 13C NMR (75 MHz,
CDCl3) d 172.02, 158.74, 154.85, 143.38, 129.79, 124.78,
124.51, 124.27, 121.97, 121.19, 119.33, 105.47, 98.99,
52.11, 14.11; IR (KBr): 3,450, 2,953, 2,940, 1,646, 1,578,
Methyl 5-hydroxy-2-methyl-2-phenyl-2,3-
1,443, 1,343, 1,241, 1,161, 1,095, 1,044, 987, 776 cm-1
;
dihydronaphtho[1,2-b]furan-4-carboxylate (19)
HRMS (EI?): m/z: calcd for C15H12O4: 256.0,736, Found
Yellow solid; Yield: 314 mg, 94 %; mp 147–148 ꢁC; 1H NMR
(300 MHz, CDCl3) d 11.72 (s, 1H), 8.28 (d, J = 8.4 Hz, 1H),
7.96 (d, J = 8.4 Hz, 1H), 7.55–7.50 (m, 1H), 7.45–7.37 (m,
3H), 7.27–7.22 (m, 2H), 7.17–7.12 (m, 1H), 3.82 (s, 3H), 3.64
(s, 2H), 1.76 (s, 3H); 13C NMR (75 MHz, CDCl3) d 171.63,
156.05, 147.26, 146.42, 129.12, 128.33, 126.98, 125.51,
124.55, 124.41, 124.37, 124.23, 121.33, 115.36, 103.25, 88.76,
52.07, 48.53, 29.32; IR (KBr): 3,420, 3,060, 3,026, 2,954,
1,665, 1,646, 1,597, 1,449, 1,379, 1,350, 1,243, 1,161, 1,066,
1,020, 863, 769, 700, 642 cm-1; HRMS (EI?): m/z: calcd for
C21H18O4: 334.1205, Found 334.1205.
256.0737.
cis-Methyl 2-ethoxy-5-hydroxy-3-methyl-2,3-
dihydronaphtho[1,2-b]furan-4-carboxylate (16)
1
Yellow solid; yield: 127 mg, 42 %; mp 103–104 ꢁC; H
NMR (300 MHz, CDCl3) d 11.89 (s, 1H), 8.35 (d,
J = 8.4 Hz, 1H), 7.89 (d, J = 7.8 Hz, 1H), 7.58 (dd,
J = 8.1, 7.2 Hz, 1H), 7.46 (dd, J = 8.1, 7.2 Hz, 1H), 5.83
(d, J = 6.6 Hz, 1H), 4.15–4.07 (m, 1H), 3.97 (s, 3H),
3.84–3.72 (m, 2H), 1.35 (t, J = 7.2 Hz, 3H), 1.20 (d,
J = 6.9 Hz, 3H); 13C NMR (75 MHz, CDCl3) d 171.44,
156.37, 144.11, 129.11, 125.65, 124.62, 124.38, 124.28,
121.41, 109.61, 102.62, 66.36, 52.05, 43.01, 42.63, 15.23,
13.60; IR (KBr): 2,974, 1,655, 1,443, 1,349, 1,233, 1,087,
916, 767 cm-1; HRMS (EI?): m/z: calcd for C17H18O5:
302.1,154, Found 302.1156.
Methyl 5-hydroxy-2-(4-methoxyphenyl)-2,3-
dihydronaphtho[1,2-b]furan-4-carboxylate (20)
1
Yellow solid; yield 294 mg, 84 %; mp 126–127 ꢁC; H
NMR (300 MHz, CDCl3) d 11.85 (s, 1H), 8.38 (d,
123

Med Chem Res
Furomollugin (3)
J = 8.4 Hz, 1H), 7.94 (d, J = 8.1 Hz, 1H), 7.59 (dd,
J = 8.1, 6.9 Hz, 1H), 7.49 (dd, J = 8.4, 6.9 Hz, 1H), 7.37
(d, J = 8.7 Hz, 2H), 6.90 (d, J = 8.7 Hz, 2H), 5.79 (dd,
J = 9.3, 8.7 Hz, 1H), 3.92–3.83 (m, 4H), 3.79 (s, 3H),
3.51–3.43 (m, 1H); 13C NMR (75 MHz, CDCl3) d 171.54,
159.30, 155.97, 147.13, 134.42, 129.05, 127.16, 125.46,
124.45, 124.26, 123.92, 121.27, 115.77, 113.89, 103.06,
83.73, 55.20, 52.00, 41.91; IR (KBr): 3,060, 3,005, 2,952,
2,836, 1,660, 1,514, 1,447, 1,348, 1,297, 1,239, 1,169,
1,089, 1,017, 832, 770, 644 cm-1; HRMS (EI?): m/z: calcd
for C21H18O5: 350.1154, Found 350.1154
To a solution of dihydronaphtho[1,2-b]furan 13 (144 mg,
0.50 mmol) in an anhydrous toluene (10.0 mL) was added
p-toluenesulfonic acid (PTSA, 18 mg, 0.10 mmol) at room
temperature. The reaction mixture was refluxed for 7 h.
After cooling to room temperature, the solution was treated
with saturated NaHCO3 solution and extracted with EtOAc
(3 9 10.0 mL). The organic layers were combined and
dried over an anhydrous MgSO4. After removal of the
drying agent and the solvent, the residue was purified by
column chromatography on silica gel using hexane–ethyl
acetate (10:1) to yield 3 (111 mg, 92 %) as a yellow solid.
MP: 114–115 ꢁC; 1H NMR (300 MHz, CDCl3) d 12.18 (s,
1H), 8.37 (d, J = 8.7 Hz, 1H), 8.08 (d, J = 8.4 Hz, 1H),
7.63 (dd, J = 8.1, 7.2 Hz, 1H), 7.61–7.59 (m, 1H), 7.46
(dd, J = 8.1, 7.2 Hz, 1H), 7.00 (s, 1H), 3.98 (s, 3H); 13C
NMR (75 MHz, CDCl3) d 171.73, 158.99, 144.05, 143.98,
129.78, 124.81, 124.69, 124.58, 122.64, 119.55, 119.43,
109.04, 98.92, 52.03; IR (KBr): 3,121, 3,092, 2,947, 1,671,
1,638, 1,575, 1,439, 1,340, 1,310, 1,227, 1,159, 1,099,
1,031, 986, 769, 695 cm-1; HRMS (EI?): m/z: calcd for
C14H10O4: 242.0579, Found 242.0579.
Methyl 5-hydroxy-7,8,9,10a-tetrahydro-6bH-
naphtho[20,10:4,5]furo[2,3-b]pyran-6-carboxylate (21)
1
Yellow solid; yield 285 mg, 95 %; mp 166–167 ꢁC; H
NMR (300 MHz, CDCl3) d 11.92 (s, 1H), 8.36 (d,
J = 8.4 Hz, 1H), 7.94 (d, J = 8.1 Hz, 1H), 7.60 (dd,
J = 8.4, 8.1 Hz, 1H), 7.49 (dd, J = 8.4, 8.1 Hz, 1H), 5.99
(d, J = 6.3 Hz, 1H), 3.99 (s, 3H), 3.95–3.92 (m, 2H),
3.53–3.46 (m, 1H), 2.22–2.15 (m, 1H), 1.78–1.65 (m, 1H),
1.63–1.54 (m, 1H), 1.45–1.32 (m, 1H); 13C NMR
(75 MHz, CDCl3) d 171.36, 156.46, 144.52, 129.36,
125.83, 124.65, 124.56, 124.34, 121.57, 121.47, 105.29,
102.35, 62.13, 52.20, 40.22, 26.10, 21.55; IR (KBr): 3,451,
2,958, 2,872, 1,652, 1,591, 1,446, 1,388, 1,349, 1,240,
1,156, 1,072, 1,021, 949, 863, 771 cm-1; HRMS (EI?): m/
z: calcd for C17H16O5: 300.0998, Found 300.0997.
Biological activity
Antioxidant screening
DPPH radical scavenging activity Methanolic solutions
of DPPH exhibit a strong purple color and strong absorption
at 517 nm, and this is reduced in the presence of an antiox-
idant (Cefarelli et al., 2006; Burits and Bucar, 2000). Thus,
there is an inverse relationship between the concentration of
remaining DPPH and antiradical activity. Various concen-
trations of the test compounds (25 to 100 lg/mL) in meth-
anol (total 1 mL) were added to 4 mL of a 0.004 % (w/v)
methanolic solution of DPPH. After a 30-min incubation
period at room temperature, absorbance was read against a
blank at 517 nm. Percentage free radical scavenging (%) was
calculated using the following equation.
Methyl 5-hydroxy-8,9-dihydro-7H-
naphtho[20,10:4,5]furo[2,3-b]pyran-6-carboxylate (22)
To a solution of methyl 1,4-dioxo-1,4-dihydronaphthalene-
2-carboxylate (6a, 216 mg, 1.0 mmol) and 3,4-dihydro-
2H-pyran (168 mg, 2.0 mmol) in MeCN (10.0 mL) was
added CAN (1.10 g, 200 mol%) at room temperature. The
progress of the reaction was monitored by TLC. After all
the starting material was consumed, water (10 mL) was
added and the solution was extracted with ethyl acetate
(10 mL 9 3). Evaporation of the solvent and purification
by column chromatography on silica gel using hexane–
ethyl acetate (10:1) gave 22 (253 mg, 85 %) as a yellow
Â
Ã
Scavenging % ¼ ðAcontrolꢁAsampleÞ=Acontrol ꢂ 100
1
where Acontrol is the absorbance of DPPH solution (con-
taining all reagents except the test compound), and Asample
is the absorbance of the tested sample. Tests were con-
ducted in triplicate.
solid. MP: 193–194 ꢁC; H NMR (300 MHz, CDCl3) d
12.37 (s, 1H), 8.35 (d, J = 8.4 Hz, 1H), 7.99 (d,
J = 8.4 Hz, 1H), 7.59 (dd, J = 7.8, 7.5 Hz, 1H), 7.35 (dd,
J = 7.8, 7.5 Hz, 1H), 4.40 (dd, J = 5.4, 5.1 Hz, 2H), 3.97
(s, 3H), 2.81 (t, J = 6.3 Hz, 2H), 2.08–2.01 (m, 2H); 13C
NMR (75 MHz, CDCl3) d 171.84, 159.65, 158.96, 129.91,
124.61, 124.34, 123.45, 122.15, 121.38, 120.54, 118.86,
99.43, 88.71, 69.37, 51.63, 23.23, 21.68; IR (KBr): 3,444,
3,069, 2,951, 2,863, 1,650, 1,619, 1,579, 1,442, 1,365,
1,336, 1,249, 971, 809, 774 cm-1; HRMS (EI?): m/z: calcd
for C17H14O5: 298.0841, Found 298.0843.
Nitric oxide (NO) scavenging activity Nitric oxide scav-
enging activity was measured as described by Marcocci
et al., (1994) with slight modification. NO radicals were
generated from sodium nitroprusside. Sodium nitroprusside
(10 mM, 1 mL) and 1.5 mL of phosphate buffer saline
(0.2 M, pH 7.4) were added to different concentrations
123

Med Chem Res
(25 to 100 lg/mL) of the test compounds (3 and 11–22) and
incubated for 150 min at 258C. After incubation, 1 mL of the
reaction mixture was treated with 1 mL of Griess reagent
(1 % sulfanilamide, 2 % H3PO4 and 0.1 % naphthylethyle-
nediaminedihydrochloride). Chromatophore absorbance
was measured at 546 nm. Nitric oxide scavenging activity
was calculated using the following equation.
Cultures (KCTC), were incubated at 35–37 ꢁC for 24–48 h,
and then the diameters of the inhibition zones were mea-
sured in millimeters (Bauer et al., 1966).
Standard disks of Ampicillin (antibacterial agent) served
as positive controls for antibacterial activity, but filter disks
impregnated with DMSO were used as a negative control.
The agar used was Mueller–Hinton agar that is rigorously
tested for composition and pH. Further, the depth of the
agar in the plate is a factor to be considered in the disk
diffusion method. This method is well documented and
standard zones of inhibition have been determined for
susceptible and resistant values.
Â
Ã
% of scavenging ¼ ðAcontrolꢁ AsampleÞ=Acontrol ꢂ 100
where Acontrol is the absorbance of the control reaction
(containing all reagents except the test compound), and
Asample is the absorbance of the test compound. Tests were
conducted in triplicate.
Blank paper disks with a diameter of 8.0 mm were
impregnated with 10 mL of tested concentration of the
stock solutions (100 lg/mL in DMSO). When a filter paper
disk impregnated with a tested chemical is placed on agar,
the chemical will diffuse from the disk into the agar. This
diffusion will place the chemical in the agar only around
the disk. The solubility of the chemical and its molecular
size will determine the size of the area of chemical infil-
tration around the disk. If an organism is placed on the
agar, it will not grow in the area around the disk if it is
susceptible to the chemical. This area of no growth around
the disk is known as zone of inhibition or clear zone.
Hydrogen peroxide (H2O2) scavenging activity The H2O2
scavenging ability was determined as described by Ruch
et al. (1989). In brief, a solution of H2O2 (40 mM) was
prepared in phosphate buffer (pH 7.4), and then test com-
pounds at 25 to 100 lg/mL in 3.4 mL of phosphate buffer
were added to H2O2 solution (0.6 mL, 40 mM). The
absorbance values of reaction mixtures were recorded at
230 nm. H2O2 scavenging percentages were calculated
using the following equation.
Â
Ã
% of scavenging ¼ ðAcontrolꢁAsampleÞ=Acontrol ꢂ 100
where Acontrol is the absorbance of the control reaction
Determination of the minimal inhibitory concentration
(MIC) Testing compounds and standard drug Ampicillin
were well dissolved in DMSO at the concentration of
64 lg/mL. Then, twofold dilutions of the solution were
prepared (64, 32…., 0.5 lg/mL). The microorganism sus-
pensions at 106 CFU/mL (colony forming unit/mL) con-
centrations were inoculated to the corresponding wells. The
plates were incubated at 36 ꢁC for 24 h. The MIC was
noted by observing the lowest concentration of the drug at
which there was no visible growth.
(containing all reagents except the test compound), and
A
sample is the absorbance of the test compounds. Tests were
conducted in triplicate. The antioxidant activity of each
sample was also expressed in terms of IC50 (microgram
concentration required to inhibit the DPPH radical forma-
tion by 50 %) and was calculated from the graph after
plotting inhibition percentage against log concentration.
Measurement of antibacterial activity using the disk
diffusion method
Acknowledgments This research was supported by the Nano
Material Technology Development Program through the Korean
National Research Foundation (NRF) funded by the Korean Ministry
of Education, Science, and Technology (2012M3A7B4049675).
Antibacterial activity of the tested samples was determined
using a modified Kirby-Bauer disk diffusion method
(Bauer et al., 1966). In brief, 100 mL of the test bacteria
was grown in 10 mL of fresh media until they reached a
count of approximately 106 cells/mL for bacteria (Pfaller
et al., 1988). One hundred microliters of the bacterial
suspension was spread onto agar plates corresponding to
the broth in which they were maintained. Isolated colonies
of each organism that might be playing a pathogenic role
should be selected from primary agar plates and tested. Of
the many media available, the National Committee for
Clinical Laboratory Standards (NCCLS) recommends
Mueller–Hinton agar because it results in good batch-to-
batch reproducibility (NCCLS, 1997). Gram-negative
E. coli (KCTC-1924) and gram-positive S. aureus (KCTC-
1916), both obtained from the Korean Collection for Type
References
Barclay LRC, Vinqvist MR, Mukai K, Itoh S, Morimoto H (1993)
Chain-breaking phenolic antioxidants: steric and electronic
effects in polyalkylchromanols, tocopherol analogs, hydroqui-
nones, and superior antioxidants of the polyalkylbenzochroma-
nol and naphthofuran class. J Org Chem 58:7416–7420. doi:10.
1021/jo00078a020
Barclay LRC, Edwards CD, Mukai K, Egawa Y, Nishi T (1995)
Chain-breaking naphtholic antioxidants: antioxidant activities of
a polyalkylbenzochromanol, a polyalkylbenzochromenol, and
2,3-dihydro-5-hydroxy-2,2,4-trimethylnaphtho[1,2-b]furan com-
pared to an a-tocopherol model in sodium dodecyl sulfate
micelles. J Org Chem 60:2739–2744. doi:10.1021/jo00114a022
123

Med Chem Res
Barclay LRC, Antunes F, Egawa Y, McAllister KL, Mukai K, Nishi
T, Vinqvist MR (1997) The efficiency of antioxidants delivered
by liposomal transfer. Biochim Biophys Acta Biomembr
1328:1–12. doi:10.1016/s0005-2736(97)00057-6
Jacobs J, Claessens S, Mbala BM, Huygen K, De Kimpe N (2009)
New and highly efficient synthesis of 3-substituted 1-hydroxy-
benz[g]isoquinoline-5,10-diones. Tetrahedron 65:1193–1199.
doi:10.1016/j.tet.2008.11.070
Bauer AW, Kirby WM, Sherris JC, Turck M (1966) Antibiotic
susceptibility testing by a standardized single disk method. Am J
Clin Pathol 45:493–496
Jang T-S, Zhang HY, Kim GJ, Kim DW, Min B-S, Kang W, Son KH,
Na MK, Lee SH (2012) Bioassay-guided isolation of fatty acid
synthase inhibitory diterpenoids from the roots of Salvia
miltiorrhiza Bunge. Arch Pharm Res 35:481–486. doi:10.1007/
s12272-012-0311-8
Kwiecien H, Smist M, Kowalewska M (2012) Recent development on
the synthesis of benzo[b]- and naphtho[b]furans: a review. Curr
Org Synth 9:529–560. doi:10.2174/157017912802651393
Le GR, Oger F, Lecorgne A, Dudasova Z, Chevance S, Bondon A,
Barath P, Simonneaux G, Salbert G (2009) Identification of
small molecule regulators of the nuclear receptor HNF4a based
on naphthofuran scaffolds. Bioorg Med Chem 17:7021–7030.
doi:10.1016/j.bmc.2009.07.079
Lee YR, Kim BS (2001) Efficient synthesis of cytotoxic furonaph-
thoquinone natural products. Synth Commun 31:381–386.
doi:10.1081/scc-100000527
Lee YR, Byun MW, Kim BS (1998) Efficient one-step synthesis of
2-arylfurans by ceric ammonium nitrate (CAN)-mediated cyclo-
addition of 1,3-dicarbonyl compounds to alkynes. Bull Korean
Chem Soc 19:1080–1083
Brimble MA, Burgess C, Halim R, Petersson M, Ray J (2004)
Synthesis of pyrrolo[3,2-b]benzofurans and pyrrolo[3,2-b]napht-
hofurans via addition of a silyloxypyrrole to activated quinones.
Tetrahedron 60:5751–5758. doi:10.1016/j.tet.2004.05.008
Burits M, Bucar F (2000) Antioxidant activity of Nigella sativa
essential oil. Phytother Res 14:323–328. doi:10.1002/1099-
1573(200008)14:5\323:aid-ptr621[3.0.co;2-q
Burton GW, Doba T, Gabe E, Hughes L, Lee FL, Prasad L, Ingold
KU (1985) Autoxidation of biological molecules. 4. Maximizing
the antioxidant activity of phenols.
107:7053–7065. doi:10.1021/ja00310a049
J Am Chem Soc
Cefarelli G, D’Abrosca B, Fiorentino A, Izzo A, Mastellone C,
Pacifico S, Piscopo V (2006) Free-radical-scavenging and
antioxidant activities of secondary metabolites from reddened
cv. Annurca apple fruits. J Agric Food Chem 54:803–809.
doi:10.1021/jf052632g
Chandrashekhar CH, Latha KP, Vagdevi HM, Vaidya VP (2011)
Synthesis and antimicrobial activity of chalcones of naphtho[2,1-
b]furan condensed with barbituric acid. Der Pharm Chem
3:365–369
Lee YR, Kim BS, Kim DH (2000) Ceric ammonium nitrate (CAN)-
mediated oxidative cycloaddition of 1,3-dicarbonyls to conjugated
compounds. Efficient synthesis of dihydrofurans, dihydrofurocouma-
rins, dihydrofuroquinolinones, dihydrofurophenalenones, and furo-
naphthoquinone natural products. Tetrahedron 56:8845–8853.
doi:10.1016/s0040-4020(00)00839-5
Claessens S, Kesteleyn B, Van Nguyen T, De Kimpe N (2006)
Synthesis of mollugin. Tetrahedron 62(35):8419–8424. doi:10.
1016/j.tet.2006.06.014
Debnath AK, Hansch C, Kim KH, Martin YC (1993) Mechanistic
interpretation of the genotoxicity of nitrofurans (antibacterial
agents) using quantitative structure-activity relationships and
Lee YR, Lee GJ, Kang KY (2002) Ceric ammonium nitrate(CAN)-
mediated oxidative cycloaddition of 1,3-dicarbonyls to vinyl
sulfides. Application to the synthesis of evodone and avicequi-
none-B. Bull Korean Chem Soc 23:1477–1480. doi:10.5012/
bkcs.2002.23.10.1477
comparative molecular field analysis.
1007–1016. doi:10.1021/jm00060a008
J Med Chem 36:
Einhorn J, Demerseman P, Royer R, Cavier R, Gayral P (1984) Effect
of a substituent in the b position of the heterocycle on the
antibacterial and parasiticidal properties of 2-nitronaphtho[2.1-
b]furans. Eur J Med Chem Chim Ther 19:405–410
Engler TA, LaTessa KO, Iyengar R, Chai W, Agrios K (1996)
Stereoselective syntheses of substituted pterocarpans with anti-
HIV activity, and 5-aza-/5-thia-pterocarpan and 2-aryl-2,3-
dihydrobenzofuran analogs. Bioorg Med Chem 4:1755–1769.
doi:10.1016/0968-0896(96)00192-7
Gaggi R, Giovanninetti G, Garuti L (1982) b-Adrenolytic activity of
some naphtho[2,1-b]furan derivatives. Farm Ed Sci 37:149–150
Giovanninetti G, Cavrini V, Chiarini A, Garuti L, Mannini-Palenzona A
(1974) Substances with antiviral activity. I. Naphtho[2,1-b]furan-
1(2H)-one, 1H-benz[e]indene-2,3-dihydro-1-one, and 1H-ben-
z[e]indene-1,3(2H)-dione derivatives. Farm Ed Sci 29:375–385
Habonimana P, Claessens S, De Kimpe N (2006) Efficient syntheses
of mollugin. Synlett 2472–2475. doi:10.1055/s-2006-950441
Ho L-K, Don M-J, Chen H-C, Yeh S-F, Chen J-M (1996) Inhibition
of hepatitis B surface antigen secretion on human hepatoma
Lee YR, Kang KY, Lee GJ, Lee WK (2003) Efficient synthesis of
dihydrofurans with sulfide groups by ceric(IV) ammonium
nitrate-mediated oxidative cycloaddition of 1,3-dicarbonyl com-
pounds to vinyl sulfides. Application to the synthesis of
benzo[b]naphtho[2,3-d]furan-6,11-dione and first total synthesis
of millettocalyxins C and pongamol methyl ether. Synthesis
1977–1988. doi:10.1055/s-2003-41023
Lumb J-P, Choong KC, Trauner D (2008) ortho-Quinone Methides
from para-quinones: total Synthesis of Rubioncolin B. J Am
Chem Soc 130:9230–9231. doi:10.1021/ja803498r
Mahadevan KM, Vaidya VP (2003) Synthesis and pharmacological
evaluation of some potent naphtho[2,1-b]furopyrazolyl, -ox-
adiazolyl, and -coumaryl derivatives. Indian J Pharm Sci
65:128–134
Mahadevan KM, Padmashali B, Vaidya VP (2001) Studies in
naphthofurans: Part V—synthesis of 2-aryl-1,2,3,4-tetrahydro-
pyrido(naphtho[2,1-b]furan)-4-ones and their biological activity.
Indian J Heterocycl Chem 11:15–20
Marcocci L, Maguire JJ, Droy-Lefaix MT, Packer L (1994) The nitric
oxide-scavenging properties of Ginkgo biloba extract EGb 761.
Biochem Biophys Res Commun 201:748–755. doi:10.1006/bbrc.
1994.1764
Matthews JM, Chen X, Cryan E, Hlasta DJ, Rybczynski PJ, Strauss
K, Tang Y, Xu JZ, Yang M, Zhou L, Demarest KT (2007)
Design and synthesis of indane-ureido-thioisobutyric acids: a
novel class of PPARa agonists. Bioorg Med Chem Lett
17:6773–6778. doi:10.1016/j.bmcl.2007.10.041
Nair V, Menon RS, Vinod AU, Viji S (2002) A facile three-
component reaction involving [4 ? 1] cycloaddition leading to
furan annulated heterocycles. Tetrahedron Lett 43:2293–2295.
doi:10.1016/s0040-4039(02)00226-5
cells. Components from Rubia cordifolia.
59:330–333. doi:10.1021/np960200h
J Nat Prod
Hranjec M, Grdisa M, Pavelic K, Boykin DW, Karminski-Zamola G
(2003) Synthesis and antitumor evaluation of some new
substituted amidino-benzimidazolyl-furyl-phenyl-acrylates and
naphtho[2,1-b]furan-carboxylates. Farmaco 58:1319–1324.
doi:10.1016/s0014-827x(03)00197-6
Inoue K, Shiobara Y, Nayeshiro H, Inouye H, Wilson G, Zenk MH
(1984) Quinones and related compounds in higher plants. Part
19. Biosynthesis of anthraquinones and related compounds in
Galium mollugo cell suspension cultures. Phytochemistry
23:307–311. doi:10.1016/s0031-9422(00)80323-4
123

Med Chem Res
NCCLS (National Committee for Clinical Laboratory Standards)
(1997) Performance standards for antimicrobial disk suscepti-
bility tests. Approved standard M2-A6. National Committee for
Clinical Laboratory Standards, Wayne, PA
Otani T, Sugimoto Y, Aoyagi Y, Igarashi Y, Furumai T, Saito N,
Yamada Y, Asao T, Oki T (2000) New Cdc25B tyrosine
phosphatase inhibitors, nocardiones A and B, produced by
Nocardia sp. TP-A0248: taxonomy, fermentation, isolation,
structural elucidation and biological properties. J Antibiot
53:337–344. doi:10.7164/antibiotics.53.337
Pfaller MA, Burmeister L, Bartlett MS, Rinaldi MG (1988) Multi-
center evaluation of four methods of yeast inoculum preparation.
J Clin Microbiol 26:1437–1441
Qian X, Zhang Y, Ni C (1994) The spectrum and antibacterial activity
of naphthofurans and furans. J East China Inst Chem Technol
20:336–341
Schildknecht H, Straub F (1976) Structure and synthesis of
furomollugin from rhizomes of Galium mollugo L. (Rubiaceae).
Justus Liebigs Ann Chem 1976:1772–1776
Shashikala DK, Ramaiah M, Vanita GK, Veena K, Vaidya VP (2011)
Synthesis and analgesic activity of triazolothiadiazoles and
triazolothiadiazines encompassing 3-nitronaphtho[2,1-b]furan.
J Chem Pharm Res 3:445–451
Son JK, Jung SJ, Jung JH, Fang Z, Lee CS, Seo CS, Moon DC, Min
BS, Kim MR, Woo MH (2008) Anticancer constituents from the
roots of Rubia cordifolia L. Chem Pharm Bull 56:213–216.
doi:10.1248/cpb.56.213
Srivastava V, Negi AS, Kumar JK, Faridi U, Sisodia BS, Darokar MP,
Luqman S, Khanuja SPS (2006) Synthesis of 1-(30,40,50-
trimethoxy)phenylnaphtho[2,1-b]furan as a novel anticancer
agent. Bioorg Med Chem Lett 16:911–914. doi:10.1016/j.bmcl.
2005.10.105
´
´
Ronto G, Grof P, Buisson J-P, Einhorn J, Demerseman P (1992)
Genotoxic effectivity. Comparison of 36 nitrated furan and
arenofuran derivatives on a quantitative scale. Statistical com-
parison of T7 and other short-term tests. Mutagenesis 7:243–249.
doi:10.1093/mutage/7.4.243
Velmurugan D, Selvi UM, Mythily U, Rao K, Rajarajeshwari R,
Sangeetha (2012) Structure-based discovery of anti-viral compounds
for hepatitis B & C, human immunodeficiency, and dengue viruses.
Curr Bioinform 7:187–211. doi:10.2174/157489312800604462
Wu L, Wang S, Hua H, Li X, Zhu t, Miyase T, Ueno A (1991)
6-Methoxygeniposidic acid, and iridoid glycoside from Rubia
cordifolia. Phytochemistry 30:1710–1711. doi:10.1016/0031-
9422(91)84241-j
Xia L, Lee YR (2013) A novel and efficient synthesis of diverse
dihydronaphtho[1,2-b]furans using the ceric ammonium nitrate-
catalyzed formal [3 ? 2] cycloaddition of 1,4-naphthoquinones
to olefins and its application to furomollugin. Org Biomol Chem
11:6097–6107. doi:10.1039/c3ob40977e
Ruch RJ, Cheng S, Klaunig JE (1989) Prevention of cytotoxicity and
inhibition of intercellular communication by antioxidant cate-
chins isolated from Chinese green tea. Carcinogenesis (Lond)
10:1003–1008. doi:10.1093/carcin/10.6.1003
Sastry MNV, Claessens S, Habonimana P, De Kimpe N (2010)
Synthesis of the natural products 3-hydroxymollugin and
3-methoxymollugin. J Org Chem 75:2274–2280. doi:10.1021/
jo100024b
123