Synthesis, anticancer activity, and iron affinity of the Actinoplanes metabolite 7,8-dihydroxy-1-methylnaphtho[2,3-c]furan-4,9-dione

Article abstract of DOI:10.1016/j.bmc.2010.12.012

The first synthesis of 7,8-dihydroxy-1-methylnaphtho[2,3-c]furan-4,9-dione (1), an isofuranonaphthoquinone produced by an Actinoplanes strain is described. Lactone ring opening of 6-methylfuro[3,4-c]furan-1(3H)-one (4) with ortho-lithiated veratrole (3),

Full text of DOI:10.1016/j.bmc.2010.12.012

Bioorganic & Medicinal Chemistry 19 (2011) 1264–1267
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis, anticancer activity, and iron affinity of the Actinoplanes
metabolite 7,8-dihydroxy-1-methylnaphtho[2,3-c]furan-4,9-dione
⇑
Sandra Breyer, Katharina Effenberger-Neidnicht, Sebastian Knauer, Rainer Schobert
Organic Chemistry Laboratory, University Bayreuth, Universitaetsstrasse 30, 95440 Bayreuth, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The first synthesis of 7,8-dihydroxy-1-methylnaphtho[2,3-c]furan-4,9-dione (1), an isofuranonaphtho-
quinone produced by an Actinoplanes strain is described. Lactone ring opening of 6-methylfuro
[3,4-c]furan-1(3H)-one (4) with ortho-lithiated veratrole (3), oxidation of product alcohol 5, and
Friedel–Crafts acylation of the resulting aroylcarboxylic acid 7 afforded the mono methyl ether 2 of
the target compound. The latter was obtained by demethylation of 2 with BBr3 in 14% overall yield. While
mono ether 2 was distinctly more cytotoxic than catechol 1 against a panel of five cancer cell lines, only
the latter showed a siderophore-like binding affinity for Fe(III) with a complex dissociation constant KD of
approximately 10–29 M3 (pM = 25.9).
Received 6 October 2010
Revised 1 December 2010
Accepted 4 December 2010
Available online 9 December 2010
Keywords:
Isofuranonaphthoquinone
Ortho-lithiation
Anticancer activity
Siderophore
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
aldehyde 6 using pyridinium dichromate (PDC) and further on to
acid 7 with sodium chlorite (Scheme 1).
Isofuranonaphthoquinones represent but a small group among
the naturally occurring para-quinones.1 Most of them were isolated
as secondary metabolites from herbal or fungal sources2–6 and were
found to have moderate antioxidative,1 antimicrobial,7,8 or antiplas-
modial activity.1 7,8-Dihydroxy-1-methylnaphtho[2,3-c]furan-4,
9-dione (1) was the first example to be isolated from a bacterial
source, an Actinoplanes strain, by Moore and co-workers in 2009
(Fig. 1).2 While exhibiting no antibacterial activity against Bacillus
subtilis or Escherichia coli it showed an ability to complex Fe(III)
and was believed to act as a siderophore and to be utilized by this
Actinoplanes isolate to sequester iron for growth support.2 Herein,
we report on the first synthesis of 1 via its mono methyl ether 2
and on the cytotoxicities and iron affinities of these two compounds.
The carboxylic acid 7 was then converted to its chloride 8 with
oxalyl chloride. The crude chloride 8 was cyclized via an intramolec-
ular Friedel–Crafts acylation initiated by an excess of aluminum
chloride at room temperature. This reaction proceeded with
concomitant loss of the methyl group on the ortho-oxygen and affor-
ded the quinone 2 in overall 15% yield. Demethylation of 2 was
finally achieved by reaction with boron tribromide (Scheme 2).
Compounds 1 and 2 had to be freed of traces of chelated metals by
extraction with aqueous Na2EDTA solution.
2.2. Biological evaluation
2.2.1. MTT-tests
The quinones 1 and 2 were tested for cytotoxicity against cells
of human HL-60 leukemia, 518A2 melanoma, HT-29 colon carci-
noma, and the multidrug-resistant MCF-7/Top breast and KB-V1/
Vbl cervix carcinomas, as well as non-malignant human foreskin
fibroblasts (HF) (Table 1). On average, the growth inhibitory effects
2. Results and discussion
2.1. Chemistry
Our synthesis of 1 drew on veratrole (3) and 6-methylfuro
[3,4-c]furan-1(3H)-one (4) as building blocks. The butyrolactone
4 was accessible by Ag-mediated ring closure of prop-2-ynyl
2-chloro-3-oxobutanoate according to a literature procedure.9–11
Opening of the lactone ring of 4 by ortho-lithiated veratrole led
to the primary alcohol 5, which was oxidized to the corresponding
⇑
Corresponding author. Fax: +49 921 552671.
E-mail address: Rainer.Schobert@uni-bayreuth.de (R. Schobert).
Figure 1. Molecular structures of 1 and 2.
0968-0896/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2010.12.012

S. Breyer et al. / Bioorg. Med. Chem. 19 (2011) 1264–1267
1265
Table 1
Inhibitory concentrations IC50 (72 h)a in
l
M of 1 and 2 and TQ when applied to cancer
and non-malignant cells
HL-60
HF
518A2
KB-V1/Vbl HT-29
MCF-7/Top
70 11 >100
31 15 16
1
2
TQ
89 11
69
31
33 10
6
6
84 16
>100
86 10
51 12
18
28
8
6
6
6
6
28
9
32
6
47 10
13
27
32
CDPP 6.2 2.7 91 13 1.2 0.4 8.4 2.2
3
a
Values are derived from concentration–response curves obtained by measuring
the percentual absorbance of viable cells relative to untreated controls (100%) after
72 h exposure of the cells to the test compounds in the MTT assay. Values represent
means of four independent experiments standard deviation.
Scheme 1. Reagents and conditions: (a) n-BuLi, TMEDA, Et2O, 0 °C?rt, 16 h, 55%;
(b) PDC, CH2Cl2, rt, 16 h, 42%; (c) NaClO2, NaH2PO4 ꢀ 2H2O, 2-methyl-2-butene,
t-BuOH, H2O, rt, 16 h, 93%.
of mono ether 2 were more pronounced than those of the natural
catechol 1. In the resistant cell line MCF-7/Top quinone 2 even
surpassed the activity of the clinically established anticancer drug
cisplatin (CDDP) and of thymoquinone (TQ), a para-benzoquinone
occurring in the seed oil of Nigella sativa that showed antitumor
effects in various in vitro and animal studies.12–14 Thus quinone
2 might be a promising candidate for further preclinical studies.
Whether these efficacy differences between the two test com-
pounds are a consequence of metal complexation or of a lower
cellular concentration of 1 when compared with 2 remains to be
shown.
2.2.2. Determination of Fe(III) complexation
The natural catechol 1 was analyzed for its ability to complex
Fe(III). Figure 2 (top) shows its UV–vis spectrum in DMSO as well
as a spectrum obtained upon addition of ferric chloride in 4:1
MeOH/0.1 M aqueous NaOAc buffer solution (pH 7.4). In the lat-
ter case, the low energy transition band of 1 at ca. 400 nm was
red shifted by more than 50 nm due to an increased conjugative
effect when chelate complexes are formed and to a different
electronic state of the formal catecholate dianion.15 The dissocia-
tion constant KD of the presumed octahedral complex Fe(III)(1)3
was determined from these spectra to be 1.39 0.26 x 10–29 M3
(pM = 25.9) by competition experiments with the good chelate
ligand EDTA. This dissociation constant lies in the range of that
of the complex Fe(III)(EDTA) (ca. 5 x 10–23 M; pM = 25.1). Hence
it is likely that catechol 1 is transported, distributed and stored
in a biological context mostly in form of its metal chelate com-
plexes. The UV–vis spectrum of the methyl ether 2 was not suf-
ficiently different from that recorded in the presence of ferric
chloride to allow the calculation of a KD value of a potential iron
Figure 2. UV–vis spectra of compounds 1 (top) and 2 (bottom) in the absence (—)
and in the presence (– –) of ferric chloride.
complex (Fig. 2; bottom). From systematic studies with flavo-
noids featuring a catecholic site and a b-hydroxyketone one
knows that the former is more influential for iron complexation
at the physiological pH of 7.4 than the latter.16,17 Thus compound
2 has presumably formed very little Fe(III) complex under these
conditions.
3. Experimental section
3.1. General methods
Melting points were recorded on a Büchi M-565 apparatus and
are uncorrected. IR spectra were recorded on a Perkin–Elmer
Spectrum One FT-IR spectrophotometer equipped with an ATR
sampling unit. Nuclear magnetic resonance (NMR) spectra were
recorded under conditions as indicated on a Bruker Avance 300
spectrometer. Chemical shifts are given in parts per million (d)
downfield from tetramethylsilane as internal standard for 1H and
13C. Mass spectra were recorded using a Varian MAT 311A (EI). Ele-
mental analyses were carried out on a Carlo Erba Strumentazione
Scheme 2. Reagents and conditions: (a) (COCl)2, rt, 16 h; (b) AlCl3, CH2Cl2, rt, 16 h,
67% (with respect to 7); (c) BBr3, CH2Cl2, ꢁ78 °C?rt, 16 h, 94%.

1266
S. Breyer et al. / Bioorg. Med. Chem. 19 (2011) 1264–1267
Elemental Analyzer Model 1106. For column chromatography
Merck silica gel 60 (230–400 mesh) was used. Solvents were dried
and distilled (diethyl ether over Na/K, TMEDA over Na and CH2Cl2
over CaH2) and stored under argon. All starting compounds were
purchased from the usual sources and used without further
purification.
1719, 1598, 1578, 1447, 1320, 1265, 1230, 1127, 998, 842, 752;
dH (300 MHz; CDCl3) 1.94 (3H, s, CH3), 3.79 (3H, s, OCH3), 3.89
(3H, s, OCH3), 6.83 (1H, dd, 3J = 7.5 Hz, 4J = 1.6 Hz, Har), 7.07 (1H,
dd, 3J = 8.3 Hz, 4J = 1.6 Hz, Har), 7.15 (1H, dd, 3J = 7.5 Hz,
3J = 8.3 Hz, Har), 8.10 (1H, s, CH); dC (75 MHz; CDCl3) 14.6, 55.9,
61.8, 115.3, 118.7, 119.5, 119.8, 124.9, 133.5, 145.9, 150.1, 152.9,
162.1, 165.3, 195.4; m/z (EI, 70 eV) 289 (M+ꢁ1, 86%), 240 (41%),
187 (17%), 165 (100%), 122 (34%), 107 (44%), 43 (32%). Anal. Calcd
for C15H14O6: C, 62.1; H, 4.9. Found: C, 61.8; H, 4.6.
3.2. Syntheses
3.2.1. 4-Hydroxymethyl-2-methyl-3-(2,3-dimethoxybenzoyl)-
furan (5)
3.2.4. 8-Hydroxy-7-methoxy-1-methylnaphtho[2,3-c]furan-4,9-
dione (2)
A solution of veratrole (3) (1.00 g, 7.24 mmol) and TMEDA
(1.09 mL, 7.24 mmol) in diethyl ether (50 mL) was cooled to 0 °C
and treated dropwise with n-BuLi (2.89 mL, 7.24 mmol, 2.5 M in
hexane). Stirring was continued at room temperature for 1 h. The
mixture was cooled to 0 °C and 4 (1.00 g, 7.24 mmol) was added.
After stirring at room temperature overnight it was quenched with
a saturated aqueous solution of NH4Cl and extracted with diethyl
ether. The combined organic layers were washed with 1 M HCl
and brine, dried over anhydrous Na2SO4 and evaporated to dryness.
The residue was purified by column chromatography (silica gel 60,
cyclohexane/ethyl acetate 6:1) to yield 5 (1.10 g, 3.98 mmol, 55%)
A solution of acid 7 (139 mg, 0.45 mmol) in oxalyl chloride
(3.84 mL, 44.8 mmol) was stirred at room temperature overnight
and then evaporated to dryness. The residue was redissolved in
CH2Cl2 (20 mL) and treated with AlCl3 (300 mg, 2.25 mmol). The
mixture was stirred at room temperature overnight and then
quenched with ice. It was diluted with 1 M HCl and extracted with
ethyl acetate. The combined organic layers were washed with
brine and dried over anhydrous Na2SO4. The crude product was
purified by column chromatography (Silica Gel 60, cyclohexane/
ethyl acetate 4:1), dissolved in ethyl acetate (30 mL) and extracted
with 0.05 M aqueous solution of Na2EDTA to yield 97 mg
(0.37 mmol, 83%) of a yellow solid. Rf 0.52 (cyclohexane/ethyl ace-
as
m
a colorless oil; Rf 0.35 (cyclohexane/ethyl acetate 1:1);
max/cmꢁ1 3442, 2938, 1634, 1578, 1475, 1425, 1266, 1001, 754;
dH (300 MHz; CDCl3) 1.91 (3H, s, CH3), 3.77 (3H, s, OCH3), 3.89
(3H, s, OCH3), 4.54 (2H, s, CH2), 6.84 (1H, dd, 3J = 7.5 Hz,
4J = 1.7 Hz, Har), 7.02 (1H, dd, 3J = 8.3 Hz, 4J = 1.7 Hz, Har), 7.12
(1H, dd, 3J = 7.5 Hz, 3J = 8.3 Hz, Har), 7.26 (1H, s, CH); dC (75 MHz;
CDCl3) 14.2, 55.5, 55.9, 61.9, 114.5, 119.4, 122.8, 124.5, 125.9,
135.6, 138.6, 146.3, 152.9, 161.9, 193.0; m/z (EI, 70 eV) 276 (M+,
46%), 245 (100%), 215 (15%), 165 (52%), 137 (65%), 77 (18%), 43
(29%). Anal. Calcd for C15H16O5: C, 65.2; H, 5.8. Found: C, 65.1; H,
5.6.
tate 1:1); mp 192 °C (decomp.); m
max/cmꢁ1 3443, 2924, 2850, 1670,
1560, 1456, 1309, 1274, 1254, 1223, 1051, 786; dH (300 MHz;
CDCl3) 2.74 (3H, s, CH3), 3.98 (3H, s, OCH3), 7.10 (1H, d,
3J = 8.5 Hz, Har), 7.78 (1H, d, 3J = 8.5 Hz, Har), 7.97 (1H, s, CH),
13.22 (1H, s, OH); dC (75 MHz; CDCl3) 14.0, 56.4, 115.3, 117.0,
117.9, 120.7, 123.6, 127.6, 143.3, 153.3, 153.9, 160.5, 178.3,
187.0; m/z (EI, 70 eV) 258 (M+, 100%), 229 (51%), 212 (13%), 184
(5%), 131 (6%), 103 (6%), 79 (11%), 43 (23%). Anal. Calcd for
C14H10O5: C, 65.1; H, 3.9. Found: C, 64.9; H, 3.8.
3.2.2. 3-Formyl-4-(2,3-dimethoxybenzoyl)-5-methylfuran (6)
Alcohol 5 (1.27 g, 4.59 mmol) and PDC (3.46 g, 9.19 mmol) were
dissolved in CH2Cl2 (30 mL) and stirred at room temperature over-
night. The solution was diluted with diethyl ether, filtered over a
pad of FloresilÒ and silica gel and evaporated. The residue was
purified by column chromatography (silica gel 60, cyclohexane/
ethyl acetate 4:1). Yield: 530 mg (1.93 mmol, 42%) of a colorless
3.2.5. 7,8-Dihydroxy-1-methylnaphtho[2,3-c]furan-4,9-dione (1)
A solution of methyl ether 2 (40 mg, 0.16 mmol) in CH2Cl2
(15 mL) was cooled to ꢁ78 °C, treated dropwise with BBr3
(0.46 mL, 0.46 mmol, 1 M in CH2Cl2), and was then allowed to
warm to room temperature overnight. Water was slowly added
and the mixture was extracted with ethyl acetate. The combined
organic layers were extracted with 1 M NaOH. The resulting aque-
ous layers were acidified to pH 1 with concd HCl and extracted
with CH2Cl2. The combined extracts were washed with a 0.05 M
aqueous solution of Na2EDTA, dried over anhydrous Na2SO4 and
evaporated to dryness to yield 35 mg (0.14 mmol, 89%) of an
amorphous yellow solid. Rf 0.42 (cyclohexane/ethyl acetate 1:1);
oil; Rf 0.52 (cyclohexane/ethyl acetate 1:1);
m
max/cmꢁ1 2939,
1683, 1649, 1547, 1475, 1424, 1311, 1264, 1078, 999, 751; dH
(300 MHz; CDCl3) 2.07 (3H, s, CH3), 3.67 (3H, s, OCH3), 3.79 (3H,
s, OCH3), 6.87 (1H, dd, 3J = 7.3 Hz, 4J = 1.9 Hz, Har), 6.97 (1H, dd,
3J = 8.2 Hz, J = 1.9 Hz, Har), 7.02 (1H, dd, 3J = 7.3 Hz, 3J = 8.2 Hz,
Har), 7.82 (1H, s, CH), 9.90 (1H, s, CHO); dC (75 MHz; CDCl3) 13.3,
55.7, 61.5, 115.0, 119.7, 120.9, 124.2, 126.9, 134.8, 140.2, 145.9,
152.7, 160.1, 186.1, 190.2; m/z (EI, 70 eV) 274 (M+, 96%), 243
(100%), 199 (17%), 165 (58%), 137 (34%), 107 (38%), 77 (50%), 43
(62%). Anal. Calcd for C15H14O5: C, 65.7; H, 5.2. Found: C, 65.6; H,
5.2.
m
max/cmꢁ1 3392, 2926, 2855, 1737, 1668, 1633, 1603, 1567, 1453,
1302, 1255, 1107, 1067, 1014, 805; dH (300 MHz; DMSO-d6) 2.73
(3H, s, CH3), 7.20 (1H, d, 3J = 8.3 Hz, Har), 7.62 (1H, d, 3J = 8.3 Hz,
Har), 8.54 (1H, s, CH), 10.78 (1H, s, OH), 12.92 (1H, s, OH); dC
(125.75 MHz; DMSO-d6) 13.8, 116.4, 118.1, 120.4, 120.7, 122.9,
126.2, 144.7, 151.4, 152.6, 160.4, 177.5, 186.9; m/z (EI, 70 eV)
244 (M+, 100%), 215 (34%), 187 (10%), 160 (18%), 131 (14%), 108
(100%), 103 (100%), 77 (22%), 63 (11%), 51 (20%), 43 (32%).
3.2.3. 4-(2,3-Dimethoxybenzoyl)-5-methylfuran-3-carboxylic
acid (7)
Aldehyde 6 (530 mg, 1.93 mmol) and 2-methyl-2-butene (7 mL)
were dissolved in tert-Butanol (50 mL) and H2O (10 mL) and
NaClO2 (1.64 g, 18.16 mmol) and NaH2PO4 ꢀ 2H2O (2.10 g,
13.51 mmol) were added. The mixture was stirred at room temper-
ature overnight and then all volatiles were evaporated. The
remainder was dissolved in ethyl acetate (100 mL) and washed
with H2O and brine. The organic phase was dried over anhydrous
Na2SO4 and the solvent was removed in vacuo. The crude product
was crystallized from ethyl acetate/pentane 1:9 at ꢁ20 °C to yield
3.3. Cell lines and culture conditions
The human HL-60 leukemia cells were obtained from the Ger-
man Resource Center for Biological Material (DSMZ), Braun-
schweig, Germany, the human 518A2 melanoma cells were a gift
from the Department of Oncology and Hematology of the Martin-
Luther-University Halle-Wittenberg, Germany, the human KB-V1/
Vbl cervix and MCF-7/Top breast carcinoma cells were obtained
from the Institute of Pharmacy of the University of Regensburg,
Germany, and the human colon carcinoma cells HT-29 as well as
the non-malignant foreskin fibroblasts HF were obtained from
521 mg (1.79 mmol, 93%) of
a
bright yellow solid; Rf 0.10
max/cmꢁ1 2583,
(cyclohexane/ethyl acetate 1:1); mp 133 °C;
m

S. Breyer et al. / Bioorg. Med. Chem. 19 (2011) 1264–1267
1267
the University Hospital Erlangen, Germany. HL-60 and HT-29 cells
were incubated in RPMI (Roswell Park Memorial Institute) Medium
1640 supplemented with 10% FCS (fetal calf serum), 100 IU/mL
the presence and absence of 200
l
M EDTA. At this wavelength,
absorption of the complex was at peak level, while that of the
Fe-EDTA complex was negligible. Absorption of the uncomplexed
compound was taken into account. The difference in absorbance
was then used to calculate the dissociation constant KD by the fol-
lowing equation:
penicillin G, 100
lg/mL streptomycin sulfate, 0.25
lg/mL ampho-
thericin B, and 250
lg/mL gentamycin (all from Gibco). 518A2,
HF, HT-29, and KB-V1/Vbl cells were cultured in DMEM
(Dulbecco’s modified Eagle medium; Gibco) containing 10% FCS,
3
KD ¼ KDðFe-EDTAÞ½Fe-EDTAꢂ½1ꢂ =½Fe-ð1Þ3ꢂ½EDTAꢂ
100 IU/mL penicillin G, 100
lg/mL streptomycin sulfate, 0.25
lg/
mL amphothericin B, and 250
l
g/mL gentamycin. MCF-7/Top cells
KD(Fe-EDTA) is 5 ꢀ 10ꢁ23 M, concentrations were determined
by using the measured extinction coefficient of quinone 1. Data
were representative of at least two independent experiments.19,20
were grown in EMEM (Eagle’s minimum essential medium; Sig-
ma–Aldrich) supplemented with 5% FCS, 110 mg/L sodium pyru-
vate (Gibco), and 2.2 g/L NaHCO3 (Merck). The cells were
maintained in a moisture-saturated atmosphere (5% CO2, 95%
humidity) at 37 °C.
Supplementary data
Supplementary data (1H NMR and 13C NMR spectra of com-
pounds 1 and 2) associated with this article can be found, in the
online version, at doi:10.1016/j.bmc.2010.12.012.
3.4. Inhibition of cancer cell growth (MTT assay)
MTT
[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazo-
lium hydrobromide; ABCR] was used to identify the metabolic
activity of vital cells which are capable of reducing it to a violet for-
mazan. HL-60 cells (5 ꢀ 105/mL) and cells (5 ꢀ 104/mL) of 518A2,
HF, HT-29, KB-V1/Vbl, and MCF-7/Top were seeded and cultured
for 24 h in 96-well microplates.18 Incubation of cells following
treatment with the test compounds was continued for 72 h. Blank
and solvent controls were treated identically. Then, MTT in phos-
phate-buffered saline (PBS) was added to a final concentration of
0.05% (HL-60, 518A2, HF) or 0.1% (HT-29, HT-29, KB-V1/Vbl,
MCF-7/Top). After 2 h, the precipitate of formazan was solved in di-
methyl sulfoxide (DMSO) containing 10% sodium dodecylsulfate
(SDS) and 0.6% acetic acid in the case of the suspension cells
HL-60. For the adherent cells (518A2, HF, HT-29, KB-V1/Vbl,
MCF-7/Top) the microplates were swiftly turned to discard the
medium before adding the solvent mixture. The microplates were
gently shaken in the dark for 30 min and absorbances at 570 nm
and 630 nm (background) were measured with a Tecan F-200 plate
reader (Germany). All experiments were carried out in quadrupli-
cate, and the percentage of the viable cells was calculated with Ori-
gin 8.1G as the mean standard deviation relative to the control
(100%).
References and notes
1. Bezabih, M.; Abegaz, B. M.; Dufall, K.; Croft, K.; Skinner-Adams, T.; Davis, T. M.
E. Planta Med. 2001, 67, 340.
2. Zhang, Q.; Peoples, A. J.; Rothfelder, M. T.; Millet, W. P.; Pescatore, B. C.; Ling, L.
L.; Moore, C. M. J. Nat. Prod. 2009, 72, 1213.
3. Abegaz, B. M. Phytochem. Rev. 2002, 1, 299.
4. Hanumaiah, T.; Rao, G. S. R.; Rao, C. P.; Rao, K. V. J.; Cowe, H. J.; Cox, P. J.; Howie,
R. A.; Marshall, D. S.; Thomson, R. H. Tetrahedron 1985, 41, 635.
5. Koyama, J.; Ogura, T.; Tagahara, K. Phytochemistry 1994, 37, 1147.
6. Yamamoto, Y.; Kinoshita, Y.; Thor, G. R.; Hasumi, M.; Kinoshita, K.; Koyama, K.;
Takahashi, K.; Yoshimura, I. Phytochemistry 2002, 60, 741.
7. Baker, R. A.; Tatum, J. H.; Nemec, S., Jr. Mycopathologia 1990, 111, 9.
8. Muleya, V.; Hayeshi, R.; Ranson, H.; Abegaz, B.; Bezabih, M.-T.; Robert, M.;
Ngadjui, B. T.; Ngandeu, F.; Mukanganyama, S. J. Enzyme Inhib. Med. Chem. 2008,
23, 391.
9. Mottet, C.; Hamelin, O.; Garavel, G.; Deprés, J.-P.; Greene, A. E. J. Org. Chem.
1999, 64, 1380.
10. Garanti, L.; Sala, A.; Zecchi, G. Synthesis 1975, 666.
11. Verniest, G.; Padwa, A. Org. Lett. 2008, 10, 4379.
12. Ivankovic, S.; Stojkovic, R.; Jukic, M.; Milos, M.; Milos, M.; Jurin, M. Exp. Oncol.
2006, 28, 220.
13. Dandawate, P. R.; Vyas, A. C.; Padhye, S. B.; Singh, M. W.; Baruah, J. B. Mini-Rev.
Med. Chem. 2010, 5, 436.
14. (a) Breyer, S.; Effenberger, K.; Schobert, R. ChemMedChem 2009, 4, 761; (b)
Effenberger, K.; Breyer, S.; Schobert, R. Chem. Biodiv. 2010, 7, 120; (c)
Effenberger, K.; Schobert, R. Cancer Chemother. Pharmacol. 2010. doi:10.1007/
s00280-010-1386-x.
15. Vaillancourt, F. H.; Barbosa, C. J.; Spiro, T. G.; Bolin, J. T.; Blades, M. W.; Turner,
R. F. B.; Eltis, L. D. J. Am. Chem. Soc. 2002, 124, 2485.
3.5. Determination of iron complexation
16. Engelmann, M. D.; Hutcheson, R.; Cheng, I. F. J. Agric. Food Chem. 2005, 53,
2953.
17. Bodoni, M. E.; Copia, G.; Tapia, R.; Leighton, F.; Herrera, L. Polyhedron 1999, 18,
2233.
The dissociation constant KD for the Fe(III) complex of 1 was
determined by competition with EDTA at room temperature using
a Shimadzu UV 160 A spectrophotometer. Stock solutions of ferric
chloride (10 mM) in pure methanol and of compound 1 (10 mM) in
DMSO were prepared and diluted to 150 lM and 500 lM, respec-
tively, with 4:1 MeOH/0.1 M NaOAc buffer solution (pH 7.4). The
18. Mosmann, T. J. Immunol. Methods 1983, 65, 55.
19. Kaufmann, G. F.; Sartorio, R.; Lee, S.-H.; Rogers, C. J.; Meijler, M. M.; Moss, J. A.;
Clapham, B.; Brogan, A. P.; Dickerson, T. J.; Janda, K. D. PNAS 2005, 102, 309.
20. Wang, J.; Buss, J. L.; Chen, G.; Ponka, P.; Pantopoulos, K. FEBS Lett. 2002, 529,
309.
absorbance of the formed complex was measured at 470 nm in