First synthesis of novel aminophenyl pyridinium-5-(hydroxybenzoyl)-hydrazonomethyl-2-oxothiazol-3-ide derivatives and evaluation of their anticancer activities

Article abstract of DOI:10.1248/cpb.c15-00441

The first total synthesis for large-scale production and anticancer activity of novel aminophenylpyridinium-5-(hydroxybenzoyl)hydrazonomethyl-2-oxothiazol-3-ide (PBHT) (1) and its derivatives are reported. The chemical structure of PBHT was unambiguously

Full text of DOI:10.1248/cpb.c15-00441

Vol. 63, No. 10
Chem. Pharm. Bull. 63, 843–847 (2015)
843
Note
First Synthesis of Novel Aminophenyl Pyridinium-5-(hydroxybenzoyl)-
hydrazonomethyl-2-oxothiazol-3-ide Derivatives and Evaluation of Their
Anticancer Activities
,a
Dongguk Min,a Moon Hi Han,b Seulki Lee,b and Mankil Jung*
b
a Department of Chemistry, Yonsei University; Seoul 120–749, Korea: and Proteogen, Inc.; 198–53 Hupyong-dong,
Chuncheon, Gangwon-do 200–957, Korea.
Received May 26, 2015; accepted July 7, 2015; advance publication released online August 1, 2015
The first total synthesis for large-scale production and anticancer activity of novel aminophenylpyridin-
ium-5-(hydroxybenzoyl)hydrazonomethyl-2-oxothiazol-3-ide (PBHT) (1) and its derivatives are reported. The
chemical structure of PBHT was unambiguously determined by utilization of the two-dimensional nuclear
Overhauser effect (NOE) technique. The anticancer activity against human colon adenocarcinoma (HCT15)
cells of all synthesized compounds was approximately four-fold greater than that of 5-fluorouracil, with IC50
values ranging from 10.1 to 14.2µM. The three structural determinants of hydroxybenzoyl, hydrazinylidene,
and pyridinium oxothiazole in the synthesized compounds could be indispensable for exhibiting anticancer
activity.
Key words convergent synthesis; oxothiazolide; anticancer activity; nuclear Overhauser effect
Anthracycline antibiotics, for example, daunomycin ex- 3h) afforded 4-(p-dimethyl aminophenyl)pyridine 5 in 85%
hibited anticancer activity via intercalation into the DNA yield by a known procedure.5) Heterocyclic chloro-substituted
double helix. The planar aromatic chromophore portion of the aldehyde 4 was prepared by phosphorus oxychloride (POCl3)
molecule intercalates between two base pairs of the DNA,1) in N,N-dimethylformamide (DMF) (95°C, 2h, and 112°C,
while the six-membered daunosamine sugar sits in the minor 20min) from readily available 2,4-thiazolidine-dione in 47%
groove.1) This process inhibits the progression of the enzyme yield by a known procedure.6,7)
topoisomerase II, which relaxes supercoils in DNA for tran-
Convergent and regiospecific coupling of compounds 4 and
scription.2) Doxorubicin stabilizes the topoisomerase II com- 5 in DMF (80°C) via Michael addition afforded compound
plex after it has broken the DNA chain for replication, pre- 3 in 64% yield as shown in Chart 2. In compound 5, the
venting the DNA double helix from being resealed and there- pyridine N site was more basic than the aniline N site and
by stopping the process of replication.3) It may also increase the lone pair electrons of the pyridine nitrogen attacked the
free radical production, hence contributing to its cytotoxicity.4) electrophilic C-4 site of the Michael acceptor 4, keeping the
We found a novel aminophenylpyridinium-5-(hydroxy- formyl group intact. Final hydrazonation of compound 3 with
benzoyl)hydrazonomethyl-2-oxothiazol-3-ide (PBHT) (1) and (2-hydroxybenzoyl)hydrazine 2 in ethanol (EtOH) (reflux)
its preliminary anti-cancer activity by using high-throughput gave the target hydrazinylidene PBHT (1) in 89% yield. These
screening of a library of aromatic molecules. Based on its procedures proved to be suitable for the large-scale produc-
efficient binding to cyclin-dependent kinase (Cdk) as the mode tion.
of action, along with its preliminary anticancer activity identi-
The E-configuration at the imine group and the s-cis form
fied by this laboratory, we propose PBHT (1) as a potential of the single bond between imine C-1′ and C-5 of the oxothia-
anticancer drug candidate. Synthesis and anti-cancer activity zolide part of compound 1 were unambiguously determined as
of PBHT (1) are unknown in the literatures. This paper reports shown in Chart 2 by utilization of the two dimensional nuclear
the first total synthesis for large-scale production and also the Overhauser effect (NOE) spectroscopy (NOESY)8,9) technique
first anti-cancer activity of PBHT and its derivatives. PBHT (Fig. 1). No NOE enhancement was observed between 2-H
(1) highly inhibited cancer cell growth in a dose-dependent (δ 8.94, d, J=6.8Hz) of the pyridine part and 1′-H (δ 8.21, s)
manner without any toxicity in the screening cells.
of the imine part, establishing that the configuration of the
imine was E. However, the NOE spectrum showed interaction
between 1′-H (δ 8.21, s) of the imine and N 3′-H (δ 12.14, s),
Results and Discussion
Chemistry A retrosynthetic analysis was performed to demonstrating that the N-2′ and N-3′ single bond was s-trans.
obtain a useful synthetic route as outlined in Chart 1. Target
Similarly, analogs 6 and 7 were successfully prepared from
molecule 1 was divided into two compound parts (2 and 3) compound 4 by the same procedures via intermediates 3a, b
and compound part 3 was further divided into compound in 61 and 56% yields, respectively (Chart 3).
parts 4 and 5.
Anticancer Activity In vitro anticancer activities of
Based on retrosynthetic analysis, PBHT (1) was success- benzoylhydrazinylidene phenylpyridinium oxothiazolides 1,
fully synthesized in four steps from pyridine in 23% overall 3, 6 and 7 were tested against the human cancer cell lines
yields as outlined in Chart 2. Thus, oxidative coupling of colon adenocarcinoma (HCT15) and gastric adenocarcinoma
dimethylaniline with pyridine in the presence of cyanuric (MKN74) using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-
chloride and aluminium chloride (AlCl3) (r.t., 1.5h and 100°C, tetrazolium bromide (MTT) colorimetric method. The results
*To whom correspondence should be addressed. e-mail: mjung@yonsei.ac.kr
© 2015 The Pharmaceutical Society of Japan

844
Chem. Pharm. Bull.
Vol. 63, No. 10 (2015)
Chart 1. Retrosynthesis of PBHT (1)
Chart 2. Total Synthesis of PBHT (1)
of these assays are summarized in Table 1. The drug standards that of Iressa®. The three structural determinants of hydroxy-
used for comparison were gefitinib10–14) (Iressa®) and 5-Fu.
benzoyl, hydrazinylidene and pyridinium oxothiazole in the
Under our conditions, all synthesized compounds (PBHT synthesized compounds could be indispensable for exhibiting
1, 6 and 7) exhibited good anticancer activity, even better anticancer activity. The drastic decrease of anticancer activity
than 5-Fu in both cell lines. The anticancer activities against of the compound 3 seemed to be due to the absence of hy-
HCT15 of compounds 1, 6 and 7 was approximately four droxybenzoyl determinant.
times greater than 5-Fu with IC50 values of 10.1, 13.1 and
14.2 µM, respectively. However, all compounds showed activity
Conclusion
that was comparable activity to that of Iressa®.
In conclusion, PBHT and its derivatives were synthesized
However, PBHT (1) failed to show notable activity at con- via a convergent and regiospecific synthesis for the first time
centrations below 50µM against MKN74. The N,N-dimethyl- from readily available N,N-dimethylaniline. The chemical
aniline function seems to interfere with the anticancer activ- structure of PBHT was unambiguously determined by utili-
ity in this cancer cell line (MKN74). For the MKN74 cancer zation of the two dimensional NOE technique. They showed
cell line, compounds 6 and 7 showed better activity than the potent anti-colon and anti-gastric cancer activity. It is also
reference 5-Fu with superior IC50 values of 11.1 and 33.1µM, noteworthy that the absence of the N,N-dimethylaniline func-
respectively, but displayed again a comparable activity with tionality seems to enhance dramatic increases in anticancer

Vol. 63, No. 10 (2015)
Chem. Pharm. Bull.
845
Fig. 1. NOESY Spectra of PBHT (1)
Chart 3. Synthesis of PBHT Analogs
activity against the MKN74 cancer cell line. The compounds (0.25mm thickness). Spots on the TLC plates were visualized
including PBHT deserve further evaluation as possible anti- using ultraviolet light (254nm), a cerium sulfate/ammonium
colon and anti-gastric cancer drug candidates.
dimolybdate/sulfuric acid solution followed by heating on a
hot plate. Flash column chromatography was performed with
Merck silica gel 60 (230–400 mesh) or purification of reaction
Experimental
Chemistry General Procedures Reaction solvents were mixture was recrystallized by diethyl ether in dichlorometh-
1
distilled from calcium hydride for dichloromethane and from ane. H- and 13C-NMR spectra were recorded on a Bruker
sodium metal and benzophenone for tetrahydrofuran. All other DRX 250 at 250MHz and 63MHz or 400 at 400MHz and
commercial reagents and solvents were used as received with- 100MHz, respectively. Proton chemical shifts are reported in
out further purification. The reactions were monitored and the ppm (δ) relative to internal tetramethylsilane (TMS, δ 0.00)
Rf values determined using analytical thin layer chromatog- or with the solvent reference relative to TMS employed as
raphy (TLC) with Merck silica gel 60, F-254 precoated plates the internal standard (CDCl3, δ 7.26ppm; dimethyl sulfoxide

846
Chem. Pharm. Bull.
Vol. 63, No. 10 (2015)
Table 1. Cytotoxic Activities of PBHT Analogs (1, 3, 6 and 7) against
Two Human Cancer Cells (µM)
of 2,4-thiazolidinedione (25g, 0.213mol, 1eq) in POCl3 (60mL,
0.640mol, 3eq) was cooled to 0°C with an ice bath. To this
suspension was added dropwise cool DMF (25mL, 0.320mol,
1.5eq) over 15min. The reaction mixture was heated to 90°C
for 2h then at 115°C for 20min. After 20min, the reaction
was cooled to 90°C and maintained for an additional hour.
After 1h the mixture was heated to 115°C for 15min. The hot
reaction mixture was poured into 1L of water with vigorously
stirring. After 10min the mixture is filtered. The aqueous
phase was extracted 5 times with ethyl ether (600mL) and
the organic layer was separated, dried, filtered and evaporated
under reduced pressure. The solid residue was dissolved in
a minimum volume of aq. sat. NaHCO3. The mixture was
carefully acidified with 6M HCl to pH=2, whereupon a pre-
cipitate formed after about 30min. Filtration yielded 16.3g
of pure product 4 as a yellow solid in 47% yield; mp 197°C
(lit.6,7) 219°C); IR (KBr) νmax cm−1 3450, 2980, 2860, 2672,
IC50 (µM) in cell linea)
Compounds
HCT15b)
MKN74c)
1
10.1
>50
3
>50
14.2
13.1
13.5
45.5
>50
11.1
33.3
14.7
>50
6
7
1
Iressa®
1684, 1646, 1563, 1457, 1384, 1233, 1173; H-NMR (250MHz
CDC13) δ: 9.79 (1H, s, CHO), 8.95 (1H, s, NH); 13C-NMR
(63MHz, DMSO) δ: 180.2, 180.1, 168.5, 132.5, 113.7; LC-MS
(ESI+) m/z 164.00 [M+H]+ (Calcd for C4H2ClNO2S. Found:
162.95). Anal. Calcd for C4H2ClNO2S: C, 29.45; H, 1.23; N,
5-Fu
a) Human cancer cell lines. b) HCT15 (human colon adenocarcinoma). c) MKN74
(human gastric adenocarcinoma).
(DMSO), δ 2.5ppm). Data are reported as follows: chemical 8.59%. Found: C, 29.49; H, 1.29; N, 8.44%.
shift {multiplicity [singlet (s), doublet (d), triplet (t), quartet
4-[4-(4-Dimethylamino)phenyl)pyridinium-1-yl]-5-for-
(q), and multiplet (m)], coupling constants [Hz], integration}. myl-2-oxothiazol-3-ide (3) A mixture of 4-chloro-5-for-
Carbon chemical shifts are reported in ppm (δ) relative to myl-2-thiazolinone 4 (5.0g, 30.6mmol, 1eq) and p-(N,N-
TMS with the respective solvent resonance as the internal dimethylphenyl)pyridine 5 (6.1g, 30.6mmol, 1eq) in 50mL
standard (CDCl3, δ 77.16ppm; DMSO, δ 39.52ppm). Infrared of DMF was heated at 80°C until a precipitate formed. The
(IR) spectra were recorded on a JASCO FT/IR-430 spectrom- precipitated solids were collected, washed with ether to give
eter. Data are reported in wave numbers (cm−1). Electrospray pure product 3 as a brown solid (6.4g, 64% yield); mp 230°C;
ionization (ESI)-LC-MS was recorded on a Waters ZQ 4000 IR (KBr) νmax cm−1 3437, 2928, 1585, 1540, 1495, 1384,
1
LC-MS spectrometer. Melting points (mp) were determined on 1315, 1220, 1146; H-NMR (250MHz DMSO) δ: 9.20 (1H, s,
a BIBBY Stuart Scientific melting point apparatus SMP3.
CHO), 9.04 (2H, d, J=6.7Hz), 8.34 (2H, d, J=7.0Hz), 8.13
4-p-(N,N-Dimethylaminophenyl)pyridine (5)5) A solu- (2H, d, J=9.0Hz), 6.89 (2H, d, J 8.9Hz), 3.11 (6H, s, NCH3);
tion of cyanuric chloride (22.8g, 0.124mol, 0.3eq) in pyridine 13C-NMR (100.6MHz, DMSO) δ: 176.8, 155.6, 153.6, 142.7,
(100mL, 1.24mol, 3eq) was heated at 70°C for 15min, then 130.4, 125.8, 119.8, 118.3, 112.4, 39.7; LC-MS (ESI+) m/z
cooled. To this solution was added PhNMe2 (52mL 0.413mol, 326.37 [M+H]+ (Calcd for C17H15N3O2S. Found: 325.09). Anal.
1eq) followed by AlCl3 (55g, 0.413mol, 1eq) at less than 30°C Calcd for C17H15N3O2S: C, 62.77; H, 4.62; N, 12.92%. Found:
and the mixture was stirred for 1.5h at room temp., heated at C, 62.89; H, 4.69; N, 12.94%.
100°C for 3h and cooled to room temp. The crude solids were
(E)-4-(4-(4-(Dimethylamino)phenyl)pyridin-1-ium-1-yl)-
added to 10% HCl (100mL) and dissolved at above 80°C. The 5-((2-(2-hydroxybenzoyl)hydrazono)methyl)-2-oxothiazol-
temperature of the acidic solution was decreased to warm and 3-ide (PBHT) (1) A solution of 4-(4-(4-(dimethylamino)-
the solution was filtered through Celite. The acidic filtrates phenyl)pyridinium-1-yl)-5-formyl-2-oxothiazol-3-ide 3 (6.0g,
were extracted with methylene chloride (1L). The aqueous 18.4mmol) and 2-hydroxybenzohydrazide 2 (2.8g, 18.4mmol)
layer was neutralized with sodium hydroxide (NaOH) and in EtOH (200mL) was refluxed for overnight. The solids
extracted with methylene chloride (1L×2). The organic layer were collected and washed with acetone to give product 1 as
was dried, filtered and evaporated under reduced pressure. The a brown-yellow solid (7.5g, 89% yield); mp 239°C; IR (KBr)
crude solids were dissolved with methylene chloride (500mL). νmax cm−1 3442, 2923, 1638, 1586, 1492, 1444, 1382, 1353,
1
The suspended solution was evaporated at 0°C under reduced 1315, 1218, 1172, 1139; H-NMR (250MHz DMSO) δ: 12.14
pressure. The precipitated solids were collected and washed (1H, s, NH), 11.46 (1H, s, OH), 8.94 (2H, d, J=6.8Hz), 8.36
with ether to give pure product 5 as pale pink solid (69.5g, (2H, d, J=7.0Hz), 8.21 (1H, s), 8.12 (2H, d, J=9.0Hz), 7.76
85% yield); mp 236°C (lit.5) 233–234°C); IR (KBr) νmax cm−1 (1H, d, J=7.8Hz), 7.39 (1H, t, J=7.5Hz), 6.89 (4H, m), 3.10
3436, 2923, 1608, 1592, 1562, 1535, 1493, 1446, 1365, 1293, (6H, s); 13C-NMR (100.6MHz, DMSO) δ: 174.8, 164.4, 159.6,
1229, 1176; 1H-NMR (250MHz CDC13) δ: 8.56 (2H, d, J 154.6, 153.5, 148.0, 142.7, 141.2, 133.7, 130.1, 127.6, 119.9,
5.6Hz), 7.60 (2H, d, J=8.8Hz), 7.51 (2H, d, J=6.0Hz), 6.80 118.7, 118.3, 117.4, 115.2, 112.4, 105.4, 39.6, 18.6; LC-MS
(2H, d, J=8.9Hz), 3.04 (6H, s); 13C-NMR (63MHz, CDCl3) δ: (ESI+) m/z 460.29 [M+H]+ (Calcd for C24H21N5O3S. Found:
140.1, 129.2, 120.7, 112.5, 40.2, 31.1; LC-MS (ESI+) m/z 199.18 459.14). Anal. Calcd for C24H21N5O3S: C, 62.75; H, 4.58; N,
[M+H]+ (Calcd for C13H14N2. Found: 198.12). Anal. Calcd for 15.25%. Found: C, 62.89; H, 4.59; N, 15.28%.
C13H14N2: C, 78.79; H, 7.07; N, 14.14%. Found: C, 78.83; H,
(E)-5-((2-(2-Hydroxybenzoyl)hydrazono)methyl)-2-
oxo-4-(pyridin-1-ium-1-yl)thiazol-3-ide (6) A mixture of
7.15; N, 14.07%.
2,4-Chloro-5-formyl-2-thiazolinone (4)6,7) A suspension 4-chloro-5-formyl-2-thiazolinone 4 (0.15g, 0.92mmol, 1eq)

Vol. 63, No. 10 (2015)
Chem. Pharm. Bull.
847
and pyridine (0.22mL, 2.75mmol, 3eq) in 2mL of acetone compounds were dissolved in DMSO and diluted with culture
was heated at 60°C until a precipitate formed. The solvents media. Cancer cells (1.0×104 cells/mL) were seeded onto each
were evaporated under reduced pressure. The residue was well of a 96-well plate with the respective media and incu-
added to 2-hydroxybenzohydrazide 2 (0.14g, 0.92mmol) in bated to adhere overnight. The cells were then treated with
EtOH (3mL). And the mixture was refluxed for overnight. various concentrations of each newly synthesized compound
The solids were collected and washed with acetone to give in serum-free medium for 24h. The MTT solution (20µL,
product 6 as a brown-yellow solid (190mg, 61% yield); mp 5mg/mL) was added to each well, and the cells were incu-
236°C; IR (KBr) νmax cm−1 3435, 3050, 1684, 1546, 1491, bated for 4h at 37°C. The medium was then removed, and
1467, 1351, 1308, 1221, 1171, 1107; 1H-NMR (250MHz 200µL of DMSO was added to each well. The absorbance
DMSO) δ: 12.02 (1H, s, NH), 11.83 (1H, brs, OH), 9.46 (2H, was determined at 570nm using a microplate reader (Bio-Rad
d, J=7.5Hz), 8.91 (1H, t, J=7.5Hz), 8.41 (3H, m), 7.90 (1H, Laboratories, Hercules, CA, U.S.A.).
d, J=10.0Hz), 7.41 (1H, t, J=7.5Hz), 7.01 (1H, d, J=7.5Hz),
6.90 (1H, t, J=7.5Hz); 13C-NMR (100.6MHz, DMSO) δ:
Acknowledgment This study was supported by a Grant
168.5, 164.3, 158.8, 149.1, 146.0, 138.5, 133.9, 128.8, 128.3, of the Korean Health Technology R&D Project, the Ministry
119.0, 117.3, 115.7, 111.2; LC-MS (ESI+) m/z 341.27 [M+H]+ of Health and Welfare, the Republic of Korea (Project No.
(Calcd for C16H12N4O3S. Found: 340.06). Anal. Calcd for A121443).
C16H12N4O3S: C, 56.47; H, 3.53; N, 16.37%. Found: C, 56.59;
H, 3.59; N, 16.44%.
Conflict of Interest The authors declare no conflict of
(E)-4-(4-(Dimethylamino)pyridin-1-ium-1-yl)-5-((2-(2- interest.
hydroxybenzoyl)hydrazono)methyl)-2-oxothiazol-3-ide (7)
A mixture of 4-chloro-5-formyl-2-thiazolinone 4 (0.15g,
0.92mmol, 1eq) and 4-(dimethylamino)pyridine (DMAP)
(0.34g, 2.75mmol, 3eq) in 2mL of acetone was heated at
60°C until a precipitate formed. The solvents were evaporated
under reduced pressure. The residue was added to 2-hydroxy-
benzohydrazide 2 (0.14g, 0.92mmol) in EtOH (3mL). And the
mixture was refluxed for overnight. The solids were collected
and washed with acetone to give product 7 as a brown-yellow
solid (200mg, 56% yield); mp 231°C; IR (KBr) νmax cm−1
3435, 2928, 1644, 1577, 1490, 1358, 1217, 1146; 1H-NMR
(250MHz DMSO) δ: 12.23 (1H, brs, OH), 11.42 (1H, s, NH),
8.49 (2H, d, J=7.5Hz), 8.14 (1H, s), 7.78 (1H, d, J=7.5Hz),
7.40 (1H, t, J=7.5Hz), 7.11 (2H, d, J=7.5Hz), 6.89 (2H, m),
3.28 (6H, s); 13C-NMR (100.6MHz, DMSO) δ: 175.1, 164.4,
159.8, 156.2, 148.6, 141.5, 141.4, 133.6, 127.5, 118.6, 117.4,
115.1, 107.2, 103.9, 40.1; LC-MS (ESI+) m/z 384.18 [M+H]+
(Calcd for C18H17N5O3S. Found: 383.11). Anal. Calcd for
C18H17N5O3S: C, 56.40; H, 4.44; N, 18.28%. Found: C, 56.49;
H, 4.57; N, 18.34%.
Biology Materials Dulbecco’s modified Eagle’s medium
(DMEM), DMEM/F12, fetal bovine serum (FBS), RPMI1640
were purchased from Gibco BRL (Rockville, MD, U.S.A.).
The MTT, DMSO, cholera toxin, hydrocortisone, insulin,
transferrin, and triiodothyronine (T3) were obtained from
Sigma-Aldrich Chemical (St. Louis, MO, U.S.A.).
Cell Viability Assay The viability of cancer cells was
determined via the MTT assay. HCT15 and MKN74 human
cancer cells were cultured in DMEM supplemented with 10%
FBS in a humidified atmosphere of 5% CO2 at 37°C. Cancer
cells were cultured in DMEM/F12 (3:1) supplemented with
10% FBS, 1×10−10 M cholera toxin, 0.4mg/mL hydrocortisone,
5µg/mL insulin, 5µg/mL transferrin or 2×10−11 M T3. The
References
1) Frederick C. A., Williams L. D., Ughetto G., Van der Marel G. A.,
Van Boom J. H., Rich A., Wang A. H. J., Biochemistry, 29, 2538–
2549 (1990).
2) Pommier Y., Leo E., Zhang H., Marchand C., Chem. Biol., 17,
421–433 (2010).
3) Tacar O., Sriamornsak P., Dass C. R., J. Pharm. Pharmacol., 65,
157–170 (2013).
4) “Australian Medicines Handbook,” ed. by Rossi S., The Australian
Medicines Handbook Unit Trust, Adelaide, 2013.
5) Migachev G. I., Stephaov B. I., Zhurnal Obshchei Khimil, 38,
1368–1369 (1968).
6) Aki O., Nakagawa Y., Chem. Pharm. Bull., 20, 1325–1327 (1972).
7) Xiang A. X., Webber S. E., Kerr B. M., Rueden E. J., Lennox J. R.,
Haley G. J., Wang T., Ng J. S., Herbert M. R., Clark D. L., Banh V.
N., Li W., Fletcher S. P., Steffy K. R., Bartkowski D. M., Kirkovsky
L. I., Bauman L. A., Averett D. R., Nucleosides Nucleotides Nucleic
Acids, 26, 635–640 (2007).
8) Bustos D. A., Jung M., ElSohly H. N., McChesney J. D., Hetereo-
cycles, 29, 2273–2277 (1989).
9) Blaskó G., Cordell G. A., Lankin D. C., J. Nat. Prod., 51, 1273–
1276 (1988).
10) Chandregowda V., Rao G. V., Reddy G. C., Org. Process Res. Dev.,
11, 813–816 (2007).
11) Sordella R., Bell D. W., Haber D. A., Settleman J., Science, 305,
1163–1167 (2004).
12) Pao W., Miller V., Zakowski M., Doherty J., Politi K., Sarkaria I.,
Singh B., Heelan R., Rusch V., Fulton L., Mardis E., Kupfer D.,
Wilson R., Kris M., Varmus H., Proc. Natl. Acad. Sci. U.S.A., 101,
13306–13311 (2004).
13) Kitazaki T., Soda H., Doi S., Nakano H., Nakamura Y., Kohno S.,
Lung Cancer, 50, 19–24 (2005).
14) Knesl P., Röseling D., Jordis U., Molecules, 11, 286–297 (2006).