Macrocyclic hexaoxazoles as sequence- and mode-selective G-quadruplex binders

Article abstract of DOI:10.1002/anie.200801235

(Figure Presented) In a bind: Macrocyclic hexaoxazole G-quadruplex binders with cationic side chains have been synthesized that are selective for the telo24 DNA sequence, and strongly stabilize telo24 in the antiparallel form (see picture). These compound

Full text of DOI:10.1002/anie.200801235

Angewandte
Chemie
DOI: 10.1002/anie.200801235
DNA Recognition
Macrocyclic Hexaoxazoles as Sequence- and Mode-Selective
G-Quadruplex Binders**
Masayuki Tera, Hiromichi Ishizuka, Motoki Takagi, Masami Suganuma, Kazuo Shin-ya, and
Kazuo Nagasawa*
Human telomeric DNA, which is located at the end of
chromosomes, contains repeating single-strand sequences of
(TTAGGG)n.[1] This G-rich single strand forms a character-
istic four-stranded helical conformation, called a G-quadru-
plex, in the presence of monovalent cations such as potassium
or sodium ions.[2] The formation of G-quadruplexes in vitro
has been shown to inhibit the activity of telomerase, an
enzyme related to the elongation of telomeres in tumor cells;
thus, inhibiting telomerase activity by stabilizing the telo-
meric G-quadruplex is considered to have potential for the
treatment of cancer.[1] The G-quadruplex structure exists not
only in the telomeric DNA, but also in the promoter regions
of some oncogenes, such as c-myc, c-kit, and bcl-2.[3–5] These
Scheme 1. Structure of telomestatin (1) and designed G-quadruplex
G-quadruplexes have different sequences from telomeric
DNA, and have their own inherent conformational mode,
namely, the telomere G-quadruplex has an antiparallel
mode,[2] while the c-myc[3] and c-kit[4] promoter regions have
a parallel-type mode, and the bcl-2 promoter region has a
mixed antiparallel/parallel mode.[5] These DNA sequences
also play significant roles in the inhibition of transcriptional
activity.[6] Therefore, sequence- and mode-selective G-quad-
ruplex ligands would be candidate anticancer agents as well as
biological tools. There have been a number of studies,[7] but
only a few compounds, such as 9944[8a] and 115405,[8a] have
been reported as selective binders to the telomeric G-
quadruplex.[8b,c] Herein, we describe the development of 6-
OTD (2; Scheme 1) derivatives with a macrocyclic hexaox-
azole skeleton as sequence- and mode-selective G-quadru-
plex binders.
binders 2a–2d.
Telomestatin (1, Scheme 1), which has a macrocyclic
structure containing five oxazole rings, two methyl oxazole
rings, and a thiazoline ring, is a natural product which was
isolated from Streptomyces anulatus 3533-SV4 after screening
with the telomeric repeat amplification protocol (TRAP).[9]
Telomestatin (1) shows potent telomerase inhibitory activity,
with an IC50 value of 5 nm. The macrocyclic polyoxazole
structure of 1 interacts with, and strongly stabilizes, the
telomere G-quadruplex,[10a] and 1 is widely used as a standard
G-quadruplex binder.[10] The total synthesis of 1 was accom-
plished by Doi et al. in 2006,[11] but further structural
development is difficult, because 1 has no convenient func-
tional groups, such as hydroxy and/or amine groups. Recently,
we reported the synthesis of S2A2-6OTD (2a), which is a
macrocyclic hexaoxazole with a bis(hydroxymethyl acetate)
moiety.[12] One of the stereoisomers, (2S,14S)-2a, was found to
show moderate telomerase-inhibitory activity by the TRAP
assay. On this basis, we have designed the amine and
guanidine derivatives (2S,14S)-L2H2-6OTD (2b) and
(2S,14S)-L2G2-6OTD (2c) with cationic functional groups.
These derivatives have a planar macrocyclic hexaoxazole
pharmacophore, which is expected to bind to guanine tetrads
through p–p interactions. The cationic functional groups of
2b and 2c, which are directed toward the G-quadruplex
grooves, are expected to interact with phosphate groups to
effectively stabilize the G-quadruplex structure; consequently
these compounds should exhibit potent telomerase-inhibitory
activity.
[*] M. Tera, H. Ishizuka, Prof. Dr. K. Nagasawa
Department of Biotechnology and Life Science, Faculty of Technol-
ogy
Tokyo University of Agriculture and Technology
2-24-16 Nakamachi, Koganei Tokyo 184–8588 (Japan)
Fax: (+81)42-388-7295
E-mail: knaga@cc.tuat.ac.jp
Dr. M. Takagi, Dr. K. Shin-ya
Biological Information Research Center
National Institute of Advanced Industrial Science and Technology
Koto-ku, Tokyo 135-0064 (Japan)
Dr. M. Suganuma
Research Institute for Clinical Oncology, Saitama Cancer Center
Ina, Kitaadachi-gun, Saitama 362–0806 (Japan)
[**] We thank Prof. Ikebukuro (TUAT) for helpful discussions. This work
was supported by a Grant-in-Aid for Scientific Research on Priority
Areas (17035025) from The Ministry of Education, Culture, Sports,
Science, and Technology (MEXT).
The amine and guanidine derivatives 2b and 2c were
synthesized as shown in Scheme 2. Briefly, the trioxazole 3,
which was synthesized from l-lysine and l-serine,[12] was
converted into the carboxylic acid 4 and amine 5. These two
segments were coupled to give the linear hexaoxazole 6. After
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801235.
Angew. Chem. Int. Ed. 2008, 47, 5557 –5560
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5557

Communications
Scheme 2. Synthesis of L2H2-6OTD (2b), L2G2-6OTD (2c), and L2A2-
6OTS (2d). a) LiOH, THF/H2O; b) Pd(OH)2/C, H2, THF/MeOH;
c) DMT-MM, N-methylmorpholine, THF/H2O/MeOH, 63% over
3 steps from 3; d) LiOH, THF/H2O; e) Pd(OH)2/C, H2, THF/MeOH;
f) EtiPr2N, DMAP, BOPCl, DMF-CH2Cl2, 51% over 3 steps from 6;
g) TFA, CH2Cl2 99%; h) Et3N, HgCl2, 1,3-bis(tert-butoxycarbonyl)-2-
methyl-2-thiopseudourea; i) TFA, CH2Cl2, 81% over 2 steps from 2b;
j) Ac2O, CH2Cl2 60%. Boc=tert-butyloxycarbonyl, Cbz=benzyloxycar-
bonyl, DMT-MM=4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpho-
linium chloride, DMAP=4-dimethylaminopyridine, BOPCl=bis(2-oxo-
3-oxazolidinyl)phosphinic chloride, TFA=trifluoroacetic acid.
hydrolysis of the ester group of 6 followed by cleavage of the
Cbz group, the resulting amino acid was subjected to macro-
cyclization to afford the bisamide 7. The Boc group was
removed with TFA to give 2b.[13] The guanidine 2c was
synthesized from 2b in 57% yield by reaction with 1,3-
Figure 1. CD spectra of 10 mm telo24 in Tris-HCl buffer (50 mm,
pH 7.6) in the presence of a) L2H2-6OTD (2b), b) L2G2-6OTD (2c),
and c) L2A2-6OTD (2d) (0–50 mm).
bis(tert-butoxycarbonyl)-2-methyl-2-thiopseudourea,
fol-
lowed by cleavage of the Boc groups with TFA. The N-
acetyl derivative of L2A2-6OTD (2d) was synthesized from
2b in 60% yield.
262 nm (the formation of a steady state required two equiva-
lents of 2b), while 2c was less effective (three equivalents of
2c were required to reach a steady state). However, the 2c–
telo24 complex was more stable than that with 2b. To
examine these differences in the stabilities of the 2b–telo24
and 2c–telo24 complexes, the melting temperatures Tm were
evaluated from the CD melting curves at 292 nm. The
Tm values of the complexes were found to be 38.1and
53.28C for the complexes with 2b and 2c, respectively. Since
the Tm value of 1 is 47.88C, the guanidine derivative 2c is a
stronger stabilizer than 1 of the antiparallel form of telo24.
The DNA sequence selectivity of 2b–d was next inves-
tigated. For this purpose the polymerase chain reaction
(PCR) stop assay[8a] was conducted using the DNA sequences
of telo24, telo24-mutant, c-kit, and c-kit-mutant.[14] Telomes-
tatin (1) was also evaluated for comparison, and the results
The ability of compounds 2b–d to form G-quadruplexes
was investigated by circular dichroism (CD) by using
telo24.[14] Telo24 forms an intermolecular G-quadruplex
(Figure 1, black line) in the absence of a G-quadruplex
binder, and forms an intramolecular G-quadruplex in the
their presence (Figure 1a,b). Telomeric antiparallel intra-
molecular G-quadruplexes have characteristic CD spectra
consisting of a positive signal at 292 nm and a negative signal
at 262 nm.[2b] The, intensity of the signals at 292 and 262 nm
reflect the ratios of the inter- and intramolecular G-quad-
ruplexes.[15] Telo24 interacts with 1 to form an antiparallel G-
quadruplex.[15] Figure 1shows the CD spectra of telo24 in the
presence of 2b–d at various molar ratios. Compound 2b
efficiently increased the intensity of the signals at 292 and
5558
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 5557 –5560

Angewandte
Chemie
(2b) and L2G2-6OTD (2C) were diluted with water from 10 mm
Table 1: DNA sequence selectivity of 6OTDs 2b–d, and 1 by the PCR stop
assay.
stock solutions to give a concentration of 1m m, and titrated into the
oligonucleotide samples at 1mol equiv up to 5 mol equiv (the 10 m m
stock solutions of 2b and 2c were made in 50% MeOH). The DNA
concentrations were 10 mm, and the CD spectra are representative of
three averaged scans taken at 258C. The CD melting curves of the
telo24 G-quadruplex were determined from measurements of the CD
intensity at 292 nm. The heating rate was 2.08CminÀ1. The results are
presented in the Supporting Information.
G-quadruplex binders 2b
2c
2d
IC50 [mM]
1
telo24
telo24 mut
c-kit
0.71Æ0.03 0.64Æ0.01 >75 0.65Æ0.35[a]
5.5Æ0.2
1.9Æ0.1
9.1Æ0.7
20.4Æ1.1
2.1Æ0.3
14.9Æ4.5
>75 40[a]
>75 4.1Æ0.1
c-kit mut
>75 7.2Æ0.3
PCR stop assay: The PCR stop assay was performed by means of
[a] The inhibitory activities using the TRAP G4 assay are shown.[8a]
a modification of the reported protocol.[8a] Oligonucleotides[14] and
the
corresponding
complementary
sequence
of
telo24
d[TCTCGTCTTCCCTA-A] (telo24 rev) and c-kit d[TATATATA-
TACCCTCCTC] (c-kit rev) were used. The chain-extension reaction
was performed in PCR buffer containing 0.2 mm dNTP, 5 U Taq
polymerase, 7.5 pmol oligonucleotides, and various concentrations of
2b–d and 1. The mixtures were incubated in a thermocycler with the
following cycling conditions: 948C for 2 min, followed by 30 cycles of
948C for 30 s, 478C for 30 s, and 728C for 30 s. Amplified PCR
products were resolved on 12% native polyacrylamide gels in 0.5 ”
TBE buffer and stained with ethidium bromide. The fluorescent
intensity of ethidium bromide was measured and quantified on a
phosphorimager (Typhoon 8600, Molecular Dynamics). The results
are summarized in the Supporting Information.
TRAP assay: Inhibitory effects against telomerase, which was
semipurified from cell lysates of human B lymphoma Namalwa cells,
were estimated by the telomeric repeat amplification protocol
(TRAP), which is a modified version of the method developed by
Kim et al.,[17] as described in detail previously.[18]
Cell culture: HeLa cells were cultivated in Dulbeccoꢀs modified
Eagleꢀs medium containing 10% fetal bovine serum (FBS),
50 mgmLÀ1 streptmycin, and 5 unitsmlÀ1 penicillin at 378C in a 5%
CO2 atmosphere. Cells were plated onto 96-well multiwell plates at a
density of 2 ” 103 cells per well. After the cells had been treated with
compounds (2b and 2c) for 6 days, 10 mL of (3-(4,5-dimethylthiazol-2-
yl)-2,5-diphenyl tetrasodium bromide) (MTT) solution was added to
each well. After further incubation for 4 h, the cell viability was
estimated from the optical density at 520 nm.
are summarized in Table 1. L2H2-6OTD (2b) strongly
inhibited the chain extension reaction of telo24 by a factor
of 7.7-fold compared with the telo-24-mutant DNA
(IC50 value of 0.71 mm versus 5.5 mm). In the case of L2G2-
6OTD (2c), the selectivity for telo24 versus the telo24-mutant
was increased 31-fold. However, L2A2-6OTD (2d) showed
no inhibitory activity on the chain extension reaction with
either telo24 or its mutant. L2H2-6OTD (2b) and L2G2-
6OTD (2c) also showed selective inhibitory activity toward
the chain extension reaction of c-kit (4.8-fold and 7.1-fold,
respectively, versus the c-kit-mutant oligonucleotides),
although the IC50 values of 2b and 2c for c-kit were only
moderate (1.9 and 2.1 mm, respectively). Thus, 2b and 2c were
more selective for the telo24 DNA sequence than for the c-kit
DNA sequence.
G-Quadruplex binders generally show telomerase inhib-
itory activities. Since 2b and 2c selectively interacted with
telo24, we next examined the telomerase inhibitory activities
of 2b and 2c by the TRAP assay, which readily evaluates
telomerase activity of G-quadruplex binders using cancer
cells. In this experiment, 2b and 2c inhibited the telomerase
activity of Namalwa cells at a concentration of 20 nm by 49
and 42%, respectively.[16] Consistent with the telomerase
inhibitory activities, both 2b and 2c inhibited the growth of
HeLa cells (which are well-known as telomerase-positive
cells) after 6 days incubation, with IC50 values of 7.4 and
0.5 mm, respectively.[10j,n] These results also reflect the G-
quadruplex-stabilizing abilities obtained from the CD anal-
ysis.
In conclusion, we have developed the macrocyclic hexa-
oxazole 6OTD-type G-quadruplex binders L2H2-6OTD (2b)
and L2G2-6OTD (2c). These new binders are selective for the
telo24 DNA sequence, and strongly stabilize telo24 in the
antiparallel form. These compounds also showed potent
telomerase-inhibitory activity in both cell-free and cell-
based assay systems. The G-quadruplex stabilizing ability
and the cell-based assay results for the G-quadruplex binders
were discussed for the first time using 6OTD derivatives.
Further structure–activity relationship studies of 6OTD
derivatives and investigations of the biological activities are
in progress.
Received: March 14, 2008
Published online: June 20, 2008
Keywords: G-quadruplexes · macrocycles · oxazoles ·
.
telomerase inhibitors · telomeres
[1] For recent reviews, see: a) L. Oganesian, T. M. Bryan, BioEssays
2007, 29, 155; b) L. R. Kelland, Eur. J. Cancer 2005, 41, 971.
[2] In the presence of potassium cations, the telomeric G-quad-
ruplex forms in the parallel mode, see: a) G. N. Parkinson, M. P.
Lee, S. Neidle, Nature 2002, 417, 876; in the presence of sodium
cations, the telomeric G-quadruplex forms in the antiparallel
mode, see: b) Y. Wang, D. J. Patel, Structure 1993, 1, 263.
[3] A. T. Phan, Y. S. Modi, D. J. Patel, J. Am. Chem. Soc. 2004, 126,
8710.
[4] a) A. T. Phan, V. Kuryavyi, S. Burge, S. Neidle, D. J. Patel, J. Am.
Chem. Soc. 2007, 129, 4386; b) P. S. Shirude, B. Okumus, L. Ying,
T. Ha, S. Balasubramanian, J. Am. Chem. Soc. 2007, 129, 7484;
c) A. K. Todd, S. M. Haider, G. N. Parkinson, S. Neidle, Nucleic
Acids Res. 2007, 35, 5799.
[5] a) J. Dai, T. S. Dexheimer, D. Chen, M. Carver, A. Ambrus,
R. A. Jones, D. Yang, J. Am. Chem. Soc. 2006, 128, 1 096; b) T. S.
Dexheimer, D. Sun, L. H. Hurley, J. Am. Chem. Soc. 2006, 128,
5404.
[6] a) A. S. Jain, C. L. Grand, D. J. Bearss, L. H. Hurley, Proc. Natl.
Acad. Sci. USA 2002, 99, 11593; b) C. Douarre, D. Gomez, H.
Experimental Section
CD spectroscopy: The CD titration experiment was performed with a
modification of the reported procedure.[15] Telo24[14] oligonucleotide
was dissolved in Tris-HCl buffer (50 mm, pH 7.6) and the solution was
heated to 908C for 5 min, then slowly cooled to 258C. L2H2-6OTD
Angew. Chem. Int. Ed. 2008, 47, 5557 –5560
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5559

Communications
Morjani, J. M. Zahm, M. F. OꢀDonohue, L. Eddabra, P. Mailliet,
Paterski, T. Lemartleur, K. Shin-ya, J. L. Mergny, J. F. Riou, J.
Biol. Chem. 2004, 279, 41487; g) J. Seenisamy, S. Bashyam, V.
Gokhale, H. Vankayalapati, D. Sun, A. Siddiqui-Jain, N.
Streiner, K. Shin-ya, E. White, W. D. Wilson, L. H. Hurley, J.
Am. Chem. Soc. 2005, 127, 2944; h) K. Shin-ya, Biosci. Biotech-
nol. Biochem. 2005, 69, 867; i) N. Binz, T. Shalaby, P. Rivera, K.
Shin-ya, M. A. Grotzer, Eur. J. Cancer, 2005, 41, 2873; j) H.
Tahara, K. Shin-ya, H. Seimiya, H. Yamada, T. Tsuruo, T. Ide,
Oncogene 2006, 25, 1955; k) L. Zhang, K. Tamura, K. Shin-ya, H.
Takahashi, Biochim. Biophys. Acta Mol. Cell Res. 2006, 1763, 39;
l) T. Tauchi, K. Shin-ya, G. Sashida, M. Sumi, S. Okabe, J. H.
Ohyashiki, K. Ohyashiki, Oncogene 2006, 25, 5719; m) D.
Gomez, M. F. OꢀDonohue, T. Wenner, C. Douarre, J. Macadre,
P. Koebel, M. J. Giraud-Panis, H. Kaplan, A. Kolkes, K. Shin-ya,
J. F. Riou, Cancer Res. 2006, 66, 6908; n) D. Gomez, T. Wenner,
B. Brassart, C. Douarre, M. F. OꢀDonohue, V. El Khoury, K.
Shin-ya, H. Morjani, C. Trentesaux, J. F. Riou, J. Biol. Chem.
2006, 281, 38721; o) A. Cheng, K. Shin-ya, R. Wan, S. C. Tang, T.
Miura, H. Tang, R. Khatri, M. Gleichman, X. Ouyang, D. Liu,
H. R. Park, J. Y. Chiang, M. P. Mattson, J. Neurosci. 2007, 27,
3722.
J. F. Riou, C. Trentesaux, Nucleic Acids Res. 2005, 33, 2192.
[7] For the use of cationic porphyrin derivatives as G-quadruplex
binders, see: a) K. Guo, A. Pourpak, K. Beetz-Rogers, V.
Gokhale, D. Sun, L. H. Hurley, J. Am. Chem. Soc. 2007, 129,
10220; for pyridine dicarboxamide derivatives, see: b) C. Gran-
otier, G. Pennarun, L. Riou, F. Hoffschir, L. R. Gauthier, A. D.
Cian, D. Gomez, E. Mandine, J. F. Riou, J. L. Mergny, P. Mailliet,
B. Dutrillauz, F. D. Boussin, Nucleic Acids Res. 2005, 33, 4182;
for polyoxazole derivatives, see: c) C. M. Barbieri, A. R. Srini-
casan, S. G. Rzuczek, J. E. Rice, E. J. LaVoie, D. S. Pilch, Nucleic
Acids Res. 2007, 35, 3272; d) K. Jantos, R. Rodriguez, S. Ladame,
P. S. Shirude, S. Balasubramanian, J. Am. Chem. Soc. 2006, 128,
13662; for berberine derivatives, see: e) M. Franceschin, L.
Rossetti, A. DꢀAmbrosio, S. Schirripa, A. Bianco, G. Ortaggi, M.
Savino, C. Schultes, S. Neidle, Bioorg. Med. Chem. Lett. 2006, 16,
1707; for triazole derivatives, see: f) A. D. Moorhouse, A. M.
Santos, M. Gunaratnam, M. Moore, S. Neidle, J. E. Moses, J. Am.
Chem. Soc. 2006, 128, 15972; for perylene derivatives, see; g) L.
Rossetti, M. Franceschin, S. Schirripa, A. Bianco, G. Ortaggi, M.
Savino, Bioorg. Med. Chem. Lett. 2005, 15, 413; for pentacyclic
acridine derivatives, see: h) S. M. Gowan, R. Heald, M. F.
Steaven, L. R. Kelland, Mol. Pharmacol. 2001, 60, 981; for
triazine derivatives, see: i) J. F. Riou, L. Guittat, P. Mailliet, A.
Laoui, E. Renou, O. Petitgenet, F. Megnin-Chanet, C. Helene,
J. L. Mergny, Proc. Natl. Acad. Sci. USA 2002, 99, 2672; for
fluoroquinophenoxazines, see: W. Duan, A. Rangan, H. Van-
kayalapati, M. Kim, Q. Zeng, D. Sun, H. Han, O. Y. Fedoroff, D.
Nishioka, S. Y. Rha, E. Izbicka, D. D. Von Hoff, L. H. Hurley,
Mol. Cancer Ther. 2001, 1, 103.
[11] T. Doi, M. Yoshida, K. Shin-ya, T. Takahashi, Org. Lett. 2006, 8,
4165.
[12] M. Tera, Y. Sohtome, H. Ishizuka, T. Doi, M. Takagi, K. Shin-ya,
K. Nagasawa, Heterocycles 2006, 69, 505.
[13] For the synthesis of 2b and evaluation of its G-quadruplex-
stabilizing properties by UV analysis which came to our
attention after submission of this paper, see: S. G. Rzuczek,
D. S. Pilch, E. J. LaVoie, J. E. Rice, Bioorg. Med. Chem. Lett.
2008, 18, 913.
[8] a) T. Lemarteleur, D. Gomez, R. Paterski, E. Mandine, P.
Mailliet, J. F. Riou, Biochem. Biophys. Res. Commun. 2004, 323,
802; for selective binders to the c-myc G-quadruplex (BRACO-
19) see: b) M. Read, R. J. Harrison, B. Romagnoli, F. A. Tanious,
S. H. Gowan, A. P. Reszka, W. D. Wilson, L. R. Kelland, S.
Neidle, Proc. Natl. Acad. Sci. USA 2001, 98, 4844; c) M.
Gunaratnam, O. Greciano, C. Martins, A. P. Reszka, C. M.
Schutes, H. Morjani, J. F. Riou, S. Neidle, Biochem. Pharmacol.
2007, 74. 679.
[14] The DNA oligonucleotide sequences used are shown below.
[15] E. M. Rezler, J. Seenisamy, S. Bashyam, M. Y. Kim, E. White,
W. D. Wilson, L. H. Hurley, J. Am. Chem. Soc. 2005, 127, 9439.
[16] a) IC50 valus of 1 were found to be 5 nm[9] in this protocol;
b) A. D. Cian, G. Cristfari, P. Reichenbach, E. D. Lemos, D.
Monchaud, M. P. Teulade-Fichou, K. Shin-ya, L. Lacroix, J.
Lingner, J. L. Mergny, Proc. Natl. Acad. Sci. USA 2007, 104,
17347.
[17] N. W. Kim, M. A. Piatyszek, K. R. Prowse, C. B. Harley, M. D.
Weat, P. L. C. Ho, G. M. Coviello, W. E. Wright, S. L. Weinrich,
J. W. Shay, Science 1994, 266, 2011.
[9] K. Shin-ya, K. Wierzba, K. Matsuo, T. Ohtani, Y. Yamada, K.
Furihata, Y. Hayakawa, H. Seto, J. Am. Chem. Soc. 2001, 123,
1262.
[10] a) M. Y. Kim, H. Vankayalapati, K. Shin-ya, K. Wierzba, L. H.
Hurley, J. Am. Chem. Soc. 2002, 124, 2098; b) T. Tauchi, K. Shin-
ya, G. Sashida, M. Sumi, A. Nakajima, T. Shimamoto, J. H.
Ohyashiki, K. Ohyashiki, Oncogene 2003, 22, 5338; c) D. Gomez,
N. Aouali, A. Renaud, C. Douarre, K. Shin-ya, J. Tazi, S,
Martinez, C. Trentesaux, H. Morjani, J. F. Riou, Cancer Res.
2003, 63, 6149; d) F. Rosu, V. Gabelica, K. Shin-ya, E. De Pauw,
Chem. Commun. 2003, 2702; e) M. Sumi, T. Tauchi, G. Sashida,
A. Nakajima, A. Gotoh, K. Shin-ya, J. H. Ohyashiki, K.
Ohyashiki, Int. J. Oncol. 2004, 24, 1481; f) D. Gomez, R.
[18] Y. Tabata, S. Ikegami, T. Yaguchi, T. Sasaki, S. Hoshiko, S.
Sakuma, K. Shin-ya, H. Seto, J. Antibiot. 1999, 52, 412.
5560
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 5557 –5560