C-15 thiazol-4-yl analogues of (E)-9,10-didehydroepothilone D: Synthesis and cytotoxicity

Article abstract of DOI:10.1021/ol061087h

The syntheses and biological evaluation of six epothilone D analogues are reported. These side-chain variants of the (E)-9,10-didehydroepothilone scaffold contain C-15 thiazole appendages that are derived from bromomethyl ketone intermediates. Although ea

Full text of DOI:10.1021/ol061087h

ORGANIC
LETTERS
2006
Vol. 8, No. 14
3057-3059
C-15 Thiazol-4-yl Analogues of
(E)-9,10-Didehydroepothilone D:
Synthesis and Cytotoxicity
Brian R. Hearn,* Dan Zhang, Yong Li, and David C. Myles
Kosan Biosciences, Inc., 3832 Bay Center Place, Hayward, California 94545
hearn@kosan.com
Received May 3, 2006
ABSTRACT
The syntheses and biological evaluation of six epothilone D analogues are reported. These side-chain variants of the (E)-9,10-didehydroepothilone
scaffold contain C-15 thiazole appendages that are derived from bromomethyl ketone intermediates. Although each of these analogues is less
cytotoxic than the parent (E)-9,10-didehydroepothilone D, three maintain IC50 values in the double-digit nanomolar range against both susceptible
and resistant cell lines.
The epothilones are naturally occurring polyketides that exert
antiproliferative activity through the stabilization of micro-
tubules, a mode of action shared with paclitaxel.1 Unlike the
taxanes, however, the epothilones maintain potency vs
multidrug resistant cell lines.2 The intense scientific research
that followed this finding led to the development of an
extensive epothilone SAR3 and, ultimately, to the evaluation
of several analogues as anticancer agents in human clinical
trials.4
During the search for analogues with improved therapeutic
properties, two noteworthy aspects of the epothilone SAR
were uncovered. First, Danishefsky and co-workers discov-
ered that installation of a C-9-C-10 (E)-olefin on the
epothilone scaffold increased cytotoxicity against several
human cancer cell lines by approximately 5-10-fold relative
to the parent compound.5 This minor structural change was
also associated with increased aqueous solubility and plasma
stability as well as improved in vivo antitumor activity.6
Second, we,7 as well as Altmann,8 noted that replacement
of the epothilone C-15 side chain with aryl groups resulted
in compounds that maintained potent cytotoxicity. These two
findings served as the basis of our own program to identify
structurally novel epothilone analogues with improved phar-
macokinetic profiles.
(5) (a) Rivkin, A.; Yoshimura, F.; Gabarda, A. E.; Chou, T.-C.; Dong,
H.; Tong, W. P.; Danishefsky, S. J. J. Am. Chem. Soc. 2003, 125, 2899.
(b) Yoshimura, F.; Rivkin, A.; Gabarda, A. E.; Chou, T.-C.; Dong, H.;
Sukenick, G.; Morel, F. F.; Taylor, R. E.; Danishefsky, S. J. Angew. Chem.,
Int. Ed. 2003, 42, 2518.
(1) (a) Ho¨fle, G.; Bedorf, N.; Steinmetz, H.; Schomburg, D.; Gerth, K.;
Reichenbach, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1567. (b) Gerth,
K.; Bedorf, N.; Ho¨fle, G.; Irschik, H.; Reichenbach, H. J. Antibiot. 1996,
49, 560.
(2) Bollag, D. M.; McQueney, P. A.; Zhu, J.; Hensens, O.; Koupal, L.;
Liesch, J.; Goetz, M.; Lazarides, E.; Woods, C. M. Cancer Res. 1995, 55,
2325.
(6) (a) Chou, T.-C.; Dong, H.; Rivkin, A.; Yoshimura, F.; Gabarda, A.
E.; Cho, Y. S.; Tong, W. P.; Danishefsky, S. J. Angew. Chem., Int. Ed.
2003, 42, 4762. (b) Rivkin, A.; Yoshimura, F.; Gabarda, A. E.; Cho, Y. S.;
Chou, T.-C.; Dong, H.; Danishefsky, S. J. J. Am. Chem. Soc. 2004, 126,
10913.
(7) Dong, S. D.; Sundermann, K.; Smith, K. M. J.; Petryka, J.; Liu, F.;
Myles, D. C. Tetrahedron Lett. 2004, 45, 1945.
(3) For review, see: Altmann, K.-H. Curr. Pharm. Des. 2005, 11, 1595.
(4) For information regarding human clinical trials of epothilone D
(KOS 862) and (E)-9,10-didehydroepothilone D (KOS 1584), visit:
www.kosan.com.
(8) (a) Altmann, K.-H.; Bold, G.; Caravatti, G.; Flo¨rsheimer, A.;
Guagnano, V.; Wartmann, M. Bioorg. Med. Chem. Lett. 2000, 10, 2765.
(b) Cachoux, F.; Isarno, T.; Wartmann, M.; Altmann, K.-H. Angew. Chem.,
Int. Ed. 2005, 44, 7469.
10.1021/ol061087h CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/09/2006

The C-15 thiazol-4-yl-9,10-didehydroepothilone D ana-
logues described herein were designed to evaluate C-15
thiazole replacements of the C-16-C-17 trisubstituted olefin
of 1 and are readily available via modifications to the
Danishefsky total synthesis of 1 (Figure 1).5a The simplest
These epothilone analogues were generated by two distinct
strategies that rely on the formation of C-15 bromomethyl
ketones derived from intermediates in the Danishefsky total
synthesis of 9,10-didehydroepothilone D (1).5a The first
approach involves an early-stage functionalization of the
relatively simple methyl ketone 8. By conserving the more
complex fragment 11, this route is valuable for material
throughput of a selected candidate (Scheme 1). The second
Scheme 1. Synthesis of
C-15-(2-Methylthiazol-4-yl)-9,10-didehydroepothilone D (2)
Figure 1. Structures of (E)-9,10-didehydroepothilones.
of these analogues (2) contains a 2-methylthiazol-4-yl
substituent attached directly to the C-15 position of the
epothilone scaffold.9 This structural alteration results in not
only a reduction of the steric bulk of the side chain relative
to 1 but also a repositioning of the thiazole basic nitrogen.
Functionalization at the 2-position of the thiazole can be used
to simply increase the steric bulk at C-15 as in 3 or to probe
the hydrogen bonding requirements of the side chain as well
(4-7). Nicolaou and co-workers have reported that appropri-
ate positioning of a side-chain basic nitrogen is vital to the
cytotoxic potency of epothilone B analogues.10,11 Further-
more, a recent model of epothilone A binding to tubulin
reported by Snyder and Downing, based in part on the
Nicolaou work, suggests that the thiazole nitrogen of the
natural product may accept a hydrogen bond from His227
of â-tubulin.12 In hopes of preserving this interaction, the
four biaryl analogues (4-7), each of which contains a
nitrogen-bearing distal ring, were targeted for total synthesis.
strategy is characterized by a late-stage divergence, useful
for multiple analogue generation, as detailed in Scheme 2.
The synthesis of analogue 2 as described in Scheme 1
required conversion of methyl ketone 8 to its bromomethyl
counterpart 9. This two-step procedure utilized silyl enol
ether formation with TMS triflate followed by NBS-
promoted electrophilic bromination. The reaction of the
resulting bromomethyl ketone (9) with thioacetamide gener-
ated the desired thiazole with concomitant removal of the
TBS protecting group. This cyclization product (10) was
acylated with carboxylic acid 115a to provide the desired ester
12, which was then treated with Grubbs' second-generation
ruthenium catalyst13 to afford the intermediate macrocycle.
Final acidic deprotection yielded the desired C-15 thiazole
analogue 2 by a synthetic route that was beneficial for the
development of our synthetic strategy, namely, bromination
and cyclization, but was not optimal for the synthesis of
multiple analogues.
(9) C-15-(2-Methylthiazol-4-yl)epothilone D is a known compound with
micromolar cytotoxicity: Su, D.-S.; Balog, A.; Meng, D.; Bertinato, P.;
Danishefsky, S. J.; Zheng, Y.-H.; Chou, T.-C.; He, L.; Horwitz, S. B. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2093.
(10) Nicolaou, K. C.; Scarpelli, R.; Bollbuck, B.; Werschkun, B.; Pereira,
M. M. A.; Wartmann, M.; Altmann, K.-H.; Zaharevitz, D.; Gussio, R.;
Giannakakou, P. Chem. Biol. 2000, 7, 593.
(11) For more recent work, see: (a) Bold, G.; Wojeik, S.; Caravatti,
G.; Lindauer, R.; Stierlin, C.; Gertsch, J.; Wartmann, M.; Altmann, K.-H.
ChemMedChem 2006, 1, 37. (b) Nicolaou, K. C.; Pratt, B. A.; Arseniyadis,
S.; Wartmann, M.; O’Brate, A.; Giannakakou, P. ChemMedChem 2006, 1,
41.
(12) Nettles, J. H.; Li, H.; Cornett, B.; Krahn, J. M.; Snyder, J. P.;
Downing, K. H. Science 2004, 305, 866.
A more efficient route for the rapid generation of structural
diversity is detailed in Scheme 2. Because macrocyclic
(13) (a) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett.
1999, 1, 953. (b) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A.
H. J. Am. Chem. Soc. 2000, 122, 8168.
3058
Org. Lett., Vol. 8, No. 14, 2006

Scheme 2. Synthesis of
C-15-(2-Arylthiazol-4-yl)-9,10-didehydroepothilone D
Analogues (3-7)
Table 1. Cytotoxicity of
C-15-Thiazole-9,10-didehydroepothilone D Analogues
cell line, IC50 (nM)
compd
MCF-7
NCI/ADR
A549
SKOV-3
Epo D
4
0.6
380
220
40
52
34
38
10
3
510
380
87
210
68
55
6.6
1.3
470
450
53
200
40
49
12
4.9
750
690
49
320
58
68
1
2
3
4
5
6
7
not simply to variations in the size of the C-15 side chain
(compare 1-4). Little change in the level of cytotoxicity
results from the incorporation of a redundant nitrogen in the
C2 symmetric pyrimidine (pyridine 4 vs pyrimidine 6), but
the second, non-C2 symmetric nitrogen of pyrazine 5 is
clearly detrimental. Although each of the more promising
biaryl analogues (4, 6, and 7) is a less potent cytotoxin in
vitro than either epothilone D or 9,10-didehydroepothilone
D (1), these compounds retain antiproliferative potency in
the NCI/ADR multidrug resistant cell line.
methyl ketone 135a contains three enolizable carbonyl groups,
a modified bromination protocol was required to achieve high
levels of selectivity. Treatment of macrocycle 13 with the
kinetic base LiHMDS resulted in selective deprotonation and
subsequent silylation of the methyl ketone. After exposing
the intermediate silyl enol ether to NBS, the resulting
bromomethyl ketone 14 was treated with a series of aromatic
thioamides to provide intermediate thiazoles in modest to
good yield. These silyl protected intermediates were then
desilylated to afford the targeted C-15 thiazole analogues.
Using this strategy, the syntheses of analogues 3-7 required
four steps each from known ketone 13 but only two steps
from the C-15 bromomethyl ketone common intermediate
14.
Analysis of the cytotoxicity data presented in Table 1
provides several conclusions. Three members of this series
(4, 6, and 7) are potent cytotoxins, having IC50 values in the
double-digit nanomolar range. These results indicate that the
C-15 thiazole is a reasonable, but not optimal, replacement
of the C-16-C-17 olefin (specifically, 1 vs 7). Compounds
2 and 3 demonstrate substantially diminished cytotoxicity
relative to 1. This loss of potency is attributed to the absence
of an appropriately positioned side-chain basic nitrogen but
In conclusion, we have utilized bromomethyl ketone
intermediates for the synthesis of epothilone analogues
containing substituted C-15 thiazole side chains. This
synthetic strategy could be easily adapted, however, to the
synthesis of various other side chains including imidazoles
and oxazoles. The thiazole-containing analogues described
herein retain potent cytotoxicity, with the potential for
improved pharmacokinetic properties.
Acknowledgment. We thank Dr. J. Carney, C. Tran, and
N. Viswanathan of the Department of Analytical Chemistry
at Kosan Biosciences for analytical support of this work and
F. Liu in the Pharmacology Department at Kosan Biosciences
for performing cell viability assays.
Supporting Information Available: Experimental pro-
cedures and characterization data, including copies of 1H and
13C NMR spectra, are provided. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL061087H
Org. Lett., Vol. 8, No. 14, 2006
3059