Design, synthesis, and biological evaluation of simplified analogues of (+)-discodermolide. Additional insights on the importance of the diene, the C(7) hydroxyl, and the lactone

Article abstract of DOI:10.1021/ol052070m

(Chemical Equation Presented) The design, synthesis, and biological evaluation of seven totally synthetic analogues of the antitumor agent (+)-discodermolide are reported. Saturation of the terminal diene system, alteration of the substituents on the lact

Full text of DOI:10.1021/ol052070m

ORGANIC
LETTERS
2005
Vol. 7, No. 23
5229-5232
Design, Synthesis, and Biological
Evaluation of Simplified Analogues of
(+)-Discodermolide. Additional Insights
on the Importance of the Diene, the C(7)
Hydroxyl, and the Lactone
Amos B. Smith, III* and Ming Xian
Department of Chemistry, Monell Chemical Senses Center, and Laboratory for
Research on the Structure of Matter, UniVersity of PennsylVania, Philadelphia,
PennsylVania 19104
smithab@sas.upenn.edu
Received August 26, 2005
ABSTRACT
The design, synthesis, and biological evaluation of seven totally synthetic analogues of the antitumor agent (
+
)-discodermolide are reported.
Saturation of the terminal diene system, alteration of the substituents on the lactone, and alkylation of the C(7)-hydroxyl group reveal significant
structure activity relationships.
−
(+)-Discodermolide (1), a polyketide natural product isolated
from the marine sponge Discodermia dissolute,1 displays both
potent cytotoxic activity against a wide variety of human
tumor cell lines and significant in vivo antitumor activity.2
The mode of action, similar to that of paclitaxel, comprises
binding and stabilization of microtubules leading to mitotic
arrest and cell death.3 In 2000, the Horwitz group in
collaboration with our laboratory and that of Danishefsky
demonstrated that discodermolide and paclitaxel display
significant synergistic antiproliferative cooperativity.4 Further
collaboration with the Horwitz group led to the discovery
that discodermolide, but not paclitaxol, processes a second
significant mechanism of tumor cell growth inhibition,
specifically induction of an accelerated senescence pheno-
type, which may play a role in the observed synergy.5
The remarkable biological profile and novel structure of
(+)-discodermolide (1, Figure 1) make it a promising
(3) (a) ter Harr, E.; Kowalski, R. J.; Hammel, E.; Lin, C. M.; Longley,
R. E.; Gunaskera, S. P.; Rosenkranz, H. S.; Day, B. W. Biochemistry 1996,
35, 243-250. (b) Hung, D. T.; Chen, J.; Schrieber, S. L. Chem. Biol. 1996,
3, 287-293.
(4) Martello, L. A.; McDiad, H. M.; Regl, D. L.; Yang, C. H.; Meng,
D.; Pettus, T. R.; Kaufman, M. D.; Arimoto, H.; Danishefsky, S. J.; Smith,
A. B., III; Horwitz, S. B. Clin. Cancer Res. 2000, 6, 1978-1987.
(1) (a) Gunasekera, S. P.; Gunasekera, M.; Longley, R. E.; Schulte, G.
K. J. Org. Chem. 1990, 55, 4912-4915. Additions and corrections: J. Org.
Chem. 1991, 56, 1346. (b) Gunasekera, S. P.; Pomponi, S. A.; Longley, R.
E. U.S. Patent No. US5840750, Nov 24, 1998.
(2) Smith, A. B., III; Freeze, B. S.; LaMarche, M. J.; Sager, J.; Kinzler,
K. W. Bioorg. Med. Chem. Lett. 2005, 15, 3623-3626.
10.1021/ol052070m CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/08/2005

additional structure activity relationships (SAR). Herein we
describe the synthesis and biological evaluation of a series
of seven potent congeners of (+)-2 and (+)-3,9 several of
which display equal or enhanced cytotoxicity over (+)-
discodermolide employing a range of drug-sensitive human
tumor cell lines, as well as the NCI/ADR drug-resistant cell
line. Importantly, access to the seven analogues was only
possible due to the wide variety of advanced intermediates
available from our gram-scale synthesis.6a
We first explored the effect of saturation of the terminal
diene system (Scheme 1). Early results by Gunasekera et al.
Figure 1. (+)-Discodermolide and simplified analogues.
Scheme 1
candidate for clinical development as a cancer chemothera-
peutic agent. Along with the scarcity of the natural material,
these properties have stimulated intensive synthetic effort,
including the gram-scale syntheses achieved by our labora-
tory and that of the Novartis group.6 Additional endeavors
have focused on the design and synthesis of structurally
simplified analogues of this potentially important chemo-
therapeutic agent.7
Recently, in collaboration with Kosan Bioscience, Inc.,
we reported a series of analogues in which the lactone moiety
was significantly simplified.8 Among the analogues prepared,
the 2-normethyl-3-deoxy congener (+)-2 and the 2-nor-
methyl-2,3-anhydro counterpart (+)-3 (Figure 1) revealed
improved activity over (+)-discodermolide (1), despite the
removal of two stereogenic centers, suggesting that the
2-methyl and 3-hydroxyl groups do not play a critical role
in the observed cytotoxicity. Given the enhanced cytotoxicity
and simplification in structure, analogues (+)-2 and (+)-3
were selected as leads for further modification to explore
(5) Klein, L. E.; Freeze, B. S.; Smith, A. B., III; Horwitz, S. B. Cell
Cycle 2005, 4, 501-507.
(6) (a) Smith, A. B., III; Beauchamp, T. J.; LaMarche, M. J.; Kaufman,
M. D.; Qiu, Y. P.; Arimoto, H.; Jones, D. R.; Kobayashi, K. J. Am. Chem.
Soc. 2000, 122, 8654-8664. (b) Paterson, I.; Florence, G. J.; Gerlach, K.;
Scott, J. P.; Sereing, N. J. Am. Chem. Soc. 2001, 123, 9535-9544. (c)
Harried, S. S.; Yang, G.; Strawn, M. A.; Myles, D. C. J. Org. Chem. 1997,
62, 6098-6099. (d) Marshall, J. A.; Johns, B. A. J. Org. Chem. 1998, 63,
7885-7892. (e) Mickel, S. J.; Sedelmeier, G. H.; Niederer, D.; Daeffler,
R.; Osmani, A.; Schreiner, K.; Seeger-Weibel, M.; Be´rod, B.; Schaer, K.;
Gamboni, R.; Chen, S.; Chen, W.; Jagoe, C. T.; Kinder, F. R.; Loo, M.;
Prasad, K.; Shieh, W.-C.; Wang, R.-M.; Waykole, L.; Xu, D. D.; Xue, S.
Org. Proc. Res. DeV. 2004, 8, 92-130. (f) Arefolov, A.; Panek, J. J. Am.
Chem. Soc. 2005, 127, 5596-5603. (g) Nerenberg, J. B.; Hung, D. T.;
Somers, P. K.; Schreiber, S. L. J. Am. Chem. Soc. 1993, 115, 12621-
12622. (h) Smith, A. B., III; Freeze, S. B.; Xian, M.; Hirose, T. Org. Lett.
2005, 7, 1825-1828.
demonstrated that the saturated C(21)-C(24) variant of (+)-
discodermolide (1) is more active than the parent molecule,
when assayed against the P388 and A549 human tumor cell
lines.10 Toward this end, hydrogenation of known alcohol
(+)-46h followed by iodination at the primary hydroxyl
provided (+)-5. Subsequent Suzuki coupling with known
vinyl iodide (+)-6,6h followed by removal of the primary
(7) (a) Hung, D. T.; Nerenberg, J. B.; Schreiber, S. L. Chem. Biol. 1994,
1, 67-71. (b) Choy, N.; Shin, Y.; Nguyen, P. Q.; Curran, D. P.;
Balachandran, R.; Madiraju, C.; Day, B. W. J. Med. Chem. 2003, 46, 2846-
2864. (c) Martello, L. A.; LaMarche, M. J.; He, L.; Beauchamp, T. J.; Smith,
A. B., III; Horwitz, S. B. Chem. Biol. 2001, 8, 843-855. (d) Smith, A. B.,
III; Freeze, B. S.; LaMarche, M.; Hirose, T.; Brouard, I.; Xian, M.;
Sundermann, K. F.; Shaw, S. J.; Burlingame, M. A.; Horwitz, S. B.; Myles,
D. C. Org. Lett. 2005, 7, 315-318. (e) Smith, A. B., III; Freeze, B. S.;
LaMarche, M.; Hirose, T.; Brouard, I.; Xian, M.; Sundermann, K. F.; Shaw,
S. J.; Burlingame, M. A.; Horwitz, S. B.; Myles, D. C. Org. Lett. 2005, 7,
311-314.
(9) Previous results from our collaboration with Kosan Biosciences (ref
8) also revealed that butyrolactone derivatives of discodermolide exhibit
improved activity. Similar SAR study employing butyrolactone analogues
as leads is currently undergoing. Results will be reported elsewhere.
(10) Gunasekera, S. P.; Longley, R. E.; Isrucker, R. A. J. Nat. Prod.
2002, 65, 1830-1837.
(8) Shaw, S. J.; Sundermann, K. F.; Burlingame, M. A.; Myles, D. C.;
Freeze, B. S.; Xian, M.; Brouard, I.; Smith, A. B., III. J. Am. Chem. Soc.
2005, 127, 6532-6533.
5230
Org. Lett., Vol. 7, No. 23, 2005

TBS silyl ether, iodination, and treatment with tri-
phenylphosphine, led to Wittig salt (+)-7. Union with known
aldehyde (-)-8,8 and in turn removal of the PMB moiety,
installation of the carbamate moiety, and global deprotection,
then furnished the fully saturated diene analogue (-)-10.
The partially saturated C(23)-C(24) analogue (+)-13
(Scheme 2) was next prepared from the corresponding alkyl
hydroxyl as the TBS ether in known ester (+)-14,11 followed
by rhodium-catalyzed hydroboration12 and oxidation with
SO3‚pyridine,13 efficiently furnished aldehyde (-)-15 as a
precursor for the unsubstituted lactone moiety of 18. Wittig
union with known phosphonium salt (+)-16,6h followed by
removal of the PMB moiety, carbamate installation, and
global deprotection with concomitant lactone ring formation,
produced analogue (+)-18.
The Gunasekera group had also prepared the C(7)-acetate
analogues of discodermolide which proved to be almost 10-
fold more potent than the parent compound against P388
leukemia cells.14 Based on this observation, we reasoned that
the C(7)-hydroxyl group on (+)-2 (Scheme 4) might prove
Scheme 2
Scheme 4
iodide (+)-12 via the same synthetic sequence, with a similar
overall efficiency. In this case, iodide (+)-12 was obtained
from known aldehyde (+)-116h in three steps: Wittig
reaction, removal of the trityl group, and iodination.
Early results from both our and the Kosan laboratories
suggested that the C(4)-methyl group does not play a critical
role for potent cytotoxicity against selected cancer cell lines.8
We therefore turned to the construction of the C(4)-normethyl
analogue 18 (Scheme 3). Protection of the secondary
to be an attractive site for further modification. Toward this
end, the C(7)-MOM analogue (-)-19 was prepared chemo-
selectively via treatment of (+)-2 with MOMCl.
The C(7)-methoxy analogue (+)-23 was also prepared,
albeit begining with lactone (-)-20,7d as outlined in Scheme
5. In this case, removal of the TBS silyl ether, followed by
Scheme 5
Scheme 3
methylation and hydrogenation yielded alcohol (-)-21, which
upon Dess-Martin oxidation, union with Wittig salt (+)-
16, and removal of the PMB group furnished (+)-22.
Carbamate installation followed by global deprotection
completed construction of the C(7)-methoxy analogue (+)-
23.
Org. Lett., Vol. 7, No. 23, 2005
5231

To explore the effect of stereogenicity on the lactone
ring,12 the 4,5-epimers of (+)-2 and (+)-3 were constructed.
For this goal, the secondary alcohol (+)-24, isolated as a
minor diastereomeric product en route to lactone (-)-20,7d
was acylated with acryloyl chloride (Scheme 6). Ring-closing
Table 1. Cytotoxicity Observed for Synthetic Analogues
cytotoxicity IC50, nM
MCF-7
NCI/ADR
A549
CCRF-CEM
(+)-1
(+)-2
(+)-3
(-)-10
(+)-13
(+)-18
(-)-19
(+)-23
(+)-26
(+)-28
28
2.7
2.1
17
3.9
23
3.4
1.3
130
4.7
240
150
95
2000
650
330
130
29
>2000
1000
22
6.0
3.7
71
16
1.5
2.7
Scheme 6
13
160
5.8
4.0
430
13
6.3
3.6
57
methyl and C(7)-hydroxyl groups [(+)-18] restores potency
nearly to the levels displayed by discodermolide (+)-1 in
the MCF-7, CCRF-CEM, and most importantly in the NCI/
ADR cell line, while activity in the A549 cell line is
somewhat depressed.
The cytotoxicity of the C(7)-modified analogues (-)-19
and (+)-23 also proved highly informative. As stated above
the Gunasekera group had observed that acetylation of the
C(7)-hydroxyl group significantly enhanced the cytotoxicity
over (+)-discodermolide (1).14 Similar introduction of the
MOM or methoxyl group on the C(7)-hydroxyl of (+)-2
resulted in analogues [(-)-19 and (+)-23], which also
displayed enhanced activity over the parent compound,
especially in the multi-drug resistant NCI/ADR cell line.
Indeed, analogue (+)-23 is currently our most potent
congener. Finally, inversion of the stereogenicity at C(4) and
C(5) of (+)-2, as in (+)-28, led only to a slight loss of
potency in the MCF-7 and A549 cell lines, but to significant
loss of activity in the NCI/ADR cell line. Interestingly,
identical modification of the unsaturated congener (+)-3 as
with (+)-26 resulted in a significant loss of activity (ca.
average of 2 orders of magnitude).
In summary, seven new, totally synthetic analogues of (+)-
discodermolide (1) have been prepared and evaluated for
tumor cell growth inhibitory activity in four human tumor
cell lines. The results indicate that the terminal diene system
does not contribute significantly to the activity. Additionally,
substitution on the C(7)-hydroxyl group appears to have a
very positive impact on cytotoxicity.
metathesis employing the second-generation Grubbs cata-
lyst,15 followed by removal the PMB ether furnished alcohol
(+)-25; hydrogenation then readily provided the correspond-
ing saturated alcohol (+)-27. Alcohols (+)-25 and (+)-27
in turn were subjected to our now standard five-step endgame
synthesis to furnish congeners (+)-26 and (+)-28, respec-
tively.
The synthetic analogues were evaluated for tumor cell
growth inhibitory activity against three drug sensitive cancer
cell lines (MCF-7, A549, and CCRF-CEM), as well as
against the multidrug resistant NCI/ADR cell line. The
results, presented in Table 1, clearly demonstrate that further
simplification of analogue (+)-2 is possible with retention
and in some cases enhanced tumor cell growth inhibitory
activity. For example, although saturation of the terminal
diene [analogues (-)-10 and (+)-13] resulted in a loss of
activity against the NCI/ADR cell line, these congeners
retained comparable cytotoxicity to that of discodermolide
(+)-1 against the drug-sensitive cell lines MCF-7 and A549.
Further simplification of (+)-2 by removal of the C(4)-
Acknowledgment. Financial support was provided by the
National Institutes of Health (Institute of General Medical
Sciences) through Grant No. GM-29028. We also thank our
colleagues at Kosan Biosciences for discussions and Fenghua
Liu for the determination of cytotoxicities.
(11) Ramacandran, P. V.; Krzeminski, M. P.; Reddy, M. V. R.; Brown,
H. C. Tetrahedron: Asymmetry 1999, 10, 11-15.
(12) Evans, D. A.; Fu, G. C.; Anderson, B. A. J. Am. Chem. Soc. 1992,
114, 6679-6685.
(13) Parikh, J. R.; Doering, W. von E. J. Am. Chem. Soc. 1967, 89, 5505-
5507.
(14) Gunasekera, S. P.; Longley, R. E.; Isrucker, R. A. J. Nat. Prod.
2001, 64, 171-174.
(15) Scholl, S.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
953-956.
Supporting Information Available: Representative pro-
cedures, spectral data, and analytical data for all compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL052070M
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Org. Lett., Vol. 7, No. 23, 2005