Design, synthesis, and evaluation of analogues of (+)-14- normethyldiscodermolide

Article abstract of DOI:10.1021/ol0476873

(Chemical Equation Presented) The design, syntheses, and biological evaluation of nine totally synthetic analogues of the microtubule-stabilizing agent (+)-14-normethyldiscodermolide (2) are reported. Simplification at the C(21)-C(24) terminal diene and a

Full text of DOI:10.1021/ol0476873

ORGANIC
LETTERS
2005
Vol. 7, No. 2
315-318
Design, Synthesis, and Evaluation of
Analogues of
(+)-14-Normethyldiscodermolide
Amos B. Smith, III,*,† B. Scott Freeze,† Matthew J. LaMarche,† Tomoyasu Hirose,†
Ignacio Brouard,† Ming Xian,† Kurt F. Sundermann,‡ Simon J. Shaw,‡
Mark A. Burlingame,‡ Susan Band Horwitz,§ and David C. Myles*,‡
Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104, Kosan Biosciences, Inc., 3832 Bay Center Place,
Hayward, California 94545, and Department of Molecular Pharmacology,
Albert Einstein College of Medicine, Bronx, New York 10461
smithab@sas.upenn.edu
Received November 10, 2004
ABSTRACT
The design, syntheses, and biological evaluation of nine totally synthetic analogues of the microtubule-stabilizing agent
normethyldiscodermolide (2) are reported. Simplification at the C(21) C(24) terminal diene and at the C(1) C(5) lactone moieties reveals significant
structure activity relationships.
(+)-14-
−
−
−
(+)-Discodermolide (1, Scheme 1), a microtubule-binding
antimitotic agent isolated in 1990 from the deep sea marine
sponge Discodermia dissoluta,1 was concurrently shown in
1996 by ter Haar and co-workers and Schreiber et al. to
possess potent antimitotic activity, with a mechanism of
action similar to the clinically proven anticancer agent Taxol
(e.g., the binding and stabilization of microtubules).2 To date,
thirteen total syntheses have been published.3 Additional
effort in (+)-discodermolide has focused on the synthesis
of structurally and thereby synthetically simplified analogues
of this potentially important chemotherapeutic agent.4
In 2001, we reported the rational design and synthesis of
(+)-14-normethyldiscodermolide (2),4f wherein replacement
of the C(13)-C(14) (Z)-trisubstituted olefin found in (+)-
discodermolide with a cis-disubstituted counterpart enabled
a significant simplification to our initial synthetic route to
this class of antitumor agents. Subsequent biological evalu-
ation revealed that the tubulin polymerization and cytotox-
icity of (+)-2 were only slightly depressed relative to the
Scheme 1
† University of Pennsylvania.
‡ Kosan Biosciences, Inc.
§ Albert Einstein College of Medicine.
(1) Gunasekera, S. P.; Gunasekera, M.; Longley, R. E.; Schulte, G. K.
J. Org. Chem. 1990, 55, 4912-4915. Correction: J. Org. Chem. 1991, 56,
1346.
(2) (a) ter Haar, E.; Kowalski, R. J.; Hamel, E.; Lin, C. M.; Longley, R.
E.; Gunasekera, S. P.; Rosenkranz, H. S.; Day, B. W. Biochemistry 1996,
35, 243-250. (b) Hung, D. T.; Chen, J.; Schreiber, S. L. Chem. Biol. 1996,
3, 287-293.
10.1021/ol0476873 CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/22/2004

natural product. As part of our continuing program directed
at the production of analogues designed to probe the
structure-activity relationships, as well as to define the
minimum structure element necessary for tumor cell growth
inhibitory activity, we report herein the results of simplifica-
tion of the C(1)-C(5) lactone and C(21)-C(24) dienyl
subunits of (+)-14-normethyldiscodermolide (2).
Modifications of each of these regions of (+)-discoder-
molide have been reported previously. In 1996, Schreiber
and co-workers disclosed that extension of the dienyl unit
with long side chains led to analogues with potency only
moderately diminished relative to the natural product and
that substitution of the lactone carboxyl with either a
thiophenyl S,O-acetal or a short-chain O,O-acetal produced
compounds equipotent with the natural product.4b Subse-
quently, Gunasekera and co-workers revealed the isolation
of three new naturally occurring congeners of (+)-discoder-
molide (1) bearing variations in the lactone region, namely,
2-epi-discodermolide, 2-normethyldiscodermolide, and the
open-chain methyl ester 5-hydroxymethyldiscodermolate, all
of which proved to be 3- to 6-fold less potent than
discodermolide.4i Separately, via partial hydrogenation of the
natural product, the Gunasekera group prepared 21,23-
tetrahydrodiscodermolide.4h Importantly, the saturated variant
proved to be more actiVe than (+)-discodermolide when
assayed against the P388 and A549 human tumor cell lines.4h
Utilizing the synthetically simplified 14-normethyl-
discodermolide4f framework for our studies, we first sought
to explore the effects of saturation and truncation of the
terminal diene. To begin, we undertook the synthesis of the
diene-deletion congener (+)-7 (Scheme 2), which proved to
Scheme 2
(3) (a) Nerenberg, J. B.; Hung, D. T.; Somers, P. K.; Schreiber, S. L. J.
Am. Chem. Soc. 1993, 115, 12621-12622. (b) Smith, A. B., III; Qiu, Y.;
Jones, D. R.; Kobayashi, K. J. Am. Chem. Soc. 1995, 117, 12011-12012.
(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) Smith, A. B., III; Kaufman, M. D.; Beauchamp, T. J.;
LaMarche, M. J.; Arimoto, H. Org. Lett. 1999, 1, 1823-1826. (f) Paterson,
J.; Florence, G. J.; Gerlach, K.; Scott, J. Angew. Chem., Int. Ed. 2000, 39,
377-380. (g) Smith, A. B., III; Beauchamp, T. J.; LaMarche, M. J.;
Kaufman, M. D.; Qiu, Y.; Arimoto, H.; Jones, D. R.; Kobayashi, K. J. Am.
Chem. Soc. 2000, 122, 8654-8664. (h) Paterson, I.; Florence, G. J.; Gerlach,
K.; Scott, J. P.; Sereinig, N. J. Am. Chem. Soc. 2001, 123, 9535-9544. (i)
Harried, S. S.; Lee, C. P.; Yang, G.; Lee, T. I. H.; Myles, D. C. J. Org.
Chem. 2003, 68, 6646-6660. (j) Paterson, I.; Delgado, O.; Florence, G. J.;
Lyothier, I.; Scott, J. P.; Sereinig, N. Org. Lett. 2003, 5, 35-38. (k) Smith,
A. B., III; Freeze, B. S.; Brouard, I.; Hirose, T. Org. Lett. 2003, 5, 4405-
4408. (l) Mickel, S. J.; Sedelmeier, G. H.; Niederer, D.; Daeffler, R.; Osmani,
A.; Schreiner, K.; Seeger-Weibel, M.; Berod, B.; Schaer, K.; Gamboni, R.;
Chen, S.; Chen, W.; Jagoe, C. T.; Kinder, F. R., Jr.; Loo, M.; Prasad, K.;
Repic, O.; Shieh, W.-C.; Wang, R.-M.; Waykole, L.; Xu, D. D.; Xue, S.
Org. Proc. Res. DeV. 2004, 8, 92-130. (m) Paterson, I.; Lyothier, I. Org.
Lett. 2004, 6, 4933-4936.
be readily accessible via an advanced intermediate in our
synthesis of (+)-14-normethyldiscodermolide. Toward this
end, mesylation of known alcohol (+)-3,4f followed in turn
by displacement with lithium aluminum hydride, removal
of the primary trityl ether, iodination, and treatment with
triphenylphosphine, provided Wittig salt (+)-4. Subsequent
Wittig coupling with known aldehyde (-)-5,3e DDQ-medi-
ated removal of the secondary p-methoxybenzyl ether,
carbamate installation, and global deprotection completed
construction of (+)-7, in which the C(21) through C(24)
substructure had been removed.
We next turned attention to the construction of the highly
elaborated aldehyde (+)-10 (Scheme 3), from which we
envisioned diene replacement analogues would readily
emerge. Iodination of the previously reported alcohol (+)-
(4) (a) Hung, D. T.; Nerenberg, J. B.; Schreiber, S. L. Chem. Biol. 1994,
1, 67-71. (b) Hung, D. T.; Nerenberg, J. B.; Schreiber, S. L. J. Am. Chem.
Soc. 1996, 118, 11054-11080. (c) Paterson, I.; Florence, G. J. Tetrahedron
Lett. 2000, 41, 6935-6939. (d) Gunasekera, S. P.; Longley, R. E.; Isbrucker,
R. A. J. Nat. Prod. 2001, 64, 171-174. (e) Isbrucker, R. A.; Gunasekera,
S. P.; Longley, R. E. Cancer Chemother. Pharmacol. 2001, 48, 29-36. (f)
Martello, L. A.; LaMarche, M. J.; He, L.; Beauchamp, T. J.; Smith, A. B.,
III; Horwitz, S. B. Chem. Biol. 2001, 8, 843-855. As they were not included
in the original paper, full experimental details for the compounds reported
therein are provided in the Supporting Information of this work. (g) Curran,
D. P.; Furukawa, T. Org. Lett. 2002, 4, 2233-2235. (h) Gunasekera, S. P.;
Longley, R. E.; Isbrucker, R. A. J. Nat. Prod. 2002, 65, 1830-1837. (i)
Gunasekera, S. P.; Paul, G. K.; Longley, R. E.; Isbrucker, R. A.; Pomponi,
S. A. J. Nat. Prod. 2002, 65, 1643-1648. (j) Minguez, J. M.; Giuliano, K.
A.; Balachandran, R.; Madiraju, C.; Curran, D. P.; Day, B. W. Mol. Cancer
Ther. 2002, 1, 1305-1313. (k) Shin, Y.; Choy, N.; Balachandran, R.;
Madiraju, C.; Day, B. W.; Curran, D. P. Org. Lett. 2002, 4, 4443-4446.
(l) Choy, N.; Shin, Y.; Nguyen, P. Q.; Curran, D. P.; Balachandran, R.;
Madiraju, C.; Day, B. W. J. Med. Chem. 2003, 46, 2846-2864. (m)
Minguez, J. M.; Kim, S.-Y.; Giuliano, K. A.; Balachandran, R.; Madiraju,
C.; Day, B. W.; Curran, D. P. Bioorg. Med. Chem. 2003, 11, 3335-3357.
(n) Paterson, I.; Delgado, O. Tetrahedron Lett. 2003, 44, 8877-8882. (o)
Burlingame, M. A.; Shaw, S. J.; Sundermann, K. F.; Zhang, D.; Petryka,
J.; Mendoza, E.; Liu, F.; Myles, D. C.; LaMarche, M. J.; Hirose, T.; Freeze,
B. S.; Smith, A. B., III. Bioorg. Med. Chem. Lett. 2004, 14, 2335-2338.
(p) Gunasekera, S. P.; Mickel, S. J.; Daeffler, R.; Niederer, D.; Wright, A.
E.; Linley, P.; Pitts, T. J. Nat. Prod. 2004, 67, 749-756.
Scheme 3
316
Org. Lett., Vol. 7, No. 2, 2005

84f and treatment with triphenylphosphine yielded the cor-
responding Wittig salt. Union with known aldehyde (-)-
5,3e reductive opening of the PMP acetal (with concomitant
reduction of the lactone to a mixture of lactols), and Dess-
Martin oxidation furnished (+)-10. Parallel Wittig coupling
with methyl, ethyl, and n-propyl triphenylphosphonium
bromide, respectively, followed by deprotection, carbamate
installation, and global deprotection, produced analogues (+)-
11, (+)-12, and (+)-13.
analogous series of 14-normethyl lactone-replacement con-
geners. To this end, R,â-unsaturated lactone aldehyde (-)-
19 was constructed from known alcohol 165 (Scheme 5).
Scheme 5
In vitro evaluation of the tumor cell growth inhibitory
activity in the multidrug-resistant cell line NCI/ADR revealed
that the 14-normethyl analogues of (+)-discodermolide (7,
11-13) are essentially inactive at submicromolar levels
(Table 1). Likewise, the most truncated analogues, (+)-7 and
(+)-11, display no appreciable cytotoxicity in any of the cell
lines tested. Reincorporation of carbon C(23) in the form of
a monoene [(+)-12] revealed moderate cytotoxicity in
MCF-7 and SKOV-3 cells, while (+)-13, which retains the
full carbon skeleton of (+)-2, but with a saturated terminus,
displayed potency rivaling that of the natural product. Cell
growth inhibition of congeners (+)-12 and (+)-13 proved
to be somewhat lower in the A549 cell line.
Protection of the secondary alcohol in (-)-16 as the tert-
butyldimethylsilyl ether, followed by ozonolysis and Brown
crotylation, furnished (-)-17 in both good yield and dia-
stereoselectivity.6 Acylation of the resultant secondary hy-
droxyl with acryloyl chloride, followed by ring-closing
metathesis,7 led to lactone (-)-18. DDQ-mediated removal
of the p-methoxybenzyl ether and Dess-Martin periodinane
oxidation furnished the desired aldehyde (-)-19. Wittig union
with known phosphonium salt (+)-204f (Scheme 6), followed
by oxidative removal of the PMB ether, carbamate instal-
lation, and global deprotection, ultimately produced lactone
congener (+)-21. The C(2) methyl derivative (+)-22 was
accessed via base-promoted â-elimination from the corre-
sponding tetra-TBS lactone, followed by global deprotection.
The results of biological evaluation are presented in Table 2.
Table 1. Cytotoxicity Observed for Analogues 7 and 11-13
cytotoxicity IC50, nM
MCF-7
NCI/ADR
A549
SKOV-3
(+)-1
(+)-2
(+)-7
(+)-11
(+)-12
(+)-13
28
46
>1000
>1000
72
240
8200
>1000
>1000
>1000
>1000
22
21
35
>1000
>1000
130
50
>1000
>1000
570
28
100
20
Scheme 6
In the discodermolide series [i.e., intact C(14) methyl
group], simplification of the lactone moiety previously
resulted in enhanced cytotoxicity. For example, in 2001, we
reported that (+)-2,3-anhydrodiscodermolide (14, Scheme
4) displays in vitro activity superior to that of the natural
product.4f Subsequently, studies from our laboratory revealed
that (+)-15 and several related congeners retained significant
cell growth inhibitory activity despite removal of fiVe
stereogenic centers!4o
To extend these promising results to the 14-normethyl-
discodermolide scaffold, we designed and synthesized an
Encouraged also by the potency of the previously reported
(+)-discodermolide analogue (+)-15 (Scheme 4, Table 2),
in which the entire C(1)-C(7) region had been replaced with
a m-hydroxyphenylethylene unit, we turned to the construc-
tion of additional simplified C(1)-C(7) congeners employing
the (+)-14-normethyldiscodermolide skeleton (2). Synthetic
routes to the requisite aldehydes for this venture are presented
in Scheme 7. Known benzyl bromide8 23 was subjected to
allylmagnesiumbromide, followed by ozonolysis to provide
aldehyde 25, which incorporates a protected m-phenol as the
carbocycle. In similar fashion, treatment of the known aniline
bromide 269 with allylmagnesium bromide followed by
ozonolysis afforded aldehyde 28. Finally, aldehyde 31,
Scheme 4
Org. Lett., Vol. 7, No. 2, 2005
317

required for the pyridol-derived δ-lactone mimic, was pre-
pared from commercially available 2-hydroxy-6-methyl-
pyridine 29. Protection of 29 as the methoxyethoxymethyl
(MEM) ether followed by deprotonation and alkylation at
the benzylic position afforded olefin 30. Two-step oxidative
cleavage then furnished the desired aldehyde 31. Wittig
reaction of the three aldehydes with salt (+)-20, followed
by deprotection, carbamate installation, and acid-mediated
deprotection, yielded phenol (+)-32, aniline (+)-33, and
pyridone (+)-34, respectively. In general, the synthetic
sequences proved to be efficient.
the hydroxyphenylethylene unit replaces the southern lactone.
While the 14-methylated congener of this compound [(+)-
15] displays significant potency, no cytotoxicity is observed
with the 14-normethyl variant (+)-32 at levels up to 1 µM.
Replacement of the phenol with an aniline moiety, as in (+)-
33, likewise leads to an inactive congener, as does wholesale
substitution of the lactone with a pyridone ring [(+)-34].
Table 2. Cytotoxicity for Analogues 21, 22, and 32-34
cytotoxicity IC50, nM
MCF-7
NCI/ADR
A549
SKOV-3
(+)-1
(+)-2
28
46
5.6
130
45
240
8200
463
22
50
8.6
190
300
21
35
3.4
40
50
Scheme 7
(+)-14
(+)-15
(+)-21
(+)-22
(+)-32
(+)-33
(+)-34
390
>1000
>1000
>1000
>1000
>1000
185
536
82
>1000
>1000
>1000
>1000
>1000
>1000
>1000
790
>1000
In summary, nine new totally synthetic analogues of (+)-
14-normethyldiscodermolide (2) have been prepared and
evaluated for tumor cell growth inhibitory activity in four
human tumor cell lines. The results clearly exhibit a trend
of decreasing activity as the C(24) terminus is truncated,
while isosteric replacement of the terminal olefin with a
saturated counterpart preserves cell growth inhibitory activity.
Unlike the highly potent discodermolide congener (+)-2,3-
anhydrodiscodermolide (14), (+)-2,3-anhydro-14-normethyl-
discodermolide (22) exhibits significantly reduced cytotox-
icity, especially in the A549 human tumor cell line. Interest-
ingly, removing the C(2) methyl group from 22 restores
potency in the drug-sensitive cell lines, but further simpli-
fication (32-34) proves quite detrimental. Finally, the 14-
normethyl congeners of (+)-discodermolide demonstrate a
general trend of relative inactivity against the NCI/ADR
multidrug-resistant cell line. The underlying nature of these
results and the subtle interplay between structure and function
in these systems continues to be the subject of active
investigation in our laboratories.
Acknowledgment. Financial support was provided by the
National Institutes of Health (Institute of General Medical
Sciences) through Grant GM-29028, the Department of the
Army through Grant DAMD 17-00-1-0404, and by a
Sponsored Research Agreement between the University of
Pennsylvania and Kosan Biosciences, Inc., where Professor
Smith is a member of the Scientific Advisory Board. We
thank Pharmacia for a Graduate Scholar Fellowship to M.J.L.
and the Ministerio de Educacio´n Cultura y Deportes (Spain)
for a Postdoctoral Fellowship to I.B.
This series of analogues again demonstrated the impotency
of the 14-normethyl variants of (+)-discodermolide (1) in
the multidrug-resistant NCI/ADR cell line (Table 2). Interest-
ingly, while dehydration of the lactone of (+)-discodermolide
(1) to form (+)-14 leads to a more potent compound in the
three drug-sensitive cell lines, identical modification to the
14-normethyl scaffold, as in (+)-22, results in a significant
decrease in tumor cell growth inhibition. Further simplifica-
tion by removal of the C(2) methyl group [(+)-21] restores
potency to the levels displayed by the parent (+)-14-
normethyldiscodermolide (2) in the MCF-7 and SKOV-3
lines, while activity in the A549 cell line is relatively
depressed, a trend also seen in the diene congeners (+)-12
and (+)-13. An even more striking contrast between the
discodermolide and the 14-normethyl scaffolds appears when
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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.
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Org. Lett., Vol. 7, No. 2, 2005