Design, synthesis, and evaluation of carbamate-substituted analogues of (+)-discodermolide

Article abstract of DOI:10.1021/ol047686a

(Chemical Equation Presented) The design, syntheses, and biological evaluation of 22 totally synthetic analogues of the potent microtubule- stabilizing agent (+)-discodermolide (1) have been achieved. Structure-activity relationships of the C(19) carbamat

Full text of DOI:10.1021/ol047686a

ORGANIC
LETTERS
2005
Vol. 7, No. 2
311-314
Design, Synthesis, and Evaluation of
Carbamate-Substituted Analogues of
(+)-Discodermolide
Amos B. Smith, III,*,† B. Scott Freeze,† Matthew J. LaMarche,† Tomoyasu Hirose,†
Ignacio Brouard,† Paul V. Rucker,† 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, 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 22 totally synthetic analogues of the potent microtubule-stabilizing agent (
(1) have been achieved. Structure activity relationships of the C(19) carbamate were defined, exploiting two synthetically simplified scaffolds,
as well as the parent ( )-discodermolide framework.
+)-discodermolide
−
+
In 1990, Gunasekera and co-workers reported the isolation
and structural elucidation of (+)-discodermolide (1, Scheme
1) from the deep sea marine sponge Discodermia dissoluta.1
Several years later (1996), Ter Haar and co-workers and
Schreiber et al. independently revealed that (+)-discoder-
molide possesses potent antimitotic activity,2 with a mech-
anism of action that, akin to the clinically proven anticancer
agent Taxol (2), entails binding and stabilization of micro-
tubules.2 Importantly, (+)-discodermolide displays both
significant tumor cell growth inhibitory activity against
Taxol-resistant cells3 and cytotoxic synergy with Taxol in a
variety of cell lines.4 Interest in (+)-discodermolide both as
a synthetic target5 and a lead chemotherapeutic agent6
remains strong, as evidenced by five total syntheses5i-n and
ten papers devoted to analogues6g-p published since 2001.
In parallel with our efforts to develop an ever more
practical (i.e., scalable) total synthesis of (+)-discodermolide
† 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.
(3) Kowalski, R. J.; Giannakakou, P.; Gunasekera, S. P.; Longley, R.
E.; Day, B. W.; Hamel, E. Mol. Pharm. 1997, 52, 613-622.
(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.
10.1021/ol047686a CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/22/2004

Scheme 1
Scheme 2
inhibitory activity rivaling that of the natural product.6f Herein
we describe the design, syntheses, and biological evaluation
of derivatives of (+)-1, (+)-3, and (+)-4, which explore the
influence of substitution at the C(19) carbamate.
Employing the synthetically simplified 14-normethyl-
discodermolide scaffold for our initial investigations, we
treated the previously disclosed secondary alcohol (+)-56f
with a range of sterically and electronically varied isocyan-
ates, followed by global deprotection to furnish the corre-
sponding carbamates 6-16 in good to excellent yield
(Scheme 3). Additionally, direct deprotection of (+)-5
furnished descarbamoyl congener (+)-17.
to provide material for clinical development, we broadened
our program to include the production of analogues designed
to probe the structure-activity relationship, as well as to
define the minimum critical structural element necessary for
tumor cell growth inhibition.
Toward this end, we reported in 2001 that the simplified
congeners (+)-14-normethyldiscodermolide (3, Scheme 2)
and (+)-2,3-anhydrodiscodermolide (4) display cell growth
(5) (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) Halstead,
D. P. Ph.D. Thesis, Harvard University, Cambridge, MA, 1999. (g) Paterson,
J.; Florence, G. J.; Gerlach, K.; Scott, J. Angew. Chem., Int. Ed. 2000, 39,
377-380. (h) 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. (i) Paterson, I.; Florence, G. J.; Gerlach,
K.; Scott, J. P.; Sereinig, N. J. Am. Chem. Soc. 2001, 123, 9535-9544. (j)
Harried, S. S.; Lee, C. P.; Yang, G.; Lee, T. I. H.; Myles, D. C. J. Org.
Chem. 2003, 68, 6646-6660. (k) Paterson, I.; Delgado, O.; Florence, G.
J.; Lyothier, I.; Scott, J. P.; Sereinig, N. Org. Lett. 2003, 5, 35-38. (l)
Smith, A. B., III; Freeze, B. S.; Brouard, I.; Hirose, T. Org. Lett. 2003, 5,
4405-4408. (m) 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 and references therein.
(n) Paterson, I.; Lyothier, I. Org. Lett. 2004, 6, 4933-4936.
Scheme 3
(6) (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, 6, 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.
In three of the four cell lines tested (Table 1), attachment
of the phenyl, p-fluorophenyl, p-bromophenyl, or pyridyl
substituents (6-8, 12) led to a slight reduction in cell growth
inhibition (a factor of 2-3), whereas carbamates possessing
either a benzophenone moiety (10), electron-rich aromatics
(9, 11), or nonpolar substituents such as n-propyl, allyl, and
benzyl (13-15), yielded analogues with potency comparable
to that of the natural product. Alternatively, the phenyl
sulfonamide congener 16 displays significantly reduced
cytotoxicity, as does the carbamate-deletion analogue 17.7
312
Org. Lett., Vol. 7, No. 2, 2005

Table 1. Cytotoxicity Observed for Analogues 6-17
Table 2. Cytotoxicity Observed for Analogues 21-25
cytotoxicity IC50, nM
cytotoxicity IC50, nM
MCF-7
NCI/ADR
A549
SKOV-3
MCF-7
NCI/ADR
A549
SKOV-3
(+)-1
(+)-3
(+)-6
(+)-7
(+)-8
(+)-9
(+)-10
(+)-11
(+)-12
(+)-13
(+)-14
(+)-15
(+)-16
(+)-17
28
46
48
42
84
23
23
6.2
27
22
240
8200
22
21
35
47
47
90
23
36
12
38
(+)-1
(+)-4
28
5.6
5.6
120
290
630
660
240
463
260
22
8.6
9.4
400
410
2000
2000
21
3.4
7.6
250
380
790
610
50
135
125
247
89
67
27
110
73
55
>1000
>1000
>1000
>1000
>1000
2000
>1000
>1000
4000
(+)-21
(+)-22
(+)-23
(+)-24
(+)-25
>1000
>1000
>1000
>1000
20
10
30
bamoyl derivatives thereof were selected for synthesis and
biological evaluation. Required for this venture was second-
ary alcohol (+)-19 (Scheme 4A), accessible via Wittig
reaction of the previously described phosphonium salt (+)-
AB5e with the corresponding R,â-unsaturated lactone alde-
hyde (-)-18, followed by oxidative removal of the secondary
PMB ether. Alternatively, (+)-19 can be obtained via base-
promoted elimination of TBSOH from lactone (+)-20, again
an advanced intermediate in our gram-scale synthesis of (+)-
discodermolide,5e followed by DDQ-mediated removal of the
PMB group. Aldehyde (-)-18, the requisite C(1)-C(8)
fragment for our modular synthesis, was available in seven
steps from known alcohol (-)-268 (Scheme 4B).
17
27
>1000
365
4000
>1000
>1000
51
>1000
>1000
>1000
200
Also of significance, in marked contrast with (+)-discoder-
molide (1), which retains significant cell growth inhibitory
activity in the Taxol-resistant p-glycoprotein-overexpressing
cell line (NCI/ADR), (+)-14-normethyldiscodermolide and
carbamoyl derivatives thereof generally demonstrate cyto-
toxicity only at concentrations greater than 1 µM in this cell
line.
(+)-2,3-Anhydrodiscodermolide (4), isolated as a minor
byproduct of global deprotection during our successful
campaign to synthesize one gram of (+)-discodermolide, was
found to be a more potent inhibitor of tumor cell growth
than the natural product.6f Thus, analogue (+)-4 and car-
In this series, the presence of the N,N-dimethylaniline
moiety as in (+)-21 produced an analogue that was again
superior in potency to the natural product (+)-1 in three of
the four tested cell lines (Table 2). However, carbamates with
pyridyl, benzyl, allyl, or propyl substituents [22-25], changes
Scheme 4
Org. Lett., Vol. 7, No. 2, 2005
313

that had minimal effect on the cytotoxicity in the 14-
normethyl series (Table 1, 12-15), severely degraded cell
growth inhibitory activity when applied to the R,â-unsatu-
rated lactone scaffold.
Table 3. Cytotoxicity Observed for Analogues 30-34
cytotoxicity IC50, nM
MCF-7
NCI/ADR
A549
SKOV-3
To extend the above carbamoyl substitutions to the natural
(+)-discodermolide scaffold, the n-propyl, benzophenone,
and N,N-dimethylaniline congeners 30-32 were prepared
from the corresponding secondary alcohol (+)-29, a related
late-stage intermediate in our synthesis of (+)-discodermolide5e
(Scheme 5). In addition, to probe further the effect of
heteroatom incorporation in the carbamate moiety, imidazole
(+)-33 and morpholine (+)-34 were also constructed and
evaluated.
(+)-1
28
27
8.4
1.9
33
240
>1000
>1000
450
>1000
>1000
22
54
10
5.1
58
30
21
34
8
1.8
31
14
(+)-30
(+)-31
(+)-32
(+)-33
(+)-34
10
tumor cell growth inhibitory activity in four human tumor
cell lines. The NCI/ADR multidrug-resistant cell line appears
to be very strict with respect to substitution at the carbamate,
as only two analogues (21 and 32) retained submicromolar
activity in this cell line. Potent cytotoxicity is, however,
retained in the drug-sensitive cell lines when the carbamate
is appended with a range of sterically and electronically
varied substituents. In general, compounds derived from the
parent (+)-discodermolide scaffold (1) and from the 14-
normethyl variant (3) were highly potent, while analogues
stemming from (+)-2,3-anhydrodiscodermolide (4), which
is itself more cytotoxic than the natural product, proved to
be over an order of magnitude less active than (+)-
discodermolide. In conclusion, on the basis of the tolerance
demonstrated by substitution at the C(19) carbamate, we
believe that this region of the discodermolide skeleton holds
considerable promise as a structural locale to alter favorably
the pharmacokinetic profile of this potentially important class
of chemotherapeutic agents.
Scheme 5
In this series, activity was again diminished relative to
(+)-discodermolide (1) in the NCI/ADR drug-resistant cell
line (Table 3). Conversely, in the three sensitive cell lines,
the n-propyl-substituted analogue (+)-30 proved to be only
slightly less potent than (+)-discodermolide (1), while the
benzophenone [(+)-31] and N,N-dimethylaniline [(+)-32]
derivatives displayed a 3- to 14-fold increase in cell growth
inhibitory activity. Introduction of alternate heteroatomic
substructures as in (+)-33 and (+)-34 led to compounds that
were generally equipotent with the natural product in the
sensitive cell lines.
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 Fengua Liu for performing the cytotoxicity assays. We
also thank Pharmacia for a Graduate Scholar Fellowship to
M.J.L., the NIH for a Postdoctoral Fellowship to P.V.R., and
Ministerio de Educacio´n Cultura y Deportes (Spain) for a
Postdoctoral Fellowship to I.B.
To summarize, 22 new, totally synthetic analogues of (+)-
discodermolide (1) have been prepared and evaluated for
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.
(7) For comparison, Gunasekera and co-workers recently reported the
isolation of the natural product 19-descarbamoyldiscodermolide, which
exhibits cytotoxicity 4-5-fold lower than that of (+)-discodermolide in
the P388 and A549 cell lines. Gunasekera, S. P.; Paul, G. K.; Longley,
R. E.; Isbrucker, R. A.; Pomponi, S. A. J. Nat. Prod. 2002, 65, 1643-
1648.
(8) Keck, G. E.; Krishnamurthy, D. Org. Synth. 1998, 75, 12-18.
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