Design and synthesis C6-C8 bridged epothilone a

Article abstract of DOI:10.1021/ol800422q

A conformationally restrained epothilone A analogue (3) with a short bridge between methyl groups at C6 and C8 was designed and synthesized. Preliminary biological evaluation indicates 3 to be only weakly active (IC₅₀ = 8.5 μM) against the A2780 human ovarian cancer cell line.

Full text of DOI:10.1021/ol800422q

ORGANIC
LETTERS
2008
Vol. 10, No. 8
1565-1568
Design and Synthesis of C6
Epothilone A
−C8 Bridged
Weiqiang Zhan,† Yi Jiang,† Peggy J. Brodie,‡ David G. I. Kingston,‡
Dennis C. Liotta,† and James P. Snyder*,†
Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322, and Department
of Chemistry, Virginia Polytechnic Institute and State UniVersity,
Blacksburg, Virginia 24061
jsnyder@emory.edu
Received January 29, 2008
ABSTRACT
A conformationally restrained epothilone A analogue (3) with a short bridge between methyl groups at C6 and C8 was designed and synthesized.
Preliminary biological evaluation indicates 3 to be only weakly active (IC50
) 8.5 µM) against the A2780 human ovarian cancer cell line.
Epothilones A (EpoA, 1) and B (EpoB, 2) (Figure 1) are
polyketide macrolides isolated in 1993 from the myxobac-
established by Bollag et al.3 In distinct contrast to paclitaxel,
the epothilones possess improved water solubility and activity
against drug-sensitive and multidrug-resistant human cancer
cells both in vitro and in vivo.4 These exceptional advantages,
combined with the ease of synthesis by comparison with
paclitaxel have evoked a vast research effort within academic
and pharmaceutical research groups5 that include numerous
total and partial syntheses,6 extensive structure-activity
relationship (SAR) studies,2,7 and conformational modeling.8,9
Importantly, these contributions have resulted in at least
seven compounds in advanced clinical trials, one of which
(1) (a) Gerth, K.; Bedorf, N.; Ho¨fle, G.; Irschik, H.; Reichenbach, H. J.
Antibiot. 1996, 49, 560. (b) Ho¨fle, G. H.; Bedorf, N.; Steinmetz, H.;
Schomburg, D.; Gerth, K.; Reichenbach, H. Angew. Chem., Int. Ed. Engl.
1996, 35, 1567.
(2) For a review of the chemical biology of epothilones, see: Nicolaou,
K. C.; Roschangar, F.; Vourloumis, D. Angew. Chem., Int. Ed. 1998, 37,
2015.
Figure 1. Structures of EpoA/B and C6-C8 bridged EpoA.
terial strain Sorangium celluosum by Reichenbach, Ho¨fle,
and co-workers.1 The intriguing biological activity2 against
a wide variety of cancer cell lines by stabilizing microtubules
and populating the taxane binding site on â-tubulin was first
(3) 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.
(4) (a) Giannakakou, P.; Sackett, D. L.; Kang, Y. K.; Zhan, Z.; Buters,
J. T.; Fojo, T.; Poruchynsky, M. S. J. Biol. Chem. 1997, 272, 17118. (b)
Kowalski, R. J.; Giannakakou, P.; Hamel, E. J. Biol. Chem. 1997, 272,
2534. (c) Altmann, K. H.; Wartmann, M.; O’Reilly, T. Biochim. Biophys.
Acta 2000, 1470, M79. (d) Wartmann, M.; Altmann, K. H. Curr. Med.
Chem. Anticancer Agents 2002, 2, 123.
† Emory University.
‡ Virginia Polytechnic Institute and State University.
10.1021/ol800422q CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/21/2008

has recently been approved by FDA as anti-cancer drug
(Ixabepilone).10
In addition, SAR studies have suggested that the C1-C8
sector is critical for maintenance of biological activity and
is not amenable to significant change.7 However, certain
modifications within C1-C8 have yielded potent ana-
logues.12 An important data point is available from the work
of Martin et al. who introduced a six-membered ring between
C4-C6 from the pro-R methyl at C4 in the corresponding
EpoB analogue.13 The compound proved to be inactive
against the MCF-7 tumor cell line. The electron crystal-
lographic structure9 suggests a pro-S attachment to be the
compatible link. Stereochemical inversion might then be
responsible for the lack of activity. In this context, EpoA
analogue 3 was conceived as a potential diagnostic test of
the electron crystallographic epothilone binding model.
The retrosynthesis of compound 3 is summarized in
Scheme 1. The approach adopts a Suzuki-Miyaura coupling
Recently, our group proposed a unique EpoA conformation
and microtubule binding model based on electron crystal-
lography (EC), NMR conformer deconvolution, and SAR
analysis.9 A peculiar feature of the proposed binding
conformer is the presence of a syn-pentane interaction
between methyl groups at C6 and C8 that can be locked in
place by incorporating the corresponding carbons in a six-
membered ring (3, Figure 1). Optimization of 3 in the
proposed binding form with OPLS200111 indicated it to be
a stable local minimum (Figure 2). Furthermore, docking
Scheme 1. Retrosynthesis of 3
Figure 2. Docking poses of 1 (yellow) and 3 (cyan) in the EC-
determined tubulin binding site. The shortest epo-tubulin H-H
contact for 3 is 2.3 Å; the sum of the van der Waals radii.
the structure into â-tubulin suggested that the additional CH2
in the newly installed cyclohexane ring would not experience
steric congestion with the protein (Figure 2).
(5) For recent general reviews, see: (a) Altmann, K. H.; Florsheimer,
A.; Bold, G.; Caravatti, G.; Wartmann, M. Chimia 2004, 58, 686. (b)
Altmann, K. H. Cur. Pharm. Des. 2005, 11, 1595. (c) Altmann, K. H.;
Pfeiffer, B.; Arseniyadis, S.; Pratt, B. A.; Nicolaou, K. C. Chemmedchem
2007, 2, 396. (d) Feyen, F.; Cachoux, F.; Gertsch, J.; Wartmann, M.;
Altmann, K. Acc. Chem. Res. 2008, 41, 21.
(6) For recent reviews on the synthesis of epothilones and analogs, see:
(a) Rivkin, A.; Cho, Y. S.; Gabarda, A. E.; Yoshimura, F.; Danishefsky, S.
J. J. Nat. Prod. 2004, 67, 139. (b) Luduvico, I.; Le Hyaric, M.; De Almeida,
M. V.; Da, Silva, A. D. Mini-ReV. Org. Chem. 2006, 3, 49. (c) Watkins, E.
B.; Chittiboyina, A. G.; Avery, M. A. Eur. J. Org. Chem. 2006, 4071.
(7) (a) Nicolaou, K. C.; Vourloumis, D.; Li, T. H.; Pastor, J.; Winssinger,
N.; He, Y.; Ninkovic, S.; Sarabia, F.; Vallberg, H.; Roschangar, F.; King,
N. P.; Finlay, M. R. V.; Giannakakou, P.; VerdierPinard, P.; Hamel, E.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2097. (b) Su, D. S.; Balog, A.;
Meng, D. F.; Bertinato, P.; Danishefsky, S. J.; Zheng, Y. H.; Chou, T. C.;
He, L. F.; Horwitz, S. B. Angew. Chem., Int. Ed. Engl. 1997, 36, 2093. (c)
For reviews, see: ref 5c.
strategy initially developed by Danishefsky for the synthesis
of epothilones A and B.14 The advanced intermediate 6, in
which the cyclohexane core structure has been constructed,
was conceived to derive from 7 utilizing sequential substrate
directed epoxidation and epoxide opening.15 Homoallylic
alcohol 7 is accessible from aldehyde 8 by Brown’s method
for preparing 1-(2-cyclohexenyl)-1-alkanols.16
(12) Regueiro-Ren, A.; Leavitt, K.; Kim, S. H.; Hofle, G.; Kiffe, M.;
Gougoutas, J. Z.; DiMarco, J. D.; Lee, F. Y. F.; Fairchild, C. R.; Long, B.
H.; Vite, G. D. Org. Lett. 2002, 4, 3815.
(8) (a) Carlomagno, T.; Blommers, M. J. J.; Meiler, J.; Jahnke, W.;
Schupp, T.; Petersen, F.; Schinzer, D.; Altmann, K. H.; Griesinger, C.
Angew. Chem., Int. Ed. 2003, 42, 2511. (b) Taylor, R. E.; Chen, Y.; Beatty,
A.; Myles, D. C.; Zhou, Y. Q. J. Am. Chem. Soc. 2003, 125, 26. (c) Heinz,
D. W.; Schubert, W. D.; Hofle, G. Angew. Chem., Int. Ed. 2005, 44, 1298.
(d) For reviews, see: ref 5c.
(9) Nettles, J. H.; Li, H. L.; Cornett, B.; Krahn, J. M.; Snyder, J. P.;
Downing, K. H. Science 2004, 305, 866.
(10) (a) Klar, U.; Buchmann, B.; Schwede, W.; Skuballa, W.; Hoffinann,
J.; Lichtner, R. B. Angew. Chem., Int. Ed. 2006, 45, 7942. (b) Conlin, A.;
Fornier, M.; Hudis, C.; Kar, S.; Kirkpatrick, P. Nat. ReV. Drug DiscoVery
2007, 6, 953.
(13) Martin, H. J.; Pojarliev, P.; Kahlig, H.; Mulzer, J. Chem. Eur. J.
2001, 7, 2261.
(14) (a) Meng, D. F.; Bertinato, P.; Balog, A.; Su, D. S.; Kamenecka,
T.; Sorensen, E. J.; Danishefsky, S. J. J. Am. Chem. Soc. 1997, 119, 10073.
(b) Chemler, S. R.; Trauner, D.; Danishefsky, S. J. Angew. Chem., Int. Ed.
2001, 40, 4544.
(15) (a) For a review of substrate directed reactions, see: Hoveyda, A.
H.; Evans, D. A.; Fu, G. C. Chem. ReV. 1993, 93, 1307. For substituent
directed epoxide opening, see: (b) Flippin, L. A.; Brown, P. A.; Jalaliaraghi,
K. J. Org. Chem. 1989, 54, 3588. (c) Chini, M.; Crotti, P.; Flippin, L. A.;
Gardelli, C.; Macchia, F. J. Org. Chem. 1992, 57, 1713.
(16) (a) Brown, H. C.; Jadhav, P. K.; Bhat, K. S. J. Am. Chem. Soc.
1985, 107, 2564. (b) Brown, H. C.; Bhat, K. S.; Jadhav, P. K. J. Chem.
Soc., Perkin Trans. 1 1991, 2633.
(11) MacroModel 9.0 (Maestro 7.0 interface), supplied by Schro¨dinger,
Portland, OR; www.schrodinger.com.
1566
Org. Lett., Vol. 10, No. 8, 2008

Our synthesis commenced with the known aldehyde 9,17
which was first converted to an enantiomerically enriched
homoallylic alcohol intermediate (98% yield, ee > 95%,
Mosher ester determination) by reaction with (+)-allyldi-
isopinocampheylborane prepared from (-)-chlorodiisopi-
nocampheylborane and allylmagnesium bromide.18 The
homoallylic alcohol intermediate was subsequently subjected
to silylation with TBSOTf to give silyl ether 10 in quantita-
tive yield (Scheme 2). Ozonolysis of 10 followed by a Wittig
Scheme 3. Synthesis of Diene 6
Scheme 2. Synthesis of Aldehyde 8
reaction furnished the desired gem-dimethyl olefin 11 in 80%
yield (2 steps).19 By exposure to HF/pyr, the primary silyl
ether of 11 was selectively demasked in 72% yield,20 and
aldehyde 8 was achieved by subsequent Swern oxidation
(quantitative yield).
epoxide 13 in 93% yield (dr > 20:1 by 1H NMR). The crucial
regiocontrolled alkyl opening of the epoxide was successfully
performed by treatment of 13 with allylmagnesium bromide
in the presence of CuCN (10 mol %) to give the desired
diol 14 (90% yield) along with a trace of C7-alkylated isomer
and bromohydrin.22 It is worth noting that an excess of
Grignard reagent (8 equiv) was required to reduce the
formation of bromohydrin. We reasoned that the regiose-
lectivity of this metal-catalyzed epoxide opening was con-
trolled not only by the Fu¨rst-Plattner rule,23 which favors a
diaxial orientation, but also by stereoelectronic factors
implicated in a chelation process.15b,c Selective silylation of
the sterically less hindered OH group in 14, followed by
Swern oxidation afforded the desired keto diene 6 in 72%
yield (2 steps). The relative configuration of 15 was
confirmed by NOESY cross-peak analysis. To further
confirm the absolute configuration, the conversion of olefin
6 to carboxylic acid 16 was carried out in three steps: (i)
regioselective Sharpless asymmetric dihydroxylation24 which
led to a mixture of diastereometric diols (79% yield, ca. 5:1
Preparation of the Suzuki-Miyaura coupling precursor 6
was undertaken as shown in Scheme 3. Aldehyde 8 was
combined with freshly prepared B-2-cyclohexen-1-yldiiso-
pinocampheylborane 12 followed by oxidative cleavage of
the B-O bond to provide intermediate 7 (96% yield, dr >
20:1 by 1H NMR).16 Surprisingly, both C-C bond formation
and B-O bond cleavage by H2O2 in this reaction were
unexpectedly sluggish (see Supporting Information), but
nonetheless, the reaction gives satisfactory yield and selectiv-
ity. Stereochemistries at C5 and C6 were assigned on the
basis of Brown’s study.16
Homoallylic alcohol directed epoxidation of 7 was achieved
by a vanadium-catalysis strategy21 to provide the hydroxy
(17) Richter, F.; Bauer, M.; Perez, C.; Maichle-Mossmer, C.; Maier, M.
E. J. Org. Chem. 2002, 67, 2474.
(18) (a) Jadhav, P. K.; Bhat, K. S.; Perumal, P. T.; Brown, H. C. J. Org.
Chem. 1986, 51, 432. (b) Nicolaou, K. C.; Ninkovic, S.; Sarabia, F.;
Vourloumis, D.; He, Y.; Vallberg, H.; Finlay, M. R. V.; Yang, Z. J. Am.
Chem. Soc. 1997, 119, 7974.
(19) (a) Lin, M. Y.; Das, A.; Liu, R. S. J. Am. Chem. Soc. 2006, 128,
9340. (b) Takenaka, Y.; Ito, H.; Iguchi, K. Tetrahedron 2007, 63, 510.
(20) Nelson, T. D.; Crouch, R. D. Synthesis 1996, 1031.
(21) (a) Mihelich, E. D.; Daniels, K.; Eickhoff, D. J. J. Am. Chem. Soc.
1981, 103, 7690. (b) Bolm, C. Coord. Chem. ReV. 2003, 237, 245.
(22) Carre, M. C.; Houmounou, J. P.; Caubere, P. Tetrahedron Lett. 1985,
26, 3107.
(23) Eliel, E. L.; Allinger, N. L.; Angyal, S. J.; Morrison, G. A.
Conformational Analysis; Interscience: New York, 1965; p 102.
(24) For Sharpless asymmetric dihydroxylation rate of substituted olefins,
see: (a) Andersson, P. G.; Sharpless, K. B. J. Am. Chem. Soc. 1993, 115,
7047. (b) For a review, see: Becker, H.; Soler, M. A.; Sharpless, K. B.
Tetrahedron 1995, 51, 1345.
1
ratio by H NMR), (ii) cleavage of glycol to aldehyde with
NaIO4, and (iii) Pinnick oxidation25 with NaClO2 (56%, 2
steps). Single crystals of 16 were obtained from hexanes.
X-ray crystallography confirmed that the desired stereo-
chemistry has been maintained (see Supporting Infor-
mation).
For the Suzuki-Miyaura cross coupling, vinyl iodide 5
was prepared from the previously reported aldehyde 1718b
(85% yield, Z/E ) 10:1) using the Stork and Zhao olefination
(25) Bal, B. S.; Childers, W. E.; Pinnick, H. W. Tetrahedron 1981, 37,
2091.
Org. Lett., Vol. 10, No. 8, 2008
1567

protocol (Scheme 4).26 The geometry of the CdC was
olefin 4 in 92% yield. The gem-dimethyl olefin of triene 4
was regioselectively dihydroxylated by the Sharpless protocol
to give diol 18 as a mixture of diastereomers (36% yield,
1
confirmed by H NMR (3J ) 7.5 Hz).26 With the requisite
1
78% BORSM, ca. 5:1 ratio by H NMR). Diols 18 were
cleaved to carboxylic acid 19 (78%, 2 steps) in a fashion
similar to that utilized in the preparation of carboxylic acid
16.
Scheme 4. Synthesis of Vinyl Iodide 5
Completion of the synthesis of bridged epothilone 3
entailed the conversion of 19 to dihydroxy lactone 20 by
employing a procedure used by Nicolaou et al.18b Selective
desilylation with TBAF, followed by Yamaguchi lactoniza-
tion and global desilylation in the presence of freshly
prepared TFA/CH2Cl2 (v/v, 1:4) gave dihydroxy macrolac-
tone 20 in 44% overall yield, which is a bridged epothilone
C analogue.27 Finally, we obtained the C6-C8 bridged
epothilone 3 by treatment with 3,3-dimethyldioxirane (DMDO)
as described by Danishefsky14a to afford a mixture of 3 and
its cis-epoxide diastereomer 3′ in a ca. 2:1 ratio by 1H NMR.
Fortunately, these two diastereomers were separatable by
preparative thin-layer chromatography. The stereochemistry
of the epoxide was determined by NOESY analysis.
A preliminary evaluation of the potency of compound 3
was probed with the A2780 ovarian cancer cell line. Bridged
EpoA 3 is only weakly active with an IC50 ) 8.5 µM. This
corresponds to a potency loss of 3900-fold in comparison
with the activity of EpoA in the isogenic 1A9 cell line.2
Syntheses of other conformationally restrained epothilone
analogs are currently being pursued. If low potency against
tumor cells for such epo-modifications persists, it may
necessitate a re-examination of the electron crystallographic
epothilone-tubulin binding representation.9
coupling precursors in hand, the final steps in the synthesis
of bridged epothilone 3 were carried out as depicted in
Scheme 5. After regioselective hydroboration in the presence
Scheme 5. Complete Synthesis of 3
Acknowledgment. The work was supported in part by
the NIH Grant No. CA-69571. We thank Kenneth Hardcastle
(Emory University) for determination of the X-ray crystal
structure of compound 16.
Supporting Information Available: Molecular modeling
and docking; experimental details, characterization data of
all compounds, and NMR spectra of key intermediates; and
X-ray crystallography data of 16. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL800422Q
(26) Stork, G.; Zhao, K. Tetrahedron Lett. 1989, 30, 2173.
(27) Tang, L.; Shah, S.; Chung, L.; Carney, J.; Katz, L.; Khosla, C.;
Julien, B. Science 2000, 287, 640.
of 9-BBN, olefin 6 was coupled with vinyl iodide 5 following
an approach reported by Danishefsky et al.14 to furnish cis-
1568
Org. Lett., Vol. 10, No. 8, 2008