From meiogynin A to the synthesis of dual inhibitors of Bcl-xL and Mcl-1 anti-apoptotic proteins

Article abstract of DOI:10.1039/c4cc01830c

The synthesis of one of the most potent dual inhibitors of the anti-apoptotic proteins Bcl-xL and Mcl-1 is reported. This analogue of a natural sesquiterpenoid dimer meiogynin A was elaborated by a convergent asymmetric synthesis with 36% yield in ten steps. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01830c

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
From meiogynin A to the synthesis of dual
inhibitors of Bcl-xL and Mcl-1 anti-apoptotic
proteins†
Cite this: Chem. Commun., 2014,
50, 8593
Received 11th March 2014,
Accepted 8th June 2014
S. Desrat, C. Remeur, C. Geny, G. Riviere, C. Colas, V. Dumontet, N. Birlirakis,
´
`
B. I. Iorga and F. Roussi*
DOI: 10.1039/c4cc01830c
www.rsc.org/chemcomm
The synthesis of one of the most potent dual inhibitors of the anti- Bcl-2 and Mcl-1.6 To date, only four distinct families of such
apoptotic proteins Bcl-xL and Mcl-1 is reported. This analogue of a pan-Bcl-2 inhibitors have been described: natural gossypol and
natural sesquiterpenoid dimer meiogynin A was elaborated by a its derivatives,7 obatoclax,8 the acetonaphtho-type compounds9
convergent asymmetric synthesis with 36% yield in ten steps.
and the ABT-263 derived molecules.10 Nevertheless, their flat
poly-aromatic structure limits the chemical space explored.11
A few years ago, meiogynin A 1, a natural Bcl-xL and
Natural products possess a vast chemical diversity and cover
a large chemical space. For these reasons, they still play Mcl-1 inhibitor, was isolated from a Malaysian tree bark12
a significant role in the drug discovery and development and synthesized by our team by a biomimetic approach.13
process.1 Thus, from the 1940s to date, 75% of the 175 small We explored different modifications on the skeleton and we
molecules used in cancer therapy are either natural products found that aromatic functionalization of the lateral chain gave
or derivatives of natural products.2 Screening of plant extracts, the most promising results about the inhibition of anti-
marine organisms or microorganisms can provide highly apoptotic proteins. Therefore, several analogues have been
original and functionalized bioactive molecules that are unli- prepared through a protecting group free and convergent
kely to be obtained by the screening of synthetic libraries. synthesis involving the condensation of three fragments: triene
In fact, chemical complexity is often a criterion of specificity 3, dienophile 4 and various aromatic partners 5 (Fig. 1).
for the target of interest.
The triene 3 was prepared in five linear steps (38%
Bcl-2 family proteins are important regulators of apoptosis overall yield) with a perfect control of the double bond stereo-
used by multicellular organisms to regulate tissue homeostasis chemistry (Scheme 1). We decided to introduce a chlorine atom
through the elimination of useless or potentially harmful cells. in the benzylic position to increase the triene reactivity in the
These proteins, divided into anti-apoptotic and pro-apoptotic Diels–Alder reaction as conjugated aromatic trienes are quite
members, are often overexpressed in many kinds of cancer or unreactive,14 and also for possible chemical modifications.
are involved in the resistance to chemotherapy. Targeting these
proteins is a highly promising strategy for anticancer therapy
that has emerged over the last decades.3 Particularly, the
development of small molecules that are able to bind the
hydrophobic cleft of the anti-apoptotic proteins, liberating
the pro-apoptotic ones and restoring apoptosis, is an enormous
challenge. So far, more than a dozen of small molecule inhibi-
tors (SMIs) have been reported.4 Most of them are Bcl-xL and/or
Bcl-2 selective, resulting in an upregulation of Mcl-1 that elicits
resistance to many cancer cell lines.5 Therefore, effective treat-
ment may require simultaneous inhibition of at least Bcl-xL or
Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles (ICSN),
CNRS UPR 2301, 1 av de la Terrasse, 91 198 Gif-sur-Yvette Cedex, France.
E-mail: fanny.roussi@cnrs.fr
Fig. 1 Pan-Bcl-2 inhibitor 2a derived from the structure of meiogynin A 1
† Electronic supplementary information (ESI) available: Experimental proce-
and its disconnection approach.
dures, supplementary data and NMR spectra. See DOI: 10.1039/c4cc01830c
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 8593--8596 | 8593

View Article Online
Communication
ChemComm
Scheme 3 Formation of the cis-decalin 17 by Diels–Alder cycloaddition
using 2-bromophenylboronic acid as the catalyst.
Scheme 1 Synthesis of the triene 3: (a) (triisopropylsilyl)acetylene (1.1 eq.),
CuI (20 mol%), Pd(PPh3)2Cl2 (5 mol%), Et3N, RT, 18 h; (b) NBS (1.1 eq.), AgF
(1.1 eq.), MeCN, RT, 18 h; (c) LiCl (2 eq.), [(allyl)PdCl]2 (5 mol%), cis,cis-1,5-
cyclooctadiene (10 mol%), HOAc, 80 1C, 18 h (80%); (d) LiOHÁH2O (5 eq.), THF/
H2O (2:1), RT, 6 h; (e) Pd(PPh3)4 (5 mol%), K3PO4 (5 eq.), THF/water (2 : 1),
50 1C, dark, 6 h; (f) Cp2ZrHCl (5 mol%), pinacolborane (1.05 eq.), neat, RT, 24 h.
triflate intermediate using Comins’ reagent21 and subsequent
palladium-catalyzed carbonylation.22
Having the two partners 3 and 4 in hand, the Diels–Alder
cycloaddition reaction was performed using a substoichio-
metric amount of 2-bromophenylboronic acid (Scheme 3),
which lowers the LUMO energy level of a,b-unsaturated carb-
oxylic acids as dienophiles through a monoacylboronate
intermediate.23 The cycloaddition reaction could be run on a
multi-gram scale and the desired cis-decalin 17 was obtained in
80% yield after 18 h under reflux in 1,2-dichloroethane
(Scheme 3). The reaction was totally regioselective since the
dienophile 4 reacts exclusively with the less hindered diene of
3. In addition, an excellent stereoselectivity was observed to the
extent that more than 95% of the endo product was obtained.
The lateral chain of 17 was then selectively extended by
Buchwald–Hartwig coupling with amide, sulfonamide, alkyl-
amine, aniline, or phenol (Scheme 4). The cross coupling
products 2a–g were obtained utilizing a monodentate biaryl
phosphine–Pd catalyst 1824 in good to excellent yields, except
for the biaryl derivative 2c.
After a Sonogashira coupling on methyl p-iodobenzoate 6,
the (triisopropylsilyl)acetylene obtained was directly converted
to the bromoalkyne 7 in 93% yield by a desilylative bromination
using a stoichiometric amount of AgF and NBS.15 Then, a
stereoselective Pd-catalyzed hydrochloration involving LiCl as
the chlorine source exclusively led to the (Z)-1-bromo-2-chloro-
alkene 8 in good yield.16 A saponification of the ester followed by
a Suzuki coupling with the freshly prepared boronic ester 12 gave
the desired chlorinated triene 3 in 52% yield, which was directly
engaged in the next Diels–Alder cycloaddition step.
In parallel, an efficient asymmetric synthesis of dienophile
4 was carried out in 8 steps with 73% yield from p-methoxy-
phenethyl alcohol 13 (Scheme 2). A Birch reduction followed by
cleavage of the methyl enol ether and reduction of the remain-
ing double bond provided 14 in 87% yield.17 The terminal
alcohol was then esterified with p-chlorobenzoyl chloride for
further extension of the lateral chain. The prochiral cyclohexa-
none 15 was converted to the cyclohexenone 16 in 92% yield
with an excellent e.r. (94 : 6) in a two-step procedure.18 Indeed,
enantioselective deprotonation with a chiral lithium amide19
and in situ enolate quench with trimethylsilyl chloride afforded
the corresponding chiral silyl enol ether which was then con-
verted to 16 by Saegusa oxidation using a catalytic amount of
palladium(II) acetate under an O2 atmosphere.20 Finally, the
dienophile 4 was obtained in 94% yield after formation of a
These meiogynin A analogues 2 and their precursor 17 were
evaluated for binding affinities to anti-apoptotic proteins Bcl-xL
and Mcl-1 (Table 1). First of all, 17 shows a higher affinity for
both proteins than the natural meiogynin A 1 with a sub-
micromolar value (entry 2). Second, all elongated compounds
2 exhibit a significant binding affinity although less than the
activity of 17, except for 2a and 2c. Indeed, the pyridine
Scheme 2 Synthesis of the dienophile 4: (a) Li (6 eq.), t-BuOH, NH3,
À78 1C, 5 h; (b) H2SO4, THF, RT, 2 h; (c) Pd/C, EtOAc, H2 atm, RT, 18 h;
(d) Et3N (3 eq.), p-chlorobenzoyl chloride (1.05 eq.), CH2Cl2, 0 1C, 2 h;
(e) bis[(R)-1-phenylethyl]amine (1.1 eq.), n-BuLi (1.2 eq.), THF, À78 1C,
30 min, then TMSCl (5 eq.), 15 in THF, À78 1C, 1 h; (f) Pd(OAc)2 (5 mol%),
DMSO, O2 atm, RT, 18 h; (g) NaHMDS (2 eq.), THF, À78 1C, 1 h, then
Comins’ reagent (3 eq.), À78 1C, 3 h; (h) KOAc (4 eq.), Pd(OAc)2 (5 mol%),
PPh3 (10 mol%), DMF, CO atm, RT, 7 h.
Scheme 4 Functionalization of the chlorobenzyl ester 2 by Buchwald–
Hartwig cross coupling.
8594 | Chem. Commun., 2014, 50, 8593--8596
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
Table 1 Biological evaluation of the ligands on Bcl-xL/Bak and Mcl-1/Bid
displacement assays
Bcl-xL/Bak binding
Mcl-1/Bid binding
affinitya Ki (nM)
affinitya Ki (nM)
b
b
Entry
Compound
1
2
3
4
5
6
7
8
9
1
8300 Æ 1200
511 Æ 79
5200 Æ 1200
494 Æ 63
106 Æ 10
252 Æ 38
460 Æ 62
465 Æ 21
1022 Æ 48
622 Æ 35
559 Æ 76
17
2a
2b
2c
2d
2e
2f
153 Æ 5
1044 Æ 48
423 000
1079 Æ 91
1829 Æ 30
1909 Æ 43
1013 Æ 236
2g
a
Binding affinities were measured by fluorescence polarization25 after
b
competition between the ligand and a fluorescein-labeled peptide. Ki is
the concentration of the ligand corresponding to 50% of the binding of the
labelled reference compound, corrected for experimental conditions.26
derivative 2a exhibits an excellent affinity for Bcl-xL and Mcl-1
(entry 3) of the order of 100 nM, so fifty times more active
than 1. These very good results show that an elongation of the
molecule on the side chain could improve the affinity, and the
nitrogen atom of the pyridyl substituent seems to be very
important for the interaction with the binding sites of Bcl-xL
and Mcl-1. Compound 2c, with three aromatic rings on this
side chain, shows selectivity for Mcl-1 with an affinity of 460 nM
(entry 5).
To get insight into the molecular determinants of the
interaction between these compounds and their protein
targets, molecular docking of compound 2a was carried out
with a conformer of Bcl-xL that is appropriate for the inter-
action with the meiogynin A family, generated previously27
using a ligand-driven conformer selection from molecular
dynamics simulations. The global positioning of the ligand in
the binding site is similar to those observed for meiogynin A,27
showing hydrophobic interactions with most of the residues
from the binding site, and additional hydrogen bond inter-
actions of the biaryl ether oxygen and the pyridyl nitrogen with
the side chains of residues Arg100 and Tyr195, respectively
(Fig. 2A). These interactions might explain, in part, the inter-
esting inhibitory activity determined for compound 2a. Com-
pound 17 exhibits a similar binding mode (Fig. 2B), although
no hydrogen bond is established with the protein, and this
probably contributes to the decreased inhibitory activity deter-
mined for 17 compared to 2a. In agreement with the biological
data, compound 2c could not be positioned in the binding site
as it was evidenced by carrying out the docking simulations
with constraints (Fig. 2C).
Fig. 2 Docking complexes of compounds 2a (A), 17 (B) and 2c (C) with a
selected conformer of Bcl-xL.
Mcl-1 pan inhibitors 2a was elaborated, as well as a selective
Mcl-1 inhibitor 2c. In this way, the affinity for Bcl-xL and Mcl-1
could be modulated by a simple modification on the side chain
of the molecule. Further biological evaluations are underway.
The authors thank the French Agence Nationale pour la
Recherche (ANR2010-JCJC-702-1) for financial support.
Notes and references
´
1 (a) J. Rosen, J. Gottfries, S. Muresan, A. Backlund and T. I. Oprea,
J. Med. Chem., 2009, 52, 1953–1962; (b) J. Gu, Y. Gui, L. Chen,
G. Yuan, H.-Z. Lu and X. Xu, PLoS One, 2013, 8, 1–10.
2 D. J. Newman and G. M. Cragg, J. Nat. Prod., 2012, 75, 311–335.
3 (a) M. Davids and A. G. Letai, J. Clin. Oncol., 2012, 30, 3127–3135;
(b) G. Lessene, P. E. Czabotar and P. M. Colman, Nat. Rev. Drug
Discovery, 2008, 7, 989–1000; (c) C. Billard, Mol. Cancer Ther., 2013,
12, 1691–1700.
4 N. Bajwa, C. Liao and Z. Nikolovska-Coleska, Expert Opin. Ther. Pat.,
2012, 22, 37–55.
5 M. F. V. Delft, A. H. Wei, K. D. Mason, C. J. Vandenberg, L. Chen,
P. E. Czabotar, S. N. Willis, C. L. Scott, C. L. Day, S. Cory, J. M.
Adams, A. W. Roberts and D. C. S. Huang, Cancer Cell, 2006, 10, 389–399.
6 (a) P. S. Jeng and E. H. Cheng, Nat. Chem. Biol., 2013, 9, 351–352;
(b) A. S. Hazmi and R. M. Mohammad, J. Cell. Physiol., 2009, 218,
13–21.
7 J. Wei, S. Kitada, J. L. Stebbins, W. Placzek, D. Zhai, B. Wu, M. F. Rega,
Z. Zhang, J. Cellitti, L. Yang, R. Dahl, J. C. Reed and M. Pellecchia,
J. Med. Chem., 2010, 53, 8000–8011 and references cited.
8 C. A. Goard and A. D. Schimmer, Core Evidence, 2013, 8, 15–26 and
references cited.
9 T. Song, X. Li, X. Chang, X. Liang, Y. Zhao, G. Wu, S. Xie, P. Su,
Z. Wu, Y. Feng and Z. Zhang, Bioorg. Med. Chem., 2013, 21, 11–20
and references cited.
10 Y. Tanaka, K. Aikawa, G. Nishida, M. Homma, S. Sogabe, S. Igaki,
Y. Hayano, T. Sameshima, Y. Miyahisa, T. Kawamoto, M. Tawada,
Y. Imai, M. Inazuka, N. Cho, Y. Imaeda and T. Ishikawa, J. Med.
Chem., 2013, 56, 9635–9645.
These studies make obvious that compound 2c cannot adopt
the same binding mode as 2a or 17 because of a very important
steric clash between the protein surface (mainly the side chains
of Arg100 and Tyr195) and the 4-amido substituent.
In conclusion, an efficient asymmetric synthesis of various
meiogynin A analogues has been developed based on a multiple
11 M. Dow, M. Fisher, T. James, F. Marchetti and A. Nelson,
Org. Biomol. Chem., 2012, 10, 17–28.
fragment strategy. Their precursor 17 was prepared on a multi- 12 M. Litaudon, H. Bousserouel, K. Awang, O. Nosjean, M.-T. Martin,
´
M. E. Tran Huu Dau, H. A. Hadi, J. A. Boutin, T. Sevenet and
gram scale via a key Diels–Alder cycloaddition reaction with
an excellent yield of 44% in nine steps and with a very good e.r.
Through this approach, one of the most potent Bcl-xL and
´
F. Gueritte, J. Nat. Prod., 2009, 72, 480–483.
´ ´
´
´
13 D. F. Fotsop, F. Roussi, A. Leverrier, A. Breteche and F. Gueritte,
J. Org. Chem., 2010, 75, 7412–7415.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 8593--8596 | 8595

View Article Online
Communication
ChemComm
´
14 J. Dardenne, S. Desrat, F. Gueritte and F. Roussi, Eur. J. Org. Chem., 21 D. L. Comins and A. Dehghani, Tetrahedron Lett., 1992, 33, 6299–6302.
2013, 2116–2122.
22 J. N. Freskos, S. A. Laneman, M. L. Reilly and D. H. Ripin, Tetra-
hedron Lett., 1994, 35, 835–838.
23 R. M. Al-Zoubi, O. Marion and D. G. Hall, Angew. Chem., 2008, 47,
2876–2879.
24 T. Ikawa, T. E. Barder, M. R. Biscoe and S. L. Buchwald, J. Am. Chem.
Soc., 2007, 129, 13001–13007.
15 T. Lee, H. R. Kang, S. Kim and S. Kim, Tetrahedron, 2006, 62,
4081–4085.
16 G. Zhu, D. Chen, Y. Wang and R. Zheng, Chem. Commun., 2012, 48,
5796–5798.
17 (a) J. H. Fassnacht and N. A. Nelson, J. Org. Chem., 1962, 27,
1885–1887; (b) M. Ihara, T. Taniguchi, K. Makita, M. Takano, 25 J. Qian, M. J. Voorbach, J. R. Huth, M. L. Coen, H. Zhang, S.-C. Ng,
M. Onishi, N. Taniguchi, K. Fukumoto and C. Kabuto, J. Am. Chem.
Soc., 1993, 115, 8107–8115.
K. M. Comess, A. M. Petros, S. H. Rosenberg, U. Warrior and
D. J. Burns, Anal. Biochem., 2004, 328, 131–138.
18 J. S. Clark, D. Vignard and A. Parkin, Org. Lett., 2011, 13, 3980–3983. 26 Y. Cheng and W. H. Prusoff, Biochem. Pharmacol., 1973, 22,
19 K. Sugasawa, M. Shindo, H. Noguchi and K. Koga, Tetrahedron Lett.,
1996, 37, 7377–7380.
20 (a) Y. Ito, T. Hirao and T. Saegusa, J. Org. Chem., 1978, 43,
1011–1013; (b) R. C. Larock, T. R. Hightower, G. A. Kraus, P. Hahn
and D. Zheng, Tetrahedron Lett., 1995, 36, 2423–2426.
3099–3108.
27 C. Colas, F. Roussi and B. I. Iorga, Focused ligand libraries as tools
for in silico design of anti-apoptotic proteins inhibitors, in Chemistry
for Life Sciences, ed. T. Kiss and A. Perczel, Medimond, Bologna,
2011, pp. 41–46.
8596 | Chem. Commun., 2014, 50, 8593--8596
This journal is ©The Royal Society of Chemistry 2014