Synthetic approach to enyne and enediyne analogues of anticancer agents

Article abstract of DOI:10.1016/j.tetlet.2005.10.001

The synthesis and the anticancer activity of a new kind of enynes 2 and 3 and enediynes 4, analogues of Combretastatin A-4 1 are reported and discussed.

Full text of DOI:10.1016/j.tetlet.2005.10.001

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8547–8550
Synthetic approach to enyne and enediyne analogues
of anticancer agents
Olivier Provot,* Anne Giraud, Jean-Franc¸ois Peyrat, Mouaˆd Alami* and
Jean-Daniel Brion
Laboratoire de Chimie The´rapeutique, BioCIS-CNRS (UMR 8076), Universite´ Paris-Sud, Faculte´ de Pharmacie,
ˆ
rue J.B. Cle´ment, 92296 Chatenay-Malabry Cedex, France
Received 6 September 2005; revised 29 September 2005; accepted 3 October 2005
Available online 20 October 2005
Abstract—The synthesis and the anticancer activity of a new kind of enynes 2 and 3 and enediynes 4, analogues of Combretastatin
A-4 1 are reported and discussed.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Combretastatin A-4 (CA-4) 1, a natural stilbene, has
been extracted from the South African willow Combre-
tum caffrum in 19871 and its synthesis has been pub-
lished in 1995.2 This substance, likeness to colchicine,
has been found to be a cytotoxic agent which strongly
inhibits the tubulin polymerization by binding to the
colchicine site2,3 and a pro-drug of CA-4, the water-solu-
ble phosphate derivative CA-4P is now in phase II clin-
ical trials. The structural simplicity of CA-4 combined
with excellent antitumoural and antineoangiogenic
activities encouraged the scientific community to synthe-
size numerous analogues. Among various studies on the
structure activity relationship of CA-4, most of them
concern the modification in aromatic rings and in linker
alkene.4 To our knowledge, very few studies concerning
the elongation of the linker between the two aryl groups
have been already published.
the four bridge analogue was four times less effective
than CA-4. Since it has been also well established that
the cis-orientation of the two aryl rings in CA-4 is
important for biological activity,6 we were interested
to investigate the structural changes of the linker having
four or six carbon units in which the spacer constrains
the two aryl rings in quasi ÔcisÕ orientation that appears
to be necessary.
In order to preserve the (Z) stereochemistry of the stil-
bene double bond and to maintain the p-conjugated sys-
tem, we have prepared a series of CA-4 analogues
incorporating between the double bond and the aro-
matic rings one or two triple bonds. These structural
changes of the linker length would contribute to better
understanding the structure–activity relationships asso-
ciated with the presence of triple bonds in derivatives
2–4. In this letter, we report the stereocontrolled synthe-
sis of previously unknown trimethoxy substituted stil-
bene enyne 2 and 3 as well as enediyne 4 (Scheme 1)
CA-4 analogues and comment on their biological activ-
ity to act as potential antimitotic agents and their ability
to inhibit cancer cellular growth.
Hamel and co-workers5 prepared a series of CA-4 ana-
logues which differed only in the number of methylene
units (ranging from none to four) separating the aryl
moieties. With the exception of the biphenyl compound,
all the synthetic combretastatin analogues had activity
as inhibitors of tubulin polymerization and for example,
A straightforward approach to enynes and enediynes
2–4 could be the palladium-mediated coupling reactions
from readily available Z-chloroenynes, an interesting
class of compounds,7 which are not photosensitive and
more stable than the corresponding iodide and bromide
derivatives. Previously,8 we showed that the carbon–
chlorine in these compounds is not inert to further cou-
pling reactions and under appropriate transition metal
Keywords: Combretastatin A-4; Enynes; Enediynes; Tubulin; Poly-
merization; Cytotoxicity.
*
Corresponding authors. Tel.: +33 1 4683 5847; fax: +33 1 4683
5828; e-mail addresses: olivier.provot@cep.u-psud.fr; mouad.alami@
cep.u-psud.fr
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.001

8548
O. Provot et al. / Tetrahedron Letters 46 (2005)8547–8550
H
H
NHAc
catalysis, isomerically pure chloroenynes react rapidly
and cleanly with organometallic reagents to afford the
corresponding coupling products. Thus, in the presence
of Et3N and a catalytic amount of PdCl2(PPh3)2, iso-
merically pure conjugated enynes were obtained from
Grignard reagents. Moreover, under weakly ligated
palladium complexes PdCl2(PhCN)2 and CuI, they
MeO
MeO
A
B
MeO
MeO
OH
OMe
O
OMe
OMe
OMe
Colchicine
Combretastatin A-4 (CA-4)
1
MeO
OH
Linker
MeO
OCH3
OMe
Cl
MeO
MeO
(i)
Linker = Z-enyne or Z-enediyne
MeO
77%
MeO
5
6
OMe
OMe
R2
MeO
MeO
2
1
Cl
R1
OMe
MeO
MeO
R2
R
R
2
(i)
R
X
2
3
OMe
R1
X
1
R
X
7
8
7a X = CH, R1 = OMe, R2 = H
7b X = CH, R1- R2 = (-CH-)4
7c X = N, R1- R2 = (-CH-)4
8a 72%
8b 51%
8c 73%
R2
MeO
MeO
X
Scheme 2. Reagents and conditions: (i) 5 mol % PdCl2(PPh3)2,
10 mol % CuI, 2 equiv (Z)-1,2-dichloroethylene, 2 equiv n-BuNH2,
Et2O, 20 ꢁC.
4
R2
OMe
Scheme 1.
Table 1. Coupling reaction of Z-chloroenyne 6 and 8 with Grignard reagents under palladium catalysisa
Entry
Chloroenyne
Grignard reagent
Enyne 2a–c, 3a–c15
Yieldb (%)
56
MgBr
1
6
OMe
MeO
OMe
2a
2b
MeO
OMe
OMe
MgBr
2
3
6
6
93
64
Me
MeO
MeO
Me
MgBr
MeO
MeO
2c
OMe
OMe
MgBr
MeO
MeO
4
5
6
8a
8b
8c
61
51
77
MeO
MeO
MeO
OMe
OMe
OMe
OMe
3a
OMe
MgBr
MeO
MeO
OMe
OMe
OMe
3b
3c
MgBr
MeO
MeO
N
OMe
a All reactions were performed with 2 equiv of ArMgBr, 5 mol % of PdCl2(PPh3)2 in THF at room temperature.
b Isolated yields.

O. Provot et al. / Tetrahedron Letters 46 (2005)8547–8550
8549
undergo rapid coupling with terminal alkynes to afford
enediynes derivatives in high yields.9 Therefore, the
scope of these methods has been explored towards the
synthesis of target enynes and enediynes 2–4 in the con-
text of a brief structure–activity relationship study of
analogues of CA-4 1.
vergent than the route leading to enynes 2, it was never-
theless efficient. It should be noted that Z-enyne
derivatives 2 and 3 were stable and no isomerization of
the Z-double bond has been noticed after a prolonged
exposure at room temperature.
Next, unsymmetrical (Z)-enediynes 4 (opening form of
biologically active triphenylene analogues)13 have been
investigated. For their preparation, we planned to intro-
duce alkynyl moieties by means of the palladium-medi-
ated S–L coupling reaction starting from key
precursors pure Z-chloroenynes 6 and 8. To this end,
we showed previously that the use of PdCl2(PhCN)2 as
catalyst associated with piperidine or pyrrolidine
improved the reactivity of terminal alkynes with vinyl
The key precursors pure Z-chloroenynes 6 and 8 were
obtained in good yields (51–77%) as already reported
by Sonogashira–Linstrumelle (S–L)10 coupling reactions
between (Z)-1,2-dichloroethylenes with terminal alkynes
5
11 and 7 in the presence of bis(triphenylphosphine)-pal-
ladium chloride, copper iodide and n-butylamine in
diethylether (Scheme 2).
The target enyne derivatives were then obtained by fur-
ther coupling of the remaining carbon–chlorine bond
according to our previous synthetic strategy. Thus,
reactions of Z-chloroenyne 6 with various aryl Grignard
reagents12 under a catalytic amount of PdCl2(PPh3)2,
in THF at room temperature furnished the expected
biaryl-enyne adducts 2 in moderate to good yields and
with retention of the configuration of the Z-double bond
(Table 1). Under similar conditions, aryl-enynes 3 have
been successfully prepared by the coupling of chloroen-
ynes 8a–c with 3,4,5-trimethoxyarylmagnesium bromide.
Although the synthetic strategy to enyne 3 was less con-
I
I
(i)
(i)
(ii)
(iii)
R
OAc
NH
2
OCH
OCH
OCH
3
3
3
9
7
10
7d: R = OH
7e: R = NH2
Scheme 3. Reagents and conditions: (i) CuI 10 mol %, PdCl2(PPh3)2
5 mol %, trimethylsilylacetylene 2 equiv, piperidine, THF, 12 h; (ii)
NaOH, MeOH, 20 ꢁC; (iii) K2CO3, MeOH, 0 ꢁC.
Table 2. Coupling reaction between (Z)-chloroenyne and 1-alkyne: synthesis of enediyne 4a–e
Entry
Chloroenyne
Alkyne
Enediyne 415
Yielda (%)
1
8a
5
86
MeO
OMe
MeO
OMe
4a
4b
4c
4d
4e
2
3
4
6
6
6
6
7
7
7
7
b
71
38
73
61
MeO
MeO
OMe
OMe
OMe
OMe
c
MeO
N
MeO
d
MeO
OH
OMe
MeO
5
e
MeO
NH
2
OMe
MeO
a Isolated yields.

8550
O. Provot et al. / Tetrahedron Letters 46 (2005)8547–8550
chlorides since large rate enhancements were observed
reducing the reaction time to 0.5–2 h at room tempera-
ture compared to 5–12 h when using palladium catalyst
ligated with triphenylphosphine.9 Consequently, we
undertook to use these conditions in our synthesis of
CA-4 analogues.
Acknowledgements
´
Mrs. Sylvianne Thoret, Dr. Daniel Guenard and Dr.
Thierry Cresteil, from the ICSN-CNRS, are gratefully
acknowledged for accurate tubulin tests and in vitro
cytotoxicity evaluations.
The desired Z-enediynes 4a–e were prepared from pure
trimethoxyphenylchloroenyne 6. The synthesis of func-
tionalized 1-alkynes 7d and 7e as second partners for these
coupling reactions were obtained from functionalized aryl
iodides 9 and 10 using the two steps sequence S–L-cou-
pling with trimethylsilylacetylene14 followed by desilyl-
ation under alkaline conditions as outlined in Scheme 3.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.10.001.
When Z-trimethoxychloroenyne 6 was treated under the
reaction conditions previously reported (5 mol %
PdCl2(PhCN)2, 10 mol % CuI, piperidine), we were
pleased to observe that the expected cross-coupled
enediynes 4 were obtained in reasonable to good yields
(38–73% unoptimized). In order to check the versatility
of this coupling methodology, we have prepared ene-
diyne 4a this time, from trimethoxyaryl alkyne 5 and
the Z-chloroenyne 8a in an excellent overall yield
(86%, Table 2, entry 1).
References and notes
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Compounds 2, 3 and 4 were tested for their ability to
interact with tubulin (polymerization and depolymeriza-
tion) and for cytotoxicity against KB, MCF7 and
MCF7R cell lines.
However, most compounds did not inhibit the tubulin
polymerization and depolymerization. Only enyne 2b
and enediyne 4e were found to be active (e.g.,
6.5 · 10À5 and 6.8 · 10À5 M, respectively), but 50-fold
less active than CA-4. Moreover, if the cytotoxicity of
compounds 2–4 against KB, MCF7 and MCF7R cell
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´
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In summary, the purpose of this study was to investigate
and develop methods for the preparation of original
enynes and enediynes as analogues of Combretastatin
A-4. We have presented here, the synthesis and the eval-
uation of new enynes 2 and 3 as well as enediynes 4. We
have developed an efficient route to the preparation of
eleven new compounds via palladium cross-coupling
reactions. Enyne 2b and enediyne 4e inhibited tubulin
polymerization with an IC50 60–70 lM while the other
compounds did not reveal any significant activity. At a
concentration of 10À5 M, these two compounds showed
a marginal activity towards KB, MCF7 and MCF7R
cells. In this series of enynes and enediynes-types ana-
logues, examination of the anticancer results revealed
that, in these analogues, the incorporation of one or
two triple bonds does not play a role for maximal anti-
cancer activity.
´
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