A new synthesis of combretastatins A-4 and AVE-8062A

Article abstract of DOI:10.1016/j.tetlet.2007.07.151

Combretastatins A-4 and AVE-8062A have been synthesized by coupling 3,4,5-trimethoxyphenylacetylene with the corresponding iodomethoxyphenol and bromomethoxyaniline, followed by hydrolysis of the thermally-stable Ti(II)-alkyne complexes.

Full text of DOI:10.1016/j.tetlet.2007.07.151

Tetrahedron Letters 48 (2007) 7007–7010
A new synthesis of combretastatins A-4 and AVE-8062A
*
´
Francisco Lara-Ochoa and Georgina Espinosa-Perez
Instituto de Quı´mica, Universidad Nacional Auto´noma de Me´xico, Circuito Exterior, Ciudad Universitaria,
Coyoaca´n 04510, Me´xico, D.F., Mexico
Received 17 May 2007; revised 24 July 2007; accepted 24 July 2007
Available online 28 July 2007
Abstract—Combretastatins A-4 and AVE-8062A have been synthesized by coupling 3,4,5-trimethoxyphenylacetylene with the cor-
responding iodomethoxyphenol and bromomethoxyaniline, followed by hydrolysis of the thermally-stable Ti(II)–alkyne complexes.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Combretastatins are an important group of anticancer
drugs,1 isolated from the African tree Combretum caff-
rum. In particular, cis-combretastatins A-4 and AVE-
8062A disrupt tubulin aggregation, thereby preventing
metastasis and angiogenesis, which places them among
the most active of antineoplastic agents.2 An efficient
synthesis of these compounds would offer an opportu-
nity to perform wider studies, given their low availability
due to the poor bark content. Specifically, as part of our
project aimed at improving the low water solubility of
these compounds, we required sufficient quantities of
both combretastatins. Therefore, a new synthetic meth-
od that offers access to both cis-A-4 and cis-AVE-8062A
is described herein. We also include other examples of
cis-stilbenes, which were prepared in order to test the
wider applicability of our method.
lyst,4 or by Wittig condensation;5 all of these methods
are associated with different complications. Mixtures
of Z- and E-isomers are inevitably formed from Wittig
reactions, which require complex chromatographic
separations.1c
Partial hydrogenation provides combretastatin A-4
together with a minor amount of the corresponding
alkane, which can only be removed by preparative
HPLC4 or else requires additional protection to
facilitate separation. On looking for another way to
selectively obtain alkenes 3, we found a report6 on a
cross-coupling reaction of thermally stable tita-
nium(II)–alkyne complexes with aryl halides, and we
tested this method as a means of obtaining the corre-
sponding combretastatins. The procedure involves
Pri-O O-iPr
H
H
Ti
R4
R4
R5
R7
R1
R3
R4
R6
R1
R2
R5
R6
Ti(O-iPr)4/nBuLi
R6
-78˚C to 50˚C
H2O
R2
ð1Þ
R1
R5
R3
R7
R2
R3 R7
2
1
3
Cis-isomers 3 are usually synthesized by alkyne hydro-
hydrolysis of the corresponding Ti(II)–diarylalkyne
complexes 2, prepared from diarylalkynes 1, Ti(O–
iPr)4, and nBuLi at ꢀ78 ꢁC. The thermally stable
Ti(II)–alkyne complexes 2 were successfully generated
by slow addition of 4 equiv of nBuLi as a reducing agent
to a mixture of Ti(O–iPr)4 (2 equiv) and alkyne 1
boration,3 selective alkyne reduction using Lindlar’s cata-
*
Corresponding author. Tel.: +52 55 56 22 44 56; fax: +52 55 56 16 22
17; e-mail: flara@servidor.unam.mx
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.07.151

7008
F. Lara-Ochoa, G. Espinosa-Pe´rez / Tetrahedron Letters 48 (2007) 7007–7010
Table 1. Selective reduction of diarylalkynes
H
H
3a COMBRETASTATIN A-4
R1
R5
3b COMBRETASTATIN AVE-8062A
R2
R3
R4
3
Entry
R1
R2
R3
R4
R5
Mpc (ꢁC)
Yielda (%)
3a
3b
3c
3d
3e
3f
OMe
OMe
OMe
OMe
NH2
H
OMe
OMe
OMe
OMe
H
OMe
OMe
OMe
OMe
H
OMe
OMe
H
H
OMe
OMe
OH
NH2
H
NH2
OMOM
OH
83.81d
Oil
Oil
79.11
Oil
94b
86b
90
92
79
H
H
43.62
83
a Yields refer to isolated yields.
b Both combretastatins gave satisfactory 1H NMR and IR spectral data.3a,5,9
c Melting points were determined on a Mettler-Toledo 822e differential scanning calorimeter.
d Lit.: 84.5–85.5 ꢁC.1b
desired product, but when the hydroxyl was protected
as an MOM ether, the precursor of combretastatin A-
4 was obtained in 80% yield. 5-Bromo-1,2,3-trimethoxy-
benzene was not reactive under these conditions (entry
C). Therefore, the combretastatin AVE-8062A precur-
sor, which also needs to be prepared from a brominated
compound, could not be obtained in this way. Results
pertaining to Method 1 are listed in Table 2.
In a second attempt (Method 2), we employed a method
described for the coupling of vinyl and aryl halides with
terminal alkynes,13 using Pd(OAc)2 and triphenylphos-
phine in refluxing pyrrolidine. These reaction conditions
worked very well for both the iodinated and brominated
precursors, and hydroxyl protection was not necessary.
When the starting materials corresponding to entries A
and C in Table 2 were subjected to the conditions of
Method 2, the isolated yields were 95% and 93%, respec-
tively,14 clearly indicating the advantage of this method.
The precursor of combretastatin AVE-8062A (entry H)
was obtained in low yield (58%), probably due to the
unstable nature of the bromomethoxyaniline under
these reaction conditions.
Figure 1. X-ray crystal structure of combretastatin A-4.7
(1 equiv) in THF at ꢀ78 ꢁC, and then increasing the
temperature to 50 ꢁC (Eq. 1). The solution of 2 in
THF thus obtained was stirred for an additional 1 h at
50 ꢁC and was then hydrolyzed to selectively afford
cis-stilbenes 3 in good yields and high purity (Table 1).
Indeed, this high purity enabled us to perform the first
single-crystal X-ray structure determination of combre-
tastatin A-4 (Fig. 1).7 Previously, the combretastatin
pioneering group of Pettit has reported a crystal struc-
ture of cis-A-4 phosphate.8
Several iodinated and brominated compounds were cou-
pled with different terminal arylalkynes, and the results
are shown in Table 2.
As regards the synthesis of diarylalkynes 1, we employed
an approach similar to that described recently.3a,c How-
ever, our protocol does not involve the use of a Cu cat-
alyst, which necessitates special experimental conditions,
and we found a way of avoiding the use of a protecting
group.
In summary, two combretastatins, A-4 and AVE-
8062A, have been synthesized by means of a new and
simple method, in yields comparable to those of previ-
ous literature syntheses, enabling a more effective pro-
duction of these antineoplastics, in respect to more
predictable yields of the cis-isomers.
In a first attempt (Method 1), we used a Sonogashira
coupling10 of arylalkynes 411 and aryl halides 512 in
the presence of Pd(OAc)2, DABCO, and Cs2CO3, with
acetonitrile as solvent, for the synthesis of the requisite
diarylalkynes. These reaction conditions proved to be
very efficient for iodinated precursors (Table 2, entries
B, D, and E). Nonetheless, the strongly basic conditions
necessitated hydroxyl protection. Reaction in the case of
the substituents indicated in Entry A did not give the
Typical procedure for the preparation of 2-(300-hydroxy-
400-methoxyphenyl)-1-(30,40,50-trimethoxyphenyl)ethyne
(Method 2, entry A)
To a stirred solution of 3,4,5-trimethoxyphenylethyne3a
(576 mg, 3.0 mmol) and 5-iodo-2-methoxyphenol3a

F. Lara-Ochoa, G. Espinosa-Pe´rez / Tetrahedron Letters 48 (2007) 7007–7010
7009
Table 2. Palladium-catalyzed coupling of arylalkynes and arylhalides
R4
R4
R1
R1
R5
R2
+
X
R5
R2
R6
R3
R6
R3
5
4
1
Entry
R1
R2
R3
X
R4
R5
R6
Mpf (ꢁC)
Yieldg (%)
Aa,c
Ba
OMe
OMe
H
OMe
OMe
H
H
H
OMe
OMe
H
H
H
OMe
OMe
H
H
H
OMe
OMe
H
H
H
I
I
Br
I
I
I
I
Br
I
I
OH
OMOM
OMe
OMOM
OMOM
OH
OMOM
OMe
OMOM
OMOM
OMe
OH
NH2
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
H
H
OMe
H
H
H
H
OMe
H
H
OMe
H
—
105.38
—
0
80
0
Ca
Da
Ea
H
57.9
85
49
95
71
93
98
84
88
88
58
NH2
OMe
OMe
H
110.2
97.4e
105.3
73.6
59.17
109.7
125.03
109.5
96.29
Ab,c
Bb
Cb
Db
Eb
H
NH2
NH2
H
Fb
H
H
OMe
H
H
OMe
Br
I
Br
Gb
Hb,d
OMe
H
a Method 1: Pd(OAc)2, DABCO, Cs2CO3, CH3CN, room temperature to reflux, 24–72 h.10
b Method 2: Pd(OAc)2, PPh3, pyrrolidine, reflux.13
c Combretastatin A-4 precursor.
d Combretastatin AVE-8062A precursor.
e Lit.: 96–98 ꢁC.4
f Melting points were determined on a Mettler-Toledo 822e differential scanning calorimeter.
g Yields refer to isolated yields.
Toki, B. E.; Herald, D. L.; Boyd, M. R.; Hamel, E.; Pettit,
R. K.; Chapuis, J. C. J. Med. Chem. 1999, 42, 1459; (d)
Pettit, G. R.; Grealish, M. P.; Herald, D. L.; Boyd, M. R.;
Hamel, E.; Pettit, R. K. J. Med. Chem. 2000, 65, 7438.
3. (a) Lawrence, N. J.; Ghani, F. A.; Hepworth, L. A.;
Hadfield, J. A.; McGown, A. T.; Pritchard, R. G.
Synthesis 1999, 9, 1656; (b) Bui, V. P.; Hudlicky, T.;
Hansen, T. V.; Stenstrom, Y. Tetrahedron Lett. 2002, 43,
2839; (c) Odlo, K.; Klaveness, J.; Rongved, P.; Hansen, T.
V. Tetrahedron Lett. 2006, 47, 1101.
(500 mg, 2.0 mmol) in pyrrolidine (5.0 mL) under an
argon atmosphere were added PPh3 (21 mg, 0.08 mmol)
and Pd(OAc)2 (9.0 mg, 0.04 mmol). After stirring at
reflux for 30 min, the mixture was hydrolyzed with a sat-
urated aqueous solution of ammonium chloride
(30 mL) and extracted with dichloromethane (30 mL).
The organic extract was dried over Na2SO4 and the
solvent was removed in vacuo. Chromatography (silica
gel; hexane/EtOAc, 8:2) gave diarylalkyne 1[Ab] (600 mg,
95%) as a thick oil that subsequently solidified. Recrys-
tallization (hexane/dichloromethane) gave colorless
crystals, mp 97.4 ꢁC.
4. Furstner, A.; Nikolakis, K. Liebigs Ann. 1996, 2107–2113.
¨
5. Obsumi, K.; Tsuji, T.; Morinaga, Y.; Ohishi, K. U.S.
Patent 5,525,632, 1996.
6. Obora, Y.; Moriya, H.; Tokunaga, M.; Tsuji, Y. Chem.
Commun. 2003, 2820.
7. X-ray structure analysis of combretastatin A-4: Bruker
Smart CCD diffractometer, Mo-Ka-radiation (k =
Acknowledgments
˚
0.71073 A), T = 298 (2) K. Crystal size: 0.33 · 0.16 ·
The authors would like to thank DGAPA for support-
ing this research, Grant PAPIIT IN214805, and to A.
0.08 mm3, colorless, prism, space group P21/n, mono-
clinic, a = 6.816 (8)ꢁ, b = 24.265 (3)ꢁ, c = 10.4219 (13)ꢁ,
3
˚
b = 107.604 (2)ꢁ, V = 1642.5 (10) A , Z = 4, qcal
=
´
´
Pena, E. Huerta, J. Perez-Flores and S. Hernandez-Ort-
˜
1.279 g/cm3, hrange = 2.22–24.39ꢁ, 13021 reflections col-
lected, 2889 independent (Rint = 0.0498), 215 parameters
and 99.9% completeness to theta = 25.0ꢁ. Final R indices
[I/2r], R = 0.65, Rw = 0.11. Structure solution: direct
methods (SHELXTL V 6.14), refinement on F2 (SHELEXTL
V 6.14). Crystallographic data (excluding structure fac-
tors) for the structures in this paper have been deposited
with the Cambridge Crystallographic Data Centre as
supplementary publication CCDC 646589.
ega for their technical support.
References and notes
1. (a) Pettit, G. R.; Singh, S. B.; Niven, M. L.; Hamel, E.;
Schmidt, J. M. J. Nat. Prod. 1987, 50, 119; (b) Pettit, G.
R.; Singh, S. B.; Hamel, E.; Lin, C. M.; Alberts, D. S.;
´
Garcıa-Kendall, D. Experientia 1989, 45, 209; (c) Pettit, G.
8. Pettit, G. R.; Rhodes, M. R.; Herald, D. L.; Hamel, E.;
Schmidt, J. M.; Pettit, R. K. J. Med. Chem. 2005, 48,
4087–4099.
R.; Singh, S. B.; Boyd, M. R.; Hamel, E.; Pettit, R. K.;
Schmidt, J. M.; Hogan, F. J. Med. Chem. 1995, 38, 1666.
2. (a) Lin, C. M.; Ho, H. H.; Pettit, G. R.; Hamel, E.
Biochemistry 1989, 28, 6984; Review: (b) Sackett, D. L.
Pharmacol. Ther. 1993, 59, 163–228; (c) Pettit, G. R.;
9. Spectral data of combretastatins and cis-arylalkenes:
Compound 3a: IR (KBr) m = 3453 (br), 3006 (m), 1616
(w), 1580 (s), 1327 (m), 1238 (s), 1126 (s), 1010 (w) cmꢀ1
.

7010
F. Lara-Ochoa, G. Espinosa-Pe´rez / Tetrahedron Letters 48 (2007) 7007–7010
1H NMR (300 MHz; CDCl3): d = 3.698 (6H, s, OMe),
(1H, dd, J = 8.4, 2.0 Hz, H-600), 7.046 (1H, t, J = 8.2 Hz,
H-50), 7.107 (1H, d, J = 7.2 Hz, H-200). MS (m/z) EI = 285
(M+).
3.842 (3H, s, OMe), 3.866 (3H, s, OMe), 5.520 (1H, s,
OH), 6.408 (1H, d, J = 12 Hz, CH@C), 6.473 (1H, d,
J = 12 Hz, C@CH), 6.527 (2H, s, H-20, 60), 6.731 (1H, d,
J = 8.1 Hz, H-500), 6.798 (1H, dd, J = 8.5, 1.9 Hz, H-600),
6.923 (1H, d, J = 2.0 Hz, H-200).
Compound 3f: IR (KBr) m = 3417 (br), 3023 (m), 2841
(m), 1589 (m), 1513 (m),1343 (m), 1119 (s), 1023 (s) cmꢀ1
.
1H NMR (300 MHz; CDCl3): d = 3.854 (3H, s, OCH3),
5.480 (1H, s, OH), 6.472 (1H, d, J = 12.0 Hz, CH@C),
6.519 (1H, d, J = 12.3 Hz, C@CH), 6.691 (1H, d,
J = 8.4 Hz, H-500), 6.748 (1H, dd, J = 8.4, 1.8 Hz, H-600),
6.835 (1H, d, J = 2.1 Hz, H-200), 7.229 (5H, m, H-Ph0). MS
(m/z) EI = 226 (M+).
Compound 3b: IR (film) m = 3473 (w), 3370 (w), 3001 (m),
2936 (m),1613 (m), 1578 (s), 1326 (m), 1233 (s), 1005
1
(w) cmꢀ1. H NMR (200 MHz; CDCl3): d = 3.708 (6H, s,
OMe), 3.834 (3H, s, OMe), 3.842 (3H, s, OMe), 6.372 (1H,
d, J = 12.2 Hz, CH@C), 6.466 ( 1H, d, J = 12 Hz,
C@CH), 6.551 (2H, s, H-20, 60), 6.696 ( 2H, m, H-500,
600), 6.747 (1H, s, H-200).
10. Li, J.-H.; Zhang, X.-D.; Xie, Y.-X. Synthesis 2005, 5, 804–
808.
Compound 3c: IR (film) m = 2997 (w), 2936 (w), 1579 (s),
11. Arylalkyne of entries A, B, and H were synthesized as
described in Ref. 3a. Arylalkynes of entries C and E were
purchased from Aldrich Chemical Co. and used as
received.
12. Aryliodide of entries A and G were synthesized as
described in Ref. 3a. Arylbromide of entry H was
synthesized according to: Choi, H. Y.; Chi, D. Y. J. Am.
Chem. Soc. 2001, 123, 9202–9203. Arylbromide of entries
C and F were purchased from Aldrich Chemical Co. and
used as received.
1327 (m), 1236 (s), 1125 (s), 1006 (w) cmꢀ1 1H NMR
.
(200 MHz; CDCl3): d = 3.648 (6H, s, OMe), 3.832 (3H, s,
OMe), 6.464 ( 2H, s, H-20-60), 6.495 (1H, d, J = 12.4 Hz,
CH@C), 6.610 (1H, d, J = 12.4 Hz, C@CH), 7.244 (5H,
m, H-Ph00). MS (m/z) EI = 270 (M+).
Compound 3d: IR (KBr) m = 3447 (s), 3365 (s), 3008 (m),
1633 (m), 1597 (m), 1582 (m), 1326 (m), 999 (m) cmꢀ1. 1H
NMR (200 MHz; CDCl3): d = 3.680 (6H, s, OMe), 3.833
(3H, s, OMe), 6.436 (1H, d, J = 12.0 Hz, CH@C), 6.533
(1H, d, J = 12.0 Hz, C@CH), 6.520 (2H, s, H-20, 60), 6.540
(d, 1H, J = 7.6, H-600), 6.634 (1H, m, H-200), 6.706 (1H, d,
J = 7.6 Hz, H-400), 7.063 (1H, t, J = 7.7 Hz, H-500). MS (m/
z) EI = 285 (M+).
Compound 3e: IR (film) m = 3457 (w), 3367 (w), 3002 (w),
2953 (w), 1619 (m), 1597 (m), 1580 (m), 1508 (s), 1261 (s),
993 (s) cmꢀ1. 1H NMR (200 MHz; CDCl3):d = 3.430 (3H,
s, OCH3), 3.574 (2H, br, NH2), 3.857 (3H, s, OCH3), 5.069
(2H, s, CH2), 6.451 (2H, s, CH@CH), 6.531 (1H, ddd,
J = 7.8, 2.4, 1.0 Hz, H-60), 6.656 (1H, m, H-20), 6.699 (1H,
t, J = 1.2 Hz, H-40), 6.761 (1H, d, J = 8.2 Hz, H-500), 6.920
13. Nwokogu, G. C. Tetrahedron Lett. 1984, 25, 3263–
3266.
14. Spectral data of diarylalkynes: entry A: IR (KBr) m = 3479
(br), 2935 (m), 2218 (w), 1575 (s), 1506 (s), 1233 (s), 1125
1
(s) cmꢀ1. H NMR (200 MHz; CDCl3): d = 3.869 (3H, s,
OCH3), 3.882 (6H, s, OCH3), 3.922 (3H, s, OCH3), 5.614
(1H, s, OH), 6.757 (2H, s, H-20, 60), 6.823(1H, d,
J = 8.0 Hz, H-500), 7.071 ( 1H, dd, J = 8.0 Hz, 2.0 Hz, H-
600), 7.09 (1H, d, J = 2.0 Hz, H-20). Entry B: IR (KBr)
m = 2956 (br), 2836 (m), 2211 (w), 1574 (s), 1511 (s), 1244
(s), 1129 (s) cmꢀ1
.