Novel syntheses of cis and trans isomers of combretastatin A-4

Article abstract of DOI:10.1021/jo015959z

A high-yielding, two-step stereoselective synthesis of the anticancer drug (Z)-combretastatin A-4 (1) has been devised. The method uses the Perkin condensation of 3,4,5-trimethoxyphenylacetic acid and 3-hydroxy-4-methoxybenzaldehyde followed by decarboxylation of the cinnamic acid intermediate using copper and quinoline. The iodine-catalyzed isomerization of the Z isomer 1 results in complete conversion to the E isomer. The Suzuki cross-coupling of an aryl boronic acid and vinyl bromide has also been successfully employed to produce both Z and E isomers of combretastatin A-4 stereoselectively. Both methods are far superior to the current five-step Wittig synthesis in which both isomers are produced nonstereoselectively.

Full text of DOI:10.1021/jo015959z

J . Org. Chem. 2001, 66, 8135-8138
8135
Novel Syn th eses of Cis a n d Tr a n s Isom er s of Com br eta sta tin A-4
Keira Gaukroger,†,‡ J ohn A. Hadfield,*,§, Lucy A. Hepworth,‡ Nicholas J . Lawrence,‡,| and
Alan T. McGown†
CRC Drug Development Group and CRC Radiochemical Targeting and Imaging Group,
Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester, M20 4BX, U.K.,
Department of Chemistry, UMIST, P.O. Box 88, Sackville Street, Manchester, M60 1QD, U.K.,
Chemistry Department, Cardiff University, P.O. Box 912, Cardiff, CF10 3TB, U.K., and Centre for
Molecular Drug Design, Department of Chemistry, University of Salford, Manchester, M5 4WT, U.K.
j.a.hadfield@salford.ac.uk
Received J uly 23, 2001
A high-yielding, two-step stereoselective synthesis of the anticancer drug (Z)-combretastatin A-4
(1) has been devised. The method uses the Perkin condensation of 3,4,5-trimethoxyphenylacetic
acid and 3-hydroxy-4-methoxybenzaldehyde followed by decarboxylation of the cinnamic acid
intermediate using copper and quinoline. The iodine-catalyzed isomerization of the Z isomer 1 results
in complete conversion to the E isomer. The Suzuki cross-coupling of an aryl boronic acid and vinyl
bromide has also been successfully employed to produce both Z and E isomers of combretastatin
A-4 stereoselectively. Both methods are far superior to the current five-step Wittig synthesis in
which both isomers are produced nonstereoselectively.
In tr od u ction
Most syntheses of the combretastatins and analogues
utilize the Wittig reaction.2c,5 The route developed by
Pettit et al.5c is representative. They reported a five-step
method from 3,4,5-trimethoxybenzyltriphenylphosphonium
bromide and isovanillin using n-butyllithium as the base.
Both cis 1 and trans 3 isomers of combretastatin A-4 were
produced in a ratio of 1:1.5, with the overall yield of the
cis isomer being 19%. The best selectivity reported was
obtained on reaction of 3,4,5-trimethoxybenzyltriphen-
ylphosphonium bromide with the bis-tert-butylphosphate
ester of isovanillin.5f A 4:1 mixture of Z:E stereoisomers
was obtained in a route to the prodrug 2. The Wittig
method is, therefore, problematic on two counts. First,
owing to the lack of or poor stereoselectivity, the yield of
the desired Z isomer is reduced, and, second, a separation
process is required. The isomers of combretastatin A-4
are difficult to separate, particularly by chromatography.
The combretastatins are a group of antimitotic agents
isolated from the bark of the South African tree Com-
bretum caffrum. The most potent of these is combret-
astatin A-41 which has been found to be a potent cytotoxic
agent which strongly inhibits the polymerization of
tubulin by binding to the colchicine site.2 Combretastatin
A-4 (1) is also able to elicit irreversible vascular shutdown
within solid tumors, leaving normal vasculature intact.3
A prodrug of combretastatin A-4, the water soluble phos-
phate derivative4 2 is now in phase II of clinical trials.
(1) (a) Lin, C. M.; Ho, H. H.; Pettit, G. R.; Hamel, E. Biochemistry
1989, 28, 6984. (b) Pettit, G. R.; Cragg, G. M.; Singh, S. B. J . Nat.
Prod. 1987, 50, 386. (c) Pettit, G. R.; Singh, S. B.; Cragg, G. M. J . Org.
Chem. 1985, 50, 3404. (d) Pettit, G. R.; Cragg, G. M.; Herald, D. L.;
Schmidt, J . M.; Lohavanijaya, P. Can. J . Chem. 1982, 60, 1374. (e)
Pettit, G. R.; Singh, S. B.; Hamel, E.; Lin, C. M.; Alberts, D. S.; Garcia-
Kendall, D. Experientia 1989, 45, 209.
(2) (a) Hamel, E., Lin, C. M. Biochem. Pharmacol. 1983, 32, 3863.
(b) McGown, A. T.; Fox, B. W. Anti-Cancer Drug Des. 1989, 3, 249. (c)
Lin, C. M.; Singh, S. B.; Chu, P. S.; Dempcy, R. O.; Schmidt, J . M.;
Pettit, G. R.; Hamel, E. Mol. Pharmacol. 1988, 34, 200.
(3) (a) Chaplin, D. J .; Pettit, G. R.; Parkins, C. S.; Hill, S. A. Br. J .
Cancer 1996, 74, S86. (b) Dark, G. G.; Hill, S. A.; Prise, V. E.; Tozer,
G. M.; Pettit, G. R.; Chaplin, D. J . Cancer Res. 1997, 57, 1829.
(4) (a) Pettit, G. R.; Temple, C.; Narayanan, V. L.; Varma, R.;
Simpson, M. J .; Boyd, M. R.; Rener, G. A.; Bansal, N. Anti-Cancer Drug
Des. 1995, 10, 299. (b) Pettit, G. R.; Rhodes, M. R. Anti-Cancer Drug
Des. 1998, 13, 183.
(5) (a) Cushman, M.; Nagarathnam, D.; Gopal, D.; Chakraborti, A.
K.; Lin, C. M.; Hamel, E. J . Med. Chem. 1991, 34, 2579. (b) Cushman,
M.; Nagarathnam, D.; Gopal, D.; He, H.-M.; Lin, C. M.; Hamel, E. J .
Med. Chem. 1992, 35, 2293. (c) Pettit, G. R.; Singh, S. B.; Boyd, M. R.;
Hamel, E.; Pettit, R. K.; Schmidt, J . M.; Hogan, F. J . Med. Chem. 1995,
38, 1666. (d) Pettit, G. R. U.S. Patent 5,561,222, 1996. (e) Woods, J .
A.; Hadfield, J . A.; Pettit, G. R.; Fox, B. W.; McGown, A. T. Br. J .
Cancer 1995, 71, 705. (f) Bedford, S. B.; Quaterman, C. P.; Rathbone,
D. L.; Slack, J . A.; Griffin, R. J .; Stevens, M. F. G. Bioorg. Med. Chem.
Lett. 1996, 6, 157.
Structure activity relationship analyses of the com-
bretastatins indicate that the cis configuration of the
stilbene unit is the most important factor for inhibition
of cancer cell growth.5 E stilbenes show a dramatic
decrease in their inhibitory effects on cancer cell growth
and tubulin polymerization when compared to their
corresponding Z isomers.
† CRC Drug Development Group, Paterson Institute for Cancer
Research.
‡ UMIST.
§ CRC Radiochemical Targeting and Imaging Group, Paterson
Institute for Cancer Research.
| Cardiff University.
University of Salford.
10.1021/jo015959z CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/02/2001

8136 J . Org. Chem., Vol. 66, No. 24, 2001
Gaukroger et al.
Sch em e 1. Syn th esis of
Furstner et al.6 have developed a stereoselective method
based on the Lindlar-type semihydrogenation of an
alkyne precursor (assembled by 9-MeO-9-BBN-mediated
Suzuki-type cross-coupling). This was Z selective with a
small amount of the corresponding alkane also isolated
((Z)-combretastatin A-4:alkane 86:14). This methodology
has been developed further, eliminating the use of pro-
tecting groups and constructing the alkene moiety with
greater stereoselectivity.7 However, both these methods6,7
have shortcomings and are not ideal for large-scale
applications.
(Z)-5-(2′-Br om oeth en yl)-2-m eth oxyp h en ol fr om
3-Hyd r oxy-4-m eth oxyben za ld eh yd e
The purpose of this study was, therefore, to investigate
novel methods of stereoselective syntheses of cis 1 and
trans 3 isomers of combretastatin A-4. The development
of a route amenable to the large-scale production of
combretastatin A-4 was our main goal. It was also
desirable to develop a stereoselective synthesis of the E
isomer, which could be applied to the preparation of other
related stilbenes. This is most important for their ac-
curate biological assessment, as it is critical that the
trans isomer is not contaminated by trace quantities of
their more active cis counterparts. In some cases, we have
found that the cis isomer is 10 000 times more active than
the trans isomer.
Sch em e 2. Syn th esis of th e Z Isom er of
Com br eta sta tin A-4 Usin g th e
P a lla d iu m -Ca ta lyzed Su zu k i Cr oss-Cou p lin g of
(Z)-5-(2′-Br om oeth en yl)-2-m eth oxyp h en ol a n d
3,4,5-Tr im eth oxyben zen e Bor on ic Acid
Resu lts a n d Discu ssion
There are a number of examples in the literature of
substituted stilbenes being produced using the Suzuki
cross-coupling of an ethenyl halide and aryl boronic acid,8
and we considered that the synthesis of combretastatin
A-4 and its trans equivalent might similarly be achieved.
(Z)-5-(2′,2′-Dibromoethenyl)-2-methoxyphenol (5) was
synthesized using the Corey-Fuchs Wittig-like bromi-
nation of 3-hydroxy-4-methoxybenzaldehyde (4).9 The
yield was poor (ca. 20%), and, consequently, the reaction
was repeated with silyl protection of the phenol.1c,19
Deprotection of the dibromostyrene (7), derived from the
aldehyde 6, using tetrabutylammonium fluoride gave an
improved overall yield of the required phenol 5 (ca. 40%).
Stereoselective reduction of the phenol 5 using tributyltin
hydride and tetrakis(triphenylphosphine)palladium(0)10
afforded (Z)-5-(2′-bromoethenyl)-2-methoxyphenol (8) in
good yield (ca. 60%) following purification by column
chromatography and recrystallization (Scheme 1).
The (Z)-vinyl bromide 8 was reacted with the boronic
acid 9 in 1,2-dimethoxyethane (DME), according to the
method of Strachan et al.11 Tetrakis(triphenylphosphine)-
palladium(0) was employed as the catalyst for the reac-
Sch em e 3. Syn th esis of
(E)-5-(2′-Br om oeth en yl)-2-m eth oxyp h en ol fr om
3-Hyd r oxy-4-m eth oxycin n a m ic Acid
tion, and aqueous Na2CO3 provided the basic environ-
ment required. Flash column chromatography, followed
by recrystallization, afforded (Z)-combretastatin A-4 (1)
in good yield (ca. 70%) (Scheme 2). GC analysis of the
compound following purification indicated that the Z:E
ratio was 99:1. The combretastatin A-4 sample was found
to contain <0.5% tin and <0.4% palladium. The overall
yield of combretastatin A-4 from isovanillin (4) was 16%.
The synthesis of (E)-combretastatin A-4 (3) by the same
method required the (E)-vinyl bromide 11. Bromination
of inexpensive 3-hydroxy-4-methoxycinnamic acid (10) in
acetic acid produced this vinyl bromide 11 in moderate
yield (ca. 30%) (Scheme 3).12 The cis and trans configura-
tions of the vinyl bromides 8 and 11 were confirmed by
measuring the characteristic coupling constants of the
olefinic protons in the NMR spectra. These were 8 Hz
for 8 and 14 Hz for 11.
(6) Furstner, A.; Nikolakis, K. Liebigs Ann. Chem. 1996, 2107.
(7) Lawrence, N. J .; Ghani, F. A.; Hepworth, L. A.; Hadfield, J . A.;
McGown, A. T.; Pritchard, R. G. Synthesis 1999, 1656.
(8) (a) Abe, S.; Miyaura, N.; Suzuki, A. Bull. Chem. Soc. J pn. 1992,
65, 2863. (b) Potter, G. A.; McCague, R. J . Org. Chem. 1990, 55, 6184.
(c) Willson, T. M.; Henke, B. R.; Momtahen, T. M.; Charifson, P. S.;
Batchelor, K. W.; Lubahn, D. B.; Moore, L. B.; Oliver, B. B.; Sauls, H.
R.; Triantafillou, J . A.; Wolfe, S. G.; Baer, P. G. J . Med. Chem. 1994,
37, 1550. (d) Shimizu, M.; Yamada, N.; Takebe, Y.; Hata, T.; Kuroboshi,
M.; Hiyama, T. Bull. Chem. Soc. J pn. 1998, 71, 2903. (e) Chen, C.;
Wilcoxen, K.; Strack, N.; McCarthy, J . R. Tetrahedron Lett. 1999, 40,
827. (f) Sharp, J . T.; Wilson, P.; Parsons, S.; Gould, R. O. J . Chem.
Soc., Perkin Trans. 1 2000, 1139.
(9) (a) Corey, E. J .; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769.
(b) Upasani, R. B.; Yang, K. C.; Acosta-Burruel, M.; Konkoy, C. S.;
McLellan, J . A.; Woodward, R. M.; Lan, N. C.; Carter, R. B.; Hawk-
inson, J . E. J . Med. Chem. 1997, 40, 73.
(E)-5-(2-Bromoethenyl)-2-methoxy-phenol (11) was re-
acted with 3,4,5-trimethoxybenzene boronic acid (9) as
described above. The reaction produced trans combret-
(10) Uenishi, J .; Kawahama, R.; Shiga, Y.; Yonemitsu, O.; Tsuji, J .
Tetrahedron Lett. 1996, 37, 6759.
(11) Strachan, J . P.; Sharp, J . T.; Parsons, S. J . Chem. Soc., Perkin
Trans. 1 1998, 807.
(12) Scho¨pf, C.; Brass, E.; J acobi, E.; J orde, W.; Mocnik, W.; Neuroth,
L.; Salzer, W. J ustus Liebigs Ann. Chem. 1940, 544, 30.

Cis and Trans Isomers of Combretastatin A-4
J . Org. Chem., Vol. 66, No. 24, 2001 8137
Sch em e 4. Syn th esis of th e E Isom er of
Com br eta sta tin A-4 Usin g th e
P a lla d iu m -Ca ta lyzed Su zu k i Cr oss-Cou p lin g of
(E)-5-(2′-Br om oeth en yl)-2-m eth oxyp h en ol a n d
3,4,5-Tr im eth oxyben zen e Bor on ic Acid
is E. Cinnamic acid 14 was first converted to the methyl
ester5b 15 and then reduced to the alcohol16 16 (Scheme
6). The stereochemistry of this compound was determined
by assessing diagnostic cross-peak signals in the NOESY
1H spectra of alcohol 16. In the spectra, the CH2 protons
of alcohol 16 form cross-peaks with both the olefinic
proton and aromatic protons, 2′ and 6′ on the A ring,
indicating that the aromatic rings are cis to each other
and that the overall configuration is E.
Decarboxylation of the acid 14 was achieved using
powdered copper and quinoline at 230 °C, following the
method of Molho et al.17 (Scheme 7). Initially cis com-
bretastatin A-4 (1) was purified in good yield (ca. 70%)
using flash column chromatography (Z:E 99.4:0.6). A
small amount of the trans isomer 3 was also isolated (ca.
10%), but on repetition of the reaction, it was found that
the cis isomer could be purified by recrystallization alone
(ca. 72%). GC analysis of the crude reaction mix showed
a Z:E ratio of 88:12, but following recrystallization this
changed to 99.8:0.2. The sample was analyzed for copper
content, but only a trace amount could be detected
(0.006%). Clearly this represents an efficient stereose-
lective synthesis of cis combretastatin A-4 (1), which
avoids the use of protecting groups and chromatography.
This efficient synthesis of (Z)-combretastatin A-4 (1)
renders it suitable for the synthesis of the trans isomer.
There is a literature precedent for the iodine-catalyzed
isomerization of cis stilbenes.18 When iodine (10 mol %)
is added to a solution of cis combretastatin A-4 (1) in
chloroform and the resulting mixture is stirred at room
temperature for 30 min, complete isomerization to trans
combretastatin A-4 (3) occurs (Scheme 8). Following
workup, trans combretastatin A-4 is afforded in quanti-
tative yield [E:Z, 99.8:0.2 (determined by GC)].
Sch em e 5. P er k in Con d en sa tion of
3,4,5-Tr im eth oxyp h en yla cetic Acid a n d
3-Hyd r oxy-4-m eth oxyben za ld eh yd e
astatin A-4 (3) in moderate yield (ca. 40%) and 3,3′,4,4′,-
5,5′-hexamethoxybiphenyl20 (12) in low yield (ca. 6%)
(Scheme 4).
Con clu sion s
While the Suzuki cross-coupling syntheses of cis and
trans combretastatin A-4 were successful, we were keen
to develop more efficient processes. We next investigated
the synthesis of the stilbenes using the Perkin condensa-
tion,13 described by Letcher et al.14 The reaction involved
heating 3,4,5-trimethoxyphenylacetic acid (13) (2 equiv),
3-hydroxy-4-methoxybenzaldehyde (4) (1 equiv), triethyl-
amine, and acetic anhydride at reflux. Upon cooling, the
reaction mix was acidified with concentrated hydrochloric
acid, and the cinnamic acid, 14, precipitated from the
solution. This acid 14 was isolated and recrystallized
from ethanol in good yield (ca. 60%) (Scheme 5).
We were unable to grow a suitable crystal of cinnamic
acid 14 for X-ray crystallography or obtain useful data
on the stereochemistry from an NOE spectrum. The
extinction coefficient, measured from the UV spectra, is
known to be much greater for stilbenes in which the
aromatic rings are trans to each other (ꢀ ca. 28 000),
rather than for those in which the aromatic rings are cis
to each other (ꢀ ca. 10 000).15 We found the extinction
coefficient for cinnamic acid 14 to be 11 100, which
supports our belief that the configuration of the molecule
We have devised two methods of synthesis of cis
combretastatin A-4 (1) in which the stereoselectivity is
much improved on the current Wittig method of synthe-
sis. The route based on the Perkin condensation, which
avoids the use of protecting groups and chromatography,
is ideally suited for large-scale applications.
Exp er im en ta l Section
Gen er a l Meth od s. All reagents and chromatography grade
solvents were obtained from commercial sources and used
without further purification unless indicated. Flash column
chromatography was performed on silica gel [Fluka Silica gel
60 220-440 mesh (35-70 µm)], and TLC was carried out using
silica (0.2 mm, 60 F254)-precoated, aluminum-backed plates.
Elemental analyses were carried out by the Microanalytical
department at UMIST and are within 0.4% of theoretical
values.
(16) Ohsumi, K.; Nakagawa, R.; Fukuda, Y.; Hatanaka, T.; Mori-
naga, Y.; Nihei, Y.; Ohishi, K.; Suga, Y.; Akiyama, Y.; Tsuji, T. J . Med.
Chem. 1998, 41, 3022.
(17) Molho, D.; Coillard, J . Bull. Soc. Chim. Fr. 1956, 78.
(18) (a) Giacomelli, G.; Lardicci, L.; Saba, A. J . Chem. Soc., Perkin
Trans. 1 1978, 314. (b) Saltiel, J .; Ganapathy, S.; Werking, C. J . Phys.
Chem. 1987, 91, 2755. (c) Ali, M. A.; Tsuda, Y. Chem. Pharm. Bull.
1992, 40, 2842. (d) Battersby, A. R.; Greenock, I. A. J . Chem. Soc. 1961,
2592.
(13) (a) Perkin, W. H. J . Chem. Soc. 1868, 21, 181. (b) Perkin, W.
H. J . Chem. Soc. 1877, 31, 388.
(14) Letcher, R. M.; Nhamo, L. R. M.; Gumiro, I. T. J . Chem. Soc.,
Perkin Trans. 1 1972, 206.
(15) (a) Riezebos, G.; Havinga, E. Recl. Trav. Chim. Pays-Bas 1961,
80, 446. (b) Lambert, J . B.; Shurvell, H. F.; Lightner, D. A.; Cooks, R.
G. Organic Structural Spectroscopy; Prentice Hall: New York, 1998;
p 289.
(19) Orsini, F.; Pelizzoni, F.; Bellini, B.; Miglierini, G. Carbohydr.
Res. 1997, 301, 95.
(20) (a) Bowden, B. F.; Read, R. W.; Taylor, W. C. Aust. J . Chem.
1981, 34, 799. (b) Graebe, S. J ustus Liebigs Ann. Chem. 1905, 340,
230. (c) Stjernstroem, N. E.; Tengedal, J . E. Acta Chem. Scand. 1970,
24, 338.

8138 J . Org. Chem., Vol. 66, No. 24, 2001
Gaukroger et al.
Sch em e 6. Ester ifica tion a n d Red u ction of Cin n a m ic Acid 14
afforded stilbene 3 (158 mg, 40%) as an off-white solid. mp
102-104 °C (lit. mp5c 103-104 °C). Rf ) 0.41 (SiO2 petrol:
EtOAc 1:1). δH (300 MHz, CDCl3): 3.88 (3H, s), 3.94 (9H, s),
5.63 (1H, s), 6.73 (2H, s), 6.86 (1H, d, J ) 8.3), 6.89 (1H, d, J
) 16.2), 6.95 (1H, d, J ) 16.2), 6.99 (1H, dd, J ) 8.3, 1.9), 7.16
(1H, d, J ) 1.9). Further elution afforded 3,3′,4,4′,5,5′-
hexamethoxybiphenyl20 (12) as an orange solid (30 mg, 6%).
mp 127-129 °C (lit. mp 128-130 °C).20a Rf ) 0.45 (SiO2 petrol:
EtOAc 1:1). δH (300 MHz): 3.92 (6H, s), 3.96 (12H, s), 6.74
(4H, s).
Sch em e 7. Syn th esis of Com br eta sta tin A-4 by
Deca r boxyla tion of Cin n a m ic Acid 14
Cis Com br eta sta tin A-4 (1). (E)-3-(3′-Hydroxy-4′-meth-
oxyphenyl)-2-(3′′,4′′,5′′-trimethoxyphenyl)prop-2-enoic acid, 14,
(2 g, 5.56 mmol) was added to powdered copper (1.84 g, 28.8
mmol) in quinoline (20 mL, 21.9 g, 0.17 mol), and the resulting
mixture was heated at 200 °C for 2 h. Upon cooling, ether was
added, and the copper was filtered off through Celite. The
filtrate was washed with 1 M hydrochloric acid, and the
aqueous layer was separated and extracted with ether. The
combined organic layers were washed with saturated aqueous
sodium carbonate, water, brine, dried (MgSO4), and concen-
trated in vacuo. Flash column chromatography (SiO2 petrol:
EtOAc 7:3) and recrystallization from ethyl acetate and petrol
afforded cis combretastatin A-4 (1) as a pale yellow crystalline
solid (1.19 g, 68%). mp 117-118 °C (lit. mp6 116 °C). Spectro-
scopic data were identical to that obtained previously. El-
emental analysis of copper content: trace amounts detected
(0.006%). Further elution afforded a mixture of the cis 1 and
trans 3 stilbenes (182 mg, 10%).
Tr a n s Com br eta sta tin (3). To a solution of cis combret-
astatin A-4 (1) (200 mg, 0.63 mmol) in chloroform (10 mL) was
added iodine (16 mg, 0.06 mmol, 10 mol %). The resulting
solution was stirred at room temperature for 30 min, after
which the solution was washed thoroughly with saturated
aqueous sodium metabisulfite, destroying the remaining io-
dine. The yellow solution was washed with water, dried
(MgSO4), and concentrated in vacuo furnishing the E stilbene
3 as an off-white solid (198 mg, 99%). mp 102-104 °C (lit. mp5c
103-104 °C). Spectroscopic data were identical to that ob-
tained previously.
Sch em e 8. Syn th esis of th e E Isom er of
Com br eta sta tin A-4 by Isom er iza tion of th e Z
Isom er Usin g Iod in e
Cis Com br eta sta tin A-4 (1). (Z)-5-(2-Bromoethenyl)-2-
methoxyphenol (8) (300 mg, 1.31 mmol) and tetrakis(triphen-
ylphosphine)palladium(0) (75 mg, 0.065 mmol) were stirred
in 1,2-dimethoxyethane (20 mL) under argon for 20 min. 3,4,5-
Trimethoxybenzene boronic acid (315 mg, 1.49 mmol) and
sodium carbonate (140 mg, 1.32 mmol) in water (12 mL) were
added, and the mixture was heated at reflux overnight. The
aqueous layer was separated and extracted with chloroform.
The combined organic layers were washed with water and
brine, dried (MgSO4), and concentrated in vacuo. Following
flash column chromatography (SiO2 petrol:EtOAc 7:3), cis
combretastatin A-4 (1) was isolated as a pale yellow crystalline
solid (295 mg, 71%). mp 117-118 °C (lit. mp5c 116 °C). Rf )
0.46 (SiO2 petrol:EtOAc 1:1). δH (300 MHz, CDCl3): 3.72 (6H,
s), 3.86 (3H, s), 3.89 (3H s), 5.53 (1H, s), 6.42 (1H, d, J ) 12.4),
6.49 (1H, d, J ) 12.4), 6.55 (2H, s), 6.75 (1H, d, J ) 8.3), 6.82
(1H, dd, J ) 8.3, 1.9), 6.94 (1H, d, J ) 1.9). Elemental analysis
of palladium content: none detected (<0.4%). Elemental
analysis of tin content: none detected (<0.5%).
Tr a n s Com br eta sta tin A-4 (3). (E)-5-(2-Bromoethenyl)-
2-methoxyphenol (11) (285 mg, 1.24 mmol) and tetrakis-
(triphenylphosphine)palladium(0) (72 mg, 0.062 mmol) were
stirred in 1,2-dimethoxyethane (20 mL) under argon for 20
min. 3,4,5-Trimethoxybenzene boronic acid (9) (300 mg, 1.42
mmol) and sodium carbonate (131 mg, 1.24 mmol) in water
(11 mL) were added, and the mixture was heated at reflux
overnight. The aqueous layer was separated and extracted
with chloroform. The combined organic layers were washed
with water and brine, dried (MgSO4), and concentrated in
vacuo. Flash column chromatography (SiO2 petrol:EtOAc 7:3)
Ack n ow led gm en t. This research was made possible
by grants from the Association for International Cancer
Research (studentship, K.G.) and the EPSRC (Total
Technology studentship, L.A.H.).
Su p p or tin g In for m a tion Ava ila ble: Experimental re-
sults and characterization data for compounds 5-8, 11, 14-
16. This material is available free of charge via the Internet
at http://pubs.acs.org.
J O015959Z