A Ramberg-Baecklund route to the stilbenoid anti-cancer agents combretastatin A-4 and DMU-212

Article abstract of DOI:10.1039/b702411h

A concise route to combretastatin A-4, a potent inhibitor of tubulin polymerisation, using a Ramberg-Baecklund reaction to form the key (Z)-stilbene unit has been developed; this Ramberg-Baecklund approach has also been extended to prepare the (E)-stilbene DMU-212, which also possesses interesting growth inhibitory properties. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702411h

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A Ramberg–Ba¨cklund route to the stilbenoid anti-cancer agents
combretastatin A-4 and DMU-212{
James E. Robinson and Richard J. K. Taylor*
Received (in Cambridge, UK) 15th February 2007, Accepted 9th March 2007
First published as an Advance Article on the web 20th March 2007
DOI: 10.1039/b702411h
compounds,5 and have recently reported high levels of unexpected
(Z)-stereoselectivity in the construction of novel stilbene systems.6
A concise route to combretastatin A-4, a potent inhibitor of
tubulin polymerisation, using a Ramberg–Ba¨cklund reaction to
These results prompted us to investigate the development of a
flexible synthetic route to the combretastatins, as shown for
combretastatin A-4 3 in Scheme 1.
form the key (Z)-stilbene unit has been developed; this
Ramberg–Ba¨cklund approach has also been extended to
prepare the (E)-stilbene DMU-212, which also possesses
interesting growth inhibitory properties.
The combretastatins, isolated from the African tree Combretum
caffrum, are a potent class of anti-mitotic and anti-vascular agents
that have attracted considerable interest in recent times.1 Although
a large family are now known (e.g. 1–11, Fig. 1) the most potent
cancer cell growth inhibitor is the (Z)-stilbene combretastatin A-4
3,2 which has been shown to act as an inhibitor of tubulin
polymerisation by binding at the same site as colchicine.
Combretastatin A-4 has been described as ‘‘the simplest natural
product known with such potent antitubulin effects’’.3
Scheme 1 Retrosynthesis of combretastatin A-4 3.
The combination of potent biological properties and relatively
straightforward molecular structures has resulted in the develop-
ment of a number of synthetic routes to the natural products, and
to a wide range of analogues.1–3 Structure–activity relationship
studies4 strongly indicate that a (Z)-stilbene configuration is
essential for the cytotoxicity of the combretastatins, as is the 3,4,5-
trimethoxy substitution pattern on the A-ring.
The route hinges on the use of a Ramberg–Ba¨cklund reaction
for the formation of the required stilbene from the sulfone
precursor 12. It was envisaged that the requisite sulfone 12 would
in turn be available via the coupling of benzylic thiol 13 and
bromide 14.
The synthesis of combretastatin A-4 3 therefore commenced
with the coupling of thiol 13, prepared via treatment of
commercially available 3,4,5-trimethoxybenzyl alcohol with
Lawesson’s reagent, and the known bromide 147 using potassium
hydroxide in ethanol (Scheme 2). Oxidation of the resultant
sulfide8 with m-chloroperoxybenzoic acid afforded the correspond-
ing sulfone 12 in 49% yield over 2 steps. With sulfone 12 in hand,
the tandem halogenation–Ramberg–Ba¨cklund reaction was carried
out, initially using the conditions devised by Chan et al.9 (CF2Br2,
tBuOH, KOH–Al2O3) as shown in Scheme 2. We were delighted
We have an interest in the application of the Ramberg–
Ba¨cklund reaction to the synthesis of biologically active
Fig. 1 Selected members of the combretastatin family.
Scheme 2 Reagents and conditions: i. KOH, EtOH, 0 uC to rt, 12 h; ii.
m-CPBA, NaHCO3, CH2Cl2, 0 uC to rt, 12 h, 49% over 2 steps; iii. Chan
conditions: CF2Br2, tBuOH, KOH–Al2O3, 0 uC to rt, 12 h, 15 81% (E : Z =
Department of Chemistry, University of York, Heslington, York, UK
YO10 5DD. E-mail: rjkt1@york.ac.uk; Tel: +44 (0)1904 432606
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b702411h
t
90 : 10); Franck conditions: C2F4Br2, BuOH, KOH–Al2O3, D, 12 h, 16
72% (E : Z = 85 : 15); iv. TBAF–SiO2, THF, 0 uC, 12 h, 72%.
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to observe that the one-pot halogenation–Ramberg–Ba¨cklund
reaction proceeded extremely efficiently to give the O-silylated
stilbene 153 ((E)-15: J = 16.2 Hz, lit.3 J = 16.3 Hz; (Z)-15: J =
12.2 Hz, lit.3 J = 12.2 Hz) as a 90 : 10 mixture of (E)- and (Z)-
isomers in 81% yield. Desilylation of 15 using tetrabutylammo-
nium fluoride on silica provided the corresponding phenols 16
and 3 in 72% yield. Both combretastatin A-4 3 and the (E)-
combretastatin analogue 16 have previously been prepared by a
Wittig approach.3 Conducting the reaction under the conditions
reported by Franck et al.10 similarly provided 16 and 3 as an
85 : 15 mixture of (E)- and (Z)-isomers in 72% yield.
We next investigated the original halogenation–Ramberg–
t
t
Scheme 3 Reagents and conditions: i. Meyers conditions: CCl4, BuOH,
KOH, H2O, D, 12 h, 69% (16 : 3 = 47 : 53); ii. Pd/C, H2, EtOAc, 99%.
Ba¨cklund conditions (CCl4, BuOH, KOH, H2O) reported by
Meyers et al.11 Under these conditions in situ deprotection of the
tert-butyldimethylsilyl ether occurred but, more importantly, the
required stilbene was produced in 69% yield as a 47 : 53 mixture of
inseparable (E)- and (Z)-isomers 16 and 32,3 (J = 12.2 Hz, lit.3 J =
12.2 Hz) (Scheme 3). The 1H and 13C NMR data for the mixture
of 16 and 3 was in agreement with that reported in the literature.2,3
Hydrogenation of either pure (E)-stilbene 16 or a mixture of the
(E)- and (Z)-isomers 16 and 3 proceeded in quantitative yield to
furnish the known3 dihydrostilbene 17.
favour of the (Z)-isomer, all proceed with electron rich aromatic
systems under Meyers’ conditions. Further study is needed to fully
understand this observation but it is of obvious applicability in
terms of (Z)-stilbene targets such as the combretastatins.
We therefore went on to apply this Ramberg–Ba¨cklund route
to the synthesis of other combretastatin analogues (Table 1,
entries 2–4). First, we investigated the preparation of the
methylated analogues (E)- and (Z)-2012 (entry 2). Thus, the
a-methylated sulfone 19 was prepared from the known13 bromide
17 and the novel thiol 18 (readily prepared from the corresponding
alcohol using Lawesson’s reagent) following the earlier sequence.
Moving on to the halogenation–Ramberg–Ba¨cklund sequence,
Meyers’ conditions again gave in situ desilylation to produce the
expected stilbene 20 (70%) as a separable mixture of the novel (E)-
isomer and known12 (Z)-isomer (65 : 35). Franck conditions also
The formation of a significant quantity of (Z)-stilbene 3 in this
Ramberg–Ba¨cklund sequence under Meyers’ conditions deserves
further comment. Until the recent publications6 from our
laboratory, it was assumed that the use of benzyl benzyl sulfones
in the Ramberg–Ba¨cklund reaction would always produce the (E)-
stilbene, if not exclusively, then as the major isomeric product.5
These recent examples, leading to an unexpectedly high ratio in
Table 1 Preparation of and Ramberg–Ba¨cklund reaction of sulfones 12, 19, 23, 25
Entry
1
Coupling partners
Sulfone (% over 2 steps)
Stilbene
Reaction conditions (%, E : Z)b
Meyers (69, 47 : 53) (R = H 3 and 16)
Chan (81, 90 : 10) (R = TBS 15)
Franck (72, 85 : 15) (R = H 3 and 16)
2
Meyers (70, 65 : 35) (R = H 20)
Chan (59, 69 : 31) (R = TBS 21)
Franck (59, 90 : 10) (R = H 20)
3
Meyers (40, 59 : 41)
Chan (49, 76 : 24)
Franck (74, 68 : 32)
4
Meyers No reaction
Chan (24, 92 : 8)
Franck (53, 85 : 15)
a
b
Yield over 3 steps as desilylation was effected prior to conducting the Ramberg–Ba¨cklund reactions. (E) : (Z) Ratios quoted as a percentage
composition of total yield as estimated from 1H-NMR spectra.
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Scheme 4 Reagents and conditions: i. KOH, EtOH, 0 uC to rt, 12 h; ii. m-CPBA, NaHCO3, CH2Cl2, 0 uC to rt, 12 h, 49% over 2 steps; iii. Meyers
conditions: CCl4, tBuOH, KOH, H2O, D, 12 h, 38% (E : Z = 42 : 58); Chan conditions: CF2Br2, tBuOH, KOH–Al2O3, 0 uC to rt, 12 h, 47% (E : Z = 91 : 9);
Franck conditions: C2F4Br2, BuOH, KOH–Al2O3, D, 12 h, 89% (E : Z = 97 : 3) then recrystallisation (EtOH), 87% (E : Z = 100 : 0).
t
provided an (E)- and (Z)-isomeric mixture (90 : 10) of 20 in 59%
yield. With substrate 19, however, the Chan conditions produced a
mixture of alkene isomers of stilbene 21 (59%, (E) : (Z) = 69 : 31).
Desilylation of 21 proceeded smoothly in 72% yield to afford 20,
which was hydrogenated to the corresponding novel dihydrostil-
bene in quantitative yield.
combretastatin A-4 3 and DMU-212 27, as well as to several novel
analogues. During the course of these investigations further insight
has been gained into the scope and limitations of the Ramberg–
Ba¨cklund reaction for stilbene synthesis, particularly with respect
to issues concerning stereoselectivity.
We thank the EPSRC and MOD for postdoctoral support.
In a similar manner, the novel sulfone 23 was prepared by
coupling of thiol 22 with bromide 17 followed by oxidation of the
resultant sulfide and removal of the tert-butyldimethylsilyl ether to
furnish sulfone 23 in 29% yield over 3 steps. Thiol 22 was obtained
from 2-acetylbenzoic acid by an efficient four step sequence (56%;
reduction–selective protection–Mitsunobu coupling with thiolace-
tic acid–thioacetate reduction). However, the Ramberg–Ba¨cklund
reaction of sulfone 23 proceeded with little selectivity (Table 1,
entry 3); Meyers conditions gave stilbene 24 in 40% yield as a 59 :
41 mixture of (E)- and (Z)-isomers, and using the Chan conditions
stilbene 24 was isolated in 49% yield as a 76 : 24 mixture of (E)-
and (Z)-isomers. Franck conditions, while providing stilbene 24 in
74% yield, gave a similar mixture of (E)- and (Z)-isomers (68 : 32).
The (Z)-stereoselectivity was even worse when the Ramberg–
Backlund reaction of sulfone 25 was carried out (Table 1, entry 4);
under Chan conditions stilbene 26 was obtained in low yield
predominantly as the (E)-isomer, as was 26 under Franck
conditions (53%, (E) : (Z) = 85 : 15). Notably, no reaction was
observed for sulfone 25 under Meyers conditions.
Notes and references
1 For recent reviews see: G. C. Tron, T. Pirali, G. Sorba, F. Pagliai,
S. Busacca and A. A. Genazzani, J. Med. Chem., 2006, 49, 3033–3044;
H. P. Hsieh, J. P. Liou and N. Mahindroo, Curr. Pharm. Des., 2005, 11,
1655–1677.
2 G. R. Pettit, S. B. Singh, C. M. Lin, D. S. Alberts and D. Garcia-
Kendal, Experientia, 1985, 45, 209–211.
3 G. R. Pettit, S. B. Singh, M. R. Boyd, E. Hamel, R. K. Pettit,
J. M. Schmidt and F. Hogan, J. Med. Chem., 1995, 38, 1666–1672.
4 J. A. Woods, J. A. Hadfield, G. R. Pettit, B. W. Fox and
A. T. McGowan., Br. J. Cancer, 1995, 71, 705–711.
5 For recent reviews on the Ramberg–Ba¨cklund reaction see: R. J. K.
Taylor, G. D. McAllister and R. W. Franck, Carbohydr. Res., 2006,
1298–1311; R. J. K. Taylor and G. Casy, Org. React., 2003, 62,
357–475.
6 J. S. Foot, G. M. P. Giblin, A. C. Whitwood and R. J. K. Taylor, Org.
Biomol. Chem., 2005, 3, 756–763; J. S. Foot, G. M. P. Giblin and
R. J. K. Taylor, Org. Lett., 2003, 23, 4441–4444.
7 G. R. Pettit, S. B. Singh and G. M. Cragg, J. Org. Chem., 1985, 50,
3404–3406.
8 All novel compounds were characterised by IR, 1H and 13C NMR
spectroscopy, MS and HRMS.
The ease with which several of these stilbene systems were
obtained with high (E)-stereoselectivity prompted an investigation
concerning the use of the Ramberg–Backlund reaction to prepare
the (E)-stilbene 27,14 known as DMU-212. DMU-212 is a
synthetic analogue of resveratrol, a naturally occurring phytoalexin
with cancer chemoprotective activity.15 However, DMU-212 has
been reported to possess chemoprotective activity superior to that
of resveratrol, and as such has shown excellent promise as an anti-
cancer agent.16 This activity contrasts with the low activity of the
(E)-combretastatin analogues compared to their (Z)-counterparts.4
Accordingly, 4-methoxybenzyl mercaptan 28, readily available
from treatment of 4-methoxybenzyl bromide with thiolacetic acid
and potassium hydrogen carbonate, was reacted with bromide 17
(Scheme 4). Oxidation of the resultant sulfide furnished the desired
sulfone 29, the Ramberg–Ba¨cklund reaction of which using
Meyers conditions gave DMU-212 27 in 38% yield as a 42 : 58
mixture of (E)- and (Z)-isomers. Gratifyingly, Chan conditions
afforded a 91 : 9 mixture of (E)- and (Z)-isomers in 47% yield.
Conducting the reaction using the conditions of Franck provided
DMU-212 27 in 89% yield and enhanced the (E) : (Z) ratio to 97 :
3. Recrystallisation from ethanol gave colourless crystals of only
(E)-27 (87%, m.p. 157–158 uC, lit.17 m.p. 160–161 uC).
9 T.-L. Chan, S. Fong, Y. Li, T.-O. Man and C.-D. Poon, J. Chem. Soc.,
Chem. Commun., 1994, 1771–1772.
10 G. Yang, R. W. Franck, H.-S. Byun, R. Bittman, P. Samadder and
G. Arthur, Org. Lett., 1999, 1, 2149–2151.
11 C. Y. Meyers, A. M. Malte and W. S. Matthews, J. Am. Chem. Soc.,
1969, 91, 7510–7512.
12 The synthesis of (Z)-20 via palladium cross coupling has been reported
by J. A. Hadfield, A. T. McGown, S. P. Mayalarp, E. J. Land,
I. Hamblett, K. Gaukroger, N. J. Lawrence, L. A. Hepworth and
J. Butler, PCT Int. Appl., 2002, WO 0250007; Chem. Abstr., 137,
47052.
13 Y. Asakawa, K. Tanikawa and T. Aratani, Phytochemistry, 1976, 15,
1057–1059.
14 For previous preparations see: G. G. Cross, C. R. Eisnor, R. A. Gossage
and H. A. Jenkins, Tetrahedron Lett., 2006, 47, 2245–2247; M. Murias,
N. Handler, T. Erker, K. Pleban, G. Ecker, P. Saiko, T. Szekeres and
W. Ja¨ger, Bioorg. Med. Chem., 2004, 12, 5571–5578; U. Azzena,
G. Dettori, M. V. Idini, L. Pisano and G. Sechi, Tetrahedron, 2003, 59,
7961–7966; K. M. Brown, N. J. Lawrence, J. Liddle, F. Muhammad
and D. A. Jackson, Tetrahedron Lett., 1994, 35, 6733–6736;
M. Cushman, D. Nagarathnam, D. Gopal, A. K. Chakraborti,
C. M. Lin and E. Hamel, J. Med. Chem., 1991, 34, 2579–2588.
15 M. Jang, L. Cai, G. O. Udeani, K. V. Slowing, C. F. Thomas, C. W. W.
Beecher, H. H. S. Fong, N. R. Farnsworth, D. Kinghorn, R. G. Mehta,
R. C. Moon and J. M. Pezzuto, Science, 1997, 275, 218–220.
16 See S. Sale, R. G. Tunstall, K. C. Ruparelia, G. A. Potter, W. P. Steward
and A. J. Gescher, Int. J. Cancer, 2005, 115, 194–201 and references
therein.
In summary, the Ramberg–Ba¨cklund reaction has been utilised
as part of a short and flexible route to the anti-cancer stilbenes
17 J. W. Cook and L. L. Engel, J. Chem. Soc., 1940, 198–200.
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