Practical and green synthesis of combretastatin A-4 and its prodrug CA4P using renewable biomass-based starting materials

Article abstract of DOI:10.1055/s-0030-1258358

A practical and green protocol for the synthesis of vascular disrupting agent combretastatin A-4 (CA4) and its water soluble prodrug CA4P is described. Starting from the biomass-based compound anethole, which is abundantly and sustainably available from Chinese star anise (Illicium verum Hook. f.), the key intermediate 3-hydroxy-4-methoxyphenylacetic acid can be obtained within five steps. Perkin condensation between this acid and another naturally derived compound 3,4,5-trimethoxybenzaldehyde, followed by decarboxylation gives combretastatin A-4 in good overall yield. The phosphate produrg CA4P can be prepared under simple and mild conditions in a sequential one-pot two-step reaction. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1258358

PAPER
217
Practical and Green Synthesis of Combretastatin A-4 and Its Prodrug CA4P
Using Renewable Biomass-Based Starting Materials
a
a,b
a
G
Y
reenSynthesis of
u
CombretastatinA
C
a
Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, P. R. of China
Fax +86(21)85231119; E-mail: zou_jinan@163.com
b
c
Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. of China
Second Affiliated Hospital, College of Medicine, Zhejiang University, Zhejiang 310009, P. R. of China
Received 13 September 2010; revised 4 November 2010
MeO
MeO
MeO
MeO
Abstract: A practical and green protocol for the synthesis of vascu-
lar disrupting agent combretastatin A-4 (CA4) and its water soluble
prodrug CA4P is described. Starting from the biomass-based com-
pound anethole, which is abundantly and sustainably available from
Chinese star anise (Illicium verum Hook. f.), the key intermediate 3-
hydroxy-4-methoxyphenylacetic acid can be obtained within five
steps. Perkin condensation between this acid and another naturally
derived compound 3,4,5-trimethoxybenzaldehyde, followed by de-
carboxylation gives combretastatin A-4 in good overall yield. The
phosphate produrg CA4P can be prepared under simple and mild
conditions in a sequential one-pot two-step reaction.
O
P
OMe
OMe
OH
O
ONa
OMe
combretastatin A-4 (1)
OMe
ONa
combretastatin A-4 phosphate (2)
Figure 1 Chemical structures of CA4 and CA4P
were totally relying on petrochemicals, which were far
Key words: combretastatin A-4, combretastatin A-4 phosphate, re- from sustainable, thus the present synthetic protocols for
newable sources, green synthesis, Perkin reaction
CA4 and analogues have not fully met the concepts of
modern ‘green chemistry’ and still remains to be im-
proved.
Combretastatin A-4 (CA4, Figure 1) is a potent tubulin-
Feedstocks derived from renewable sources play an in-
binding agent first isolated from the bark of the South Af-
creasingly important role in the modern chemical and
rican bush willow Combretum caffrum by Pettit and co-
pharmaceutical industry, the strategy of which is high-
1
workers in 1989. Since then, it has received great and
lighted in the ‘Twelve Principles of Green Chemistry’ in
continuous attention owing to the strong inhibition of tu-
which Article VII mentions that: a raw material or feed-
bulin polymerization and selective targeting of tumor vas-
stock should be renewable rather than depleting whenever
cular systems, and has commonly been recognized to be
the original lead compound for vascular disrupting agents
10
technically and economically practicable. Consequent-
ly, in the course of our ongoing efforts to develop natural-
ly occurring stilbenes and derivatives from renewable
biomass-based materials endemic to China, we herein re-
port a practical and green protocol using Perkin condensa-
tion for the synthesis of CA4/CA4P. The synthesis starts
from the biomass-based and most importantly, the sus-
tainable-supplied compound anethole (6, derived from
2
3
(
VDAs). The water soluble prodrug of CA4, namely
combretastatin A-4 phosphate (CA4P, Figure 1), which is
endowed with a much improved pharmacokinetic profile
and being developed by Oxigene, has proved to be the
first VDA candidate entering into phase III clinical trials
for treatment of cancers as well as ophthalmological dis-
4
eases (myopic and wet macular degeneration).
11
Chinese star anise) and 3,4,5-trimethoxybenzaldehyde
1
2
The contradiction between encouraging biological results (5, derived from Chinese gallnut), providing a good ex-
and the extreme scarcity in nature concerning about CA4/ ample for the development of bioactive compounds in a
CA4P have stimulated the synthetic studies. As a green chemistry manner and revealing the potential in
polyphenolic stilbene with cis-configuration, the synthet- large-scale applications (Scheme 1).
ic methodologies for CA4 are mainly based on Wittig
Our strategy is based on Perkin reaction, which has com-
5
6
7
reaction, Perkin reaction, Suzuki-type reaction, Ramberg–
monly been recognized to be stereoselective (cis/trans:
95/5 after one crystallization), easy to handle, high-
8
Backlund reaction, etc. Among them, the Perkin conden-
>
6
sation developed by Gaukroger et al. seems to be more
yielding, and atom economic. Retrosynthetic analysis that
led to the Perkin-based strategy for the synthesis of CA4
is outlined in Scheme 1. The decarboxylation precursor 3
would be prepared through Perkin condensation between
the key building blocks 3-hydroxy-4-methoxyphenylace-
tic acid (4) and 3,4,5-trimethoxybenzaldehyde (5), which
could be further traced back to biomass-based raw mate-
rials anethole (6) and gallic acid (7), respectively.
convenient and cis-selective. Our group has also reported
a concise route for CA4 and its dihydro analogue erianin
9
utilizing the same portocol. It could be noticed that the
starting materials of the above mentioned methodologies
SYNTHESIS 2011, No. 2, pp 0217–0222
x
x
.
x
x
.2
0
1
1
Advanced online publication: 08.12.2010
DOI: 10.1055/s-0030-1258358; Art ID: F16410SS
Georg Thieme Verlag Stuttgart · New York
©

2
18
Y. Chen et al.
PAPER
MeO
tion was complete, the crude oil of anisaldehyde (8) could
be obtained by filtration and separation. The acidic aque-
ous phase and the excess slag of pyrolusite, which still
contained approximately 25% manganese dioxide, were
treated with oxalic acid at 80 °C for 30 minutes. A non-
manganese, nonhazardous residue (mainly consisting of
SiO and BaSO ) was then obtained and separated by fil-
COOH
MeO
OMe
OH
HO
OH
MeO
OMe
OH
1
6
7
2
4
tration. The filtrate was treated by successive addition of
H O and aqueous ammonia to give a solution with a pH
2
2
of 5.4–6.0, which resulted in the precipitation of Al(OH)3
COOH
and Fe(OH) . After filtration, the filtrate was treated with
3
MeO
COOH
OH
CHO
5
% aq (NH ) S, and most of the heavy metal ions except
4 2
2+
2+
2+
Mn were precipitated as sulfide MS (M = Pb , Cd ,
Zn , Cu , Ni , etc.). Then, a calculated amount of
(NH ) SO was added to the filtrate to form an equivalent
+
MeO
2
+
2+
2+
OMe
OH
MeO
OMe
4
2
4
OMe
OMe
OMe
content of (NH ) SO and MnSO in the solution. To our
4 2 4 4
4
3
5
delight, when the solution was evaporated partially and
repeatedly, a pale reddish, single crystal with high purity
were steadily obtained. Indeed, the high purity of the inor-
ganic compound enabled us to perform a single-crystal X-
ray diffraction analysis to determine its chemical composi-
Scheme 1 Biomass-based retrosynthetic analysis of CA4
Anethole (4-propenylanisole, 6) is the major volatile com-
ponent of Chinese star anise (Illicium verum Hook. f.) and
could be obtained sustainably in large quantity from
fruits, branches, and leaves of the plant. The content of
anethole could be as high as 8.88% in the fruit of Illicium
17
tion, and it was confirmed to be (NH ) SO ·MnSO ·6H O.
4
2
4
4
2
It crystallizes in the monoclinic P2 /c space group with
1
half a molecule in the asymmetric unit. The molecule pos-
sesses crystallographic inversion symmetry. The Mn(II)
1
1
verum and 92% in volatile essential oil. It is worth men-
tioning that the plant of Illicium verum belongs to the cat-
egory of non-corn biomass and that the anethole is not
obtained from barks, stems, and roots of the plant, thereby
no ethical problems are caused by competition with food,
and the acquisition process of anethole brings no damage
to the plant sources. Hence, the anethole is deserved to be
an ideal renewable biofeedstock and served as a typical
example for elaborating the concept of sustainable (green)
center is six-coordinated by six O donors from H O with
Mn–O distances of 2.15–2.19 Å. Interestingly, the six
2
H O molecules are connected with Mn(II) through coor-
2
dination bonds rather than hydrogen bonds as we expected
(
Figure 3). The packing in the crystals is determined by
rather complex intermolecular networks of hydrogen
bonds (Figure 4). The strongest hydrogen bonds in the
crystal structure are formed between the hydrogen of H O
2
2–
and oxygen of SO4 (O–H···O: 1.87 Å, 177°), in which
1
3
chemistry. Similarly, gallic acid (7) is also a sustainable
the coordinated H O molecule acts as H-bond acceptor.
2
biomass-based feedstock obtained in large amounts from
Moreover, the recovery of manganese in the form of
1
4
Chinese Gallnuts and Tara (from Peru), and has found
wide applications as a starting material for the develop-
(
NH ) SO ·MnSO ·6H O can be as high as 90% of the to-
4 2 4 4 2
tal amount of manganese in form of MnO existed in py-
2
1
2a
ment of a series of fine chemicals. 3,4,5-Trimethoxy-
benzaldehyde (5) is, for example, a well-known
pharmaceutical intermediate derived from gallic acid (7)
rolusite. In addition, (NH ) SO ·MnSO ·6H O can be
4
2
4
4
2
further converted into high purity MnCO by treatment
3
with a stoichiometric amount of 10% of aqueous
NH HCO .
1
2
and the synthetic approach has been well established. As
a matter of fact, from modern green chemistry point of
view, the biomass-based feedstocks such as anethole, gal-
lic acid, shikimic acid, pinene, etc., which are endowed
with the unambiguous character of sustainability, deserve
further research and applications as alternative resources
in place of petroleum-based feedstocks for modern chem-
ical and pharmaceutical industry.
4
3
p-Anisaldehyde (8) was converted into sodium 4-meth-
oxymandelate by TEBA-catalyzed phase-transfer reac-
tion with chloroform in aqueous NaOH, followed by a
hydrogenolytic reduction by means of stannous chloride
dihydrate in the presence of concentrated hydrochloric
acid. It is noteworthy that, owing to the strong acidic con-
ditions, the crude product of sodium 4-methoxymandelate
could be directly used in hydrogenolytic step without be-
ing further transformed into its acidic form and purified,
thus readily affording 4-methoxyphenylacetic acid (9) in
The synthesis (Scheme 2) of the key intermediate 3-hy-
droxy-4-methoxyphenylacetic acid (4) starting from ane-
thole (6) commenced with a simple and conventional
oxidation reaction in the presence of natural pyrolusite
7
6% yield. Subsequently, 9 was readily brominated to
(
containing 60–70% MnO ) in sulfuric acid to afford p-
2
give 3-bromo-4-methoxyphenylacetic acid (10) in 96%
1
5
anisaldehyde (8) in good isolated yield (75%). Our goal
was, however, to find a way to minimize the disposal
problems of large amount of acidic waste water and resi-
due in this reaction and to recover the manganese element
18
yield.
In 1984, Weller et al.18a reported the transformation reac-
tion of 3-bromo-4-methoxyphenylacetic acid (10) to 3-
hydroxy-4-methoxyphenylacetic acid (4) under harsh
1
6
mostly. As shown in Figure 2, when the oxidation reac-
Synthesis 2011, No. 2, 217–222 © Thieme Stuttgart · New York

PAPER
Green Synthesis of Combretastatin A-4
219
COOH
COOH
CHO
a
c
b
Br
OMe
OMe
OMe
OMe
6
8
9
10
COOH
OH
MeO
MeO
COOH
d
e
OMe
OH
OMe
OMe
4
3
MeO
MeO
MeO
MeO
f
g
O
P
OMe
OMe
OH
O
ONa
OMe
OMe
ONa
1
2
Scheme 2 Synthesis of combretastatin A-4 and its prodrug CA4P. Reagents and conditions: (a) pyrolusite, 32% H SO , 50–100 °C, 75%; (b)
2
4
i. 50% aq NaOH, CHCl , TBAB, 60 °C, 4.5 h, ii. concd HCl, SnCl ·2H O, 80 °C, 2 h, 71%; (c) Br , AcOH, 0 °C to r.t., 96%; (d) bis(8-quino-
3
2
2
2
linolate)copper(II) catalyst, 30% aq NaOH, 110 °C, 6 h, 98%; (e) 3,4,5-trimethoxybenzaldehyde (5), Et N, Ac O, 110 °C, 6 h, 67%; (f) Cu,
3
2
quinoline, 200 °C, 3 h, 70%; (g) i. Et N, POCl , 25 °C, 5 h, ii. 1 M aq NaOH, r.t., 10 h, 86%.
3
3
Mn-containing waste
1
2
. treatment with oxalic acid at 80 °C
for 40 min
. filtration
filtrate
residue
(SiO , BaSO )
2 4
1
. addition of 30% H2O2
. neutralisation with aq NH3
to pH 5.4~6 at 50 °C, 1 h
. filtration
2
3
residue
filtrate
[
Fe(OH)3, Al(OH)3] 1.addition of 5% (NH ) S at 60–65 °C,
4 2
pH 5~6, 2 h
. filtration
2
Figure
3
Metal coordination in the crystal structure of
(
NH ) SO ·MnSO ·6H O
4
2
4
4
2
residue
MnSO4
MS (M = Pb , Cd2+
2
+
the transformation of bromo group to hydroxy group pro-
ceeded smoothly under atmospheric pressure at 110 °C
within six hours, giving 3-hydroxy-4-methoxyphenylace-
tic acid (4) in 98% yield. The catalytic bis(8-quinolino-
late)copper(II) complex (oxine-copper) can be readily
obtained by reacting 8-quinolinol with a copper(II) salt
such as CuSO ·5H O, CuCl ·2H O and Cu(NO ) ·3H O
in water, methanol, or ethanol. For practical applica-
tions, lifetime of catalyst and the levels of reusability are
very important factors. We therefore conducted a set of
experiments to investigate the reusability of the catalyst
for the transformation reaction of compound 10 to 4 under
similar optimized conditions. After completion of the first
reaction in 98% yield, the catalyst was recovered by filtra-
,
Zn2+, Cu2+, Ni2+,
etc.)
1
2
. treatment with (NH4)2SO4
. concentration
1. treatment
with aq 10% NH HCO3
4
2
. wash with H2O
(
NH4)2SO4⋅MnSO4⋅6H2O MnCO3
4
2
2
2
3 2
2
1
9
Figure 2 Protocol for waste-product treatment and manganese re-
covery in the oxidation of anethole
conditions by means of CuSO /NaOH at 150 °C in a
sealed stainless steel bomb for 36 h. To simplify the pro-
cedure, we used bis(8-quinolinolate)copper(II) in 10%
4
molar ratio instead of CuSO as the catalyst and found that
4
Synthesis 2011, No. 2, 217–222 © Thieme Stuttgart · New York

2
20
Y. Chen et al.
PAPER
benzylchlorophosphite followed by cleavage of the ben-
zyl ester protecting group with trimethyliodosilane. The
phosphoric acid intermediate was treated with sodium
methoxide to complete a practical route to the sodium
3
phosphate prodrug. In this study, phosphorylation of
combretastatin A-4 by means of phosphorus oxychloride
followed by hydrolysis, gave combretastatin A-4 dihydro-
gen phosphate, which is readily alkalized by means of so-
dium hydroxide in ethanol to give CA4P (2) in 86% yield
in a more cost-competitive way, compared with Pettit’s
method. Particularly noteworthy was that these reaction
conditions were apparently quite sensitive to temperature
and pH value. When the reaction temperature was above
3
5 °C or hydrolysis of the phosphoryl chloride had gone
to completion in the dilute acid solution, isomerization of
CA4P was observed and less active trans-isomer of CA4P
was obtained. The cis or trans geometries were deter-
1
mined by their characteristic H NMR coupling constants
Figure 4 Packing diagram of (NH ) SO ·MnSO ·6H O showing the
hydrogen-bond interactions
4
2
4
4
2
for the olefinic protons in conformity with this class of
compounds of approximately 12 Hz for the cis and 16–17
Hz for the trans isomers.3
,5
tion, washed with water, and dried. Then the used catalyst
was employed in another reaction run with fresh reactants In conclusion, starting from the renewable biomass-based
under the similar conditions. To our delight, oxine-copper compounds anethole (6) and 3,4,5-trimethoxybenzalde-
showed excellent recoverability and reusability over six hyde (5), a practical and green protocol for the synthesis
successive runs, which confirmed the strong coordination of CA4 and CA4P in satisfactory overall yields has been
and stability of copper in the catalyst.
achieved. Our work has not only provided an alternative
route for the synthesis of natural compounds with medical
interests, but also provided a typical example for the sus-
tainable development of valuable compounds in green
chemistry manner without using petroleum-based starting
materials. In addition, the minimization of disposal prob-
lems of pyrolusite/sulfuric acid promoted oxidation of
anethole (6) and the successful recovery of manganese el-
(
E)-2-(3¢-Hydroxy-4¢-methoxyphenyl)-3-(3¢,4¢,5¢-trimeth-
oxyphenyl)acrylic acid (3) was synthesized by means of
Perkin reaction between 3-hydroxy-4-methoxyphenylace-
tic acid (4) (1.1 equiv) and 3,4,5-trimethoxybenzaldehyde
(
5) (1.0 equiv) in the presence of acetic anhydride and tri-
ethylamine at 110 °C, which runs as a much improved
counterpart to the method previously reported by
6
ement in form of (NH ) SO ·MnSO ·6H O, the formation
Gaukroger et al. After the reaction, the mixture was acid-
4 2 4 4 2
of the hydroxy intermediate 4 using oxine-copper as cata-
lyst under mild conditions and the convenient phosphory-
lation process to form CA4P have also introduced aspects
of green chemistry into the synthesis.
ified with ice-cold dilute hydrochloric acid, deacetylated,
and discolored in basic conditions, and then acidified to
give 3. The cis-stilbene configuration of this compound
1
was unambiguously established by H NMR spectrum, on
the basis of typical chemical shifts observed for the ethyl-
enic proton (=CH) signal. Due to the carboxy group field Anethole (6) was kindly supplied by Guangxi Wanshan Spice Co.
effect in the cis-stilbene, a remarkable downfield shift of Ltd. as a natural product with chromatography grade. 3,4,5-Tri-
methoxybenzaldehyde (5) was purchased from Zhangjiajie
the acrylic alkene proton (=CH) at d = 7.78 is resulted.
Maoyuan Chemical Industry Co., Ltd. and proved to be a biomass-
based product derived from natural gallic acid. Other reagents and
chromatography grade solvents were obtained from commercial
sources and used without further purification, unless otherwise stat-
The decarboxylation of 3 was carried out in the presence
of quinoline and copper powder at 220 °C under nitrogen,
to afford the combretastatin A-4 in 70% yield. Combret-
astatin A-4 (1) was purified by solvent extraction with pe-
ed. Petroleum ether (PE) used refers to the boiling fraction 60–90
troleum ether and subsequent recrystallization from ethyl °C. Reactions were monitored by TLC using silica gel (0.2 mm,
acetate and petroleum ether, which avoided the use of GF254) percolated, glass-backed plates. The purity of products was
analyzed by Shimadzu GC-9A gas chromatograph. The melting
flash column chromatography and thus be amenable to
points were determined on Thiele apparatus and are uncorrected. IR
large-scale preparation. The geometrical configuration of
1
1
spectra were recorded on an Analect RFX-65A IR spectrometer. H
NMR was obtained from a Bruker DRX-400 MHz spectrometer.
Chemical shifts (d) are given in ppm downfield from TMS as inter-
nal standard and coupling constants (J values) are in hertz (Hz). EI/
MS analyses were carried out using a Shimadzu GCMS-QP5050A
mass spectrometer. Elemental analyses were carried out by Elemen-
tar Vario EL element analyzer. The single X-ray diffraction mea-
surement was performed on a Bruker SMART 1000 CCD X-ray
diffractometer.
combretastatin A-4 was assigned by the characteristic H
NMR coupling constants of the olefinic protons
(
JCH=CH = 12.4 Hz for cis-stilbene).
The clinically applicable form of combretastatin A-4,
namely CA4P (2) is designed as the phosphate prodrug
with excellent water solubility and pharmacokinetic pro-
file. At present, the most effective route was accom-
plished by phosphorylation with in situ formation of
Synthesis 2011, No. 2, 217–222 © Thieme Stuttgart · New York

PAPER
-Methoxybenzaldehyde (8)
Green Synthesis of Combretastatin A-4
221
+
+
4
MS (EI): m/z (%) = 244 (50, [M] ), 246 (48, [M + 2] ), 199 (100),
To a stirred mixture of pyrolusite (containing 56.44% of MnO , 160
201 (98), 121 (10), 105 (30), 77 (70), 51 (45).
2
g, 1.04 mol) in H O (360 g) was added anethole (6; 60 g, 0.41 mol)
at 50 °C. The resulting mixture was heated to 65 °C, and 32% aq
2
3-Hydroxy-4-methoxyphenylacetic Acid (4)
H SO (600 g, 1.96 mol) was added dropwise over 30 min. After
stirring at 100 °C for 1 h, the separated pyrolusite was filtered off
Compound 10 (24.5 g, 0.1 mol), 30% aq NaOH (90 mL), and bis(8-
quinolinolate)copper (II) (3.52 g, 0.01 mol) were loaded into a
stainless steel bomb. The resulting mixture was refluxed under N2
at atmospheric pressure for 6 h. After completion of the reaction
(monitored by TLC), the reaction mixture was cooled to r.t. and
neutralized by the addition of concd HCl. The precipitated bis(8-
quinolinolate)copper(II) was separated by filtration and was recy-
cled. The resulting filtrate was acidified and extracted with EtOAc
(3 × 30 mL). The combined organic layers were washed with brine
2
4
and washed with CHCl (3 × 30 mL). The organic layer was sepa-
3
rated from the filtrate and neutralized to pH 8–9 with sat aq Na CO ,
2
3
and allowed to stay overnight. The organic layer was separated,
washed neutral with H O, and concentrated under reduced pressure
2
to afford a crude oily product, which was purified by fractional dis-
tillation under reduced pressure to give 8 as a colorless viscous liq-
uid (41.2 g, 75%) (GC assay 98.9%).
(
2 × 10 mL), dried (MgSO ). and evaporated under reduced pres-
4
Removal of the excess slag of pyrolusite and recycling of these salts
sure to afford a yellow solid, which was recrystallized with EtOAc–
back to MnCO began with combining the filtered pyrolusite with
3
PE to afford 4 as white crystals (17.93 g, 98%); mp 127.5–129 °C
the aqueous layer after removal of anisaldehyde (8) by separation.
This mixture was treated with oxalic acid (25 g, 0.198 mol) for 0.5
h at 80 °C followed by a filtration to remove the residue (mainly
consisting of SiO and BaSO ). The filtrate was treated by the suc-
1
8a
(
Lit. mp 128–130 °C).
1
H NMR (400 MHz, acetone-d ): d = 7.54 (s, 1 H, OH), 6.86 (d,
6
J = 8.0 Hz, 1 H, 5-ArH), 6.81 (d, J = 2.0 Hz, 1 H, 2-ArH), 6.72 (dd,
2
4
cessive addition of 30% aq H O (10 mL) and 25% aq NH (142
1 H, J = 8.0, 2.0 Hz, 6-ArH), 3.80 (s, 3 H, 4-OCH ), 3.55 (s, 2 H,
2
2
3
3
mL) to give a solution of pH 5.4–6.0, which resulted in the precipi-
tation of Al(OH) and Fe(OH) . After filtration, the filtrate was
CH2).
3
3
+
+
MS (EI): m/z = 182 [M] , 183 [M + 1] , 137, 122, 94, 77.
treated with 5% aq (NH ) S (4 mL), and the heavy metal ion were
4
2
2
+
2+
2+
2+
2+
precipitated as sulfide MS (M = Pb , Cd , Zn , Cu , Ni , etc.).
After filtration, the filtrate was treated by the addition of calculated
amount of (NH ) SO over 0.5 h, giving a homogeneous solution,
(
E)-2-(3¢-Hydroxy-4¢-methoxyphenyl)-3-(3¢,4¢,5¢-trimethoxy-
phenyl)acrylic Acid (3)
4
2
4
To a solution of compounds 4 (4.01 g, 0.022 mol) and 5 (3.92 g,
which
after
concentration
by
heating,
afforded
0
.02 mol) in Ac O (12 mL) was added Et N (12 mL) under N . The
2
3
2
(
NH ) SO ·MnSO ·6H O (365.3 g, 90%) as pale reddish single
4 2 4 4 2
mixture was stirred at 110 °C for 6 h. The mixture was poured into
ice-cold H O (20 mL) and acidified with dil. HCl with stirring. A
crystal with high purity. Furthermore, (NH ) SO ·MnSO ·6H O can
4
2
4
4
2
2
be converted into high quality MnCO by treatment with stoichio-
3
yellow solid was obtained, which was collected by filtration. It was
dissolved in 10% aq NaOH (100 mL) and decolored by extracting
with EtOAc (2 × 15 mL). The organic layers were separated and the
aqueous layer was acidified with concd HCl to pH 2–3. The precip-
itated crude product was collected by filtration and recrystallized
metric amount of 10% aq NH HCO .
4
3
4
-Methoxyphenylacetic Acid (9)
To a mixture of compound 8 (12.3 mL, 0.1 mol) and Bu NBr
4
(
TBAB, 1.6 g, 0.005 mol) in CHCl (16 mL) was added dropwise
3
from EtOH to afford 3 as pale yellow crystals (4.82 g, 67%); mp
5
0% aq NaOH (18 mL, 0.5 mol) at 60 °C under N over 1.5 h. The
9
2
2
33–234 °C (Lit. mp 233–234 °C).
resulting mixture was stirred at 60 °C for 3 h. Upon cooling, the re-
action mixture was filtered and the precipitate was collected,
IR (KBr): 3401, 2998, 2944, 2838–2518, 1679, 1581, 1508, 1459,
–
1
1
378, 1334, 1184, 998, 946 cm .
washed with CHCl (3 × 10 mL), and dried. The obtained white sol-
3
id, which was a mixture of sodium 4-methoxymandelate and NaCl
1
H NMR (400 MHz, CDCl ): d = 7.78 (s, 1 H, CH=), 6.89 (d,
3
(
ratio ~1:3) could be used in the next step without further purifica-
J = 8.4 Hz, 1 H, 5¢-ArH), 6.85 (d, J = 2.0 Hz, 1 H, 2¢-ArH), 6.75 (dd,
J = 8.4, 2.0 Hz, 1 H, 6¢-ArH), 6.39 (s, 2 H, 2¢,6¢-ArH), 5.76 (br, 2 H,
CO H, OH), 3.89 (s, 3 H, 4¢-OCH ), 3.81 (s, 3 H, 4¢-OCH ), 3.57 (s,
tion. To a solution of the above crude sodium 4-methoxymandelate
in concd HCl (100 mL) was added SnCl ·2H O (33.9 g, 0.15mol).
2
2
2
3
3
The reaction mixture was stirred vigorously for 2 h at 80 °C. After
stirring further for 1 h, the mixture was poured into ice-water and
6
H, 3¢,5¢-OCH3).
+
+
MS (EI): m/z (%) = 360 (100, [M] ), 345 (20, [M – OCH ] ), 327
3
the precipitate was collected, washed with ice-cold H O (3 × 10
2
(5), 285 (15).
Anal. Calcd for C19H O : C, 63.3; H, 5.6. Found: C, 63.3; H, 5.6.
20 7
mL), and dried. The crude product was recrystallized from H O to
2
afford 9 as light white crystals (14.2 g, 71% from 8); mp 86–87 °C.
1
H NMR (400 MHz, acetone-d ): d = 10.66 (s, 1 H, CO H), 7.22 (d,
6
2
Combretastatin A-4 (1)
J = 8.0 Hz, 2 H, 2,6-ArH), 6.85 (d, J = 8.0 Hz, 2 H, 3,5-ArH), 3.76
To a suspension of Cu powder (1.5 g, 23.3 mmol) in quinoline (11.0
g, 85 mmol) was added compound 3 (1.0 g, 2.8 mmol) and the re-
(
s, 3 H, 4-OCH ), 3.54 (s, 2 H, CH ).
3
2
+
MS (EI): m/z = 166 [M] , 121, 106, 91, 77.
sulting mixture was stirred under N at 200 °C for 3 h. Upon cool-
2
ing, the mixture was diluted with EtOAc (50 mL), and the Cu was
filtered off. The filtrate was washed with aq 1 M HCl (3 × 20 mL),
and the aqueous layer was separated and extracted with EtOAc
(3 × 10 mL). The combined organic layers were washed with brine
3
-Bromo-4-methoxyphenylacetic Acid (10)
To a stirred solution of 4-methoxyphenylacetic acid 9 (9.6 g, 57.8
mmol) in AcOH (25 mL) was added Br (3.1 mL, 60 mmol) drop-
wise over 1 h. After stirring for 3 h at r.t., the reaction mixture was
quenched with an excess of 5% aq Na SO and cooled at 0 °C. The
precipitate was collected, washed with ice-cold H O (3 × 10 mL),
and dried to obtain the crude product 10 (13.6 g, 96%), which could
be used directly in the next step. The crude product was recrystal-
2
(
2 × 10 mL), dried (MgSO ), and concentrated under reduced pres-
4
2
3
sure to afford a brown viscous solid. This was purified by extraction
2
with PE and subsequent recrystallization from EtOAc–PE to give 1
5a
as colorless crystals (0.62 g, 70%); mp 116–117 °C (Lit. mp 116
°
C).
lized from EtOH–H O to afford 10 as white plates; mp 113–114 °C
2
Lit.18 mp 113–115 °C).
IR (KBr): 3424, 3002, 2938, 2836, 1610, 1579, 1508, 1459, 1419,
(
–
1
1
328, 1182, 1025, 1004, 944, 881, 854, 796 cm .
1
H NMR (400 MHz, acetone-d ): d = 7.52 (d, J = 2 Hz, 1 H, 2-
6
1
H NMR (400 MHz, CDCl ): d = 6.91 (d, J = 2.0 Hz, 1 H, 2¢-ArH),
3
ArH), 7.27 (dd, J = 8.4, 2.0 Hz, 1 H, 6-ArH), 7.02 (d, J = 8.4 Hz, 1
6
.79 (dd, J = 8.0, 2.0 Hz, 1 H, 6¢-ArH), 6.71 (d, J = 8.0 Hz, 1 H, 5¢-
H, 5-ArH), 3.87 (s, 3 H, 4-OCH ), 3.58 (s, 2 H, CH ).
3
2
Synthesis 2011, No. 2, 217–222 © Thieme Stuttgart · New York

2
22
Y. Chen et al.
PAPER
ArH), 6.51 (s, 2 H, 2¢,6¢-ArH), 6.45 (d, J = 12.4 Hz, 1 H, CH=), 6.42
d, J = 12.4 Hz, 1 H, CH=), 5.49 (s, 1 H, OH), 3.89 (s, 3 H, 4¢-
OCH ), 3.84 (s, 3 H, 4¢-OCH ), 3.68 (s, 6 H, 3¢,5¢-OCH ).
(3) (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.Paradise
Valley Arizona,US Patent 5561122 (A), 1996; Chem. Abstr.
(
3
3
3
+
MS (EI): m/z (%) = 316 (100, [M] ), 301 (75), 241 (8), 226 (6), 211
5), 141 (12), 115 (8), 93 (5), 57 (8).
1
996, 125, 309027.
(
(
4) (a) Vincent, L.; Kermani, P.; Young, L. M.; Cheng, J.;
Zhang, F.; Shido, K.; Lam, G.; Bompais-Vincent, H.; Zhu,
Z.; Hicklin, D. J.; Bohlen, P.; Chaplin, D. J.; May, C.; Rafii,
S. J. Clin. Invest. 2005, 115, 2992. (b) Tozer, G. M.;
Kanthou, C.; Baguley, B. C. Nat. Rev. Cancer 2005, 5, 423.
(c) Pettit, G. R.; Minardi, M. D.; Hogan, F.; Price, P. M.
J. Nat. Prod. 2010, 73, 399.
Anal. Calcd for C H O : C, 68.4; H, 6.3. Found: C, 68.3; H, 6.3.
18
20
5
Combretastatin A-4 Disodium Phosphate (2)
A solution of compound 1 (3.16 g, 10 mmol) in CH Cl (15 mL)
2
2
was added dropwise to a stirred solution of POCl (5.5 mL, 60
3
mmol) in CH Cl (15 mL) at r.t. over 1 h. A solution of Et N (3.5
2
2
3
mL, 24 mmol) in CH Cl (5 mL) was then added dropwise over 2 h.
2
2
(5) (a) 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. (b) Lawrence, N. J.; Ghani, F. A.; Hepworth, L. A.;
Hadfield, J. A.; McGown, A. T.; Pritchard, R. G. Synthesis
1999, 1656.
The resulting mixture was stirred at r.t. for 3 h until completion of
the reaction, then quenched by the addition of H O (20 mL). The
2
aqueous layer was extracted with CH Cl (3 × 10 mL) and the com-
2
2
bined organic layers were dried (MgSO ), filtered, and concentrated
4
in vacuo to afford a colorless oil. The crude combretastatin A-4 di-
hydrogen phosphate (10 mmol) obtained was dissolved in EtOH (20
mL) at r.t. and 1 M aq NaOH (12 mL, 25 mmol) was added drop-
wise. The resulting mixture was stirred at r.t. for 10 h, then filtered,
concentrated, and the residue was recrystallized from MeOH–ace-
tone to afford 2 as a white solid (1.2 g, 86%); mp 194 °C (dec.)
(
6) Gaukroger, K.; Hadfield, J. A.; Hepworth, L. A.; Lawrence,
N. J.; McGown, A. T. J. Org. Chem. 2001, 66, 8135.
7) Fürstner, A.; Nikolakis, K. Liebigs Ann./Recl. 1996, 2107.
8) Robinson, J. E.; Taylor, R. J. K. Chem. Commun. 2007,
(
(
1617.
(9) Zou, Y.; Xiao, C. F.; Zhong, R. Q.; Wei, W.; Huang, W. M.;
(
Lit.3b mp 190–195 °C).
He, S. J. J. Chem. Res., Synop. 2008, 354.
(
10) Jessop, P. G.; Trakhtenberg, S.; Warner, J. The Twelve
Principles of Green Chemistry, In Innovations in Industrial
and Engineering Chemistry, Vol. 1000; Flank, W. H.;
Abraham, M. A.; Mathews, M. A., Eds.; American Chemical
Society: Washington D.C., 2008, 401.
11) (a) Ohira, H.; Torii, N.; Aida, T. M.; Watanabe, M.; Smith,
R. L. Sep. Purif. Technol. 2009, 69, 102. (b) Zou, J. M.; Lü,
G. R.; Zhong, X. Q.; Wen, H. J. Chin. Med. Mater. 2005, 28,
106.
IR (KBr): 3386, 3002, 2937, 2836, 1579, 1509, 1458, 1428, 1411,
–
1
1
329, 1267, 1236, 1126, 997, 952, 856, 829, 792 cm .
1
H NMR (400 MHz, D O): d = 7.27 (s, 1 H, 2¢-ArH), 6.77 (d,
2
J = 8.4 Hz, 1 H, 6¢-ArH), 6.73 (d, J = 8.4 Hz, 1 H, 5¢-ArH), 6.55 (d,
J = 12.0 Hz, 1 H, CH=), 6.54 (s, 2 H, 2¢,6¢-ArH), 6.40 (d, J = 12.0
Hz, 1 H, CH=), 3.764 (s, 3 H, OCH ), 3.64 (s, 3 H, OCH ), 3.59 (s,
(
3
3
6
H, 2 × OCH3).
FAB-MS: m/z = 441 [M]+.
(
12) (a) Wu, G.; Guo, H. F.; Gao, K.; Liu, Y. N.; Bastow, K. F.;
Morris-Natschke, S. L.; Lee, K. H.; Xie, L. Bioorg. Med.
Chem. Lett. 2008, 18, 5272. (b) Erofeev, Y. V.; Afanas’eva,
V. L.; Glushkov, R. G. Pharm. Chem. J. 1990, 24, 501.
13) Centi, G.; Perathoner, S. Catal. Today 2003, 77, 287.
14) (a) Leston, G. In Kirk-Othmer Encyclopedia of Chemical
Technology, Vol. 19; Kroschwitz, J. I.; Howe-Grant, M.,
Eds.; Wiley: New York, 1996, 778. (b) Kambourakis, S.;
Frost, J. W. J. Org. Chem. 2000, 65, 6904.
Acknowledgment
We thank the Science and Technology Program of Guangdong Pro-
vince, Strategic Cooperation Program between Guangdong Pro-
vince, Chinese Academy of Sciences, and National Key
Technology R & D Program, P. R. China (2003B31603,
(
(
2
006B35604002, 2009B091300125 and 2007BAD82B02) for fi-
nancial support.
(
15) Zou, Y.; Du, J. L.; Chen, D. F. Chinese Patent CN
20081028683, 2008; Chem. Abstr. 2008, 150, 5385.
(
16) Zou, Y.; He, S. J.; Huang, T. K.; Liu, X. K.; Yang, S. F.; Liu,
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Synthesis 2011, No. 2, 217–222 © Thieme Stuttgart · New York