Synthesis and crystal structures of cis- and trans-1-(3′-N,N- dimethylthiocarbamoyl-4′-methoxy)-2-(3″, 4″, 5″-trimethoxyphenyl) ethene

Article abstract of DOI:10.1007/s10870-005-2961-6

The combretastatin A-4 analogue cis-1-(3′-N,N-dimethylthiocarbamoyl- 4′-methoxy)-2-(3″,4″,5″-trimethoxyphenyl) ethene (1) and its trans stereoisomer (2) were synthesized. The molecular structures of these compounds were obtained by single-crystal X-ray di

Full text of DOI:10.1007/s10870-005-2961-6

Journal of Chemical Crystallography, Vol. 35, No. 3, March 2005 (ꢀC 2005)
DOI: 10.1007/s10870-005-2961-6
Synthesis and crystal structures of cis- and trans-1-(3ꢀ-N,
N-dimethylthiocarbamoyl-4ꢀ-methoxy)-2-(3ꢀꢀ, 4ꢀꢀ, 5ꢀꢀ-
trimethoxyphenyl) ethene
Daniel A. Ramirez,(1) Jianxing Zhang,(1) Kevin Klausmeyer,(1) and Robert R. Kane(1)∗
Received August 22, 2004; accepted October 17, 2004
The combretastatin A-4 analogue cis-1-(3ꢁ-N,N-dimethylthiocarbamoyl-4ꢁ-methoxy)-2-
(3ꢁꢁ,4ꢁꢁ, 5ꢁꢁ-trimethoxyphenyl) ethene (1) and its trans stereoisomer (2) were synthesized. The
molecular structures of these compounds were obtained by single-crystal X-ray diffraction.
Crystallization of 1 occurs in the centrosymmetric monoclinic space group C2/c (No. 15)
with a = 21.468(5), b = 7.932(1), c = 23.949(3); and β = 100.75(1)◦ and Z = 8. Crys-
tallization of 2 occurs in the centrosymmetric monoclinic space group P21/n (No. 14) with
a = 11.7825(7), b = 11.562(1), c = 14.911(1) and β = 93.294(6); and Z = 4. Details of
the synthesis and the structural characterization of the title compounds are presented and
discussed.
KEY WORDS: combretastatin; stilbene; thiocarbamate.
(CA4) (3).3–5 While this compound is known to
be an extremely effective inhibitor of the assem-
bly of tubulin into microtubules,6 its potential as a
vascular targeting agent was only revealed after its
formulation into the water-soluble prodrug, dis-
odium phosphate Combretastatin A-4 (CA4DP)
(4).7 Although the prodrug 4 is inactive towards
tubulin, it is dephosphorylated in the neovascu-
lature of the tumor to reveal the parent tubulin
polymerization inhibiting drug 3. The resulting
disruption of the microtubule skeleton of the en-
dothelial cells in the neovasculature causes them
to lose their characteristic flat shape and become
bloated, thereby occluding the narrow capillaries.
The loss of blood flow to the tumor mass results
in extensive necrotic cell death.8
Introduction
The use of vascular targeting agents is a rel-
atively recent approach to the treatment of solid
tumors.1,2 This approach to cancer therapy takes
advantage of the observation that tumors that en-
large beyond a certain mass can no longer be
supported by one main blood vessel, and there-
forebecomevascularized. Disruptionofthesenew
tumor-associated capillaries (neovascalature) pre-
vents the blood flow necessary to feed the tumor,
resulting in tumor cell starvation, the build-up of
toxins, and massive necrotic cell death.
One of the best characterized small molecule
vascular targeting agents is Combretastatin A-4
(1) Department of Chemistry and Biochemistry and the Center for
Drug Discovery, Baylor University, Waco, Texas 76798-7348.
∗ To whom correspondence should be addressed at Department
of Chemistry and Biochemistry, Baylor University, Waco, Texas
76798; e-mail: Bob Kane@baylor.edu.
The amine analogue (5) of CA4 is also a very
active tubulin polymerization inhibitor.9 Formu-
lated as the serine amide 6 this compound also
shows significant promise as a vascular targeting
227
ꢀ
C
1074-1542/05/0300-0227/0 2005 Springer Science+Business Media, Inc.

228
Ramirez, Zhang, Klausmeyer, and Kane
agent for the treatment of cancer.10 As is true for
the CA4DP prodrug of CA4, compound 6 is inac-
tive until converted to the parent drug 5. However,
the activation chemistry for the two prodrugs is
verydifferent(dephosphorylationfor4and3com-
paredtodeacylationfor 6and 5). Thispromptedus
to investigate the thiol analogue (7) of compounds
3 and 5, and to study the formulation of this novel
compound into reductively activated prodrugs. In
the course of this work a crystalline derivatives of
compound 7 (its N,N-dimethylcarbamate 1) and
its trans stereoisomer (derivative 2) were isolated
and structurally characterized in order to unequiv-
ocally confirm their molecular structure and to
gather information for our molecular recognition
studies.
(85%) of 3-(N,N-dimethylthiocarbamoyloxy)-
4-methoxybenzaldehyde. H NMR (300 MHz,
1
CDCl3, δ relative to residual CHCl3 at 7.26 ppm):
δ 9.86 (1H, s; CHO), 7.77 (1H, dd, J = 8.5 and
2.0 Hz; ArH), 7.57 (1H, d, J = 2.0 Hz; ArH),
7.06 (1H, d, J = 8.5 Hz; ArH), 3.90 (3H, s;
OCH3), 3.45 (3H, s; N(CH3)), and 3.35 (3H, s;
N(CH3)).
3-(N,N-dimethylcarbamoylthio)-4-methoxy-
benzaldehyde. A solution of 3-(N,N-dimethyl-
carbamoylthio)-4-methoxybenzaldehyde (4.93
g, 0.0206 mol) in 200 mL of diphenyl ether was
heated to 250◦C under argon for 2 h. The reaction
mixture was cooled to room temperature and
added to 1 L of hexanes. The precipitate that
formed was collected by suction filtration, washed
with hexanes, and dried under vacuum. Yield
3.77 g (77%) of 3-(N,N-dimethylcarbamoylthio)-
4-methoxybenzaldehyde as a light brown solid.
1H NMR (300 MHz, CDCl3, δ relative to residual
CHCl3 at 7.26 ppm): δ 9.87 (1H, s; CHO), 7.99
(1H, d, J = 2.1 Hz; ArH), 7.94 (1H, dd, J = 2.1
and 8.5 Hz; ArH), 7.06 (1H, d, J = 8.5 Hz;
ArH), 3.95(3H, s; OCH3), 3.08 (6H, 2 broad s;
N(CH3)2).
3, 4, 5-Trimethoxybenzyl triphenylphospho-
nium bromide. To solution of CBr4 (22.9 g, 0.069
mol) in anhydrous acetone (170 mL) at 0◦C un-
der argon, 3,4,5-trimethoxybenzyl alcohol (10 g,
0.050 mol) and triphenylphosphine (17.99 g,
0.068 mol) were added. After the reaction was
stirred at room temperature overnight the solvent
was removed under reduced pressure to obtain
a crude sample of 3,4,5-trimethoxybenzyl bro-
mide as a brown gel. That crude product was im-
mediately dissolved in toluene (200 mL), triph-
enylphosphine (14.54 g, 0.055 mol) was added,
and the reaction was refluxed for 2 h. The reaction
was then allowed to mix overnight, resulting in
the formation of a sticky orange precipitate. The
solvent was removed under reduced pressure and
the residue dried under vacuum. The crude prod-
uct was recrystalized from ethanol, and the re-
sulting white crystals dried under vacuum. Yield
17.64 g (66.8%) of 3,4,5-trimethoxybenzyl triph-
enylphosphonium bromide.
Experimental
Synthesis
3-(N,N-dimethylthiocarbamoyloxy)-4-meth-
oxybenzaldehyde. A solution of N,N-dimethyl-
thiocarbamoyl chloride (4.07 g, 0.0329 mmol)
in THF was added dropwise at 0◦C to a so-
lution of 3-hydroxy-4-methoxybenzaldehyde
(5.03 g, 0.0329 mol) and potassium hydroxide
(1.84 g, 0.0329 mol) in water (22 mL). The
resulting white precipitate was filtered, washed
with water and dried by vacuum. Yield 6.72 g

1-(3ꢀ-N,N-dimethylthiocarbamoyl-4ꢀ-methoxy)-2-(3ꢀꢀ,4ꢀꢀ,5ꢀꢀ-trimethoxyphenyl) ethene
229
(E) and (Z)-1-(3ꢁ-N, N-dimethylthiocar-
bamoyl-4ꢁ-methoxy)-2-(3ꢁꢁ,4ꢁꢁ,5ꢁꢁ-trimethoxyphen-
yl) ethene. 3,4,5-trimethoxybenzyl triphenyl-
phosphonium bromide (4.37 g, 8.35 mol) was
added to approximately 170 mL of anhydrous
THF under argon and stirred vigorously at −20◦C.
A 2.0 M solution of n-BuLi in cyclohexane (0.535
g, 4.18 mL 8.36 mmol) was then added dropwise
to the solution over a period of 15 minutes.
The resulting blood-red solution was allowed to
warm to RT and 3-(N,N-dimethylcarbamoylthio)-
4-methoxybenzaldehyde (1 g, 4.18 mmol) in
30 mL of anhydrous THF was added dropwise
over 20 minutes, during which time the solution
became dark orange in color. The reaction was
followed by monitoring the disappearance of the
aldehyde by thin layer chromatography (40%
EtOAc/60% hexanes, approx. 1.5 h). When the
reaction reached completion it was quenched by
the careful addition of 210 mL ice-cold water
and the organic products extracted (4 extractions
using 250 mL of diethyl ether). The ether extracts
were then washed with 200 mL of ice-cold water
and dried over sodium sulfate. Removal of the
solvent in vacuo resulted in the isolation of a yel-
OCH3), 3.70 (6H, s; OCH3), 3.03 (6H, broad s;
N(CH3)2).
13C NMR Z-isomer (75 MHz, CDCl3, δ
relative to CHCl3 at 77 ppm): δ 14.21, 21.07,
36.96, 56.00, 56.23, 60.41, 60.91, 105.83, 111.11,
116.56, 128.60, 129.21, 129.98, 132.31, 132.65,
137.12, 138.53, 152.99, 159.32, and 165.93.
1H NMR E-isomer (300 MHz, CDCl3, δ rel-
ative to residual CHCl3 at 7.26 ppm): δ 7.64 (1H,
d, J = 2.3 Hz; ArH), 7.51 (1H, dd, J = 8.6 and
2.3 Hz; ArH), 6.94 (1H, d, J = 8.6 Hz; ArH),
6.93 (1H, d, J = 16.1 Hz; vinylic H), 6.89 (1H,
d, J = 16.1Hz;vinylicH), 6.69(2H, s;ArH), 3.90
(6H, s; OCH3), 3.89 (3H, s; OCH3), 3.85 (3H, s;
OCH3), 3.11 (6H, broad s, N(CH3)2).
13C NMR E-isomer (75 MHz, CDCl3, δ rel-
ative to CHCl3 at 77 ppm): δ 37.06, 56.13, 56.31,
60.99, 103.41, 111.66, 117.26, 127.02, 127.36,
129.50, 130.52, 133.30, 136.05, 137.77, 153.40,
and 159.59.
X-ray. A summary of crystal data is pre-
sented in Table 1. Diffracted intensities were col-
lected on an Enraf-Nonius CAD4 diffractome-
ter using graphite-monochromated Mo Kα- X-
radiation. Final unit cell dimensions were deter-
mined from the setting angles of 25 reflections for
1 and 2. The data were corrected for Lorentz and
polarization effects.
Both structures were solved by direct meth-
ods which gave the positions of all non-hydrogen
atoms using SHELXTL ver. 6.12.11 Refinements
were made by full-matrix least squares on all F2
data using SHELXL. Anisotropic thermal param-
eters were included for all non-hydrogen atoms.
All hydrogen atoms were included in calculated
positions and allowed to ride on their parent car-
bon atom with fixed isotropic thermal parameters
[Uiso(H) = 1.2Uiso(parent)].
1
low gel. H NMR analysis of this crude product
revealed the presence of a 2:1 mixture of the Z
and E isomers of the product. The crude mixture
was subjected to flash chromatography (Biotage
Flash40S system, 40 g silica gel, 40% EtOAc/60%
hexanes), resulting in the isolation of white and
yellow crystalline solids, corresponding to E and
Z isomers respectively, which were recrystalized
from methanol. Yields were 0.71 g (42%) of Z-
1-(3ꢁ-N,N-dimethylthiocarbamoyl-4ꢁ-methoxy)-
2-(3ꢁꢁ,4ꢁꢁ,5ꢁꢁ-trimethoxyphenyl) ethene and 0.34
g (21%) of E-1-(3ꢁ-N,N-dimethylthiocarbamoyl-
4ꢁ-methoxy)-2-(3ꢁꢁ,4ꢁꢁ,5ꢁꢁ-trimethoxyphenyl) eth-
ene.
1H NMR Z-isomer (300 MHz, CDCl3, δ
relative to residual CHCl3 at 7.26 ppm): δ 7.44
(1H, d, J = 2.3 Hz; ArH), 7.31 (1H, dd, J =
8.6 and 2.3 Hz; ArH), 6.82 (1H, d, J = 8.6
Hz; ArH), 6.51 (2H, s; ArH), 6.45 (1H, d, 1H,
J = 12.3 Hz; vinylic H), 6.40 (1H, d, J = 12.3
Hz; vinylic H), 3.84 (3H, s; OCH3), 3.82 (3H, s;
Results and discussion
The synthesis of the two title com-
pounds was relatively straightforward. A ther-
mal Newmann-Kwart rearrangement12 efficiently
transformed isovanillin into the desired pro-
tected thiol. Wittig reaction of this aldehyde

230
Ramirez, Zhang, Klausmeyer, and Kane
Table 1. Crystal Data and Structure Refinement
1
2
CCDC deposit no.
Color/Shape
247276
247137
Colorless/prism
0.31 × 0.20 × 0.13
C21H25NO5S
403.48
298(2)
Monoclinic
C2/c
Colorless/prism
0.59 × 0.52 × 0.34
C21H25NO5S
403.48
298(2)
Monoclinic
P21/n
Crystal size (mm)
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space Group
˚
a (A)
21.468(5)
7.932(1)
23.949(3)
100.75(1)
4006(1)
11.7825(7)
11.562(1)
14.911(1)
93.294(6)
2027.8(3)
4
˚
b (A)
˚
c (A)
β (◦)
3
˚
V (A )
Z
8
Dc (Mg M−3
µ (Mo Kα) (mm−1
)
1.338
0.194
1.322
0.192
)
Diffractometer/scan
ꢀθ for data collection (◦)
Reflections measured
h, k, l limits
Enraf-Nonius CAD4/ω
1.73–24.98
3885
−7 ≤ h ≤ 25, 0 ≤ k ≤ 9, −28 ≤ l ≤ 27
3520 (Rint = 0.0123)
2491 [I > 2σ(I)]
3520/0/254
Enraf-Nonius CAD4/ω
2.14–24.93
4028
−13 ≤ h ≤ 13, −2 ≤ k ≤ 13, −2 ≤ l ≤17
3527 (Rint = 0.0081)
2105 [I > 2σ(I)]
3527/0/253
1.011
R1 = 0.0558, wR2 = 0.11127
R1 = 0.1002, wR2 = 0.1281
0.194
Unique Reflections
Observed reflections
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
1.028
R1 = 0.0480, wR2 = 0.1044
R1 = 0.0744, wR2 = 0.1134
0.307
−3
˚
Largest diff. peak (eA
)
with 3,4,5-trimethoxybenzyl triphenylphospho-
nium bromide afforded a 2:1 mixture of the Z and
E stilbene isomers which could be readily sepa-
rated by column chromatography.
˚
˚
1.480(4) A and for 2 it is 1.464(4) A. The longer
bond lengths for the cis isomer are likely due to
Thermal ellipsoid plots of compounds 1 and
2 are shown in Figs. 1 and 2 respectively and
selected geometric parameters are presented in
Table 2. The geometric parameters of the two
structures match up very closely, for example the
˚
S C(ring) bond length are identical at 1.771(3) A,
and the C O(carbonyl) bond lengths are nearly iden-
˚
tical at 1.213(3) A for 1 and 1.212(3) for 2.
The only bond lengths that show a significant
deviation are those associated with the alkene dou-
ble bond, which is 1.332(4) for 1 and 1.313(4) for
2. Smaller differences are noticed in the C(alkene)
–
C
˚
(ring) bond lengths with C6 C11 being 1.476(4)
˚
A for 1 and the same bond being 1.466(6) A for
2. Similarly the C12 C13 bond length for 1 is
Fig. 1. A view of the crystallographic orientation of 1.
Thermal ellipsoids are drawn at the 50% probability level.

1-(3ꢀ-N,N-dimethylthiocarbamoyl-4ꢀ-methoxy)-2-(3ꢀꢀ,4ꢀꢀ,5ꢀꢀ-trimethoxyphenyl) ethene
231
Fig. 2. A view of the crystallographic orientation of 2. Thermal ellipsoids are drawn
at the 50% probability level.
◦
˚
Table 2. Selected Geometric Parameters for 1 and 2 in (A) and ( )
Selected Geometric Parameters for 1
S(1) C(4)
O(1) C(1)
N(1) C(3)
C(6) C(11)
1.771(3)
1.213(3)
1.459(3)
1.476(4)
1.480(4)
S(1) C(1)
N(1) C(1)
N(1) C(2)
C(11) C(12)
1.816(3)
1.347(3)
1.462(3)
1.332(4)
C(12) C(13)
C(4) S(1) C(1)
C(1) N(1) C(3)
C(3) N(1) C(2)
O(1) C(1) S(1)
C(11) C(12) C(13)
C7 C6 C11 C12
C18 C13 C12 C11
100.7(1)
124.1(2)
117.0(2)
121.6(2)
126.5(2)
152.34(0.27)
122.78(0.29)
C(9) O(2) C(10)
C(1) N(1) C(2)
O(1) C(1) N(1)
N(1) C(1) S(1)
C(12) C(11) C(6)
C6 C11 C12 C13
C12 C11 C6 C5
117.0(2)
118.7(2)
125.1(3)
113.3(2)
128.7(2)
−9.80(0.45)
−35.13(0.41)
◦
˚
Selected Geometric Parameters for 2 in (A) and ( )
S(1) C(4)
O(1) C(1)
N(1) C(3)
C(6) C(11)
1.771(3)
1.212(3)
1.448(4)
1.466(4)
S(1) C(1)
N(1) C(1)
N(1) C(2)
C(11) C(12)
1.793(3)
1.346(4)
1.457(4)
1.313(4)
C(12) C(13)
1.464(4)
C(4) S(1) C(1)
C(1) N(1) C(3)
C(3) N(1) C(2)
O(1) C(1) S(1)
C(11) C(12) C(13)
C7 C6 C11 C12
C6 C11 C12 C13
99.84(14)
124.7(3)
116.8(3)
122.0(2)
129.1(3)
C(9) O(2) C(10)
C(1) N(1) C(2)
O(1) C(1) N(1)
N(1) C(1) S(1)
C(12) C(11) C(6)
C12 C11 C6 C5
C11 C12 C13 C18
117.7(2)
118.4(3)
123.5(3)
114.5(2)
125.9(3)
2.27(0.50)
5.51(0.54)
−177.78(0.31)
−176.84(0.30)

232
Ramirez, Zhang, Klausmeyer, and Kane
These data can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html [or from the Cambridge Crystallographic
Data Centre (CCDC), 12 Union Road, Cambridge CB2
1EZ,UK; fax +44(0)1223–336033; e-mail: deposit@ccdc.ccam.
ac.uk].
steric repulsions of the rings that are now on the
same side of the double bond. These steric forces
also likely cause the major difference between the
two structures which is the ring tilt about the dou-
ble bond. In the structure of 1, which consists of
a cis arrangement of the rings across the double
bond, the planes of the two rings are 71.2◦ to
each other, while for 2, the trans structure they
are much closer to coplanar at only 6.7◦. The tor-
sion angle reflects this twist as well, for 1 the angle
C6 C11 C12 C13 is 9.8 (5)◦ and in 2 the corre-
sponding angle (C6 C11 C12 C13) is 3.2 (3)◦.
The twist is more clearly seen by looking at the
torsion angles C(alkene)–C(alkene)–C(ring)–C(ring), for
1 these angles range about from 27◦ to 57◦ from
coplanar. The angles are much more acute for 2
with a range from about 2.2◦ to 5.5◦ from coplanar.
References
1. Burrows, F.J.; Thorpe, P.E. Pharmacol. Ther. 1994, 64(1), 155–
174.
2. Siemann, D.W.; Chaplin, D.J.; Horsman, M.R. Cancer 2004,
100(12), 2491–2499.
3. Pettit, G.R.; Cragg, G.M.; Herald, D.L.; Schmidt,
J.M.; Lohavanijaya, P. Can. J. Chem. 1982, 60, 1374–
1376.
4. Pettit, G.R.; Cragg, G.M.; Singh, S.B. J. Nat. Prod. 1987, 50,
386–391.
5. Pettit, G.R.; Lippert, J.W., III; Herald, D.C. J. Org. Chem. 2000,
65, 7438–7444.
6. Hamel, E.; Lin, C.M. Biochem. Pharmacol. 1983, 32, 3864–
3867.
7. Pettit, G.R.; Rhodes, M.R. Anticancer Drug Des. 1998, 13, 183–
191.
8. Molema, G. Pharm. Res. 2002, 19(9), 1251–
Acknowledgments
1258.
9. Ohsumi, K.;Nakagawa, R.;Fukuda, Y.;Hatanaka, T.;Morinaga,
Y.; Nihei, Y.; Ohishi, K.; Suga, Y.; Akiyama, Y.; Tsuji, T. J. Med.
Chem. 1998, 41(16), 3022–3032.
10. Hori, K.; Saito, S.; Nihei, Y.; Suzuki, M.; Sato Y. Jpn. J. Cancer
Res. 1999, 90, 1026–1038.
RRK and KK thank the Robert A. Welch
Foundation for support (Grants #AA-1355 and
AA-1508, respectively).
11. Sheldrick, G.M. SHELXTL, version 6.12; Bruker AXS, Inc.:
Madison, WI, 2001.
Supplementary material CCDC-247276 and CCDC-247137
contain the supplementary crystallographic data for this paper.
12. Newman, M.S.;Karnes, H.A. J. Org. Chem. 1966, 31(12), 3980–
3984.