A novel palladium-mediated coupling approach to 2,3-disubstituted benzo[b]thiophenes and its application to the synthesis of tubulin binding agents

Article abstract of DOI:10.1021/ol0067179

Flexible, convergent access to 2,3-disubstituted benzo[b]thiophenes has been developed. The most concise approach involves sequential coupling of o-bromoiodobenzenes with benzylmercaptan and zinc acetylides to give benzyl o-ethynylpnenyl sulfides which re

Full text of DOI:10.1021/ol0067179

ORGANIC
LETTERS
2001
Vol. 3, No. 5
651-654
A Novel Palladium-Mediated Coupling
Approach to 2,3-Disubstituted
Benzo[b]thiophenes and Its Application
to the Synthesis of Tubulin Binding
Agents
,†
Bernard L. Flynn,* Pascal Verdier-Pinard,‡ and Ernest Hamel‡
Department of Chemistry, The Faculties, Australian National UniVersity,
Canberra, ACT, 0200, Australia, and Screening Technologies Branch,
DeVelopmental Therapeutics Program, DiVision of Cancer Treatment and Diagnosis,
National Cancer Institute, Frederick Cancer Research and DeVelopment Center,
Frederick, Maryland 21702
flynn@rsc.anu.edu.au
Received October 10, 2000
ABSTRACT
Flexible, convergent access to 2,3-disubstituted benzo[b]thiophenes has been developed. The most concise approach involves sequential
coupling of o-bromoiodobenzenes with benzylmercaptan and zinc acetylides to give benzyl o-ethynylphenyl sulfides which react with iodine
to give 3-iodobenzo[b]thiophenes in a 5-endo-dig iodocyclization. These iodides can be further elaborated using palladium-mediated coupling
and/or metalation techniques. This method has been applied to the synthesis of some novel tubulin binding agents.
Benzo[b]thiophenes serve as very useful heterocyclic cores
to a host of drugs and drug candidates as well as providing
useful properties in the development of new and interesting
materials.1 Benzo[b]thiophene derivatives currently in phar-
maceutical use or development include estrogen receptor
antagonists,2 antimitotic agents,3 modulators of multidrug
resistance,4 angiogenesis inhibitors,5 cognition enhancers,6
and antifungal7 and antiinflammatory8 agents to name but a
few. A specific example is the tubulin binding agent 1, an
analogue of the natural product combretastatin A-4 (2)
(Figure 1).3a,b The emergence of such valuable chemothera-
(3) (a) Pinney, K. G.; Bounds, A. D.; Dingeman, K. M.; Mocharla, V.
P.; Pettit, G. R.; Bai, R.; Hamel, E. Bioorg. Med. Chem. Lett. 1999, 9,
1081. (b) Pinney, K. G.; Pettit, G. R.; Mocharla, V. P.; Del, P. M. M.;
Shirali, A. PCT Int. Appl. WO 98 39 323 (Chem. Abstr. 1998, 129,
245037c). (c) Zhang, S.-X.; Bastow, K. F.; Tachibana, Y.; Kuo, S.-C.;
Hamel, E.; Mauger, A.; Narayanan, V. L.; Lee, K.-H. J. Med. Chem. 1999,
42, 4081.
(4) Norman, B. H.; Dantzig, A. H.; Kroin, J. S.; Law, K. L.; Tabas, L.
B.; Shepard, R. L.; Palkowitz, A. D.; Hauser, K. L.; Winter, M. A.; Sluka,
J. P.; Starling, J. J. Bioorg. Med. Chem. Lett. 1999, 9, 3381.
(5) (a) Boschelli, D. H.; Kramer, J. B.; Connor, D. T.; Lesch, M. E.;
Schrier, D. J.; Ferin, M. A.; Wright, C. D. J. Med. Chem. 1994, 37, 717.
(b) Cobb, R. R.; Felts, K. A.; McKenzie, T. C.; Parry, G. C. N.; Mackman,
N. FEBS Lett. 1996, 382, 323. (c) Gualberto, A.; Marquez, G.; Garballo,
M.; Youngblood, G. L.; Hunt, S. W., III; Baldwin, A. S.; Sobrino, F. J.
Biol. Chem. 1998, 273, 7088.
† Department of Chemistry, Australian National University.
‡ Screening Technologies Branch, NCI.
(1) For reviews on the synthesis and application of benzo[b]thiophenes,
see: (a) Zhang, T. Y.; O’Toole, J.; Proctor, C. S. Sulfur Rep. 1999, 22, 1.
(b) Pelkey, E. T. Prog. Heterocycl. Chem. 1999, 11, 102. (c) Bianchini,
C.; Meli, A. Synlett 1997, 643. (d) Russell, R. K.; Press, J. B. Compr.
Heterocycl. Chem. II 1996, 2, 679. (e) Irie, M.; Uchida, K. Bull. Chem.
Soc. Jpn. 1998, 71, 985.
(2) (a) Magarian, R. A.; Overacre, L. B.; Singh, S.; Meyer, K. L. Curr.
Med. Chem. 1994, 1, 61. (b) Jones, C. D.; Jevnikar, M. G.; Pike, A. J.;
Peters, M. K.; Black, L. J.; Thompson, A. R.; Falcone, J. F.; Clemens, J.
A. J. Med. Chem. 1984, 27, 1057. (c) Bryant, H. U.; Dere, W. H. Proc.
Soc. Exp. Biol. Med. 1998, 217, 45.
(6) (a) Martel, A. M.; Prous, J.; Castaner-Prous, J. Drugs Future 1997,
22, 386. (b) Miyazaki, H.; Murayama, T.; Ono, S.; Narita, H.; Nomura, Y.
Biochem. Pharmacol. 1997, 53, 1263.
10.1021/ol0067179 CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/06/2001

Scheme 1a
Figure 1.
peutic agents has stimulated new investigations into the
concise synthesis of 2,3-disubstituted benzo[b]thiophenes.1,9
Here we report a highly convergent protocol for rapid
construction of 2,3-disubstituted benzo[b]thiophenes. The
approach involves a combination of palladium-mediated
coupling and iodocyclization reactions. The scope of this new
method has been explored in the context of a brief structure-
activity relationship study of analogues of 1. Compounds
with enhanced affinity for tubulin have been discovered.
The synthesis of 1 and analogues 11, 14, and 16 com-
menced with readily available 2-iodo-5-methoxyaniline (3)
(Scheme 1).10 A sequence involving diazotation, xanthate
substitution, methanolysis, and benzylation converted 3 into
the benzyl sulfide 4 in an overall 55% yield.11,12 This
multistep conversion of 3 to 4 is amenable to large scale
preparations and requires chromatography only after the last
step. Iodide 4 was coupled to ethynyl zinc species 6 (obtained
directly from â,â-dibromostyrene 5 by addition of 2 equiv
of n-BuLi and zinc chloride), giving 7 in an excellent yield
(95%).13,14 Reaction of 7 with iodine led to a rapid 5-endo-
dig iodocyclization to give 3-iodobenzo[b]thiophene 8 in an
almost quantitative yield (98%).15,16 Lithiation of 8 and
reaction with commercially available 3,4,5-trimethoxyben-
zoyl chloride 9 afforded 1 in high yield (96%). Negishi
coupling of 8 with arylzinc chloride 10 gave the non-carbonyl
a Reagents and conditions: i. HBF4, NaNO2, H2O; ii. KSC-
(C)OEt, DMF; iii. MeOH, KOH; iv. KOH(aq), BnCl, n-Bu4NHSO4
cat., CH2Cl2; v. 2 × n-BuLi, THF, then ZnCl2, Pd(PPh3)2Cl2 2 mol
%, 4; vi. I2, CH2Cl2; vii. 2 × t-BuLi, THF, 9; viii. 10 (from 3,4,5-
trimethoxyiodobenzene, 2 × t-BuLi, THF and ZnCl2), Pd(PPh3)2Cl2
2 mol %, ix. AlCl3 3 equiv, CH2Cl2.
containing analogue 11 (91%). This synthetic approach to 1
and 11 was repeated using the different â,â-dibromostyrene
1213a to afford 13 and 15. The isopropyl ethers in 13 and
1512 were selectively cleaved using aluminum trichloride to
provide 14 and 16, respectively.12,17
(7) (a) Thesen, R. Pharm. Ztg. 1995, 140, 44. (b) Raga, M.; Palacin, C.;
Castello, J. M.; Ortiz, J. A.; Cuberes, M. R.; Moreno-Manas, M. Eur. J.
Med. Chem.-Chim. Ther. 1986, 21, 329.
The bromo equivalent of iodide 4, benzyl 5-bromo-3-
methoxyphenyl sulfide (19), proved even more accessible
(Scheme 2). Regioselective bromination of commercially
available 3-iodoanisole (17) with NBS to give 18 is followed
by chemoselective substitution of the iodide in 18 with
benzylmercaptan under palladium catalysis to give 19 (96%
from 17).18,19 Although less reactive than the corresponding
aryl iodide 4, the bromide 19 could still be efficiently coupled
with zinc acetylides (derived from â,â-dibromostyrenes)
using a modification of the procedure described above for
the coupling of 4 and 5 to give 7. After conversion of 20 to
the corresponding zinc acetylide (not shown), bromide 19,
Pd(PPh3)2Cl2, and triphenylphosphine were added. The
(8) (a) Bleavins, M. R.; de la Igelsia, F. A.; McCay, J. A.; White, L.;
Kimber, L., Jr.; Munson, A. E. Toxicology 1995, 98, 111. (b) Wright, C.
D.; Stewart, S. F.; Kuipers, P. J.; Hoffman, M. D.; Devall, L. J.; Kennedy,
J. A.; Ferin, M. A.; Theuson, D. O.; Conroy, M. C. J. Leukocyte Biol. 1994,
55, 443.
(9) (a) Bradley, D. A.; Godfrey, A. G.; Schmid, C. R. Tetrahedron Lett.
1999, 40, 5155. (b) Gallagher, T.; Pardoe, D. A.; Porter, R. A. Tetrahedron
Lett. 2000, 41, 5415. (c) Arnau, N.; Moreno-Manas, M.; Pleixats, R.
Tetrahedron 1993, 49, 11019. (d) McDonald, F. E.; Burova, S. A.; Huffman,
L. G., Jr. Synthesis 2000, 970.
(10) Sakamoto, T.; Kondo, Y.; Uchiyama, M.; Yamanaka, H. J. Chem.
Soc., Perkin Trans. 1 1993, 1941.
(11) Xiao, W.-J.; Alper, H. J. Org. Chem. 1999, 64, 9646.
(12) See also Supporting Information.
(13) (a) Banwell, M. G.; Flynn, B. L.; Willis, A. C.; Hamel, E. Aust. J.
Chem. 1999, 52, 767. (b) Banwell, M. G.; Flynn, B. L.; Hockless D. C. R.
Chem. Commun. 1997, 2259.
(14) Salaun, J. J. Org. Chem. 1977, 42, 28.
(15) For some other examples of iodocyclization involving alkynyl benzyl
sulfides, see: (a) Ren, X.-F.; Turos, E. Tetrahedron Lett. 1993, 34, 1575.
(b) Ren, X.-F.; Turos, E.; Lake, C. H.; Churchill, M. R. J. Org. Chem.
1995, 60, 6468.
(16) For related iodocyclizations of o-ethynylphenols to give 3-iodo-
benzo[b]furans, see ref 13a and the following: Arcadi, A.; Cacchi, S.;
Giancarlo, F.; Marinelli, F.; Moro, L. Synlett 1999, 1432.
(17) Banwell, M. G.; Flynn, B. L.; Stewart, S. G. J. Org. Chem. 1998,
63, 9139.
(18) Carreno, M. C.; Garcia Ruano, J. L.; Sanz, G.; Toledo, M. A.;
Urbano, A. J. Org. Chem. 1995, 60, 5328.
(19) Ciattini, P. G.; Morera, E.; Ortar, G. Tetrahedron Lett. 1995, 36,
4133.
652
Org. Lett., Vol. 3, No. 5, 2001

Scheme 2a
Scheme 3a
a Reagents and conditions: i. 4-methoxyethynylbenzene, Pd(PPh3)2-
Cl2 1.5 mol %, CuI 3 mol %, DMF, Et3N; ii. n-BuLi, THF,
BnSSBn; iii. I2, CH2Cl2; v. n-BuLi, THF, 9.
o-bromoiodobenzenes. This has been demonstrated in the
synthesis of 33 from 18 (Scheme 3). Coupling 18 with
4-methoxyphenylacetylene under Sonogashira conditions
gave 30. This product was lithiated and reacted with dibenzyl
disulfide to give 31 and iodocyclized to give 32.20 Lithiation
of 32 and reaction with 3,4,5-trimethoxybenzoyl chloride (9)
provided 33 in excellent yield (99%). Note that this process
has resulted in the methoxy group being located at the C-5
postion rather than at C-6, as was the case in the preparation
of 24 from the same o-bromoiodobenzene 18 (Scheme 2).
a Reagents and conditions: i. NBS, DMF, 80 °C, 4 h; ii. Pd(dba)2
3 mol %, dppf 3 mol %, BnSH, DMF, Et3N, 70 °C, 3 h; iii. 20, 2
× n-BuLi, THF, then ZnCl2, Pd(PPh3)2Cl2 2 mol %, PPh3 4 mol %
17, DMF, 100 °C, 3 h; iv. I2, CH2Cl2; v. n-BuLi, THF, 22, then
KOH in MeOH; vi. DDQ, CH2Cl2; vii. 12, 2 × n-BuLi, THF, then
ZnCl2, Pd(PPh3)2Cl2 2 mol %, PPh3 4 mol %, 26, DMF, 100 °C, 3
h; viii. i-PrMgCl, THF, 9; ix. AlCl3, 3 equiv, CH2Cl2.
Biological Studies. Tubulin binding agents are of interest
in view of their potential to act as both antimitotics and as
selective inhibitors of tumor vasculature growth.21,22 Com-
bretastatin A-4 (2) is an example of such a tubulin binding
agent which is currently undergoing clinical trials as an
anticancer agent. The poor solubility of 2 in suitable solvents,
as well as its tendency to isomerize to its inactive E-isomer,
has prompted a number of studies directed at the preparation
of more soluble, configurationally stable analogues.23 Pinney
and co-workers have developed the benzo[b]thiophene
analogue 1 as a ring fused, configurationally stable analogue
of 2 (Figure 1).3a,b
resultant solution was diluted with an equal volume of DMF
and heated to 100 °C under a slight flow of N2 to remove
THF. Under these conditions coupling proceeded smoothly
and the crude product was iodocyclized to afford directly
the 3-iodobenzo[b]thiophene 21 in a 78% overall yield from
19. Lithiation of 21 and reaction with the acetylisovanilin
22 and in situ methanolysis of the acetate gave 23 (74%).
Alcohol 23 was readily oxidized to ketone 24 using DDQ
(98%).
The dioxolane-fused benzo[b]thiophene derivative 29 was
prepared from commercially available 4-bromo-5-iodo-1,3-
benzodioxolane (25) (Scheme 2). Chemistry similar to that
described in the synthesis of 23 was employed. The 3-iodo-
benzo[b]thiophene 27 was prepared from a sequence of
iodide substitution to afford 26 and coupling to the zinc
acetylide derived from 12 and iodocyclization. Metalation
of 27 and reaction with 9 gave 28, which gave 29 upon
deprotection.
We have used our novel approach to benzo[b]thiophenes
to prepare some analogues of 1, including some which more
closely resemble the cis-stilbene pharmacophore of combre-
tastatin A-4 (2), compounds 11, 13-16, 23, 24, 29, and 33.24
These compounds were first evaluated for inhibition of
(20) Attempts to introduce the benzylthiol by palladium-mediated
substitution of the aryl bromide in 30 with benzyl mercaptan have so far
failed.
(21) For a review, see: Sackett, D. L. Pharmacol. Ther. 1993, 59, 163.
(22) Dark, G. G.; Hill, S. A.; Prise, V. E.; Tozer, G. M.; Pettit, G. R.;
Chaplin, D. J. Cancer Res. 1997, 57, 1829.
(23) See ref 13a and references cited therein.
(24) During the course of these studies, Pinney and co-workers also
prepared compound 14: Pinney, K. G.; Chen, Z.; Mocharla, V. P.; Choony,
N.; Strong, T. Synthesis of a Benzo[b]thiophene-Based Vascular Targeting
Prodrug and Related Anti-Tubulin Ligands. Abstracts of Papers, 220th
National Meeting of the American Chemical Society, Division of Organic
Chemistry; Washington, D.C., August 20-24, 2000; American Chemical
Society: Washington, D.C., Abstract No. 196.
The relative placement of substituents in the benzene ring
of benzo[b]thiophenes can be varied by changing the order
of introduction of the alkyne and the benzyl sulfide in certain
Org. Lett., Vol. 3, No. 5, 2001
653

Table 1. Effects of Benzo[b]thiophenes on Tubulin Polymerization, Colchicine Binding, and Growth of Burkitt Lymphoma CA46
Cells
inhibition of tubulin
polymerizationa
inhibition of colchicine binding (% inhibition)b
inhibition of cell growth
IC50 (nM)
compd
IC50 (µM)
5 µM inhibitor
50 µM inhibitor
1c
2
14
23
29
>40*d
28
2000
1
500
>1000
>1000
2.1 ( 0.1
3.5 ( 0.3
6.1 ( 0.8
>40*d
94
6
5
61
73
31
2
a The tubulin concentration was 10 µM. Inhibition of extent of assembly was the parameter measured. b The tubulin concentration was 1.0 µM and the
[3H]colchicine concentration was 5.0 µM. cData from ref 3a. d The asterisk indicates that the rate but not the extent of assembly was inhibited by compound
concentrations as high as 40 µM.
tubulin assembly, and those that displayed inhibitory effects
were also examined for inhibitory effects on the binding of
[3H]colchicine to tubulin and for cytotoxicity in human
Burkitt lymphoma CA46 cells (Table 1; methodologies as
described in ref 3a). The newly synthesized compounds were
compared in contemporaneous experiments to the potent
antimitotic combretastatin A-4 (2), generously provided by
Prof. G. R. Pettit, Arizona State University.
CA46 cells. Compound 14, however, was far less active than
combretastatin A-4 (2) in these latter assays.
In SAR terms, vicinal hydroxyl and methoxyl groups in
the C ring (as in 2) enhance the activity and perhaps the
solubility of this class of compounds (14 vs 1). The bridged
carbon between the B and D rings is essential (cf. 11 and 1;
14 and 16). The relative positioning of the C and D rings on
the 3-carboxybenzo[b]thiophene skeleton is important (cf.
24 and 14). Fusion of a 1,3-dioxolane ring to the A ring is
undesirable (29 vs 14), as is moving the methoxyl group
from position C-6 to C-5 (33 vs 1).
Conclusion. A flexible method for the construction of
benzo[b]thiophenes has been developed. The protocol gives
direct access to a range of substitution patterns making it an
ideal process for structure-activity relationship studies of
biologically active molecules. This point has been demon-
strated in the structural refinement of a known lead com-
pound 1, giving more active compounds 14 and 23.
Compounds 11, 13, 15, 16, 24, and 33 showed no effect
on tubulin assembly at concentrations as high as 40 µM and
were not further studied and are not included in Table 1.
Compound 1 had shown3a an unusual inhibitory effect on
tubulin polymerization in that low concentrations (up to about
3-4 µM) inhibited the rate of assembly. Little further effect
on either rate or extent of assembly occurred at concentra-
tions as high as 40 µM. It was postulated that this
phenomenon was due to limited aqueous solubility of 1.
Compound 29 showed a similar inhibitory pattern on
assembly, but in direct comparisons to 1 at low concentra-
tions, 29, was clearly less active than 1, and therefore rate
effects were not examined in detail. Two compounds, 14
and 23, displayed typical progressive inhibition of all
parameters of assembly (initiation, assembly rate, assembly
extent), and quantitative measurements of the assembly extent
were performed relative to 2 (IC50, 2.1 µM). Compound 14
was about half as active (IC50, 3.5 µM) and 23 about one-
third as active (IC50, 6.1 µM) as 2. In addition, compound
14 (but not 23) was distinctly more active than 1 as both an
inhibitor of colchicine binding to tubulin and of growth of
Acknowledgment. The authors thank the Australian
Research Council for financial support including an Aus-
tralian Research Fellowship to B.L.F.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for compounds involved in
the synthetic sequence leading to 14 and 16 from 3. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL0067179
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Org. Lett., Vol. 3, No. 5, 2001