The synthesis of indanones related to combretastatin A-4 via microwave-assisted Nazarov cyclization of chalcones

Article abstract of DOI:10.1016/j.tetlet.2005.12.110

A fast and efficient microwave-assisted synthesis of combretastatin A-4-like indanones has been developed. Microwave irradiation provides a useful alternative to traditional heating techniques to promote the TFA-catalyzed Nazarov cyclization of chalcones.

Full text of DOI:10.1016/j.tetlet.2005.12.110

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 1637–1640
The synthesis of indanones related to combretastatin A-4
via microwave-assisted Nazarov cyclization of chalcones
Nicholas J. Lawrence,a,* E. Simon M. Armitage,a,b Benjamin Greedy,a,b Darren Cook,c
Sylvie Duckic and Alan T. McGownc
aDrug Discovery Program, H. Lee Moffitt Cancer Center and Research Institute, Department of Interdisciplinary Oncology,
University of South Florida, Tampa, FL 33612-9497, USA
bSchool of Chemistry, Cardiff University, PO Box 912, Cardiff CF10 3TB, UK
cCentre for Molecular Drug Design, Cockcroft Building, University of Salford, Salford M5 4WT, UK
Received 5 November 2005; revised 13 December 2005; accepted 21 December 2005
Abstract—A fast and efficient microwave-assisted synthesis of combretastatin A-4-like indanones has been developed. Microwave
irradiation provides a useful alternative to traditional heating techniques to promote the TFA-catalyzed Nazarov cyclization of
chalcones.
ꢀ 2006 Elsevier Ltd. All rights reserved.
As part of a program investigating the tubulin binding
properties of combretastatin A-4 11 (CA-4) and related
analogs,2 we had need for a rapid method for the prep-
aration of indanones. We have found that the a-methyl
chalcone 2 has biological activities comparable, and in
some cases superior, to those of CA-4 1 and colchicine
3 (Fig. 1).3 These compounds bind strongly to a,b-tubu-
lin, causing microtubules to depolymerize, which in turn
causes arrest at the G2/M stage of the cell cycle.4 The
compounds cause selective damage to tumor vascula-
ture, an effect that is related to their tubulin binding
properties.5,6 Compounds such as these that target
tumor vasculature clearly have significant clinical prom-
ise for the treatment of cancer.7 Our research has shown
that the s-trans conformation of the chalcone 2 has pro-
ven to be important to its biological activity.2a Therefore
we initiated a study of conformationally constrained
analogs that mimic this s-trans arrangement. We have
recently described the synthesis and biological proper-
ties of combretastatin-like flavones8 and we next concen-
trated our attention on indanones. Indanones
4
represent a class of analogs in which the b-carbon of
its corresponding chalcone is attached directly to C-2
of the A-ring. This will provide a series of conformation-
ally constrained chalcone analogs incorporating a five-
membered ring. Being chiral, the indanones will also
serve as stereochemical probes for studying the colchi-
cine-binding site of tubulin. In this letter, we describe
a convenient synthesis of CA-4-like indanones.
The indanone scaffold can be constructed from
chalcones by way of the Nazarov cyclization.9,10 This
O
O
MeO
MeO
O
MeO
MeO
MeO
NH
O
A
MeO
MeO
OH
2
β
MeO
OH
OMe
OMe
B
OMe
OMe
R
OMe
OMe
OMe
4
Combretastatin A-4 (1)
2
Colchicine (3)
Figure 1. Structure of tubulin-binding agents.
*
Corresponding author. Fax: +1 813 979 6700; e-mail: Lawrennj@moffitt.usf.edu
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.12.110

1638
N. J. Lawrence et al. / Tetrahedron Letters 47 (2006) 1637–1640
Table 2. Yields of indanones 4a–s
is convenient since chalcones are themselves easy to pre-
pare and we already had a large number in hand.11 The
conventional method for the preparation of indanones
involves heating a precursor chalcone in a strong acid,
such as TFA, at high temperatures (120–130 ꢁC) for
extended periods of time (4–24 h).12 We were intrigued
to see whether microwave heating would significantly
shorten the reaction time and thereby provide a conve-
nient process for the preparation of CA-4-like inda-
nones. The use of microwave irradiation is now widely
adopted in organic synthesis and the drug discovery pro-
cess as an alternative to traditional heating techniques.
The process often results in higher yields and purer
products as well as significantly shorter reaction times,13
and is more energy efficient than conventional heating.14
Route R1
Ar1
Yield (%)
72
4a
4b
4a
4b
4c
4d
4e
4f
i
i
OMe 4-Methoxyphenyl
OMe 3-Hydroxy-4-methoxyphenyl 63
OMe 4-Methoxyphenyl 60
OMe 3-Hydroxy-4-methoxyphenyl 56
ii
ii
ii
ii
ii
ii
ii
ii
ii
ii
ii
ii
OMe 4-Chlorophenyl
53
59
69
54
33
26
25
95
83
23
39
37
7
OMe 3-Bromo-4-methoxyphenyl
OMe 3,4-Dichlorophenyl
OMe Benzo[d][1,3]dioxol-5-yl
OMe 4-Bromophenyl
OMe Phenyl-4-oxyacetic acid
OMe 2,3,4-Trimethoxyphenyl
OMe 2,6-Dichlorophenyl
OMe 2,4-Dichlorophenyl
OMe Phenyl
4g
4h
4i
4j
4k
4l
4m ii
OMe 3-Nitro-4-methoxyphenyl
OMe 3-Fluoro-4-methoxyphenyl
The precursor chalcones (5a–s) were synthesized in the
usual way by the NaOH-catalyzed Claisen–Schmidt
condensation of appropriate acetophenones and benzal-
dehydes (Scheme 1).2a The yields of these chalcones (5a–
4n
4o
4p
4q
4r
4s
ii
ii
ii
ii
ii
ii
H
H
H
H
H
4-Methoxyphenyl
4-Bromophenyl
4-Chlorophenyl
2,6-Dichlorophenyl
3,4,5-Trimethoxyphenyl
52
34
71
41
1
s) are shown in Table 1. Inspection of the H NMR
spectra of the chalcones indicated that they were all geo-
metrically pure (E)-isomers as evident from the alkene
1H–1H coupling (3JHH 15–16 Hz).
The first attempt at the microwave-assisted reaction was
performed with chalcone 5a on a 600 mg scale. The chal-
cone in TFA was heated at 60 ꢁC (power 50 W) for
5 min. Analysis of the resulting mixture by TLC showed
that only a small amount of the indanone 4a had been
formed. Heating at 70 ꢁC for 10 min (100 W) also
resulted in an incomplete reaction. The mixture was
heated at 120 ꢁC for 10–20 min (100 W), after which
TLC revealed that the reaction was complete. The inda-
none was then isolated after column chromatography
(SiO2, EtOAc/hexane, 1:3) in 60% yield. These condi-
tions proved to be sufficiently general for the synthesis
of other indanones as shown in Table 2 (route ii, Scheme
2).
We first prepared the indanones using the technique
used by Rice and co-workers.12e The chalcones 5a and
5b were dissolved in neat TFA and heated to 120–
130 ꢁC in a sealed tube for 4 h. This technique gave rea-
sonable yields of indanones 4a and 4b, respectively (see
Table 2, route i). The technique was then modified for
microwave synthesis.
O
O
O
MeO
MeO
MeO
MeO
1
1
Ar
H
Ar
NaOH,
MeOH
5a-s
1
1
R
R
The TFA used in the microwave reaction should ideally
be anhydrous. The synthesis of indanone 4s using TFA
as supplied was problematic. The 1H NMR spectrum of
the crude product showed formation of a 2.5:1 mixture
of 3,4-dimethoxyacetophenone and 4s. It appeared that
the acetophenone is derived from 5s by a water-pro-
moted retro-Michael retro-aldol process. We dried the
TFA by stirring over phosphorus pentoxide for 12 h
under an atmosphere of argon followed by distillation
(bp 74–5 ꢁC/1 atm.).15 When the reaction of 5s was
repeated with this dry TFA the product indanone 4s
was isolated in 41% yield. There was no acetophenone
Scheme 1. Precursor chalcone synthesis.
Table 1. Yields of chalcones 5a–s
Chalcone
R1
Ar1
Yield (%)
5a
5b
5c
5d
5e
5f
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
H
4-Methoxyphenyl
3-Hydroxy-4-methoxyphenyl
4-Chlorophenyl
74
69
55
75
50
73
88
52
78
82
83
76
68
87
67
76
76
77
90
3-Bromo-4-methoxyphenyl
3,4-Dichlorophenyl
Benzo[d][1,3]dioxol-5-yl
4-Bromophenyl
Phenyl-4-oxyacetic acid
2,3,4-Trimethoxyphenyl
2,6-Dichlorophenyl
2,4-Dichlorophenyl
Phenyl
3-Nitro-4-methoxyphenyl
3-Fluoro-4-methoxyphenyl
4-Methoxyphenyl
4-Bromophenyl
4-Chlorophenyl
1
5g
5h
5i
evident in the H NMR spectrum of the crude reaction
5j
O
5k
5l
O
MeO
MeO
MeO
MeO
1
Ar
5m
5n
5o
5p
5q
5r
5s
i or ii
1
1
1
Ar
R
R
H
H
H
H
5a-s
4a-s
2,6-Dichlorophenyl
3,4,5-Trimethoxyphenyl
Scheme 2. Reagents and conditions: (i) TFA, sealed tube, 120 ꢁC, 4 h;
(ii) TFA, lwave, 100 W, 120 ꢁC, 20 min.

N. J. Lawrence et al. / Tetrahedron Letters 47 (2006) 1637–1640
Table 3. Cell growth inhibitiona against the K562 cell line
1639
mixture. The use of the microwave reactor is ideal for
handling potentially hazardous reagents such as TFA.
The reactions can be performed safely: both the temper-
ature and the pressure of the reaction vial are monitored
continuously. The exposure to pressurized reaction
vessels containing TFA is thereby significantly reduced.
IC50 (lM)
IC50 (lM)
IC50 (lM)
4a
4b
4c
4d
4e
4f
0.49
0.13
30
1.2
24
4g
4h
4i
1.9
>10
2.8
7.4
2.0
4m
4n
4o
4p
4q
4s
0.08
0.1
0.17
5.4
>10
34
4j
4k
4l
2.4
>10
The process conveniently provides a library of inda-
nones possessing di- and trimethoxy substituted A-rings.
The presence of the methoxy groups is highly beneficial
for good tubulin-binding properties. This feature is pres-
ent in many ligands that target the colchicine-binding
site of b-tubulin. Acceptable yields of indanones were
obtained for a variety of functional groups and substitu-
tion patterns present on the B-ring substituent.
a As measured by the MTT assay after 5 days of incubation of the drug
with the cells cultured at 37 ꢁC.
bearing the greatest resemblance to combretastatin A-4
displays high activity. This level of activity is potentially
useful and indicates that the indanone scaffold is a
framework for further design. Further details of this
work will be revealed in due course.
In general, the reaction works best for electron deficient
B-ring substituents. The presence of a p-alkoxy group is
detrimental to the yield of indanone. This is consistent
with the expected reactivity of the electrophilic substitu-
tion process that requires an electron-rich A-ring and
electron deficient enone reaction partner. In this case,
the methoxy groups of the A-ring aid the required reac-
tivity of the chalcones as well as being necessary for
good tubulin binding properties. Indeed the reaction
works well for the synthesis of 4b in which both the A
and B-rings are the same as those of 1. The indanone
skeleton is an excellent scaffold for further modification.
Additional substituents can easily be appended via addi-
tion to the carbonyl or alkylation at the a-carbon. The
microwave-assisted Nazarov cyclization provides easy
access to this important template.12
In conclusion, we have shown that indanones can be
conveniently and safely prepared by the TFA micro-
wave-assisted Nazarov cyclization of chalcones. The
indanones have been shown to possess potentially useful
antiproliferative properties. Further biological effects of
this library of indanones are under study and the tubu-
lin-binding properties will be reported in due course.
General procedure for the microwave-assisted synthesis
of indanones 4a–s: The precursor chalcone (ꢀ250 mg)
was dissolved in dry TFA (0.3 mL) and sealed in a pres-
sure-rated reaction vial (10 mL). The reaction tube was
irradiated in a self-tuning single-mode CEM DiscoverTM
Focused Synthesiser. The reaction was maintained at
120 ꢁC (power: 100 W) for 20 min. The mixture was then
rapidly cooled to room temperature and poured into
H2O (5 mL), extracted with EtOAc (3 · 5 mL) and
washed with saturated NaHCO3 (aq) (2 · 3 mL). The
organic extract was dried over MgSO4 and concentrated
in vacuo. The crude product was purified by column
chromatography (SiO2, EtOAc/hexane, 1:3 to 1:1) to
afford the product indanone.
Since chalcones can be prepared under acid-catalyzed
conditions,16 we attempted a one-pot tandem condensa-
tion Nazarov reaction (Scheme 3). We were able to iso-
late the indanone 4a but in only 31% yield. Additionally,
the separation from the starting materials was not triv-
ial, so we did not develop this further.
The cell growth inhibitory properties of selected inda-
nones (summarized in Table 3) were determined in the
K562 human chronic myelogenous leukaemia cell line
using the MTT assay from a previously reported proce-
dure.2b The IC50 value represents the concentration,
which results in a 50% inhibition in cell growth after 5
days of incubation. The 3,4,5-trimethoxy substituted
indanones (4a–n) generally have greater cell growth
inhibitory properties than their corresponding 3,5-di-
methoxy counterparts. The indanone 4b (IC50 130 nM)
Acknowledgements
The support of our work by the EPSRC (Research grant
GR/S25456/01: 500 MHz NMR spectrometer; the
EPSRC Chemical Database Service at Daresbury,17
the EPSRC national mass spectrometry centre at Swan-
sea University for high-resolution mass spectrometry),
BBSRC (Research grant B18009 to BG & B18017 to
D.C.), Cardiff University (Senior Research Fellowship
to N.J.L., and studentship to E.S.A.) and the Moffitt
Cancer Center is gratefully acknowledged.
O
O
MeO
MeO
MeO
p-MeOC6H4CHO
TFA, µwave,
References and notes
MeO
100 W, 120 C
˚
1. (a) Gaukroger, K.; Hadfield, J. A.; Lawrence, N. J.;
Nolan, S.; McGown, A. T. Org. Biomol. Chem. 2003, 1,
3033–3037; (b) Lawrence, N. J.; Hepworth, L. A.; Ren-
nison, D.; McGown, A. T.; Hadfield, J. A. J. Fluorine
Chem. 2003, 123, 101–108; (c) Gaukroger, K.; Hadfield, J.
A.; Hepworth, L. A.; Lawrence, N. J.; McGown, A. T.
J. Org. Chem. 2001, 66, 8135–8138.
MeO
20 mins
MeO
31%
4a
OMe
Scheme 3. The one-pot Claisen–Schmidt–Nazarov-like route to
indanones.

1640
N. J. Lawrence et al. / Tetrahedron Letters 47 (2006) 1637–1640
2. (a) Ducki, S.; Forrest, R.; Hadfield, J. A.; Kendall, A.;
C. D.; McDonnell, P. D.; Weatherstone, T. A. P.
Tetrahedron Lett. 1983, 24, 2851–2854; (d) Elliott, J. D.;
Lago, M. A.; Cousins, R. D.; Gao, A.; Leber, J. D.;
Erhard, K. F.; Nambi, P.; Elshourbagy, N. A.; Kumar, C.;
Lee, J. A.; Bean, J. W.; DeBrosse, C. W.; Eggleston, D. S.;
Brooks, D. P.; Feuerstein, G.; Ruffolo, R. R., Jr.;
Weinstock, J.; Gleason, J. G.; Peishoff, C. E.; Ohlstein,
E. H. J. Med. Chem. 1994, 37, 1553–1557; (e) Gu, X.-H.;
Yu, H.; Jacoboson, A. E.; Rothman, R. B.; Dersch, C. M.;
George, C.; Flippen-Anderson, J. L.; Rice, K. C. J. Med.
Chem. 2000, 43, 4868–4876.
Lawrence, N. J.; McGown, A. T.; Rennison, D. Bioorg.
Med. Chem. Lett. 1998, 8, 1051–1056; (b) Lawrence, N. J.;
McGown, A. T.; Ducki, S.; Hadfield, J. A. Anti-Cancer
Drug Des. 2000, 15, 135–141; (c) Lawrence, N. J.;
Rennison, D.; Woo, M.; McGown, A. T.; Hadfield, J.
A. Bioorg. Med. Chem. Lett. 2001, 11, 51–54.
3. Lawrence, N. J.; McGown, A. T. Curr. Pharm. Design
2005, 11, 1679–1693.
4. Chaplin, D. J.; Hill, S. A. Int. J. Radiat. Oncol. Biol. Phys.
2002, 54, 1491–1496.
5. Dark, G. G.; Hill, S. A.; Prise, V. E.; Tozer, G. M.; Pettit,
G. R.; Chaplin, D. J. Cancer Res. 1997, 57, 1829–1834.
6. Tozer, G. M.; Kanthou, C.; Baguley, B. C. Nat. Rev.
Cancer 2005, 5, 423–435.
7. Li, Q.; Sham, H. L. Expert Opin. Ther. Patents 2002, 12,
1663–1702.
8. Lawrence, N. J.; Rennison, D.; McGown, A. T.; Hadfield,
J. A. Bioorg. Med. Chem. Lett. 2003, 13, 3759–3763.
9. Habermas, K. L.; Denmark, S. E.; Jones, T. K. Org.
React. 1994, 45, 1–158.
13. (a) Caddick, S. Tetrahedron 1995, 51, 10403–10432; (b)
Nuchter, M.; Muller, U.; Ondruschka, B.; Tied, A.;
¨
Lautenschla¨ger, W. Chem. Eng. Technol. 2003, 26, 1207–
1216; (c) Mavandadi, F.; Lidstrom, P. Curr. Top. Med.
Chem. 2004, 4, 773–792; (d) Nuchter, M.; Ondruschka, B.;
Bonrath, W.; Gum, A. Green Chem. 2004, 6, 128–141; (e)
Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250–
6284.
14. Gronnow, M. J.; White, R. J.; Clark, J. H.; Macquarrie,
D. J. Org. Proc. Res. Dev. 2005, 9, 516–518.
10. (a) Pellissier, H. Tetrahedron 2005, 61, 6479–6517; (b)
Frontier, A.; Collinson, C. Tetrahedron 2005, 61, 7577–
7606.
15. Armarego, W. L. F.; Perrin, D. D. Purification of
Laboratory Chemicals, 4th ed.; Butterworth Heinemann:
Oxford, 1996.
11. Lawrence, N. J.; Rennison, D.; McGown, A. T.; Ducki,
S.; Gul, L. A.; Hadfield, J. A.; Khan, N. J. Comb. Chem.
2001, 3, 421–426.
12. For relevant examples of indanone synthesis see: (a)
Woltersdorf, O. W., Jr.; deSolms, S. J.; Schultz, E. M.;
Cragoe, E. J., Jr. J. Med. Chem. 1977, 20, 1400–1408; (b)
McDonald, E.; Smith, P. J. Chem. Soc., Perkin Trans. 1
1980, 4, 837–842; (c) Bradley, J. P.; Jarvis, T. C.; Johnson,
16. Dimmock, J. R.; Kandepu, N. M.; Hetherington, M.;
Quail, J. W.; Pugazhenthi, U.; Sudom, A. M.; Chaman-
khah, M.; Rose, P.; Pass, E.; Allen, T. M.; Halleran, S.;
Szydlowski, J.; Mutus, B.; Tannous, M.; Manavathu, E.
K.; Myers, T. G.; De Clercq, E.; Balzarini, J. J. Med.
Chem. 1998, 41, 1014–1026.
17. Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem.
Inf. Comp. Sci. 1996, 36, 746–749.