Synthesis of 3-methoxy-9-(3,4,5-trimethoxyphenyl)-6,7-dihydro-5H-benzo[7] annulen-4-ol, a potent antineoplastic benzosuberene derivative for anti-cancer chemotherapy

Article abstract of DOI:10.1016/j.tetlet.2011.10.145

Benzosuberene 8 is a tubulin polymerization inhibitor that possesses remarkable potency against a variety of cancer cell lines. Herein, an efficient synthetic route for 8 is described featuring a Claisen rearrangement/RCM sequence and a late stage Suzuki

Full text of DOI:10.1016/j.tetlet.2011.10.145

Tetrahedron Letters 53 (2012) 64–66
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of 3-methoxy-9-(3,4,5-trimethoxyphenyl)-6,7-dihydro-5H-
benzo[7]annulen-4-ol, a potent antineoplastic benzosuberene derivative
for anti-cancer chemotherapy
⇑
Zecheng Chen, Christopher J. O’Donnell, Andreas Maderna
Pfizer Global Research and Development, Pfizer Inc., 445 Eastern Point Road, Groton, CT 06340, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzosuberene 8 is a tubulin polymerization inhibitor that possesses remarkable potency against a vari-
ety of cancer cell lines. Herein, an efficient synthetic route for 8 is described featuring a Claisen rearrange-
ment/RCM sequence and a late stage Suzuki coupling as the key steps. By adopting this strategy 8 was
synthesized in eight steps in 18% overall yield starting from known compound 18.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 11 October 2011
Revised 24 October 2011
Accepted 26 October 2011
Available online 3 November 2011
Keywords:
Benzosuberene
Tubulin polymerization inhibitor
Colchicine binding site
Suzuki coupling
Microtubules are cytoskeletal polymers of tubulin
a
b heterodi-
great interest in this class of compounds for cancer chemotherapy
based on the observation that agents that bind to the colchicine
binding site act as vascular disrupting agents and block the blood
supply to the tumor vasculature, in addition to their cytotoxic
effects.8,9
mers that are essential in several cellular functions such as cell
division and morphogenesis.1 The functional assembly–disassem-
bly cycle of
ab tubulin units includes activation by GTP binding
at the b-subunit, polymerization into microtubules, GTP hydroly-
sis, depolymerization of GDP-tubulin, followed by replacement
with GTP. Compounds that interfere with the polymerization/
depolymerization dynamics of tubulin represent an important
class of antineoplastic agents. Four major classes of microtubule
active agents have been identified in terms of four different bind-
ing sites within b-tubulin.2 The vinca alkaloids (e.g., vincristine,
vinblastine, vindesine, and vinorelbine), peptide inhibitors (e.g.,
dolastatins) and maytansines bind to two overlapping sites on
b-tubulin and induce microtubule depolymerization. The taxoids
paclitaxel (Taxol) and docetaxel (Taxotere), as well as the epothil-
ones, bind to b-tubulin and induce microtubule polymerization.
The fourth class of agents, typified by the natural products such
as colchicine (1) and podophyllotoxin (2) (Fig. 1), bind at the
OH
O
O
MeO
MeO
MeO
MeO
O
NHAc
O
O
OMe
MeO
OR
OMe
MeO
OMe
OMe
3
4
: R = H (CA4)
: R = PO3Na2 (CA4P)
OMe
Colchicine (1)
Podophyllotoxin (2)
Combretastatin A4
OMe
MeO
MeO
OMe
MeO
MeO
MeO
a
-b-subunit intradimer interface and include the combretastatins,
OMe
MeO
which also promote tubulin depolymerization. Combretastatins, as
exemplified by combretastatin A4 (3) and its phosphorylated pro-
drug CA4P (4), represent a class of drugs that are in clinical trials as
antitumor agents.3,4 A large array of compounds have been re-
ported that bind to the colchicine binding site such as chalcone
5,5 benzofurans (6, 7)6 and benzosuberene 87 (Fig. 1). There is a
O
O
Me
O
MeO
MeO
OR
OH
OH
MeO
6: R = H
7: R = PO3Na2
8
5
⇑
Corresponding author.
E-mail address: Andreas.Maderna@pfizer.com (A. Maderna).
Figure 1. Selected compounds that bind to the colchicine binding site of b-tubulin.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.145

Z. Chen et al. / Tetrahedron Letters 53 (2012) 64–66
65
OMe
MeO
MeO
MeO
OMe
OMe
1. Hydroxylation
2. Protection
3. Oxidation
TfO
O
CH2
Wittig
68%
B(OH)2
MeO
15%
MeO
MeO
MeO
OiPr
10
OMOM
OiPr
11
MeO
9
OH
15
8
O
O
1. Reduction
2. Oxidation
Ring-Expansion
33%
O
OH
MeO
OiPr
MeO
2.5%
OiPr
12
13
MeO
MeO
OMOM
16
OMOM
17
MeO
MeO
OMe
OMe
MeO
MeO
1. Dehydration
2. Deprotection
Li
14%
Ref.12
CHO
CHO
8
OH
56%
MeO
MeO
MeO
OiPr
14
OH
OH
18
Isovanillin
Scheme 1. Pinney’s synthesis of benzosuberene (8).7
Scheme 2. Retrosynthetic analysis of benzosuberene (8).
O
O
OH
O
H
H
a
H
Ref.11
b
MeO
MeO
MeO
MeO
OH
OH
OMOM
OMOM
Isovanillin
17
18
19
HO
O
TfO
OMe
d, e
c
f
MeO
OMe
+
MeO
MeO
MeO
OMOM
20
OMOM
21
OMOM
15
B(OH)2
MeO
MeO
OMe
OMe
MeO
MeO
h
g
MeO
MeO
OMOM
22
OH
8
Scheme 3. New synthesis of benzosuberene 8. Reagents and conditions: (a) MOM–Br, iPr2NEt, CH2Cl2, rt (78%); (b) allylMgBr, THF, À78 °C (85%); (c) Grubb’s 2nd catalyst
(3 mol %), CH2Cl2, 40 °C, 1 h (90%); (d) TsNH–NH2, NaOAc, DME, 80 °C; (e) Dess–Martin periodinane, CH2Cl2, rt (58% for two steps); (f) PhNTf2, KHDMS, THF, À78 °C; (g)
Pd(PPh3)4 (3 mol %), Ba(OH)2, DME, H2O, 70 °C (62% for two steps); (h) HCl, MeOH, 60 °C (85%).
Recently, Pinney and co-workers discovered the benzosuberene
analog 8 (Fig. 1) as a potent tubulin polymerization inhibitor at
concentrations comparable to that of combretastatin A4 (CA4).7
Furthermore, this compound demonstrated remarkable cytotoxic-
ity against a variety of cancer cell lines in the low nanomolar range.
As shown Scheme 1, this 10-step synthesis included five low yield-
ing steps and provided the target compound 8 in 0.01% overall
yield from tetraline 9.7
install the trimethoxyphenyl group is highly desirable. Addition-
ally, a better synthesis should be able to access diverse structural
analogs to facilitate the SAR study for lead optimization. As such
we envisioned utilizing
a Claisen rearrangement/ring closing
metathesis (RCM) sequence to construct the benzocycloheptene
ring system and a late stage Suzuki coupling to install the trime-
thoxyphenyl ring as the key steps.
Scheme 2 shows the retrosynthetic analysis of our designed
route for 8. We envisioned preparing 8 through a late stage Suzuki
coupling of trimethoxy phenylboronic acid with enol triflate 15.
In considering an alternative strategy to the synthesis of
benzosuberene 8, a higher yielding C–C bond forming reaction to

66
Z. Chen et al. / Tetrahedron Letters 53 (2012) 64–66
The benzosuberone 16 could be obtained from diene 17 via a RCM
reaction.10,11 Diene 17 could be elaborated from isovanillin by
adopting an O-allylation/Claisen rearrangement sequence via the
known compound 18.12
Acknowledgments
The authors would like to thank Russell Dushin, Christoph Zapf
and Jeremy Levin for discussions around benzosuberenes and other
VDA’s. The authors would also like to thank Justin Stroh and Geeta
Yalamanchi for HRMS data and Frank Loganzo for discussions
about tubulin biology.
Our synthesis commenced with the O-allylation of the phenol
group in isovanillin followed by Claisen rearrangement under ther-
mal conditions to provide 18 in high yield (Scheme 3).11,12 Protec-
tion of the phenol group in 18 as its MOM ether through reaction
with methoxymethyl bromide gave 19, which was reacted with
allylmagnesium bromide to produce the corresponding secondary
alcohol 17. The subsequent RCM reaction of 17 was performed in
dichloromethane using Grubb’s 2nd generation catalyst13
(3 mol % loading) to give the bicyclic compound 20 in 90% isolated
yield. It was found that Grubb’s 1st generation catalyst also can be
employed for this RCM reaction, but the catalyst loading needs to
be higher (from 3 mol % to 6 mol %), and the reaction time needs
to be longer (from 1 h to 6 h) for completion. Diimide reduction
of the double bond in 20 and oxidation of the benzylic hydroxyl
group by Dess–Martin periodinane gave the MOM-protected ben-
zosuberone 21 in moderate yield over two steps. Reaction of 21
with potassium hexamethydisilazane and N-phenyl triflimide re-
sulted in the corresponding enol triflate 15, which was directly
used for the succeeding Suzuki coupling14 with 3,4,5-trimethoxy-
phenyl boronic acid to provide the MOM protected benzosuberene
precursor 22 in 62% yield over two steps. Finally, deprotection
using methanolic hydrogen chloride provided the target benzos-
uberene 3-methoxy-9-(3,4,5-trimethoxyphenyl)-6,7-dihydro-5H-
benzo[7]-annulen-4-ol (8) in 85% isolated yield.15 Overall, the
target compound 8 was synthesized in eight steps and 18% yield
starting from the literature known compound 18.
References and notes
1. Dumontet, C.; Jordan, M. A. Nature Rev. Drug. Disc. 2010, 9, 790–803.
2. Li, Q.; Sham, H. L. Exp. Opin. Ther. Patents 2002, 12, 1663–1702.
3. Siemann, D. W.; Chaplin, D. J.; Walicke, P. A. Exp. Opin. Investig. Drugs 2009, 18,
189–197.
4. Tron, G. C.; Pirali, T.; Sorba, G.; Pagliai, F.; Busacca, S.; Genazzani, A. A. J. Med.
Chem. 2006, 49, 3033–3044.
5. Ducki, S.; Mackenzie, G.; Greedy, B.; Armitage, S.; Fournier Dit Chabert, J.;
Bennett, E.; Nettles, J.; Snyder, J. P.; Lawrence, N. J. Bioorg. Med. Chem. 2009, 17,
7711–7722.
6. Kremmidiotis, G.; Leske, A. F.; Lavranos, T. C.; Beaumont, D.; Gasic, J.; Hall, A.;
O’Callaghan, M.; Matthews, C. A.; Flynn, B. Mol. Cancer. Ther. 2010, 9,
1562–1573.
7. Sriram, M.; Hall, J. J.; Grohmann, N. C.; Strecker, T. E.; Wootton, T.; Franken, A.;
Trawick, M. L.; Pinney, K. G. Bioorg. Med. Chem. 2008, 16, 8161–8171.
8. Bonezzi, K.; Taraboletti, G.; Borsotti, P.; Bellina, F.; Rossi, R.; Giavazzi, R. J. Med.
Chem. 2009, 52, 7906–7910.
9. Tozer, G. M.; Kanthou, C.; Baguley, B. C. Nature Rev. Cancer 2005, 5, 423–435.
10. Huang, K.-S.; Wang, E.-C.; Chen, H.-M. J. Chin. Chem. Soc. 2004, 51, 807–816.
11. Bracca, A. B. J.; Kaufman, T. S. Eur. J. Org. Chem. 2007, 31, 5284–5293.
12. Yang, W.; Liu, J.; Zhang, H. Tetrahedron Lett. 2010, 51, 4874–4876.
13. Grubb’s 2nd generation catalyst: benzylidene[1,3-bis(2,4,6-trimethylphenyl)-2-
imidazolidene]dichloro(tricyclohexylphosphine) ruthenium, CAS: 246047-72-
3.
14. Carreno, M. C.; Gonzalez-Lopez, M.; Latorre, A.; Urbano, A. J. Org. Chem. 2006,
71, 4956–4964.
15. Compound 8: White solid, mp 130–131 °C. 1H NMR (400 MHz, CDCl3) d 1.97 (q,
J = 7.0 Hz, 2H), 2.15 (quin, J = 6.9 Hz, 2H), 2.77 (t, J = 6.9 Hz, 2H), 3.81 (s, 6H),
3.87 (s, 3H), 3.92 (s, 3H), 5.75 (s, 1H), 6.34 (t, J = 7.0 Hz, 1H), 6.51 (s, 2H), 6.58
(d, J = 8.2 Hz, 1H), 6.72 (d, J = 8.2 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 23.2,
25.4, 33.3, 55.6, 55.8 (2C), 60.6, 104.9 (2C), 107.3, 120.5, 126.9, 127.4, 133.9,
136.9, 138.2, 141.9, 142.4, 144.7, 152.5 (2C); MS (ESI) m/z 357 (M+H); HRMS
calcd for C21H24O5 (M+H) 357.1696, obsd 357.1699.
In summary, we have developed a novel and efficient synthesis
for the benzosuberene analog 3-methoxy-9-(3,4,5-trimethoxy-
phenyl)-6,7-dihydro-5H-benzo[7]-annulen-4-ol (8),
a powerful
antineoplastic agent that binds to the colchicine binding domain
of tubulin. Efforts on the application to the synthesis of other ben-
zosuberene analogs with the structural modifications in the B-ring
for further biological studies are undergoing in our laboratory, and
the results will be reported in due course.