Total synthesis of diptoindonesin G and its analogues as selective modulators of estrogen receptors

Article abstract of DOI:10.1039/c6ob01657j

We have developed a versatile synthetic strategy for the synthesis of the natural product diptoindonesin G and its analogues as selective modulators of estrogen receptors. The strategy involves a regioselective dehydrative cyclization of arylacetals, a regioselective bromination of benzofurans, a sequential cross-coupling of bromo-benzofurans with aryl boronic acids, and a BBr₃-mediated tandem cyclization and demethylation. Preliminary biological studies uncovered the critical and dispensable phenolic hydroxyl groups in the natural product and also revealed unexpected selectivity for isoforms of estrogen receptor.

Full text of DOI:10.1039/c6ob01657j

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Biomolecular Chemistry
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Total synthesis of diptoindonesin G and its
analogues as selective modulators of estrogen
Cite this: DOI: 10.1039/c6ob01657j
receptors†
Received 1st August 2016,
Accepted 5th September 2016
Ji-tian Liu,a,b Truman J. Do,c Christopher J. Simmons,a,d John C. Lynch,a Wen Gu,a,e
DOI: 10.1039/c6ob01657j
Zhi-Xiong Ma,f Wei Xuc and Weiping Tang*a,d,f
www.rsc.org/obc
We have developed a versatile synthetic strategy for the synthesis
of the natural product diptoindonesin G and its analogues as selec-
tive modulators of estrogen receptors. The strategy involves a
regioselective dehydrative cyclization of arylacetals,
selective bromination of benzofurans, a sequential cross-coupling
of bromo-benzofurans with aryl boronic acids, and BBr3-
a regio-
a
mediated tandem cyclization and demethylation. Preliminary bio-
logical studies uncovered the critical and dispensable phenolic
hydroxyl groups in the natural product and also revealed un-
expected selectivity for isoforms of estrogen receptor.
The natural product diptoindonesin (Dip) G 1 (Scheme 1) was
isolated from tree barks of Hopea mengarawan in Indonesia
together with nine other oligostilbenoids in 2009.1 Around the
same time, the same molecule was also isolated from Hopea
chinensis stem barks in China.2 Dip G has a rather unusual
tetracyclic core with A–D rings bearing a ketone and three phe-
nolic OH groups and an additional E-ring with one more phe-
nolic OH group. Dip G showed anti-proliferation effect in
murine leukemia P-388 cells1 and immunosuppressant activity
in a concanavalin A induced proliferation of mouse splenic
lymphocytes (T cells) assay.2 Recently, we reported that Dip G
could regulate the stability of estrogen receptor α (ERα) and
estrogen receptor β (ERβ), two members of the steroid nuclear
receptor superfamily with opposing effect on cell prolifer-
ation.3 ERα promotes cell proliferation, while ERβ has an anti-
Scheme 1 Retrosynthetic analysis of Dip G.
proliferative effect in breast cancer cells.4 Interestingly, Dip G
decreases the stability of the oncogenic ERα and increases the
stability of ERβ, a tumor suppressor in breast cancer. Instead
of directly interacting with ERs, Dip G was found to target the
E3 ubiquitin ligase C-terminus of HSC70-interacting protein
(CHIP), also known as STIP1 homology and U-Box containing
protein 1 (STUB1).3 By reciprocally regulating ER levels,
natural product Dip G or its analogues may be developed as
novel therapeutics for the treatment of breast cancers.
aSchool of Pharmacy, University of Wisconsin-Madison, Madison, WI, 53705, USA
bSchool of Pharmaceutical Science and Technology, Tianjin University, Tianjin,
300072, P. R. China
cMcArdle Laboratory for Cancer Research, Wisconsin Institute for Medical Research,
The first and only total synthesis of Dip G was reported by
Kim’s group in 2010.5 The key features of this synthesis
include an elegant domino cyclization to construct both B and
C rings of 2a from diaryl ether 2b and a Pd-catalyzed C–H ary-
lation to install the E-ring in the penultimate step. For further
biological studies, we are in need of a new synthetic strategy
that is flexible for the practical synthesis of Dip G and its ana-
logues as selective modulators of ERs. Ring C of 1 could be
constructed by Friedel–Crafts acylation of a carboxylic acid
University of Wisconsin-Madison, Madison, WI 53705, USA
dDepartment of Chemistry, University of Wisconsin-Madison, Madison, WI, 53706,
USA
eJiangsu Key Lab of Biomass-based Green Fuels and Chemicals, College of Chemical
Engineering, Nanjing Forestry University, Nanjing 210037, PR China
fMedicinal Chemistry Center, School of Pharmacy, University of Wisconsin-Madison,
Madison, WI, 53706, USA. E-mail: wtang@pharmacy.wisc.edu
†Electronic supplementary information (ESI) available: 1H NMR, 13C NMR,
HRMS, and IR data and copies of NMR specta for all starting materials and pro-
ducts. See DOI: 10.1039/c6ob01657j
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derivative of 2. We envisioned that the permethylated inter- to do the sequential Pd-catalyzed cross-coupling reaction for
mediate 2 could be derived from dibromobenzofuran 3 dibromobenzofuran 3,8 the yield was low for the first step.
efficiently by sequential cross-coupling with two aryl boronic
During the course of optimizing the dibromination of
acids. A sequence of alkylation, cyclization, and dibromination benzofuran 4, we found that mono-bromination occurred
can convert resorcinol derivative 5 to dibromobenzofuran 3 via quickly. The second bromination was very slow, and decompo-
benzofuran 4. The modularity of this strategy will allow us to sition of brominated products started to occur at high temp-
access various Dip G analogues by simply changing the aryl erature. 2-Bromobenzofuran 9 was isolated in high yield as the
boronic acid reagents at the late stage.
only isomer when we used dichloroethane as the solvent and
Mono-protected resorcinol derivative 5 is commercially DMF as the catalyst. The direct bromination of benzofurans
available. It can also be prepared conveniently from methyl generally occurred on the more reactive 3-position.9
3,5-dihydroxybenzoate. The benzofuran core 4 was synthesized 2-Bromobenzofurans were usually prepared by lithiation and
efficiently from 5 by the sequence of alkylation with bromo- quenching with electrophilic bromination reagents.10 We
dimethylacetal and cyclodehydration (Scheme 2) using hypothesized that the carboxylate ester substituent on the
Amberlyst-15.6 The cyclization occurred regioselectively, which 4-position deactivated the 3-position for bromination and this
is consistent with similar reactions reported previously.6
led to the formation of 2-bromobenzofuran only. Pd-catalyzed
Although there are a number of reports on dibromination cross-coupling of 2-bromobenzofuran 9 with 4-methoxyphenyl
of unsubstituted benzofurans,7 we were not able to prepare 3 boronic acid occurred smoothly to afford product 10.
from 4 directly in reliable yields after screening various bromi- 3-Bromobenzofuran 8 could be obtained in high yield from 10
nation reagents, solvents, bases, and temperature (Scheme 3). after stopping the reaction at around 10 min. Longer reaction
The carboxylate ester substituent on the 4-position appeared time led to lower yield of product 8. The second cross-coupling
to interfere with the second bromination. The cross-coupling with 3,5-dimethoxyphenyl boronic acid occurred at higher
occurred selectively on the 2-position of benzofuran 3. temperature to yield penultimate intermediate 2. Natural
However, the yield of product 8 was low. Although we were able product Dip G 1 was prepared from 2 by BBr3 mediated
tandem cyclization and demethylation following literature pro-
cedure.5 Up to 0.4 g of Dip G precursor 2 was prepared using
this synthetic route.
To examine the effect of the four phenolic OH groups on
biological activity, we next prepared Dip G analogues with just
two or three phenolic OH groups as shown in Scheme 4.
Simply replacing the para-methoxyphenyl boronic acid by
phenyl boronic acid, analogue 14 lacking the phenolic OH
group in the E-ring of Dip G was prepared from bromobenzo-
furan 9 in four steps according to the sequence outlined in
Scheme 3 for the preparation of 1 from 9. The bromination of
2-phenyl substituted benzofuran 11 proved to be more difficult
than the bromination of substrates 4 or 10. Only trace amount
of 3-bromobenzofuran 12 was obtained under the conditions
shown in Scheme 3 for substrates 4 or 10. The yield could be
improved to 50% by using DMF as the co-solvent.
Scheme 2 Synthesis of the benzofuran core of Dip G.
Analogue 15 that missed both phenolic OH groups in the
D-ring was synthesized from intermediate 8 in just two steps.
We were pleased to find that the Friedel–Crafts cyclization still
worked smoothly for the formation of 15 after replacing the
electron-rich dimethoxyphenyl group in Dip G 1 by a simple
phenyl group in 15. Finally, we prepared compound 16 with
just one methoxy group in the D-ring from intermediate 8.
Ideally, we want to access both products 17 and 18 to investi-
gate the effect of each one of the two phenolic OH groups in
the D-ring of Dip G. As expected, both products were formed
and the acylation occurred preferentially on the less-hindered
position of the D-ring.
The biological activity of the above four analogues were
then compared with the parent compound Dip G following our
previous protocol (Scheme 5).3 Cells were treated with each
compound at 10 μM concentration in order to evaluate how
well each modified the stability of ERα and ERβ. In MCF7
Scheme 3 Synthesis of Dip G 1 by sequential bromination of benzo-
furans and Pd-catalyzed cross-couplings.
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Scheme 4 Synthesis of Dip G analogues.
Scheme 5 Activities of Dip G analogues to modify ER stability.
cells, Dip G and all four analogs showed decreased ERα stabi-
lity with the largest change occurring with analogues 17 fol-
lowed by 18 and Dip G. In Hs578T-ERβLuc Dox-inducible
lines, ERβ was strongly stabilized by Dip G and compound 17.
ERβ was moderately stabilized by compound 14. Surprisingly,
compounds 15 and 18 both destabilized ERβ, suggesting that
the hydroxyl group para to the ketone in the D-ring of Dip G
was critical for stabilizing ERβ. Compound 15 is interesting as
it does not significantly modulate the stability of ERα and this
compound can be a useful chemical probe for the study of the
function of ERβ. The hydroxyl group ortho to the ketone in the
D-ring of Dip G is dispensable as the activity of compound 17
was slightly better than Dip G for destabilizing ERα and stabi-
lizing ERβ. This is very critical for further improving the
pharmacological properties of Dip G analogues and accessing
different chemical probes.
discovered in this study. This paved the way for further structure
activity relationship studies and the development of new chemi-
cal probes for ERs, which would be reported in due course.
Acknowledgements
We thank the University of Wisconsin-Madison for financial
support. W. X. thanks Department of Defense ERA of Hope
Award (W81XWYH-11-1-0237). J.-t. Liu thanks Chinese
Scholarship Council for a pre-doctoral fellowship. W. Gu
thanks Jiangsu government for financial support of a visiting
scholar position at UW-Madison. This study made use of the
Medicinal Chemistry Center at UW-Madison instrumentation,
funded by the Wisconsin Alumni Research Foundation (WARF)
and the UW School of Pharmacy.
In summary, we have developed a practical and flexible syn-
thetic strategy for the preparation of Dip G. This new strategy
allows us to access Dip G analogues with just two or three of the
four phenolic hydroxyl groups systematically. Preliminary bio-
logical evaluation of these new analogues revealed that not all
four phenolic hydroxyl groups were required for activity.
Selective modulators for both isoforms of ERα and ERβ were
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