Design and synthesis of 3′,5′-ansa-adenosines as potential Hsp90 inhibitors

Article abstract of DOI:10.1016/j.tetlet.2009.06.094

3′,5′-Ansa-adenosine derivatives, rationally designed as an Hsp90 inhibitor by extracting and fusing a natural product, geldanamycin, and a natural substrate, ATP, were efficiently synthesized by the ring-closing metathesis assisted by the 2,4-dimethoxybe

Full text of DOI:10.1016/j.tetlet.2009.06.094

Tetrahedron Letters 50 (2009) 5102–5106
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Design and synthesis of 30,50-ansa-adenosines as potential Hsp90 inhibitors
*
Kazuhiro Muranaka, Satoshi Ichikawa, Akira Matsuda
Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan
a r t i c l e i n f o
a b s t r a c t
30,50-Ansa-adenosine derivatives, rationally designed as an Hsp90 inhibitor by extracting and fusing a
natural product, geldanamycin, and a natural substrate, ATP, were efficiently synthesized by the ring-
closing metathesis assisted by the 2,4-dimethoxybenzyl group. This simpler scaffold design provides a
practical synthesis of a set of analogs and demonstrates synthetic innovation.
Article history:
Received 29 May 2009
Revised 17 June 2009
Accepted 19 June 2009
Available online 24 June 2009
Ó 2009 Elsevier Ltd. All rights reserved.
The heat shock protein 90 (Hsp90) family is a group of molecu-
lar chaperones that play a key role in the folding of polypeptide,
called as client proteins.1 Many of the Hsp90 client proteins are
oncogenic and are also considered hallmarks of the disease.2 Inhi-
bition of Hsp90 leads to degradation of these various client pro-
teins by a proteasome. The simultaneous combinatorial depletion
of many cancer-causing client proteins and the modulation of all
of the hallmarks of cancer are the major advantages of Hsp90
inhibitors. Geldanamycin (GDM, Fig. 1, 1)3 is known to be an inhib-
itor of Hsp90 and exhibits anti-proliferation activity against a
range of tumor cell lines.4 The 17-allylamino derivative (17-AAG,
2) and the 17-dimethylaminoethylamino derivative (17-DMAG,
3) of GDM are currently in phase II clinical studies for anti-cancer
chemotherapy.5 However, it has been revealed that, as a potential
drug, GDM has certain drawbacks such as solubility and toxicity.
Chemical modifications of GDM are limited, raising the problem
of providing a range of analogs by total synthesis.6 GDM is a com-
petitive inhibitor with ATP, which is a natural substrate of the
Hsp90 N-terminal domain.7,8 Interaction of GDM in the N-terminal
ATP-binding pocket of Hsp90 has been revealed by X-ray crystal
structure analysis.9,7c,10 The key features of the interaction upon
binding may be described as follows: (a) the bound GDM adopts
an overall folded conformation with a cis-amide bond, (b) the car-
bamate moiety at the 7-position mimics the adenine moiety with
the key hydrogen bonding to the pocket, and (c) the benzoquinone
moiety is found at the top of the binding pocket, where the b- and
GDM overlaps with the 30-position of the ATP. Considering these
structural features of ATP and GDM, we designed 30,50-ansa-aden-
osine derivatives containing the benzoquinone moiety (Fig. 2).
Although chimeric inhibitors of Hsp90 exemplified by two natural
products, GDM and radicicol, have been reported,12 there have
been no Hsp90 inhibitors designed by extracting and fusing a nat-
ural product and a natural substrate. The aim of this study was to
establish a synthetic route to this class of molecules, for example,
4.
Scheme 1 describes the preparation of the phosphonate 9 bear-
ing the N-aryl carbamoyl substituent, which was used in the Horn-
er–Wadsworth–Emmons (HWE) olefination13 with the adenosine
50-aldehyde derivative. 2-Methoxy-1,4-bis-methoxymethyloxy-
phenyl-3-cuprate,14 which was prepared by an ortho-lithiation of
5 followed by metal exchange, was reacted with allyl bromide to
give 6 in 66% yield. According to the procedure previously re-
ported,14 a nitro group was introduced at the 5-position of 6 under
mild conditions using ammonium nitrate and trifluoroacetic anhy-
dride in THF at ꢀ20 °C to give 7 in 41% yield. The nitro group in 7
was reduced by zinc to afford the aniline 8 in 85% yield. The result-
ing amine 8 was acylated with diethyl phosphonoacetic acid and
EDCI in the presence of DMAP to give the N-aryl phosphonoaceta-
mide 9 in 95% yield.
The convergent assembly of the key N-aryl unsaturated com-
pound 15 from 9 and the labile adenosine 50-aldehyde derivative
14 is summarized in Scheme 2. The previous method for the
c-phosphate groups of the ATP lie. As opposed to enzymes, such as
preparation of 30-O-allyladenosine 11 required
a
multi-step
protein kinases which utilize ATP as a substrate, the N-terminal
ATP-binding pocket of Hsp90 adopts a rather unusual Bergerat
fold.11 When inside the pocket, the ATP adopts a U-shaped bend
at the phosphate moiety and the 30-endo-conformation at the ribo-
furanose moiety. The superimposition of the bound ATP and the
bound GDM in the N-terminal ATP-binding pocket of Hsp90 pro-
vides additional information, where the 14- or 15-position of
procedure, including a protection–deprotection sequence and
chromatographic separation,15 and thus was not suitable for a
large-scale synthesis. We prepared 11 simply by allylation of
the 20,30-O-stannyleneadenosine16 followed by crystallization.
Although the yield was not high, this method allowed us to oper-
ate on a 0.5 mol scale to obtain pure 11 without any chromatog-
raphy. 30-O-Allyladenosine 11 was sequentially protected with
TBS and Tr groups to give the suitably protected 12. Selective re-
moval of the TBS-protecting group at the 50-hydroxy group was
conducted by TBAF at low temperature to give the corresponding
* Corresponding author. Tel.: +81 11 706 3228; fax: +81 11 706 4980.
E-mail address: matuda@pharm.hokudai.ac.jp (A. Matsuda).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.06.094

K. Muranaka et al. / Tetrahedron Letters 50 (2009) 5102–5106
5103
Figure 1. Structural comparison of geldanamycin and ATP bound to the N-terminal ATP-binding site of Hsp90.
ation. The modified HWE reaction developed by Schauer et al.17
(Zn(OTf)2, Et3N, TMEDA, THF) was applied, and the desired unsat-
urated-E-amide 15, a precursor to the cyclization, was selectively
obtained in good yield (88% over two steps) without any epimer-
ization at the 40-position.
Ring-closing metathesis (RCM)18 to provide the 14-membered
cyclophane was next examined. The RCM of 15 promoted by the
first generation Grubbs’ catalyst 1619 gave none of the desired
cyclophane, and only a mixture of the dimers20 was obtained
in approximately 33% yield along with recovery of 15 in 34%
yield. On the other hand, the use of the Grubbs’ second gener-
ation catalyst 1721 afforded only the dioxabicyclo[4.3.0]nonene
derivative 1822 in 44% yield. The conformational analysis of
the RCM precursor 15 by 1H NMR provided insight into the fail-
ure of the RCM as follows. First, the chemical shift of the amide
proton was observed at d 8.60 ppm in CDCl3, which is lower
than that of the typical N-aryl amide proton, and strong NOEs
were observed between the amide proton and both the methyl
Figure 2. Structures of 30,50-ansa-adenosine derivatives.
alcohol 13 in 56% yield along with recovery of 12 in 34% yield.
The resulting 50-hydroxyl group of 13 was oxidized to the alde-
hyde 14 by IBX. Since 14 was labile under basic conditions lead-
ing to the epimerization at the 40-position or b-elimination, mild
reaction conditions would likely be necessary for the HWE olefin-
Scheme 1. Preparation of 9.

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K. Muranaka et al. / Tetrahedron Letters 50 (2009) 5102–5106
Scheme 2. Initial approach to synthesize 4.
and the methylene protons of the MOM group at the 200-position
of the phenyl ring. These observations indicate that the amide
proton forms a hydrogen bonding with the two oxygen atoms
at the 200-substituent. The second is the geometry around the
amide. Observation of the NOE between the amide proton and
the 60-proton indicates that compound 15 exists predominantly
as the trans-conformer. These conformational features suggest
that the RCM precursor 15 adopts an extended conformation,
where the two olefins found in 15 are far apart. Unable to react
with each other, the desired RCM providing the cyclophane thus
failed to occur.
In order to circumvent the disfavored conformation of 15 for
the RCM, we next planned to examine the RCM with the sub-
strate 21, which has an acid-removable 2,4-dimethoxybenzyl
group (DMB) at the amide site23 (Scheme 3). Installation of the
substituent on the nitrogen atom of the amide would free the
precursor to the RCM from the intramolecular hydrogen bonding
by the methoxymethyloxy group. It is also expected that the
geometry around the amide bond would change to the confor-
mation that is advantageous for the desired RCM providing the
cyclophane 22. The DMB group was introduced on 8 by reduc-
tive amination with 2,4-dimethoxybenzaldehyde to give 19 in
near quantitative yield. The phosphonate 20 was obtained by
acylation of 19 (BrCH2CO2H, Et3N, CH2Cl2, 97%) followed by the
Arbuzov reaction with P(OEt)3. The HWE reaction of 14 with
20 provided the DMB-protected unsaturated amide 21 (95%) in
a manner similar to the preparation of 15. The amide 21 exists
as an approximately 3:2 mixture of rotamers in CDCl3 observed
by 1H NMR at room temperature. The RCM of 21 was then
examined. Treatment of 21 with 10 mol % of the catalyst 16 in
refluxing CH2Cl2 (1 mM) resulted in complete consumption of
21, and the desired cyclophanes 22 were obtained as a mixture
of trans- and cis-cyclophanes 22a and 22b (22a:22b = 2.9:1). The
22a included both R- and S-atropisomers, which were separated
by chromatography. The impact of the newly introduced DMB
group on the RCM proved to be immense and the conversion
to the highly strained 14-membered cyclophanes containing
two olefins within the macrocycle was nearly quantitative. The
TBS group at the 20-hydroxyl of 22a24 was removed to give
23a, and no atropisomerization was observed during the course
of the reaction. Global deprotection of 23a followed by the oxi-
dation of the resulting dihydroquinone moiety successfully affor-
ded trans-4 in 18% yield over 3 steps. In a manner similar to the
synthesis of trans-4, cis-4 was synthesized from 22b. This syn-
thetic strategy is quite effective and enabled us to prepare 4
over 10 steps from 5.25
In conclusion, 30,50-ansa-adenosine derivatives, which were
rationally designed by fusing the natural product, GDM, and the
natural substrate, ATP, were synthesized very efficiently via RCM
assisted by the DMB group. This simpler scaffold design provides
a practical synthesis of a set of analogs and demonstrates synthetic
innovation.

K. Muranaka et al. / Tetrahedron Letters 50 (2009) 5102–5106
5105
Scheme 3. Synthesis of 4.
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Acknowledgments
This work was supported by grants-in-aid for scientific research
from the Ministry of Education. We thank Ms. S. Oka (Center for
Instrumental Analysis, Hokkaido University) for measurement of
the mass spectra.
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