Synthesis of a versatile metacyclophane macrolactam

Article abstract of DOI:10.1016/j.tetlet.2007.10.150

As Hsp90 has emerged as a promising target for the development of cancer chemotherapeutics, so has the need for systematic evaluation of structure-activity relationships between natural product inhibitors and this molecular chaperone. Utilizing our chimer

Full text of DOI:10.1016/j.tetlet.2007.10.150

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 141–144
Synthesis of a versatile metacyclophane macrolactam
Mingwen Wang and Brian S. J. Blagg*
Department of Medicinal Chemistry, The University of Kansas, 1251 Wescoe Hall Drive, Malott 4070,
Lawrence, KS 66045-7563, United States
Received 24 April 2007; revised 30 October 2007; accepted 31 October 2007
Available online 4 November 2007
Abstract—As Hsp90 has emerged as a promising target for the development of cancer chemotherapeutics, so has the need for sys-
tematic evaluation of structure–activity relationships between natural product inhibitors and this molecular chaperone. Utilizing our
chimera approach, which encompasses the quinone moiety of geldanamycin and the resorcinol moiety of radicicol, molecules have
been produced that are highly effective inhibitors of the Hsp90 protein folding machinery. These chimeric inhibitors contain meta-
cyclophane macrolactams that are difficult to cyclize and modify for incorporation of functional diversity. To circumvent this prob-
lem, a highly diversifiable a-bromo-a,b-unsaturated ester has been prepared, which allows for the introduction of various
functionalities that enable elucidation of structure–activity relationships between chimeric compounds and Hsp90. The rationale,
synthesis, and optimization for such a molecule is reported herein.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
dous interest in the development of new inhibitory scaf-
folds of the Hsp90 protein folding machinery.
Recently, the 90 kDa heat shock proteins (Hsp90) have
attracted a great deal of interest as targets for the devel-
opment of cancer chemotherapeutics.1–3 Likewise, the
discovery and synthesis of novel Hsp90 inhibitors have
also become increasingly important.4 The two natural
product inhibitors, radicicol and geldanamycin (Fig. 1)
have been shown to inhibit the Hsp90-mediated protein
folding process at low to mid nanomolar concentrations
against various cancer cell lines.2 However, structure–
activity relationships for geldanamycin have been limi-
ted by the fact that only one total synthesis of this mole-
cule has been reported to date and it required an excess
of 40 synthetic transformations.5 As a result, derivatives
of geldanamycin reported thus far represent only those
that are accessible through modification of the natural
product. In contrast, the total synthesis of radicicol
has been reported and the most recently utilized a
[4+2] cycloaddition for installation of the resorcinol ring
as reported by Danishefsky and co-workers.6,7 Unfortu-
nately, radicicol manifests no activity in vivo as it is rap-
idly converted to inactive species as a result of its highly
electrophilic nature (a,b,c,d-unsaturated ketone and
allylic epoxide).8 Consequently, there remains a tremen-
To circumvent these issues and to elucidate structure–
activity relationships between these natural products
and Hsp90, we have proposed the construction of
chimeric inhibitors that contain the resorcinol moiety
O
OMe
O
O
N
H
O
HO
O
O
Cl
O
O
O
O
NH2
HO
OH
Radicicol
O
Geldanamycin
O
O
OH
OMe
HN
OMe
HN
OMe
O
HN
O
O
O
O
O
HO
O
Cl
CO2Me
OH
Cl
Cl
HO
O
O
HO
HO
OH
Radester
OH
Radanamycin
Radamide
*
Corresponding author. Tel.: +1 785 864 2288; fax: +1 785 864
5326; e-mail: bblagg@ku.edu
Figure 1. Natural product and chimeric inhibitors of Hsp90.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.150

142
M. Wang, B. S. J. Blagg / Tetrahedron Letters 49 (2008) 141–144
O
O
of radicicol and the quinone ring of geldanamycin.
Appropriately, the chimeric molecules were named
radanamycin9 for the macrocyclic compound and rada-
mide/radester10,11 for the seco-derivatives as shown in
Figure 1.
MeO
MeO
MeO
MeO
Br
Br
O
O
OMe
OMe
HN
NH2
HOOC
1
7
O
O
MeO
MeO
OH
Br
Of the three chimeric species reported thus far, the mac-
rocyclic compound, radanamycin, manifested the most
potent activity against several cancer cell lines with an
IC50 value in the mid to high nanomolar range.9b Upon
further inspection of the co-crystal structures of radici-
col and geldanamycin bound to Hsp90, it was realized
that additional functionalities may be tolerated or lead
to increased inhibitory activity by modification of the
resorcinol moiety of radanamycin.
MeO
R
+
OMe
9
(E)
NO2
8
Initial attempts to prepare the a,b-unsaturated ester
involved the use of a modified Horner–Wadsworth–
Emmons reagent 11,12 which gave predominately the
(Z)-configured bromoacrylate under all conditions
investigated. An example of which is given in Eq. 1,
wherein treatment of lactol 10 with 11 produced a 4:1
mixture of isomers in favor of (Z)-12 (Eq. 1).
It was hoped that a macrocyclic inhibitor could be pre-
pared through a mechanism that allowed for modifica-
tion of the resorcinol moiety, as this functionality
appears important for inhibitory activity. Therefore,
we chose to prepare a macrocyclic precursor (1) to
radanamycin, which could then be diversified via a num-
ber of chemical transformations (Scheme 1). For exam-
ple, it was envisioned that 1 could undergo a [4+2]
cycloaddition with 2 or 3 to provide the corresponding
phenols upon rearomatization of the Diels–Alder
cycloadducts. Likewise, it was proposed that the electro-
philic nature of the a-bromo-a,b-unsaturated ester could
allow for introduction of various functionalities at both
the 3- and 4-positions and enable survey of additional
structure–activity relationships for this class of inhibi-
tors and the proximal region of the binding site.
MeO2C
K-OtBu
Br
MeO2C
Br
O
P
O
+
HO
CO2Me
+
OHHO
(EtO)2
THF, -78 oC
4 : 1
Br
11
(Z )-12
10
(E)-1
2
ð1Þ
In contrast, when the bromo substituted bis(trifluoro-
ethoxy)phosphonoacetate (14) was prepared as reported
by McKenna13 and reacted with the appropriate alde-
hyde (13) under Still–Genari conditions,14 the desired
(E)-a-bromoacrylate, 15, was afforded in excellent yield
following purification by flash chromatography (Scheme
2). The corresponding acid 9 was provided upon treat-
ment of 15 with lithium hydroxide in a heterogeneous
solution of tetrahydrofuran and water (3:1). Interest-
ingly, when a homogenous solution of tetrahydrofuran,
water, and methanol (3:1:1) was used, isomerization
readily occurred to give (Z)-acid 9 exclusively.
OMe
O
OH
R
OTMS MeO
O
OMe
TMSO
Br
MeO
2
R
OH
OH
HN
HN
O
OMe
4
O
P
O
O
Br
K-OtBu, 18-C-6
OMe
O
RO
MeO
MeO
CHO+
13
(CF3CH2O)2
O
THF, -78 oC
MeO
MeO
O
OMe
14
Br
TMSO
O
OMe
3 R
15
LiOH,
98%
, R = Me, 95%, E/Z >95
HN
9, R = H
Heck, Suzuki, or
Stille coupling
O
R
5
1
O
Scheme 2.
O
MeO
R
and/or
nucleophilic
addition (Nu)
O
OMe
MeO
With the bromoacrylate in hand, the nitrated trimeth-
oxyphenethanol 8 was prepared as previously descri-
bed.9b However, due to incorporation of the a,b-
unsaturated ester into the macrocyclic product, it was
necessary to reduce the nitro substituent prior to cou-
pling with the acrylate. Therefore, the nitro group was
reduced to the corresponding amine and subsequently
masked as the t-butyl carbamate 16 as shown in Eq. 2.
Nu
HN
6
O
Scheme 1.
Retrosynthetically, 1 was envisioned to be constructed
through a macrolactamization, which was aided by the
(E)-configuration of the a,b-unsaturated ester, allowing
for closure of the metacyclophane 14-membered ring.
The homobenzylic alcohol was prepared as previously
described9b with minor modification, however, the a-
bromo-a,b-unsaturated ester required ample develop-
ment before a succinct protocol was produced.
OMe
OMe
1) Pd(C)/H2
MeO
MeO
OH
OMe
OH
OMe
2) (Boc)2O, NaOH
80%, over 2 steps
8
16 NBoc
NO2
ð2Þ

M. Wang, B. S. J. Blagg / Tetrahedron Letters 49 (2008) 141–144
143
Coupling of acid 9 with homobenzylic alcohol 16 proved
difficult as the olefin underwent isomerization under the
majority of conditions that produced the ester product.
For example, when the more frequently used coupling
reagents were employed such as DCC/DMAP, EDCI/
DMAP, or oxalyl chloride/triethylamine, only the un-
desired (Z)-a-bromoacrylic ester 17 was obtained as
depicted in Eq. 3.
using a Still–Gennari reaction and a Mukayama
reagent-promoted lactamization as pivotal steps.
Construction of this 14-membered ring allows for incor-
poration of multiple functionalities that will enable
elucidation of structure–activity relationships between
chimeric compounds and Hsp90. Utilization of this scaf-
fold and biological evaluation of new Hsp90 inhibitory
scaffolds are now in progress and will be reported in
due course.
MeO
O
MeO
MeO
O
O
OMe
OH
OMe
Br
+
Br
HO
MeO
a, b, or c
Acknowledgment
NBoc
16 NBoc
9
Z-17
conditions: a) DCC/DMAP; b) EDCI/DMAP; c) (COCl)2/TEA
The authors gratefully acknowledge financial support of
this project by NIH CA109265.
ð3Þ
Instead of using standard coupling protocols, which
utilize the activated carboxylate as the electrophile, we
chose to reverse the reactive nature of these partners.
Under Mitsunobu conditions,15 homobenzylic alcohol
16 underwent smooth displacement by carboxylate 9 to
afford the corresponding ester (18) in high yield and
without isomerization of the double bond. Subsequent
chemoselective oxidation of the terminal olefin to the
requisite aldehyde (19) was accomplished via osmium-
mediated dihydroxylation,16 followed by oxidative
cleavage with sodium periodate.9 The aldehyde was
then oxidized to acid 20 under relatively neutral condi-
tions,9,17 before the t-butyl carbamate was removed
upon exposure to trifluoroacetic acid. The resulting ami-
no acid (21) was treated with various coupling reagents
that proved to be unproductive, as undesired compounds
represented the vast majority of products. However,
Mukayama’s reagent18,19 proved especially effective in
this transformation and in the presence of triethylamine,
the metacyclophane 14-membered macrolactam, 1,20 was
produced in exceptionally good yield (Scheme 3).
References and notes
1. Neckers, L. Trends Mol. Med. 2002, 8, S55–S61.
2. Blagg, B. S. J.; Kerr, T. D. Med. Res. Rev. 2006, 26, 310–
338.
3. Workman, P. Trends Mol. Med. 2004, 10, 47–51.
4. (a) Burlison, J. A.; Neckers, L. M.; Smith, A.; Maxwell,
A.; Blagg, B. S. J. J. Am. Chem. Soc. 2006, 128, 15529–
15536; (b) Yu, X.-M.; Shen, G.; Cronk, B.; Marcu, M.;
Holzberlein, J.; Neckers, L. M.; Blagg, B. S. J. J. Am.
Chem. Soc. 2005, 127, 12778–12779.
5. Andrus, M. B.; Hicken, E. J.; Meredith, E. L.; Simmons,
B. L.; Cannon, J. F. Org. Lett. 2003, 5, 3859–3862.
6. Garbaccio, R. M.; Stachel, S. J.; Baeschlin, D. K.;
Danishefsky, S. J. J. Am. Chem. Soc. 2001, 23, 10903–
10908.
7. Yamamoto, K.; Garbaccio, R. M.; Stachel, S. J.; Solit, D.
B.; Chiosis, G.; Rosen, N.; Danishefsky, S. J. Angew.
Chem., Int. Ed. 2003, 42, 1280–1284.
8. Geng, X.; Yang, Z.-Q.; Danishefsky, S. J. Synlett 2004, 8,
1325–1333.
9. (a) Wang, M.; Shen, G.; Blagg, B. S. J. Bioorg. Med.
Chem. Lett. 2006, 16, 2459–2462; (b) Shen, G.; Wang, M.;
Blagg, B. S. J. J. Org. Chem. 2006, 7618–7631.
10. Clevenger, R. C.; Blagg, B. S. J. Org. Lett. 2004, 6, 4459–
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11. Shen, G.; Blagg, B. S. J. Org. Lett. 2005, 7, 2157–2160.
12. (a) Maryanoff, B. E.; Reitz, A. Chem. Rev. 1989, 89, 863–
927; (b) Semmelhack, M. F.; Brickner, S. J. J. Am. Chem.
Soc. 1981, 103, 3945; (c) Danishefsky, S.; Chackalamannil,
S.; Harrison, P.; Silvestri, M.; Cole, P. J. Am. Chem. Soc.
1985, 107, 3474.
OMe
O
MeO
DIAD/PPh3
0 oC, 92%
Br
1) OsO4/NMO
2) NaIO4
O
16
+
9
OMe
18
79% over
2 steps
NBoc
OMe
O
O
OMe
O
NaClO2
NaH2PO4
MeO
Br
MeO
Br
O
13. McKenna, C. E.; Khawli, L. A. J. Org. Chem. 1986, 51,
5467.
95%
OMe
OMe
14. Tago, K.; Kogen, H. Tetrahedron 2000, 56, 8825–8831.
15. (a) Mitsunobu, O. Synthesis 1981, 1; (b) Hughes, D. L.
Org. React. 1992, 42, 335–656.
16. Tan, Q.; Danishefsky, S. J. Angew. Chem., Int. Ed. 2000,
112, 4683.
17. Yang, Z.; He, Y.; Vourloumis, H.; Nicolaou, K. C.
Angew. Chem., Int. Ed. 1997, 109, 170.
18. Mukayama, T.; Usui, M.; Saigo, K. Chem. Lett. 1975,
1045.
20
HOOC
19
OHC
NBoc
NBoc
I
OMe
NH3
O
MeO
Br
O
N
Cl
TFA
90%
Me
1
OMe
Et3N
86%
21
O2C
Scheme 3.
19. Nicolaou, K. C.; Bunnage, M. E.; Koide, K. J. Am. Chem.
Soc. 1994, 116, 8402.
20. Compound 1 was prepared and fully characterized as
described below: to a mixture of 21 (246 mg, 0.57 mmol)
and Et3N in ClCH2CH2Cl (300 mL) was slowly added 2-
chloro-1-methylpyridinium iodide (220 mg, 0.86 mmol).
The mixture was stirred for 20 h at rt. The solvent was
2. Conclusion
In this Letter, a highly diversifiable a-bromo-a,b-unsat-
urated metacyclophane macrolactam has been prepared

144
M. Wang, B. S. J. Blagg / Tetrahedron Letters 49 (2008) 141–144
removed under reduced pressure and the residue was
2.14 (m, 2H), 1.93 (m, 1H); 13C NMR (CDCl3, 100 MHz)
d 176.9, 163.8, 150.4, 149.8, 147.6, 142.3, 126.1, 125.7,
114.2, 111.2, 66.8, 61.8, 61.4, 56.4, 34.0, 27.4, 23.8; IR
(film) mmax 3380, 2340, 1714, 1668, 1489, 1463, 1344, 1244,
1217, 1172, 1095, 1009 cmꢀ1; HRMS (TOF-ES+) found
414.0579 (M+H+), calcd 414.0552 for C17H21NO6Br.
purified via chromatography (SiO2, 60% EtOAc in hex-
anes) to afford 1 (206 mg, 86%) as a colorless solid: 1H
NMR (CDCl3, 400 MHz) d 7.48 (s, 1H), 6.67 (s, 1H), 6.46
(t, J = 7.4 Hz, 1H), 4.60 (m, 1H), 4.43 (m, 1H), 3.94 (s,
3H), 3.85 (s, 3H), 3.65 (s, 3H), 3.31 (m, 1H), 3.01 (m, 2H),