An improved synthesis of resorcylic acid macrolactone inhibitors of Hsp90

Article abstract of DOI:10.1055/s-0029-1217329

A synthesis of resorcylic acid macrolactone analogues of the natural product radicicol is described in which the key steps are the acylation and ring opening of a homophthalic anhydride to give an isocoumarin, followed by a ring-closing metathesis to form

Full text of DOI:10.1055/s-0029-1217329

LETTER
1567
An Improved Synthesis of Resorcylic Acid Macrolactone Inhibitors of Hsp90
R
esorcylic
a
Acid Mac
m
rolactone Inhibitors
e
of
H
sp90 s E. H. Day, Alexander J. Blake, Christopher J. Moody*
School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
Fax +44(115)9513564; E-mail: c.j.moody@nottingham.ac.uk
Received 23 March 2009
Radicicol (1), also known as monorden, was originally
Abstract: A synthesis of resorcylic acid macrolactone analogues of
the natural product radicicol is described in which the key steps are
the acylation and ring opening of a homophthalic anhydride to give
an isocoumarin, followed by a ring-closing metathesis to form the
macrocycle.
isolated from the fungus Monocillium nordinii over 50
years ago,15 is a member of a larger class of natural prod-
ucts known as resorcylic acid lactones, many of which
also exhibit potent biological activity.16–18 The compound
has attracted the interest of synthetic chemists and has
been the subject of total synthesis by the groups of Lett,19–22
Danishefky,23 and Winssinger.24
Key words: macrocyclic lactones, isocoumarins, ring-closing
metathesis reaction
Recently we reported the synthesis and biological evalua-
tion of a simple analogue of radicicol, NP261 (3), which
not only showed the established molecular signature of
Hsp90 inhibition, that is, depletion of client proteins with
upregulation of Hsp70, but also bound to the protein in a
similar way to the structurally more complex natural prod-
uct.25 Our original approach to the synthesis of radicicol
analogues, such as NP261 (3), was based on Danishefsky
and co-workers’ first-generation synthesis of radicicol it-
self,23 employing a ring-closing metathesis (RCM) reac-
tion to form the macrocycle, a tactic commonly used in
related syntheses.24,26–28 We have now developed a more
versatile route to the RCM precursor that is based on iso-
coumarin chemistry, and incorporates the necessary chlo-
rine from an early stage, thereby avoiding a late stage
chlorination on every analogue.
Heat shock protein 90 (Hsp90) is an ATP-dependent
chaperone that plays a central role in regulating the stabi-
lization, activation, and degradation of a range of client
proteins,1,2 including a number of known over-expressed
or mutant oncogenic proteins such as C-RAF, B-RAF,
ERBB2, AKT, telomerase, and p53, many of which are
associated with the six hallmarks of cancer.3,4 It is one of
the most abundant proteins in eukaryotic cells, and has
emerged as a very attractive target for novel cancer thera-
peutic agents, with several Hsp90 inhibitors having now
entered clinical trial.5–10 Early pioneering work on Hsp90
inhibition was carried out with two natural products,
radicicol (1) and geldanamycin (2, Figure 1), both of
which, in common with most other inhibitors, bind to the
ATP-binding pocket in the N-terminal domain of the pro-
tein.11–14
Our pivotal intermediate was the 5-chlorohomophthalic
anhydride 8, available by adaptation of chemistry reported
by Bauta et al.29 The starting point for the synthesis was
diester 4,29 obtained in two steps from 4-chlororesorcinol,
using the addition of malonate anion to 3,5-dimethoxy-
benzyne and subsequent rearrangement to the homo-
phthalate ester 4, a reaction originally developed by
Danishefsy.30 The removal of both methoxy groups of
diester 4 using aluminium chloride in dichloromethane
proved to be problematic when increasing the scale of the
reaction (>5.0 g). However, using aluminium bromide
(1.2 equiv) to remove the methoxy next to the ester first,
followed by a large excess of aluminium chloride (8.0
equiv) to remove the second methyl group, reproducible
results and a good yield of the resorcylic compound 5 was
achieved (71%). The chlorination of compound 5 was car-
ried out by cooling to low temperature (–30 °C) and using
a slight excess of chlorinating agent (1.1 equiv). At higher
temperatures (–10 °C and 0 °C), double chlorination was
observed. The protection of the chlororesorcinol (6) with
two methoxymethyl (MOM) groups proceeded in excel-
lent yield. Cleavage of both esters using sodium hydrox-
ide and dehydration with acetic anhydride gave the key
homophthalic anhydride intermediate 8 in 59% yield over
two steps (Scheme 1).31
O
MeO
O
OH
O
Me
H
Me
N
H
O
O
O
Me
MeO
Me
H
HO
OH MeO
Me
Cl
O
radicicol (1)
OH
O
OCONH2
geldanamycin (2)
O
HO
Cl
O
NP261 (3)
Figure 1 Structures of the naturally occurring Hsp90 inhibitors ra-
dicicol (1) and geldanamycin (2), together with the simpler radicicol
analogue NP261 (3)
SYNLETT 2009, No. 10, pp 1567–1570
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x
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x
.2
0
0
9
Advanced online publication: 02.06.2009
DOI: 10.1055/s-0029-1217329; Art ID: D07809ST
© Georg Thieme Verlag Stuttgart · New York

1568
J. E. H. Day et al.
LETTER
Our projected conversion of anhydride 8 into an isocou- the ethyl malonyl chloride 10 bearing a pentenyl side
marin suitable for a subsequent RCM reaction to give the chain. This was obtained by monohydrolysis of the corre-
macrolactone 3 (and analogues) required its acylation by sponding malonate 9, itself obtained by alkylation of tri-
ethyl methanetricarboxylate, followed by removal of one
of the ester groups.32 The mono ethyl malonate 9 was con-
OMe
OH
verted into the acid chloride 10 using thionyl chloride.
With the synthesis of both the anhydride 8 and acid chlo-
ride 10 complete, both were subjected to the reaction con-
ditions for formation of the isocoumarin developed by
Bauta et al. (Scheme 2),29 using tetramethylguanidine
(TMG) to effect the initial acylation. In this process, the
initial product 11 of acylation undergoes further base-
mediated cyclization to give 12, ring opening of which
followed by loss of carbon dioxide gives the isocoumarin
(13, Scheme 2). The presence of two MOM-protected
phenols and the chlorine adjacent to the reaction center
had no adverse effect, and a good yield of the isocoumarin
(13) was obtained (75%).33
CO2Et
CO2Et
Cl
a, b
MeO
HO
4
c
OH
OH
d
CO2Et
CO2Et
CO2Et
CO2Et
HO
HO
5
Cl
6
e
Next, efforts turned to the conversion of isocoumarin (13)
into radicicol analogue NP261 (3), and started with the
hydrolysis of the ester – lactone. It was found that this
could be achieved using an excess of lithium hydroxide
(20 equiv, Scheme 3), and resulted in hydrolysis of the
lactone and ester moieties with concomitant decarboxyla-
tion of the resulting b-keto acid fragment to give the de-
sired keto acid 14 directly without the need for a separate
decarboxylation step. The keto acid 14 was used directly
for the Mitsunobu reaction [diisopropyl azodicarboxylate
MOMO
MOMO
O
CO2Et
CO2Et
f
O
MOMO
MOMO
O
Cl
Cl
7
8
Scheme 1 Reagents and conditions: (a) Me2SO4, aq NaOH, heptane
(88%); (b) diethyl malonate, LDA, THF, –78 °C (57%); (c) (i) AlBr3,
CH2Cl2, r.t. to 45 °C, (ii) AlCl3, r.t. (71%); (d) SO2Cl2, THF, –30 °C
(81%); (e) MOMCl, i-Pr2NEt, DMF, 0 °C (85%); (f) (i) NaOH, THF–
MeOH–H2O, 70 °C, (ii) Ac2O, PhMe, 125 °C (59% over two steps).
MOMO
O
MOMO
O
MOMO
O
a
O
OH
XOC
CO2Et
CO2Et
O
c
MOMO
MOMO
Cl
Cl
MOMO
O
13
O
O
Cl
14
b
O
9 X = OEt
a, b
CO2Et
10 R = Cl
11
MOMO
MOMO
O
base
R
c
O
O
MOMO
O
R
MOMO
EtO2C
R
MOMO
O
O
O–
O
Cl
Cl
CO2Et
O
O
MOMO
15a R = H (67%, 2 steps)
15b R = Me (65%, 2 steps)
16a R = H (69%)
16b R = Me (92%)
Cl
O–
O
O
12
d
– CO2
OH
Cl
O
MOMO
O
O
R
O
CO2Et
HO
MOMO
Cl
O
3 R = H (72%)
17 R = Me (60%)
13
Scheme 3 Reagents and conditions: (a) LiOH, THF–MeOH–H2O,
r.t.; (b) H2C=CHCHRCH2OH, Ph3P, DIAD, PhMe, r.t.; (c) Grubbs II
catalyst (5 mol%), CH2Cl2, 45 °C; (d) HCl, dioxane, r.t.
Scheme 2 Reagents and conditions: (a) KOH, EtOH, 0 °C to r.t.
(98%); (b) SOCl2, 70 °C (not purified); (c) 8, TMG, MeCN, 0 °C, then
Et3N, 0 °C to r.t. (75%).
Synlett 2009, No. 10, 1567–1570 © Thieme Stuttgart · New York

LETTER
Resorcylic Acid Macrolactone Inhibitors of Hsp90
1569
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Figure 2 X-ray crystal structure of the ( )-resorcylic macrolactone
1735
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(DIAD), Ph3P and toluene] with 3-butenol and its 2-meth-
yl derivative, which gave the esters 15a and 15b in rea-
sonable yield (65–67%). The critical ring-closing
metathesis using Grubbs second-generation catalyst {ben-
zylidene[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinyl-
idene]dichloro(trichlorocyclohexylphosphine)ruthenium}
proceeded in good yield to give the macrolactones 16 iso-
lated as the E-isomer after chromatography. Cleavage of
the MOM groups could be achieved in good yield under
acidic conditions to give the previously synthesized
NP261 (3) and its novel methyl analogue, the resorcylic
macrolactone 17 (Scheme 3), the structure of which was
confirmed by X-ray crystallography (Figure 2).
(30) Shair, M. D.; Yoon, T. Y.; Mosny, K. K.; Chou, T. C.;
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(31) 5-Chloro-6,8-bis(methoxymethoxy)isochroman-1,3-
dione (8)
Mp 96–98 °C. MS: m/z calcd for C13H1335ClNaO7: 339.0242;
found: 339.0229 [M + Na]. IR (CH2Cl2): nmax = 1796, 1756,
1593 cm–1. 1H NMR (400 MHz, CDCl3): d = 7.18 (1 H, s,
ArH), 5.48 (2 H, s, OCH2O), 5.39 (2 H, s, OCH2O), 4.20 (2
Thus, we have established a new route to the Hsp90 inhib-
itor 3 and its new analogue 17,34 that proceeds in an elev-
en-step linear sequence in an overall yield of 4%.
H, s, CH2), 3.53 (3 H, s, OCH3), 3.52 (3 H, s, OCH3). 13
C
NMR (100 MHz, acetone-d6): d = 165.3 (C), 160.6 (C),
159.2 (C), 157.1 (C), 137.7 (C), 114.6 (C), 107.1 (C), 103.4
(CH), 96.3 (CH2), 96.1 (C), 57.0 (CH3), 56.9 (CH3), 34.2
(CH2). ESI-MS: m/z (%) = 341/339 (23/69) [M + Na], 327
(14), 309 (15).
Acknowledgment
This work was supported by Cancer Research UK [CUK] grant
number C215/A7606.
(32) Shimamoto, T.; Chimori, M.; Sogawa, H.; Yamamoto, K.
J. Am. Chem. Soc. 2005, 127, 16410.
(33) Ethyl 2-[5-Chloro-6,8-bis(methoxymethoxy)-1-oxo-1H-
References and Notes
isochromen-3-yl]hept-6-enoate (13)
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Opin. Pharm. 2008, 8, 370.
(9) Drysdale, M. J.; Brough, P. A. Curr. Top. Med. Chem. 2008,
8, 859.
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17, 2225.
(11) Prodromou, C.; Roe, S. M.; O’Brien, R.; Ladbury, J. E.;
Piper, P. W.; Pearl, L. H. Cell 1997, 90, 65.
Mp 50–52 °C. MS: m/z calcd for C22H2735ClNaO8: 477.1292;
found: 477.1326 [M + Na]. IR (CH2Cl2): nmax = 2987, 1736,
1661, 1585 cm–1. 1H NMR (400 MHz, acetone-d6): d = 7.15
(1 H, s, ArH), 6.85 (1 H, s, ArH), 5.89–5.77 (1 H, m,
CH=CH2), 5.45 (2 H, s, OCH2O), 5.34 (2 H, s, OCH2O),
5.05–4.93 (2 H, m, CH=CH2), 4.18 (2 H, q, J = 7.1 Hz,
CO2CH2CH3), 3.66 (1 H, t, J = 7.5 Hz, CHCH2), 3.51 (3 H,
s, OCH3), 3.51 (3 H, s, OCH3), 2.13 (2 H, dt, J = 7.1, 7.5 Hz,
CH2), 2.00–1.89 (2 H, m, CH2), 1.49 (2 H, quin, J = 7.1,
CH2), 1.22 (2 H, q, J = 7.1 Hz, CO2CH2CH3). 13C NMR (100
MHz, CDCl3): d = 171.1 (C), 160.5 (C), 159.2 (C), 157.2
(C), 158.15 (C), 139.1 (CH), 138.6 (C), 115.3 (CH2), 110.9
(C), 105.9 (C), 103.7 (CH), 101.2 (CH), 96.4 (CH2), 96.0
(CH2), 61.8 (CH2), 56.9 (CH3), 56.8 (CH3), 50.5 (CH), 34.0
(CH2), 27.2 (CH2), 14.5 (CH3), 1 × CH2 unobserved. ESI-
MS: m/z (%) = 475/477 (36/100) [M + Na].
Synlett 2009, No. 10, 1567–1570 © Thieme Stuttgart · New York

1570
J. E. H. Day et al.
LETTER
(34) (E)-1-Chloro-2,4-dihydroxy-7,8,11,12,13,14-hexahydro-
6-oxa-16H-benzocyclotetradecene-5,15-dione (3)
Mp 174–176 °C (lit.25 mp 168–169 °C). MS: m/z calcd for
C17H1935ClNaO5: 361.0813; found: 361.0817 [M + Na]. IR
(CH2Cl2): nmax = 3722, 3156, 1712, 1659, 1603 cm–1. 1H
NMR (300 MHz, CDCl3): d = 11.80 (1 H, s, OH), 6.61 (1 H,
s, ArH), 6.17 (1 H, s, OH), 5.56–5.39 (2 H, m, CH=CH),
4.44 (2 H, t, J = 5.3 Hz, CO2CH2), 4.29 (2 H, s, CH2), 2.58
(2 H, t, J = 6.5 Hz, CH2), 2.44 (2 H, dt, J = 5.3, 5.3 Hz, CH2),
2.10 (2 H, dt, J = 6.2, 4.8 Hz, CH2), 1.71 (2 H, quin, J = 6.2
Hz, CH2), 1.58 (2 H, m, CH2). 13C NMR (75 MHz, CDCl3):
d = 206.2 (C), 170.7 (C), 163.0 (C), 156.4 (C), 136.2 (CH),
134.1 (C), 126.8 (C), 114.7 (C), 107.3 (CH), 65.8 (CH2),
46.8 (CH2), 41.0 (CH2), 32.2 (CH2), 31.54 (CH2), 25.5
(CH2), 22.1 (CH2). ESI-MS: m/z (%) = 363/361 (31/100) [M
+ Na], 341/339 (4/12).
(E)-1-Chloro-2,4-dihydroxy-7,8,11,12,13,14-hexahydro-
8-methyl-6-oxa-16H-benzocyclotetradecene-5,15-dione
(17)
Mp 163–164 °C. MS: m/z calcd for C18H2135ClNaO5:
375.0970; found: 375.0959 [M + Na]. IR (CH2Cl2): nmax
=
3686, 3512, 1720, 1658, 1604 cm–1. 1H NMR (400 MHz,
CDCl3): d = 11.78 (1 H, s, OH), 6.63 (1 H, s, ArH), 6.00 (1
H, br s, OH), 5.46 (1 H, dt, J = 15.6, 7.5 Hz, =CH), 5.34 (1
H, dt, J = 15.6, 7.3 Hz, =CH), 4.27 (1 H, dd, J = 10.7, 3.0 Hz,
CO2CH), 4.28 (2 H, s, CH2), 3.99 (1 H, t, J = 10.7 Hz,
CO2CH), 2.58 (3 H, m, CH2, CH), 2.13 (3 H, m, CH2, CH),
1.72 (3 H, m, CH2, CH), 1.04 (3 H, d, J = 7.0 Hz, CH3). 13
C
NMR (125 MHz, CDCl3): d = 205.7 (C), 171.8 (C), 163.8
(C), 158.9 (C), 138.3 (C), 134.3 (CH), 132.5 (CH), 115.9
(C), 107.8 (C), 103.6 (CH), 71.1 (CH2), 47.1 (CH2), 41.2
(CH2), 36.7 (CH), 32.7 (CH2), 26.6 (CH2), 23.0 (CH2), 17.4
(CH3). ESI-MS: m/z (%) = 377/375 (33/100) [M + Na], 355/
353 (8/23), 335/337 (31/10).
(35) Crystallographic data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge, CB21EZ, UK; fax: +44 (1223)336033; or
deposit@ccdc.cam.ac.uk].
Synlett 2009, No. 10, 1567–1570 © Thieme Stuttgart · New York

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