Concise synthesis of pochonin A, an HSP90 inhibitor

Article abstract of DOI:10.1021/ol052263+

(Chemical Equation Presented) An expedient synthesis of (-)-pochonin A is reported (seven steps). This natural product is closely related to radicicol and was shown to be a 90 nM inhibitor of HSP90.

Full text of DOI:10.1021/ol052263+

ORGANIC
LETTERS
2005
Vol. 7, No. 25
5637-5639
Concise Synthesis of Pochonin A, an
HSP90 Inhibitor
Emilie Moulin, Sofia Barluenga, and Nicolas Winssinger*
Institut de Science et Inge´nierie Supramoleculaires, UniVersite´ Louis Pasteur,
8 alle´e Gaspard Monge, 67000 Strasbourg, France
winssinger@isis.u-strasbg.fr
Received September 18, 2005
ABSTRACT
An expedient synthesis of (−)-pochonin A is reported (seven steps). This natural product is closely related to radicicol and was shown to be
a 90 nM inhibitor of HSP90.
Pochonin A (Figure 1) is part of a new family of resorcyclic
macrolides isolated by Hellwig and co-workers from the
fermentation broths of Pochonia chlamydosporia.1 These
compounds were identified from a high throughput screen
for inhibition of the herpes simplex virus (HSV) helicase
primase, an ATPase-dependent enzyme complex. Our interest
in these compounds stems from the discovery that radicicol2
(Figure 1) is a competitive antagonist of ATP for HSP903
(an ATPase-dependent chaperone) as there is no obvious
structural or topological relationship between the resorcy-
clides and ATP. Interestingly, the pochonins were also shown
to be devoid of the estrogenic activity found with the
structurally related zearalenone.4 The different biological
activity found with the structurally similar resorcyclides may
be in part rationalized by the conformational diversity that
is achievable within this class of compounds as we have
shown for pochonin C versus radicicol.5 Conformational
analysis of radicicol and related analogues led us to speculate
that pochonin D might also be an HSP90 inhibitor, a fact
that was subsequently verified experimentally.6 The high
potential and widespread interest in HSP90 inhibition7,8
recently led us to examine pochonin A.
A direct conversion of pochonin D into pochonin A via
epoxidation (DMDO, mCPBA) was poorly stereoselective
(1) Hellwig, V.; Mayer-Bartschmid, A.; Mueller, H.; Greif, G.; Kley-
mann, G.; Zitzmann, W.; Tichy, H.-V.; Stadler, M. J. Nat. Prod. 2003, 66,
829.
(2) Delmotte, P.; Delmotte-Plaquee, J. Nature 1953, 171, 344.
(3) Roe, S. M.; Prodromou, C.; O’Brien, R.; Ladbury, J. E.; Piper, P.
W.; Pearl, L. H. J. Med. Chem. 1999, 42, 260.
(4) Anke, H. In Fungal Biotechnology; Anke, T., Ed.; Chapman &
Hall: Weinheim, Germany, 1997; p 186.
(5) Barluenga, S.; Moulin, E.; Lopez, P.; Winssinger, N. Chem. Eur. J.
2005, 11, 4935.
Figure 1. Structures of the pochonin family members.
10.1021/ol052263+ CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/16/2005

and did not appear to be improvable due to the lack of
solubility of pochonin D at low temperature. Epoxidation
of the previously reported EOM9-protected pochonin D (8,
Scheme 1) afforded a 1:1 ratio of diastereoisomers 9.
Scheme 2. Synthesis of Pochonin A
Scheme 1. From Bis-EOM Pochonin D to Pochonin A
Deprotection of the EOM group using sulfonic acid resin
(Novabiochem, 70-90 mesh, MP) in methanol led to epoxide
opening (10), as well as conjugate addition of methanol (11).
A screen of other protic acids such as acetic acid or
hexafluoro-2-propanol and Lewis acids such as Sc(OTf)3 did
not yield a suitable solution. We therefore turned our
attention toward a synthesis that would rely on silyl-protected
phenols (Figure 2). Thus, persilylation of benzoic acid 14
esterification with alcohol 15. The low yield obtained was
attributed to the lability of the ortho TBS group, although it
should be noted that these three steps were carried out
without purification of the intermediates and as such this
sequence was quite practical. Deprotonation of the benzylic
methylene followed by reaction with Weinreb amide 13
afforded metathesis precursor 16 in modest yield. Ring-
closing metathesis using second generation Grubbs catalyst11
under thermodynamic conditions12 afforded macrocyle 17
in good yield and excellent cis/trans ratio (<5% cis).
Epoxidation of the unconjugated olefin was optimal when
carried out with methyl(trifluoromethyl)-dioxirane generated
in situ.13 It afforded protected pochonin A in excellent yield
as an inseparable 3:1 diastereomeric mixture. It is interesting
to note that epoxidation with mCPBA or DMDO only
proceeded at room temperature and did not give any
diastereomeric excess. Attempts to further improve the
stereoselectivity of the epoxidation using epoxone14 were not
productive. Deprotection of the products obtained from the
epoxidation afforded a separable diastereomeric mixture and
confirmed that the major product was indeed the desired
pochonin A.15 Although the overall synthetic sequence was
short and could be carried out in only a few days, the poor
yield in the acylation reactions led us to consider alternative
protecting groups. Eventually, we decided to use SEM ethers
Figure 2. General retrosynthetic analysis.
(Scheme 2), followed by “acid-free” conversion of the silyl
ester to the acyl chloride10 yielded key intermediate 12 upon
(6) Moulin, E.; Zoete, V.; Barluenga, S.; Karplus, M.; Winssinger, N. J.
Am. Chem. Soc. 2005, 127, 6999.
(7) Neckers, L.; Neckers, K., Heat-shock protein 90 inhibitors as novel
cancer chemotherapeutics-an update. Exp. Opin. Emerg. Drugs 2005, 10,
137. Workman, P. Curr. Cancer Drug Targets 2003, 3, 297.
(8) For recent examples, see: Shen, G.; Blagg, B. S. Org. Lett. 2005, 7,
2157. Yu, X. M.; Shen, G.; Neckers, L.; Blake, H.; Holzbeierlein, J.; Cronk,
B.; Blagg, B. S. J. Am. Chem. Soc. 2005, 127 (37), 12778. Cheung, K.-M.;
Matthews, T. P.; James, K.; Rowlands, M. G.; Boxall, K. J.; Sharp, S. Y.;
Maloney, A.; Roe, S. M.; Prodromou, C.; Pearl, L. H.; Aherne, G. W.;
McDonald, E.; Workman, P. Bioorg. Med. Chem. Lett. 2005, 15, 3338.
Dymock, B. W. et al J. Med. Chem. 2005, 48, 4212. Llauger, L.; He, H.;
Kim, J.; Aguirre, J.; Rosen, N.; Peters, U.; Davies, P.; Chiosis, G., J. Med.
Chem. 2005, 48, 2892. Kreusch, A.; Han, S.; Brinker, A.; Zhou, V.; Choi,
H.-S.; He, Y.; Lesley, S. A.; Caldwell, J.; Gu, X.-J., Bioorg. Med. Chem.
Lett 2005, 15, 1475. Vilenchik, M.; Solit, D.; Basso, A.; Huezo, H.; Lucas,
B.; He, H.; Rosen, N.; Spampinato, C.; Modrich, P.; Chiosis, G. Chem.
Biol. 2004, 11 (6), 787.
(10) Wissner, A.; Grudzinskas, C. V. J. Org. Chem. 1978, 43, 3972.
(11) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
953. Chatterjee, A. K.; Morgan, J. P.; Scholl, M.; Grubbs, R. H. J. Am.
Chem. Soc. 2000, 122, 3783.
(12) Lee, C. W.; Grubbs, R. H. Org. Lett. 2000, 2, 2145.
(13) Yang, D.; Wong, M.-K.; Yip, Y.-C. J. Org. Chem. 1995, 60 (12),
3887.
(14) Tu, Y.; Wang, Z.-X.; Shi, Y. J. Am. Chem. Soc. 1996, 118, 9806.
Wang, Z. X.; Tu, Y.; Frohn, M.; Shi, Y. J. Org. Chem. 1997, 62, 2328.
(15) 1H NMR of pochinin A was kindly provided by Dr. Stadler (Bayer
Health Care, Wuppertal, Germany).
(9) EOM (ethoxymethyl) because of the easier availability for EOM-Cl
compared to MOM-Cl.
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Org. Lett., Vol. 7, No. 25, 2005

on account of their stability toward basic conditions and their
capacity for deprotection with TBAF (Scheme 3). Thus after
temperature afforded the desired pochonin A and its dias-
tereoisomer as a separable mixture. The yield was limited
by reaction conversion, since longer times generated the
corresponding bromohydrine by opening of the oxirane ring.
It is important to note that in the case of EOM-protected
product 9, treatment with MgBr2 was found to open the
epoxide faster than to deprotect the EOM.
Scheme 3. Synthesis of Bis-SEM Protected Pochonin A
Pochonin A (1) and its diol analogue 10 were evaluated
for HSP90 affinity in a competition assay with geldanamycin
using a previously described assay.16 Interestingly, pochonin
A was a good ligand (90 nM), while analogue 10 was
inactive. The inactivity of diol 10 may be rationalized by
the different conformational profile of this compound relative
to pochonin A or D as was calculated for radicicol and its
diol-analogue.6
The synthesis described herein represents the first synthesis
of pochonin A, thereby assigning a negative specific optical
rotation to this product. It also led to the discovery that
pochonin A is a good ligand for HSP90. The ability of these
compounds to antagonize ATP and inhibit ATPase activity
may be related to their inhibition of HCV helicase. The
difference of selectivity in the epoxidation reaction depending
on the phenol protecting groups may be ascribed to the
conformational impact of the bulky silyl group on the ortho
phenol.
Acknowledgment. A BDI fellowship from CNRS and
region d’Alsace to E.M. is gratefully acknowledged. We
thank Merck Bioscience/Novabiochem for samples of resins
and Xiang-ju Gu and Francisco Adrian for their assistance
with the biological screening. This work was partly supported
by grants from the European Commission (MIRG-CT-2004-
505317), Association pour la Recherche sur le Cancer (ARC)
and is part of the COST network D28.
selective Mitsunobu reaction of benzoic acid 14 and alcohol
19 using polymer-bound DEAD and subsequent protection
with SEM-Cl, ester 20 was isolated in 72% yield.6 Depro-
tonation of 20 followed by addition of Weinreb amide 13
afforded the metathesis precursor 21 in 60% yield. Treatment
of 21 with the second generation Grubbs catalyst under
thermodynamic conditions afforded the corresponding mac-
rocycle in 87% yield (<5% cis olefin), which was epoxidized
under the same conditions as for the TBS-protected com-
pound 17 [methyl(trifluoromethyl)-dioxirane], yielding com-
pound 22 in 83% yield albeit in 1:1 diastereomeric ratio
(inseparable). All attempts to deprotect the SEM groups using
TBAF were unsuccessful, with the formation of coumarin
being the major undesired reaction. To our great delight, we
found that 8 equiv of MgBr2 in dichloromethane at room
Supporting Information Available: Experimental pro-
cedures utilized and compound characterization including
NMR of pochonin A. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL052263+
(16) Zhou, V.; Han, S.; Brinker, A.; Klock, H.; Caldwell, J.; Gu, X.-j.
Anal. Biochem. 2004, 331, 349.
Org. Lett., Vol. 7, No. 25, 2005
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