Total synthesis and biological evaluation of verticipyrone and analogues

Article abstract of DOI:10.1021/ol0626140

(Chemical Equation Presented) Total synthesis of verticipyrone, a novel NADH-fumarate reductase inhibitor, has been accomplished by a convergent approach using novel "Reverse Julia olefination" method. During total synthetic studies, we also prepared and evaluated several synthetic verticipyrone analogues, some of which exhibited more potent antiparasitic activity than the natural verticipyrone.

Full text of DOI:10.1021/ol0626140

ORGANIC
LETTERS
2007
Vol. 9, No. 1
65-67
Total Synthesis and Biological
Evaluation of Verticipyrone and
Analogues
Hiroyuki Shimamura,† Toshiaki Sunazuka,†,‡,§ Takashi Izuhara,†
Tomoyasu Hirose,§ Kazuro Shiomi,†,‡,§ and Satoshi Omura*,†,‡,§
h
School of Infection Control Sciences and Kitasato Institute for Life Sciences, Kitasato
UniVersity, and The Kitasato Institute, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641,
Japan
omura-s@kitasato.or.jp
Received October 24, 2006
ABSTRACT
Total synthesis of verticipyrone, a novel NADH-fumarate reductase inhibitor, has been accomplished by a convergent approach using novel
“Reverse Julia olefination” method. During total synthetic studies, we also prepared and evaluated several synthetic verticipyrone analogues,
some of which exhibited more potent antiparasitic activity than the natural verticipyrone.
In the course of our screening for NADH-fumarate reductase
(NFRD)1 inhibitors, verticipyrone (1) was isolated from the
fermentation broth of a fungal strain, Verticillium sp. FKI-
1083.2 Verticipyrone (1) inhibited NFRD of Ascaris suum
with an IC50 value of 4.1 nM and showed selective inhibition
against its target enzyme, complex I, of helminth mitochon-
dria. Furthermore, verticipyrone (1) showed anthelmintic
activity against Caenorhabditis elegans and Artemia salina.
It therefore may become a good candidate as a new
antiparasitic agent. These useful biological activities of 1
attracted our attention and recently prompted us to undertake
a total synthesis study.3 In this paper, we wish to report the
total synthesis and biological evaluation of verticipyrone and
its analogues.
bination of pyrones and side chains. Our synthetic plan is
shown in Scheme 1. Verticipyrone (1) would be obtained
Scheme 1. Retrosynthetic Analysis of Verticipyrone (1)
We envisioned developing a convergent synthesis of 1 that
would provide verticipyrone analogues with varying com-
* Corresponding author.
† School of Infection Control Sciences, Kitasato University.
‡ Kitasato Institute for Life Sciences, Kitasato University.
§ The Kitasato Institute.
(1) For recent reviews, see: (a) Komuniecki, R.; Tielens, A. G. M. In
Molecular Medical Parasitology; Marr, J. J., Nilsen, T. W., Komuniecki,
R. W., Eds.; Academic Press: London, 2003; Chapter 14, p 339. (b) Kita,
K.; Nihei, C.; Tomitsuka, E. Curr. Med. Chem. 2003, 10, 2535. (c) Shiomi,
K.; Ohmura, S. Proc. Jpn. Acad., Ser. B 2004, 80, 245.
via aldol reaction between γ-pyrone 3 and aldehyde 4
followed by regioselective dehydration of the resulting
alcohol 2.
10.1021/ol0626140 CCC: $37.00
© 2007 American Chemical Society
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On the basis of this synthetic plan, we initially prepared
γ-pyrone 3 as follows (Scheme 2). Known â-ketoacid 54 was
oxidation10 of resulting alcohol 12 gave the desired aldehyde
4 quantitatively.
Aldol reaction between γ-pyrone 3 and aldehyde 4 with
LHMDS gave aldol adduct 2 in 74% yield (Scheme 4). The
Scheme 2. Synthesis of γ-Pyrone (3)
Scheme 4. Aldol Reaction of γ-Pyrone (3) and Aldehyde (4)
treated with mixture of acetone and Ac2O in the presence of
5
catalytic amount of concd H2SO4 to give the dioxinone 6
desired nonconjugated olefin of 1 from aldol adduct 2 was
unsuccessful, though elimination of mesylate under basic
conditions and thermal decomposition of xanthate were
attempted. Further isomerization11 of 13 into 1 was also
unsuccessful.
in 92% yield. Dioxinone 6 was converted into ketone 7 in
64% yield for two steps by aldol reaction with acetaldehyde
followed by oxidation6 of the resulting alcohol under Swern
conditions.7 Ketone 7 was then treated with sodium meth-
oxide in methanol to give the γ-hydroxy-R-pyrone 8 in 83%
yield via deprotection of the acetonide group and successive
cyclization of the resulting diketoester. Regioselective O-
methylation of 8 with methylfluorosulfonate8 provided
R-methoxy-γ-pyrone 3 in 93% yield.
Although aldol adduct 2 could not be converted to the
desired non-conjugated olefin, the aldol reaction between
γ-pyrone and aldehyde was useful for SAR studies on
verticipyrone libraries. This observation prompted us to
design an appropriate aldehyde to selectively generate a
nonconjugated olefin after aldol reaction. Therefore, R-phe-
nylsulfonyl aldehyde 16 was designed as it would allow
reductive elimination of â-hydroxysulfone via a novel
“Reverse Julia olefination” process after aldol reaction.
R-Phenylsulfonyl ester 14 was alkylated sequentially to
give the R-dialkyl ester 15 in 82% overall yield (Scheme
5). Reduction of the ethyl ester by DIBAL afforded R-phe-
nylsulfonyl aldehyde 16 in 99% yield. Coupling the γ-pyrone
3 and aldehyde 16 using LDA gave the Julia hydroxysulfone
17 in 74% yield. Finally, acetylation and reductive elimina-
tion afforded nonconjugated olefin 1 (59%) and its Z-isomer
18 (33%). Synthetic verticipyrone (1) was identical to the
natural product in all respects (1H and 13C NMR, IR, FAB-
MS, and inhibitory activity against NFRD).
The side chain aldehyde 4 was prepared as illustrated in
Scheme 3. Alkylation of diethyl methylmalonate 9 with NaH
Scheme 3. Synthesis of Aldehyde (4)
(4) Tirpak, R. E.; Olsen, R. S.; Rathke, M. W. J. Org. Chem. 1985, 50,
4877.
(5) Sato, M.; Ogasawara, H.; Oi, K.; Kato, T. Chem. Pharm. Bull. 1983,
31, 6.
(6) Direct acetylation of 6 was unsuccessful and gave only O-acetylated
product.
(7) Omura, K.; Swern, D. Tetrahedoron 1978, 34, 1651.
(8) Beak, P.; Lee, J.; McKinnie, G. J. Org. Chem. 1978, 43, 1367.
(9) Krapcho, A. P.; Weimaster, J. F.; Eldridge, J. M.; Jahngen, E. G. E.,
Jr.; Lovey, A. J.; Stephens, W. P. J. Org. Chem. 1978, 43, 138.
(10) Parikh, J. R.; Doering, W. v. E. J. Am. Chem. Soc. 1967, 89,
5505.
(11) Fletcher, S. R.; Baker, R.; Chambers, M. S.; Herbert, R. H.; Hobbs,
S. C.; Thomas, S. R.; Verrier, H. M.; Watt, A. P.; Ball, R. G. J. Org. Chem.
1994, 59, 1771.
and iodooctane gave 10 quantitatively. Decarboxylation9 of
10 with LiCl in aqueous DMSO solution afforded 11 in 86%
yield. Reduction of the ethyl ester 11 by LiAlH4 and
(2) Ohmura, S.; Shiomi, K.; Masuma, R. PCT WO 2003050104, 2004.
(3) Recent synthetic approach for the other complex I inhibitors: (a)
Ichimaru, N.; Abe, M.; Murai, M.; Senoh, M.; Nishioka, T.; Miyoshi, H.
Bioorg. Med. Chem. Lett. 2006, 16, 3555. (b) Schnermann, M. J.; Romero,
F. A.; Hwang, I.; Nakamaru-Ogiso, E.; Yagi, T.; Boger, D. L. J. Am. Chem.
Soc. 2006, 128, 11799.
66
Org. Lett., Vol. 9, No. 1, 2007

Scheme 5. Total Synthesis of Verticipyrone (1)
Figure 1. Inhibitory activity of synthetic 1 and analogues against
NFRD.
and 3,5-dimethyl groups on a γ-pyrone moiety were an
essential factor for inhibitory activity against NFRD. The
γ-pyrone 3, however, showed no activity. Thus, the long side
chain was also important for the inhibitory activity against
NFRD.
In conclusion, we have achieved a concise total synthesis
of verticipyrone (1) using a reverse Julia olefination method.
We have also synthesized some analogues and established a
SAR profile, and found more potent active analogues, 2 and
20, than the natural product. Additional investigation of the
SAR and biological studies of verticipyrone are currently
underway.
We then evaluated the NFRD inhibitory activities of
several synthetic analogues13,14 of 1 in order to establish a
SAR profile (Figure 1). The assay method for the inhibitory
activities (IC50) against A. suum NFRD was the same as that
reported previously.15
First, olefin isomers 13 and 18 and alkyl side chain
analogue 19 showed similar NFRD inhibitory activities to
1. This result suggested that the olefin in the side chain was
not particularly important to activity. Fortunately, alcohol
20 showed 10-fold greater potency than 1. Furthermore,
alcohol 2 also showed potent activity. This suggested that
the newly introduced hydroxyl group on the side chain might
contribute to the observed activity. Surprisingly, analogue
21 showed no inhibitory activity. This finding suggested that
the most important active center was the γ-pyrone moiety,
Acknowledgment. This work was supported by the Grant
of 21st Century COE Progam, Ministry of Eduction, Culture,
Sports, Science and Technology. We also thank Ms. Naka-
gawa, Ms. C. Sakabe, and Ms. N. Sato (School of Pharma-
ceutical Sciences, Kitasato University) for various instru-
mental analyses.
(12) Julia, M.; Paris, J. M. Tetrahedron. Lett. 1973, 49, 4836.
(13) Syntheses of the verticipyrone anlogues are described in the
Supporting Information.
Supporting Information Available: Experimental pro-
cedures and spectral data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(14) The analogues 2 and 21 were evaluated as a diastereomeric mixture.
The diastereomeric ratio of both 2 and 21 was approximately 2:1, determined
1
from H NMR.
(15) Shiomi, K.; Ui, H.; Suzuki, H.; Hatano, H.; Nagamitsu, T.; Takano,
D.; Miyadera, H.; Yamashita, T.; Kita, K.; Miyoshi, H.; Achim, H.; Tomoda,
H.; Ohmura, S. J. Antibiot. 2005, 58, 50.
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