Total synthesis of (+)-stachyflin: A potential anti-influenza A virus agent

Article abstract of DOI:10.1039/c000193g

The first enantioselective total synthesis of (+)-stachyflin, a potential anti-influenza A virus agent, was achieved; the method features a BF ₃·Et₂O-induced domino epoxide-opening/ rearrangement/cyclization reaction to stereoselecti

Full text of DOI:10.1039/c000193g

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Total synthesis of (+)-stachyflin: a potential anti-influenza A virus
agentw
Kazuhiro Watanabe, Junji Sakurai, Hideki Abe and Tadashi Katoh*
Received (in Cambridge, UK) 6th January 2010, Accepted 4th March 2010
First published as an Advance Article on the web 1st April 2010
DOI: 10.1039/c000193g
The first enantioselective total synthesis of (+)-stachyflin,
a potential anti-influenza A virus agent, was achieved; the
method features a BF3ꢀEt2O-induced domino epoxide-opening/
rearrangement/cyclization reaction to stereoselectively form the
requisite pentacyclic ring system in one step.
ABCDE ring system was built step by step starting from
2,3-dimethylcyclohexanone, which corresponds to the B ring
of 1. We have recently demonstrated the feasibility of our
synthetic strategy towards (+)-1 through construction of the
tetracyclic model compound (ABCD ring system).9 Herein, we
have described the realization of the first enantioselective total
synthesis of (+)-1.
Influenza has always remained a major health problem. The
emergence of the fatal H5N1 avian influenza virus and the
recent outbreak of the new H1N1 human influenza virus have
raised worldwide concerns regarding the development of
alternative and more efficacious anti-influenza agents.1
Scheme 1 outlines our synthetic plan for (+)-1 based on a
novel and expeditious approach: synthesizing the target
molecule 1 from epoxide 2 through an acid-induced domino
epoxide-opening/rearrangement/cyclization reaction (see 2 -
[I - II - III] - 19a,b in Scheme 4) to stereoselectively
construct the requisite ABCDE ring system with cis-fused AB-
and BC-ring junctions in one step. Epoxide 2 possessing the
trans-fused decalin system, in turn, could be prepared through
stereocontrolled reductive alkylation of the known enone 310
with bromide 4 accessible from 3,5-dihydroxybenzoic acid (5).
We first pursued the synthesis of bromide 4 starting from
commercially available 5 (Scheme 2). The Kolbe–Schmitt
reaction of 5 followed by methyl esterification produced
dimethyl ester 6, which was then converted to acetonide 7
via a three-step sequence involving selective reduction of the
C4 methyl ester function, acetonide formation and methyl
etherification. Subsequent regioselective bromination at C2 in
7 followed by reaction with cuprous cyanide provided nitrile 8.
Isoindolinone 9 was elaborated by hydrogenation of 8 and
subsequent lactamization with a base. After protection of the
lactam amide moiety in 9,11 the resulting 3,4DMB amide 1012
Stachyflin (1, Scheme 1) is a novel sesquiterpenoidal alkaloid
isolated from a culture broth of Stachybotrys sp. RF-7260 by
the Shionogi research group.2 This substance was found to
exhibit extremely potent antiviral activity against influenza
A/WSN/33 (H1N1) virus (IC50 = 0.003 mM) with a unique
mode of action.3,4 The antiviral action of 1 has been shown to
inhibit the hemagglutinin (HA)-mediated fusion process
between the viral envelop and the endosome constituting the
cell membrane, which is an essential step in the entry of the
influenza virus into host cells.5 This mechanism is quite
different from that of the approved anti-influenza virus agents
such as amantadine (Symmetrels), zanamivir (Relenzas) and
oseltamivir (Tamiflus).6 Stachyflin, therefore, is anticipated to
be a promising candidate or a new lead for novel molecular-
targeted anti-influenza A virus agents.6
The gross structure and relative stereochemistry of 1 has
been determined by extensive spectroscopic studies (COSY,
HMBC and NOESY spectra) and X-ray crystallographic
analysis.2a,b This natural product possesses a novel pentacyclic
3H-naphtho[10,80a:5,6]pyrano[2,3-e]isoindol-3(2H)-one skeleton
(ABCDE ring system) involving five asymmetric carbons, with
the special characteristic features of cis-fused AB and BC
rings, and an ether bond at the bridgehead of the AB ring
junction.2a,b The absolute configuration of 1 was determined
by circular dichroism (CD) spectrum as depicted in Scheme 1.2b
These desirable biological properties and unique structural
features prompted us to undertake a project directed towards
the total synthesis of (+)-1 in an enantiomerically pure form.7
To date, only one total synthesis of racemic (ꢁ)-1 has been
reported by the Shionogi research group,8 wherein the
Laboratory of Synthetic Medicinal Chemistry, Department of
Chemical Pharmaceutical Science, Tohoku Pharmaceutical University,
4-4-1 Komatsushima, 981-8558, Aoba-ku, Sendai, Japan.
E-mail: katoh@tohoku-pharm.ac.jp; Fax: +81-22-727-0135;
Tel: +81-22-727-0135
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds along
Scheme 1 Synthetic plan for (+)-stachyflin (1). 3,4DMB = 3,4-
dimethoxybenzyl, TBS = tert-butyldimethylsilyl.
1
with copies of H and 13C NMR spectra. See DOI: 10.1039/c000193g
ꢂc
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Scheme 2 Synthesis of bromide 4. Reagents and conditions: (a) CO2,
KHCO3, glycerol, 180 1C; (b) Me2SO4, KHCO3, acetone, reflux, 64%
(2 steps); (c) NaBH4, THF–H2O, rt; (d) 2,2-dimethoxypropane,
p-TsOH, rt; (e) MeI, K2CO3, acetone, rt, 79% (3 steps); (f) NBS,
MeCN, 0 1C; (g) CuCN, DMF, 120 1C, 66% (2 steps); (h) H2, PtO2,
EtOH–CHCl3, rt; (i) NaOMe, MeOH, rt, 98% (2 steps); (j) 3,4-
dimethoxybenzyl chloride, NaN(SiMe3)2, Bu4NI, THF, 0 1C to rt,
78%; (k) 2 M HCl, THF, rt; (l) TBSCl, imidazole, DMF, rt, 75%
(2 steps); (m) 48% aq HF, MeCN, 0 1C; (n) PPh3, CBr4, CH2Cl2, rt,
86% (2 steps). p-TsOH = p-toluenesulfonic acid, NBS = N-bromo-
succinimide, DMF = N,N-dimethylformamide.
was transformed to bis-TBS ether 11 by acetonide cleavage
and silylation of the two liberated hydroxy groups. Subsequent
regioselective desilylation of 11 followed by bromination of
the resulting benzylic alcohol provided the requisite bromide
4.
After synthesizing bromide 4, we performed the synthesis of
epoxide 2, the substrate for the key acid-induced domino
reaction (Scheme 3). The critical reductive alkylation of enone
310 (>99% ee) with 4 under Birch conditions proceeded
smoothly and cleanly.13 The expected coupling product 12
was obtained as the single diastereomer in 76% yield.
Deprotection of the TBS group in 12 followed by Wittig
methylenation of the resulting hemiacetal 13 produced
exo-olefin 14. To establish the C8 stereogenic center, the
ethylene acetal moiety in 14 was first removed by acid treatment,
and the resulting ketone 15 was subjected to hydrogenation
[H2 (1 atm), 10% Pd/C, Et3N/MeOH 50 : 1, rt], which afforded
the desired product 16 in 96% yield as the single diastereomer.
Subsequent Wittig methylenation of 16 provided exo-olefin 17,
which was then converted to endo-olefin 18 by isomerization of
the olefinic double bond. Finally, epoxidation of 18 with
mCPBA afforded the desired epoxide 2 in 86% yield as an
inseparable mixture of a- and b-epoxides (7 : 1 by 400 MHz
1H NMR).
Scheme 3 Synthesis of epoxide 2. Reagents and conditions: (a) Li, liq.
NH3/THF, ꢃ78 to ꢃ30 1C; isoprene, H2O, 4, ꢃ30 1C to rt, 76%;
(b) TBAF, THF, 0 1C to rt, 88%; (c) Ph3P+CH3Brꢃ, t-BuOK, benzene,
reflux, 86%; (d) 4 M HCl, THF, rt, 98%; (e) H2 (1 atm), 10% Pd/C,
Et3N/MeOH 50 : 1, rt, 96%; (f) Ph3P+CH3Brꢃ, t-BuOK, benzene,
reflux, 74%; (g) RhCl3ꢀ3H2O, EtOH, reflux, 100%; (h) mCPBA,
NaHCO3, CH2Cl2, ꢃ40 to ꢃ5 1C, 86% (a-/b-epoxide 7 : 1). TBAF =
tetrabutylammonium fluoride, mCPBA = m-chloroperbenzoic acid.
carbocation intermediates such as I, II and III.15 Thus, the first
coordination-activation between the Lewis acid and the
epoxide moiety of 2 would lead to an epoxide ring-opening
and the formation of intermediate I, which would further
produce intermediate II via migration of the C5 methyl group
to the C4 carbocation centre. Intermediate II would undergo a
1,2-hydride shift from the C10 position to the C5 carbocation
centre on the a-face of the molecule to provide intermediate
III, wherein the C10 carbocation centre would be trapped by
the inner phenolic hydroxy group to deliver, after the elimination
of the Lewis acid, the desired cyclized products 19a,b.
With epoxide 2 in hand, the stage was set for the most
crucial acid-induced domino epoxide-opening/rearrangement/
cyclization event (Scheme 4). This domino reaction was
successfully performed by treating 2 (a-/b-epoxide 7 : 1) with
BF3ꢀEt2O14 (10 equiv) in CH2Cl2 at ꢃ40 1C to room temperature
for 2 h, which resulted in the formation of the desired
products—19a (C3 a–OH) and 19b (C3 b–OH), which were
readily separated by silica gel column chromatography
(66% yield for 19a, 9% yield for 19b). We believe that this
domino sequence proceeds in a stepwise manner through the
To continue the synthesis, inversion of the configuration at
the C3 hydroxy group in 19a was performed by Dess–Martin
oxidation (94% yield) followed by lithium tri-tert-butoxy-
aluminium hydride reduction of the resulting ketone 20 with
complete stereoselectivity, providing the desired 19b (96%
yield). Finally, compound 19b was converted to (+)-stachyflin
(1)16 via a two-step operation involving deprotection of
the 3,4DMB group using a hypervalent iodine(III) reagent17
ꢂc
This journal is The Royal Society of Chemistry 2010
4056 | Chem. Commun., 2010, 46, 4055–4057

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Scientific Research (C) (No. 18590013 and No. 21590018) and
a Grant-in-Aid for High Technology Research Program from
the Ministry of Education, Culture, Sports, Science and
Technology of Japan (MEXT).
Notes and references
1 For a review on influenza virus infections, see: G. Neumann,
T. Noda and Y. Kawaoka, Nature, 2009, 459, 931–939.
2 (a) T. Kamigauchi, T. Fujiwara, H. Tani, Y. Kawamura and
I. Horibe, Shionogi & Co., Ltd., Japan, PCT WO 9711947 A1,
April 3, 1997; (b) K. Minagawa, S. Kouzuki, J. Yoshimoto,
Y. Kawamura, H. Tani, T. Iwata, Y. Terui, H. Nakai, S. Yagi,
N. Hattori, T. Fujiwara and T. Kamigauchi, J. Antibiot., 2002, 55,
155–164; (c) K. Minagawa, S. Kouzuki and T. Kamigauchi,
J. Antibiot., 2002, 55, 165–171.
3 It has been reported that the anti-influenza A virus activity of 1 is
1760 times more than that of amantadine (IC50 = 5.3 mM) and 250
times more than that of zanamivir (IC50 = 0.75 mM), see ref. 2b.
4 The in vivo anti-influenza virus activity of 1 has been also reported;
see: (a) J. Yoshimoto, S. Yagi, J. Ono, K. Sugita, N. Hattori,
T. Fujioka, T. Fujiwara, H. Sugimoto and N. Hashimoto,
J. Pharm. Pharmacol., 2000, 52, 1247–1255; (b) S. Yagi, J. Ono,
J. Yoshimoto, K. Sugita, N. Hattori, T. Fujioka, T. Fujiwara,
H. Sugimoto, K. Hirano and N. Hashimoto, Pharm. Res., 1999,
16, 1041–1046.
5 (a) J. Yoshimoto, M. Kakui, H. Iwasaki, H. Sugimoto,
T. Fujiwara and N. Hattori, Microbiol. Immunol., 2000, 44,
677–685; (b) J. Yoshimoto, M. Kakui, H. Iwasaki, T. Fujiwara,
H. Sugimoto and N. Hattori, Arch. Virol., 1999, 144, 865–878.
6 For a recent review on the strategies for discovery of anti-influenza
virus agents, see: H.-P. Hsieh and J. T.-A. Hsu, Curr. Pharm. Des.,
2007, 13, 3531–3542.
7 It has been reported that the anti-influenza A virus activity of
racemic (ꢁ)-1 is about half of that of natural (+)-1 (see ref. 8),
implying that unnatural (–)-1 shows no potent anti-influenza A
virus activity.
8 T. Taishi, S. Takechi and S. Mori, Tetrahedron Lett., 1998, 39,
4347–4350.
9 M. Nakatani, M. Nakamura, A. Suzuki, M. Inoue and T. Katoh,
Org. Lett., 2002, 4, 4483–4486.
10 T. Oguchi, K. Watanabe, K. Ohkubo, H. Abe and T. Katoh,
Chem.–Eur. J., 2009, 15, 2826–2845.
Scheme 4 Synthesis of (+)-stachyflin (1) through the key BF3ꢀEt2O-
induced domino epoxide-opening/rearrangement/cyclization reaction
of 2. Reagents and conditions: (a) BF3ꢀEt2O, CH2Cl2, ꢃ40 1C to rt,
66% for 19a, 9% for 19b; (b) Dess–Martin periodinane, CH2Cl2, rt,
94%; (c) LiAlH(t-BuO)3, THF, ꢃ20 1C, 96%; (d) PIFA, CH2Cl2, rt,
54%; (e) n-BuSLi, HMPA, 110 1C, 80%. PIFA = [bis(trifluoro-
acetoxy)iodo]benzene, HMPA = hexamethylphosphorous triamide.
11 Protection of the lactam amide was indispensable due to the low
solubility of 9 in organic solvents.
12 After screening several protecting groups, the 3,4DMB group was
found to be suitable for a later deprotection step (see 19b - 21 in
Scheme 4).
13 This type of reductive alkylation has been successfully utilized as a
crucial step in our total synthesis of marine sesquiterpene quinones
and hydroquinones; see: J. Sakurai, T. Oguchi, K. Watanabe,
H. Abe, S. Kanno, M. Ishikawa and T. Katoh, Chem.–Eur. J.,
2008, 14, 829–837.
(54% yield) followed by cleavage of the O-methyl moiety in
the resulting lactam 21 (80% yield).
In conclusion, we accomplished the enantioselective
total synthesis of (+)-stachyflin (1) in a highly efficient and
convergent manner. The key transformation involved a novel
acid-induced domino epoxide-opening/rearrangement/cyclization
reaction (2 - [I - II - III] - 19a,b). Importantly, the
synthesis has the potential for producing stachyflin analogues
for the development of novel anti-influenza virus agents.
We are grateful to Dr Kazuyuki Minagawa, Shionogi &
Co., Ltd., for providing us with natural (+)-stachyflin. We
also thank Professor Masayuki Inoue, the University of
Tokyo, for useful discussion and suggestion. This work was
supported by a Grant-in-Aid for Scientific Research on Priority
Area (No. 17035073 and No. 18032065), a Grant-in-Aid for
14 In our preliminary model studies (see ref. 9), BF3ꢀEt2O gave the
best result among several Lewis and Brønsted acids examined.
Detailed results and discussion will be reported in a full account.
15 For a recent review on cationic domino reactions, see: L. F. Tietze,
G. Brasche and K. M. Gericke, in Domino Reactions in Organic
Synthesis, Wiley-VCH, Weinheim, 2006, pp. 11–42.
16 The optical rotation was [a]2D4+133.3 (c 0.46 in MeOH), and that
ofnatural 1 was [a]2D4 + 138.7 (c 1.00 in MeOH). The spectroscopic
properties (IR, 1H and 13C NMR, and HRMS) of synthetic sample
1 were identical to those of the natural product, which was kindly
provided by Dr Kazuyuki Minagawa, Shionogi & Co., Ltd. (see
ESIw).
17 The use of standard reagents such as ammonium cerium(IV) nitrate
(CAN) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
resulted in failure. In the case of CAN, decomposition products were
generated, while as per DDQ the starting material was recovered.
ꢂc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4055–4057 | 4057