Convergent synthesis and biological activity of the WXYZA′B′ C′ ring system of maitotoxin

Article abstract of DOI:10.1021/ol801369g

(Chemical Equation Presented) The WXYZA′B′C′ ring system (1) of maitotoxin (MTX) was synthesized in a convergent manner via successive coupling of the W, Z, and C′ ring fragments through construction of the XY and A′B′ ring systems. The synthetic segment

Full text of DOI:10.1021/ol801369g

ORGANIC
LETTERS
2008
Vol. 10, No. 16
3599-3602
Convergent Synthesis and Biological
Activity of the WXYZA′B′C′ Ring System
of Maitotoxin
Tohru Oishi,* Futoshi Hasegawa, Kohei Torikai, Keiichi Konoki,
Nobuaki Matsumori, and Michio Murata
Department of Chemistry, Graduate School of Science, Osaka UniVersity,
1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
oishi@chem.sci.osaka-u.ac.jp
Received June 17, 2008
ABSTRACT
The WXYZA′B′C′ ring system (1) of maitotoxin (MTX) was synthesized in a convergent manner via successive coupling of the W, Z, and C′ ring
fragments through construction of the XY and A′B′ ring systems. The synthetic segment 1 blocked the hemolytic activity elicited by MTX.
Maitotoxin (MTX) was first discovered as one of the toxins
responsible for ciguatera seafood poisoning and later found
to be a product of the epiphytic dinoflagellate Gambierdiscus
toxicus.1 MTX is the largest non-biopolymer known to date
(MW 3422),2 and its remarkable biological activities have
been reported, namely, that (i) it elicited hemolysis of mouse
blood cells at 15 nM,3 (ii) it induced Ca2+ influx in rat glioma
C6 cells at 0.3 nM,4 and particularly, (iii) its lethality against
mammals was extremely potent (50 ng/kg, mice, i.p.).2c,5
The structure of MTX was elucidated by the Yasumoto group
through extensive NMR analysis,2 and its complete stereo-
chemistry was determined independently by the Tachibana6
and Kishi7 groups. Its unique, gigantic molecular structure
has attracted considerable attention from the synthetic
community, and recent syntheses of partial structures of MTX
by the Nicolaou8 and Nakata9 groups have given support to
the originally proposed structure.10 Despite a large number
of pharmacological and biophysical investigations, the precise
mode of action of MTX has not been elucidated at the
molecular level. Preparation of photoaffinity probes and
(1) Yasumoto, T.; Bagnis, R.; Vernoux, J. P. Bull. Jpn. Soc. Sci. Fish.
1976, 42, 359–365.
(7) (a) Zheng, W.; DeMattei, J. A.; Wu, J.-P.; Duan, J. J.-W.; Cook,
L. R.; Oinuma, H.; Kishi, Y. J. Am. Chem. Soc. 1996, 118, 7946–7968. (b)
Cook, L. R.; Oinuma, H.; Semones, M. A.; Kishi, Y. J. Am. Chem. Soc.
1997, 119, 7928–7937.
(2) (a) Murata, M.; Iwashita, T.; Yokoyama, A.; Sasaki, M.; Yasumoto,
T. J. Am. Chem. Soc. 1992, 114, 6594–6596. (b) Murata, M.; Naoki, H.;
Iwashita, T.; Matsunaga, S.; Sasaki, M.; Yokoyama, A.; Yasumoto, T. J. Am.
Chem. Soc. 1993, 115, 2060–2062. (c) Murata, M.; Naoki, H.; Matsunaga,
S.; Satake, M.; Yasumoto, T. J. Am. Chem. Soc. 1994, 116, 7098–7107.
(d) Satake, M.; Ishida, S.; Yasumoto, T.; Murata, M.; Utsumi, H.; Hinomoto,
T. J. Am. Chem. Soc. 1995, 117, 7019–7020.
(8) (a) Nicolaou, K. C.; Frederick, M. O.; Burtoloso, A. C. B.; Denton,
R. M.; Rivas, F.; Cole, K. P.; Aversa, R. J.; Gibe, R.; Umezawa, T.; Suzuki,
T. J. Am. Chem. Soc. 2008, 130, 7466–7476. (b) Nicolaou, K. C.; Cole,
K. P.; Frederick, M. O.; Aversa, R. J.; Denton, R. M. Angew. Chem., Int.
Ed. 2007, 46, 8875–8879. (c) Nicolaou, K. C.; Frederick, M. O. Angew.
Chem., Int. Ed. 2007, 46, 5278–5282. (d) Nicolaou, K. C.; Postema,
M. H. D.; Yue, E. W.; Nadin, A. J. Am. Chem. Soc. 1996, 118, 10335–
10336.
(3) Igarashi, T.; Aritake, S; Yasumoto, T. Nat. Toxins 1999, 7, 71–79.
(4) Konoki, K.; Hashimoto, M.; Nonomura, T.; Sasaki, M.; Murata, M.;
Tachibana, K. J. Neurochem. 1998, 70, 409–416.
(5) For a review, see: Gusovsky, F.; Daly, J. W. Biochem. Pharmacol.
(9) (a) Satoh, M.; Koshino, H.; Nakata, T. Org. Lett. 2008, 10, 1683–
1685. (b) Morita, M.; Haketa, T.; Koshino, H.; Nakata, T. Org. Lett. 2008,
10, 1679–1682. (c) Morita, M.; Ishiyama, S.; Koshino, H.; Nakata, T. Org.
Lett. 2008, 10, 1675–1678. (d) Sakamoto, Y.; Matsuo, G.; Matsukura, H.;
Nakata, T. Org. Lett. 2001, 3, 2749–2752.
1990, 39, 1633–1639
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(6) (a) Sasaki, M.; Matsumori, N.; Maruyama, T.; Nonomura, T.; Murata,
M.; Tachibana, K.; Yasumoto, T. Angew. Chem., Int. Ed. Engl. 1996, 35,
1672–1675. (b) Nonomura, T.; Sasaki, M.; Matsumori, N.; Murata, M.;
Tachibana, K.; Yasumoto, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 1675–
(10) Gallimore, A. R.; Spencer, J. B. Angew. Chem., Int. Ed. 2006, 45,
4406–4413.
1678
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10.1021/ol801369g CCC: $40.75
Published on Web 07/23/2008
2008 American Chemical Society

radioisotope-labeled derivatives for identification of its target
molecules has been hampered chiefly due to the short supply
of this material from natural sources, although non-selective
cation channels are proposed to be its target protein.11 It is
reported that Ca2+ influx in rat glioma C6 cells induced by
MTX was inhibited by brevetoxin-B (BTXB) and the EC50
value was estimated to be 13 µM.4 The hydrophobic part of
MTX resembles other ladder-shaped polyethers such as
BTXB, which is known to bind to voltage-sensitive sodium
channels.12 Therefore, this part was expected to be the
binding domain for target (transmembrane) proteins and to
be utilized as a molecular probe to identify these proteins.13
Herein, we describe the convergent synthesis of the
WXYZA′B′C′ ring system (1, Scheme 1) corresponding to
ethers14 to construct the heptacyclic ether 1, which was to
be derived from the C′ (2) and WXYZ (3) ring units through
construction of the A′B′ ring system. In turn, the tetracyclic
ether 3 was to be synthesized in an analogous sequence from
the W and Z ring units via formation of the XY ring system.
However, it remained uncertain whether our method was
applicable to the WXYZ ring system with its contiguous
angular methyl groups.
As shown in Scheme 2, synthesis of the WXYZ ring
system 3 started with coupling of the Z ring diol 415 and the
W ring aldehyde 515 through (i) acetal formation, (ii)
regioselective opening of the seven-membered ring acetal
using TMSCN and Sc(OTf)3,16 and (iii) elimination of the
resulting primary alcohol, giving a terminal olefin by
Nishizawa-Grieco protocol,17 to yield R-cyano ether (6) as
an inseparable mixture of C106 diastereomers (70%, three
steps).18 Reduction of the nitrile 6 with DIBALH gave
aldehyde 7 (70%), which was treated with vinyllithium to
afford allylic alcohol 8 (86%). Ring-closing metathesis
(RCM) of the diene 8 by the action of second-generation
Grubbs catalyst 919 proceeded smoothly in toluene under
reflux for 20 min. Oxidation of the alcohol with Dess-Martin
periodinane (DMP)20 giving enone, followed by hydrogena-
tion of the double bond using PtO2 catalyst, furnished
saturated ketones as an inseparable mixture of the desired
10 and its C106 epimer in a 1:2 ratio. Subsequent DBU-
mediated isomerization in toluene at 110 °C improved the
ratio to 8:1. Removal of the NAP21 group of 10 with DDQ
afforded 11 (73%), which was separated from C106 epimer
(12%) by silica gel chromatography. When hydroxyketone
11 was treated with ethanethiol in the presence of Zn(OTf)2,
the reaction was sluggish, and mixed thioacetal 12 was
obtained in 42% yield with recovery of the starting material,
in contrast to the behavior of a similar system lacking angular
methyl groups.22 Recovered 11 was recycled (twice) to
provide 12 in 64% total yield as an inseparable mixture of
isomers (5:1) with respect to the stereogenic center on the
acetal carbon, with concomitantly formed dithioacetal 13
(12%) and recovery of 11 (10%). The crucial step of the
present synthesis, introduction of the angular methyl group
in axial orientation,23 was achieved under carefully controlled
conditions. Oxidation of the mixed thioacetal to the sulfone
was carried out with MCPBA in dichloromethane at -78 to
-40 °C. The reaction mixture was treated with Me3Al at
Scheme 1
.
Synthesis Plan for the WXYZA′B′C′ Ring System
(1) of MTX
a portion of the hydrophobic part of MTX and the evaluation
of its biological activity.
From the synthetic point of view, it is a daunting challenge
to construct the WXYZ ring system in a convergent manner
because of the presence of contiguous angular methyl groups,
i.e., C157, C158, and C159, on the Y and Z rings, although
a linear synthesis of the WXYZA′ ring system has recently
been reported.9b Our synthesis plan of the WXYZA′B′C′ ring
system (1) of MTX is shown in Scheme 1. We envisaged
extensive utilization of the convergent method via R-cyano
(15) The W, Z, and C′ ring fragments were synthesized from 2-deoxy-
D-ribose by using Nicolaou’s procedure: Nicolaou, K. C.; Nugiel, D. A.;
Couladouros, E.; Hwang, C.-K. Tetrahedron 1990, 46, 4517–4552. See
Supporting Information.
(11) (a) Escobar, L. I.; Salvador, C.; Martinez, M.; Vaca, L. Neurobi-
ology 1998, 6, 59–74. (b) Dietl, P.; Voelkl, H. Mol. Pharmacol. 1994, 45,
300–305. (c) Soergel, D. G.; Yasumoto, T.; Daly, J. W.; Gusovsky, G. Mol.
Pharmacol. 1992, 41, 487–493.
(16) Fukuzawa, S.-I.; Tsuchimoto, T.; Hotaka, T.; Hiyama, T. Synlett
1995, 1077–1078.
(17) Grieco, P. A.; Gilman, S.; Nishizawa, M. J. Org. Chem. 1976, 41,
1485–1486.
(12) Catterall, W. A.; Risk, M. Mol. Pharmacol. 1981, 19, 345–348.
(13) It is reported that gambierol, gambieric acid-A, and brevenal elicit
antagonistic activity against the binding of PbTx-3 (a BTXB derivative) to
voltage-sensitive sodium channels; see: (a) Inoue, M.; Hirama, M.; Satake,
M.; Sugiyama, K.; Yasumoto, T. Toxicon 2003, 41, 469–474. (b) Bourdelais,
A. J.; Jacocks, H. M.; Wright, J. L. C.; Bigwarfe, P. M., Jr.; Baden, D. G.
J. Nat. Prod. 2005, 68, 2–6.
(18) The carbon numbering of compounds in this paper corresponds to
that of MTX.
(19) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953–956.
(20) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155–4156.
(21) Gaunt, M. J.; Yu, J.; Spencer, J. B. J. Org. Chem. 1998, 63, 4172–
4173.
(14) (a) Oishi, T.; Watanabe, K.; Murata, M. Tetrahedron Lett. 2003,
44, 7315–7319. (b) Oishi, T.; Suzuki, M.; Watanabe, K.; Murata, M.
Heterocycles 2006, 69, 91–98. (c) Watanabe, K.; Suzuki, M.; Murata, M.;
Oishi, T. Tetrahedron Lett. 2005, 46, 3991–3995.
(22) Nicolaou, K. C.; Veale, C. A.; Hwang, C.-K.; Hutchinson, J.; Prasad,
C. V. C.; Ogilvie, W. W. Angew. Chem., Int. Ed. Engl. 1991, 30, 299–303.
(23) Nicolaou, K. C.; Prasad, C. V. C.; Hwang, C.-K.; Duggan, M. E.;
Veale, C. A. J. Am. Chem. Soc. 1989, 111, 5321–5330.
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Scheme 2
-40 °C and warmed to 0 °C to give 14 as a single isomer
16 (16:regioisomer g10:1). Conversion of alcohol 16 to
aldehyde 3 was performed by standard procedures: (i)
triflation, (ii) substitution with NaCN giving nitrile 17 (83%,
two steps), and (iii) DIBALH reduction of 17 (72%).
Having synthesized the WXYZ ring fragment, we turned
our attention to the WXYZA′B′C′ ring system (Scheme 3).
The C′ ring diol 215 and the WXYZ ring aldehyde 3 were
coupled through (i) acetal formation, (ii) regioselective
cleavage, and (iii) elimination of the primary alcohol to afford
in 73% yield along with the sulfoxide intermediate 15 (23%).
The structure of 14 was unambiguously determined by NOE
experiments. Thus, convergent synthesis of the WXYZ ring
system possessing the requisite five angular methyl groups
was achieved. Side chain elongation of the Z ring was carried
out next. Removal of the TIPS groups of 14 was followed
by 2-naphthylidene acetal formation and regioselective
reductive cleavage24 of the acetal to provide primary alcohol
Scheme 3
Org. Lett., Vol. 10, No. 16, 2008
3601

R-cyano ether 18 (67%, three steps). Reduction of nitrile 18
with DIBALH was not an easy task, in contrast to the
previous results, furnishing aldehyde 19 in 51% yield with
concomitant formation of primary amine 20 (42%) due to
overreduction, which was recovered by Dess-Martin oxida-
tion to give nitrile 18 in 77% yield.25 Treatment of the
aldehyde 19 with allylmagnesium bromide resulted in the
formation of homoallylic alcohol 21. RCM of the diene 21
with Grubbs catalyst 9 was followed by Dess-Martin
oxidation to yield eight-membered ring enones as a mixture
of 22 and its C116 epimer in a 1:2 ratio. Isomerization by
treatment with DBU in toluene at 110 °C afforded 22 as the
major product in 85% yield, and removal of the NAP group
with DDQ gave hydroxyketone 23 (70%), which was
separated from C116 epimer (12%).
The final step, construction of the A′ ring by reductive
etherification of the hydroxyketone 23, turned out to be
problematic.26 Treatment of 23 with Et3SiH in the presence
of BF3·OEt2 was followed by removal of the TIPS groups
with TBAF to furnish 24 in low yield (19%) as a mixture of
C116 epimers (2:1) with concomitant formation of diol 25
(41%, a mixture of diastereomers). Although conversion of
23 to mixed thioacetal under the identical conditions for
transforming 11 to 12 (Scheme 2) using EtSH/Zn(OTf)2
failed, with recovery of starting material, treatment of 23
with CH(OMe)3 in the presence of PPTS in CH2Cl2/MeOH
under reflux for 65 h afforded methylacetal 26 in 59% yield.
Reduction of the methylacetal27 with Et3SiH/BF3·OEt2 and
subsequent removal of the TIPS groups with TBAF afforded
the WXYZA′B′C′ ring system (1) as a single isomer (86%,
two steps), whose structure was determined by NOESY
experiments.
Figure 1. Differences in proton (500 MHz, 1:1 C5D5N/CD3OD)
and carbon (125 MHz, 1:1 C5D5N/CD3OD) NMR chemical shifts
between MTX and the WXYZA′B′C′ ring system (1). The x- and
y-axes represent carbon number and ∆δ (∆δ ) δMTX - δsynthetic
1 in ppm), respectively.
extensive utilization of the R-cyano ether method through
union of the W, Z, and C′ ring fragments via construction
of the XY and A′B′ ring systems. Further studies on the
biological activities of 1 and synthesis of the W-F′ ring
system of MTX to improve the antagonistic activity are
currently in progress in our laboratory.
Differences in proton (500 MHz) and carbon (125 MHz)
NMR chemical shifts in 1:1 C5D5N/CD3OD between MTX
and the synthetic WXYZA′B′C′ fragment 1 are shown in
1
Figure 1. The H and 13C NMR chemical shifts of the
XYZA′B′ ring moiety were in good accordance with those
of MTX, supporting the proposed structure, although the val-
ues corresponding to both terminal rings (W and C′) were
different due to the absence of the D′E′F′ and UV ring
systems.7
The biological activity of 1 was evaluated next (see
Supporting Information).3,4 MTX induced hemolysis of
human red blood cells at 10 nM, and this value was taken
as 100%. The WXYZA′B′C′ moiety 1 blocked this hemolytic
activity in a dose-dependent manner, and ca. 80% inhibition
was observed at 10 µM. However, 1 itself induced no
hemolysis at the same concentration. Considering that BTXB
did not inhibit MTX-induced hemolytic activity even at 10
µM, the hydrophobic portions (e.g., 1) of MTX are expected
to be promising molecular probes for identifying the target
proteins of MTX.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No.
16073211) from MEXT, Japan.
Supporting Information Available: Experimental details
and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL801369G
(24) Oikawa, M.; Liu, W.-C.; Nakai, Y.; Koshida, S.; Fukase, K.;
Kusumoto, S. Synlett 1996, 1179–1180.
(25) Nicolaou, K. C.; Mathison, C. J. N. Angew. Chem., Int. Ed. 2005,
44, 5992–5997.
(26) Nicolaou, K. C.; Hwang, C.-K.; Duggan, M. E.; Nugiel, D. A.;
Abe, Y.; Reddy, K. B.; DeFrees, S. A.; Reddy, D. R.; Awartani, R. A.;
Conely, S. R.; Rutjes, F. P. J. T.; Theodorakis, E. A. J. Am. Chem. Soc.
1995, 117, 10227–10238.
In conclusion, a highly convergent synthesis of the
WXYZA′B′C′ ring system of MTX was achieved by
(27) Oishi, T.; Nagumo, Y.; Shoji, M.; Le Brazidec, J.-Y.; Uehara, H.;
Hirama, M. Chem. Commun. 1999, 2035–2036.
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