Stereocontrolled synthesis of 8,11-dideoxytetrodotoxin, unnatural analogue of puffer fish toxin

Article abstract of DOI:10.1021/ol026177a

(Matrix presented) 8,11-Dideoxytetrodoxin, an unnatural tetrodoxin analogue, was synthesized in a highly stereoselective manner from a common intermediate in our synthetic studies on tetrodoxin. The synthesis features neighboring group participation of tr

Full text of DOI:10.1021/ol026177a

ORGANIC
LETTERS
2002
Vol. 4, No. 16
2679-2682
Stereocontrolled Synthesis of
8,11-Dideoxytetrodotoxin, Unnatural
Analogue of Puffer Fish Toxin
Toshio Nishikawa, Daisuke Urabe, Kazumasa Yoshida, Tomoko Iwabuchi,
Masanori Asai, and Minoru Isobe*
Laboratory of Organic Chemistry, Graduate School of Bioagricultural Sciences,
Nagoya UniVersity, Chikusa, Nagoya 464-8601, Japan
isobem@agr.nagoya-u.ac.jp
Received May 13, 2002
ABSTRACT
8,11-Dideoxytetrodotoxin, an unnatural tetrodotoxin analogue, was synthesized in a highly stereoselective manner from a common intermediate
in our synthetic studies on tetrodotoxin. The synthesis features neighboring group participation of trichloroacetamide for stereoselective
hydroxylation, protection of ortho ester, and guanidine installation with Boc-protected isothiourea.
Tetrodotoxin 1 (Figure 1),1 originally isolated as a toxic
principle of puffer fish poisoning, is one of the most famous
and important marine natural products because of its novel
structure and potent biological activity.2 Since the mechanism
of action of this molecule was revealed to be the specific
blockage of voltage-dependent sodium channels that are
responsible for nerve and muscle excitability, the toxin has
been widely employed as an indispensable biochemical tool
for neurophysiological study.3 However, the detail of the
bound structure has not been described to date despite many
efforts such as photoaffinity labeling and site-directed
mutagenesis;4 severe limitations regarding the structural
modifications needed for preparing potent probe molecules
and the lack of the tertiary structure of the sodium channel
(1) (a) Goto, T.; Kishi, Y.; Takahashi, S.; Hirata, Y. Tetrahedron 1965,
21, 2059. (b) Tsuda, K.; Ikuma, S.; Kawamura, M.; Tachikawa, R.; Sakai,
K.; Tamura, C.; Amakasu, O. Chem. Pharm. Bull. 1964, 12, 1357. (c)
Woodward, R. B. Pure Appl. Chem. 1964, 9, 49.
Figure 1. Structures of tetrodotoxin and its analogues.
(2) Review: (a) Kao, C. Y.; Lovinson, S., Ann. N.Y. Acad. Sci. 1986,
479, 1. (b) Kobayashi, J.; Ishibashi, M. In ComprehensiVe Natural Products
Chemistry; Pergamon: 1999; p 480.
(3) (a) Narahashi, T. Physiol. ReV. 1974, 54, 813. (b) Hucho, F. Angew.
protein have hindered elucidation of the bound structure of
this toxin.5,6 In the course of our synthetic studies on
Chem., Int. Ed. Engl. 1995, 34, 39. (c) Anger, T.; Madge, D. J.; Mulla, M.;
Riddall. D. J. Med. Chem. 2001, 44, 115.
10.1021/ol026177a CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/18/2002

tetrodotoxin, we succeeded in the asymmetric syntheses of
5,11-dideoxytetrodotoxin 47 and 11-deoxytetrodotoxin 2.8-10
In this letter, we describe an efficient synthesis of 8,11-
dideoxytetrodotoxin 3, which was extremely difficult to
prepare from naturally occurring tetrodotoxin. This study
might shed light on the role played by the 8-hydroxyl group
in the toxicity of the tetrodotoxin family.11
On the basis of the successful syntheses of 5,11-dideoxy-
tetrodotoxin 4 and 11-deoxytetrodotoxin 2, the synthesis
started with dibromide 6, which was readily prepared by
bromination of the common key intermediate 5, as described
in our previous studies (Scheme 1).12 In the previous
afford an allylic alcohol at the C-8 position.7 On the other
hand, we found that the same dibromide 6 was treated with
K2CO3 in MeOH to give a bicyclic iminoether 8 in good
yield. The oxidation state of 8 has been set for 8,11-
dideoxytetrodotoxin 3.
The cyclic iminoether 8 was transformed to allylic alcohol
9 in good overall yield through three steps: (i) acid
hydrolysis of iminoether 8,13 (ii) trichloroacetylation of the
resulting amino alcohol, and (iii) methanolysis of trichloro-
acetate (Scheme 2). Epoxidation of allylic alcohol 9 with
Scheme 2a
Scheme 1a
a Conditions: (a) AcOH, THF, H2O, rt. (b) CCl3COCCl, Py, rt.
(c) K2CO3, MeOH, rt. (d) MCPBA, Na2HPO4, CH2Cl2, rt. (e) PCC,
4 Å MS, CH2Cl2, rt. (f) NaBH4, MeOH, 5 °C. (g) TMSCl, Et3N,
THF, rt. (h) O3, CH2Cl2, -78 °C; Et3N.
a Conditions: (a) PyH‚Br3, K2CO3, CH2Cl2, 10 °C. (b) DBU,
DMF, rt. (c) K2CO3, MeOH, rt.
MCPBA gave â-epoxide 10 in 85% yield. The configuration
of the C-7 position was inverted by oxidation with PCC and
subsequent reduction with NaBH4 to give R-alcohol 11 in
89% overall yield from 10. The resulting alcohol 11 was
protected as TMS ether.14 The remaining vinyl group was
then ozonized upon treatment of Et3N as a reductant15 to
give unstable aldehyde 12.
The epoxyaldehyde 12 was transformed to lactone 15, as
shown in Scheme 3. The aldehyde 12 reacted with lithium
trimethylsilylacetylide to give propargyl alcohol 13 as a
single stereoisomer.16 As expected, the configuration of the
syntheses of 2 and 4, hydroxylation at the C-8 position of 6
was accomplished by a novel neighboring group participation
of trichloroacetamide with DBU in DMF to give oxazoline
7, which was hydrolyzed under mild acidic conditions to
(4) For example: (a) Nakayama, H.; Hatanaka, Y.; Takai, M.; Yoshida,
E.; Kanaoka, Y. Ann. N.Y. Acad. Sci. 1993, 707, 349. (b) Noda, M.; Suzuki,
H.; Numa. S.; Stuehmer, W. FEBS Lett. 1990, 259, 213. (c) Terlau, H.;
Heinemann, S. H.; Stuehner, W.; Pusch, M.; Conti, F.; Imoto, K.; Numa,
S. FEBS Lett. 1991, 293, 93. (d) Lipkind, G. M.; Fozzard, H. A. Biophys.
J. 1994, 66, 1.
(5) Catterall, W. A. Neuron 2000, 26, 13.
(6) Recently, the gross three-dimensional structure at 19 Å was reported.
See: Sato, C.; Ueno, Y.; Asai, K.; Takahashi, K.; Sato, M.; Engel, A.;
Fujiyoshi, Y. Nature 2001, 409, 1047.
(7) (a) Nishikawa, T.; Asai, M.; Ohyabu, N.; Yamamoto, N.; Isobe, M.
Angew. Chem., Int. Ed. 1999, 38, 3081. (b) Asai, M.; Nishikawa, T.; Ohyabu,
N.; Yamamoto, N.; Isobe, M. Tetrahedron 2001, 57, 4543.
(8) Nishikawa, T.; Asai, M.; Isobe, M. J. Am. Chem. Soc. 2002, 124,
7847.
(9) A single total synthesis of the racemate was reported. See: (a) Kishi,
Y.; Aratani, M.; Fukuyama, T.; Nakatsubo, F.; Goto, T.; Inoue, S.; Tanino,
H.; Sugiura, S.; Kakoi, H. J. Am. Chem. Soc. 1972, 94, 9217. (b) Kishi, Y.;
Fukuyama, T.; Aratani, M.; Nakatsubo, F.; Goto, T.; Inoue, S.; Tanino, H.;
Sugiura, S.; Kakoi, H. J. Am. Chem. Soc. 1972, 94, 9219.
(10) For leading references from other laboratories, see: (a) Noya, B.;
Paredes, M. D.; Ozores, L.; Alonso, R. J. Org. Chem. 2000, 65, 5960. (b)
Burgey, C. S.; Vollerthun, R.; Fraser-Reid, B. J. Org. Chem. 1996, 61,
1609. (c) Sato, K.; Kajihara, Y.; Nakamura, Y.; Yoshimura, J. Chem. Lett.
1991, 1559. (d) Nachman, R. J.; Ho¨nel, M.; Williams, T. M.; Halaska, R.
C.; Mosher, H. S. J. Org. Chem. 1986, 51, 4802. (e) Keana, J. F. W.; Bland,
J. S.; Boyle, P. J.; Erion, M.; Hartling, R.; Husman, J. R.; Roman, R. B. J.
Org. Chem. 1983, 48, 3621, 3627. (f) Speslacis, J. Ph.D. Thesis, Harvard
University, Cambridge, MA, 1975.
(11) For a review on tetrodotoxin analogues, see: Yotsu-Yamashita, M.
J. Toxicol. Toxin ReV. 2001, 20, 51.
(12) The numbering used in this paper corresponds to that of tetrodotoxin.
(13) Hydrolysis of 7 under acidic conditions gave allylic alcohol bearing
trichloroacetamide,7 while hydrolysis of 8 under the same conditions did
not proceed. Attempted one-step conversion of 8 to 9 under forcing
hydrolytic conditions failed.
(14) The protective group was critical not only for the further function-
alization, but for final successful deprotection.
(15) (a) Isobe, M.; Iio, H.; Kawai, T.; Goto, T. Tetrahedron Lett. 1977,
703. (b) Iio, H.; Isobe, M.; Kawai, T.; Goto, T. Tetrahedron 1979, 35, 941.
(16) Use of magnesium acetylide in THF gave a decreased diastereo-
1
selectivity (ca. 4:1 from H NMR).
2680
Org. Lett., Vol. 4, No. 16, 2002

the less sterically hindered front side. The alkynyl group of
14 was oxidatively cleaved with RuO4 to give carboxylic
acid.22 Since spontaneous lactonization through epoxide
opening was very sluggish under the conditions, the car-
boxylic acid was treated with acid to afford the lactone 15
in good yield. At this stage, the configuration of the C-9
position was confirmed by observing W-shaped long-range
coupling (J ) 1 Hz) between H-9 and H-4a of 15. Prior to
installation of the guanidine functionality, the hydroxyl group
at the C-9 position was protected as an intramolecular mixed
acetal with the aldehyde at the C-4 position in three steps.
Selective deacetylation at C-9, oxidative cleavage of ac-
etonide,23 and acetalization in refluxing methanol provided
a 4:1 separable mixture of 16a and 16b.24 Now these
compounds were set for guanidinylation.
Scheme 3a
In the total synthesis of 11-deoxytetrodotoxin 2,8 a two-
step deprotection of fully functionalized intermediate 17
furnished 11-deoxytetrodotoxin 2 and 4,9-anhydro-11-deoxy-
tetrodotoxin 19, as shown in Scheme 4. Therefore, we
Scheme 4a
a Conditions: (a) TMS-CtC-Li, THF, -78 °C. (b) PDC, 3 Å
MS, CH2Cl2, rt. (c) NaBH4, CeCl3(H2O)7, MeOH, 0 °C. (d) Ac2O,
DMAP, Py; H2O, rt. (e) RuCl3(H2O)n, NaIO4, CCl4, H2O, CH3CN,
rt. (f) PPTS, CH2Cl2, rt. (g) K2CO3, MeOH, 0 °C. (h) HIO4(H2O)2,
AcOMe, rt; MeOH, reflux.
secondary alcohol (C9-OH) was proved to be the opposite
of that of tetrodotoxin.17 Consequently, the configuration was
inverted by oxidation with PDC followed by the Luche
reduction18 to give an unstable propargyl alcohol,19 which
was subjected to acetylation to afford diacetate 1420 in 71%
yield.21 The stereochemical outcomes of these highly stereo-
selective reactions can be uniformly explained by invoking
the similar chelation intermediates a and b, as depicted in
Figure 2. In this model, the carbonyl oxygen, amide nitrogen,
a Conditions: (a) aq NH3, H2O, MeOH, rt. (b) TFA, H2O, rt.
synthesized a similar intermediate 18 from the major isomer
16a in a manner analogous to that described for the synthesis
of 2 and 19.25,26 Surprisingly, deprotection of 18 did not give
either 8,11-dideoxytetrodotoxin 3 or its anhydro derivative
20 under the same reaction conditions as those of 11-
deoxytetrodotoxin 2.27 This unexpected result implied that
(20) We found that a small amount of water drove the in situ deprotection
of TMS ether and subsequent acetylation to give 14 in one pot.
(21) A series of these reactions were performed without purification,
because of the instability of the intermediates.
Figure 2.
(22) Carisen, P. H.; Katsuki, T.; Martin, V. S.; Sharpless, K. B. J. Org.
Chem. 1981, 46, 3936.
(23) Xie, M.; Berges, D. A.; Robins, M. J. J. Org. Chem. 1996, 61, 5178.
(24) Configurations of the C-4 position of 16a and 16b were established
to be S and R by coupling constants between H-4 and H-4a (0 and 4 Hz,
respectively). See ref 8.
and one oxygen of the acetal functionality were ligated by
the metals (Li and Ce). The steric hindrance of the acetonide
led nucleophiles such as acetylide and hydride to attack from
(17) Determination of the configuration of C-9 is included in Supporting
Information.
(18) Luche, J. L.; Gamal, A. L. J. Am. Chem. Soc. 1979, 101, 5848.
(19) The TMS group of acetylene was removed when the crude ynone
was dissolved in MeOH dried over 3 Å molecular sieves.
(25) Details will be disclosed in a full account of this study.
(26) Nishikawa, T.; Ohyabu, N.; Yamamoto, N.; Isobe, M. Tetrahedron
1999, 55, 4325.
(27) Structures of the products have not been fully elucidated, because
of the instability.
Org. Lett., Vol. 4, No. 16, 2002
2681

31
S-methylisothiourea30 in the presence of HgCl2 to afford
22 in high overall yield. Toward the goal, 22 was treated
with aqueous TFA to furnish 8,11-dideoxytetrodotoxin 3 and
4,9-anhydro-8,11-dideoxytetrodotoxin 20 in 43 and 32%
yields, respectively.32 The structures of these products were
Scheme 5. Synthesis of 8,11-Dideoxytetrodotoxina
1
confirmed by full assignment of H and 13C NMR spectra
by extensive two-dimensional NMR experiments.
In summary, we have accomplished the synthesis of
8,11-dideoxytetrodotoxin in a highly stereoselective manner.
This study provides a novel and efficient route for the
synthesis of tetrodotoxin analogues through protection of the
ortho ester and subsequent guanidine installation. Total
synthesis of tetrodotoxin 1 along this line is currently under
investigation.
Acknowledgment. This work was financially supported
by JSPS-RFTF, a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports, and Culture of
Japan, a Pfizer Award in Synthetic Organic Chemistry, and
the Mitsubishi Chemical Corporation Fund.
a Conditions: (a) K2CO3, MeOH, rt. (b) TBS-OTf, Py, CH3CN,
rt. (c) DIBAL-H, CH2Cl2, -78 °C. (d) BocNdC(SMe)NHBoc,
HgCl2, Et3N, DMF, rt. (e) TFA, MeOH, H2O, rt.
the hydroxyl group at the C-8 position might contribute to
the stabilization of the tetrodotoxin molecule, especially
under basic conditions.
Supporting Information Available: Proof of the con-
figuration of the C-9 position of 13 and characterization data
Considering the inapplicability of the deprotection pro-
cedure in this synthesis, we slightly modified the synthetic
plan to protect the ortho ester function itself instead of
protecting the δ-hydroxy lactone. Fortunately, upon treatment
of 16a with K2CO3 in MeOH at room temperature, acetate
at the C-7 position was hydrolyzed to afford ortho ester,
which was protected as silyl ether 21.28 The trichloroaceta-
mide was reductively removed with DIBAL-H29 to give an
amine, which was subsequently guanidinylated with bis Boc-
1
of all new compounds, including copies of the H and 13C
NMR spectra of 3 and 20. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL026177A
(29) Oishi, T.; Ando, K.; Inomoya, K.; Sato, H.; Iida, M.; Chida, N.
Org. Lett. 2002, 4, 151.
(30) Bergeron, R. J.; McManis, J. S. J. Org. Chem. 1987, 52, 1700.
(31) Kim, K. S.; Qian, L. Tetrahedron Lett. 1993, 34, 7677.
(32) Due to the interconvertible nature of the tetrodotoxin analogue, 4,9-
anhydro-8,11-dideoxytetrodotoxin 20 was equilibrated in 1% TFA-d/D2O
at room temperature for one week to reach a mixture of 3 and 20 in a 7:1
(28) Heathcock, C. H.; Young, S. D.; Hagen, J. P.; Pilli, R.; Badertscher,
U. J. Org. Chem. 1985, 50, 2095.
1
ratio (from H NMR).
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Org. Lett., Vol. 4, No. 16, 2002