Total synthesis of (-)-5,6,11-trideoxytetrodotoxin and its 4-epimer

Article abstract of DOI:10.1021/ol062098d

(Chemical Equation Presented) The first total synthesis of 5,6,11-trideoxytetrodotoxin (1) and its 4-epimer were achieved. The synthesis is characterized by the stereoselective construction of the quaternary amino carbon center at C8a by an asymmetric tra

Full text of DOI:10.1021/ol062098d

ORGANIC
LETTERS
2006
Vol. 8, No. 21
4971-4974
Total Synthesis of
)-5,6,11-Trideoxytetrodotoxin and Its
(−
4-Epimer
Taiki Umezawa, Toshihiro Hayashi, Hiroshi Sakai, Hidetoshi Teramoto,
Takeshi Yoshikawa, Masashi Izumida, Yoshinori Tamatani, Tadashi Hirose,
Yasufumi Ohfune,* and Tetsuro Shinada*
Graduate School of Science, Osaka City UniVersity, 3-3-138, Sugimoto, Sumiyoshi,
Osaka 558-8585, Japan
ohfune@sci.osaka-cu.ac.jp; shinada@sci.osaka-cu.ac.jp
Received August 25, 2006
ABSTRACT
The first total synthesis of 5,6,11-trideoxytetrodotoxin (1) and its 4-epimer were achieved. The synthesis is characterized by the stereoselective
construction of the quaternary amino carbon center at C8a by an asymmetric transferring Strecker synthesis and the highly efficient conversion
of cyanohydrin 4 to 1 via intramolecular cyclization reactions.
5,6,11-Trideoxytetrodotoxin (trideoxyTTX) (1) and its
4-epimer 2 were isolated from the ovaries of the puffer fish,
Fugu poecilonotus, by Yamashita and Yasumoto (Scheme
1).1 These are natural congeners of tetrodotoxin (TTX),
known as a principal toxin of puffer fish poisoning.2 TTX
exhibits specific blocking activities of the voltage-dependent
Na+ channels. Structure-activity relationship studies with
natural TTX congeners indicated that the sodium channel
blocking activity of 1 was much less potent than that of
TTX.2 Recently, Yamashita and co-workers analyzed TTX
and its congeners in eggs of the puffer fish by a novel ESI-
LC/MS method. It is interesting to note that large amounts
Scheme 1. Synthetic Plan
(1) (a) Yotsu-Yamashita, M.; Yamagishi, Y.; Yasumoto, T. Tetrahedron
Lett. 1995, 36, 9329-9332. (b) Nakagawa, T.; Jang, J.; Yotsu-Yamashita,
M. Anal. Biochem. 2006, 352, 142-144.
(2) (a) Kao, C. Y.; Lovinson, S., Eds. In Tetrodotoxin, Saxitoxin and
the Molecular Biology of the Sodium Channels. Ann. N.Y. Acad. Sci. 1986,
479, 52-67. (b) Kobayashi, J.; Ishibashi, M. In ComprehensiVe Natural
Products Chemistry; Pergamon: Oxford, UK, 1999; Vol. 8, p 480. (c) Yotsu-
Yamashita, M.; Sugimoto, A.; Takai, A.; Yasumoto, T. J. Pharmacol. Exp.
Ther. 1999, 289, 1688-1696. (d) Cestele, S.; Catterall, W. A. Biochimie
2000, 82, 883-892. (e) Wood, J. N. Curr. Opin. Pharmacol. 2001, 1, 17-
21. (f) Yotsu-Yamashita, M. J. Toxicol. Toxin ReV. 2001, 20, 51-66.
10.1021/ol062098d CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/15/2006

Scheme 2. Synthesis of the Key Intermediates 15 and 5
of trideoxyTTX 1 were accumulated in the eggs and its
Herein, we report the first total synthesis of (-)-5,6,11-
trideoxyTTX (1) and its 4-epimer 2. Our total synthesis
features a simple approach to 1 and 2 via the intramolecular
cyclization of a nitrile 3 placed on a conformationally rigid
concentration was greater than that of TTX.1b These results
indicated a possibility that 1 and 2 could be an important
biosynthetic precursor of TTX or an antidotal metabolite of
TTX.3
(5) Total sythensis of TTX and its congeners, 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, 9217-9219. (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, 9219-9221. (c) Asai,
M.; Nishikawa, T.; Ohyabu, N.; Yamamoto, N.; Isobe, M. Tetrahedron 2001,
57, 4543-4558. (d) Nishikawa, T.; Asai, M.; Isobe, M. J. Am. Chem. Soc.
2002, 124, 7847-7852. (e) Nishikawa, T.; Urabe, D.; Yoshida, K.; Iwabuchi,
T.; Asai, M.; Isobe, M. Org. Lett. 2002, 4, 2679-2682. (f) Ohyabu, N.;
Nishikawa, T.; Isobe, M. J. Am. Chem. Soc. 2003, 125, 8798-8805. (g)
Hinman, A.; Du Bois, J. J. Am. Chem. Soc. 2003, 125, 11510-11511. (h)
Nishikawa, T.; Urabe, D.; Isobe, M. Angew. Chem., Int. Ed. 2004, 43, 4782-
4785. (i) Nishikawa, T.; Urabe, D.; Yoshida, K.; Iwabuchi, T.; Asai, M.;
Isobe, M. Chem. Eur. J. 2004, 10, 452-462. (j) Sato, K.; Akai, S.; Sugita,
N.; Ohsawa, T.; Kogure, T.; Shoji, H.; Yoshimura, J. J. Org. Chem. 2005,
70, 7496-7504. For a recent review see: (k) Koert, U. Angew. Chem., Int.
Ed. 2004, 43, 5572-5576.
TTX and its congeners are a class of highly functionalized
natural products possessing a variety of functional groups
that are densely juxtaposed on the cyclohexane skeleton.
Substantial efforts have been devoted to the total synthesis
of TTX and its congeners4,5 due to their structural and
pharmacological interests. On the other hand, the total
synthesis of the naturally occurring 5-deoxy congeners has
not been reported.6
(3) The biosynthesis and metabolic pathways of TTX are unknown and
remain a challenging subject in this field as mentioned in ref 4h.
(4) Total synthesis of (-)-5,11-dideoxyTTX as an unnatural TTX
analogue was reported by Isobe and Nishikawa in 2001. See ref 4c.
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Org. Lett., Vol. 8, No. 21, 2006

cyclohexane platform. The cyclic aminal 3 would be derived
from 4 by oxidative cleavage of the olefin followed by
removal of the protecting groups. Inspection of molecular
models suggested that the cyanide moiety and C7 hydroxy
group of 3 are in close proximity, facilitating the desired
lactonization. The cyanohydrin 4 would be obtained from 6
via the aldehyde 5. We postulated that an asymmetric
transferring Strecker synthesis (ATS) of R-acyloxyketone 7
would effect the stereocontrolled construction of the C8a
quaternary amino carbon center of 6.7 L-Phe was chosen as
the chirality transferring group, which would induce a perfect
diastereofacial selectivity upon cyanide addition to C8a (vide
supra).7b
We began the synthesis with the triol 8,8 derived from
(-)-quinic acid, which was converted into the ATS precursor
7 by the following sequence of transformations: (1) initial
protection of 8 and subsequent conversion to diol 10, (2)
regioselective installation of Boc-L-Phe to 10, and (3)
oxidation (Scheme 2). Removal of the Boc group with
TMSOTf and 2,6-lutidine9 followed by treatment with
TMSCN in the presence of ZnCl2 gave the amino nitrile 12
as a single diastereomer. The stereochemical outcome was
rationalized by the preferential approach of the cyanide ion
from the upper face of the ketimine intermediate A having
a stacking conformation as was discovered in a simple
cyclohexane system.7b Removal of the chirality transferring
group from 12 was performed by the initial oxidation to
R-imino nitrile and subsequent hydrolysis. Under these
conditions, replacement of the primary acetate by a chlorine
atom occurred concomitantly. Protection of the resulting
amino acid 13 with a Boc group followed by esterification
produced the N-Boc pyrrolidine 15. This structure was
unambiguously confirmed by X-ray analysis.10
unsaturated ketone 17, which was stereoselectively reduced
to 18. Mislow-Evans rearrangement of the sulfoxide 19 (a
1:1 mixture of diastereomers), synthesized from 18, gave
the olefin 20 bearing an equatorial hydroxy group at C7.12
Hydrogenation of the TBS ether 21 in the presence of Pd/C
proceeded in a stereoselective manner to afford 22 (20:1).13
The resulting trans-1,2-diol was converted to the TBS ether
24 via the acetonide 23 in 3 steps. The pyrrolidine ring was
successfully opened by initial RuO4 oxidation of 24 and
subsequent reduction of the resulting lactam 25 to give the
alcohol 26, which, following a selenylation/ deselenylation
sequence, yielded 27 possessing the requisite vinyl group.
Inversion of the C7 hydroxy group was performed by the
successive oxidation and reduction of 27 to afford 28 (7â:
7R ) 4.5:1). The major isomer was converted to the aldehyde
5 in 3 steps via protection of the diol moiety with an
acetonide.
The aldehyde 5 was transformed to the cyanohydrin 4 by
treatment of 5 with TMSCN in the presence of Et3N followed
by installation of a Boc protected guanidine moiety (Scheme
3). With the key precursor (9S)-4 in hand,14 this was
Scheme 3. Total Synthesis of TrideoxyTTXs (1) and Its
4-Epimer 2 from 5
With the protected amino acid 15 in hand, efforts focused
on the synthesis of the aldehyde 5. The conversion was
accomplished by a series of sequential transformations: (i)
introduction of a C7 hydroxy group by the Mislow-Evans
rearrangement,11 (ii) stereoselective construction of the C6
stereogenic center, (iii) pyrrolidine ring opening to install a
vinyl group at C4a, and (iv) introduction of the axial hydroxy
group at C7. Along this line, 15 was transformed into the
ketone 16 via selective reduction of the R-hydroxy ester
moiety. Treatment of 16 with DBU provided the R,â-
(6) Synthetic studies of TTX and its congeners, see: (a) Keana, J. F.
W.; Bland, J. S.; Boyle, P. J.; Erion, M.; Hartling, R.; Husman, J. R.; Roman,
R. B.; Ferguson, G.; Parvez, M. J. Org. Chem. 1983, 48, 3627-3631. (b)
Sato, K.; Kajihara, Y.; Nakamura, Y.; Yoshimura, J. Chem. Lett. 1991,
1559-1562. (c) Fraser-Reid, B.; Burgey, C. S.; Vollerthun, R. Pure Appl.
Chem. 1998, 70, 285-288. (d) Noya, B.; Paredes, M. D.; Ozores, L.; Alonso,
R. J. Org. Chem. 2000, 65, 5960-5968. (e) Itoh, T.; Watanabe, M.;
Fukuyama, T. Synlett 2002, 1323-1325. (f) Ohtani, Y.; Shinada, T.; Ohfune,
Y. Synlett 2003, 619-622. (g) Taber, D. F.; Storck, P. H. J. Org. Chem.
2003, 68, 7768-7771.
(7) (a) Ohfune, Y.; Shinada, T. Eur. J. Org. Chem. 2005, 5127-5143.
(b) Shinada, T.; Kawakami, T.; Sakai, H.; Matsuda, H.; Umezawa, T.;
Kawasaki, M.; Namba, K.; Ohfune, Y. Bull. Chem. Soc. Jpn. 2006, 79,
768-774.
(8) Toscano, M. D.; Frederickson, M.; Evans, D. P.; Coggins, J. R.; Abell,
C.; Gonzalez-Bello, C. Org. Biomol. Chem. 2003, 1, 2075-2083.
(9) Sakaitani, M.; Ohfune, Y. J. Org. Chem. 1990, 55, 870-876.
(10) CCDC 610332 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.cam.ac. uk/data _request/cif.
(11) Evans, D. A.; Andrews, G. C. Acc. Chem. Res. 1974, 7, 147-155.
subjected to ozonolysis and the resulting aldehyde cyclized
spontaneously to produce the acetal 30. To our delight, it
was found that the treatment of 30 with 20% aqueous TFA
afforded a mixture of trideoxyTTX 1, 4-epimer 2, and
anhydro-derivative 31 [10:20:70]. These results indicated that
(12) The stereochemical outcome is attributed to the steric effect of the
methoxycarbonyl group at C8a.
(13) The hydrogenation of 20 gave a 10:1 mixture of 22 and its R-isomer.
(14) As a preliminary experiment, we found that the product ratio of
(9S)-4 and (9R)-4 was improved to be 85:15 by using NaCN in the presence
of MgCl2 in MeOH. The synthesis of 13C-labeled 1 and 2 under this
condition is in progress. The full details will be reported in due course.
Org. Lett., Vol. 8, No. 21, 2006
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the following sequential transformations were effective under
the reaction conditions: (i) removal of the Boc group, (ii)
an acetal exchanging reaction to form the conformationally
rigid 3, (iii) removal of the acetonide, (iv) formation of
imidate, (v) hydrolysis of the resulting imidate, and (vi)
hydrolysis of the aminal.15 Yields of the desired natural
products 1 and 2 were improved by an equilibrium experi-
ment with 1% aqueous TFA [1:2:31 ) 40:26:34]. The
mixture was purified by ion-exchange column chromatog-
raphy. Analytical data of the synthetic 1 and 2 were identical
with those of the authentic samples.
In conclusion, we have achieved the first total synthesis
of 5,6,11-trideoxyTTX (1) and its 4-epimer 2. The synthesis
was highlighted by the efficient final approach to furnish
the total synthesis with use of the cyanohydrin 4. This
strategy opens the way to supply the 13C-labeled trideoxy-
TTX as a biological probe for elucidation of the biosynthetic
pathway of TTX.
Acknowledgment. We thank Professor M. Yamashita
(Tohoku University) for the kind gifts of the 1H NMR spectra
and authentic samples of 1 and 2, and Professors M. Isobe
and T. Nishikawa (Nagoya University) for valuable discus-
sions. We thank Dr. N. Hamanaka (Ono Pharmaceutical Co.
Ltd.) for the X-ray analysis of 15. This work was supported
by the JSPS KAKENHI (Nos. 16201045 and 16073214), a
JSPS Grant from the Research for the Future Program
(99L01204), and a SUNBOR grant.
Supporting Information Available: Experimental details
and NMR, IR, and MS data of new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
(15) Treatment with 80% aq TFA gave 31 as a sole product.
OL062098D
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Org. Lett., Vol. 8, No. 21, 2006