Synthesis of an advanced intermediate bearing two hydroxy groups for (-)-tetrodotoxin and its analogs

Article abstract of DOI:10.1246/bcsj.20090223

A new intermediate for tetrodotoxin and its analogs was synthesized from a bicyclic lactam intermediate, which was prepared from a known enone compound by a novel neighboring group participation as a key step.

Full text of DOI:10.1246/bcsj.20090223

66
Bull. Chem. Soc. Jpn. Vol. 83, No. 1, 66–68 (2010)
Short Articles
H
H
N
HO
N
NH2
HO
NH2
Synthesis of an Advanced
Intermediate Bearing Two Hydroxy
Groups for (¹)-Tetrodotoxin
and Its Analogs
NH
NH
5
R2
OH
OH
HO
11
O
8
11
CH3
O
OH
O
OH
O
O
R1
tetrodotoxin (1) R1 = R2 = OH
11-deoxytetrodotoxin (2) R1 = H, R2 = OH
8,11-dideoxytetrodotoxin (3) R1 = R2 = H
5,11-dideoxytetrodotoxin (4)
Figure 1. Structures of tetrodotoxin and its analogs.
Toshio Nishikawa,* Yuya Koide, Masaatsu Adachi,
and Minoru Isobe*
O
O
O
O
O
O
Laboratory of Organic Chemistry, Graduate School
of Bioagricultural Sciences, Nagoya University,
Chikusa-ku, Nagoya 464-8601
COCCl3
NH
PG
NH
COCCl3
NH
11
8
8
11
11
CH3
OTES
OTES
OH
CH3
R
Received August 24, 2009
E-mail: nisikawa@agr.nagoya-u.ac.jp, isobem@ff.iij4u.or.jp
OR
5a R = H
5b R = OH
6
7
Figure 2. The synthetic intermediates for tetrodotoxin and
its analogs.
A new intermediate for tetrodotoxin and its analogs was
synthesized from a bicyclic lactam intermediate, which
was prepared from a known enone compound by a novel
neighboring group participation as a key step.
O
O
O
O
COCCl3
NH
COCCl3
NH
5a
11
In the course of our synthetic studies on tetrodotoxin (1), a
CH3
O
O
well-known marine natural product,1,2 we have employed a
simple compound 5a as a common key intermediate3 for the
synthesis of 11-deoxy analogs of tetrodotoxin such as 11-
deoxytetrodotoxin (2),4 8,11-dideoxytetrodotoxin (3),5 and
5,11-dideoxytetrodotoxin (4)6 (Figures 1 and 2). Fortunately,
the intermediate 5a could also be used for total synthesis of
tetrodotoxin (1),7 for which additional hydroxy group was
introduced at the C-11 position by allylic oxidation with SeO2
of the olefinic methyl group of an intermediate 6 prepared from
5a. However, similar allylic oxidation of the other substrates,
such as 5a and 5b, proved to be difficult. Our recent interest in
elucidating the biosynthetic and metabolic pathway of tetrodo-
toxin prompted us to develop a more general and flexible
synthetic route for tetrodotoxin analogs, and particularly for
11-hydroxylated analogs of tetrodotoxin. For this purpose, we
designed an advanced intermediate as 7 possessing two
hydroxy groups at the C-11 and C-8 positions. This short
article describes the synthesis of compound 7 from a known
intermediate 5a by using a novel neighboring group partic-
ipation of the trichloroacetamide.
The common intermediate 5a was transformed to enone 8 by
a method developed in our laboratory,6 and hydroxylation at the
C-11 position was then attempted. Direct allylic oxidation of
the methyl group with SeO2 gave a complex mixture. Two-step
hydroxylation including formation of vinyl silyl ether from 8
followed by oxidation with MCPBA was also unsuccessful.
The regioselectivity of the enolization might be one of the
reasons. During examination of conditions for the enolization,
we were surprised to find that treatment of enone 8 with
Na2CO3 in refluxing DMF gave bicyclic lactam 10 in moderate
yield (67-75%). The condition has been employed for
OH
8
Na2CO3
DMF, reflux
O
O
C
N
HMBC
NH
C
5
H
O
CH3
O
O
CH3
O
(67-75%)
11
O
O
10
9
Scheme 1. Synthesis of bicyclic £-lactam intermediate 10.
generation of isocyanate from trichloroacetamide,8 therefore
involvement of an isocyanate intermediate 9 would promote the
intramolecular trapping by dienolate emerging from the enone
to result in affording 10. The structure of 10 was determined by
extensive NMR analysis; the newly formed C-C bond was
confirmed by HMBC correlation between the methine proton at
the C-5 position and the lactam carbonyl carbon as shown in
Scheme 1. Since the bicyclic structure of the product 10 allows
exclusive enolization at the C-11 position, hydroxylation of the
C-11 position was attempted again by the above-mentioned
procedure. The lactam of 10 was protected with a Boc group,
and then the product 11 was used for preparation of the
corresponding vinyl silyl ether (Scheme 2). Upon treatment
of 11 with TBSOTf in the presence of Et3N, the desired vinyl
silyl ether 12 was obtained as an unstable product, which
was therefore immediately epoxidized with MCPBA for this
hydroxylation. We found that the oxidation was best carried out
Published on the web December 18, 2009; doi:10.1246/bcsj.20090223

T. Nishikawa et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 1 (2010)
67
O
O
Na2SO4, and concentrated. The residue was purified by column
chromatography (SiO2 gel 250 g, AcOEt:hexane = 1:1) to afford
bicyclic lactam 10 (8.30 g, 67%) as yellow crystals. Mp 165.7-
NR
NBoc
1) TBSOTf
Et3N
2) MCPBA
CH3
O
OTBS
24
EtOH
0 °C
166.5. ½¡ꢀD ¹371 (c 1.00, CHCl3). IR (KBr): ¯max 3306, 3084,
CH2Cl2
-78 °C
O
O
O
O
2987, 2936, 2879, 2814, 1722, 1682, 1622, 1435, 1380, 1070
¹1
.
1H NMR (400 MHz, CDCl3): ¤ 1.29 (3H, s, CH3 of
(75% in 2 steps)
cm
Boc2O, DMAP
THF (93%)
12
10 R = H
11 R = Boc
acetonide), 1.36 (3H, s, CH3 of acetonide), 2.11 (3H, d, J = 1.5
Hz, CH=CCH3), 2.80 (1H, dd, J = 9, 4 Hz, NH-C-CH), 2.87 (1H,
dd, J = 4, 0.5 Hz, CH=CCH3CH), 3.56 (1H, dd, J = 8, 6.5 Hz, O-
CHAHB-CH-O), 3.97 (1H, dd, J = 8, 6 Hz, O-CHAHB-CH-O),
4.21 (1H, dt, J = 9, 6 Hz, O-CH2-CH-O), 5.36 (1H, d, J = 11 Hz,
CH=CHCHD), 5.39 (1H, d, J = 17.5 Hz, CH=CHCHD), 5.66 (1H,
br s, NH), 5.90 (1H, m, CH=CCH3), 6.45 (1H, dd, J = 17.5,
11 Hz, CH=CH2). 13C NMR (100 MHz, CDCl3): ¤ 24.1, 25.6,
26.6, 52.2, 61.3, 68.4, 69.0, 72.5, 110.4, 115.2, 124.0, 133.9,
156.6, 175.0, 191.9. Anal. Calcd for C15H19NO4: C, 64.97; H,
6.91; N, 5.05%. Found: C, 64.98; H, 6.83; N, 5.31%.
O
O
i) 1M LiOH
THF,
rt, 15 min
O
NBoc
Boc
NH
NaHCO3
MeOH, rt
O
ii) neutralization
with HCl aq., rt
OR
O
O
O
(98% in 4 steps
OEE
from 13)
15
EVE, PPTS
CH2Cl2, rt
13 R = H
14 R = EE
O
O
O
N-Boc-bicyclic Lactam 11. A solution of bicyclic lactam 10
(7.10 g, 25.6 mmol), Boc2O (6.5 mL, 28.2 mmol), and DMAP
(312 mg, 2.56 mmol) in THF (200 mL) was stirred at reflux
temperature for 30 min. The reaction mixture was diluted with
Et2O, washed with H2O (©2) and brine (©1), dried over anhydrous
Na2SO4, and concentrated. The residue was purified by column
chromatography (SiO2 gel 300 g, AcOEt:hexane = 1:2) to afford
N-Boc-bicyclic lactam 11 (9.0 g, 93%) as yellow crystals. Mp
O
Boc
NH
Boc
4a
LiAlH4
NH
8
Et2O
-78 °C
11
OH
O
OEE
OEE
(96%)
17
16
Scheme 2. Synthesis of the new intermediate 17.
25
128.9-129.6. ½¡ꢀD ¹331 (c 0.74, CHCl3). IR (KBr): ¯max 2987,
1
1787, 1698, 1624, 1372, 1298 cm¹1. H NMR (400 MHz, CDCl3):
in ethanol solvent for spontaneous hydrolysis to give the
desired allylic alcohol 13 in a good yield. After protection of
the alcohol as 1-ethoxyethyl (EE) ether, the lactam ring of the
resulting 14 was cleaved with aqueous lithium hydroxide.
Subsequent decarboxylation occurred upon careful neutraliza-
tion to yield ¢,£-unsaturated ketone 15, which was treated with
NaHCO3 in methanol to afford ¡,¢-unsaturated ketone 16 in an
excellent overall yield. Reduction of the unsaturated ketone 16
with LiAlH4 in ether as a solvent proceeded stereoselectively to
furnish 17 (7: R = EE) in a high yield (96% as a single
product). The configuration of the C-8 position was determined
by observing the NOESY correlation between the protons of
the C-8 and the C-4a positions.
In summary, we have developed a route for the synthesis of
the advanced intermediate 17 from enone 8. The salient feature
of this route is utilization of the bicyclic intermediate, which
was incidentally obtained from 8. In our previous synthetic
studies on tetrodotoxin, the trichloroacetyl group has been used
not only as a protective group for the amino function, but also
for other purposes such as guanidine synthesis and hydroxyl-
ation through a neighboring group participation. The synthesis
described in this short article demonstrates a new use for the
trichloroacetamide group. The compound 17 possessing two
hydroxy groups at the C-8 and -11 positions should play a
crucial role for expeditious synthesis of a variety of tetrodo-
toxin derivatives for biochemical experiments.
¤ 1.27 (3H, s, CH3 of acetonide), 1.34 (3H, s, CH3 of acetonide),
1.42 (9H, s, t-butyl), 2.10 (3H, d, J = 1.5 Hz, CH=CCH3), 2.82
(1H, dd, J = 8, 4 Hz, NBoc-C-CH), 2.96 (1H, dd, J = 4, 1 Hz,
CH=CCH3CH), 3.55 (1H, dd, J = 8, 6.5 Hz, O-CHAHB-CH-O),
3.94 (1H, dd, J = 8, 6 Hz, O-CHAHB-CH-O), 4.11 (1H, dt, J = 8,
6.5 Hz, O-CH2-CH-O), 5.34 (1H, d, J = 18 Hz, CH=CHCHD),
5.36 (1H, d, J = 11 Hz, CH=CHCHD), 5.90 (1H, m, CH=CCH3),
6.70 (1H, dd, J = 18, 11 Hz, CH=CH2). 13C NMR (100 MHz,
CDCl3): ¤ 23.8, 25.5, 26.4, 27.7, 52.2, 58.6, 68.3, 71.4, 72.0, 84.6,
110.3, 115.1, 124.5, 133.7, 148.6, 154.9, 170.9, 189.8. Anal. Calcd
for C20H27NO6: C, 63.64; H, 7.21; N, 3.71%. Found: C, 63.44; H,
7.20; N, 3.88%.
11-Hydroxy-N-Boc-bicyclic Lactam 13. A flask was charged
with N-Boc-bicyclic lactam 11 (310 mg, 0.822 mmol) and con-
nected to a vacuum/nitrogen line. The flask was evacuated and
then filled with nitrogen. The substrate was dissolved in dry
CH2Cl2 (9.3 mL). This solution was cooled to ¹78 °C and treated
with Et3N (0.57 mL, 4.11 mmol) and TBSOTf (0.57 mL, 2.47
mmol). After stirring at ¹78 °C for 3 h 10 min, the reaction mixture
was diluted with Et2O. The mixture was quenched with sat.
NaHCO3 solution and extracted with Et2O (©3). The combined
organic extracts were washed with sat. NH4Cl solution (©2), sat.
NaHCO3 solution (©2), H2O (©2) and brine (©1), dried over
anhydrous Na2SO4, and concentrated. This crude vinyl silyl ether
12 (577 mg) was used for the next step without further purification.
To a stirred solution of MCPBA (213 mg, 1.23 mmol) in EtOH
(10 mL) cooled at 0 °C was added a solution of the crude vinyl
silyl ether 12 (577 mg) in EtOH (7 mL) cooled at 0 °C. After
stirring for 17.5 h, the reaction mixture was poured into sat.
Na2SO3 solution and sat. NaHCO3 solution. The mixture was
extracted with Et2O (©3), and the combined organic layer was
washed with H2O (©2) and brine (©1), dried over anhydrous
Na2SO4, and concentrated. The residue was purified by column
chromatography (SiO2 gel, 9 g, AcOEt:hexane = 1:1) to afford
lactam 13 (243 mg, 75% in 2 steps) as white crystals. Mp 139.7-
Experimental
Bicyclic Lactam 10. To a solution of the enone 8 (17.5 g,
44.3 mmol) in DMF (500 mL) was added Na2CO3 (9.40 g, 88.6
mmol). After stirring at 170 °C for 15 min, the reaction mixture
was poured into an ice-cold sat. NH4Cl solution. The mixture was
extracted with AcOEt (©3), and the combined organic layer was
washed with H2O (©2) and brine (©1), dried over anhydrous

68
Bull. Chem. Soc. Jpn. Vol. 83, No. 1 (2010)
© 2010 The Chemical Society of Japan
25
140.1. ½¡ꢀD ¹316 (c 0.995, CHCl3). IR (KBr): ¯max 3524, 2985,
(SiO2 gel 36 g, AcOEt:hexane = 1:3 ¼ 1:1) to afford allylic
25
¹1
2874, 1782, 1698, 1372, 1307, 1156 cm
.
1H NMR (400 MHz,
alcohol 17 (1.16 g, 96%) as a colorless amorphous solid. ½¡ꢀD
CDCl3): ¤ 1.26 (3H, s, CH3 of acetonide), 1.34 (3H, s, CH3 of
acetonide), 1.41 (9H, s, t-butyl), 2.86 (1H, dd, J = 8.5, 4 Hz,
NBoc-C-CH), 3.08 (1H, dd, J = 4, 0.5 Hz, CH=CCH2OHCH),
3.56 (1H, dd, J = 8, 6.5 Hz, O-CHAHB-CH-O), 3.98 (1H, dd,
J = 8, 6 Hz, O-CHAHB-CH-O), 4.11 (1H, dt, J = 8.5, 6 Hz, O-
CH2-CH-O), 4.32 (1H, dd, J = 16.5, 2 Hz, CHCHDOH), 4.40 (1H,
dd, J = 16.5, 1.5 Hz, CHCHDOH), 5.35 (1H, d, J = 18 Hz,
CH=CHCHD), 5.36 (1H, d, J = 11 Hz, CH=CHCHD), 6.12 (1H,
m, CH=CCH2OH), 6.71 (1H, dd, J = 18, 11 Hz, CH=CH2).
13C NMR (100 MHz, CDCl3): ¤ 25.5, 26.4, 27.8, 47.8, 58.6, 63.5,
68.2, 71.9, 72.2, 84.9, 110.4, 115.1, 122.1, 133.7, 148.6, 156.2,
171.4, 189.7. Anal. Calcd for C20H27NO7: C, 61.06; H, 6.92; N,
3.56%. Found: C, 61.05; H, 6.87; N, 3.69%.
11-Protected-Hydroxy-N-Boc Enone 16. To a solution of 13
(2.47 g, 6.28 mmol) in CH2Cl2 (70 mL) were added ethyl vinyl
ether (1.80 mL, 18.9 mmol) and pyridinium p-toluenesulfonate
(2.37 g, 9.42 mmol). After stirring at rt for 15 min, the reaction
mixture was diluted with Et2O, washed with H2O (©2) and brine
(©1), dried over anhydrous Na2SO4, and concentrated. The crude
product 14 (2.95 g) was dissolved in THF (88 mL), and 1 M LiOH
solution (12.6 mL, 12.6 mmol) was added. After vigorous stirring
at rt for 15 min, the reaction mixture was carefully neutralized to
pH 7 by slow addition of 1 M HCl into cold reaction mixture with
stirring, and the stirring was continued at rt for 14 h 40 min. The
mixture was diluted with AcOEt, washed with H2O (©2) and brine
(©1), dried over anhydrous Na2SO4, and concentrated. The crude
product 15 (2.81 g) was dissolved in MeOH (84 mL), and NaHCO3
(791 mg, 9.42 mmol) was added. After vigorous stirring at rt for
21.5 h, the reaction mixture was quenched with sat. NH4Cl
solution. The mixture was extracted with AcOEt (©3), and the
combined organic layer was washed with H2O (©2) and brine
(©1), dried over anhydrous Na2SO4, and concentrated. The
residue was purified by column chromatography (SiO2 gel 55 g,
AcOEt:hexane = 1:2) to afford enone 16 (2.7 g, 98% in 4 steps)
¹2.0 (c 1.03, CHCl3). IR (KBr): ¯max 3362, 2984, 2864, 1699,
1
1538, 1370, 1286 cm¹1. H NMR (300 MHz, CDCl3): ¤ 1.19 (3H,
t, J = 7.5 Hz, OCH2CH3), 1.30 (3H, d, J = 5.5 Hz, OCHCH3),
1.38 (3H, s, CH3 of acetonide), 1.41 (3H, s, CH3 of acetonide),
1.44 (9H, s, t-butyl), 1.55-1.72 (1H, m, CH=CCHAHBCH), 1.78-
1.88 (1H, m, CH=CCHAHBCH), 1.89-2.01 (1H, m, CH2CHC-
NBoc), 3.45 (1/2H, q, J = 7 Hz, OCHCHDCH3), 3.48 (1/2H, q,
J = 7 Hz, OCHCHDCH3), 3.57-3.67 (2H, m, OCHCHDCH3, O-
CHEHF-CH-O), 3.83-4.07 (4H, m, CH2OEE, O-CHEHF-CH-O,
O-CH2-CH-O), 4.44 (1H, br s, CHOH), 4.68 (1H, q, J = 5.5 Hz,
O-CH-O), 5.29 (1H, br d, J = 17.5 Hz, CH=CHIHJ), 5.41 (1H, br
d, J = 11 Hz, CH=CHIHJ), 5.63 (1H, s, CH=CCH2OEE), 5.79
(1H, ddd, J = 17.5, 11, 1 Hz, CH=CH2), 6.72 (1H, s, OH), 7.52
(1H, s, NH). Anal. Calcd for C23H39NO7: C, 62.56; H, 8.90; N,
3.17%. Found: C, 62.56; H, 9.17; N, 3.28%.
We are grateful to Mr. S. Kitamura (analytical laboratory in
this school) for performing the elemental analyses. This work
was financially supported by the PRESTO Program of the
Japan Science and Technology Agency, by a Grant-in-Aid for
Scientific Research and G-COE Program of MEXT, and by the
Naito Foundation.
Supporting Information
Copies of 1H NMR and 13C NMR spectra of all new com-
pounds. This material is available free of charge on the web at
http://www.csj.jp/journals/bcsj/.
References
1
a) T. Goto, Y. Kishi, S. Takahashi, Y. Hirata, Tetrahedron
1965, 21, 2059. b) K. Tsuda, S. Ikuma, M. Kawamura, R.
Tachikawa, K. Sakai, C. Tamura, O. Amakasu, Chem. Pharm. Bull.
1964, 12, 1357. c) R. R. Woodward, Pure Appl. Chem. 1964, 9, 49.
24
as a colorless amorphous solid. ½¡ꢀD +7.3 (c 1.08, CHCl3). IR
2
For reviews on tetrodotoxin, see: a) Issue of Tetrodotoxin,
(KBr): ¯max 3398, 2984, 2873, 1718, 1496, 1369 cm¹1. H NMR
Saxitoxin, and the Molecular Biology of the Sodium Channel:
Ann. N.Y. Acad. Sci. 1986, 479. doi:10.1111/j.1749-6632.1986.
tb15556.x b) T. Noguchi, O. Arakawa, Mar. Drugs 2008, 6, 220.
c) T. Narahashi, Proc. Jpn. Acad., Ser. B 2008, 84, 147.
1
(300 MHz, CDCl3): ¤ 1.19 (3/2H, t, J = 7 Hz, OCH2CH3), 1.20
(3/2H, t, J = 7 Hz, OCH2CH3), 1.33 (3H, d, J = 5 Hz, OCHCH3),
1.36 (3H, s, CH3 of acetonide), 1.39 (3H, s, CH3 of acetonide),
1.44 (9H, s, t-butyl), 2.19 (2H, m, CH=CCH2CH), 3.25 (1H, q,
J = 8 Hz, CH2CHCNBoc), 3.42-3.54 (1H, m, OCHAHBCH3),
3.56-3.68 (1H, m, OCHAHBCH3), 3.66 (1H, t, J = 8 Hz, O-
CHCHD-CH-O), 4.00-4.28 (4H, m, O-CHCHD-CH-O, O-CH2-
CH-O, CH2OEE), 4.76 (1H, q, J = 5 Hz, O-CH-O), 5.09 (1H,
br s, NH), 5.29 (1H, dd, J = 17.5, 1 Hz, CH=CHEHF), 5.35
(1H, d, J = 11 Hz, CH=CHEHF), 5.88 (1H, ddd, J = 17.5, 11,
1.5 Hz, CH=CH2), 6.27 (1H, m, CH=CCH2OEE). Anal. Calcd for
C23H37NO7: C, 62.85; H, 8.48; N, 3.19%. Found: C, 62.85; H,
8.61; N, 3.19%.
Allylic Alcohol 17. A flame-dried flask was charged with
LiAlH4 (207 mg, 5.47 mmol), and evacuated and then filled with
nitrogen. Dry Et2O (20 mL) was added, and the suspension was
cooled to ¹78 °C. To this flask was added the enone 16 (1.20 g,
2.73 mmol) in dry Et2O (16 mL). After stirring at ¹78 °C for 1 h,
the reaction was quenched with H2O. The mixture was stirred with
AcOEt, H2O, and aqueous potassium sodium tartrate for an
additional 10 min, and then extracted with AcOEt (©3). The
combined organic layer was dried over anhydrous Na2SO4, and
concentrated. The residue was purified by column chromatography
3
T. Nishikawa, M. Asai, N. Ohyabu, N. Yamamoto, Y.
Fukuda, M. Isobe, Tetrahedron 2001, 57, 3875.
T. Nishikawa, M. Asai, M. Isobe, J. Am. Chem. Soc. 2002,
124, 7847.
4
5
a) T. Nishikawa, D. Urabe, K. Yoshida, T. Iwabuchi, M.
Asai, M. Isobe, Org. Lett. 2002, 4, 2679. b) T. Nishikawa, D.
Urabe, K. Yoshida, T. Iwabuchi, M. Asai, M. Isobe, Chem.®Eur. J.
2004, 10, 452.
6
a) T. Nishikawa, M. Asai, N. Ohyabu, N. Yamamoto,
M. Isobe, Angew. Chem., Int. Ed. 1999, 38, 3081. b) M. Asai, T.
Nishikawa, N. Ohyabu, N. Yamamoto, M. Isobe, Tetrahedron
2001, 57, 4543.
7
a) T. Nishikawa, D. Urabe, M. Isobe, Angew. Chem., Int.
Ed. 2004, 43, 4782. b) D. Urabe, T. Nishikawa, M. Isobe, Chem.
Asian J. 2006, 1, 125.
8
a) D. Urabe, K. Sugino, T. Nishikawa, M. Isobe, Tetra-
hedron Lett. 2004, 45, 9405. b) T. Nishikawa, D. Urabe, M.
Tomita, T. Tsujimoto, T. Iwabuchi, M. Isobe, Org. Lett. 2006, 8,
3263.