The Chemistry and Biology of Saxitoxin
Burke Group Literature Seminar
February 1, 2008
Hosted by Tom Anderson
Guanidinium Toxin Family
(+)-Saxitoxin
IC50 = 5 nM
(–)-Tetrodotoxin
Alexandrium catanella
Taricha torosa
Atelopus varius
Geographical Distribution of Paralytic
Shellfish Poison Red Tides
Moving Ions Across the Membrane
K+ Channel
Gramicidin D
Amphotericin B
Doyle, D. et al Science 1998, 280, 69-77
Wallace, B.A.; Ravikumar, K. Science 1988, 241, 182-187
Palacios, D.S.; Anderson, T.M.; Burke, M.D. J. Am. Chem. Soc. 2007, 129, 13804-13805
Functional Studies of Ion Channels
Ruta, V. et al Nature (London) 2003, 422, 180-185
Na+ Channels and Action Potentials
Voltage-Gated Na+ and Medicine
Myotonia is a paralysis inducing
channelopathy of skeletal muscle Na+
channels
Long QT syndrome produces in
ventricular arrhytmias which can result
in loss of consciousness or sudden
death
Lidocaine
Ashcroft, F.M. Ion Channels and Disease; Academic Press, San Diego, 2000.
Novocaine
Anatomy of a Sodium Channel
O
Saxitoxin (+2)
Tetrotodoxin (+1)
-O
Ca
Na
EXTERNAL
‐Dissociation constants
versus voltage curves
of both STX and TTX
have essentially
identical slopes
INTERNAL
αS589 Mutants Change Ion Permeability
Kellenberger, S.; Gautschi, I.; Schild, L. Proc. Natl. Acad. Sci. 1999, 96, 4170-4175.
Ion Radius Effects on Permeability
Kellenberger, S.; Gautschi, I.; Schild, L. Proc. Natl. Acad. Sci. 1999, 96, 4170-4175.
Cryo-EM in the Voltage-Gated Na+ Channel
Sato, C.; et al. Nature 2001, 409, 1047-1051
Enantioselective Total Synthesis of
(-)-Decarbamoylsaxitoxin
Kishi, Y. Heterocycles 1980, 14, 1477-1495.
Hong, C. Y., Kishi, Y. J. Am. Chem. Soc. 1992, 114, 7001-7006.
Synthesis of Pre-cyclization Intermediate
Kishi, Y. Heterocycles 1980, 14, 1477-1495.
Trimolecular Cyclization to Develop
C6 Stereocenter
Hong, C. Y., Kishi, Y. J. Am. Chem. Soc. 1992, 114, 7001-7006.
Formation of Ureacycle
• Model system: acid-promoted cyclization
A
B
Cyclization conditions
Ratio (A:B)
AcOH, 50 °C
>>20:1
EtOH, PhMe, reflux
>>20:1
TFA, rt
2:3
Lack of interconversion of A and B under reaction conditions led
to dual mechanism proposal
Kishi, Y. Heterocycles 1980, 14, 1477-1495.
Dual Cyclization Manifold
H
HN
O
Me
N
H
N
O
NH2
TFA
H
HN
O
Me
H H
N
N
OH
NH
H
HN
O
Me
H H
N
O
N
N
H
Cyclization in actual system proved problematic, presumably because of
steric interaction with thioacetal
Kishi, Y. Heterocycles 1980, 14, 1477-1495.
Characterization of urea-thiourea
intermediate
BnO
H H
N
HN
S
optically active sample
in CDCl3
O
N
N
H
S
S
urea-thiourea 3
Observations:
• NMR identical for both samples in DOMe
• NMRs completely different in CDCl3
• CDCl3 spectra of racemate is concentration
dependant
racemic sample
in CDCl3
Hong, C. Y., Kishi, Y. J. Am. Chem. Soc. 1992, 114, 7001-7006.
Characterization of urea-thiourea
intermediate
BnO
H H
N
HN
S
O
N
N
H
S
S
urea-thiourea 3
Proposed binding scheme
BnO
H H
N
HN
S
O
N
N
H
S
OMe
S
key intermediate of 15
gonyautoxin synthesis
Hong, C. Y., Kishi, Y. J. Am. Chem. Soc. 1992, 114, 7001-7006.
Jacobi Synthetic Strategy
Kishi, JACS, 1977, 22, 99, 2818
Converted to Saxitoxin in 4 steps
(Avg . Yield: 47%)
Overall: 18 steps, 0.14% yield (73% average)
Jacobi, P.A.; Martinelli, M.J.; Polanc, S. J. Am. Chem. Soc. 1984, 106, 5594-5598
Forward Synthesis
• Scalability
• No chromatography required
10 grams
Stereoselective Cyclization
Jacobi, P.; Martinell, M.; Polanc,S. J. Am. Chem. Soc. 1984, 106, 5594-5598
Jacobi, P.; Brownstein, A.; Martinell, M.; Grozinger, K. J. Am. Chem. Soc. 1981, 103, 239-241
Epimerization of β-ester
NMR comparison
Jacobi, P. A.; Martinelli, M. J.; Polanc, S. J. Am. Chem. Soc. 1984, 106, 5594-5598.
Du Bois Synthetic Strategy
Fleming, J.J.; Du Bois, J. J. Am. Chem. Soc. 2006, 128, 3926-3927
Fleming, J.J.; McReynolds, M.D.; Du Bois, J. J Am. Chem. Soc. 2007, 129, 9964-9975
Nine-memberd ring formation
Alkene Ketohydroxylation
Mechanistic skeleton:
Du Bois Second Generation Synthesis
References
1. (a) Kreuger, B.K.; Worley, III, J.F.; French, R.J. Block of Sodium Channels in
Planar Lipid Bilayers by Guanidinium Toxins and Calcium. In Tetrodotoxin,
Saxitoxin, and the Molecular Biology of the Sodium Channel. Kao, C.Y.,
Levinson, S.R., Eds.; Annals of the New York Academy of Sciences: New York,
1986; pp. 257-268. (b) Kellenberger, S.; Gautschi, I.; Schild, L. Proc. Natl.
Acad. Sci. U.S.A. 1999, 96, 4170-4175. (c) Sato, C.; Ueno, Y.; Asai, K.;
Takahashi, K.; Sato, M.; Engel, A.; Fujiyoshi, Y.; Nature 2001, 409, 1047-1051.
2. (a) Kishi, Y. Heterocycles 1980, 14, 1477-1495. (b) Chang, C.Y.; Kishi, Y. J
Am. Chem. Soc. 1992, 114, 7001-7006.
3. Jacobi, P.A.; Martinelli, M.J.; Polanc, S. J. Am. Chem. Soc. 1984, 106, 55945598.
4. (a) Fleming, J.J.; McReynolds, M.D.; Du Bois, J. J Am. Chem. Soc. 2007, 129,
9964-9975. (b) Fleming, J.J.; Du Bois, J. J. Am. Chem. Soc. 2006, 128, 39263927.