J. Am. Chem. Soc. 2001, 123, 8851-8853
8851
Communications to the Editor
Stereoselective Total Synthesis of the Cyanobacterial
Hepatotoxin 7-Epicylindrospermopsin: Revision of
the Stereochemistry of Cylindrospermopsin
Geoffrey R. Heintzelman, Wen-Kui Fang, Stephen P. Keen,
Grier A. Wallace, and Steven M. Weinreb*
Department of Chemistry
The PennsylVania State UniVersity
UniVersity Park, PennsylVania 16802
ReceiVed May 29, 2001
A serious outbreak of hepatoenteritis in 1979 on Palm Island
(Queensland, Australia) requiring the hospitalization of about 100
people was found to be due to drinking water in which the
cyanobacterium (blue-green alga) Cylindrospermopsin raciborskii
was growing.1 It was discovered that this freshwater alga produces
a toxic substance causing hepatotoxicity symptoms in mice
identical to those that afflicted the human victims. In 1992, Moore
and co-workers2 described the isolation of the toxin, which was
named cylindrospermopsin, and using extensive NMR evidence
proposed the tetracyclic structure and stereochemistry shown in
1 for this metabolite. More recently, the same hepatotoxin was
isolated from the alga Umezakia natans collected in Lake Mikata
(Fukui, Japan)3 and from Aphanizomenon oValisporum found in
Lake Kinneret in Israel.4 The latter cyanobacterium was also found
to coproduce a minor metabolite 7-epicylindrospermopsin, for-
mulated as 2, which was reported to be as toxic as 1.5 A key
premise in the assignment of stereochemistry at C-7 for 1 and 2
is that the molecules exist in the rigid conformations shown,
enforced by a hydrogen bond between an enolic uracil D-ring
tautomer and the guanidine C-ring. Such a conformation was used
to rationalize the observed C-7,8 proton coupling constants in
the two isomers. A third metabolite in the series, 7-deoxycylin-
drospermopsin (3), was also recently isolated from C. raciborskii.6
Interestingly, this latter compound proved to be nontoxic. Cy-
lindrospermopsin continues to be a serious public health problem,
particularly in tropical areas, and has recently been traced to the
deaths of livestock in Australia.7 On the basis of work reported
by Runnegar and co-workers it appears that cylindrospermopsin
exerts its toxic effects by inhibiting biosynthesis of cell-reduced
glutathione.8
We9 and others10 have described studies on the synthesis of
cylindrospermopsin, and the Snider group has recently reported
a total synthesis of this structurally unique natural product.10c In
this paper we disclose a synthesis which completely controls the
six stereogenic centers of the proposed cylindrospermopsin
structure 1 and which now proves that the stereochemical
assignments at C-7 in fact have been reversed in cylindrosper-
mopsin and the 7-epi compound. Thus, cylindrospermopsin has
the constitution shown in 2 and 7-epicylindrospermopsin is 1 (vide
infra). Our approach utilizes a novel stereospecific intramolecular
[4 + 2]-cycloaddition of an N-sulfinylurea heterodienophile11 and
application of our new efficient uracil synthesis9c as key strategic
steps.
Construction of the requisite Diels-Alder precursor 12 with
the attendant four stereogenic centers contained in the piperidine
A-ring was effected as outlined in Scheme 1. Using the methodol-
ogy of Comins,12 an efficient high-yield sequence was developed
for preparation of vinylogous urethane 4 involving N-acylation
of 4-methoxypyridine with benzyl chloroformate, followed by
addition of (allyldimethylsilyl)methylmagnesium bromide,13 and
subsequent trans-selective enolate methylation of the resulting
enone product. Conjugate addition of a vinyl goup to enone 4
cleanly and stereospecifically9b,12 afforded the desired ketone 5,
which could be cleanly reduced with L-Selectride, and the
resulting alcohol protected to produce benzyl ether 6. Tamao
oxidation13 of silane 6, followed by in situ cyclization of the
resulting alcohol provided carbamate 7, and subsequent hydro-
boration then yielded alcohol 8. Swern oxidation of 8 to the
(1) Hawkins, P. R.; Runnegar, M. T. C.; Jackson, A. R. B.; Falconer, I. R.
Appl. EnViron. Microbiol. 1985, 50, 1292.
(2) Ohtani, I.; Moore, R. E.; Runnegar, M. T. C. J. Am. Chem. Soc. 1992,
114, 7941. Moore, R. E.; Ohtani, I.; Moore, B. S.; DeKoning, C. B.; Yoshida,
W. Y.; Runnegar, M. T. C.; Carmichael, W. W. Gazz. Chim. Ital. 1993, 123,
329.
(3) (a) Harada, K.; Ohtani, I.; Iwamoto, K.; Suzuki, M.; Watanabe, M. F.;
Watanabe, M.; Terav, K. Toxicon 1994, 32, 73. (b) Terav, K.; Ohmori, S.;
Igarashi, K.; Ohtani, I.; Watanabe, M. F.; Harada, K. I.; Ito, E.; Watanabe,
M. Toxicon 1994, 32, 833 and references therein.
(4) Banker, R.; Carmeli, S.; Hadas, O.; Teltsch, B.; Porat, R.; Sukenik, A.
J. Phycol. 1997, 33, 613.
(5) Banker, R.; Teltsch, B.; Sukenik, A.; Carmeli, S. J. Nat. Prod. 2000,
63, 387. See also: Banker, R.; Carmeli, S.; Werman, M.; Telsch, B.; Porat,
R.; Sukenik, A. J. Toxicol. EnViron. Health 2001, 62, 281.
(6) Norris, R. L.; Eaglesham, G. K.; Pierens, G.; Shaw, G. R.; Smith, M.
J.; Chiswell, R. K.; Seawright, A. A.; Moore, M. R. EnViron. Toxicol. 1999,
14, 163.
(9) (a) Heintzelman, G. R.; Parvez, M.; Weinreb, S. M. Synlett 1993, 551.
(b) Heintzelman, G. R.; Weinreb, S. M.; Parvez, M. J. Org. Chem. 1996, 61,
4594. (c) Keen, S. P.; Weinreb, S. M. Tetrahedron Lett. 2000, 41, 4307.
(10) (a) Snider, B. B.; Harvey, T. C. Tetrahedron Lett. 1995, 36, 4587. (b)
Snider, B. B.; Xie, C. Tetrahedron Lett. 1998, 39, 7021. (c) Xie, C.; Runnegar,
M. T. C.; Snider, B. B. J. Am. Chem. Soc. 2000, 122, 5017. The stereochem-
istry generated at C-7 in the Snider approach was not independently established.
(d) McAlpine, I. J.; Armstrong, R. W. Tetrahedron Lett. 2000, 41, 1849. (e)
White, J. D.; Hansen, J. D. Abstracts of Papers, 219th National Meeting of
the American Chemical Society, San Francisco, CA; American Chemical
Society: Washington, DC, 2000; ORGN 812. (f) Djung, J. F.; Hart, D. J.;
Young, E. R. R. J. Org. Chem. 2000, 65, 5668. (g) Looper, R. E.; Williams,
R. M. Tetrahedron Lett. 2001, 42, 769.
(11) For reviews of N-sulfinyl dienophile Diels-Alder-based methodology,
see: (a) Weinreb, S. M. Acc. Chem. Res. 1988, 21, 313. (b) Boger, D. L.;
Weinreb, S. M. Hetero Diels-Alder Methodology in Organic Synthesis;
Academic Press: San Diego, 1987; Chapter 1. (c) Weinreb, S. M. Hetero-
dienophile Additions to Dienes. In ComprehensiVe Organic Synthesis; Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 5, p 401.
(12) Comins, D. L.; LaMunyon, D. H.; Chen, X. J. Org. Chem. 1997, 62,
8182 and references therein.
(7) Saker, M. L.; Thomas, A. D.; Norton, J. H. EnViron. Toxicol. 1999,
14, 179.
(8) Runnegar, M. T.; Kong, S.-M.; Zhong, Y.-Z.; Ge, J.-L.; Lu, S. C.
Biochem. Biophys. Res. Commun. 1994, 201, 235. Runnegar, M. T.; Kong,
S.-M.; Zhong, Y.-Z.; Lu, S. C. Biochem. Pharmacol. 1995, 49, 219.
(13) Tamao, K.; Ishida, N. Tetrahedron Lett. 1984, 25, 4249.
10.1021/ja011291u CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/18/2001