Total synthesis of (+)-kalkitoxin

Article abstract of DOI:10.1039/b306124h

The neurotoxic lipopeptide (+)-kalkitoxin was synthesized by a route which employed asymmetric organocopper conjugate addition followed by in situ enolate alkylation to install the anti,anti-1,2,4-trimethyl relationship of the toxin; the synthesis of kalkitoxin required sixteen steps and proceeded in 3% overall yield.

Full text of DOI:10.1039/b306124h

Total synthesis of (+)-kalkitoxin†
James D. White,* Chang-Sun Lee and Qing Xu
Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, USA.
E-mail: james.white@orst.edu
Received (in Cambridge, UK) 30th May 2003, Accepted 23rd June 2003
First published as an Advance Article on the web 4th July 2003
The neurotoxic lipopeptide (+)-kalkitoxin was synthesized
by a route which employed asymmetric organocopper
conjugate addition followed by in situ enolate alkylation to
install the anti,anti-1,2,4-trimethyl relationship of the toxin;
the synthesis of kalkitoxin required sixteen steps and
proceeded in 3% overall yield.
The lipopeptide kalkitoxin (1) was isolated from an extract of
the cyanobacterium Lyngbya majuscula collected in the Car-
ibbean1 and has been shown to possess potent neurotoxicity in
a primary cell culture of rat neurons (LC50 3.86 nM).2
Kalkitoxin is also highly active in an inflammatory disease
model which measures IL-11b-induced sPLA2 secretion from
Scheme 2
HepG2 cells (IC50 27 nM),3 and there is evidence to suggest that
1 is a potent blocker of the voltage sensitive sodium ion channel
in mouse neuro-2a cells (EC50 1 nM; cf. EC50 of saxitoxin is 8
was reacted with (S)-N-(trans-crotonyl)-4-phenyloxazolidin-
2-one (6)8 (Scheme 2).
nM).4
The anti-1,3-dimethyl array of enolate 7 generated from 5
and 6 represents a matched case and has precedent in studies by
Williams9 who has proposed a transition state model for this
process.10 The use of Hruby’s 4-phenyloxazolidinone auxiliary
is of crucial importance in this asymmetric conjugate addition
since the Evans auxiliary, in which the phenyl substituent is
replaced by benzyl, gave significantly lower asymmetric
induction. Quenching of 7 and subsequent methylation of the
enolate in a separate step was found to yield a mixture of 8 and
9 favoring the former (2.8 : 1), but slightly improved
stereoselectivity occurred (3.6 : 1) when enolate 7 from
organocopper conjugate addition to 6 was trapped directly with
methyl iodide. After chromatographic separation from its
isomer 9, oxazolidinone 8 was crystallized and its structure was
confirmed by X-ray crystallographic analysis (Fig. 1).‡
Reductive removal of the benzyl ether from 8 and Swern
oxidation of the resultant primary alcohol gave aldehyde 10,
which was subjected to reductive amination with methylamine
and sodium cyanoborohydride (Scheme 3). The resulting
secondary amine 11 was coupled to (R)-2-methylbutyric acid11
in a process that required HOAt12 as an activating agent to yield
amide 12 (mixture of rotamers). Reductive cleavage of the
chiral adjuvant with lithium borohydride then furnished primary
alcohol 13. This substance required a one-carbon homologation
before proceeding toward construction of the thiazoline moiety
of 1, and for this purpose, 13 was oxidized to the corresponding
aldehyde. The latter was subjected to Wittig olefination with the
ylide from methoxymethyltriphenylphosphonium chloride13 to
furnish a mixture of (E)- and (Z)-enol ethers 14. Hydrolysis of
These properties make kalkitoxin and analogues attractive
targets for synthesis, and we report herein a concise route to the
natural (3R,7R,8S,10S,2AR) configuration of 1 employing a
tandem organocopper conjugate addition-alkylation sequence
for installing the 1,2,4-anti,anti-trimethyl relationship of the
toxin. This strategy diverges from a previous route to 1 used to
establish the structure and absolute configuration of kalkitoxin
in which the C7, C8, and C10 methyl substituents were
introduced sequentially.1
Our synthesis of 1 commenced from the lithium enolate of
(1S,2S)-N-propionylpseudoephedrine (2)5 which was alkylated
with 2-iodoethyl benzyl ether6 (Scheme 1). The resulting amide
3 was cleaved reductively with lithium amidotrihydroborate7 to
yield alcohol 4. The latter was converted to bromide 5 which,
after transformation to the corresponding organocopper species,
Scheme 1
† Electronic supplementary information (ESI) available: experimental
procedures and spectroscopic data. See http://www.rsc.org/suppdata/cc/b3/
b306124h/
Fig. 1 X-Ray crystal structure of 8 (ORTEP at 50% probability level).
2012
CHEM. COMMUN., 2003, 2012–2013
This journal is © The Royal Society of Chemistry 2003

Scheme 5
excellent yield, and subsequent reductive cleavage of the benzyl
thioether with sodium–ammonia furnished thiol 22 (Scheme 5).
Finally, exposure of 22 to titanium tetrachloride yielded
kalkitoxin (1, [a]D20 + 11.5 (c 0.34, CHCl3)), whose 1H and 13
C
NMR spectra precisely matched those of the natural material.
The sixteen-step sequence to kalkitoxin described above
proceeds in ca. 3% overall yield (Scheme 5) and has provided
sufficient material for comprehensive studies of neurotoxic and
cytotoxic properties. The results of these studies will be
published in due course.
We are grateful to Professor Alexandre F. T. Yokochi,
Oregon State University, and Dr Lee M. Daniels, Molecular
Structure Corporation, for the X-ray crystal structure of 8, and
to Professor William H. Gerwick, College of Pharmacy, Oregon
State University, for NMR spectra of natural kalkitoxin.
Financial support was provided by the National Science
Foundation (01076103-CHE), the National Institutes of Health
(GM58889), and the Marine-Freshwater Biomedical Sciences
Center of Oregon State University (ES03850).
Notes and references
‡ Crystal data for 8: M = 425.53; monoclinic, space group C2 (no. 5); a =
29.048(7), b = 5.7108(6), c = 14.709(3) Å, b = 102.212(8)°, V =
2384.8(8) Å3, T = 93(2) K; Z = 4; m(Cu-Ka) = 0.629 mm21; reflections:
total = 3851, unique = 2613 (Rint = 0.0317); residuals (all data, Shelxl):
R1 = 0.0862, wR2 = 0.1453; Absolute structure parameter (Flack) =
0.1(6) (the absolute structure cannot be uniquely determined based on
diffraction data alone). CCDC 211921. See http://www.rsc.org/suppdata/cc/
b3/b306124h/ for crystallographic data in CIF or other electronic format.
Scheme 3
14 and oxidation of the resulting aldehyde produced carboxylic
acid 15.
1 M. Wu, T. Okino, L. M. Nogle, B. L. Marquez, R. T. Williamson, N.
Sitachitta, F. W. Berman, T. F. Murray, K. McGough, R. Jacobs, K.
Colsen, T. Asano, F. Yokokawa, T. Shioiri and W. H. Gerwick, J. Am.
Chem. Soc., 2000, 122, 12041.
2 F. W. Berman, W. H. Gerwick and T. F. Murray, Toxicon, 1999, 37,
1645.
3 L. T. Tan, R. T. Williamson, K. S. Watts, W. H. Gerwick, K. McGough
and R. Jacobs, J. Org. Chem., 2000, 65, 419.
4 R. L. Manger, L. S. Leja, S. Y. Lee, J. M. Hungerford, Y. Hokama, R.
W. Dickey, H. R. Granade, R. Lewis, T. Yasumoto and M. M. Wekell,
J. AOAC Intern., 1995, 78, 521.
5 A. G. Myers, B. H. Yang, H. Chen, L. McKinstry, D. J. Kopecky and J.
L. Gleason, J. Am. Chem. Soc., 1997, 119, 6496.
6 S. Mahboobi and K. Bernauer, Helv. Chim. Acta, 1988, 71, 2034.
7 A. G. Myers, B. H. Yang and D. J. Kopecky, Tetrahedron Lett., 1996,
37, 3623.
For synthesis of the thiazoline portion of kalkitoxin we
employed a route based on titanium tetrachloride-mediated
cyclization of an amido thiol.14,15 Thus, (R)-3-amino-4-thio-
benzylbut-1-ene (16) required for coupling with 15 was
prepared from (R)-cysteine which was first protected as its N-
Boc-S-benzyl derivative 17 (Scheme 4).16
This carboxylic acid was converted to its Weinreb amide
18,17 reduction of which gave aldehyde 19. Wittig methylen-
ation of 19 produced 20, and subsequent removal of the Boc
protection afforded 16. Coupling of this amine with carboxylic
acid 15, using HATU for activation,18 led to bis-amide 21 in
8 E. Nicolas, K. C. Russell and V. J. Hruby, J. Org. Chem., 1993, 58,
766.
9 D. R. Williams, W. S. Kissel, J. J. Li and R. Mullins, Tetrahedron Lett.,
2002, 43, 3723.
10 D. R. Williams and R. A. Turske, Org. Lett., 2000, 2, 3217.
11 D. A. Evans, K. T. Chapman and J. Bisaha, J. Am. Chem. Soc., 1988,
110, 1238.
12 L. A. Carpino, J. Am. Chem. Soc., 1993, 115, 4397.
13 H. Takayama, T. Koike, N. Aimi and S. Sakai, J. Org. Chem., 1992, 57,
2176.
14 M. A. Walker and C. H. Heathcock, J. Org. Chem., 1992, 57, 5566.
15 P. Raman, H. Razavi and J. W. Kelly, Org. Lett., 2000, 2, 3289.
16 K. Koerber-Ple and G. Massiot, J. Heterocycl. Chem., 1995, 32,
1309.
17 G. Brenner-Wei, A. Giannis and K. Sandhoff, Tetrahedron, 1992, 48,
5855.
Scheme 4
18 L. A. Carpino and A. El-Faham, J. Org. Chem., 1995, 60, 3561.
CHEM. COMMUN., 2003, 2012–2013
2013