Stereospecific total synthesis of somocystinamide A

Article abstract of DOI:10.1021/ol8016947

(Chemical Equation Presented) The first total synthesis of somocystinamide A, a disulfide dimer with extremely labile enamide functional groups, was accomplished in a concise and stereospecific manner. Somocystinamide A is reported to possess exceptionall

Full text of DOI:10.1021/ol8016947

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4449-4452
Stereospecific Total Synthesis of
Somocystinamide A
Takashi L. Suyama and William H. Gerwick*
Scripps Institution of Oceanography and Skaggs School of Pharmacy and
Pharmaceutical Sciences, UniVersity of California at San Diego, 9500 Gilman DriVe,
La Jolla, California 92093
wgerwick@ucsd.edu
Received July 24, 2008
ABSTRACT
The first total synthesis of somocystinamide A, a disulfide dimer with extremely labile enamide functional groups, was accomplished in a
concise and stereospecific manner. Somocystinamide A is reported to possess exceptionally potent antiangiogenic and tumoricidal activities.
The current work should enable further pharmacological investigation of this important natural product.
Marine cyanobacteria are a rich source of biomedically
relevant secondary metabolites that are of unique molecular
architecture.1 In line with this theme, somocystinamide A
(1, Figure 1) was isolated from a mixed assemblage of
Schizothrix and Lyngbya and shown to possess remarkable
biological properties.2,3 Initially, 1 only showed moderate
activity against mouse neuroblastoma cells (Neuro-2a).3 In
subsequent studies, however, its IC50 against human umbilical
vein endothelial cells (HUVECs) was found to be 500 fM.2
This astonishing in vitro finding was verified in zebra fish
wherein antiangiogenetic effects were observed at 80 nM
media concentration. Despite this potency, 1 was shown to
have no observable adverse effects on zebra fish even at 30
µM. In addition, 1 was shown to trigger apoptosis in tumor
cells via caspase-8 activation.2 This activity profile supports
Figure 1. Examples of disulfide-containing natural products.
the development of 1, or analogues thereof, for potential use
in cancer treatment.
Biosynthetically, somocystinamide A appears to be as-
sembled through alternating NRPS-PKS elements with a
unique termination of a PKS unit via decarboxylation and
dehydration to furnish the terminal olefin as seen in curacin
A.4 Methylation of the enamide using S-adenosyl methionine,
a signature decoration in marine cyano-bacterial natural
products,5 produces the tertiary enamide. Secondary enam-
(1) Gerwick, W. H.; Tan, L. T.; Sitachitta, N. The Alkaloids; Cordell,
G. A., Ed.; Academic Press; San Diego, CA 2001; Vol. 57, pp 75-184.
(2) Wrasidlo, W.; Mielgo, A.; Torres, V. A.; Barbero, S.; Stoletov, K.;
Suyama, T. L.; Klemke, R. L.; Gerwick, W. H.; Carson, D. A.; Stupack,
D. G. Proc. Nat. Acad. Sci. U.S.A. 2008, 105, 2313–2318
.
(3) Nogle, L. M.; Gerwick, W. H. Org. Lett. 2002, 4, 1095–1098
.
(4) Moore, B. S. Nat. Prod. Rep. 2005, 22, 580–593.
10.1021/ol8016947 CCC: $40.75
Published on Web 09/13/2008
2008 American Chemical Society

ides have been observed in many natural products, and their
preparation has been studied extensively in recent years.6
Tertiary enamides, however, are encountered very rarely in
natural products, and as such, the strategies for their
preparation are relatively undeveloped and scarce.6,7 Fur-
thermore, the presence of the disulfide group in 1 requires
great care and consideration during the course of synthesis.8
For example, synthetic investigation of epidithiapiperazinedi-
one natural products (such as 2) has met with much difficulty
in the installation of the disulfide.8,9 To date, only one
complete total synthesis of a compound of this class has been
reported.10 Another case in point is psammaplin A (3); in
all three of the published syntheses of 3, the sulfur atoms
were introduced as a disulfide in the final step so as to avoid
side reactions.11
Scheme 1. Cross-Metathesis to 8
It was envisioned in the synthesis of 1 that the key
carbon-carbon connection at the internal olefin would be
made by olefin cross metathesis using a ruthenium catalyst.12
Accordingly, terminal olefin 6 was prepared from the known
aldehyde 513 via a Wittig reaction (Scheme 1).14 Thiazolidine
was chosen as the protecting group for the thiol of 4 because
of its relative stability and its tandem protection of the
carbamate proton.
Screening of commercially available ruthenium catalysts
revealed that the second-generation Hoveyda-Grubbs cata-
lyst (11) was optimal (Table 1).12c Furthermore, this reaction
was optimized for multigram scale by adjusting the concen-
tration and number of equivalents of 7. Good stereoselectivity
was observed in all cases (e.g., trans:cis ) 18:1, entry 8).
Minimizing the amount of 7 facilitated the purification
Table 1. Olefin Cross-Metathesis with Various Conditions To
Produce 8
esterb,
equiv
yield of
8c (%)
catalyst (mol %)
conca
1
2
3
4
9, 20
10, 20
10, 4
11, 5
11, 5
11, 2.5
11, 5
11, 5
0.04
0.04
0.04
0.03
0.03
0.03
0.04
0.2
7, 10
7, 10
7, 3
7, 3
12, 1.5
7, 3
0
26 (na)
23 (57)
81 (94)
53 (55)
44 (na)
73 (83)
82 (82)
5
6
7d
8e
7, 3
7, 2.2
a Concentration of 6 (M). b Equivalents of 7 or 12 with respect to 6.
c Isolated yields. Yields in parentheses are based on recovered starting
material. d 3.2 g scale (ca. 10 times more than entries 1-6). e 5.5 g scale.
(5) Chang, Z; Sitachitta, N.; Rossi, J.; Roberts, M. A.; Flatt, P. M.; Jia,
J.; Sherman, D. H.; Gerwick, W. H. J. Nat. Prod. 2004, 67, 1356–1367.
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L.; Slafer, B. W.; Zhang, Y. Sci. Synth. 2005, 21, 387–475. (b) Jiang, L.;
Job, G. E.; Klapars, A.; Buchwald, S. L. Org. Lett. 2003, 5, 3667–3669.
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Legros, J.; Bolm, C. Chem. Commun. 2005, 973–986. (e) Pan, X.; Cai, Q.;
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Cook, G. R.; Barta, N. S.; Stille, J. R. J. Org. Chem. 1992, 57, 461–467.
(d) Couture, A.; Dubiez, R.; Lablache-Combier, A. J. Org. Chem. 1984,
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1478–1497. (f) Morales, O.; Seide, W.; Watson, S. E. Synth. Commun. 2006,
process because the major side product of the reaction, dimer
12, closely eluted with the desired product 8 during chro-
matography. In line with recent reports that some ruthenium
catalysts are more functional group-tolerant than initially
suspected,15 alkyl sulfides are apparently very well tolerated
by 11, but not by 9, suggesting that there may be competition
between tricyclohexylphosphine and the sulfide 6 for binding
as a ligand on ruthenium.16
The methyl ester 8 was hydrolyzed to obtain the carboxylic
acid 13 in order to avoid undesired reduction to the primary
alcohol in the next step. Afterward, the thiol and the
carbamate of 13 were reductively deprotected by sodium in
liquid ammonia (Scheme 2).13a Reprotection of the carboxy-
lic acid as a methyl ester, deprotection of the amine, and
36, 965–973
.
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2045–2078.
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(14) Rh-catalyzed methylenation with TMSCHN2 gave poor results. (a)
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(16) To our knowledge, this is the first example of a successful cross-
metathesis of an alkyl sulfide substrate by ruthenium carbene catalysis.
4450
Org. Lett., Vol. 10, No. 20, 2008

Scheme 2
.
Synthesis of Disulfide 15 (Somocystinoic Acid)
Scheme 3. Possible Mechanisms for Enamide Formation
acetylation yielded 14 in a good yield. Simultaneous basic
hydrolysis of the methyl ester and the thioacetate in the
presence of O2 cleanly caused dimerization to the disulfide
in one pot to give 15. However, attempts to effect the
dimerization by conventional means, such as treatment with
I2, did not give good yields.17
With 15 (named “somocystinoic acid”) in hand, various
conditions were investigated to couple the in situ generated
imine 16 to the corresponding diacyl chloride 20, the
formation of which was verified by the reaction with
methylamine to produce 17. In most cases, the starting
material decomposed while in some cases a trace amount of
1 was observed. This result was curious because there are
reports of synthesis of simple enamides via acylation of the
corresponding acid chloride with imine.7,18 It is possible that
the putative acyl iminium ion intermediate 21 is intercepted
via an intramolecular reaction due to its dimeric nature
(Scheme 3).18 In support of this hypothesis, only tautomer
16 and not 19 was observed by 1H and 13C NMR in CD2Cl2.19
We then turned our attention to the recently developed
Cu-mediated vinylation reaction.6 It has been reported,
although with little experimental evidence, that this approach
is inapplicable to acylic secondary amides.6b,d,f Therefore,
coupling between a simple amide 23 and a commercially
available vinyl bromide 24 was investigated, but found
ineffective, giving support to these earlier reports.
was found to be a convenient vessel for small scale reactions
and allowed the use of solvents heavier than water.22
Observing that the putative intermediate 21 did not yield 1,
we hypothesized that the E1 pathway was not viable (Scheme
3). In support of this hypothesis, use of a more polar solvent,
THF, decreased the yield in comparison to 1,2-dichloroet-
hane, a less polar solvent.23 The best result was obtained
when TsOH was used as the catalyst, which gave a 41%
yield. Further investigation of this reaction is underway on
model systems.
The analytical data (1H and 13C NMR, MS, UV, IR, and
optical rotation) for synthetic 1 were essentially identical to
those for natural somocystinamide A,24 thus confirming the
originally assigned structure.3 The bioactivity of the synthetic
product 1 was evaluated in the murine Neuro-2a cancer cell
line, but its activity was highly variable,25 possibly due to
the unusual solubility properties of 1, or to factors which
we do not currently understand. We also tested synthetic 1
in the brine shrimp toxicity model26 and observed signifi-
cantly impaired motility in treated (at 1, 10, and 100 µg/
mL) versus control shrimps (DMSO). We also noted a much
decreased quanitity of intestinal contents in the treated group.
These observations underscore the value of a synthetic supply
of somocystinamide A for it is clear that it possesses
biological properties not yet understood.
Observation that the hydrolytic decomposition of 1 to 17
occurs with relatively low activation energy3 inspired us to
carry out the opposite reaction,20 specifically condensation
of the aldehyde 1821 with 17. A Soxhlet extraction apparatus
(17) Atmospheric air as oxidant only worked for small scale reactions
(ca. 10 mg). Larger scale reactions were run with bubbling O2 (see the
experimental procedures in the Supporting Information). For I2 examples,
see: Bourles, E.; Alves de Sousa, R.; Galardon, E.; Selkti, M.; Tomas, A.;
Artaud, I. Tetrahedron 2007, 63, 2466–2471.
(22) Instead of a thimble, glass wool and molecular sieves were used
to capture any moisture distilled as an azeotrope.
(18) The acetamide, disulfide, or already formed enamide of the other
half of the dimeric intermediate may act as a nucleophile to attack the
iminium ion. For examples of nucleohilic additions to iminium ions, see:
(23) (a) Saunders, W. H., Jr. Acc. Chem. Res. 1976, 9, 19–25. (b) Carey,
F. A.; Sundberg, R. J. AdVanced Organic Chemistry Part A, 4th ed.; Kluwer
Academic/Pleunum Publishers: New York, 2000; pp 378-383.
(24) The 1H and 13C were identical between natural and synthetic
samples except the shift of C-16 showed a 0.2 ppm downfield shift in the
synthetic sample. The UV maximum for synthetic somocystinamide was
at 234 nm while natural was reported at 241 nm, both samples in MeOH.
(25) IC50 against Neuro-2a cells, synthetic 1 ) 0.2 to >10 µM; natural
1 ) 1.8 µM (ref 3).
Moonen, K.; Stevens, C. V. Synthesis 2005, 20, 3603–3612
.
(19) In separate studies, reaction of 16 with simple unfunctionalized
acid chlorides formed enamides, thus indicating the presence of an
acyliminium ion intermediate like 21. Because 19 was not observed by
NMR, an acylenaminium intermediate is not likely involved.
(20) In ref 3, it was found that 1 decomposed after a few days in CDCl3
due to the residual acid and water present in this solvent.
(21) Griffith, G. A.; Percy, J. M.; Pintat, S.; Smith, C. A.; Spencer, N.;
Uneyama, E. Org. Biomol. Chem. 2005, 3, 2701–2712.
(26) Meyer, B. N.; Ferrigni, N. R.; Putnam, L. B.; Jacobson, L. B.;
Nichols, D. E.; McLaughlin, J. L. Planta Med. 1982, 45, 31–34.
Org. Lett., Vol. 10, No. 20, 2008
4451

In conclusion, the first total synthesis of somocystinamide
A was achieved despite the presence of two quite challenging
functional groups in the molecule (Scheme 4). The current
compound should now be possible. However, our work
demonstrates the need for a more robust reaction to prepare
tertiary enamides, a development which would open the door
for synthesis of other natural products possessing this
functional group, such as the laingolides.27
Scheme 4. Synthesis of Somocystinamide A (1)
Acknowledgment. T.L.S. thanks D. Carson and the
Moores Cancer Center for a student fellowship. Funding is
acknowledged from NIH Grant No. NS 053398. We thank
J. Wingerd at SIO/UCSD for the Neuro-2a assay and Y. Su
at UCSD for the mass spectroscopic analyses. We thank K.
Schwartz at Oregon State University for suggesting the use
of a Soxhlet extractor, and the J. Bada Laboratory (SIO) for
use of liquid NH3.
Supporting Information Available: Experimental pro-
1
cedures, compound characterization, and H and 13C NMR
spectra for compounds 6, 8, 13-15, 17, and 1. Pictures and
video of somocystinamide A and control treated brine shrimp.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL8016947
(27) (a) Klein, D.; Braekman, J.; Daloze, D.; Hoffman, L.; Demoulin,
V. Tetrahedron Lett. 1996, 37, 7519–7520. (b) Klein, D.; Braekman, J.;
Daloze, D.; Hoffman, L.; Castillo, G.; Demoulin, V. J. Nat. Prod. 1999,
62, 934–936.
synthesis is fairly robust for all steps but the last two such
that >1 g of 15 has been prepared smoothly, and scale-up
necessary for in vivo testing of this biomedically exciting
4452
Org. Lett., Vol. 10, No. 20, 2008