Versatile routes to marine sponge metabolites through benzylidene rhodanines

Article abstract of DOI:10.1021/ol303057a

The first total synthesis of the marine natural products Psammaplin C and Tokaradine A is described. Benzylidene rhodanines were utilized as versatile intermediates toward the synthesis of seven brominated marine sponge metabolites through the optimizatio

Full text of DOI:10.1021/ol303057a

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Versatile Routes to Marine Sponge
Metabolites through Benzylidene
Rhodanines
Suresh K. Kottakota,† Mathew Benton,† Dimitrios Evangelopoulos,‡,§
Juan D. Guzman,‡ Sanjib Bhakta,‡ Timothy D. McHugh,§ Mark Gray,†
Paul W. Groundwater,^ Emma C. L. Marrs, John D. Perry, and
J. Jonathan Harburn*,r
Sunderland Pharmacy School, Department of Pharmacy, Health & Well Being,
University of Sunderland, Wharncliffe Street, Sunderland, SR1 3SD, U.K.,
Department of Biological Sciences, Institute of Structural and Molecular Biology,
Birkbeck, University of London, Malet Street, Bloomsbury London, WC1E 7HX, U.K.,
Centre for Clinical Microbiology, Department of Infection, Royal Free Campus,
University College London, London, NW3 2PF, U.K., Faculty of Pharmacy,
The University of Sydney, Sydney, NSW 2006, Australia, Microbiological Department,
Freeman Hospital, Newcastle-upon-Tyne, NE7 7DN, U.K., and School of Medicine,
Pharmacy and Health, Wolfson Research Institute for Health and Wellbeing,
Durham University, TS17 6BH, U.K.
jonathan.harburn@durham.ac.uk
Received November 8, 2012
ABSTRACT
The first total synthesis of the marine natural products Psammaplin C and Tokaradine A is described. Benzylidene rhodanines were utilized as
versatileintermediatestoward the synthesisofseven brominatedmarine spongemetabolites through the optimization of protection groupstrategies.
Spermatinamine demonstrated good inhibition of all cancer cell lines tested, in particular the leukemia K562 and colon cancer HT29 cell lines.
Over the past 40 years marine sponges of the order
Verongida have provided a wealth of brominated com-
pounds with a broad range of biological activities.1 Some
of the most notable tyrosine oxime metabolites include the
dimeric Spermatinamine,2 the sulfur containing dimeric
Psammaplin A,3 Bisaprasin,4 and the unusual N-substituted
pyridinium containing Tokaradine A (Figure 1).5
General routes to synthesizing R-oximino amide de-
rived natural products from precursor benzaldehydes
have included azlactone hydrolysisꢀoximation, Hornerꢀ
WadsworthꢀEmmons (HWE), and cyano-ylide chemistry.6
A major drawback to using these methods is the harsh
conditions required (TFA, LDA, and O3 respectively),
which requires specific protectionꢀdeprotection strategies if
the starting substituted benzaldehyde contains sensitive
† University of Sunderland.
‡ University of London.
§ University College London.
^ The University of Sydney.
Freeman Hospital.
r Durham University.
(1) Peng, J.; Li, J.; Hamann, M. The Alkaloids 2005, 61, 59 and
references therein.
(2) Buchanan, M. S.; Caroll, A. R.; Fechner, G. A.; Boyle, A.;
Simpson, M. M.; Addepalli, R.; Avery, V. M.; Hooper, J. N. A.; Su,
N.; Chen, H.; Quinn, R. J. Bioorg. Med. Chem. Lett. 2007, 17, 6860.
(3) Arabshahi, L.; Schmitz, F. J. J. Org. Chem. 1987, 52, 3584.
(4) Rodriguez, A. D.; Akee, R. K.; Scheuer, P. J. Tetrahedron Lett.
1987, 28, 4989.
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R. W. M. Tetrahedron 2001, 57, 7507.
(6) Hentschel, F.; Lindel, T. Synthesis 2010, 2, 181 and references
therein.
r
10.1021/ol303057a
XXXX American Chemical Society

groups. In order to reach some of the more sensitive meta-
bolites in larger quantities for biological evaluation, a general
method is required which involves stable intermediates.
Starting with the appropriate benzaldehyde 1aꢀe it was
envisaged that the corresponding O-protected oximino
acids 3aꢀe could be prepared through a two-step procedure
and then utilized in a subsequent coupling/deprotection to
give the sponge metabolites (Scheme 1).
All benzaldehydes 1aꢀe were converted to their benzy-
lidene rhodanines 2aꢀe in good to excellent yield on a
multigram scale involving a simple workup.
The three-step conversion of 2aꢀe, involving hydrolysis,
acidification, and subsequent oximation, gave O-benzyl
protected oximino acids 3aꢀe on a gram scale, in good
yields after chromatography. The intermediate 3-arylmer-
captoacrylic acids were not isolated but converted imme-
diately to avoid oxidative dimerization.
20-N-Methylpurpuramine E 6 is an analogue of purpur-
amine E11 which has been isolated from the Okinawan
sponge Pseudoceratina purpurea and has weak cytotoxicity
(IC50 = 4.3 μg mLꢀ1) against HeLa S3 cells.12 Aplysamine-
2 8 was isolated from the Australian sponge Aplysina sp.
and was previously synthesized in our group via the amina-
tion of the oximino methyl ester. Gram positive bacteria
demonstrate moderate susceptibility to aplysamine-2.13,11a
Carbodiimide coupling of 3a or 3b with purpuramine E,
followed by benzyl deprotection, provided 20-N-methyl-
purpuramine E 6 (61%, two steps) and aplysamine-2 8
(38%, 2 steps), the structures of which were confirmed by
comparison of their spectra data with those of the natural
products (Scheme 2, Tables S1 and S2).
Figure 1. Brominated tyrosine oxime metabolites.
Benzylidene rhodanines are easily prepared through the
condensation of an appropriate benzaldehyde with rhoda-
nine and hydrolysis under basic conditions to give the cor-
responding 3-aryl-mercaptoacrylic acids, which are prone
to spontaneous dimerization under oxidative conditions.7
Oximation of 3-arylmercaptoacrylic acids with hydroxyla-
mine has been shown to proceed in high yields8 and could
thus provide a reliable route to O-substituted oximino acids,
by analogy to work using arylpyruvic acids.9 The previous
formation of substituted O-substituted oximino acids has
focused upon O-PMB-, O-THP-, and O-benzyl-substituted
hydroxylamines due to ease of synthesis and availability.10
Scheme 2. Synthesis of 20-N-Methylpurpuramine E and
Aplysamine-2
Scheme 1. Synthesis of O-Benzyl Protected Oximes
5-Bromoverongamine 10 was isolated from Caribbean
sponge Pseudoceratina sp. and reported to inhibit the
settlement of barnacle larvae, to be bactericidal toward
methicillin resistant S. aureus (MRSA) and slightly cytotoxic
(10) (a) Hentschel, F.; Sasse, F.; lindel, T. Org. Biomol. Chem. 2012,
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S.; McHugh, T. D.; Gray, M.; Groundwater, P. W.; Marrs, E. C. L.;
Perry, J. D.; Spilling, C. D; Harburn, J. J. J. Nat. Prod. 2012, 75, 1090.
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Barluenga, S.; Roecker, A. J. Chem.;Eur. J. 2001, 7, 4280. (c) Nicolaou,
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B
Org. Lett., Vol. XX, No. XX, XXXX

to HeLa cells (IC50> 50 μg mLꢀ1).14 Spermatinamine
12 was isolated from Pseudoceratina sp. collected in
Australia and was shown to be the first natural product
inhibitor (IC50 = 1.9 μM) of isoprenylcysteine carboxyl
methyltransferase (Icmt). It has been synthesized via oxi-
dation of the tyrosine precursor and azlactone hydrolysisꢀ
oximation.15
The synthesis of 5-bromoverongamine 10 (64%, two
steps, Scheme 3) proceeded from 3c in a manner analogous
to that for the preparation of 7 and 8. Under similar
carbodiimide coupling conditions (amine, EDC, HOBt,
Et3N, CH2Cl2) 3c failed to couple with N-4,N-9-
dimethylspermine,15c presumably due to solubility issues.
Changing the solvent system to dioxane/methanol (9:1)
provided the desired product 11 in good yield, and this
was subsequently deprotected to give spermatinamine 12
(46%, two steps) (Scheme 3). The structures of both 10 and
12 were again confirmed through direct comparison with the
spectral data for the natural products (Tables TS3 and TS4).
respectively (Scheme 4). Several attempts at the deprotec-
tion of 13 and 15 using hydrogenolysis resulted in no
reaction or decomposition, possibly as a result of catalyst
poisoning, desulfurization, and disulfide bond breakage.
Benzyl group deprotection with BCl3 SMe2 in DCM has
3
produced excellent results for similar compounds21 but, in
our hands, resulted in decomposition.
Deprotection of 13 and 15 using TMSI22 in CH2Cl2
proceeded smoothly to provide psammaplin C 14 (76%)
and psammaplin A (16, 80%), respectively. Once again,
the structures of both 14 and 16 were confirmed by direct
comparison with the spectral data for the natural products
(Tables TS5 and TS6).
Tokaradine A 20 was isolated from Pseudoceratina
purpurea collected in Japan and found to be lethal to the
crab Hemigrapsus sanguineus.5
The synthesis of tokaradine A (Scheme 5) proceeded
through the preparation of amine 18 as the ditriflate salt
from the alkylation of N-Boc-3,5-dibromo-4-hydroxyphe-
nethylamine23 with 1-(3-bromopropyl)pyridinium bromide,24
to give 17 (87%). Boc deprotection to give 18 (97%), followed
by carbodiimide coupling with acid 3e, provided the Cbz-
benzyl protected derivative 19 (71%) in good yield after
chromatography.
Scheme 3. Synthesis of 5-Bromoverongamine and
Spermatinamine
Scheme 4. Synthesis of Psammaplin C and Psammaplin A
Psammaplin C 14 has been isolated from the sponge
Pseudoceratina purpurea, but, due to the compound’s
scarcity, broad biological evaluation has not been deter-
mined.16 In contrast, psammaplin A 16 has been isolated in
abundance from a number of sponges, including Pseudo-
ceratina purpurea,3,4,16a,16b,17 and has been shown to in-
hibit mycothiol-S-conjugate amidase,18 DNA gyrase,19
topoisomerase II,20 and histone deacetylases (HDACs)
as well as DNA-methyltransferases (DNMTs).17
Hydrogenation of19underthestandardconditionsused
for the preparation of 5, 6, 9, and 11 proved unsuccessful,
resulting in complex mixtures of O- and N-deprotection
and decomposition. Deprotection using TMSI in CH2Cl2
(12 h at 40 ꢀC or rt up to 14 days) also resulted in mixtures
of deprotected (O-, N-) oxime reduced material and trace
amounts of 20 (detected by MALDI-MS).
The use of AlCl3 and anisole in CH2Cl2/CH3NO2 has
been reported for the removal of N-Cbz and O-PMB
protecting groups from oxime-containing bromotyrosines.11b
In our hands these conditions, at rt for 48 h, yielded a mixture
of 20, the O-Bn analogue, and oxime reduced products which
proved difficult to separate through column chromatog-
raphy. These results suggest the use of either AlCl3 or
The coupling of 3d with cystamine and β-aminomethane-
sulfonamide proceeded in good yield to give the benzyl
protected sponge metabolites 15 (78%) and 13 (69%)
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(16) (a) Jimenez, C.; Crews, P. Tetrahedron 1991, 47, 2097. (b) Pina,
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Kobayashi, M. Tetrahedron 2005, 61, 7211.
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Pfefferkorn, J. A.; Barluenga, S.; Nicolaou, K. C.; Bewley, C. A. Bioorg.
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Duke, S. O.; Hamann, M. T. J. Nat. Prod. 2012, 75, 1090.
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Org. Lett., Vol. XX, No. XX, XXXX
C

MICs against mycobacteria were determined for 6, 8, 10,
16, and 20 using a spot culture growth inhibition as-
say (SPOTi),27 which has been previously used to eval-
uate antitubercular natural products28 and novel syn-
thetic compounds.29 20-N-Methylpurpuramine E 6 and
tokaradine A 20 showed potent growth activity against
M. bovis BCG (MIC = 5 μg/mL) and M. tuberculosis
H37Rv (MIC = 16 μg/mL) respectively. Only 20 dis-
played weak antifungal activity against two pathogenic
strains tested.
Scheme 5. Synthesis of Tokaradine A
Due to its promising biological activity demonstrated in
the initial one-dose screen against the NCI 60 human
tumor cell line panel, and as a known Icmt inhibitor,
spermatinamine 10 was selected for rescreening by the
Biological Evaluation Committee over the ranges given
in Table TS15. Spermatinamine inhibited all cell lines at
single-digit micromolar concentrations (average GI50 =
0.23 μM) or below (Leukemia K562 GI50 = 0.93 μM,
Colon cancer HT29 GI50 = 0.59 μM).
In summary, this paper describes the first syntheses of
the marine sponge secondary metabolites psammaplin C
14 and tokaradine A 20 and a robust synthetic route to a
range of brominated marine sponge metabolites, including
the more problematic sulfur containing analogues. Sper-
matinamine 10 showed promising results against the NCI
60 cell line cancer screen, and we are currently exploiting
the versatility of our synthetic strategy to produce a range
of analogues of the compounds reported here in order to
obtain SAR data.
TMSI does not afford complete deprotection, promoting
N-deprotection and O-deprotection/reduction respec-
tively. TMSI and AlCl3/anisole together have been re-
ported to remove N-Cbz and O-Bn protecting groups
successfully from macrolide antibiotics,25 and a two-step,
one-pot procedure provided tokaradine A 20 (13%) after
column chromatography. Direct comparison with the
spectral data of the extracted natural product confirmed
the structure (Table TS7).
All synthesized natural products (6, 8, 10, 12, 14, 16, 20)
were assessed for their antibacterial and antifungal proper-
ties in accordance with the recommendations of the British
Society of Antimicrobial Chemotherapy (Table TS8).26
Apart from psammaplin C 14, all the other natural products
demonstrated medium to good activity against Gram
positive bacteria, most noteworthy tokaradine A (20,
MIC = 8 μg/mL against S. epidermidis, S. aureus, and
MRSA), 5-bromoverongamine 10 (MIC e 8 μg/mL
against S. pyogenes), and psammaplin A 16 (MIC e
1 μg/mL against S. epidermidis). Only 6, 10, and 20 demon-
strated weak activity against Gram negative bacteria.
Acknowledgment. This work was supported by a
Postgraduate Academic Assistantship (KSK) from the
Sunderland Pharmacy School and was performed in col-
laboration with Tuberculosis Drug Discovery Consortium
(TBD-UK) and Developmental Therapeutics Program,
National Cancer Institute, Bethesda, USA (http://dtp.
cancer.gov/).
Supporting Information Available. Synthetic proce-
dures, analytical and biological data for compounds
2aꢀe, 3aꢀe, 5ꢀ20. This material is available free of
charge via the Internet at http://pubs.acs.org.
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The authors declare no competing financial interest.
D
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