Latonduines A and B, new alkaloids isolated from the marine sponge Stylissa carteri: Structure elucidation, synthesis, and biogenetic implications

Article abstract of DOI:10.1021/ol034950b

(Matrix Presented) Latonduines A (6) and B (7), two new alkaloids with unprecedented heterocyclic skeletons, have been isolated from the Indonesian marine sponge Stylissa carteri. The structures of the latonduines were elucidated by analysis of spectroscopic data and confirmed by the total synthesis of latonduine A (6). It is proposed that ornithine is the biogenetic precursor to the aminopyrimidine fragment of the latonduines.

Full text of DOI:10.1021/ol034950b

ORGANIC
LETTERS
2003
Vol. 5, No. 15
2735-2738
Latonduines A and B, New Alkaloids
Isolated from the Marine Sponge
Stylissa carteri: Structure Elucidation,
Synthesis, and Biogenetic Implications
Roger G. Linington,† David E. Williams,† Akbar Tahir,‡ Rob van Soest,§ and
Raymond J. Andersen†,*
Departments of Chemistry and Earth and Ocean Sciences,
UniVersity of British Columbia, VancouVer, B.C., Canada V6T 1Z1,
Faculty of Marine Science and Fisheries, UniVersity of Hasanuddin,
Ujung Pandang, Indonesia, 90245, and Zoologisch Museum,
UniVersity of Amsterdam, Amsterdam, The Netherlands
randersn@interchange.ubc.ca
Received May 29, 2003
ABSTRACT
Latonduines A (6) and B (7), two new alkaloids with unprecedented heterocyclic skeletons, have been isolated from the Indonesian marine
sponge Stylissa carteri. The structures of the latonduines were elucidated by analysis of spectroscopic data and confirmed by the total
synthesis of latonduine A (6). It is proposed that ornithine is the biogenetic precursor to the aminopyrimidine fragment of the latonduines.
The C11N5 skeleton of oroidin (1)1 represents the formal
biogenetic building block for a diverse family of monomeric
and dimeric sponge alkaloids2 that includes dibromophakellin
(2),3 hymenialdisine (3),4 and sceptrin (4).5 The “oroidins”
hymenialdisine (3) and debromohymenialdisine (5)6 have
attracted particular attention7 because of their ability to inhibit
protein kinases,8,9 modulate the proinflammatory transcription
factor NF-κΒ,10 inhibit the G2 cell-cycle checkpoint,9 and
slow joint and cartilage deterioration associated with osteo-
arthritis in animal models.11 As part of an ongoing investiga-
tion of bioactive sponge metabolites,12 we were prompted
to investigate an extract of the Indonesian sponge Stylissa
(7) For syntheses, see: (a) Xu, Y.-Z.; Yakushijin, K.; Horne, D. A. J.
Org. Chem. 1997, 62, 456. (b) Sosa, A. C. B.; Yakushijin, K.; Horne, D.
A. J. Org. Chem. 2000, 65, 610. (c) Lindel, T.; Hochgurtel, M. J. Org.
Chem. 2000, 65, 2806. (d) Annoura, H.; Tatsuoka, T. Tetrahedron Lett.
1995, 36, 413.
(8) Tasdemir, D.; Mallon, R.; Greenstein, M.; Feldberge, L.; Kim, S.
C.; Collins, K.; Wojciechowicz, D.; Mangalindan, G. C.; Concepcion, G.
P.; Harper, M. K.; Ireland, C. M. J. Med. Chem. 2002, 45, 529.
(9) Curman, D.; Cinel, B.; Williams, D. E.; Rundle, N.; Block, W. D.;
Goodarzi, A. A.; Hutchins, J. R.; Clarke, P. R.; Zhou, B. B.; Lees-Millar,
S. P.; Andersen, R. J.; Roberge, M. J. Biol. Chem. 2001, 276, 17914.
(10) Breton, J. J.; Chabot-Fletcher, M. C. J. Pharmacol. Exp. Therap.
1997, 282, 459.
† University of British Columbia.
‡ University of Hasanuddin.
§ University of Amsterdam.
(1) (a) Forenza, S.; Minale, L.; Riccio, R.; Fattorusso, E. J. Chem. Soc.,
Chem. Commun. 1971, 1129. (b) Garcia, E. E.; Benjamin, L. E.; Fryer, R.
I. Chem. Commun. 1973, 78.
(2) Mourabit, A. A.; Potier, P. Eur. J. Org. Chem. 2001, 237.
(3) Sharma, G. M.; Burkholder, P. R. Chem. Commun. 1971, 151.
(4) Cimino, G.; De Rosa, S.; De Stephano, S.; Mazzarella, L.; Puliti,
R.; Sodano, G. Tetrahedron Lett. 1982, 23, 767.
(5) Walker, R. P.; Faulkner, D. J.; Van Engen, D.; Clardy, J. J. Am.
Chem. Soc. 1981, 103, 6772.
(11) OsteoArthritis Sciences, Inc. The Regents of the University of
California. The Use of Debromohymenialdisine for Treating Osteoarthritis.
U.S. Patent 5,591,740, 1997.
(6) Sharma, G. M.; Buyer, J. S.; Pomerantz, M. W. J. Chem. Soc., Chem.
Commun. 1980, 435.
(12) For the previous paper in the series, see: Williams, D. E.; Roberge,
M.; Van Soest, R.; Andersen, R. J. J. Am. Chem. Soc. 2003, 125, 5296.
10.1021/ol034950b CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/26/2003

carteri because it showed significant in vitro cytotoxicity
against human cancer cell lines. Bioassay-guided fraction-
ation of the extract gave complex cytotoxic mixtures that
are still under investigation and an inactive fraction contain-
ing the two novel alkaloids latonduines A (6) and B (7).
The structures of latonduines A (6) and B (7) were elucidated
by spectroscopic analysis and confirmed by the total synthesis
of latonduine A (6) as described below.
Hz, 1H), and 13.10 (s, 1H) in the 1H NMR spectrum (DMSO-
d6) of 6, accounting for the remaining hydrogen atoms. The
COSY spectrum contained a single cross-peak confirming
the scalar coupling between the aliphatic methylene protons
(δ 3.90) and the exchangeable proton at δ 8.14.
A previous chemical investigation of S. carteri resulted
in the isolation of several oroidin alkaloids, including (Z)-
hymenialdisine (3) and (Z)-3-bromohymenialdisine (10).13
The HMBC spectrum of latonduine A (6) contained cor-
relations that were consistent with the presence of the 2,3-
dibomo-4-alkyl-5-amido fragment found in 10. Thus, the
exchangeable resonance at δ 13.10, assigned to the pyrrole
NH, showed HMBC correlations to carbon resonances at δ
96.0 and 120.0, assigned to C-3 and C-4, respectively, and
the exchangeable resonance at δ 8.14, assigned to the N-7
amide NH, showed correlations to carbon resonances at δ
125.1, assigned to C-5, and 46.4, assigned to C-8. By analogy
with 3-bromo hymenialdisine (10), the carbon resonance at
δ 107.9 in the spectrum of 6 was assigned to C-2.
The remaining fragment of latonduine A (6) had to account
for four sp2-hybridized carbons (1 x CH, 3 x C), an isolated
aromatic proton (δ 8.76), three nitrogen atoms, two equiva-
lent exchangeable protons (δ 6.88), and five sites of
unsaturation. These structural requirements could be satisfied
by linking the C-4 position of the pyrrole and the C-8
methylene to adjacent carbons of a 2-aminopyrimidine
fragment as shown in 6. An HMBC correlation observed
between the aromatic methine resonance at δ 8.76 (H-11)
and the carbon resonance at δ 120.0, assigned to C-4,
tentatively suggested the orientation of the 2-aminopyrimi-
dine ring as shown. Additional HMBC correlations observed
between the C-9 resonance at δ 163.7 and the H-11 (δ 8.76),
H-8/H8′ (δ 3.90), and NH2-15 (δ 6.88) resonances, between
the C-10 resonance at δ 113.4 and the H-11 (δ 8.76) and
H-8/H-8′ (δ 3.90) resonances and between the C-11 reso-
nance at δ 155.9 and the NH2-15 (δ 6.88) resonance, were
consistent with the assigned structure 6. The four-bond NH2-
15 to C-11 and C-9 correlations seemed to be reasonable
since “W coupling” pathways existed for both long-range
correlations.
The alternate structure 11 could also account for all of
the spectroscopic data obtained for latonduine A. However,
the biogenetic oroidin building block has a minimum four-
carbon linear chain separating the amide nitrogen (N-7) and
the first point of attachment (C-11) of a guanidine nitrogen.
Therefore, on biogenetic grounds (Figure 1), 6 appeared to
be the most likely structure for latonduine A. To verify this
proposal, structure 6 was synthesized as shown in Scheme
1.
Specimens of S. carteri (Dendy) (Demospongiae, order
Halichondrida, family Dictyonellidae) were collected by hand
using SCUBA on shallow reefs off of Latondu Island, Taka
Bonerate, Indonesia. Freshly collected sponge (50 g) was
preserved on site in EtOH for 2 days at rt after which the
EtOH was discarded and the sample was frozen for transport
to Vancouver. The frozen sponge was subsequently extracted
exhaustively with MeOH. Concentration of the MeOH extract
in vacuo gave an aqueous suspension that was partitioned
between H2O and EtOAc. Fractionation of the EtOAc soluble
materials by sequential application of Sephadex LH20
chromatography (eluent: 80% MeOH/20% CH2Cl2) and
reversed-phase HPLC (eluent: 45% MeOH/55% H2O) gave
pure samples of latonduine A (6) (2.9 mg), latonduine B
ethyl ester (8) (2.8 mg), and latonduine B methyl ester (9)
(0.5 mg) as pale yellow crystalline solids.
Latonduine A (6) gave a 1:2:1 M+ ion cluster at m/z 371,
373, and 375 in the LREIMS, indicating that the molecule
contained two bromine atoms. A HREIMS analysis of
6 showed that the mass of the M+ cluster peak at m/z
372.9002 was appropriate for a molecular formula of
C10H7N5O79Br81Br (calcd 372.8997) requiring seven sites of
unsaturation. The 13C NMR spectrum (DMSO-d6) of 6
contained 10 well-resolved resonances consistent with the
HREIMS data, and the HMQC spectrum demonstrated that
only three of the protons in the molecule were attached to
carbon (1 x CH2 (δ 1H 3.90 (d, J ) 5.2 Hz, 2H), 13C 46.4);
1 x CH (δ 1H 8.76 (s), 13C 155.9); 8 x C (δ 13C 96.0, 107.9,
113.4, 120.0, 125.1, 161.8, 162.1, 163.7)). Exchangeable
resonances were observed at δ 6.88 (s, 2H), 8.14 (t, J ) 5.2
The synthesis of 6 started with amide 12 prepared
according to literature procedures.7a Treatment of neat 12
with excess methanesulfonic acid at 35 °C for 7 days gave
the cyclized product 13 in 69% yield.7b Hydroboration of
13 with catecholborane in THF, in the presence of a catalytic
amount of LiBH4, followed by oxidative workup, gave the
(13) Eder, C.; Proksch, P.; Wray, V.; Steube, K.; Bringmann, G.; van
Soest, R. W. M.; Sudarsono; Ferdinandus, E.; Pattisina, L. A.; Wiryow-
idagdo, S.; Moka, W. J. Nat. Prod. 1999, 62, 184.
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Org. Lett., Vol. 5, No. 15, 2003

Scheme 1a
a Reaction conditions: (a) Neat, excess CH3SO3H, 35 °C, 7 days.
(b) (i) Catecholborane, LiBH4, THF, rt, 1 h; (ii) excess 1 M NaOH,
5 min; (iii) excess H2O2. (c) Dess-Martin periodinane (1.2 equiv),
THF, rt, 2 h. (d) Excess ethylorthoformate, TFA, reflux, 18 h. (e)
Guanidine, K2CO3, THF/H2O 6:1, reflux, 1 h.
alcohol 14 in 84% yield. Dess-Martin oxidation of the
secondary alcohol 14 gave the corresponding ketone 15 in
nearly quantitative yield. Reaction of the ketone 16 with neat
ethylorthoformate in the presence of TFA catalysis proceeded
cleanly to give the enol ether 16 in modest yield. Treatment
of 16 with excess guanidine in refluxing THF/H2O (6:1) for
1 h gave a high yield of latonduine A (6), identical by NMR,
MS, and TLC comparison with the natural product.
Figure 1. Proposed biogenesis of latonduines A (6) and B (7).
Latonduine B ethyl ester (8) gave a M+ ion at m/z
444.9193 in the EIHRMS appropriate for a molecular
formula of C13H11N5O379Br81Br (calcd 444.9195) that re-
quired eight sites of unsaturation. The NMR data obtained
for 8 showed strong similarities to the data for latonduine A
(6) (Supporting Information), indicating that the two com-
pounds were closely related. The major differences in the
NMR data for 8, compared to the data for 6, were the absence
of an aromatic resonance in the region of δ 8.76, assigned
to H-11 in 6, and the presence of a complex two-proton
multiplet at δ 4.19 and a three-proton triplet at 1.19, both
assigned to an ethoxy fragment. An HMBC correlation from
the ethoxy methylene at δ 4.19 to a carbon resonance at δ
165.0 demonstrated that the ethoxy fragment was part of an
ethyl ester. The ethyl ester had to be attached at C-11 in 8
to explain the absence of the H-11 methine resonance. All
of the additional two-dimensional NMR data obtained for
latonduine B ethyl ester was consistent with the proposed
structure 8. Interestingly, the H-8/H-8′ protons in ester 8 gave
latonduine B from an extract of S. carteri that had been
exposed to both ethanol and methanol during workup
indicated that the free acid 7 is the natural product and that
the esters are artifacts. Latonduine A (6) and latonduine B
ethyl ester (8) were tested for in vitro cytotoxicity against a
panel of human cancer cell lines and for enzyme inhibition
against a panel of protein kinases. Both molecules were
inactive in all of the assays.
Although the latonduines appear to be biogenetically
related to the oroidin family of alkaloids, their skeletons
cannot be derived from the C11N5 building block of the
oroidins.2 Instead, alkaloids 6 and 7 have an unprecedented
heterocyclic skeleton that contains a six-membered amino-
pyrimidine substructure in the place of the five-membered
aminoimidazole substructure common to the oroidin family.
Figure 1 shows a proposed biogenesis for latonduine B (7)
that suggests that the building blocks of the molecule are
4,5-dibromopyrrole-2-carboxylic acid, ornithine, and guani-
dine. Latonduine A (6) would arise from decarboxylation
of lantonduine B (7). The biogenetic scheme also suggests
that the hymenialdisines might arise from an alternate
cyclization of an intermediate like 18 that involves the
ornithine carboxyl functionality and a guanidine nitrogen to
give the five-membered oxoaminoimidazole ring found in
(E)-3-bromohymenialdisine (21).
1
a pair of H NMR resonances at δ 3.77 (dd, J ) 14.5, 7.1
Hz) and 4.09 (dd, J ) 14.5, 3.1 Hz), indicating that an
unfavorable steric interaction between the ester substituent
at C-11 and the bromine at C-3 severely hinders rotation
about the C-4/C-10 bond, preventing H-8 and H-8′ from
interchanging rapidly on the NMR time scale. This is in
contrast to the situation in latonduine A (6) where H-8 and
H-8′ are magnetically equivalent.
The structure of latonduine B methyl ester (9), obtained
in very small amounts from the crude extract, was routinely
assigned by analysis of its spectroscopic data (Supporting
Information). Isolation of the ethyl and methyl esters of
Kerr and co-workers have shown that radiolabeled orni-
thine, proline, and histidine are incorporated into the hy-
menisaldisine analogue stevensine (22) in a cell culture of
Org. Lett., Vol. 5, No. 15, 2003
2737

the sponge Teichaxinella morchella but that arginine is not.14
They proposed that ornithine was incorporated into stevensine
via initial conversion to proline followed by oxidation to
pyrrole-2-carboxylic acid. However, they did not degrade
their radiolabeled stevensine, so the location of the ornithine
label in the final molecule is only conjecture. The discovery
of the latonduines, which appear to have a straightforward
biogenesis from ornithine (Figure 1), suggests an alternate
possibility for its incorporation in oroidin alkaloids via the
aminoimidazole fragment rather than, or likely in addition
to, incorporation via the pyrrolecarboxylic acid fragment. The
two possible modes of incorporation of ornithine would be
easily probed by doing feeding studies with oroidin (1),
where the aminoimidazole and pyrrolecarboxylic acid frag-
ments could be readily cleaved.
Acknowledgment. Financial support was provided by the
Natural Sciences and Engineering Research Council of
Canada. The authors thank M. LeBlanc (UBC), S. Yusuf
(UNHAS), and J. Setiadi (UNHAS) for assisting the collec-
tion of S. carteri.
Supporting Information Available: Experimental and
NMR spectra for 6, 8, and 9. This material is available free
of charge via the Internet at http://pubs.acs.org.
(14) Andrade, P.; Willoughby, R.; Pomponi, S. A.; Kerr, R. G.
Tetrahedron Lett. 1999, 40, 4775.
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