Total syntheses of the bromotyrosine-derived natural products ianthelline, 5-bromoverongamine and JBIR-44

Article abstract of DOI:10.1016/j.tetlet.2010.07.016

The total syntheses of the bromotyrosine-derived natural products ianthelline, 5-bromoverongamine and JBIR-44 are described and their cytotoxic activity in a cervical cancer (HeLa) cell line and human umbilical vein endothelial cells (HUVECs) are reported

Full text of DOI:10.1016/j.tetlet.2010.07.016

Tetrahedron Letters 51 (2010) 4812–4814
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total syntheses of the bromotyrosine-derived natural products ianthelline,
5-bromoverongamine and JBIR-44
James W. Shearman a, Rebecca M. Myers a, Thomas M. Beale a, James D. Brenton b, Steven V. Ley a,
*
a Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
b Cancer Research UK Cambridge Research Institute, Li Ka Shing Centre, Robinson Way, Cambridge CB2 0RE, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The total syntheses of the bromotyrosine-derived natural products ianthelline, 5-bromoverongamine and
JBIR-44 are described and their cytotoxic activity in a cervical cancer (HeLa) cell line and human umbilical
vein endothelial cells (HUVECs) are reported.
Received 28 May 2010
Revised 19 June 2010
Accepted 2 July 2010
Available online 8 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Ianthelline
5-Bromoverongamine
JBIR-44
Bromotyrosine
Anticancer
The Verongida order of marine sponges is a rich source of com-
pounds that are characterised by tyrosine metabolites where
dibromotyrosine is a signature motif.1 Such compounds have been
found to act as antimicrobials,2 antifoulants,3 antifungals4 and
antibacterials with varying degrees of potency.5 In terms of broad
anticancer activity, a large number of these compounds are cyto-
toxic and antiproliferative.6 However, some have been identified
as having specific antitubulin,7 antiangiogenic8 and protein inhib-
itory9 activity. For example, ceratamine A (1) and B (2), isolated
from Pseudoceratina sp. in 2003,7a were found to have antimitotic
(Fig. 1) has largely been unexplored owing to the small amount
of material obtained from the natural sources.
To enable access to all the three compounds a convenient syn-
thetic strategy involving a common a-oximino acid intermediate 6
was envisaged which would provide sufficient material for preli-
minary assays (Fig. 2). In previous reports ianthelline (3) (isolated
from Ianthella ardis, 1986) had been found to be moderately active
OMe
O
R
Br
Br
O
properties against a breast cancer (MCF-7) cell line (IC50 10 lg/
Br
N
mL) (Fig. 1). The treatment of MCF-7 cells with 1 also resulted in
an unusual bundling of cellular microtubules around cell nuclei.7b
Other related compounds such as the bisulfide mono-bromotyro-
sine derivatives bisaprasin and psammaplin A act as dual inhibitors
of histone deacetylase (HDAC) and DNA methyltransferase in the
low nanomolar range,9a and aplysamine-6 is one of a few small
molecules to be discovered that inhibits, in the low micromolar
range, the relatively new cancer target isoprenylcysteine carboxyl
methyltransferase (Icmt).9b In summary, bromotyrosine-derived
natural products are interesting chemical starting points for
exploring potential anticancer activities.
N
MeO
NH2
N
Br
MeHN
N
N
N
H
H
N
HO
1, Ceratamine A (R = Me)
2, Ceratamine B (R = H)
3, Ianthelline
OMe
3
OMe
Br
Br
Br
Br
O
Br
OH
Br
N
O
The anticancer activity of the three dibromotyrosine com-
pounds ianthelline (3), 5-bromoverongamine (4) and JBIR-44 (5)
8
12
N
N
N
H
H
H
N
N
HO
HO
4, 5-Bromoverongamine
5, JBIR-44
* Corresponding author. Tel.: +44 1223 336398; fax: +44 1223 336442.
E-mail address: svl1000@cam.ac.uk (S.V. Ley).
Figure 1. Structures of ceratamine A and B, ianthelline, 5-bromoverongamine and
JBIR-44.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.016

J. W. Shearman et al. / Tetrahedron Letters 51 (2010) 4812–4814
4813
most common approach via an azlactone intermediate,14 other
pathways include cyano ylide couplings15 and conversion of either
OMe
OMe
Br
Br
O
Br
Br
O
amines16 or
a
-keto compounds17 directly to their corresponding
+
7, 8 or 9
oxime. One convenient approach adopts a Horner–Wadsworth–
Emmons reaction to couple a functionalised dimethylphosphate
to an aldehyde which has been used to good effect in the synthesis
of purealidin N18 and bastadin analogues.19 Full details of these
methods are discussed in a recent review.1a
As discussed above, access to all the three natural products 3, 4
and 5 was envisaged by elaborating the a-oximino acid derivative
6 (Fig. 2). Amide coupling of 6 with the amine salts 7, 8 or 9 fol-
lowed by the removal of the O-para methoxybenzyl (PMB) group
would furnish the desired bromotyrosine derivatives.
R
N
N
OH
H
N
N
HO
PMBO
6
3, 4, and 5
Br
OH
Br
N
NH2
N
H
N
H2N
.2HCl
H2N
.2HCl
H2N
.HBr
H
7
8
9
The a-oximino acid intermediate 6 was accessed in three steps
from the commercially available 3,5-dibromo-4-hydroxybenzalde-
hyde 10 (Scheme 1). Methylation of the phenol group of 10 with
methyl iodide under refluxing conditions20 and conversion of the
product, after isolation, to the corresponding azlactone 11 was
achieved by the treatment with N-acetylglycine and sodium ace-
tate in acetic anhydride (Erlenmeyer conditions).21 These two steps
proceeded in an overall yield of 82% and the geometry of 11 was
confirmed by X-ray crystallographic analysis to be the thermody-
namically favoured Z-isomer.22 The one-pot saponification of the
azlactone 11 with barium hydroxide octahydrate and subsequent
condensation with O-para methoxybenzyl hydroxylamine23 affor-
ded the oxime acid intermediate 6 on gram-scale in 54% yield.
The oxime acid 6 was then coupled with the appropriate amine
(7, 823 or 924) using N,N0-dicyclohexylcarbodiimide (DCC) and
N-hydroxybenzotriazole (HOBt) to furnish the O-PMB-protected
oximes 12, 13 and 14, respectively. Removal of the PMB group with
AlCl3 and anisole14b proceeded smoothly to furnish the bromotyro-
sine-derived products ianthelline (3), 5-bromoverongamine (4)
and JBIR-44 (5) in 78%, 44% and 85% yield, respectively.
Figure 2. Retrosynthetic analysis for the proposed synthesis of 3, 4 and 5 involving
a common -oximino acid intermediate 6.
a
as an antibacterial and antifungal agent against Staphylococcus
aureus and Candida albicans.10 5-Bromoverongamine (4) (isolated
from Pseudoceratina sp., 1998) had been reported to inhibit the set-
tlement of barnacle larvae at 10 mg/mL (EC50 1.03 mg/mL)11 and
was bactericidal towards methicillin-resistant S. aureus (MRSA)
(MIC 0.0625–0.5 mg Lꢀ1).12 Both 3 and 4 are purported to share a
biogenetic relationship to the ceratamines and can be envisioned
as open chain cyclisation precursors. JBIR-44 (5) (isolated from
Psammaplysilla purpurea, 2009) was active against a cervical cancer
cell line (HeLa, IC50 3.7
Synthetic routes to
6 have been widely discussed in the literature. In addition to the
l
a
M).13
-oximino acids and esters closely related to
OH
OMe
OMe
Br
Br
Br
Br
Br
Br
O
(a), (b)
(c)
The 1H and 13C NMR spectra for synthetic ianthelline (3) and
JBIR-44 (5) matched those of the natural materials. However, in
the case of synthetic 5 the original assignments of C-8 (dC 153.8)
and C-3 (dC 152.0) should be reversed on the basis of HMBC spec-
troscopy (see Supplementary data).25 In addition, whilst the 1H
NMR spectrum of synthetic 5-bromoverongamine (4) agreed with
that of the natural material, the assignment of the C-12 quaternary
centre (dC 134.0) in the natural material did not correlate with the
shift of C-12 (dC 135.9) in the synthetic material.26 However, X-ray
crystallographic analysis confirmed that the structure of synthetic
4 matched with that proposed for the natural material (Fig. 3). Fur-
thermore, this data also confirmed the E-geometry of the oxime
and the tautomeric form of the imidazole.27
N
O
O
OH
N
PMBO
O
10
11
6
(d)
(f)
OMe
Br
Br
O
N
N
N
(h)
H
H
N
RO
In the MTS assays28 that were conducted (Table 1), JBIR-44 (5)
12; R = PMB
4; R = H
was found to be cytotoxic against HeLa cells (IC50 14
was within the range reported for the natural material against
the same cell line (IC50 3.7 M). Ianthelline (3) was moderately
lM). This
(e)
l
OMe
OMe
Br
Br
O
Br
Br
Br
OH
Br
N
O
NH2
N
N
N
H
H
H
N
N
RO
(g)
RO
13; R = PMB
3; R = H
14; R = PMB
5; R = H
(i)
Scheme 1. Synthetic routes to ianthelline (3), 5-bromoverongamine (4) and JBIR-44
(5). Reagents and conditions: (a) MeI, K2CO3, acetone, reflux 6 h; (b) N-acetylgly-
cine, NaOAc, Ac2O, 120 °C, 6 h (82% over two steps); (c) Ba(OH)2ꢁ8H2O, 1,4-dioxane/
H2O (1:1) 60 °C, 1 h then PMBO–NH2ꢁHCl, 14 h (54%); (d) DCC, HOBt, 7, iPr2EtN,
CH2Cl2 (81%); (e) AlCl3, anisole, CH2Cl2, 5 min, (78%); (f) DCC, HOBt, 8, iPr2EtN,
CH2Cl2/DMF, (41%); (g) AlCl3, anisole, CH2Cl2, 5 min (44%); (h) DCC, HOBt, 9, iPr2EtN,
CH2Cl2 (42%); (i) AlCl3, anisole, CH2Cl2, 5 min (85%).
Figure 3. X-ray structure of 5-bromoverongamine (4).

4814
J. W. Shearman et al. / Tetrahedron Letters 51 (2010) 4812–4814
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72, 1211–1217.
Cytotoxic activities of natural products 3, 4, and 5 against HeLa and HUVEC cell lines
Compound
Cell line [IC50
(l
M)]
HUVEC
HeLa
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D. Tetrahedron Lett. 2001, 42, 5265–5267.
Ianthelline (3)
5-Bromoverongamine (4)
JBIR-44 (5)
35
>50
14
74
36
24
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cytotoxic (IC50 35
active (IC50 >50 M). Since the related dibromotyrosine derivative
aeroplysinin-1 has been reported to inhibit the proliferation of the
M),29 it was also of inter-
lM) and 5-bromoverongamine (4) was the least
l
8. Córdoba, R.; Salvador Tormo, N.; Medarde, A. F.; Plumet, J. Bioorg. Med. Chem.
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bovine arterial endothelial cells (IC50 ꢂ2
l
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Antibiot. 2009, 62, 393–395.
est to investigate the activity against endothelial cells. To this end,
an xCELLigence assay using human umbilical vein endothelial cells
(HUVECs) was conducted and all the three compounds (Table 1)
were found to be cytotoxic to HUVECs in the micromolar range
(IC50 24–74 lM).
In conclusion, the total syntheses of the bromotyrosine-derived
products ianthelline (3), 5-bromoverongamine (4) and JBIR-44 (5),
obtained in five steps from the commercially available aldehyde
10, have been reported. Ianthelline and JBIR-44 exhibit anticancer
activity in HeLa cells and all the three compounds were cytotoxic
towards HUVECs. In addition, this short synthetic route will allow
for rapid development of analogues to explore structure–activity
relationships.
14. (a) Nishiyama, S.; Yamamura, S. Bull. Chem. Soc. Jpn. 1985, 3453–3456; (b) Zhu,
G.; Yang, F.; Balachandran, R.; Höök, P.; Vallee, R. B.; Curran, D. P.; Day, B. W. J.
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Karpf, A. R.; Sufrin, J. R. Bioorg. Med. Chem. Lett. 2006, 16, 3330–3333.
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20. Okamoto, K. J.; Clardy, J. Tetrahedron Lett. 1987, 28, 4969–4972.
21. Erlenmeyer, E. Justus Liebigs Ann. Chem. 1893, 275, 1–8.
22. Crystal data for 11: C12H9Br2NO3, M = 375.02, monoclinic, space group P2(1)/c,
a = 11.5810(3) Å, b = 14.3132(3) Å, c = 7.6379(2) Å, V = 1261.35(8) Å3, Z = 4,
Dc = 1.975 Mg mꢀ3, T = 180(2) K. CCDC 778410.
23. Prepared following the procedure described in Yang, F. Ph.D. Thesis, University
of Pittsburgh, 2006.
Acknowledgements
This research was funded by the Cambridge Cancer Research UK
PhD Training Programme in Medicinal Chemistry (J.W.S. and
T.M.B.). The authors thank Dr. John E. Davies of the X-ray facility
of the Department of Chemistry for collecting the crystallographic
data.
24. Prepared following the procedure described in Ullah, N.; Arafeh, K. M.
Tetrahedron Lett. 2009, 50, 158–160.
Supplementary data
25. For the natural sample, the authors report a strong m-coupling between
aromatic protons H-1 and H-5 (dH 7.52) and the aromatic carbon (dC 152.0) and
also observe the long range coupling of methylene protons H–7 (dH 3.87) to C-8
(dC 153.8). However, in the synthetic sample, we observe a strong m-coupling
between aromatic protons H-1 and H-5 (dH 7.52) and the signal at (dC 153.8)
Supplementary data (synthesis procedures, spectral data, 13C
NMR for natural and synthetic 4, HMBC spectra for synthetic 5, bio-
logical assay protocols) associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.07.016.
which we have assigned to C-3. There is also
a long range coupling of
methylene protons H-7 (dH 3.87) to the signal at (dC 152.0) which we have
assigned to C-8. The HMBC spectra for the natural sample are not available for
direct comparison.
26. The original 13C NMR spectrum for the natural material was kindly provided by
Professor Jean Claude Braekman for comparison and is included in the
Supplementary data.
References and notes
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27. Crystal data for 4: C15H16Br2N4O3, M = 460.14, monoclinic, space group P2(1)/n,
a = 8.9668(2) Å, b = 21.2760(3) Å, c = 9.3347(2) Å, V = 1752.45(6) Å3, Z = 4,
Dc = 1.744 Mg mꢀ3, T = 180(2) K. CCDC 777660.
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nyl)-2H-tetrazolium) (MTS) assay gives a colourimetric measure of the number
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