Application of a 6π-1-azatriene electrocyclization strategy to the total synthesis of the marine sponge metabolite ageladine A and biological evaluation of synthetic analogues

Article abstract of DOI:10.1021/jo0707232

(Chemical Equation Presented) A 12-step synthesis of the angiogenesis inhibitory marine metabolite ageladine A is reported. The key steps include a 6π-1-azatriene electrocyclization for formation of the pyridine ring and a Suzuki-Miyaura coupling of N-Boc

Full text of DOI:10.1021/jo0707232

Application of a 6π-1-Azatriene Electrocyclization Strategy to the
Total Synthesis of the Marine Sponge Metabolite Ageladine A and
Biological Evaluation of Synthetic Analogues
Matthew L. Meketa,† Steven M. Weinreb,*,† Yoichi Nakao,‡ and Nobuhiro Fusetani§
Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802,
School of AdVanced Science and Engineering, Waseda UniVersity, 3-4-1, Okubo, Shinjuku-ku,
Tokyo, 169-8555, Japan, and Graduate School of Fisheries Sciences, Hokkaido UniVersity,
3-1-1, Minato-cho, Hakodate, 041-8611, Japan
smw@chem.psu.edu
ReceiVed April 6, 2007
A 12-step synthesis of the angiogenesis inhibitory marine metabolite ageladine A is reported. The key
steps include a 6π-1-azatriene electrocyclization for formation of the pyridine ring and a Suzuki-Miyaura
coupling of N-Boc-pyrrole-2-boronic acid with a chloroimidazopyridine. In addition, an assessment of
the biological activity of a variety of synthetic analogues of ageladine A prepared during this synthesis
is described.
Introduction
MMP inhibitors, kinetic analysis showed that the compound
does not inhibit MMP-2 in a competitive manner.
Ageladine A (1) was isolated in 2003 by Fusetani, Nakao,
and co-workers by bioassay-guided extraction and repetitive
reverse-phase HPLC purification of the hydrophilic extract from
the marine sponge Agelas nakamurai, which was obtained off
the coast of Kuchinoerabu-jima Island in southern Japan
(Scheme 1).1 Marine sponges of the genus Agelas have been
reported to contain many bioactive polycyclic pyrrole-imidazole
alkaloids. Interestingly, ageladine A, whose structure was
established primarily by 2D NMR studies, is the first example
of this family to contain a 2-aminoimidazopyridine core.2
Ageladine A is an inhibitor of matrix metalloproteinases at
micromolar levels (MMPs), a family of enzymes involved in
metastasis and tumor angiogenesis (vide infra).3 Unlike other
To date, three total syntheses of ageladine A have been
reported. In 2006, we described the first synthesis of this marine
metabolite using a 6π-1-azatriene electrocyclization and a
Suzuki-Miyaura coupling involving a 2-chloropyridine deriva-
tive as key steps.4 Shortly afterward an alternative route to
ageladine A was completed by Shengule and Karuso, in which
a pivotal Pictet-Spengler reaction between 2-aminohistamine
and 4,5-dibromo-2-formylpyrrole was used, ultimately leading
to the natural product.5 Recently, we described a second
generation synthesis of 1 using a biomimetically inspired 6π-
2-azatriene electrocyclization as the key step for construction
of the imidazopyridine core.6,7 In this paper we describe the
full details of our first total synthesis of ageladine A, along with
an assessment of the biological activity of a variety of synthetic
analogues of 1.
† The Pennsylvania State University.
‡ Waseda University.
§ Hokkaido University.
(1) (a) Fujita, M.; Nakao, Y.; Matsunaga, S.; Seiki, M.; Itoh,
Y.; Yamashita, J.; van Soest, R. W. M.; Fusetani, N. J. Am. Chem.
Soc. 2003, 125, 15700. (b) Nakao, Y.; Fusetani, N. J. Nat. Prod. 2007, 70,
689.
(2) Mourabit, A. A.; Potier, P. Eur. J. Org. Chem. 2001, 237.
(3) (a) Whittaker, M.; Floyd, C. D.; Brown, P.; Gearing, A. J. H. Chem.
ReV. 1999, 99, 2735. (b) Matter, A. Drug DiscoVery Today 2001, 6, 1005.
(4) Meketa, M. L.; Weinreb, S. M. Org, Lett. 2006, 8, 1443.
(5) Shengule, S. R.; Karuso, P. Org. Lett. 2006, 8, 4083.
(6) Meketa, M. L.; Weinreb, S. M. Org. Lett. 2007, 9, 853.
(7) For a brief review of the three ageladine A total syntheses, see:
Weinreb, S. M. Nat. Prod. Rep. In press.
10.1021/jo0707232 CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/01/2007
4892
J. Org. Chem. 2007, 72, 4892-4899

Synthesis of the Marine Sponge Metabolite Ageladine A
SCHEME 1
SCHEME 2
Results and Discussion
with various electrophiles.13,14 Thus, using known BOM-
protected tribromoimidazole (5a),15 metalation with 1 equiv of
n-butyllithium, followed by addition of dimethyl disulfide,
selectively introduced a thiomethyl moiety at C(2) (Scheme 2).
Without workup, the imidiazole was then lithiated at C(5)
by using a second equivalent of n-butyllithium. Subsequent
addition of DMF then afforded aldehyde 6a in 91% overall yield.
The corresponding PMB-protected imidazole aldehyde 6b
was generated in 86% yield by using an identical series of
reactions.
Stille coupling of bromoimidazole 6a and vinyltributylstan-
nane proceeded smoothly to give vinylimidazole 7. The highly
functionalized vinylimidazole aldehyde 7 reacted with 2-lithio-
N-benzenesulfonylpyrrole16 to yield alcohol 8, which was
oxidized to the corresponding ketone 9 by using Dess-Martin
periodinane. Interestingly, oxidation of alcohol 8 with manga-
nese dioxide produced an inseparable 2:1 mixture of ketone 9
along with pyrrole aldehyde 11, while PCC oxidation afforded
exclusively pyrrole aldehyde 11 as the only isolable product
(Scheme 3).
Our initial retrosynthetic analysis for ageladine A involved
a pivotal 6π-1-azaelectrocyclization of vinylimidazole oxime
derivative 3 to give key tricyclic intermediate 2 (Scheme 1).8
Vinylimidazole oxime 3 would be derived from the correspond-
ing ketone 4. Hibino and co-workers have previously used 6π-
azaelectrocyclizations of related 1-azatrienes in the synthesis
of various imidazopyridines.9 In addition, Trost and Gutierrez
recently reported a novel route to pyridines utilizing a ruthenium-
catalyzed cycloisomerization/6π-azaelectrocyclization sequence.10
Although the basic pericyclic reaction has been known for a
number of years,11 few natural product syntheses have utilized
1-azaelectrocyclizations. One notable case is the recent biomi-
metically patterned synthesis of grossularine-1 by the Horne
group.12
Our synthetic work initially focused on the formation of biaryl
oxime 3. Two different imidazole nitrogen protecting groups
were investigated in these studies on ageladine A (1): benzy-
loxymethyl (BOM) and p-methoxybenzyl (PMB). Iddon and co-
workers have shown that 2,4,5-tribromoimidazoles can be
sequentially and predictably metalated, followed by trapping
Unfortunately ketone 9 could not be converted to the desired
oxime (10, R ) H) or methoxime (10, R ) Me). Moreover,
formation of a hydrazone derivative with use of phenylhydrazine
or semicarbazide could not be effected. Oxime formation from
the analogous N-Boc-protected or N-H pyrrole ketone substrates
(8) For a review of electrocyclizations of 1-azatrienes, see: Okamura,
W. H.; de Lera, A. R. 1,3-Cyclohexadiene Formation Reactions. In
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon: Oxford, UK, 1991; Vol. 5, pp 699-750.
(9) (a) Yoshioka, H.; Choshi, T.; Sugino, E.; Hibino, S. Heterocycles
1995, 41, 161. (b) Yoshioka, H.; Matsuya, Y.; Choshi, T.; Sugino, E.;
Hibino, S. Chem. Pharm. Bull. 1996, 44, 709. (c) Kumemura, T.; Choshi,
T.; Yukawa, J.; Hirose, A.; Nobuhiro, J.; Hibino, S. Heterocycles 2005,
66, 87.
(10) Trost, B. M.; Gutierrez, A. C. Org. Lett. 2007, 9, 1473.
(11) (a) Maynard, D. F.; Okamura, W. H. J. Org. Chem. 1995, 60, 1763.
(b) Tanaka, K.; Mori, H.; Yamamoto, M.; Katsumura, S. J. Org. Chem.
2001, 66, 3099. (c) Tanaka, K.; Kobayashi, T.; Mori, H.; Katsumura, S. J.
Org. Chem. 2004, 69, 5906.
(12) Miyake, F. Y.; Yakushijin, K.; Horne, D. A. Angew. Chem., Int.
Ed. 2005, 44, 3280.
(13) (a) Iddon, B.; Khan, N. J. Chem. Soc., Perkin Trans. 1 1987, 1445.
(b) Lipshutz, B. H.; Hagen, W. Tetrahedron Lett. 1992, 33, 5865. (c) Chen,
Y.; Dias, H. V. R.; Lovely, C. J. Tetrahedron Lett. 2003, 44, 1379. (d)
Carver, D. S.; Lindell, S. D.; Saville-Stones, E. A. Tetrahedron 1997, 53,
14481.
(14) Groziak, M. P.; Wei, L. J. Org. Chem. 1991, 56, 4296.
(15) (a) Preparation of 5a from commercially available 2,4,5-tribro-
moimidazole: Schumacher, R. W.; Davidson, B. S. Tetrahedron 1999, 55,
935. (b) Preparation of 5b: Iddon, B.; Khan, N.; Lim, B. L. J. Chem. Soc.,
Perkin Trans. 1 1987, 1437.
(16) (a) Hasan, I.; Marinelli, E. R.; Lin, L.-C. C.; Fowler, F. W.; Levy,
A. B. J. Org. Chem. 1981, 46, 157. (b) Lautens, M.; Fillion, E. J. Org.
Chem. 1997, 62, 4418.
J. Org. Chem, Vol. 72, No. 13, 2007 4893

Meketa et al.
SCHEME 3
SCHEME 5
SCHEME 4
dazole 16, obtained in low yield from Stille reaction of 14c
with vinyltributylstannane (Scheme 5). In all other cases, the
only detectable product was nitrile imidazole 15, derived from
reductive elimination of the halomethoxime starting material.19,20
Interestingly, Kim and co-workers did not report observing such
a nitrile product during their studies.
As a result of the failure of the halomethoxime cross-coupling
reactions, we turned to 6π-1-azaelectrocyclizations related to
those of Hibino.9 Thus, heating dilute solutions of halom-
ethoximes 14b and 14c in o-xylene at 150 °C gave cyclized
haloimidazopyridines 17b (73%) and 17c (46%), respectively
(Scheme 6). When the cyclization of 14b was performed in a
microwave reactor, the product yield increased slightly to 81%.
However, due to scale-up problems associated with the micro-
wave, this procedure was not routinely used. Similarly, thermal
cyclization of chloromethoxime 14a in o-xylene at reflux
afforded chloro imidazopyridine 17a in 84% yield.
With the halopyridines 17a-c in hand, cross couplings with
the pyrrole moiety were then explored. Initial attempts at both
Suzuki-Miyaura and Stille couplings with N-Boc-pyrrole-2-
boronic acid (18)21 or the analogous 2-tributylstannylpyrrole,
respectively, afforded only the starting halopyridines. Negishi
cross couplings of N-benzenesulfonylpyrrole-2-zinc chloride
with the halopyridines were also unsuccessful.22 After extensive
experimentation, it was discovered that a Suzuki-Miyaura cross
coupling with Buchwald’s 2-biphenyldicyclohexylphosphine
ligand afforded the desired product.23 Thus, refluxing bromopy-
ridine 17c with pyrrole boronic acid 18 in 1,4-dioxane for 20 h
in the presence of 25% Pd(dba)2, the Buchwald ligand, and
potassium phosphate afforded tricycle 19 in 60% yield, along
with 20% of recovered bromide (Scheme 6). Chloropyridine
17b underwent cross coupling in higher yield (70%) along with
26% of recovered chloride. Interestingly, when the coupling of
17b with boronic acid 18 was performed in a microwave
reactor,24 the yield of 19 increased to 91% with only 5%
recovered starting material. It should be noted that the Lakshman
also failed. Only the starting ketone or the deprotected pyrrole
was recovered in all of these reactions. These results are not
too surprising since carbonyl compound 9 is hindered and is
also a vinylogous amide. Due to the inability to form the
requisite oxime, an alternative synthetic route was therefore
devised.
Our revised strategy was to form the pyridine ring prior
to introducing the pyrrole moiety. Synthetic routes were per-
formed with both the BOM- and PMB-protected imida-
zoles, which gave comparable product yields for most steps.
Bromo vinylimidazoles 12a and 12b were produced in high
yields from aldehydes 6a and 6b via a Wittig reaction (Scheme
4). Bromides 12a and 12b were then lithiated with n-
butyllithium, followed by treatment with carbon dioxide, to
afford carboxylic acids 13a and 13b, respectively. The yields
of the acids were significantly higher when freshly crushed
dry ice was added directly to a solution of the lithiated
imidazole, rather than by bubbling CO2 into the reaction mixture.
Carboxylic acids 13a and 13b were then converted into
the corresponding halomethoximes 14a-c in a single step by
using the methodology of Kikugawa and co-workers.17
Chloro- or bromomethoximes 14 could be produced in good
yield by using either carbon tetrachloride or tetrabromide,
respectively.
(19) The conversion of a halomethoxime to a nitrile has been performed
under basic conditions,20a,b photolytically,17a,20c or with a zinc/acetic acid/
DMF protocol.17c
(20) (a) Shustov, G. V.; Kachanov, A. V.; Kostyanovsky, R. G. IzV. Akad.
Nauk, Ser. Khim. 1992, 11, 2584. (b) Nie, H.; Wu, Y.-W.; Mei, H.-S. Zh.
Yiyao Gongye Zazhi 1999, 30, 469. (c) Sakamoto, T.; Okamoto, K.;
Kikugawa, Y. J. Org. Chem. 1992, 57, 3245.
(21) Kelly, T. A.; Fuchs, V. U.; Perry, C. W.; Snow, R. J. Tetrahedron
1993, 49, 1009.
On the basis of the work of Kim et al., cross coupling of the
halomethoximes 14b and 14c with a wide variety of stannanes,
boronic acids, and organozinc halides was explored.18 However,
the only coupled product that could be isolated was vinylimi-
(22) Dinsmore, A.; Billing, D. G.; Mandy, K.; Michael, J. P.; Mogano,
D.; Patil, S. Org. Lett. 2004, 6, 293.
(23) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J.
Am. Chem. Soc. 2005, 127, 4685.
(24) Song, Y. S.; Kim, B. T.; Heo, J.-N. Tetrahedron Lett. 2005, 46,
5987.
(17) (a) Kikugawa, Y.; Fu, L. H.; Sakamoto, T. Synth. Commun. 1993,
23, 1061. (b) Einhorn, J.; Einhorn, C.; Luche, J.-L. Synth. Commun. 1990,
20, 1105. (c) Sakamoto, T.; Mori, H.; Takizawa, M.; Kikugawa, Y. Synthesis
1991, 750.
(18) Chang, S.; Lee, M.; Kim, S. Synlett 2001, 1557.
4894 J. Org. Chem., Vol. 72, No. 13, 2007

Synthesis of the Marine Sponge Metabolite Ageladine A
SCHEME 6
SCHEME 7
SCHEME 8
with sodium azide in DMF at 90 °C afforded the desired azide
24 in 64% yield.28 Removal of the Boc protecting group of 24
under acidic conditions then generated N-H pyrrole 25. Unfor-
tunately, removal of the PMB protecting group of imidazopy-
ridine 25 either by treatment with acid or by catalytic hydro-
genation failed. Similar attempts to remove the PMB group from
precursors 19 and 23 also failed. In light of these difficulties,
the PMB series was abandoned in favor of the BOM-protected
compounds.
However, it was found that all attempts to cross couple the
BOM-protected chloropyridine 17a with boronic acid 18 under
microwave irradiation at 150 °C resulted in a complex mixture
of products (Scheme 9). In addition to recovered starting
material, 16% of the C(2) coupled product 27 and a desulfurized
coupled product 28 (12%) were also isolated. Presumably these
products result via initial BOM protecting group coordination
with the palladium, placing the metal in close proximity to the
C-S bond, thereby increasing the thiophilicity of the metal. In
addition, the pyrrole Boc protecting group was also lost in both
isolated products, probably due to the high temperature of the
reaction. In light of these results, the synthetic strategy for
ageladine A required further modification.
group has noted improved product yields for chloride over
bromide substrates in purine derivatives when performing
Suzuki-Miyaura cross couplings.25 However, the reason for this
trend is currently unclear.
To continue the synthesis, tricyclic sulfide 19 could be
desulfurized with nickel boride26 followed by thermolytic
removal of the Boc protecting group to produce N-H pyrrole
20 (Scheme 7). We hoped it would then be possible to
selectively tribrominate compound 20 to produce 21. The
bromine at the C(2) position of the imidazopyridine could
subsequently serve as a handle for the introduction of the amino
group of ageladine A (1). Unfortunately, with use of Br2 in acetic
acid, N-H pyrrole 20 underwent tribromination at the three
pyrrole carbons to give tricycle 22, with no bromination
observed at the imidazopyridine C(2) position. Bromination
attempts with NBS gave an inseparable mixture of di- and
tribrominated products. As a result of the high reactivity of the
pyrrole toward bromination, an alternative route had to be
devised for the selective dibromination of the pyrrole and
introduction of the imidazole amino moiety.
To circumvent the above problems, we decided to install
the imidazopyridine C(2)-amino group prior to introducing the
pyrrole fragment. Thus, Oxone oxidation of sulfide 17a ef-
ficiently produced sulfoxide 29 (Scheme 10). Displacement of
the sulfoxide moiety of 29 with sodium azide in DMF could be
effected at room temperature in this case to afford azide 30. A
competing reaction at temperatures above 23 °C was hydrolysis
of the imidazole sulfoxide by adventitious moisture to produce
the corresponding 2-imidazolone. Interestingly, the analogous
imidazolone byproduct had not been observed in the previous
conversion of sulfoxide 23 to azide 24 (cf. Scheme 8). Catalytic
Toward this end, we hoped to initially introduce the amine
moiety at C(2) of imidazopyridine via displacement of an
appropriate thio derivative. Therefore, methyl sulfide 19 was
oxidized to the corresponding sulfoxide 23 with Oxone in high
yield (Scheme 8).27 Attempts to further oxidize the sulfoxide
to the corresponding sulfone with Oxone or m-CPBA failed,
resulting only in decomposition. On the basis of the work of
Jarosinski and Anderson, displacement of the sulfoxide moiety
(25) Lakshman, M. K.; Hilmer, J. H.; Martin, J. Q.; Keeler, J. C.; Dinh,
Y. Q. V.; Ngassa, F. N.; Russon, L. M. J. Am. Chem. Soc. 2001, 123, 7779.
(26) Back, T. G.; Baron, D. L.; Yang, K. J. Org. Chem. 1993, 58, 2407.
(27) Oxidation of methyl sulfide 19 with m-CPBA only led to decom-
position.
(28) Jarosinski, M. A.; Anderson, W. K. J. Org. Chem. 1991, 56, 4058.
J. Org. Chem, Vol. 72, No. 13, 2007 4895

Meketa et al.
SCHEME 9
SCHEME 10
SCHEME 11
hydrogenation of azide 30 cleanly produced 2-amino-
imidazopyridine 31. With the amino group in place, Suzuki-
Miyaura cross coupling with pyrrole boronic acid 18 then
produced Boc-protected tricyclic pyrrole 32 and N-H compound
33 as a separable 2:1 mixture in good total yield. We were
pleased to find that no arylation of the amino group of 31 with
boronic acid 18 had occurred.29
Next, Boc-protected tricyclic pyrrole 32 was brominated with
tetrabutylammonium tribromide (TBATB), which to our surprise
gave the 3,5-dibromopyrrole 35 in 18% yield along with
monobromopyrrole 34 as the major product (74%).30 The
monobromopyrrole 34 could be brominated to generate ad-
ditional dibromopyrrole 35. Bromination of tricyclic pyrrole 32
with Br2 in carbon tetrachloride led to a mixture of starting
material, along with the same mono- and dibrominated products
but in poorer yields. The Boc protecting group of tricycle 35
was removed with refluxing 6 N HCl in ethanol to give the
3,5-dibromo regioisomer 36 of ageladine A. The positions of
the bromines in bromopyrroles 34, 35, and 36 were established
by using 1H NMR chemical shift data and proton-proton
coupling constants.31 At this point, we do not have a good
rationale for the observed regioselectivity in the bromination
step since there is little literature precedent for dibromination
of pyrrole derivatives bearing a C(2) aryl substituent.32
Since the Boc-protected pyrrole 32 led to the undesired
dibromination product, we decided to investigate the bromina-
tion of the analogous unprotected system. Thus, the crude
mixture of 2:1 Boc protected/N-H pyrrole tricycle products 32/
33 was hydrolyzed to afford the fully deprotected tricycle 37
in 67% overall yield from chloropyridine 31 (Scheme 11).
Bromination of the N-H pyrrole 37 with either TBATB or NBS
resulted in complex mixtures of dibromo and tribrominated
pyrrole products. After some experimentation, the optimal
conditions found for halogenation were to treat pyrrole 37 with
Br2 in an acetic acid/methanol solvent mixture at 0 °C. These
conditions produced ageladine A (1) in 17% yield, along with
recovered starting material (29%), monobromopyrrole 38 (50%),
(29) For analogous chemoselectivity see: Thompson, A. E.; Hughes, G.;
Batsanov, A. S.; Bryce, M. R.; Parry, P. R.; Tarbit, B. J. Org. Chem. 2005,
70, 388.
(30) Chaudhuri, M. K.; Khan, A. T.; Patel, B. K. Tetrahedron Lett. 1998,
39, 8163.
(31) Pretsch, E.; Simon, W.; Seibel, J.; Clerc, T. In Tables of Spectral
Data for Structure Determination of Organic Compounds, 2nd ed.;
Fresenius, W., Huber, J. F. K., Pungor, E., Rechnitz, G. A., Simon, W.,
West, T. S., Eds.; Springer-Verlag: New York 1989; p H265.
(32) (a) El’chaninov, M. M.; Simonov, A. M.; Oleinikova, L. Y. Chem.
Heterocycl. Compd. 1980, 16, 71. (b) Chakrabarti, J. K.; Goulding, R. W.;
Todd, A. J. Chem. Soc., Perkin Trans. 1 1973, 21, 2499. (c) Blache, Y.;
Gueiffier, A.; Chavigon, O.; Teulade, J. C.; Milhavet, J. C.; Voils, H.;
Chapat, J. P. J. Heterocycl. Chem. 1994, 31, 161.
4896 J. Org. Chem., Vol. 72, No. 13, 2007

Synthesis of the Marine Sponge Metabolite Ageladine A
and tribromopyrrole 39 (2%). Purification of this mixture was
accomplished via reverse-phase HPLC. Ageladine A (1) and
the tribrominated product 39 were very difficult to separate since
they have similar retention times. Bromination conditions,
therefore, had to be optimized such that the maximum amount
of ageladine A was produced, while minimizing the formation
of undesired tribromopyrrole 39. Both the recovered starting
material and monobromopyrrole 38 could be resubjected to the
same bromination conditions to produce additional ageladine
A. Synthetic ageladine A had proton and carbon NMR spectra,
as well as UV and fluorescence maxima, identical with those
reported for the natural product.1
Biological Activity of Ageladine A and Analogues
Ageladine A (1) has been shown to be a unique inhibitor of
matrix metalloproteinases (MMPs), particularly MMP-2.1 In
addition to significant inhibition of MMP-2 (2.0 µg/mL), natural
ageladine A has been found to display inhibitory activity against
MMP-1, -8, -9, -12, and -13 with IC50 values of 1.2, 0.39, 0.79,
0.33, and 0.47 µg/mL, respectively. Matrix metalloproteinases
are a family of zinc-containing enzymes that regulate multiple
steps of tumor metastasis and angiogenesis.3 These enzymes
are involved in degradation of most components of the extra-
cellular matrix (ECM) including collagens, elastins, and fi-
bronectins, thereby allowing for tumor growth and expansion.
The entire family of MMPs in humans now includes over two
dozen related enzymes that are commonly grouped as either
collagenases, gelatinases, stromelysins, or membrane-type MMPs
(MT-MMPs). One such gelatinase is MMP-2, which in addition
to being involved in tumor metastasis and angiogenesis, is
known to complex with MT1-MMP via TIMP at the migration
front of tumor or endothelial cells.1,3 In addition, the ratio of
activated to total MMP levels, particularly MMP-2, can be
correlated with tumor aggressiveness.33 As a result, MMP-2
inhibitors are presumed to be both antiangiogenic and antime-
tastatic, making MMP-2 inhibition an important area of cancer
research.
The vast majority of MMP inhibitors act by chelation of the
enzyme active-site zinc(II) ion and such compounds usually
contain chelating groups like carboxylates, hydroxamates, or
thiols. Interestingly, studies have revealed that ageladine A is
not capable of zinc binding and that its MMP-2 inhibition is
noncompetitive by kinetic analysis.1 Therefore ageladine A is
believed to have a novel and as yet unknown mode of MMP
inhibition.
Our total synthesis of ageladine A supplied us not only with
samples of synthetic natural product, but also with several
structural analogues that were screened for MMP inhibition
using procedures previously described.1 The compounds which
were evaluated include synthetic ageladine A (1), 3,5-dibromo
regioisomer 36, monobromopyrrole 38, its fully protected
version 35, tricyclic analogue 37, and 2-chloropyridine 31
(Figure 1). The biological screening focused on MMP-2 and
MT1-MMP.1
FIGURE 1. MMP inhibition results for synthetic analogues of
ageladine A.
bromine atoms impacts on MMP inhibition. Both brominated
analogues 36 and 38 inhibited MMP-2 and MT1-MMP, but with
about a 5-fold decrease relative to 1. The complete removal of
bromine from the molecule (i.e., 37) appeared to substantially
decrease MMP inhibition. However, due to the strong fluores-
cence of 37, which interferes with the inhibition assay, the data
are not entirely reliable. The data also indicated that the presence
of the nitrogen protecting groups in 35 negatively impacts MMP
inhibition, as does removal of the pyrrole ring (cf. 31).
Conclusion
A 12-step total synthesis of the tricyclic marine metabolite
ageladine A (1) has been developed starting from 2,4,5-
tribromoimidazole, utilizing a 6π-1-azaelectrocyclization and
a Suzuki-Miyaura coupling of N-Boc-pyrrole-2-boronic acid
(18) with a chloro imidazopyridine as key steps. The synthesis
has led to the production of several structural analogues of 1
that were subjected to MMP inhibition testing. Although none
of the analogues prepared to date are as potent as the natural
product, this route, along with our recently reported second
generation synthesis of ageladine A,6 allows for the facile
synthesis of additional compounds for further biological testing.
Experimental Section
3-Benzyloxymethyl-5-bromo-2-methylsulfanylimidazole-4-
carboxaldehyde (6a). To a solution of tribromoimidazole 5a (6.00
g, 14.1 mmol) in THF (35 mL) was added n-BuLi in hexanes (2.4
M, 5.88 mL, 14.1 mmol) at -78 °C and the reaction mixture was
stirred for 15 min.34 Dimethyl disulfide (1.33 g, 14.1 mmol) was
then added dropwise at -78 °C. The reaction mixture was stirred
at -78 °C for 15 h, and n-BuLi in hexanes (2.4 M, 5.88 mL, 14.1
mmol) was added dropwise. After the mixture was stirred for 15
min at -78 °C, DMF (3.09 g, 3.29 mL, 42.4 mmol) was added
slowly. The reaction mixture was stirred at -78 °C for 30 min and
The synthetic ageladine A displayed the same level of
inhibition as the isolated natural product against MMP-2 and
MT1-MMP. However, as indicated from the testing results, none
of our analogues was a more potent inhibitor than ageladine A
(Figure 1). It appears that the quantity and location of the
(33) Zucker, S.; Cao, J.; Wen-Tien, C. Oncogene 2000, 19, 6642 and
references cited therein.
(34) For general procedures and HPLC conditions, see the Supporting
Information.
J. Org. Chem, Vol. 72, No. 13, 2007 4897

Meketa et al.
then warmed to rt. The mixture was diluted with H2O and extracted
with CH2Cl2. The combined organic extracts were dried (MgSO4)
and concentrated. Purification of the residue by column chroma-
tography (hexanes/EtOAc, 6/1) afforded aldehyde 6a (4.39 g, 91%)
127.3, 104.1, 72.5, 70.4, 14.3; ESI (+) [M + H]+ calcd for
C15H15N3OSCl 320.0624, found 320.0616.
1-Benzyloxymethyl-4-chloro-2-methanesulfinylimidazopyri-
dine (29). To a solution of sulfide 17a (100 mg, 0.31 mmol) in
MeOH (3 mL) at -20 °C was added Oxone (Aldrich, 577 mg,
0.19 mmol) in H2O (3 mL). The mixture was stirred at -20 °C for
2.5 h, after which the MeOH was removed in vacuo. The mixture
was diluted with H2O and extracted with CH2Cl2. The combined
organic extracts were dried (MgSO4) and concentrated. Purification
of the residue by preparative TLC (hexanes/EtOAc, 1/1) afforded
1
as a yellow solid. H NMR (400 MHz, CDCl3) δ 2.63 (s, 3H),
4.52 (s, 2H), 5.68 (s, 2H), 7.21-7.23 (m, 5H), 9.52 (s, 1H); 13C
NMR (100 MHz, CDCl3) δ 178.5, 155.3, 137.1, 132.1, 128.8, 128.6,
128.4, 128.1, 74.3, 71.8, 15.1; ESI (+) [M + Na]+ calcd for
C13H13N2O2BrSNa 362.9779, found 362.9775.
1-Benzyloxymethyl-4-bromo-2-methylsulfanyl-5-vinylimida-
zole (12a). To a solution of Ph3PMeBr (6.54 g, 18.32 mmol) in
THF (35 mL) was added t-BuOK (1.81 g, 16.12 mmol) in portions
at 0 °C. The reaction mixture was stirred at rt for 30 min and cooled
to 0 °C. A solution of aldehyde 6a (2.50 g, 7.33 mmol) in THF (5
mL) was added slowly after which the reaction mixture was warmed
to rt and stirred for 3.5 h. The reaction mixture was diluted with
saturated NH4Cl and extracted with EtOAc. The combined organic
extracts were dried (MgSO4) and concentrated. Purification of the
residue by column chromatography (hexanes/EtOAc, 3/1) afforded
vinylimidazole 12a (2.39 g, 96%) as a yellow solid. 1H NMR (300
MHz, CDCl3) 2.56 (s, 3H), 4.47 (s, 2H), 5.28 (m, 3H), 5.87 (d, J
) 17.9 Hz, 1H), 6.45 (dd, J ) 17.9, 12.1 Hz, 1H), 7.18-7.28 (m,
5H); 13C NMR (75 MHz, CDCl3) δ 143.9, 135.4, 128.0, 127.5,
127.1, 126.7, 120.9, 116.1, 115.0, 72.3, 69.6, 15.0; ESI (+) [M +
H]+ calcd for C14H16N2OSBr 339.0167, found 339.0153.
1-Benzyloxymethyl-2-methylsulfanyl-5-vinylimidazole-4-car-
boxylic Acid (13a). To a solution of bromoimidazole 12a (3.50 g,
10.32 mmol) in THF (30 mL) was added n-BuLi in hexanes (2.4
M, 6.45 mL, 15.47 mmol) at -78 °C and the reaction mixture was
stirred for 1.25 h. Excess freshly crushed dry ice was then added
at -78 °C and the reaction mixture was slowly warmed to rt. The
reaction mixture was quenched with H2O and extracted with Et2O.
The aqueous layer was acidified to pH 3 with 1 M HCl and
extracted with EtOAc. The extract was dried (MgSO4) and
concentrated. Purification of the residue by recrystallization (hex-
anes/EtOAc) afforded acid 13a (3.09 g, 98%) as a white solid. 1H
NMR (400 MHz, CDCl3) δ 2.62 (s, 3H), 4.55 (s, 2H), 5.31 (s,
2H), 5.55 (dd, J ) 12.2, 1.0 Hz, 1H), 6.08 (dd, J ) 18.1, 0.9 Hz,
1H), 7.02 (dd, J ) 18.1, 12.2 Hz, 1H), 7.19-7.28 (m, 5H); 13C
NMR (100 MHz, CDCl3) δ 164.2, 146.9, 138.2, 136.7, 129.9, 129.0,
128.7, 128.2, 123.0, 122.7, 74.0, 71.5, 15.9; ESI (+) [M + H]+
calcd for C15H17N2O3S 305.0960, found 305.0947.
1-Benzyloxymethyl-2-methylsulfanyl-4-(N-methoxyimino-1-
chloromethyl)-5-vinylimidazole (14a). A solution of methoxy-
lamine hydrochloride (82 mg, 0.99 mmol) and pyridine (78 mg,
0.08 mL, 0.99 mmol) in acetonitrile (15 mL) was stirred at rt for
10 min, and acid 13a (250 mg, 0.82 mmol) and carbon tetrachloride
(505 mg, 0.32 mL, 3.29 mmol) were added. The reaction mixture
was stirred at rt for 10 min before the addition of triphenylphosphine
(862 mg, 3.29 mmol). The mixture was refluxed at 80 °C for 4 h
and then concentrated. The residue was purified by column
chromatography on Florisil (hexanes/EtOAc, 6/1) affording chlo-
romethoxime 14a (251 mg, 87%) as a yellow solid. 1H NMR (300
MHz, CDCl3) δ 2.57 (s, 3H), 3.97 (s, 3H), 4.51 (s, 2H), 5.28 (s,
2H), 5.36 (dd, J ) 12.1, 1.0 Hz, 1H), 5.83 (dd, J ) 18.0, 1.09 Hz,
1H), 6.77 (dd, J ) 18.0, 12.1 Hz, 1H), 7.19-7.25 (m, 5H); 13C
NMR (75 MHz, CDCl3) δ 146.7, 137.0, 133.3, 133.1, 132.6, 128.9,
128.5, 128.1, 123.6, 119.9, 74.1, 71.2, 63.5, 16.5; ESI (+) [M +
H]+ calcd for C16H19N3O2SCl 352.0887, found 352.0888.
1-Benzyloxymethyl-4-chloro-2-methylsulfanylimidazopyri-
dine (17a). A solution of methoxime 14a (0.50 g, 1.42 mmol) in
o-xylene (50 mL) was refluxed at 145 °C for 17 h and concentrated.
The residue was purified by column chromatography on Florisil
(hexanes/EtOAc, 2/1) affording pyridine 17a (0.38 g, 84%) as a
yellow solid. 1H NMR (300 MHz, CDCl3) δ 2.80 (s, 3H), 4.43 (s,
2H), 5.43 (s, 2H), 7.12 (d, J ) 5.5 Hz, 1H), 7.16-7.19 (m, 2H),
7.23-7.26 (m, 3H), 8.05 (d, J ) 5.5 Hz, 1H); 13C NMR (75 MHz,
CDCl3) δ 156.0, 142.1, 140.6, 139.7, 137.2, 135.3, 128.1, 127.8,
1
sulfoxide 29 (96 mg, 91%) as a white solid. H NMR (300 MHz,
CDCl3) δ 3.22 (s, 3H), 4.56 (s, 2H), 5.97 (s, 2H), 7.15-7.18 (m,
2H), 7.21-7.24 (m, 3H), 7.31 (d, J ) 5.7 Hz, 1H), 8.21 (d, J )
5.6 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 154.0, 143.1, 142.3,
141.5, 135.3, 135.1, 127.9, 127.7, 127.1, 105.3, 73.1, 70.8, 39.7;
ESI (+) [M + H]+ calcd for C15H15N3O2SCl 336.0574, found
336.0575.
2-Azido-1-benzyloxymethyl-4-chloroimidazopyridine (30).
To a solution of sulfoxide 29 (75 mg, 0.22 mmol) in DMF
(5 mL) was added NaN3 (73 mg, 1.12 mmol) at rt. The mix-
ture was stirred at rt for 14 h, diluted with H2O, and extracted with
CH2Cl2. The combined organic extracts were dried (MgSO4) and
concentrated. Purification of the residue by column chroma-
tography (hexanes/EtOAc, 1/1) afforded azide 30 (61 mg, 87%)
1
as a white solid. H NMR (300 MHz, CDCl3) δ 4.42 (s, 2H),
5.34 (s, 3H), 7.12-7.16 (m, 3H), 7.20-7.24 (m, 3H), 8.05 (d, J )
5.5 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 151.3, 143.3, 142.4,
142.0, 137.6, 137.4, 130.3, 130.2, 129.5, 107.0, 74.1, 73.1;
ESI (+) [M + H]+ calcd for C14H12N6OCl 315.0761, found
315.0765.
1-Benzyloxymethyl-4-chloroimidazopyridin-2-ylamine (31). A
solution of azide 30 (50 mg, 0.16 mmol) in EtOH (3 mL) was
reduced with 10% Pd/C (16 mg) at rt under 1 atm of H2 for 3.5 h.
The mixture was then filtered through a Celite pad, which was
washed with MeOH. The filtrate was concentrated to afford amine
31 (44 mg, 96%) as a white solid sufficiently pure for use in the
1
next step. H NMR (300 MHz, CD3OD) δ 4.55 (s, 2H), 5.55 (s,
2H), 7.23-7.25 (m, 6H), 7.88 (d, J ) 5.4 Hz, 1H); 13C NMR (75
MHz, CD3OD) δ 156.9, 141.5, 138.9, 137.1, 136.3, 136.0, 128.3,
127.9, 127.8, 104.4, 72.3, 71.0; ESI (+) [M + H]+ calcd for
C14H14N4OCl 289.0856, found 289.0866.
4-(1H-Pyrrol-2-yl)imidazopyridin-2-ylamine (37). In an oven-
dried Schlenk tube were placed chloropyridine 31 (70 mg, 0.24
mmol), boronic acid 18 (205 mg, 0.97 mmol), Pd2(dba)3 (55 mg,
0.06 mmol), (2-biphenyl)dicyclohexylphosphine (85 mg, 0.24
mmol), K3PO4 (206 mg, 0.97 mmol), and 1,4-dioxane (6 mL). The
Schlenk tube was flushed with argon, sealed, and heated at 100 °C
for 17 h. The reaction mixture was then cooled to rt and filtered
through a Celite pad, which was washed with EtOAc. The filtrate
was concentrated and the crude coupled product mixture 32 and
33 (2:1 mixture of 32:33) was dissolved in EtOH (7 mL) and 6 N
HCl (4 mL). The mixture was heated at 100 °C for 12 h and then
concentrated. The residue was redissolved in MeOH, neutralized
with 1% KOH, and concentrated again. The residue was purified
with use of a short silica plug (CH2Cl2/MeOH, 4/1) affording
deprotected imidazole 37 (32 mg, 67%) as a white solid. 1H NMR
(300 MHz, CD3OD) δ 6.41 (dd, J ) 3.9, 2.6 Hz, 1H), 7.17 (dd, J
) 3.9, 1.3 Hz, 1H), 7.23 (dd, J ) 2.6, 1.3 Hz, 1H), 7.33 (d, J )
6.5 Hz, 1H), 7.94 (d, J ) 6.5 Hz, 1H); 13C NMR (75 MHz, CD3-
OD) δ 160.3, 146.2, 136.5, 131.8, 131.5, 125.6, 123.5, 112.6, 112.0,
104.4; ESI (+) [M + H]+ calcd for C10H10N5 200.0936, found
200.0923.
Ageladine A (1). To a solution of pyrrole 37 (20 mg, 0.1 mmol)
in AcOH/MeOH (5 mL/1 mL) at 0 °C was slowly added Br2 in
glacial AcOH (20 mM, 0.36 mL, 0.07 mmol). The mixture was
stirred at 0 °C for 20 min and then concentrated. Purification of
the residue by reverse-phase HPLC34 afforded ageladine A (1, 6
mg, 17%) as a yellow solid, along with starting pyrrole 37 (6 mg,
29%), monobromopyrrole 38 (14 mg, 50%), and tribromopyrrole
4898 J. Org. Chem., Vol. 72, No. 13, 2007

Synthesis of the Marine Sponge Metabolite Ageladine A
39 (1 mg, 2%). Ageladine A (1): 1H NMR (300 MHz, CD3OD) δ
7.17 (s, 1H), 7.41 (d, J ) 6.4 Hz, 1H), 8.05 (d, J ) 6.4 Hz, 1H);
13C NMR (75 MHz, CD3OD) δ 160.8, 147.1, 136.7, 133.0, 128.5,
125.7, 115.1, 107.7, 105.4, 102.3; ESI (+) [M + H]+ calcd for
C10H8N579Br2 355.9146, found 355.9163. Monobromopyrrole 38:
1H NMR (400 MHz, CD3OD) δ 6.34 (d, J ) 6.0 Hz, 1H), 7.03 (d,
J ) 6.0 Hz, 1H), 7.30 (d, J ) 6.5 Hz, 1H), 7.91 (d, J ) 6.5 Hz,
1H); ESI (+) [M + H]+ calcd for C10H9N579Br 277, found 278.0.
Tribromopyrrole 39: 1H NMR (400 MHz, CD3OD) δ 7.44 (d, J )
6.4 Hz, 1H), 8.15 (d, J ) 6.4 Hz, 1H); ESI (+) [M +H]+ calcd for
C10H7N579Br3 433, found 433.8.
Acknowledgment. We are grateful to the National Sci-
ence Foundation (CHE-0404792) for financial support of this
research. We also thank Professor Blake Peterson and Jocelyn
Edathil for assistance with reverse-phase HPLC analysis.
Supporting Information Available: Experimental procedures
for the preparation of additional new compounds including copies
1
of H and 13C NMR spectral data. This material is available free
of charge via the Internet at http://pubs.acs.org.
JO0707232
J. Org. Chem, Vol. 72, No. 13, 2007 4899