Formal synthesis of (±)-cladoniamide G

Article abstract of DOI:10.1016/j.tetlet.2014.01.086

The formal synthesis of (±)-cladoniamide G, a natural product that exhibits cytotoxicity against human breast cancer MCF-7 cells, is presented. Our strategy employed a Suzuki-Miyaura coupling to join the two indole subunits of this natural product.

Full text of DOI:10.1016/j.tetlet.2014.01.086

Tetrahedron Letters 55 (2014) 1621–1624
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Formal synthesis of ( )-cladoniamide G
⇑
Paiboon Ngernmeesri , Sarochar Soonkit, Anothai Konkhum, Boonsong Kongkathip
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
The formal synthesis of ( )-cladoniamide G, a natural product that exhibits cytotoxicity against human
breast cancer MCF-7 cells, is presented. Our strategy employed a Suzuki–Miyaura coupling to join the
two indole subunits of this natural product.
Received 27 November 2013
Revised 10 January 2014
Accepted 21 January 2014
Available online 28 January 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Natural product synthesis
Anticancer
( )-Cladoniamide G
Suzuki–Miyaura coupling
Indolocarbazole alkaloids such as staurosporine (1) and
rebeccamycin (2) (Fig. 1) have drawn considerable attention from
the synthetic community because of their numerous interesting
biological activities, especially in the area of oncology.1–3 In 2008,
the Anderson group reported the isolation and structure determi-
nation of cladoniamides A-G from cultures of Streptomyces uncialis
obtained from the lichen, Cladonia uncialis.4 These natural products
represent a new class of alkaloids that possess an unprecedented
indolotryptoline skeleton, which has one of its indole subunits
flipped within the indolocarbazole. Investigations on these alka-
loids have revealed that only cladoniamides A (3) and G (4) possess
significant biological activities, having cytotoxicity against human
colon cancer HCT116 cells (8.8 ng/mL)5 and human breast cancer
N
N
H
N
H
N
H
indolotryptoline
indolocarbazole
H
Me
N
N
O
O
O
HO
OH
Cl
N
N
N
N
Me
H
MCF-7 cells (10 l
g/mL),4 respectively. The absolute stereochemis-
H
MeO
MeO
try of cladoniamide A has been assigned as shown in Figure 1 by
single crystal X-ray diffraction. Since cladoniamides A and G are
likely to be biosynthetically related, the absolute configuration of
cladoniamide G can be inferred.4
3
: cladoniamide A
NHMe
1: staurosporine
H
N
Our aim was to develop a strategy to synthesize cladoniamide G
and a set of its structural analogs with various substituents on both
indole moieties. Scheme 1 depicts our retrosynthetic analysis of
cladoniamide G. We envisioned that this alkaloid could be derived
from dioxo-bisindole 5, which could in turn be prepared from
bisindole 6. The two indole fragments of 6 could be joined together
by a Suzuki–Miyaura coupling reaction of bromoindole 7 and
boronic acid 8, which could both be prepared in a few steps from
commercially available indole derivatives. Although Pd-catalyzed
cross-coupling reactions have gained popularity and are widely
NHMe
O
O
O
O
HO
N
Cl
N
H
N
Cl
N
H
Cl
Cl
MeO
4
: cladoniamide G
O
HO
OH
OMe
2: rebeccamycin
⇑
Corresponding author. Tel.: +66 2 562 5555x2202; fax: +66 2 579 3955.
E-mail address: fscipbn@ku.ac.th (P. Ngernmeesri).
Figure 1. Cladoniamides A and G and related bisindole alkaloids.
http://dx.doi.org/10.1016/j.tetlet.2014.01.086
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

1622
P. Ngernmeesri et al. / Tetrahedron Letters 55 (2014) 1621–1624
NHMe
O
OMe
O
O
N
O
O
O
Br
Cl
Cl
Cl
O
HO
OMe
NH2
15
OMe
O
Cl
N
OMe
Cl
N
H
Na2CO3, 15 h,
80 oC, 94%
N
H
5
Cl
N
14
16
MeO
H
MeO
OH
+
( )-cladoniamide G
OMe
2
NaOMe, MeOH
THF
Cl
O
(MeO)2SO2
Cl
Cl
Cl
Br
1 h, Δ, 75%
acetone, rt,
18 h, 96%
N
H
OMe
HN
N
H
17
7
Cl
N
Boc
+
OMe
OMe
1. aq. NaOH, MeOH,
MeO
Cl
Cl
O
Δ , 1 h
6
2
B(OH)2
2. Cu, quinoline,
N
N
H
N
H
OMe
200 oC, 1 h
Boc
8
18
19
94%
OMe
OMe
Scheme 1. Retrosynthetic analysis of ( )-cladoniamide G.
Boc2O,
Cl
Cl
NBS, CCl4,
DMAP
Br
CH3CN, rt,
1 h, 96%
rt, 3 h,
75%
N
N
used in natural product synthesis, only a few examples of C2–C20
indole–indole couplings have been reported.6 Therefore, we were
interested in further exploring this area, and expanding upon it,
by coupling indoles with novel substitution patterns. This coupling
strategy would allow us to easily prepare a number of different
analogs by varying the substituents on the starting materials 7
and 8.
Boc
Boc
20
21
8, PdCl2(PPh3)2
Na2CO3,
Boc
N
Cl
1,4-dioxane, H2O,
80 oC, 2 h, 69%
N
Cl
Boc
OMe
22
To test the feasibility of our cross-coupling strategy, we began a
model study with 3-methoxyindole (9)7 as shown in Scheme 2.
Attempts to brominate 9 at C2 with NBS at 0 °C resulted in none
of the desired product, and when the temperature was raised to
room temperature, decomposition occurred. Surprisingly, bromin-
ation of the more electron-deficient N-Boc-3-methoxyindole (10)
took place easily to give bromoindole 11. TLC analysis showed that
the starting material was completely consumed within 15 minutes,
and the desired bromoindole was produced exclusively. However,
this compound was quite unstable and decomposed rapidly when
the crude product was either purified on silica gel or carried on to
next step. Fortunately, purification by flash chromatography on
basic alumina gave compound 11 in a decent yield (79%). To our
delight, the Suzuki–Miyaura coupling reaction of 11 with indolyl
boronic acid 128 in the presence of catalytic PdCl2(PPh3)2 gener-
ated bisindole 13 in 72% yield.9
Scheme 3. Synthesis of bisindole 22.
Nevertheless, this yield was comparable to that reported by Bös
(79% yield for the ethyl analog of 17). It should be noted that our
attempts to use NaOEt/EtOH produced both 17 and its ethyl analog
in an approximate 1:1 ratio. In addition, dimethyl sulfate was used
in place of diazomethane to generate methoxy indole 18, due to
safety concerns, and did not compromise the yield. The ester group
of compound 18 was then hydrolyzed by aq NaOH and the result-
ing carboxylic acid was decarboxylated using copper powder in
quinoline according to the method described by Cook12 to give
5-chloro-3-methoxyindole (19) in excellent yield. Similar to 9, bro-
mination of 19 resulted in decomposition, but bromination of the
N-Boc indole 20 went on smoothly to generate bromoindole 21
in 75% yield. Compared to the bromination of 10, this reaction took
With this promising result, we synthesized the 5-chloro analog
of 11 as outlined in Scheme 3. Methoxy indole ester 18 was pre-
pared according to the methods described by Dropinski10 and
Bös11 with minor modifications. In the second step, NaOMe/MeOH
was used instead of NaOEt/EtOH to generate methyl carboxylate
17. TLC analysis showed almost exclusively carboxylate 17, but
O
O
TFA,
H
N
CH2Cl2
Cl
Cl
13
0 oC to rt,
6 h, 94%
Et2O, 0 oC,
1 h, 88%
N
H
only
a 75% yield was obtained due to isolation problems.
OMe
23
O
O
N
O
O
N
OMe
2
OMe
Boc2O, DMAP
NBS, CCl4
Boc2O, DMAP
CH3CN, rt,
2 d, 72%
N
H
rt, 15 min,
79%
N
CH3CN, rt,
1 h, 95%
N
N
Boc
H
Boc
OMe
OMe
9
24
25
10
B(OH)2
NC
HO
HO
X
O
N
N
OMe
Boc
N
Boc
N
12
or
Br
PdCl2(PPh3)2
Na2CO3,
N
N
N
N
Boc
Boc
Boc
Boc
OMe
OMe
OMe
11
13
1,4-dioxane, H2O,
27
26a
: X = O
80 oC, 1 h, 72%
26b: X = H, H
Scheme 2. Model study on the Suzuki coupling reaction.
Scheme 4. Synthesis of dioxo-bisindole 25.

P. Ngernmeesri et al. / Tetrahedron Letters 55 (2014) 1621–1624
1623
much longer to go to completion (3 h vs 15 min), presumably due
to the decreased nucleophilicity of the substrate brought about by
the electron-withdrawing Cl substituent. However, both reactions
gave the desired products in comparable yields. Finally, the Suzu-
ki–Miyaura coupling of 21 and boronic acid 88 provided the desired
bisindole 22 in reasonable yield (69%).
In summary, we were able to synthesize bisindole 29 in nine
steps with 27% overall yield from methyl 2-amino-5-chlorobenzo-
ate (14). The reaction of this bisindole precursor 29 and linchpin
reagent 30 was reported earlier by Anderson and Dake to produce
( )-cladoniamide G. Although our route to prepare 29 is longer,
good-to-excellent yields were obtained in each step in the synthe-
sis resulting in a higher overall yield. In addition, our strategy to
couple two indole subunits together would allow us to vary the
substituents on each indole unit independently, and this modifica-
tion could not be achieved with the route reported by Anderson
and Dake.
As a final test of our strategy, we employed bisindole 13
(Scheme 4). Removal of the Boc groups of 13 with TFA provided
N,N0-unprotected bisindole 23 in 94% yield. When 23 was treated
with oxalyl chloride in diethyl ether at 0 °C, dioxo-bisindole 24
formed immediately and then precipitated as a red solid. This solid
was easily collected by removal of the solvent and rinsing with
ether/hexanes. 1H NMR spectroscopy of the red solid showed
exclusively the desired product so no further purification was
required. Attempts to modify the carbonyl group of 24 were prob-
lematic as this compound was not soluble in most organic solvents.
Therefore, the indole NH of 24 was reprotected with a Boc group to
make it less polar. In addition, this protecting group would serve as
an electron-withdrawing group to increase the electrophilicity of
the keto carbonyl. It should be noted that this protection reaction
took two days instead of a few minutes, presumably due to the
insolubility of 24 in CH3CN as well as the steric hindrance imparted
by the methoxy group. Although the problem with solubility was
solved, our attempts to reduce or add a nucleophile to the keto car-
bonyl group to form compounds 26a or 27 were unsuccessful. For
example, both of the carbonyl groups of 25 were easily reduced
with NaBH4 to form 26b as indicated by LRMS (m/z = 405.1) and
1H NMR (1H at 4.12 ppm, dd, J = 8.5, 11.0 Hz and 1H at 3.74 ppm,
dd, J = 4.9, 10.9 Hz) data of the isolated, but impure product. Unfor-
tunately, this product decomposed within hours after isolation, so
no further purification was performed. Moreover, an alternative
route with the addition of CNꢀ to the carbonyl in DMSO also
resulted in decomposition.
Acknowledgments
Financial support was provided by the Thailand Research
Fund (MRG5380301), the Center of Excellence for Innovation in
Chemistry (PERCH-CIC), the Office of the Higher Education
Commission, the Ministry of Education, the Faculty of Science,
Kasetsart University, and Kasetsart University Research and
Development Institute (KURDI). The authors thank the Chulabhorn
Research Institute for providing high-resolution mass spectra.
Supplementary data
Supplementary data (Experimental procedures and character-
ization data for 10–11, 13, 16–25 and 29.) associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.01.086.
References and notes
1. For a review on the synthesis of indolocarbazole alkaloids, see: Knölker, H.-J.;
Reddy, K. R. Chem. Rev. 2002, 102, 4303–4427.
2. For the synthesis of staurosporine and related compounds, see: (a) Link, J. T.;
Gallant, M.; Danishefsky, S. J.; Huber, S. J. Am. Chem. Soc. 1993, 115, 3782–3783;
(b) Link, J. T.; Raghavan, S.; Danishefsky, S. J. J. Am. Soc. Chem. 1995, 117, 552–
553; (c) Link, J. T.; Raghavan, S.; Gallant, M.; Danishefsky, S. J.; Chou, T. C.;
Ballas, L. M. J. Am. Chem. Soc. 1996, 118, 2825–2842; (d) Wood, J. L.; Stoltz, B.
M.; Goodman, S. N. J. Am. Soc. Chem. 1996, 118, 10656–10657; (e) Stoltz, B. M.;
Goodman, S. N.; Onwueme, K. J. Am. Chem. Soc. 1997, 119, 9652–9661; (f)
Moody, C. J.; Rahimtoola, K. F.; Porter, B.; Ross, B. C. J. Org. Chem. 1992, 57,
2105–2114; (g) Mahboobi, S.; Eibler, E.; Koller, M.; Kumar, K. C. S.; Popp, A. J.
Org. Chem. 1999, 64, 4697–4704; (h) Chen, S.-Y.; Uang, B.-J.; Liao, F.-L.; Wang,
S.-L. J. Org. Chem. 2001, 66, 5627–5628; (i) Rajeshwaran, G. G.;
Mohanakrishnan, A. K. Org. Lett. 2011, 13, 1418–1421.
3. For the synthesis of rebeccamycin and related compounds, see: (a) Kaneko, T.;
Wong, H.; Okamoto, K. T.; Clardy, J. Tetrahedron Lett. 1985, 26, 4015–4018; (b)
Gallant, M.; Link, J. Y.; Danishefsky, S. J. J. Org. Chem. 1993, 58, 343–349; (c)
Faul, M. M.; Winneroske, L. L.; Krumrich, C. A. J. Org. Chem. 1999, 64, 2465–
2470; (d) Saulnier, M. G.; Frennesson, D. B.; Deshpande, M. S.; Vyas, D. M.
Tetrahedron Lett. 1995, 36, 7841–7844; (e) Moreau, P.; Anizon, F.; Sancelme, M.;
Prudhomme, M.; Sevère, D.; Riou, J.-F.; Goossens, J.-F.; Hénichart, J.-P.; Bailly,
C.; Labourier, E.; Tazzi, J.; Fabbro, D.; Meyer, T.; Aubertin, A. M. J. Med. Chem.
1999, 42, 1816–1822.
During our investigations to complete the synthesis of cladonia-
mide G, Anderson and Dake reported the total synthesis of this nat-
ural product from 5-chloroindole in five steps with 15% overall
yield.13 Their strategy was to prepare 5,50-dichloroindigo (28)
and transform it into bisindole 29 (Scheme 5). This compound
was then converted into ( )-cladoniamide G in 81% yield when
treated with linchpin reagent 30, which was prepared from
dimethyl malonate and benzaldehyde in four steps. Intermediate
29 could also be produced from our synthetic route in 84% yield
when di-Boc bisindole 22 was treated with TFA. Hence, we report
here the formal synthesis of ( )-cladoniamide G.
O
H
Cl
N
4. Williams, D. E.; Davies, J.; Patrick, B. O.; Bottriell, H.; Tarling, T.; Roberge, M.;
Anderson, R. Org. Lett. 2008, 10, 3501–3504.
N
H
Cl
Cl
5. Chang, F. Y.; Brady, S. F. J. Am. Chem. Soc. 2011, 133, 9996–9999.
6. (a) Merlic, C. A.; McInnes, D. M. Tetrahedron Lett. 1997, 38, 7661–7664; (b)
Merlic, C. A.; You, Y.; McInnes, D. M.; Zechman, A. L.; Miller, M. M.; Deng, Q.
Tetrahedron 2001, 57, 5199–5212; (c) Yamabuki, A.; Fujinawa, H.; Choshi, T.;
Tohyama, S.; Matsumoto, K.; Ohmura, K.; Nobuhiro, J.; Hibino, S. Tetrahedron
Lett. 2006, 47, 5859–5861; (d) Tohyama, S.; Choshi, T.; Matsumoto, K.;
Yamabuki, A.; Hieda, Y.; Nobuhiro, J.; Hibino, S. Heterocycles 2010, 82, 397–
416; (e) Han, S.; Movassaghi, M. J. Am. Chem. Soc. 2011, 133, 10768–10771; (f)
Matsumoto, K.; Choshi, T.; Hourai, M.; Tohyama, S.; Nobuhiro, J.; Hibino, S.;
Zamami, Y.; Sasaki, K.; Abe, T.; Ishikura, M.; Hatae, N.; Iwamura, T. Bioorg. Med.
Chem. Lett. 2012, 22, 4762–4764; (g) Palmisano, G.; Santagostino, M. Helv. Chim.
Acta 1993, 76, 2356–2366; (h) La Regina, G.; Bai, R.; Rensen, W. M.; Di Cesare,
E.; Coluccia, A.; Piscitelli, F.; Famiglini, V.; Reggio, A.; Nalli, M.; Pelliccia, S.; Da
Pozzo, E.; Costa, B.; Granata, I.; Porta, A.; Maresca, B.; Soriani, A.; Iannitto, M. L.;
Santoni, A.; Li, J.; Cona, M. M.; Chen, F.; Ni, Y.; Brancale, A.; Dondio, G.;
Vultaggio, S.; Varasi, M.; Mercurio, C.; Martini, C.; Hamel, E.; Lavia, P.;
Novellino, E.; Silvestri, R. J. Med. Chem. 2013, 56, 123–149.
O
28
TFA,
H
Cl
CH2Cl2
N
22
0 oC to rt,
6 h, 84%
N
H
OMe
29
Me
N
O
OMe
Boc
Anderson and
Dake's work
O
O
30
EtOAc, Δ, 81%
+
(
)-4: cladoniamide G
-
7. Although 3-methoxyindole is commercially available, this compound was
synthesized in our laboratory according to the procedure outlined in Scheme 3.
8. Vazquez, E.; Davies, I. W.; Payack, J. F. J. Org. Chem. 2002, 67, 7551–7552.
Scheme 5. Formal synthesis of ( )-cladoniamide G.

1624
P. Ngernmeesri et al. / Tetrahedron Letters 55 (2014) 1621–1624
9. Our model study was reported at the Pure and Applied Chemistry International
Conference in January 2013 (Ubon Ratchathani, Thailand).
11. Bös, M.; Jenck, F.; Martin, J. R.; Moreau, J. L.; Mutel, V.; Sleight, A. J.; Widmer, U.
Eur. J. Med. Chem. 1997, 32, 253–261.
10. Dropinski, J. F.; Akiyama, T.; Einstein, M.; Habulihaz, B.; Doebber, T.; Berger, J.
P.; Meinke, P. T.; Shi, G. Q. Bioorg. Med. Chem. Lett. 2005, 15, 5035–5038.
12. Lui, R.; Zhang, P.; Gan, T.; Cook, J. M. J. Org. Chem. 1997, 62, 7447–7456.
13. Looseley, B. C.; Anderson, R. J.; Dake, G. R. Org. Lett. 2013, 15, 1152–1154.