(-)-Bacillamide C: The convergent approach

Article abstract of DOI:10.1039/b918214d

The newly discovered natural product bacillamide C and several derivatives were convergently synthesized for the first time and in only three steps. The key transformation constitutes a thiazole Ugi multicomponent reaction. These compounds will serve to elucidate chemical biology and SAR of this potent anti-algae natural product and shows the synthetic pathway to related natural products.

Full text of DOI:10.1039/b918214d

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www.rsc.org/obc | Organic & Biomolecular Chemistry
(-)-Bacillamide C: the convergent approach †‡
Wei Wang, Shannon Joyner, Kareem Andrew Sameer Khoury and Alexander Do¨mling*
Received 4th September 2009, Accepted 11th November 2009
First published as an Advance Article on the web 26th November 2009
DOI: 10.1039/b918214d
The newly discovered natural product bacillamide C and
several derivatives were convergently synthesized for the
first time and in only three steps. The key transformation
constitutes a thiazole Ugi multicomponent reaction. These
compounds will serve to elucidate chemical biology and
SAR of this potent anti-algae natural product and shows the
synthetic pathway to related natural products.
hypersaline microbial mat,4 thermophilic Thermoactinomyces
strain TA66-2 collected from hot spring sources of the West Ana-
tolia in Turkey5 and Microbispora aerate isolated from penguin
excrement collected on the Antarctic Livingston Island.6 In addi-
tion, the tryptamide thiazole motif observed in the bacillamides
frequently occurs as a building block in potently bioactive cyclic
peptides. Examples include the antibiotic zelkovamycin produced
by a Streptomyces sp.,7 the protein synthesis inhibitors A-21459
A and B from an Actinoplanes sp.,8 and the immunosuppressive
argyrins from the myxobacterium Archangium gephyra.9
Algae bloom has recently become an important ecological and
economical threat. Due to intensified farming more and more
phosphate and nitrogen rich run-off is accumulating in deltas and
leading to eutrophication and explosive algae bloom. Additionally,
pollution from chemical factories is often adding to the bad water
quality. This complicated process seems also to be supported
by the local and world wide increase of temperature due to
climate change. Algae species such as the harmful dinoflagellate
Cochlodinium polykrikoides a known red tide species are associated
with extensive fish kills and great economic loss e.g. recently in
Japanese and Korean waters.1 Another topical example is the
recent algae bloom (“hu tai”) in the bay of Qingdao during the
Olympic games in 2008 which severely threatened the Olympic
sailing events.
Thiazole-5-carboxy-2-ethylindole amide natural products have
been recently described under different names including bacil-
lamide A and C and microbiaeratin associated with diverse
bioactivity including algicidal and antibacterial (Fig. 1).2 These
structurally interesting natural products have been isolated from
a number of different sources, including Bacillus sp. SY-1 isolated
during the termination of bloom by C. polykrikoides in the Masan
Bay of Korea,3 Bacillus endophyticus isolated from a Bahamian
Natural products have an evolutionary “built-in” biological
activity, thus they often serve as valuable leads for medicinal
chemistry or sometimes are useful even unmodified as drugs.
Despite huge progress in synthetic methodologies, however, the
multistep total synthesis of natural products for clinical trials or
the market, e.g. epothilone derivative ZK-EPO (22 steps, 0.9%
yield) or (+)-discodermolide (39 steps, 0.65% yield) is very time
and resource demanding and rather the exception than the rule.10
Especially, when PKPD (Pharmacokinetics, Pharmacodynamics)
properties of the original natural product are unfortunate and
many derivatives have to be synthesized to overcome these
limitations, multistep sequential syntheses make these efforts often
the domain of large pharma companies. Therefore a goal of future
natural product chemistry must be the design of short, convergent
and resource saving synthetic pathways to allow rapid access to
derivatives for optimization and evaluation of their biological
activities. One synthetic methodology which is very useful to
reduce time and effort and widely applied in drug discovery is
multicomponent reactions (MCR).11 MCRs are the prototypes
of convergent reactions and can yield large libraries of bioactive
compounds in a short time based on the broad availability of
their starting materials. Despite their usefulness in drug discovery,
MCRs so far have been employed only sporadically in natural
product total synthesis.12
Thus in order to elucidate mode-of-action of these algicidal
leads and to improve bioactivity a short, efficient and versatile syn-
thetic access to natural products containing a thiazol-5-carboxy-
2-ethylindole moiety is warranted. Additionally, the tryptamide
thiazole motif caught our attention due to our recently discovered
multicomponent reaction giving convergent and efficient access to
peptide like thiazole back bones.13 Here we would like to describe
the first total synthesis of bacillamide C and the parallel synthesis
of an array of derivatives thereof.
Fig. 1 Bacillamide C and argyrin B as representative bioactive natural
products containing a tryptamide thiazole motif.
Retrosynthetically, using our MCR approach the central
fragment of the bacillamides C is accessible using the com-
mercially available starting materials Scho¨llkopf’s isocyanide 2,
acetaldehyde 3, an ammonia equivalent 4 and thioacetic acid
5 (Scheme 1).12 We first tested our synthetic idea in preparing
racemic target structure 1. A benzylamine derivative instead
of ammonia in Ugi-type reactions is required since ammonia
University of Pittsburgh, Drug Discovery Institute, Pittsburgh, PA, 15261,
USA. E-mail: asd30@pitt.edu; Fax: +1 412-383-5298; Tel: +1 412-383-
5300
† Dedicated to the life of Prof. Kris Venkat
‡ Electronic supplementary information (ESI) available: Experimental
details, NMR spectra and HPLC traces and a movie showing incubation
of compound 10j into HCT116 cell line. See DOI: 10.1039/b918214d
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Table 1 Synthesized bacillamide derivatives
Amine
Yield
70%
Amine
Yield
90%
Scheme 1 Four-component thiazole component synthesis.
often displays low reactivity.14 Thus, reaction of the four starting
materials 2–5 affords the Ugi intermediate 6 in 60% yield under
ambient conditions. The next step, the cleavage of the 2,4-
dimethoxybenzyl protecting group is usually performed with
TFA at ambient temperature.15 However, attempts to analogously
deprotect 6 under these conditions didn’t show any conversion.
90%
92%
85%
98%
98%
80%
◦
Only prolonged treatment with TFA at 50 C yielded the depro-
tected 7 in 58% yields. In order to avoid multistep saponification,
activation and coupling sequences to transform the methyl ester
into the amide we turned to a recently described procedure for
a one-pot conversion.16 Thus, 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD) catalyzed direct amidation of 7 with tryptamine 8 yielded
racemic target natural product bacillamide C 1. Overall racemic
bacillamide C is thus accessible from commercial available starting
materials in three simple to perform steps in overall 30% yield
(Scheme 2).
70%
98%
56%a
a Total yield. First treated with an excess of diamine, separated after
protection with Boc2O in situ, and then deprotected under TFA.
as chiral auxiliary, complete racemization (chiral HPLC analysis)
was observed after deprotection using TFA (see Supporting
Information). Therefore we investigated readily available chiral
4-methoxyphenylethylamine 11 as auxiliary. Thus reaction of (R)-
1-(4-methoxyphenyl)ethylamine yielded a 1 : 1 mixture of the two
diastereomers (Scheme 4) in 65% yield which can be conveniently
separated by silica gel chromatography. Additionally it is known
that Lewis acids can be used to enhance diastereoselectivity in
the Ugi reaction.17 In order to optimize diastereoselectivity we
planned to test several Lewis acids in this new thiazole Ugi varia-
tion. The first Lewis acid we tested ZnCl2 afforded no formation of
the product. Surprisingly also BF3·OEt2 yielded little of the desired
product contaminated with side products. Thus we decided to
separate the 1 : 1 mixture of the MCR without any additive Lewis
acid with the advantage of having access to both enantiomers of
the natural product. Cleavage of the chiral auxiliary lead to the
amide (R)7 without any noticeable racemisation as shown by chiral
HPLC (see Supporting Information). To the best of our knowledge
Scheme 2 Racemic four-component Bacillamide C synthesis.
In order to explore the SAR of this new natural product we
attempted to make amide derivatives based on the carboxylic acid
methyl ester 7 using the same one-pot amidation (Scheme 3).13
Thus we produced 12 amide derivatives of bacillamide C 1 shown
in Table 1. Remarkably, this procedure is fast and straightforward
and can yield a diversity of functionalized products. For example,
we synthesized 10j, which is a fluorescent dansyl derivative of 1.
Scheme 3 Fast and efficient synthesis of bacillamide C derivatives.
Next we considered a stereoselective approach towards 1.
It is well known, that stereoselectivity in the classical Ugi
reaction can be directed by the use of chiral primary amines.17
Different chiral amine components have been used in the past
to accomplish diastereoselctivity, including phenylethylamine,18
a-ferrocenylalkylamines, and anomeric glycosylamines.19 When
the widely used non-racemic 2-phenylethylamine was employed
Scheme 4 Stereoselective 3-step synthesis of bacillamide C 1.
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Table 2 Screening of coupling conditions for the final step to 1
fluorescence microscopy and found that after 100 min the
compound entered the cell almost quantitatively (see Supporting
Information Movie).
Coupling method
Yield
eea
In summary, we described the first total syntheses of the algicidal
natural product bacillamide C 1: an enantioselective and a
3-step synthesis leading to racemic 1 and derivatives. Additionally
we accomplished the racemic synthesis of a fluoro derivative
of the recently described neobacillamide A.22 Our syntheses are
convergent and very short and a key step is a modification
of the Ugi reaction leading in one step to highly substituted
thiazoles. We also describe a stereoselective synthesis of the
target compound by introducing the mildly cleavable chiral 2-
(4-methoxylphenyl)ethylamine for the first time as auxiliary in the
thiazole-Ugi reaction. Finally we prepared several derivatives and
showed that the fluorescence tagged 10j can rapidly enter cells. The
work is significant since bacillamides are potent algicidal agents
and our approach can potentially access optimized derivatives by
parallel chemistry.23 Currently SAR studies are ongoing and will
be reported in due course.
EDCI/HOBt
CDI/HOBt
IsoBuCO2Cl/NMM
TBTU
20%
30%
85%
40%
20%
95%
94%
<5%
0
HOAt/HATU
<5%
a Determined by chiral HPLC (column type: DAICEL CHIRALPAK IB).
this is the first example of the use of 4-methoxyphenylethylamine
as chiral auxiliary in Ugi-type reactions.20 We propose this to be a
useful and alternative method especially when labile racemisation-
prone substrates are employed, e.g. heteroaromatic amides. Sub-
sequent amidation under the above described TBD catalysis
conditions, however, resulted in extensive racemisation. Thus we
first saponified (R)7 with 70% yield and then coupled tryptamine
using CDI, HOBt and DIPEA to yield (-)-1 in 30% yield (retention
time: 74 min in chiral HPLC see supporting information). The
NMR spectra obtained from our total synthetic 1 compares well
with the natural product. The final peptide coupling condition
was extensively investigated because of the low yield of the
peptide coupling method and the observed racemization problem
(Table 2).
We thank Raghavan Balachandran for growing the cell line
HTC116 for confocal microscopy studies.
Notes and references
1 (a) K. Yuki and S. Yoshimatsu, in Red Tides: Biology, Environmental
Science, and Toxicology, eds. T. Okaichi, D. M. Anderson and
T. Nemoto, Elsevier, Amsterdam, 1989, pp. 451–454; (b) Y. Fukuyo, H.
Takano, M. Chihara and K. Matsuok, Red Tide Organisms in Japan.
An Illustrated Taxonomic Guide, Uchida Rokakuho Co. Ltd., Tokyo,
1990; (c) H. G. Kim, in Harmful Algae, Xunta de Galicia and Intergov-
ernmental Oceanographic Commission of UNESCO, eds. B. Reguera,
J. Blanco, M. L. Fernandez and T. Wyatt, Spain, 1998, pp. 227–
228.
2 A. M. Socha, R. A. Long and D. C. Rowley, J. Nat. Prod., 2007, 70,
1793–1795.
3 C¸ . A. Korkmaz, E. E. Hame-Kocaba, A. Uzel and E. Bedir, Magn.
Reson. Chem., 2008, 46, 80–83.
4 S.-Y. Jeong, K. Ishida, Y. Ito, S. Okada and M. Murakami, Tetrahedron
Lett., 2003, 44, 8005–8007.
5 V. Ivanova, M. Kolarova, K. Aleksieva, U. Gra¨fe, H.-M. Dahse and H.
Laatsch, Prep. Biochem. Biotechnol., 2007, 37, 161–168.
6 Y. Konda, Y. Suzuki, S. Omura and M. Onda, Chem. Pharm. Bull.
(Tokyo), 1976, 24, 92–96.
7 N. Tabata, H. Tomoda, H. Zhang, R. Uchida and S. Omura, J. Antibiot.,
1999, 52, 34–39.
A similar coupling problem with low yields was also reported
during a coupling step of the related bacillamide A.21 In our
study 15–20% yield was accomplished without racemization using
EDCI/HOBt. Iso-BuCO2Cl/NMM in ethyl acetate yielded >80%
1 but with accompanying racemization. Other conditions such as,
HOAt/HATU, TBTU, were also tried but either with low yield
or racemization results. The best result was 30% yield using CDI
accompanied by HOBt with slight racemization as determined by
HPLC.
In order to investigate the biological properties of this thiazole
natural product we are interested in derivatives able to transverse
the cell membrane and exert biological effects in the cell. Therefore
we prepared the fluorescent derivative 10j and measured the cell
viability into the HCT116 cell line (Fig. 2). Also we measured the
dynamic cellular uptake by performing time relapsed confocal
8 P. Ferrari, K. Vekey, M. Galimberti, G. G. Gallo, E. Selva and L. F.
Zerilli, J. Antibiot., 1996, 49, 150–154.
9 L. Vollbrecht, H. Steinmetz and G. Hofle, J. Antibiot., 2002, 55, 715–
721.
10 (a) U. Klar, B. Buchmann, W. Schwede, W. Skuballa, J. Hoffmann and
R. B. Lichtner, Angew. Chem., 2006, 118, 8110–8116; (b) S. J. Mickel,
D. Niederer, R. Daeffler, A. Osmani, E. Kuesters, E. Schmid, K. Schaer,
R. Gamboni, W. Chen, E. Loeser, F. R. Kinder, K. Konigsberger, K.
Prasad, T. M. Ramsey, O. Repicˇ, R.-M. Wang, G. Florence, I. Lyothier
and I. Paterson, Org. Process Res. Dev., 2004, 8, 122–130.
11 (a) J. Zhu and H. Bienayme´, Multicomponent reactions, Wiley-
VCH, Weinheim, 2005; (b) A. Do¨mling, Chem. Rev., 2006, 106, 17–
89.
12 (a) J. Isaacson, M. Loo and Y. Kobayashi, Org. Lett., 2008, 10, 1461–
1463; (b) N. Liu, S. Cao, J. Wu, J. Yu, L. Shen, X. Feng and X.
Qian, Tetrahedron, 2008, 64, 3966–3974; (c) A. Do¨mling, B. Beck, U.
Eichelberger, S. Sakamuri, S. Menon, Q.-Z. Chen, Y. Lu, Ludger and A.
Wessjohann, Angew. Chem., Int. Ed., 2007, 46, 2347–2348; (d) S. Patel,
L. Saroglou, C. D. Floyd, A. Miller and M. Whittaker, Tetrahedron
Lett., 1998, 39, 8333–8334; (e) A. Zask, G. Birnberg, K. Cheung, J.
Kaplan, C. Niu, E. Norton, R. Suayan, A. Yamashita, D. Cole, Z.
Tang, G. Krishnamurthy, R. Williamson, G. Khafizova, S. Musto,
R. Hernandez, T. Annable, X. Yang, C. Discafani, C. Beyer, L. M.
Fig. 2 Cellular uptake of 10j in HCT116 cells observed by laser confocal
microscopy. Left column: excitation with 488 nm laser; middle column:
differential interface contrast (DIC); right: merge of the two images.
Noteworthy, bacillamide derivative 10j appears to localize in specific
compartments of the cell.
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The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 529–532 | 531
©

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Greenberger, F. Loganzo and S. Ayral-Kaloustian, J. Med. Chem., 2004,
47, 4774–4786; (f) A. Endo, A. Yanagisawa, M. Abe, S. Tohma, T. Kan
and T. Fukuyama, J. Am. Chem. Soc., 2002, 124, 6552–6554; (g) B.
Beck, S. Hess and A. Do¨mling, Bioorg. Med. Chem. Lett., 2000, 10,
1701–1705.
17 I. Ugi, in Isonitrile Chemistry, ed. I. Ugi, Academic Press, New York,
1971, p. 161.
18 (a) H. Herlinger, H. Kleimann, K. Offermann, D. Ru¨cker and I. Ugi,
Justus Liebigs Ann. Chem., 1966, 692, 94–99; (b) R. Urban and I.
Ugi, Angew. Chem., 1975, 87, 67–69; (c) R. Urban, G. Eberle, D.
Marquarding, D. Rehn, H. Rehn and I. Ugi, Angew. Chem., 1976,
88, 644–646.
19 (a) H. Kunz and W. Pfrengle, Tetrahedron, 1988, 44, 5487–5494; (b) H.
Kunz and W. Pfrengle, J. Am. Chem. Soc., 1988, 110, 651–652; (c) K.
Oertel, G. Zech and H. Kunz, Angew. Chem., Int. Ed., 2000, 39, 1431–
1433.
13 (a) S. Heck and A. Do¨mling, Synlett, 2000, 424–426; (b) J. Kolb, B. Beck
and A. Domling, Tetrahedron Lett., 2002, 43, 6897–6901; (c) J. Kolb,
B. Beck, M. Almstetter, S. Heck, E. Herdtweck and A. Do¨mling, Mol.
Diversity, 2003, 6, 297–313; (d) B. Henkel, B. Westner and A. Do¨mling,
Synlett, 2003, 2410–2412; (e) B. Henkel, M. Sax and A. Domling,
Tetrahedron Lett., 2003, 44, 3679–3682.
14 R. Bossio, S. Marcaccini and R. Pepino, Liebigs Ann. Chem., 1990,
935–937.
20 U. Kazmaier, C. Hebach, A. Watzke, S. Maier, H. Mues and V. Huch,
Org. Biomol. Chem., 2005, 3, 136.
21 V. B. C. Figueira, S. Prabhakar and A. M. Lobo, Arkivoc, 2005, (xiv),
14–19.
15 A. Zhang, J. S. Ferguson, K. Yamato, C. Zheng and B. Gong, Org.
Lett., 2006, 8, 5117–5120.
16 (a) C. Sabot, K. A. Kumar, S. Meunier and C. Mioskowski, Tetrahedron
Lett., 2007, 48, 3863–3866; (b) W. Wang and A. Do¨mling, J. Comb.
Chem., 2009, 11, 403–409; (c) S. Srivastava, B. Beck, W. Wang, A.
Czarna, T. A. Holak and A. Do¨mling, J. Comb. Chem., 2009, 11, 631–
639.
22 L.-L. Yu, Z.-Y. Li, C.-S. Peng, Z.-Y. Li and Y.-W. Guo, Helv. Chim.
Acta, 2009, 92, 607–612.
23 C. Churro, E. Alverca, F. Sam-Bento, S. Paulino, V. C. Figueira, A. J.
Bento, S. Prabhakar, A. M. Lobo, A. J. Calado and P. J. Pereira, J. Appl.
Phycol., 2009, 21, 429–442.
532 | Org. Biomol. Chem., 2010, 8, 529–532
This journal is
The Royal Society of Chemistry 2010
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