Total Synthesis of Cyclomarin A, a Marine Cycloheptapeptide with Anti-Tuberculosis and Anti-Malaria Activity

Article abstract of DOI:10.1021/acs.orglett.5b03292

An efficient synthetic protocol for the stereoselective synthesis of cyclomarin A is reported. Key steps in the syntheses of the building blocks are an asymmetric chelate-enolate Claisen rearrangement, an asymmetric hydrogenation, and highly diastereosele

Full text of DOI:10.1021/acs.orglett.5b03292

Letter
pubs.acs.org/OrgLett
Total Synthesis of Cyclomarin A, a Marine Cycloheptapeptide with
Anti-Tuberculosis and Anti-Malaria Activity
Philipp Barbie and Uli Kazmaier*
Organic Chemistry I, Saarland University, Campus Building C4.2, D-66123 Saarbrucken, Germany
̈
S
* Supporting Information
ABSTRACT: An efficient synthetic protocol for the stereo-
selective synthesis of cyclomarin A is reported. Key steps in the
syntheses of the building blocks are an asymmetric chelate−
enolate Claisen rearrangement, an asymmetric hydrogenation,
and highly diastereoselective additions of organozinc and
-titanium reagents.
ost people in industrial nations believe that tuberculosis
(TB) has been wiped out, but statistically almost every
highly important human pathogens is unprecedented. To date,
M
all studies have been carried out with cyclomarin A (and
derivatives thereof) from fermentation, since no total synthesis
has been reported. Only one synthesis of the minor metabolite
cyclomarin C, lacking the epoxide functionality on the
hydroxytryptophan, has been described by Yao et al.9 Herein,
we report the first total synthesis of cyclomarin A, which should
open the way to synthetic derivatives for SAR studies to develop
efficient drugs to fight both diseases.
Some of the unusual amino acids found in cyclomarins, e.g., 5-
hydroxyleucine and the β-methoxyphenylalanine, are also
present in other natural products.10 Others, such as the tert-
prenylated β-hydroxytryptophan and the adjacent 2-amino-3,5-
dimethyl-4-hexenoic acid, are unique to cyclomarins. Although
the synthesis of cyclomarin A has not been reported, several
syntheses of building blocks have been described during the last
several years.11−14
third human is infected by Mycobacterium tuberculosis (MTB).1
Behind HIV/AIDS, TB stands at position two in the list of lethal
infections.2 Several multidrug resistant bacteria are resistant not
only toward first-line medication but also toward second-line
medicine.3 Therefore, the development of new drugs, with a new
mode of action, is highly desired.
In 1999, Clardy et al. described the isolation and structure
elucidation of the cyclomarins from the marine streptomycete
CNB-982 (Figure 1).4 The crude extract showed a moderate
We developed our own synthetic protocols toward the
required building blocks based on our long-term experience in
amino acid synthesis.15 When chelated amino acid ester enolates
are used, β-hydroxy amino acids become easily available via an
aldol reaction,16 and γ,δ-unsaturated amino acids can be obtained
either by transition-metal-catalyzed allylic alkylation17 or chelate
enolate Claisen rearrangement.18 While allylic alkylation
provides anti-configured products, the syn isomers can be
obtained via Claisen rearrangement. Therefore, this approach
was used here to generate the desired N-protected amino acid
(2S,3R)-4 (Scheme 1).
The chelate Claisen rearrangement of chiral ester (S)-1 gave
rise to the desired amino acid with perfect chirality transfer and
diastereoselectivity.18 After methylation, the double bond was
cleaved via ozonolysis. Unfortunately, attempts to introduce the
required terminal methyl group via Wittig reaction9 failed. Even
with a 5-fold excess of the Wittig reagent only 21% of the desired
product 3 could be obtained, while the large excess of basic
Wittig reagent caused complete epimerization of the β-methyl
Figure 1. Natural occurring cyclomarins.
cytotoxicity, with an IC50 of ∼2.5 μM of the major metabolite
cyclomarin A (Cyc A) toward several cancer cell lines. The minor
metabolites cyclomarin B and C (Cyc B and C) are only found in
trace amounts (2−3%). Later, a further derivate cyclomarin D
(Cyc D), missing an N-methyl group, was isolated from
Salinispora arenicola CNS-205.5 Besides the moderate cytotox-
icity, the cyclomarins show some antiviral6 and antibiotic activity,
especially against MTB, while the exact mode of action was not
clear until recently.7 Detailed studies at Novartis identified
ClpC1, a subunit of the caseinolytic protease, as the target
protein. Very recently, the same researchers also reported
significant antimalaria activity. Cyclomarin A selectively inhibits
the PfAp3Ase of Plasmodium falciparum in a nanomolar range,
but not the human homologue in FHIT.8 To the best of our
knowledge, a natural product with two different targets in two
Received: November 15, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03292
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Organic Letters
Letter
Scheme 1. Synthesis of (2S,3R)-4
Scheme 3. Synthesis of (2S,3R)-13
double bond of the prenyl group. Recently, we reported the
synthesis of an N-Boc-protected tert-prenylated β-hydroxytryp-
tophan derivative,25 which unfortunately could not be applied
here, since the Boc-protecting group could not be removed
without decomposition. Therefore, we had to develop a new
protocol, introducing the epoxy group relatively early in the
synthesis.
Recently, a new method was reported for the regioselective
tert-prenylation of electron-demanding indoles.26 We applied
this Pd-catalyzed protocol to 3-indolecarboxylate 14, giving rise
to indole 15 in almost quantitative yield (Scheme 4). The
reaction temperature was kept at 0 °C to suppress the competing
n-prenylation. After saponification of the ester, the free acid was
subjected to a thermal decarboxylation.27 Heating the neat acid
to 180 °C resulted in a clean decarboxylation in perfect yield. The
group. Better results could be obtained using a modified Julia−
Kocienski reaction,19 normally used for (E)-selective olefina-
tions. Sulfone A was deprotonated with LHMDS, and the
solution was added to a fresh prepared aldehyde (without
purification). Under these conditions, the protected amino acid 3
could be obtained without significant epimerization. During the
synthesis, we recognized that the N-Boc-protecting group could
not be removed in the peptide without several side reactions.
Therefore, we replaced the Boc-protecting group by an Alloc
group. Subsequent saponification gave rise to building block 4.
The synthesis of protected 5-hydroxyleucine 9 (Scheme 2)
started with the commercially available Roche ester 5, which was
Scheme 2. Synthesis of (2S,4R)-9
Scheme 4. Synthesis of (2S,3R)-25
protected, reduced to the aldehyde 6, and directly used in an
olefination using Schmidt’s phosphonoglycinate B.20 The α,β-
unsaturated amino acid derivative 7 obtained was subjected to a
stereoselective hydrogenation21 using (R)-monophos as a
ligand.22 Subsequent N-methylation and saponification provided
amino acid 9 in high yield and selectivity.
The third unusual building block, β-methoxyphenylalanine 13,
should be accessible via chelate-controlled arylmetal addition
toward a protected (R)-serinal, obtained from ester 10 (Scheme
3).23 Our first attempts using phenylmagnesium bromide (3
equiv) gave the addition product in acceptable yield (60%) but
only moderate diastereoselectivity (ratio 7:3). Much better
results could be obtained with the corresponding titanium
reagents,24 providing 11 as a single diastereomer. O-Methylation,
cleavage of the silyl ether, and subsequent oxidation gave rise to
the desired acid 13 in excellent yield.
By far, most of our encounteded difficulties caused the
synthesis of the tert-prenylated epoxidized β-hydroxytryptophan,
mainly because of the lability of the β-OH group and the terminal
B
DOI: 10.1021/acs.orglett.5b03292
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Scheme 5. Synthesis of Cyclomarin A
prenylated indole 16 was converted into the corresponding 3-
iodo derivative 17, and the terminal double bond was
transformed into epoxide 1814b using a Sharpless dihydrox-
ylation as a stereodetermining step. Compound 18 was lithiated
at −78 °C using BuLi and was subsequently transmetalated to
the corresponding organozinc reagent 19,28 which was added to
the solution of fresh prepared aldehyde 20. The desired product
21 was formed in acceptable yield as a 9:1 diastereomeric
mixture. Out of four possible stereoisomers, only two isomers
were obtained and the isomeric mixture was the result of the
moderate selectivity of the Sharpless dihydroxylation (80% ee).
Therefore, the carbonyl addition toward chiral aldehyde 20
proceeded with excellent diastereoselectivity. The secondary OH
functionality was subjected to silyl protection. While most
silylation reactions caused complete decomposition of the
molecule, only the combination of TBMDSOTf and lutidine
gave acceptable results. At −40 °C the side reactions could be
suppressed almost completely, and after only 15 min, the desired
silyl ether 22 was obtained in good yield. The primary silyl group
could selectively be removed in the presence of the secondary
one using NH4F in methanol.29 This reagent is less reactive than
the commonly used Bu4NF, but it is therefore more selective.
Interestingly, no complete conversion was observed, while the
primary alcohol 23 was obtained diastereomerically pure.
Obviously the silyl ether of the “wrong epoxide” does not
undergo cleavage under these reaction conditions. Alcohol 23
was oxidized to the corresponding aldehyde,30 a rather unstable
compound. Workup had to be done at 0 °C and the crude
aldehyde was directly converted into methyl ester 24 using N-
iodosuccinimide (NIS) in MeOH.31 This ester was saponified
directly before the peptide coupling to avoid decomposition of
the labile acid.
With these building blocks in hand, we started the synthesis of
the linear heptapeptide. Ring closure was planned between the
aminohexenoic acid and the N-methylated leucine. Therefore,
the synthesis started with this amino acid. BEP (2-bromo-1-
ethylpyridinium tetrafluoroborate)32 was used for the coupling
with protected valine. Surprisingly, the cleavage of the Cbz-
protecting group required a pressure of 20 atm of H2. HCl (2
equiv) was added to suppress the formation of diketopiperazine.
The dipeptide salt was subjected in the next peptide coupling
step with activated 13. Since an excellent yield was obtained, the
same protocol was also used in the next coupling step with
comparable success. HOBT was used to activate the N-methyl-4-
hydroxyleucine 9 to avoid epimerization during the coupling
step. By this sequence, pentapeptide 29 could be obtained
enantiomerically pure in gram scale. For the coupling of the
highly sensitive hydroxytryptophan, methyl ester 24 was
saponified and the crude product (without purification) was
directly coupled using the BEP protocol to give 30 in acceptable
yield. The alloc protecting group was removed under Pd
catalysis, and the final amino acid 4 was connected. After
removing the protecting groups, the deprotected heptapeptide
was slowly added to a diluted solution of PyBOP (2 equiv) and
base in CH2Cl2 (final concentration: 1 mM). Under these
conditions, O-silylated cyclomarin A (32) could be obtained in
good yield (Scheme 5). The removal of the two silyl protecting
groups was more challenging than expected, since they could not
be removed in one step without decomposition. Therefore, we
decided to use a two-step protocol where the primary OH group
was deprotected first using the NH4F method. The secondary
C
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S.; Karmakar, A.; Bhowmick, M. Tetrahedron: Asymmetry 2006, 17,
210−222.
one was removed afterward using exactly 1 equiv of TBAF at 0
°C.
(12) (2S,4R)-5-Hydroxyleucin: (a) Reference 14a. (b) Tarver, J. E., Jr.;
In conclusion, we can show that the unusual amino acids of the
cyclomarins can be synthesized in enantiomerically pure form in
high yields. With these building blocks in hand, cyclomarin A, the
most complex representative of this family of cyclopeptides,
could be obtained. Syntheses of the other family members and
derivatives thereof for SAR studies are under investigation.
́
Terranova, K. M.; Joullie, M. M. Tetrahedron 2004, 60, 10277−10284.
(c) Sathish, K.; Reddy, G. P. K.; Mainkar, P. S.; Chandrasekhar, S.
Tetrahedron: Asymmetry 2011, 22, 1568−1573.
(13) (2S,3R)-2-Amino-3,5-dimethyl-4-hexenoic acid: (a) Tarver, J. E.,
́
Jr.; Joullie, M. M. J. Org. Chem. 2004, 69, 815−820. (b) Reference 15a.
(c) Nian, Y.; Wang, J.; Zhou, S.; Wang, S.; Moriwaki, H.; Kawashima, A.;
Soloshonok, V. A.; Liu, H. Angew. Chem., Int. Ed. 2015, 54, 12918−
12922.
ASSOCIATED CONTENT
* Supporting Information
■
(14) (2S,3R)-3-Hydroxytryptophane: (a) Sugiyama, H.; Yokokawa, F.;
Aoyama, T.; Shiori, T. Tetrahedron Lett. 2001, 42, 7277−7208. (b) Della
Sala, G.; Izzo, I.; Spinella, A. Synlett 2006, 1319−1322. (c) Hansen, D.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03292.
B.; Lewis, A. S.; Gavalas, S. J.; Joullie,
2006, 17, 15−21.
́
M. M. Tetrahedron: Asymmetry
Detailed experimental procedures and copies of NMR
spectra (PDF)
(15) (a) Kazmaier, U.; Schauß, D.; Pohlman, M.; Raddatz, S. Synthesis
2000, 2000, 914−916. (b) Pohlman, M.; Kazmaier, U. Org. Lett. 2003, 5,
2631−2633. (c) Kazmaier, U.; Deska, J. Curr. Org. Chem. 2008, 12,
355−385.
AUTHOR INFORMATION
Corresponding Author
■
(16) (a) Grandel, R.; Kazmaier, U. Tetrahedron Lett. 1997, 38, 8009−
8012. (b) Kazmaier, U.; Grandel, R. Eur. J. Org. Chem. 1998, 1998,
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*E-mail: u.kazmaier@mx.uni-saarland.de.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemein-
schaft.
■
13, 6204−6211. (d) Kazmaier, U.; Stolz, D.; Kramer, K.; Zumpe, F.
̈
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