Total synthesis of NW-G01, a cyclic hexapeptide antibiotic, and 34-epi-NW-G01

Article abstract of DOI:10.1021/ol201912w

NW-G01, a cyclic hexapeptide antibiotic, and 34-epi-NW-G01 were synthesized by the highly stereoselective convergent approach for the first time, thereby unambiguously determining the absolute structure of NW-G01.

Full text of DOI:10.1021/ol201912w

ORGANIC
LETTERS
2011
Vol. 13, No. 17
4700–4703
Total Synthesis of NW-G01, a Cyclic
Hexapeptide Antibiotic, and
34-epi-NW-G01
Setsuya Shibahara, Takaaki Matsubara, Keisuke Takahashi, Jun Ishihara, and
Susumi Hatakeyama*
Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, 852-8521,
Japan
susumi@nagasaki-u.ac.jp
Received July 14, 2011
ABSTRACT
NW-G01, a cyclic hexapeptide antibiotic, and 34-epi-NW-G01 were synthesized by the highly stereoselective convergent approach for the first
time, thereby unambiguously determining the absolute structure of NW-G01.
The severe resistance of many organisms against existing
antibiotics is intensifying in the medical fields. This situa-
tion requires the advent of new classes of antibiotics. In the
course of a screening program for such antibiotics, Wu and
co-workers isolated NW-G01 from the fermentation broth
of Streptomyces alboflavus 313 in 2009.1 This compound
exhibits potent antibacterial activity against gram-positive
bacteria including methicillin-resistant Staphylococcus
aureus (MRSA) but is ineffective against gram-negative
bacteria. The comparison of the MIC value of NW-G01
(7.81 μg mLꢀ1) against MRSA with that of ampicillin
(>100 μg mLꢀ1), a representative β-lactam antibiotic,
suggests that NW-G01 is expected to provide an excellent
scaffold for thedevelopment of new antibiotics.1a Based on
the detailed NMR and X-ray analyses as well as Marfey’s
amino acid analysis, the absolute structure of NW-G01
was initially assigned to structure 1 consisting of D-valine,
N-methyl-L-alanine, (R)-piperazic acid, two molecules of
(S)-piperazic acid, and a characteristic chlorinated pyrro-
loindoline moiety.1b However, it was later revised as structure
2, a C34-epimer of 1 (Figure 1).2 The 18-membered cyclic
hexapeptide structure is related to the monomeric structures
of himastatin3,4 and chloptosin,5,6 antitumor dimeric cyclo-
hexapeptides. In connection with a project directed toward
the synthesis of chloptosin,7 we became interested in the
synthesis of NW-G01 due to its intriguing biological and
structural features. We herein describe the first total synthesis
of NW-G01 and its C34-epimer, thereby unambiguously
determining the absolute structure of NW-G01 as depicted
in 2.
(3) Leet, J. E.; Schroeder, D. R.; Krishnan, B. S.; Maston, J. A.;
Doyle, T. W.; Lam, K. S.; Hill, S. E.; Lee, M. S.; Whitney, J. L.;
Krishnan, B. S. J. Antibiot. 1996, 49, 299 and references therein.
(4) Total synthesis: (a) Kamenecka, T. M.; Danishefsky, S. J. Angew.
Chem., Int. Ed. 1998, 37, 2993. (b) Kamenecka, T. M.; Danishefsky, S. J.
Angew. Chem., Int. Ed. 1998, 37, 2995. (c) Kamenecka, T. M.; Danishefsky,
S. J. Chem.;Eur. J. 2001, 7, 41.
(5) Umezawa, K.; Ikeda, Y.; Uchihata, Y.; Naganawa, H.; Kondo, S.
J. Org. Chem. 2000, 65, 459.
(6) Total synthesis:(a) Yu, S.-M.; Hong, W.-X.; Wu, Y.; Zhong,
C.-L.; Yao, Z.-J. Org. Lett. 2010, 12, 1124. (b) Oelke, A. J.; France, D. J.;
Hofmann, T.; Wuitschik, G.; Ley, S. V. Angew. Chem., Int. Ed. 2010, 49,
6139. (c) Oelke, A. J.; France, D. J.; Hofmann, T.; Wuitschik, G.; Ley,
S. V. Chem.;Eur. J. 2011, 17, 4183.
(1) (a) Guo, Z.; Shen, L.; Ji, Z.; Zhang, J.; Huang, L.; Wu, W.
J. Antibiot. 2009, 62, 201. (b) Guo, Z.; Ji, Z.; Zhang, J.; Deng, J.; Shen,
L.; Liu, W.; Wu, W. J. Antibiot. 2010, 63, 231.
(2) Guo, Z.; Ji, Z.; Zhang, J.; Deng, J.; Shen, L.; Liu, W.; Wu, W.
J. Antibiot. 2010, 63, 733.
(7) Shibahara, S. Thesis, Nagasaki University, 2011. Our synthetic
study toward chloptosin was presented at the 36th Symposium on Progress in
Organic Reactions and Synthesis: Applications in the Life Sciences (Nagoya,
Japan, November 2, 2010, abstract paper pp 182ꢀ183).
r
10.1021/ol201912w
Published on Web 08/02/2011
2011 American Chemical Society

Scheme 1. Retrosynthetic Analysis of 1
11 with 0.6 equiv of Mg(ClO4)2 in acetonitrile at room
temperature,11 selective removal of one of the N-Boc
groups was cleanly achieved to provide 12 in almost
quantitative yield. According to the procedure reported
by Ley and co-workers,6c,12 12 was then converted to 14
diastereoselectively (dr = 23:1) via 13 by selenocyclization
followed by oxidative deselenation. For the subsequent
assembly of the cyclic hexapeptide, the methyl ester of 14
was switched to an allyl ester group by successive hydro-
lysis and allylation to give 15. In our preliminary examina-
tion, we found that a free tertiary alcohol in pyrroloind-
oline 6 (R1 = H) hampered the coupling with carboxylic
acid 7. Thus, the hydroxy group of 15 was masked as its
TES ether by treatment with TESOTf which also pro-
moted the concomitant removal of two Boc groups to
afford pyrroloindoline 16 in excellent yield.
Fragment 7 was prepared as dipeptide 23 starting with
Hale’s piperazic acid synthesis13 (Scheme 3). Oxazolidi-
none 17, prepared from 5-bromovaleric acid,13 was sub-
jected to hydrazination/cyclization conditions to furnish
piperazic acid derivative 18 in 63% yield. Hydrolysis of 18
followed by allyl esterification of the resulting carboxylic
acid afforded allyl ester 19, which was then converted to
(S)-20 via removal of the Boc groups and selective benzy-
loxycarbonylation in 65% overall yield. The enantiomeric
Figure 1. Related cyclohexapeptides with a pyrroloindoline.
Since the absolute structure of NW-G01 had been
proposed to be 11b when we started this project, we initially
addressed the synthesis of 1. Our synthetic plan is illu-
strated in Scheme 1. In the synthesis of chloptosin, Yao
and co-workers demonstrated6a that it was very difficult to
install pyrroloindoline 6 in either penta-, tetra-, or tripep-
tide residue by amidation at the N2 position due to its low
nucleophilicity. In addition, in the course of our synthetic
studies7 on this family of cyclohexapeptides, we often
observed that piperazid acid derivatives are very reluctant
coupling partners with bulky peptides. Taking into ac-
count the poor reactivities of 6 and piperazic acid, we
retrosynthetically sought to address the formation of the
macrocycle from 3. Disconnection at N8ꢀC26 affords two
key intermediates 4 and 5, which can be further discon-
nected at N2ꢀC35 and N4ꢀC16 respectively giving three
important building blocks 6, 7, and 8.
The synthesis of pyrroloindoline fragment 6 commenced
withthepreparationof10from5-chloro-2-iodoaniline and
aldehyde 98 by Zhu’s tryptophan synthesis9 (Scheme 2).
Compound 10 was then converted to N,N0-di-Boc deriva-
tive 12 by a protectionꢀdeprotection sequence. However,
the second deprotection step turned out to be problematic
under conventional acidic conditions.10 After considerable
exploration, we eventually found that, upon treatment of
ꢀ
(11) Hernandez, J. N.; Ramırez, M. A.; Martın, V. S. J. Org. Chem.
´ ´
2003, 68, 743.
(12) (a) Marsden, S. P.; Depew, K. M.; Danishefsky, S. J. J. Am.
Chem. Soc. 1994, 116, 11143. (b) Ley, S. V.; Cleator, E.; Hewitt, P. R.
Org. Biomol. Chem. 2003, 1, 3492. (c) Hewitt, P. R.; Cleator, E.; Ley,
S. V. Org. Biomol. Chem. 2004, 2, 2415. (d) Crich, D.; Huang, X. J. Org.
Chem. 1999, 64, 7218. (e) Crich, D.; Banerjee, A. Acc. Chem. Res. 2007,
40, 151.
ꢀ
(8) Padron, J. M.; Kokotos, G.; Martı
´
n, T.; Markidis, T.; Gibbons,
W. A.; Martın, V. S. Tetrahedron: Asymmetry 1998, 9, 3381.
´
(9) (a) Jia, Y.; Zhu, J. Synlett 2005, 2469. (b) Jia, Y.; Zhu, J. J. Org.
Chem. 2006, 71, 7826.
(13) Hale, K. J.; Cai, J.; Delisser, V.; Manaviazar, S.; Peak, S. A.;
Bhatia, G. S.; Collins, T. C.; Jogiya, N. Tetrahedron 1996, 52, 1047.
(14) (a) Hale, K. J.; Cai, J.; Williams, G. Synlett 1998, 149. (b) Hale,
K. J.; Lazarides, L. Org. Lett. 1998, 4, 1903.
(10) Ley and co-workers6c pointed out the difficulty of this transfor-
mation on scale-up.
Org. Lett., Vol. 13, No. 17, 2011
4701

Scheme 2. Synthesis of Pyrroloindoline 16
Scheme 3. Synthesis of Key Intermediate 25
purity of (S)-20 was determined to be 98% ee by chiral
HPLCanalysis. This compoundwas thenreacted withacid
chloride 21, derived from commercially available N-
methyl-Fmoc-L-alanine, in the presence of AgCN14 to give
22 quantitatively. Upon successive removal of the allyl and
Cbz groups, 22 furnished practically pure dipeptide 23,
which was used for the next reaction without purification.
The union of two fragments 16 and 23 was cleanly
achieved under the conditions using HATU, HOAt, and
γ-collidine to provide tripeptide 24 in 91% overall yield
from 22. Exposure of 24 to diisopropylamine afforded 25
corresponding to the envisaged key intermediate 4. It is
important to note that the unprotected free amine of 23 did
not impede the condensation at all. In addition, the Cbz
group of 22 should be removed prior to the coupling with
16, because the chlorine atom in the pyrroloindoline
moiety did not survive under hydrogenolytic deprotection
conditions.
Scheme 4. Synthesis of 34-epi-NW-G01 (1)
Another key intermediate 5 was prepared as 30from (S)-
20 following the known procedure6a (Scheme 4). After
attachment of an Fmoc group to (S)-20, palladium-cata-
lyzed cleavage of the allyl group followed by chlorination
yielded the acid chloride, which was condensed with (R)-
2015 under SchottenꢀBaumann conditions to give dipep-
tide 27 in 80% yield. After removal of the Fmoc group of
27, the resulting amine was reacted with acid chloride 28,
derived from commercially available Fmoc-D-valine, in the
presence of AgCN to afford tripeptide 29 in 95% yield.
Palladium-catalyzed cleavage of the allyl ester group of 29
and subsequent hydrogenolytic removal of the Cbz groups
furnished practically pure tripeptide 30.
With two key tripeptide fragments 25 and 30 in hand, we
then addressed the synthesis of NW-G01. Coupling of 25
(15) Prepared in 98% ee in the same manner as described for the
preparation of (S)-19.
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Org. Lett., Vol. 13, No. 17, 2011

€
with 30 was achieved using HATU, HOAt, and Hunig’s
base to afford 31 in 71% yield from 29. After the allyl and
Fmoc groups of 31 were unveiled, the resulting amino acid
was exposed to condensation conditions using HATU,
Scheme 5. Synthesis of NW-G01 (2)
€
HOAt, and Hunig’s base to form cyclic hexapeptide 32 in
moderate yield. Finally, desilylation of 32 produced the
originally proposed NW-G01 (1). Unfortunately, how-
ever, the spectral data and specific rotation of the synthetic
substance did not match with those of the natural product,
although the X-ray crystallography16 showed the structure
to be identical to the originally proposed structure 1.
Right after the completion of our total synthesis of 1,
Wu and co-workers reported the revised structure 2 for
NW-G01, which possesses the R-configuration at C34.
For the synthesis of NW-G01 (2), the required tripeptide
35 was prepared in 37% (59% brsm) overall yield starting
from (R)-20 via the attachment of N-methyl-L-alanine and
pyrroloindole moieties (Scheme 5). Then, the coupling of
two tripeptides 30 and 35 was achieved in the same manner
as described in Scheme 4 to provide hexapeptide 36 in 85%
yield. After successive removal of the allyl and Fmoc
groups, the resulting amino acid was subjected to macro-
cyclization to afford cyclohexapeptide 37 in 25% overall
yield. Finally, removal of the TES group led to the
completion of the total synthesis of NW-G01 (2). The
spectral data, melting point, and specific rotation exhibited
good agreement with those of natural NW-G01.
In conclusion, we have accomplished the first total
synthesis of NW-G01 (2) as well as 34-epi-NW-G01 (1)
in 40 steps (20 steps in the longest linear sequence) in 5%
overall yield from 5-bromovaleric acid.
University) for providing us with natural NW-G01 and
Associate Professor Hidehiro Uekusa (Department of
Chemistry and Material Sciences Tokyo Institute of
Technology) for X-ray crystallographic analysis of com-
pound 1.
Acknowledgment. Research Fellowship for Young
Scientists to S.S. (21-6107) from the Japan Society for
the Promotion of Science (JSPS) is gratefully acknowl-
edged. This work was supported by a Grant-in-Aid for
Scientific Research (A) (22249001) from JSPS. We thank
Prof. Wenjun Wu (Northwest Agricultural & Forestry
Supporting Information Available. Experimental pro-
cedures and characterization data for all new compounds,
and the X-ray crystallographic data of 1. This material is
available free of charge via the Internet at http://pubs.acs.
org.
(16) The crystallographic data (CCDC 831625) can be obtained free
of charge from the Cambridge Crystallographic Data centre via www.
ccdc.cam.ac.uk/data_request/cif.
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