First total synthesis of cyclic pentadepsipeptides Hikiamides A–C

Article abstract of DOI:10.1016/j.tetlet.2018.01.096

The first total syntheses of naturally occurring cyclodepsipeptides Hikiamides A–C are described. The key linear pentapeptide precursors, prepared efficiently via Fmoc-solid-phase synthesis, were cyclized in dilute solution to provide the target Hikiamide

Full text of DOI:10.1016/j.tetlet.2018.01.096

Accepted Manuscript
First total synthesis of cyclic pentadepsipeptides Hikiamides A−C
Donglin Fu, Xuemin Rao, Jinyi Xu, Genzoh Tanabe, Osamu Muraoka,
Xiaoming Wu, Weijia Xie
PII:
S0040-4039(18)30141-2
https://doi.org/10.1016/j.tetlet.2018.01.096
TETL 49683
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
5 December 2017
27 January 2018
30 January 2018
Please cite this article as: Fu, D., Rao, X., Xu, J., Tanabe, G., Muraoka, O., Wu, X., Xie, W., First total synthesis
of cyclic pentadepsipeptides Hikiamides A−C, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.
2
018.01.096
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Hikiamides A−C
*
*
Donglin Fu, Xuemin Rao, Jinyi Xu, Genzoh Tanabe, Osamu Muraoka, Xiaoming Wu and Weijia Xie

1
Tetrahedron Letters
First total synthesis of cyclic pentadepsipeptides Hikiamides A−C
a
a
a
b
b
a
a
Donglin Fu , Xuemin Rao , Jinyi Xu , Genzoh Tanabe , Osamu Muraoka , Xiaoming Wu  and Weijia Xie 
a
State Key Laboratory of Natural Medicines and Department of Medicinal Chemistry, China Pharmaceutical University, Nanjing, 210009,China
Faculty of Pharmacy, Kinki University, 3-4-1Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
b
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The first total syntheses of naturally occurring cyclodepsipeptides Hikiamides A−C are
described. The key linear pentapeptide precursors, prepared efficiently via Fmoc-solid-phase
synthesis, were cyclized in dilute solution to provide the target Hikiamides A−C. The structures
of the synthetic Hikiamides A−C were characterized by NMR and HRMS spectroscopy which
were in agreement with those of natural products.
Available online
2
009 Elsevier Ltd. All rights reserved.
Keywords:
Total synthesis
Hikiamides A-C
Fusarium sp.
Solid-phase peptide synthesis
Introduction
Diabetes is one of the fastest growing global public health
concerns. Type II diabetes mellitus (T2DM), once known as
non-insulin-dependent diabetes mellitus, accounts for more than
However, several serious side-effects such as macular edema
and hepatotoxicity restricted their clinical application.
1
5
Succedaneous drugs without such deleterious effects are
peremptorily needed. Hikiamides A-C (Figure 1), cyclo
depsipeptides, were isolated and identified from Fusarium
9
0% diabetes cases. It was reported that T2DM is becoming a
2
serious threat to human health. Antidiabetics in clinical
application include: insulin and its analogues, insulin
secretagogues (sulfonylureas and glinides), insulin sensitizers
6
species. TAMA 456 in 2015. Structurally, Hikiamide A and B
contain four identical residues: L-Phe, L-Phe, L-Val and S-OLeu.
The different residue is N-methylated L-Leu of Hikiamide A.
Hikiamide A and C also contain four residues in common: L-Phe,
N-Me-L-Leu, L-Val and S-OLeu. Hikiamide A differs from C by
substitution of a single amino acid residue L-Phe instead of L-
Trp. These cyclodepsipeptides induce the differentiation of
murine ST-13 preadipocytes into mature adipocytes at 2 μM and
(
Metformin and Thiazolidinediones (PPARγ agonists), -
3
glucosidase inhibitors (acarbose) et al. Compared to other anti-
diabetic agents, PPARγ agonists have the following advantages:
a) improvement in insulin sensitivity, (b) modulation of cytokine
(
secretion from adipocytes, (c) preserve cell mass and insulin
secretion capability, (d) reduction in cardiovascular morbidity
4
and mortality in high-risk T2DM patients. Thiazolidinediones
adiponectin mRNA expression (5 to 13 fold stronger than
(
TZDs) are most extensively employed PPARγ agonists to treat
type II diabetes.
6
TZDs).
To
date,
this
rare
family
of
cyclic
Figure. 1. Structures of Hikiamides (A-C) and Sansalvamide A.
—
——


Corresponding author. Tel.: +86-25-83271414; e-mail: xmwu@cpu.edu.cn
Corresponding author. Tel.: +86-25-83271414; e-mail: weijiaxie@cpu.edu.cn

2
Tetrahedron Letters
Figure. 2. synthetic strategy of Hikiamides A-C.
peptides also include sansalvamide A and N-methylsansalv
amide A as metabolites from Fusarium species. (Figure 1)
Results and discussion
7
Hikiamides (A-C) were the only cyclic peptide PPARγ agonists
isolated from Fusarium species as potential lead compounds. Our
interest toward these natural products also relied on their
intriguing structural features. It was reported that the construction
of large cyclopeptides containing more than seven amino acids is
not problematic whereas cyclization of smaller peptides with
The synthetic strategy of Hikiamides A-C was depicted in
Figure 2. Four amino acid Fmoc-L-Phe, Fmoc-L-phe(Trp),
Fmoc-L-Leu(R ), Fmoc-L-Val as well as S-OLeu were used to
2
assemble resin-bound linear pentapeptides A-C through SPPS.
After cleavage of 2-chlorotrityl chloride (2-CTC) resin, the
resulted linear pentapeptides were subjected to head-to-tail
8
8,15
tetra- or pentapeptides are usually challenging. Moreover, N-
macrolactamization
to provide target Hikiamides A-C in
alkyl or β-branched amino acids which are extensively existed in
Hikiamides (A-C) could indeed hamper the intramolecular
cyclization process. Considering the fact that synthesis of
solution phase.
9
Total synthesis of Hikiamide A and B
Hikiamides (A-C) has not been reported ever since their first
identification in 2015, in the present study, we want to challenge
the first total synthesis of Hikiamides (A-C).
Firstly, the Fmoc-L-Phe-O-Leu ester (3) was produced by
EDCI
and
N-Hydroxysuccinimide
(NHS)
mediated
16
Solid-phase peptide synthesis (SPPS), pioneered by
esterification of Fmoc-L-Phe (2) and S-OLeu in 81% yield.
3
1
0
Merrifield , is
a
practical approach to resolve some
was then treated with 2-CTC resin in the presence of N,N’-
diisopropylethylamine (DIPEA) to give resin-attached
intermediate (4). Sequence elongation of Fmoc-L-Phe-OLeu-
resin (4) with Fmoc-L-Phe, N-Me-Fmoc-L-Leu, Fmoc-L-Val in a
C to N direction was then carried out to provide resin-bound
linear pentapeptide (A). Each addition reaction involved
deprotection and coupling procedures. Fmoc protection was
removed with 20% 4-methylpiperidine in DMF. The target resin-
bound linear pentapeptide (A) was assembled by using
HBTU/DIPEA, which was reported as an efficient coupling
disadvantages inherent in solution-phase-synthesis. One of the
benefits is that it could be applied for the preparation of complex
peptide without additional purification. The strategy of SPPS has
been extensively utilized to prepare several important
11-14
cyclodepsipeptides.
Herein, we report the first total synthesis of Hikiamides A-C.
The key linear pentapeptide analogs were first constructed by
solid-phase peptide synthesis which were then subjected to a
solution phase assisted cyclization to provide Hikiamides A−C.
15,17
reagent in SPPS.
Scheme 1. Reagents and conditions: (a) EDCI, NHS, DCM, 24h, rt.; ( b) DMAP, DCM, 18h, rt., 81% (over 2 steps); (c) DIPEA, DMF, 2h, rt.,
6%; (d) Piperidine(20%), DMF, 30min, rt; (e) HBTU, DIPEA, DMF, 6h, rt.; (f) TFA, DCM, 1h, rt., (7a, 35% from 4) or (7b, 32% from 4); (g)
HATU, EDPBT, TBTU, DIPEA, DCM, 72h, rt., 25.7% (Hikiamide A) or 22.6% (Hikiamide B).
5

3
Scheme 2. Reagents and conditions: (d) Piperidine (20%), DMF, 30min, rt.; (e) HBTU, DIPEA, DMF, 6h, rt.; (f) TFA (1%), DCM, 1h, rt., (11,
1% from 4); (g) HATU, EDPBT, TBTU, DIPEA, DCM, 72h, rt, 45%., (h) TFA (50%), DCM, 30min, rt, 90%.
5
All of the amino acid coupling and deprotection reactions were
When TFA solution was diluted to 1% concentration, Boc-
protected linear peptide (11) could be obtained with the total
yield of 51% from 4. Finally, the Boc-protected cyclicpeptide
18
monitored by Kaiser or chloranil test. The cleavage between
linear peptide and 2-CTC resin was achieved by using a 5%
trifluoroacetic acid (TFA) in dichloromethane (DCM) solution
and the linear pentapeptide (7a) was obtained with the total yield
of 35% from 4. Finally, the key cyclization to afford Hikiamide
A was successfully performed by the treatment of linear
(12) was treated with 50% TFA in DCM solution to remove Boc
protection, giving Hikiamide C in 90% yield. The spectroscopic
1
13
data H NMR and C NMR of synthetic Hikiamide C were
6, 24
19
consistent with those of the natural product.
pentapeptide (7a) with a mixture of HATU, DEPBT and TBTU.
The cyclization process was finished within 72 h and the desired
Hikiamide A was obtained with the isolated yield of 25.7%. The
spectroscopic data of synthetic Hikiamide A were in agreement
The determination of an appropriate disconnection site is
critical for the construction of cyclic peptide. It has to be pointed
8
out that different ring-closure strategies have been attempted
during our synthetic process and most of them were proven to be
6, 24
with those of the natural product.
25
When Fmoc-L-Leu other than N-Me-Fmoc-L-Leu was used as
peptide building block, the above synthetic strategy was also
effective for the synthesis of Hikiamide B. Thus, the linear
pentapeptide (7b) was obtained in the total yield of 32% from 4.
The subsequent intramolecular cyclization of 7b performed under
similar condition successfully provided the desired Hikiamide B
ineffective. As for the construction of Hikiamide families, ring
closure at ester residue should be first excluded from the
potential cyclization protocol, owing to a reported risk of
26
racemization. In addition, the methylation of Leu moiety, as in
the case of Hikiamides A and C, made the certain area more
sterically encumbered which could also potentially hamper the
cyclization reaction. The intramolecular cyclization between the
N-terminus L-Phe and C-terminus L-Phe was also unsuccessful
which might be rationalized by sterical hindrance arising from
the attachment of the adjacent benzyl group.
1
13
in 22.6% yield (Scheme 1). The H NMR and C NMR spectrum
of synthetic Hikiamide B were also in accord with those of the
6, 24
natural product.
Conclusions
Total synthesis of Hikiamide C
In summary, we have established a suitable strategy for the
preparation of Hikiamides A-C by a combination of solid-phase
and solution-phase synthesis. SPPS was found to be efficient to
assemble different single amino acids to afford linear
pentapeptide intermediates which could then macrocyclized
smoothly in solution-phase system to furnish the target natural
product Hikiamide A-C. Different experiments were attempted in
order or determine an appropriated ring-closure site which might
be constructive for the synthesis of similar cyclic peptide
derivatives. Further structural modification and biological
evaluation of Hikiamide A-C will be presented in due course.
The above synthetic strategy which successfully provided us
Hikiamide A and B was then used for the preparation of
Hikiamide C. In a similar manner, 2-CTC resin attached
intermediate (4) was first assembled with Boc-protected L-Trp,
N-Me-L-Leu and L-Val to give the resin-bound linear
pentapeptide (C). The cleavage of 2-CTC resin was carried out
under the same reacting condition discussed above. Interestingly,
deprotection of Boc group on L-Trp fragment also occurred
simultaneously to give the linear pentapeptide (10) in 30% yield
together with Boc-protected linear peptide precursor (11) in 10%
yield as minor product. Since previous reports indicated the
participation of unprotected tryptophan moiety into
Acknowledgments
20-23
intramolecular cyclization.
The major product (10) was first
subjected to macrolactamization. Unfortunately, the reacting
system turned out to be a complex mixture and the desired
Hikiamide C could not be detected even in trace amount. When
the same cyclization reaction was applied to minor product (11),
the corresponding Boc-protected cyclic peptide (12) could be
obtained with the isolation yield of 45%, indicating that Boc-
protection on L-Trp residue was critical for the intramolecular
cyclization of linear pentapeptide. Thus, different reacting
conditions were scanned to optimize the reaction yield of 11.
This work was financially supported by the National Natural S
cience Foundation of China (Grant No. 81473081, 81673312), N
atural Science Foundation of Jiangsu Province for outstanding
young scholars (BK20160078), Scientific Research Foundation
for the Returned Overseas Chinese Scholars, Ministry of Educati
on (2013) and Technology Foundation for Selected Overseas Chi
nese Scholar, Ministry of Personnel of China (2013).

4
Tetrahedron
Supplementary data
Supplementary data associated with this article can be found
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Highlights
•
•
•
First total syntheses of natural cyclodepsipeptides
Hikiamides A−C are described.
Solid-phase peptide synthesis was found effective to
assemble linear pentapeptides.
Cyclization of linear pentapeptides in solution phase
provided Hikiamides A−C.

6
Tetrahedron
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First total synthesis of cyclic pentadepsipeptides
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Hikiamides A−C
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Donglin Fu, Xuemin Rao, Jinyi Xu, Genzoh Tanabe, Osamu Muraoka, Xiaoming Wu and Weijia Xie