Synthesis and biological evaluation of analogues of the marine cyclic depsipeptide obyanamide

Article abstract of DOI:10.1002/psc.1361

On the basis of the total synthesis of obyanamide, 20 analogues of this marine cyclic depsipeptide have been synthesized by (i) preparation of the tripeptide fragments in the western hemisphere using Z/OtBu protocol; (ii) preparation of the dipeptide fragments in the eastern hemisphere using Boc/OMe protocol; and (iii) fragments coupling, removal of protecting groups (Boc and OtBu, in one pot), and macrocyclizaion in the last step. The cytotoxic test showed that three synthetic compounds exhibited moderate activities against HL-60, KB, LOVO, and A549 cell lines. According to the results, the β-amino acid residue was found to play a critical role in the biological activities. Additionally, the ester bond along with the Ala(Thz) moiety was also essential for biological activities. However, it seems too early to draw a conclusion that the N-methylation of Val/Phe can lead to higher or lower cytotoxic activities.

Full text of DOI:10.1002/psc.1361

Research Article
Received: 14 December 2010
Revised: 17 January 2011
Accepted: 18 January 2011
Published online in Wiley Online Library: 14 April 2011
(wileyonlinelibrary.com) DOI 10.1002/psc.1361
Synthesis and biological evaluation
of analogues of the marine cyclic depsipeptide
obyanamide
Wei Zhang, Ning Ding and Yingxia Li∗
On the basis of the total synthesis of obyanamide, 20 analogues of this marine cyclic depsipeptide have been synthesized by
(i) preparation of the tripeptide fragments in the western hemisphere using Z/OtBu protocol; (ii) preparation of the dipeptide
fragments in the eastern hemisphere using Boc/OMe protocol; and (iii) fragments coupling, removal of protecting groups (Boc
and OtBu, in one pot), and macrocyclizaion in the last step. The cytotoxic test showed that three synthetic compounds exhibited
moderate activities against HL-60, KB, LOVO, and A549 cell lines. According to the results, the β-amino acid residue was found
to play a critical role in the biological activities. Additionally, the ester bond along with the Ala(Thz) moiety was also essential
for biological activities. However, it seems too early to draw a conclusion that the N-methylation of Val/Phe can lead to higher
c
or lower cytotoxic activities. Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd.
Keywords: obyanamide; synthesis; cyclic depsipeptides; analogues; cytotoxicity
Introduction
to generate the corresponding tripeptides 3e and 3f in high
yields during the second cycle of coupling. However, when the
N-terminus of the dipeptides was methylated (2a, 2d, and 2e), it
was necessary to mask the hydroxy group of (S)-lactic acid before
the coupling steps to avoid nucleophilic competition between the
amino group of the dipeptides and the free α-hydroxy group of
the acid. Thus, after two cycles of coupling and removals of the
N/O-protecting groups in the final step (omitted for compound
3e and 3f, see Table 1), six tripeptide fragments were obtained
smoothly.
In order to construct the eastern hemispheric dipeptides
7a–j, in first instance four thiazole/oxazole containing amino
acids (5a–d) and four β-amino acids (6b–e) were prepared
from commercially available reagents [12–15]. After Boc group
removals and EDC/HOAt intermediate couplings, ten dipeptides
building blocks were obtained smoothly in satisfactory yields
(Scheme 2).
Marine cyclic (depsi)peptides are a growing research area in the
past two decades due to their novel structures and remarkable
biological activities [1–3]. Many of them have become the focus
of recent synthetic endeavors [4–6]. Obyanamide is a cytotoxic
cyclic depsipeptide that was isolated by Moore and co-workers
in 2002 [7]. One year later, several structural analogues have also
been reported by the same [8] and other [9] groups. Interestingly,
most of them were found to have moderate activities against
several cancer cell lines. Structure–activity relationship studies on
these compounds might help to find more efficient anti-cancer
agents. Inongoingeffortsonthistarget, wehavereportedthetotal
synthesis and revised structure of obyanamide [10,11] (Figure 1).
Here we report the results of the synthesis of its analogues along
with their biological activities.
With six tripeptides (4a–d, 3e–f) and ten dipeptides fragments
(7a–j) in hand, we could now proceed toward the cyclic
(depsi)peptides (Scheme 3 and Table 2). Previous studies showed
that Yamaguchi’s procedure [16] was superior to the standard
esterification method (EDC/DMAP). Thus 19 linear pentapeptides
were obtained smoothly by this strategy, while for compounds 8l
and 8m standard peptide coupling conditions (EDC/HOAt) gave
satisfactory results. Finally, all these linear pentapeptides were
treated with TFA in DCM to remove the OtBu and N-Boc groups in
one pot and cyclized to afford the desired cyclic (depsi)peptides
(9a–u).
Results and Discussions
On the basis of the total synthesis of the natural product, we
have established a rapid and efficient route to prepare these cyclic
depsipeptides. More specifically, this route includes (i) preparation
of the tripeptide fragments in the western hemisphere using
Z/OtBu protocol; (ii) preparation of the dipeptide fragments in the
eastern hemisphere using Boc/OMe protocol; and (iii) fragments
coupling, removal of protecting groups (Boc and OtBu, in one pot),
and macrocyclizaion in the last step.
As illustrated in Scheme 1, the synthesis of tripeptide fragments
started from alanine or phenylalanine derivatives (1a–c). The Z-
protected amino acids were hydrogenated to release the free
amine compounds which were coupled with Z-Val-OH or Z-MeVal-
OHinthefollowingstep, toproducethecorresponding dipeptides
2a–e in 70–97% yields. These five dipeptides were used for the
next cycle, introducing a new amino/hydroxy acid to give six
tripeptide fragments. Notably, (S)-lactic acid was used directly
∗
Correspondence to: Yingxia Li, Department of Medicinal Chemistry, School of
Pharmacy, Fudan University, Zhangheng Rd. 826, Shanghai 201203, China.
E-mail: liyx417@fudan.edu.cn
Department of Medicinal Chemistry, School of Pharmacy, Fudan University,
Shanghai 201203, China
c
J. Pept. Sci. 2011; 17: 533–539
Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd.

ZHANG, DING AND LI
Experimental Section
"West
"Eastern
hemisphere" hemisphere"
General Information
Solvents were purified by standard methods. Amino acids and
coupling reagents were purchased from GL Biochem (Shanghai)
Ltd. TLC was carried out on Merck 60 F254 silica gel plates
and visualized by UV irradiation or by staining with iodine
absorbed on silica gel, ninhydrin solution, or with aqueous acidic
ammonium molybdate solution as appropriate. Flash column
chromatography was performed on silica gel (200–300 mesh,
Qingdao, China). Optical rotations were obtained using a JASCO
P-1020 digital polarimeter. NMR spectra were recorded on JEOL
JNM-ECP 600 MHz spectrometers. Mass spectra were obtained on
a Q-Tof Ultima Global mass spectrometer. The cytotoxic effects
were examined using SRB assay against A549, LOVO, KB, BEL-7402,
and MTT tetrazolium dye assay against P388 and HL-60 cell lines.
O
O
S
S
14
N
H
N
8
H
O
N
N
N
O
N
N
N
HN
O
HN
3
O
29
O
1
O
S
R
O
O
O
O
Obyanamide(I),
previously assumded structure
Obyanamide(II),
revised structure
Figure 1. The structure of obyanamide.
Table 1. Yields of tripeptides 4a–d and 3e–f
Yields (%) Yields (%) Yields (%) of
Synthesis of dipeptide 2a–e: general procedure (first cycle)
of first
cycle
of second
cycle
N/O-PG
removal
The amino acid derivatives 1a–c were hydrogenated over 10%
Pd/C in EtOAc until all the starting material disappeared (2–4 h).
The suspensions were filtered through a pad of celite, washed with
EtOAc, and concentrated in vacuo. The amine compounds were
dried under high vacuum for 2 h and dissolved in dry DCM. After
1 equiv. of Z-Val-OH or Z-MeVal-OH was added, the mixtures were
cooledto0 ◦C.Then15 minlater,EDC(1.2equiv.),HOAt(1.2equiv.),
and DIPEA (2 equiv.) were added, respectively. The mixtures were
stirred at 0 ◦C for 2 h and room temperature overnight. After
dilution with EtOAc, the mixtures were washed with 10% citric
acid, 5% NaHCO3, brine, dried (Na2SO4), and concentrated in
vacuo. The residues were purified by column chromatography
(EtOAc–petroleum ether) to give the dipeptides 2a–e.
R1 R2 R3 W1
W2
4a Ph Me Me OBn OH
4b Ph Me OBn OH
4c Ph Me Me NHZ NH2
4d Me Me OBn OH
3e Ph Me
3f Ph
78
89
78
70
97
92
87
100
82
98
100
100
100
–
H
H
78
H
H
OH OH
OH OH
78
H
95
–
Thecytotoxicactivitiesofthefinalcompoundsweredetermined
by the MTT or sulforhodamine B (SRB) assays, using the HL-60,
P388, BEL-7402, KB, LOVO, and A549 cell lines.Among the 21 cyclic
(depsi)peptides,3compounds(9b,9dand9p)exhibitedmoderate
cytotoxic activities (Table 3). According to the above results, the
β-amino acid residue especially its configuration and side chain
may play a critical role in the biological effect. Converting the
R-configuration(9b)toS-(9a)ledtoinactivederivative.Whilewhen
the side chain of this residue was shortened at the same time (9b
vs. 9d), the activities decreased dramatically. The ester bond along
with the Ala(Thz) moiety was also essential for biological activities.
Any changes at these positions led to inactive derivatives (9b vs.
9m;9b vs. 9i, 9j, and 9k). However, it seems too early to draw a
conclusion that the N-methylation of Val/Phe can lead to higher
or lower cytotoxic activities (9p vs. 9n and 9o).
Nα-Z-Nα-Me-L-Val-L-Phe-OtBu (2a)
Yield 89%. Colorless oil. Rf = 0.48 (EtOAc : Petroleum ether = 1 : 3);
1H NMR (CDCl3, ∗ as rotamer) δ: 0.81 and 0.83∗ (d, J = 6.6 Hz, 3H),
0.88 and 0.92∗ (d, J = 6.6 Hz, 3H), 1.42 and 1.44∗ (s, 9H), 2.20–2.24
(m, 1H), 2.71 and 2.80∗ (s, 3H), 2.91 (dd, J = 7.7, 13.9 Hz, 1H),
3.10 (dd, J = 5.8, 14.3 Hz, 1H), 3.96 and 4.09∗ (d, J = 10.6 Hz,
1H), 4.69–4.75 (m, 1 H), 5.03–5.17 (m, 2 H), 6.02 and 6.46∗ (each
d, J = 7.3 Hz, 1H), 7.00–7.39 (m, 10H); 13C NMR (CDCl3) δ: 18.5,
19.6, 25.9, 27.9, 29.5, 38.1, 53.2, 64.9, 67.5, 82.1, 126.8, 126.9, 128.1,
128.3, 128.6, 129.3, 136.2, 136.5, 157.3, 169.4, 170.3; ESI-MS (m/z)
calcd. [M+Na]+ = 491.3, [M+K]+ = 507.2, found 491.2, 507.2.
R1
O
R1
O
R1
O
R1
O
OtBu
OtBu
OtBu
O
N/O-PG
removal
1) N-PG removal
OtBu
1) N-PG removal
N
O
O
N
N
O
O
R2
R2
N
R2
2) coupled with
Z-Val-OH or
Z-MeVal-OH
2) coupled with
lactic acid or
Z-Ala-OH
R2
Z
W2
Z
W1
N
N
N
1a~c
R3
R3
2a~e
R3
1st CYCLE
1a R1=Ph, R2=H
1b R1=Ph, R2=Me
1c R1=H, R2=Me
2nd CYCLE
4a~d
3a~f
2a R1=Ph, R2=H, R3=Me
2b R1=Ph, R2=R3=H
4a R1=Ph, R2=Me, R3=Me, W2=OH
4b R1=Ph, R2=H, R3=Me, W2=OH
4c R1=Ph, R2=Me, R3=Me, W2=NH2
4d R1=H, R2=Me, R3=Me, W2=OH
3a R1=Ph, R2=R3=Me, W1=OBn
3b R1=Ph, R2=H, R3=Me, W1=OBn
3c R1=Ph, R2=R3=Me, W1=NHZ
3d R1=H, R2=R3=Me, W1=OBn
3e R1=Ph ,R2=R3=H, W1=OH
3f R1=Ph, R2=Me, R3=H, W1=OH
2c R1=Ph, R2=Me, R3=H
2d R1=Ph, R2=R3=Me
2e R1=H, R2=R3=Me
Scheme 1. (a) Conditions of PG removals: Pd/C, H2, EtOAc, rt, 2–4 h; (b) conditions of couplings: EDC/HOAt/DCM, 0 ◦C to rt, 6–14 h.
c
wileyonlinelibrary.com/journal/jpepsci Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd. J. Pept. Sci. 2011; 17: 533–539

SYNTHESIS AND CYTOTOXICITY STUDIES OF THE ANALOGUES OF OBYANAMIDE
CO2H
N
O
BocHN
X
N
NH
R2
5a~d
R1
EDC/HOAt/DIPEA
BocHN
DCM
X
80~98%
R1
O
NHBoc
R2
TFA, CH2Cl2
6a~e
O
O
O
7a R1=Me#, R2=H, X=S
7b R1=R2=Me#, X=S
5a R1=Me#, X=S
5b R1=Me#, X=O
5c R1=iPr#, X=S
5d R1=Me*, X=S
6a R2=H
7c R1=Me#, R2=Me*, X=S
7d R1=Me# ,R2=Et#, X=O
7e R1=Me#, R2=Et*, X=O
7f R1=Me#, R2=Et*, X=S
7g R1=Me*, R2=Et#, X=S
7h R1=Me*, R2=Et*, X=S
7i R1=iPr#, R2=Et#, X=S
7j R1=iPr#, R2=Et*, X=S
6b R2=Me#
6c R2=Me*
6d R2=Et#
6e R2=Et*
Scheme 2. Synthesis of dipeptide fragments 7a–j. ∗: R configuration; #: S configuration.
R1
O
OtBu
O
O
R5
N
BocHN
R1
R2
R1
O
R5
N
W2
X
X
N
H
O
N
R3
OtBu
O
NR2
NR3
N
HN R4
b
c
O
O
4a~f
O
N
O
HN
O
R2
O
R4
W
W
N
NH
O
N
R3
8a~u
a
R5
O
X
R4
9a~u
NHBoc
O
O
7a~j
Scheme 3. Reagents and conditions: (a) LiOH, THF/MeOH/H2O; (b) Yamaguchi esterification or EDC/HOAt/DIPEA (for 8l and 8m); (c) (i) TFA, DCM;
(ii) HATU/DIPEA, THF.
Nα-Z-L-Val-L-Phe-OtBu (2b)
Nα-Z-Nα-Me-L-Val-Nα-Me-L-Phe-OtBu (2d)
See Ref. 11.
Yield 92%. Pale yellow solid. Rf = 0.47 (EtOAc : Petroleum ether
= 1 : 2); [α]20 = −29.5 (c = 1.0, MeOH); 1H NMR (CDCl3) δ:
D
0.88 (d, J = 7.0 Hz, 3H), 0.94 (d, J = 6.6 Hz, 3H), 1.39 (s, 9H),
2.06–2.12 (m, 1H), 3.07 (d, J = 6.4 Hz, 2H), 4.00–4.02 (m, 1H),
4.73–4.77 (m, 1H), 5.09 (d, J = 12.4 Hz, 1H), 5.12 (d, J = 12.1 Hz,
1H), 5.38 (d, J = 8.0 Hz, 1H), 6.36 (br, 1H), 7.14–7.37 (m, 10H); 13C
NMR (CDCl3) δ: 17.6, 19.1, 27.9, 31.1, 38.0, 53.5, 60.2, 67.0, 82.4,
127.0, 128.0, 128.1, 128.4, 128.5, 129.4, 135.9, 136.2, 156.3, 170.2,
170.6. ESI-MS (m/z) calcd. [M+H]+ = 455.3, [M+Na]+ = 477.2,
[M+K]+ = 493.2, found 455.2, 477.2, 493.2.
Nα-Z-Nα-Me-L-Val-Nα-Me-L-Ala-OtBu (2e)
Yield 70%. Colorless oil. Rf = 0.31 (EtOAc : Petroleum ether = 1 : 3);
1H NMR (CDCl3, one main rotamer of four) δ: 0.89 (d, J = 6.4 Hz,
3H), 0.96 (d, J = 6.4 Hz, 3H), 1.32 (d, J = 7.3 Hz, 3H), 1.43 (s, 9H),
2.31–2.37 (m, 1H), 2.92 (s, 3H), 3.03 (s, 3H), 4.74 (d, J = 10.6 Hz,
1H), 4.98–5.27 (m, overlapped, 3H), 7.30–7.36 (m, 5H); 13C NMR
(CDCl3, one main rotamer of four) δ: 14.3, 18.4, 19.4, 27.7, 28.0,
29.1, 31.6, 53.2, 60.0, 67.4, 81.5, 127.6, 128.0, 128.5, 136.7, 156.0,
156.5, 157.0.
Nα-Z-L-Val-Nα-Me-L-Phe-OtBu (2c)
Yield 97%. Colorless oil. Rf = 0.38 (EtOAc : Petroleum ether = 1 : 3);
1H NMR (CDCl3, one main rotamer of four) δ: 0.90 (d, J = 6.6 Hz,
3H), 0.97 (d, J = 7.0 Hz, 3H), 1.42 (s, 9H), 1.91–1.97 (m, 1H), 2.96
(s, 3H), 2.91–2.93 (m, 1H), 3.32 (dd, J = 5.8, 14.6 Hz, 1H), 4.38
(dd, J = 6.2, 9.1 Hz, 1H), 5.01–5.12 (m, 2H), 5.27 (dd, J = 5.8,
10.3 Hz, 1H), 5.31 (d, J = 9.2 Hz, 1H), 7.12–7.38 (m, 10H); 13C NMR
(CDCl3, one main rotamer of four) δ: 17.3, 19.4, 27.9, 31.2, 32.6, 34.5,
55.6, 58.9, 66.7, 81.9, 126.6, 127.9, 128.1, 128.5, 128.9, 129.2, 136.4,
137.0, 156.2, 169.6, 172.3. ESI-MS (m/z) calcd. [M+Na]+ = 491.3,
[M+K]+ = 507.2, found 491.2, 507.2.
Synthesis of tripeptide 3: general procedure (second cycle)
The second cycle was performed in similar mode as the first
cycle. The dipeptides 2a–e were hydrogenated and coupled with
Z-Ala-OHorL-lacticacid(Lac)togivethecorrespondingtripeptides
3a–f.
BnO-L-Lac-Nα-Me-L-Val-Nα-Me-L-Phe-OtBu (3a)
See Ref. 11.
c
J. Pept. Sci. 2011; 17: 533–539 Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jpepsci

ZHANG, DING AND LI
J = 5.0,13.9 Hz, 1H), 3.38 (dd, J = 4.6, 14.0 Hz, 1H), 4.64–4.68 (m,
1H), 4.85 (d, J = 11.0 Hz, 1H), 5.07–5.09 (dd like, overlapped, 2H),
5.29 (dd, J = 4.6, 11.5 Hz, 1H), 5.68 (d, J = 8.2 Hz, 1H), 7.16–7.35
(m, 10H); 13C NMR (CDCl3) δ: 18.0, 18.9, 19.0, 26.7, 28.0, 30.1,
32.1, 34.4, 47.2, 58.4, 58.8, 66.8, 81.8, 126.7, 127.9, 128.1, 128.5,
128.7, 128.8, 136.4, 137.1, 155.5, 169.6, 172.9; ESI-MS (m/z) calcd.
[M+Na]+ = 576.3, [M+K]+ = 592.3, found 576.2, 592.2.
Table 2. Yields of linear pentapeptides 8a–u and cyclic
(depsi)peptides 9a–u
Yields of Yields of
R1
R2
R3
W
R4
X
R5
8 (%)
9 (%)
a
b
c
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
Ph Me Me
O
O
O
O
O
O
O
O
O
O
O
Et#
Et∗
H
S
S
S
S
O
S
S
S
O
S
S
S
S
S
S
S
S
S
S
S
S
Me#
Me#
Me#
Me#
Me#
Me∗
iPr#
Me#
Me#
iPr#
Me∗
Me#
Me#
Me#
Me#
Me#
Me#
Me#
Me#
Me#
Me#
94
98
91
54
82
95
87
96
83
80
98
100
77
87
92
100
100
77
84
72
92
59
53
36
45
35
41
43
67
67
50
52
40
71
74
88
94
24
64
75
66
47
BnO-L-Lac-Nα-Me-L-Val-Nα-Me-L-Ala-OtBu (3d)
d
e
f
Me#
Et#
Et#
Et#
Me∗
Et∗
Et∗
Et∗
Et#
Et∗
Et∗
Et∗
Et∗
Et∗
Et#
Et#
Et#
Et#
Yield 78%. Colorless oil. Rf = 0.60 (EtOAc : Petroleum ether = 1 : 1);
1H NMR (CDCl3, one main rotamer of four) δ: 0.90 (d, J = 6.6 Hz,
3H), 0.99 (d, J = 6.6 Hz, 3H), 1.30 (d, J = 7.3 Hz, 3H), 1.41 (d
like, overlapped, 3H), 1.44 (s, 9H), 2.34–2.39 (m, 1H), 3.00 (s, 3H),
3.04 (s, 3H), 4.33–4.37 (m, 1H), 4.38 (d, J = 11.3 Hz, 1H), 4.59 (d,
J = 11.3 Hz, 1H), 5.02 (q, J = 7.3 Hz, 1H), 5.23 (d, J = 11.0 Hz, 1H),
7.27–7.34 (m, 5H); 13C NMR (CDCl3, one main rotamer of four) δ:
14.3, 17.5, 18.4, 19.4, 27.1, 28.0, 29.7, 31.8, 35.4, 58.1, 70.9, 73.3,
81.5, 127.8, 128.0, 128.5, 137.5, 170.6, 171.8, 172.5
g
h
i
j
k
l
Ph Me Me NH
Ph Me Me NH
m
n
o
p
q
r
Ph
H
Me
H
O
O
O
O
O
O
O
O
Ph Me
HO-L-Lac-L-Val-L-Phe-OtBu (3e)
Ph
H
H
H
Yield 95%. White foam. Rf = 0.24 (EtOAc : Petroleum ether = 1 : 1);
Me Me
1
[α]20 = −43.4 (c = 1.0, MeOH); H NMR (DMSO-d6) δ: 0.78 (d,
D
Ph
H
Me
H
J = 6.6 Hz, 3H), 0.83 (d, J = 7.0 Hz, 3H), 1.22 (d, J = 6.6 Hz, 3H),
1.30 (s, 9H), 1.92–1.98 (m, 1H), 2.90 (dd, J = 8.4, 13.4 Hz, 1H), 2.96
(dd, J = 7.0, 13.9 Hz, 1H), 3.94–3.97 (m, 1H), 4.25 (dd, J = 6.2,
9.5 Hz, 1H), 4.35 (m, 1H), 5.69 (d, J = 5.1 Hz, 1H), 7.19–7.28 (m,
5H), 7.39 (d, J = 9.5 Hz, 1H), 8.52 (d, J = 7.7 Hz, 1H); 13C NMR
(DMSO-d6) δ: 17.6, 19.1, 21.3, 27.5, 31.5, 36.8, 54.2, 55.9, 67.3, 80.6,
126.5, 128.2, 129.1, 137.1, 170.4, 170.6, 173.9; ESI-MS (m/z) calcd.
[M+Na]+ = 415.2, [M+K]+ = 431.2, found 415.2, 431.2.
s
Ph Me
t
Ph
H
H
H
u
Me Me
∗ R configuration,
# S configuration
Table 3. IC50 (µM) values of 9b, 9d, and 9pa
HO-L-Lac-L-Val-Nα-Me-L-Phe-OtBu (3f)
HL-60
KB
LOVO
A-549
Yield 78%. Colorless oil. Rf = 0.28 (EtOAc : Petroleum ether = 1 : 1);
1H NMR (CDCl3) δ: 0.90 (d, J = 6.8 Hz, 3H), 0.96 (d, J = 6.9 Hz, 3H),
1.37 (d, J = 6.9 Hz, 3H), 1.44 (s, 9H), 1.98–2.04 (m, 1H), 2.93 (dd,
J = 5.5, 14.2 Hz, 1H), 3.32 (dd, J = 5.5, 14.7 Hz, 1H), 3.57 (br, 1H),
4.10–4.14 (m, 1H), 4.64 (dd, J = 6.9, 9.2 Hz, 1H), 5.33 (dd, J = 5.5,
10.1 Hz, 1H), 6.92 (d, J = 9.2 Hz, 1H), 7.15–7.31 (m, 5H); 13C NMR
(CDCl3) δ: 17.6, 19.4, 21.2, 28.0, 31.1, 32.6, 34.5, 53.3, 58.8, 68.3, 82.1,
126.4, 128.4, 129.1, 137.1, 169.5, 172.5, 174.3.
9b
9d
9p
6.8
42
6.7
19
43
0.78
Nd
16
Nd
Nd
Nd
0.64
a All the target compounds (9a–9u) were screened for cytotoxicities.
Other compounds not listed in the table were inactive in the test
(IC50 > 100 µM).
Nd, not detected.
Synthesis of tripeptide fragments 4a–d: general procedure
BnO-L-Lac-Nα-Me-L-Val-L-Phe-OtBu (3b)
Ten percent Pd/C was added to the solutions of tripeptides 3a–d
in EtOAc. The reaction mixtures were purged with hydrogen three
times and stirred for 2 h at room temperature. The suspensions
were filtered through a pad of celite, washed with EtOAc, and
concentrated in vacuo to give 4a–d in nearly quantitative yields.
Yield 99%. Colorless oil. Rf = 0.24 (EtOAc : Petroleum ether = 1 : 3);
1H NMR (CDCl3, one rotamer of two) δ: 0.83 (d, J = 6.6 Hz, 3H), 0.95
(d, J = 6.6 Hz, 3H), 1.33 (d, J = 6.6 Hz, 3H), 1.40 (s, 9H), 2.25–2.31
(m, 1H), 2.79 (s, 3H), 2.91–2.94 (dd like, overlapped, 1H), 3.09 (dd,
J = 5.5, 13.9 Hz, 1H), 4.24 (q, J = 7.0 Hz, 1H), 4.32 (d, J = 11.7 Hz,
1H), 4.53 (d, J = 11.7 Hz, 1H), 4.58 (d, J = 11.3 Hz, 1H), 5.71–5.75
(m, 1H), 6.51 (br, 1H), 7.13–7.36 (m, 10H); 13C NMR (CDCl3, one
rotamer of two) δ: 16.9, 18.5, 19.6, 25.5, 27.9, 30.2, 38.1, 53.2, 63.0,
70.3, 72.6, 82.1, 126.9, 127.8, 128.4, 128.5, 129.1, 129.8, 136.3, 137.5,
169.4, 170.2, 172.9.
HO-L-Lac-Nα-Me-L-Val-Nα-Me-L-Phe-OtBu (4a)
See Ref. 11.
HO-L-Lac-Nα-Me-L-Val-L-Phe-OtBu (4b)
Yield 100%. Colorless oil. Rf = 0.32 (EtOAc : Petroleum ether =
1 : 1); 1H NMR (CDCl3) δ: 0.82 (d, J = 7.0 Hz, 3H), 0.96 (d, J = 6.2 Hz,
3H), 1.18 (d, J = 6.6 Hz, 3H), 1.39 (s, 9H), 2.25–2.34 (m, 1H), 2.87 (s,
3H), 2.96 (dd, J = 7.7, 13.9 Hz, 1H), 3.06 (dd, J = 6.2, 14.3 Hz, 1H),
3.82 (d, J = 8.1 Hz, 1H), 4.41–4.17 (m, 1H), 4.65 (d, J = 11.0 Hz,
1H), 4.67–4.76 (m, 1H), 6.85 (d, J = 7.3 Hz, 1H), 7.15–7.28 (m, 5H);
13C NMR (CDCl3) δ: 18.4, 19.3, 20.5, 25.7, 27.7, 29.9, 37.9, 53.4, 63.1,
64.4, 82.1, 126.8, 128.3, 129.1, 136.1, 169.2, 170.3, 175.9.
Nα-Z-L-Ala-Nα-Me-L-Val-Nα-Me-L-Phe-OtBu (3c)
Yield 82%. Colorless oil. Rf = 0.21 (EtOAc : Petroleum ether =
1 : 2); [α]20 = −99.2 (c = 0.4, MeOH); 1H NMR (CDCl3) δ: 0.50 (d,
D
J = 6.4 Hz, 3H), 0.72 (d, J = 6.4 Hz, 3H), 1.30 (d, J = 6.8 Hz, 3H),
1.44 (s, 9H), 2.17–2.21 (m, 1H), 2.82 (s, 3H), 2.94 (s, 3H), 3.33 (dd,
c
wileyonlinelibrary.com/journal/jpepsci Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd. J. Pept. Sci. 2011; 17: 533–539

SYNTHESIS AND CYTOTOXICITY STUDIES OF THE ANALOGUES OF OBYANAMIDE
H-L-Ala-Nα-Me-L-Val-Nα-Me-L-Phe-OtBu (4c)
7d
This intermediate was directly subjected to next step without
structural characterization.
Yield 92%. Colorless oil. Rf = 0.24 (EtOAc : Petroleum ether = 1 : 1);
1H NMR (CDCl3) δ: 0.97 (t, J = 7.4 Hz, 3H), 1.46 (s, 9H), 1.54 (d,
J = 6.8 Hz, 3H), 1.65–1.70 (m, 2H), 2.63 (d, J = 5.5 Hz, 2H), 3.70
(s, 3H), 4.31–4.37 (m, 1H), 4.97–4.99 (m, 1H), 5.11 (br, 1H), 7.23 (d,
J = 9.2 Hz, 1H), 8.11 (s, 1H); 13C NMR (CDCl3) δ: 10.6, 20.0, 27.2,
28.3, 38.4, 44.7, 47.2, 51.7, 80.3, 136.0, 141.1, 155.0, 160.0, 164.5,
171.9.
HO-L-Lac-Nα-Me-L-Val-Nα-Me-L-Ala-OtBu (4d)
Yield 99%. Colorless oil. Rf = 0.24 (EtOAc : Petroleum ether = 1 : 1);
1H NMR (CDCl3, one main rotamer of four) δ: 0.88 (d, J = 7.0 Hz,
3H), 0.99 (d, J = 6.6 Hz, 3H), 1.32 (d, J = 7.0 Hz, 6H), 1.44 (s, 9H),
2.36–2.41 (m, 1H), 2.95 (s, 3H), 3.00 (s, 3H), 3.70 (d, J = 7.7 Hz, 1H),
4.48–4.54 (m, 1H), 5.00 (q, J = 7.0 Hz, 1H), 5.17 (d, J = 11.0 Hz,
1H); 13C NMR (CDCl3, one main rotamer of four) δ: 14.3, 18.3, 19.3,
21.0, 27.0, 27.9, 29.6, 31.7, 53.6, 58.6, 64.5, 81.6, 169.9, 170.4, 175.9.
7e
Yield 82%. Colorless oil. 1H NMR (CDCl3) δ: 0.97 (t, J = 7.3 Hz, 3H),
1.47 (s, 9H), 1.54 (d, J = 6.8 Hz, 3H), 1.65–1.70 (m, 2H), 2.62 (d,
J = 5.5 Hz, 2H), 3.70 (s, 3H), 4.31–4.36 (m, 1H), 4.97 (brm, 1H), 5.10
(brs, 1H), 7.23 (d, J = 9.1 Hz, 1H), 8.12 (s, 1H); 13C NMR (CDCl3) δ:
10.6, 19.9, 27.2, 28.3, 38.4, 44.8, 47.3, 51.7, 80.2, 136.0, 141.1, 154.9,
160.0, 164.6, 171.9.
Synthesis of dipeptide fragments 7a–j: general procedure
TFA was added to the solutions of 6a–e in CH2Cl2 at 0 ◦C. The
reaction mixtures were warmed to room temperature and stirred
for 2 h. The solvent was evaporated and the oily residues were
dissolved twice in CH2Cl2 with evaporation each time to give
TFA salts, which were dissolved in dry DCM. After 1 equiv. of
5a–d was added, the mixtures were cooled to 0 ◦C. Then 15 min
later, EDC (1.2 equiv.), HOAt (1.2 equiv.), and DIPEA (2 equiv.)
were added respectively. The mixtures were stirred at 0 ◦C for 2 h
and room temperature overnight. After dilution with EtOAc, the
mixtures were washed with 10% citric acid, 5% NaHCO3, brine,
dried (Na2SO4), and concentrated in vacuo. The residues were
purified by column chromatography (EtOAc–petroleum ether) to
give dipeptides 7a–j in 80-98% yield.
7f
See Ref. 11.
7g
Yield 85%. Colorless oil. Rf = 0.28 (EtOAc : Petroleum ether = 1 : 1);
1H NMR (CDCl3) δ: 0.98 (t, J = 7.3 Hz, 3H), 1.47 (s, 9H), 1.62 (d,
J = 6.2 Hz, 3H), 1.65–1.72 (m, 2H), 2.64 (dd, J = 5.5, 15.7 Hz, 1H),
2.67 (dd, J = 5.5, 15.8 Hz, 1H), 3.70 (s, 3H), 4.32–4.38 (m, 1H), 5.09
(br, 1H), 5.11 (br, 1H), 7.64 (d, J = 8.8 Hz, 1H), 8.00 (s, 1H); 13C NMR
(CDCl3) δ: 10.7, 21.6, 27.3, 28.3, 38.4, 47.6, 48.7, 51.8, 80.3, 123.1,
149.9, 155.0, 160.6, 172.0, 174.1.
7a
Yield 84%. Colorless oil. Rf = 0.24 (EtOAc : Petroleum ether = 1 : 1);
1
[α]20 = −24.9 (c = 1.0, MeOH); H NMR (CDCl3) δ: 1.47 (s, 9H),
7h
D
1.60 (d, J = 6.9 Hz, 3H), 2.67 (t, J = 6.4 Hz, 3H), 3.69–3.73 (m, 2H),
3.74 (s, 3H), 5.02–5.07 (m, 1H), 5.16 (br, 1H), 7.73 (t, J = 5.5 Hz,
1H), 8.01 (s, 1H); 13C NMR (CDCl3) δ: 21.6, 28.3, 34.0, 34.7, 48.7, 51.8,
80.3, 123.0, 149.7, 154.9, 161.1, 172.7, 174.5; ESI-MS (m/z) calcd.
[M+H]+ = 358.2, [M+Na]+ = 380.1, [M+K]+ = 396.1, found
358.7, 380.6, 396.6.
Yield 90%. Colorless oil. 1H NMR (CDCl3) δ: 0.98 (t, J = 7.2 Hz, 3H),
1.47 (s, 9H), 1.62 (d, J = 6.9 Hz, 3H), 1.67–1.71 (m, 2H), 2.64–2.66
(m, 2H), 3.70 (s, 3H), 4.33–4.37 (m, 1H), 5.08–5.14 (brm, 2H), 7.63
(d, J = 9.6 Hz, 1H), 8.00 (s, 1H); 13C NMR (CDCl3) δ: 10.7, 21.6, 27.3,
28.3, 38.5, 47.6, 48.7, 51.7, 80.3, 123.0, 149.9, 154.9, 160.6, 171.9,
174.1.
7b
7i
Yield 82%. Colorless oil. Rf = 0.26 (EtOAc : Petroleum ether =
1 : 1); [α]20 = −7.3 (c = 1.1, MeOH); 1H NMR (CDCl3) δ: 1.35
Yield 80%. Colorless oil. Rf = 0.31 (EtOAc : Petroleum ether = 1 : 1);
1H NMR (CDCl3) δ: 0.93 (d, J = 6.6 Hz, 3H), 0.97 (d, J = 7.3 Hz, 3H),
0.99 (t, J = 7.0 Hz, 3H), 1.47 (s, 9H), 1.67–1.72 (m, 2H), 2.33–2.39
(m, 1H), 2.64 (dd, J = 5.8, 15.7 Hz, 1H), 2.67 (dd, J = 5.9, 16.0 Hz,
1H), 3.70 (s, 3H), 4.32–4.38 (m, 1H), 4.88 (dd, J = 5.5, 8.8 Hz, 1H),
5.19 (d, J = 8.8 Hz, 1H), 7.62 (d, J = 9.1 Hz, 1H), 8.00 (s, 1H); 13C
NMR (CDCl3) δ:10.7, 17.4, 19.3, 27.3, 28.3, 33.2, 38.5, 47.6, 51.7, 57.9,
80.2, 122.6, 150.1, 155.4, 160.7, 171.9, 172.5.
D
(d, J = 7.0 Hz, 3H), 1.47 (s, 9H), 1.61 (d, J = 7.0 Hz, 3H), 2.61
(dd, J = 5.9, 15.4 Hz, 1H), 2.68 (dd, J = 5.5, 15.7 Hz, 1H), 3.71
(s, 3H), 4.51–4.56 (m, 1H), 5.07–5.09 (m, 1H), 5.15 (br, 1H), 7.65
(d, J = 8.5 Hz, 1H), 8.00 (s, 1H); 13C NMR (CDCl3) δ: 20.2, 21.6,
28.3, 40.2, 42.1, 48.7, 51.7, 80.3, 123.1, 149.8, 154.9, 160.3, 171.8,
174.1; ESI-MS (m/z) calcd. [M+H]+ = 372.2, [M+Na]+ = 394.1,
[M+K]+ = 410.1, found 372.7, 394.6, 410.6.
7j
7c
Yield 85%. Colorless oil. 1H NMR (CDCl3) δ: 1.35 (d, J = 6.8 Hz, 3H),
1.47 (s, 9H), 1.61 (d, J = 6.4 Hz, 3H), 2.61 (dd, J = 15.6, 6.4 Hz, 1H),
2.69 (dd, J = 15.6, 5.0 Hz, 1H), 3.71 (s, 3H), 4.52–4.56 (m, 1H), 5.08
(br, 1H), 5.14 (br, 1H), 7.67 (d, J = 7.8 Hz, 1H), 8.01 (s, 1H); 13C NMR
(CDCl3) δ: 20.2, 21.6, 28.3, 40.1, 42.1, 48.7, 51.7, 80.4, 123.1, 149.8,
154.9, 160.3, 171.8, 174.1.
Yield 87%. Colorless oil. 1H NMR (CDCl3) δ: 0.93 (d, J = 6.8 Hz,
3H), 0.97–1.01 (overlapped, 6H), 1.47, (s, 9H), 1.68–1.72 (m, 2H),
2.36–2.39 (m, 1H), 2.62–2.68 (m, 2H), 3.70 (s, 3H), 4.33–4.37 (m,
1H), 5.15 (d, J = 8.7 Hz, 1H), 7.61 (d, J = 8.7 Hz, 1H), 7.99 (s, 1H);
13C NMR (CDCl3) δ: 10.7, 17.4, 19.3, 27.3, 28.3, 29.7, 33.2, 38.5, 47.6,
51.7, 57.9, 80.2, 122.7, 150.1, 155.4, 160.7, 171.9, 172.5.
c
J. Pept. Sci. 2011; 17: 533–539 Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jpepsci

ZHANG, DING AND LI
Synthesis of linear pentapeptides 8a–k, 8n–u: general procedure
room temperature and stirred for another 4 h. Residual TFA was
removed by successive addition and evaporation of CH2Cl2. The
residues were dissolved in dry THF (ca. 1 × 10−3 mol/l) and then
HATU (5 equiv.) and DIPEA (8 equiv.) were added successively. The
resultant mixtures were stirred for 3 days at room temperature
and concentrated in vacuo. The residues were dissolved in EtOAc
and washed with 10% citric acid, 5% NaHCO3, and brine. The
organic layers were dried over Na2SO4 and concentrated in vacuo.
The residues were purified by silica gel column (CHCl3 –MeOH)
followed by Pharmadex LH-20 (CHCl3 –MeOH = 1 : 1) to give
compounds 9a–u in 24–94% yields.
LiOH (2 equiv.) was added to the solutions of dipeptides 7a–j in
THF/MeOH/H2O (cat. 0.5 M) at 0 ◦C. The reaction mixtures were
warmed to room temperature and stirred until all the starting
material disappeared. After most of the solvent was evaporated,
thesolutionswereacidifiedtopH2 with1 M HClandextractedwith
EtOAc.Theorganicextractswerecombinedandwashedwithbrine.
Evaporation of the solvent gave the corresponding carboxylic
acids, which were dried under high vacuum for 2 h and then
dissolved in dry THF followed by addition of DIPEA (4 equiv.) and
2,4,6-trichlorobenzoyl chloride (3 equiv.). The reaction mixtures
were stirred at room temperature for 3 h before concentration to
dryness under argon. The residues were dissolved in dry toluene
and 0.9 equiv. of the alcohols (3e–f, 4a–b, 4c–d) and DMAP
(4 equiv.) were added. The mixtures were stirred for 3 h at
room temperature, diluted with EtOAc, and washed with 10%
citric acid, 5% NaHCO3, and brine. The organic layers were dried
(Na2SO4) and concentrated in vacuo. The residues were purified
by column chromatography (EtOAc–petroleum) to give the linear
pentapeptides.
All the cyclic (depsi)peptides gave satisfactory analytical and
spectroscopic data in full accord with their assigned structures.
Selected data of these compounds:
9d
Yield 45%. White foam. Rf = 0.55 (CHCl3:MeOH = 20 : 1);
19
[α]D = −83.7 (c = 0.3, MeOH); 1H NMR (CDCl3) δ: 0.59 (d,
J = 6.6 Hz, 3H), 0.88 (d, J = 6.6 Hz, 3H), 1.44 (d, J = 6.6 Hz, 3H),
1.47 (d, J = 7.0 Hz, 3H), 1.54 (d, J = 6.6 Hz, 3H), 2.26–2.33 (m,
1H), 2.72 (dd, J = 4.4, 16.9 Hz, 1H), 2.80–2.84 (m, 2H), 3.08 (s, 3H),
3.22 (s, 3H), 3.44 (dd, J = 8.8, 13.6 Hz, 1H), 4.41–4.44 (m, 1H), 5.02
(d, J = 10.6 Hz, 1H), 5.16 (dd, J = 5.9, 8.8 Hz, 1H), 5.32–5.37 (m,
1H), 5.44 (q, J = 6.6 Hz, 1H), 7.15–7.24 (m, 5H), 7.96 (s, 1H), 8.01
(d, J = 8.0 Hz, 1H), 8.91 (d, J = 9.1 Hz, 1H); 13C NMR (CDCl3) δ:
16.2, 18.7, 18.9, 20.4, 23.6, 27.6, 29.0, 30.1, 36.4, 39.6, 41.5, 46.6,
57.2, 60.9, 67.5, 122.2, 127.0, 128.7, 129.4, 136.3, 150.6, 160.1, 167.2,
168.9, 169.8, 170.4, 171.9; HRESIMS calcd. for C29H39N5O6NaS [M
+ Na]+ 608.2519, found 608.2517.
For compounds 8l and 8m, general coupling methods
(EDC/HOAt/DIPEA) were adopted.
All the linear pentapeptides gave satisfactory analytical and
spectroscopic data in full agreement with the assigned structures.
Selected data of these compounds:
8d
Yield 54%. White foam. Rf = 0.14 (EtOAc : Petroleum ether = 1 : 1);
27
[α]D = −109.5 (c = 0.11, MeOH); 1H NMR (CDCl3) δ: 0.76 (d,
J = 6.6 Hz, 3H), 0.90 (d, J = 6.2 Hz, 3H), 1.20 (d, J = 6.6 Hz, 3H),
1.33 (d, J = 7.0 Hz, 3H), 1.45 (s, 9H), 1.47 (s, 9H), 1.60 (d, J = 7.0 Hz,
3H), 2.21–2.27 (m, 1H), 2.44 (s, 3H), 2.62 (dd, J = 6.2, 15.4 Hz, 1H),
2.69 (dd, J = 5.5, 15.4 Hz, 1H), 2.79 (s, 3H), 2.90–2.95 (dd like,
overlapped, 1H), 3.38 (dd, J = 4.8, 15.4 Hz, 1H), 4.54–4.60 (m, 1H),
4.96 (d, J = 10.7 Hz, 1H), 5.09 (q, J = 7.0 Hz, 1H), 5.05–5.13 (m,
overlapped, 1H), 5.53 (dd, J = 4.4, 11.7 Hz, 1H), 7.15–7.29 (m, 5H),
7.55 (d, J = 8.5 Hz, 1H), 7.98 (s, 1H); 13C NMR (CDCl3) δ: 16.2, 17.7,
19.7, 20.1, 21.5, 26.9, 28.0, 28.3, 29.2, 31.5, 34.3, 40.2, 42.2, 48.7,
57.6, 58.3, 67.1, 80.4, 82.0, 123.0, 126.6, 128.4, 128.8, 137.2, 149.9,
154.9, 160.2, 169.7, 170.1, 170.5, 170.7; ESI-MS (m/z) calcd. [M +
Na]+ = 782.4, found 782.3.
9p
Yield 94%. white foam. 1H NMR (CDCl3) δ: 0.93 (d, J = 6.9 Hz,
3H), 0.95 (d, J = 6.8 Hz, 3H), 0.96 (t, J = 7.3, 7.8 Hz, 3H), 1.17
(d, J = 6.8 Hz, 3H), 1.47 (d, J = 6.8 Hz, 3H), 1.51–1.59 (m, 2H),
2.11–2.17 (m, 1H), 2.56 (dd, J = 3.9, 12.2 Hz, 1H), 2.71 (dd, J = 4.1,
12.4 Hz, 1H), 3.06 (dd, J = 5.0, 14.2 Hz, 1H), 3.61 (dd, J = 6.2,
14.0 Hz, 1H), 4.00 (t like, J = 5.0, 4.6 Hz, 1H), 4.41–4.46 (m, 1H),
4.79–4.81 (m, 1H), 5.09 (q, J = 6.9 Hz, 1H), 5.43 (m, 1H), 6.12 (d,
J = 8.7 Hz, 1H), 6.52 (d, J = 4.6 Hz, 1H), 7.20 (t, J = 7.3 Hz, 1H),
7.24–7.26 (m, 2H), 7.29–7.31 (m, 2H), 7.42 (d, J = 6.8 Hz, 1H), 8.03
(s, 1H), 9.08 (d, J = 10.1 Hz, 1H); 13C NMR (CDCl3) δ 11.3 (C-5),
17.3 (C-28), 18.0 (C-24), 19.2 (C-25), 24.3 (C-11), 26.3 (C-4), 29.6
(C-23), 36.9 (C-14), 38.2 (C-2), 47.7 (C-3), 47.9 (C-10), 53.4 (C-13),
60.3 (C-22), 69.5 (C-27), 123.2 (C-8), 127.3 (C-18), 128.8 (C-16, C-20),
129.5 (C-17, C-19), 136.5 (C-15), 150.3 (C-7), 160.7 (C-6), 169.3 (C-9),
170.0 (C-12), 170.2 (C-21), 170.7 (C-1), 173.2 (C-26); HRESIMS calcd.
for C28H37N5O6NaS [M + Na]+ 594.2362, found 594.2354.
8t
Yield 72%. White foam. Rf = 0.31 (EtOAc : Petroleum ether =
20
1
1 : 1); [α]D = −37.7 (c = 0.12, MeOH); H NMR (CDCl3) δ: 0.92
(br, overlapped, 6H), 0.98 (t, J = 7.4 Hz, 3H), 1.39 (s, 9H), 1.42 (d,
J = 6.4 Hz, 3H), 1.47 (s, 9H), 1.60 (d, J = 6.9 Hz, 3H), 1.66–1.73 (m,
2H), 2.15–2.16 (m, 1H), 2.68 (dd, J = 6.9, 16.0 Hz, 1H), 2.77 (dd,
J = 4.6,16.0 Hz,1H),3.05–3.06(ddlike,overlapped,2H),4.21–4.24
(m, 1H), 4.43–4.49 (m, 1H), 4.71–4.74 (m, 1H), 5.07–5.09 (m, 1H),
5.15–5.16 (m, 1H), 6.43 (d, J = 6.4 Hz, 1H), 7.07 (d, J = 8.2 Hz, 1H),
7.14–7.26 (m, 5H), 7.47 (d, J = 9.2 Hz, 1H), 7.98 (s, 1H); 13C NMR
(CDCl3) δ: 10.6, 18.1, 19.1, 19.2, 21.5, 27.8, 27.9, 28.3, 31.0, 38.0, 39.4,
47.4, 48.7, 53.5, 58.3, 71.2, 123.2, 127.0, 128.4, 129.4, 141.5, 149.6,
149.7, 151.6, 160.8, 170.0, 170.2.
Conclusion
In conclusion, 20 analogues of marine cyclic depsipeptide
obyanamidehavebeensynthesizedby[3+2]strategy. Preliminary
SAR studies show that the β-amino acid residue, the ester bond,
and the Ala(Thz) moiety were essential for biological activities.
However, the role of N-methylation of Val/Phe needs further
research.
Synthesis of target compound, cyclic (depsi)peptide 9a–u: general
Acknowledgement
procedure
This work was supported by the National Natural Science
Foundation of China (81001392) and School of Pharmacy, Fudan
University.
To the solutions of the linear pentapeptides 8a–u in dry CH2Cl2
was added TFA at 0 ◦C. The reaction mixtures were warmed to
c
wileyonlinelibrary.com/journal/jpepsci Copyright ꢀ 2011 European Peptide Society and John Wiley & Sons, Ltd. J. Pept. Sci. 2011; 17: 533–539

SYNTHESIS AND CYTOTOXICITY STUDIES OF THE ANALOGUES OF OBYANAMIDE
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