Total synthesis and cytotoxicity studies of a cyclic depsipeptide with proposed structure of palau'amide

Article abstract of DOI:10.1021/ol051685g

(Chemical Equation Presented) Total synthesis of a cyclic depsipeptide with proposed structure of palau'amide is achieved, which features a stereoselective elaboration of its 5,7-dihydroxy-2,6-dimethyldodec-2-en-11-ynoic acid unit. The synthetic compound

Full text of DOI:10.1021/ol051685g

ORGANIC
LETTERS
2005
Vol. 7, No. 19
4237-4240
Total Synthesis and Cytotoxicity Studies
of a Cyclic Depsipeptide with Proposed
Structure of Palau’amide
Bin Zou, Kai Long, and Dawei Ma*
State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, China
madw@mail.sioc.ac.cn
Received July 18, 2005
ABSTRACT
Total synthesis of a cyclic depsipeptide with proposed structure of palau’amide is achieved, which features a stereoselective elaboration of
its 5,7-dihydroxy-2,6-dimethyldodec-2-en-11-ynoic acid unit. The synthetic compound has potent cytotoxicity against three tumor cell lines but
different rotation and NMR data compared to those reported for palau’amide, which implies that its conformation is close to that of palau’amide
but that some stereochemistry in palau’amide was misassigned.
Palau’amide is a cyclic depsipeptide that was isolated by
Moore and co-workers from a species of the marine
cyanobacterium Lyngbya, which showed potent cytotoxicity
to KB cells (IC50 ) 13 nM).1 From this source, several potent
antitumor agents such as lyngbyabellins2 and apratoxins3
have been discovered. These compounds have become the
focus of recent synthetic endeavors.4,5
lished as depicted in Figure 1. However, the stereochemistry
of C-37 awaited further confirmation since it could not be
rigorously established by chemical means. More interestingly,
Moore and co-workers found that there was a structurally
related minor component accompanying with palau’amide,
which showed even more potent cytotoxicity (IC50 ) 1 nM
against KB cells), but its structure was unclear because
insufficient material was available for adequate NMR
characterization.1 These unsolved structural problems, to-
gether with the remarkable biological activity displayed by
these two compounds, make palau’amide an attractive target
for total synthesis.
During investigations on the total synthesis and structure-
activity relationship (SAR) of some biologically important
cyclopeptides,5c,6 we became interested in development of a
flexible synthetic protocol to palau’amide. Our retrosynthetic
analysis for this molecule is shown in Figure 1. Macrocy-
By combination of NMR studies and chiral HPLC analysis
of the degradation of palau’amide, its structure was estab-
(1) Williams, P. G.; Yoshida, W. Y. Quon, M. K.; Moore, R. E.; Paul,
V. J. J. Nat. Prod. 2003, 66, 1545.
(2) (a) Luesch, H.; Yioshida, W. Y.; Moore, R. E.; Paul, V. J.; Moobery,
S. L. J. Nat. Prod. 2000, 63, 611. (b) Luesch, H.; Yioshida, W. Y.; Moore,
R. E.; Paul, V. J. J. Nat. Prod. 2000, 63, 1437.
(3) (a) Leusch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J.; Corbett,
T. H. J. Am. Chem. Soc. 2001, 123, 5418. (b) Leusch, H.; Yoshida, W. Y.;
Moore, R. E.; Paul, V. J. Bioorg. Med. Chem. 2002, 10, 1973.
(4) (a) Yokokawa, F.; Sameshima, H.; Shioiri, T. Tetrahedron Lett. 2001,
42, 4171. (b) Yokokawa, F.; Sameshima, H.; Katagiri, D.; Aoyama, T.;
Shioiri, T. Tetrahedron 2002, 58, 9445.
(5) (a) Chen, J.; Forsyth, C. J. J. Am. Chem. Soc. 2003, 125, 8734. (b)
Chen, J.; Forsyth, C. Proc. Natl. Acad. Sci. U.S.A., 2004, 101, 12067. (c)
Zou, B.; Wei, J.; Cai, G.; Ma, D. Org. Lett. 2003, 5, 3503. (d) Xu, Z.;
Chen, Z.; Ye, T. Tetrahedron: Asymmetry 2004, 15, 355.
(6) (a) Xie, W.; Zou, B.; Pei, D.; Ma, D. Org. Lett. 2005, 7, 2775. (b)
Yu, S.; Pan, X.; Lin, X.; Ma, D. Angew. Chem., Int. Ed. 2005, 44, 135. (c)
Zhu, J.; Ma, D. Angew. Chem., Int. Ed. 2003, 42, 5348. (d) Ma, D.; Wu,
Q. Tetrahedron Lett. 2001, 42, 5279. (e) Ma, D.; Wu, Q. Tetrahedron Lett.
2000, 41, 9089.
10.1021/ol051685g CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/11/2005

Scheme 1
Figure 1. Proposedstructure of palau’amide and its retrosynthetic
analysis.
clization was chosen at the Ile-Gly site because it is
sufficiently unhindered to allow a smooth peptide formation
reaction. Further bond disconnection revealed that a stereo-
selective elaboration of the 5,7-dihydroxy-2,6-dimethyldodec-
2-en-11-ynoic acid (Dddd) part was required. We decided
to assemble it using a vinylogous Mukaiyama aldol reaction7
as the key step.
The detailed synthesis for Dddd unit is outlined in Scheme
1. Initially, we planned to employ Oppolzer’s “anti” al-
dolization strategy to build the C37-C44 unit in the target
molecule.8 Unfortunately, reaction of 5-hexynal with (1S)-
2,10-camphorsultam-derived N-propionylsultam 2 under
standard conditions did not afford any condensation products.
However, we were pleased to find that following the
Oppolzer’s procedure for “syn” aldolization,8 5-hexynal could
react with (1R)-2,10-camphorsultam-derived N-propionyl-
sultam 4, providing aldol adduct 5 in 90% yield. This
difference might be caused by some side reactions between
TiCl4 and 5-hexynal since the N-propionylsultam was
recovered and 5-hexynal disappeared completely in the
former case.
Reduction of 5 with LAH followed by selective protection
with TBSCl produced alcohol 6, which was subjected to
Mitsunobu inversion9 to afford alcohol 7 with the desired
stereochemistry. Treatment of 7 with TBSCl to protect the
secondary hydroxyl group and subsequent selective cleavage
of silyl ether of the primary hydroxyl group with pyridine
hydrofluoric acid salt delivered alcohol 8. Next, Swern
oxidation of 8 provided an aldehyde, which was reacted with
(E)-(2-methylbuta-1,3-dienyloxy)-trimethylsilane10 mediated
with boron trifluoride diethyl etherate to furnish 9a in 65%
yield, together with its diastereomer 9b in 5% yield.7c
Oxidation of aldehydes 9 with NaClO2 provided the corre-
sponding acids, which were masked with an allyl group to
give esters 10. The stereochemistry at the newly created
sterogenic center in 9a was assumed to be R, which resulted
from a 1,3-asymmetric induction based on a model proposed
by Evans and co-workers.11 The assumption was further
confirmed using a method reported by Rychnovsky and co-
workers.12 Accordingly, cleavage of the silyl ether in 10
(7) (a) Mukaiyama, T.; Banno, K.; Narasaka, K. J. Am. Chem. Soc. 1974,
96, 7503. (b) Paterson, I.; Smith, J. D. J. Org. Chem. 1992, 57, 3261. (c)
Paterson, I.; Smith, J. D.; Ward, R. A. Tetrahedron 1995, 51, 9413. (d)
Mutou, T.; Suenaga, K.; Fujita, T.; Itoh, T.; Takada, N.; Hayamizu, K.;
Kigoshi, H.; Yamada, K. Synlett. 1997, 199. (e) Grieco, P. A.; Speake, J.
D. Tetrahedron Lett. 1998, 39, 1275.
(10) Krohn, K.; Tolkiehn, K.; Lehne, V.; Schmalle, H. W.; Gru¨tzmacher,
H.-F. Liebigs Ann. Chem. 1985, 1311.
(8) Oppolzer, W.; Lienard, P. Tetrahedron Lett. 1993, 34, 4321.
(9) Mitsunobu, O. Synthesis 1981, 1.
(11) Evans, D. A.; Duffy, J. L.; Dart, M. J. Tetrahedron Lett. 1994, 35,
8537.
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Org. Lett., Vol. 7, No. 19, 2005

produced diols, which were protected with DMP to afford
1,3-diol acetonides 11. Since the C(2)-acetals in 11a and 11b
had 13C chemical shifts of 100.75 and 98.00 and differences
between two 13C methyl signals in 11a and 11b were 0.01
and 12.42, they should have anti-acetonide and syn-acetonide
moieties, respectively.
Scheme 3
Scheme 2
With alcohol 10a in hand, we attempted an esterification
reaction of 10a and 12 accompanied by stereochemistry
inversion using Mitsunobu’s method.9 However, under
several standard conditions such as using Ph3P/DEAD or
Ph3P/DIAD as activators and THF or toluene as solvent, no
desired product 13 was isolated. In contrast, esterification
of 10a and 12 using Yamaguchi’s method13 provided ester
14 in good yield.
On the basis of the above observations, we adjusted our
synthetic plan by running stereochemistry inversion prior to
esterification. For this purpose the oxidation/reduction
strategy was considered. As demonstrated in Scheme 3, after
the aldehyde 9a was oxidized to the corresponding acid,
coupling with D-leucine-derived alcohol 15 was carried out
to afford ester 16. Dess-Martin oxidation of 16 followed
by NaBH4 reduction delivered protected syn-1,3-diol 17 as
a single product. Next, coupling of 17 with the acid 12
mediated with 2,4,6-trichlorobenzoyl chloride and diisopro-
pylethylamine in benzene resulted in ester 18. The allyl
protecting group in 18 was removed via palladium chemis-
try14 to yield an acid, which was connected with a liberated
amine from tripeptide 19 to deliver amide 20. Finally,
sequential liberation of allyl ester and Fmoc-protected amine
moieties in 20 with Pd(Ph3P)4/NMA and diethylamine
followed by macrocyclization with HATU produced a cyclic
peptide in 30% yield, which was treated with 5% HF in
acetonitrile to furnish 115 in 83% yield.
MeOH)), which clearly ruled out the proposed structure for
palau’amide. For H NMR data, the major difference came
1
from the proton signals of C-4. Compound 1 has a chemical
shift of 3.16, whereas the reported one is 3.36, which implied
that the stereochemistry of its surrounding amino acid
residues were misassigned. Similar problems have often been
observed.16 In addition, the stereochemistry of C-37 might
also cause the above difference if it was misassigned.
The synthetic 1 displayed potent cytotoxicity against Hela,
A549, and BGC cell lines with IC50 values of 39, 19, and
26 nM, respectively. Although different tumor cells were
used, these data indicated that compound 1 has a similar
potency against tumor cells as palau’amide and implied that
(15) Selected data for 1: [R]20 ) +18.3 (c 0.15, MeOH); 1H NMR
D
(500 MHz, CD3OD) δ 7.65 (m, 1H), 7.55 (m, 1H), 7.24 (t, J ) 7.3 Hz,
1H), 7.15 (d, J ) 7.0 Hz, 2H), 7.10 (t, J ) 6.8 Hz, 2H), 6.76 (m, 1H), 5.47
(dd, J ) 5.7, 9.8 Hz, 1H), 5.05 (m, 1H), 4.93 (dd, J ) 10.1, 13.7 Hz, 1H),
4.90 (m, 1H), 4.51 (q, J ) 7.0 Hz, 1H), 4.21 (d, J ) 18.7 Hz, 1H), 3.80
(m, 1H), 3.48 (m, 1H), 3.16 (s, 3H), 3.11 (d, J ) 18.6 Hz, 1H), 3.03 (s,
3H), 3.02 (dd, J ) 9.7, 14.7 Hz, 1H), 2.95 (m, 1H), 2.91 (s, 3H), 2.82 (m,
1H), 2.58 (m, 1H), 2.20 (t, J ) 2.5 Hz, 1H), 2.19 (m, 2H), 1.93 (s, 3H),
1.86 (m, 1H), 1.84 (m, 1H), 1.81 (m, 1H), 1.79 (m, 1H), 1.58 (m, 2H),
1.53 (m, 2H), 1.49 (d, J ) 7.5 Hz, 3H), 1.39 (m, 1H), 1.37 (m, 1H), 1.33
(m, 1H), 0.99 (d, J ) 7.3 Hz, 3H), 0.94 (t, J ) 7.4 Hz, 3H), 0.91 (d, J )
3.5 Hz, 3H), 0.89 (d, J ) 6.7 Hz, 3H), 0.85 (d, J ) 7.1 Hz, 3H), 0.83 (d,
J ) 7.2 Hz, 3H); ESI-MS m/z 852 (M + H)+, 874 (M + Na)+; HRMS
calcd for C46H69N5O10SiNa (M + Na)+ requires 874.4937, found 874.4938.
(16) For recent examples, see: (a) Xu, Z.; Peng, Y.; Ye, T. Org. Lett.
2003, 5, 2821. (b) Peng, Y.; Pang, H.; Ye, T. Org. Lett. 2004, 6, 3781.
Unfortunately, both rotation ([R]20 ) +18.3 (c 0.15,
D
MeOH)) and NMR data for synthetic 1 were not in agreement
with those reported for palau’amide ([R]23 ) -22 (c 0.4,
D
(12) Rychnovsky, S. D.; Rogers, B.; Yang, G. J. Org. Chem. 1993, 58,
3511.
(13) (a) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989. (b) Hikota, M.; Sakurai, Y.; Horita,
K.; Yonemitsu, O. Tetrahedron Lett. 1990, 31, 6367.
(14) Ciommer, M.; Kunz, H. Synlett 1991, 593.
Org. Lett., Vol. 7, No. 19, 2005
4239

it might share a similar conformation when interact with their
target biomolecule.
Natural Science Foundation of China (grants 20321202 and
20373074), and Science and Technology Commission of
Shanghai Municipality (grant 04DZ14901) for their financial
support.
In conclusion, we have developed a facile route to
palau’amide analogues and found that the previous assign-
ments for stereochemistry were improper. Further investiga-
tions on elaborating more analogues to establish the real
structure of this natural product, as well as exploring their
SAR, are in progress and will be reported in due course.
Supporting Information Available: Experimental pro-
cedures and characterizations for compounds 5-9, 11, 12,
16-20, and 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. The authors are grateful to the Chinese
Academy of Sciences (grant KGCX2-SW-209), National
OL051685G
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Org. Lett., Vol. 7, No. 19, 2005