Total synthesis of chloptosin, a potent apoptosis-lnducing cyclopeptide

Article abstract of DOI:10.1021/ol100135a

Chemical Equation Presentation A bidirectional total synthesis of chloptosin has been achieved in 16 operations (32 individual reactions) and 3% overall yield from the readily available materials. Palladium-catalyzed tryptophan synthesis, diastereoselective selenocyclization and oxidative deselenation successfully served as key steps In construction of the dimeric core amino acid. 2-Bromo-1-ethyl pyridinium tetrafluoroborate was efficiently employed In the peptide couplings with spatial encumbrance In this synthesis.

Full text of DOI:10.1021/ol100135a

ORGANIC
LETTERS
2010
Vol. 12, No. 5
1124-1127
Total Synthesis of Chloptosin, a Potent
Apoptosis-Inducing Cyclopeptide
Shun-Ming Yu, Wen-Xu Hong, Yuan Wu, Chun-Long Zhong, and Zhu-Jun Yao*
State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, China
yaoz@sioc.ac.cn
Received January 19, 2010
ABSTRACT
A bidirectional total synthesis of chloptosin has been achieved in 16 operations (32 individual reactions) and 3% overall yield from the readily
available materials. Palladium-catalyzed tryptophan synthesis, diastereoselective selenocyclization and oxidative deselenation successfully
served as key steps in construction of the dimeric core amino acid. 2-Bromo-1-ethyl pyridinium tetrafluoroborate was efficiently employed in
the peptide couplings with spatial encumbrance in this synthesis.
Two types of cell death have been generally identified in
metazoan animals. Among these, necrosis often triggers
an inflammatory response when the body suffers from
overwhelming cellular injuries. Apoptosis is the other
distinct programmed cell death, which is often associated
with characteristic morphological and biochemical changes.
Apoptosis plays an indispensable role during their devel-
opment, homeostasis, as well as in many diseases includ-
ing cancers.1 Many anticancer agents including adriamycin
and paclitaxel have been known to induce apoptosis in a
variety of cultured neoplastic cells.2 However, human
carcinoma cells are often resistant to apoptosis. This
phenotype may partly explain the poor therapeutic effects
of present cancer chemotherapies on most solid tu-
mors.
Chloptosin (1, Figure 1), a natural cyclopeptide isolated
as the third metabolite from the culture broth of the
MK498-98F14 strain of Streptomyces by Umzawa and
co-workers in 2000, was found to induce apoptotic activity
in the apoptosis-resistant human pancreatic adenocarci-
noma cell line AsPC-1 with an EC50 of 2.5 µg/mL after
24 h.3 Chloptosin (1) also strongly inhibited the growth
of Gram-positive bacteria, such as Staphylococcus, Mi-
crococcus and Bacillus strains (MIC range 0.20-0.78 µg/
mL), and its LD50 value was relatively high (50-100 mg/
kg) to the 4-week-old ICR male mice.3 Therefore, it shows
considerable prospects as a promising lead for drug
development and an excellent molecular tool for apoptosis
studies.
Chloptosin (1) demonstrates a rare dumbbells-shaped
C2-symmetrical bis-cyclopeptide architecture, which con-
tains a central core 3a-hydroxypyrrolo[2,3-b]indole di-
(1) (a) Steller, H. Science 1995, 267, 1445–1449. (b) Ashkenazi, A.;
Dixit, V. M. Science 1998, 281, 1305–1308.
(2) (a) Ling, Y.-H.; Priebe, W.; Perez-Soler, R. Cancer Res. 1993, 53,
1845–1852. (b) Haldar, S.; Chintapalli, J.; Croce, C. M. Cancer Res. 1996,
56, 1253–1255.
(3) Umezawa, K.; Ikeda, Y.; Uchihata, Y.; Naganawa, H.; Kondo, S. J.
Org. Chem. 2000, 65, 459–463.
10.1021/ol100135a  2010 American Chemical Society
Published on Web 02/05/2010

aryl-aryl couplings under relatively high temperatures in
the further synthesis, an alternative bidirectional strategy8
based on a Zinin rearrangement was thus developed in our
successful synthesis of the dimeric core amino acid derivative
2 (Figure 1) in 2006.7 In this communication, we report our
achievement in the total synthesis of chloptosin using a
bidirectional strategy.
Although the core amino acid derivative 3 (Figure 1)
suitable for further synthesis could be prepared after
several transformations from acid 2,7 relatively longer
steps and lower overall yield made it disadvantageous in
the material accumulation. Improvement of our previous
synthesis of this core amino acid7 was decided as the
starting point of this work. Because both reactants are
readily available, palladium-catalyzed heteroannulation9
of 2,2′-dichloro-5,5′-diiodobiphenyl-4,4′-diamine 47 and
L-pyroglutamic acid derivative 510 was then employed as
a shortcut to the requested bis-tryptophan intermediate 6a.
After careful optimization of reaction conditions, a mixture
of the needed dimeric tryptophan derivative 6a and meso-
product 6b was afforded in 42% combined yield (Scheme
1, 6a:6b > 4:1, measured by chiral HPLC). The corre-
Figure 1. Structure of chloptosin (1) and its unique core amino
acid (derivatives 2 and 3).
meric amino acid and several other unusual amino acids.
Structurally, it is similar to a previously reported natural
product himastantin, whose structure was revised via a
total synthesis by Danishefsky in 1998.4 Besides some
amino acid components, a major difference between these
two natural compounds is the central core amino acid.
Two additional chlorines are symmetrically embraced in
the diphenyl moiety of chloptosin (1) in this case. Though
a sound number of other natural products have been
isolated having the pyrroloindole architecture,5 the pres-
ence of a chlorine atom at the 6-position is apparently
rare. In addition, existence of two vicinal piperazine-3-
carboxylic acids in the cyclopeptide subunits further makes
chloptosin more spatially crowded than himastatin. All
these characteristics of the molecular architecture of
chloptosin including crowded spatial environment would
bring about some uncertain challenges during its total
synthesis, especially in the peptide couplings and the
macrocyclization.
Scheme 1. One-Step Synthesis of the Dimeric
6-Chlorotryptophan Derivatives 6a and 6b
For its unique structure and promising biological proper-
ties, chloptosin has attracted sound attentions in organic
synthesis. Several methodology developments6 and one
synthesis of the central amino acid7 have been reported in
recent years. Due to the presence of 6-chlorine, a number of
our initial attempts failed in the Csp2-Csp2 couplings to the
diphenyl core under various metal-catalyzed conditions.
Considering the stability of peptide substrates in those
sponding reaction with aldehyde 5′ gave much lower yield.
Diastereomers 6a and 6b are inseparable by silica gel
chromatography. Because of the poor resolution under
chiral HPLC conditions, the enantio purity of 6a was
finally dertemined after several further transformations
(see below text).
After treatment of the mixture of 6a/6b with Boc2O,
the resulting mixture of 7a/7b was then subjected to the
reaction with N-phenylselenophthalimide (N-PSP),11 pro-
(4) (a) Kamenecka, T. M.; Danishefsky, S. J. Angew. Chem., Int. Ed.
1998, 37, 2993–2995. (b) Kamenecka, T. M.; Danishefsky, S. J. Angew.
Chem., Int. Ed. 1998, 37, 2995–2998. (c) Kamenecka, T. M.; Danishefsky,
S. J. Chem.sEur. J. 2001, 7, 41–63.
(5) (a) Depew, K. M.; Marsden, S. P.; Zatorska, D.; Zatorski, A.;
Bornmann, W. G.; Danishefsky, S. J. J. Am. Chem. Soc. 1999, 121, 11953–
11963. (b) Nishida, A.; Ushigome, S.; Sugimoto, A.; Arai, S. Heterocycles
2005, 66, 181–185. (c) Iwaki, T.; Yamada, F.; Funaki, S.; Somei, M.
Heterocyles 2005, 65, 1811–1815. (d) Kawahara, M.; Nishida, A.; Naka-
gawa, M. Org. Lett. 2000, 2, 675–678. (e) Sun, W. Y.; Sun, Y.; Tang,
Y. C.; Hu, J. Q. Synlett 1993, 337–338.
(8) A review on the bidirectional synthesis, see: Poss, C. S.; Schreiber,
S. L. Acc. Chem. Res. 1994, 27, 9–17.
(6) (a) Hong, W.-X.; Yao, Z.-J. Chin. J. Chem. 2004, 22, 365–370. (b)
Kim, Y.-A.; Han, S.-Y. Synth. Commun. 2004, 34, 2931–2943.
(7) Hong, W. X.; Chen, L. J.; Zhong, C. L.; Yao, Z.-J. Org. Lett. 2006,
8, 4919–4922.
(9) Xia, Y. X.; Zhu, J. P. Synlett 2005, 16, 2469–2472.
(10) (a) August, R. A.; Khan, J. A.; Moody, C. M.; Young, D. W.
J. Chem. Soc., Perkin Trans. I. 1996, 507–514. (b) Johannesson, P. J. Med.
Chem. 1999, 42, 4524–4537.
Org. Lett., Vol. 12, No. 5, 2010
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viding inseparable pyrroloindoles 8a/8b in 89% yield
(Scheme 2). Oxidative deselenation of 8a/8b with excess
(BEP)12 was used as the coupling reagent in the presence of
HOAt and Pr2NEt (Scheme 3).
i
Scheme 3
.
Coupling of the Core Amino Acid 3 with the First
Amino Acid Derivative 11
Scheme 2
.
Improved Synthesis of the Central Amino Acid 3
Synthesis of the remaining tetrapeptide fragment 22 is
outlined in Scheme 4. Protection of the carboxylic acid 13
Scheme 4. Synthesis of the Tetrapeptide Fragment 22
i
of m-CPBA in the presence of aq NaHCO3 in PrOH
afforded the C2-symmetrical tertiary alcohol 9a (95.9%
ee, determined by chiral HPLC) in 75% yield and the other
meso-compound 9b in 19% yield after separation by
routine silica gel column chromatorgraphy. The N-Boc
protected compound 9a was then converted to the required
amine acid derivative 3 in 4 steps. Spectroscopic proper-
ties (1HNMR,13CNMR, IR, MS, and [R]D) of sample 3
obtained by this newly established route are in good
agreement with the one obtained from our previous syn-
thesis.7 Compared to our first synthesis, this short synthesis
greatly improves the overall yield of core amino acid and
therefore facilitates the material accumulation in our total
synthesis.
As our previous analysis, coupling of the core amino acid
derivative 3 with the remaining pentapeptide was surprisingly
difficult under various conditions. The initial attempts led
to the decomposition of pentapeptide substrates. Shortening
the pentapeptide to corresponding tetrapeptide, tripeptide, and
dipeptide could not improve the results. Using the active ester
or mixed anhydride of single amino acid threonine derivative
11 as the reactant, no desired product was given, either. When
a combination of HATU-HOAt-iPr2NEt was employed as the
coupling reagents, the double-coupling product 12 was
provided in 29% yield, along with the corresponding mono
product in 28% yield. The yield of 12 was finally improved
to 63% when 2-bromo-1-ethyl pyridinium tetrafluoroborate
with allyl bromide gave ester 14. Treatment of acid 1513
with oxalyl chloride (to give the corresponding acid chloride)
followed by reaction with amine 14 in the presence of
NaHCO3 provided the dipeptide 16.14 Removal of the
(11) Ley, S. V.; Cleator, E.; Hewitt, P. R. Org. Biomol. Chem. 2003, 1,
3492–3494.
(12) (a) Li, P.; Xu, J. C. Tetrahedron 2000, 56, 8119–8131. (b) Li, P.;
Xu, J. C. Chem. Lett. 2000, 204–205.
(13) (a) Hale, K. J.; Lazarides, L. Org. Lett. 2002, 4, 1903–1906. (b)
Hale, K. J.; Delisser, V. M.; Yeh, L.-K.; Peak, S. A.; Manaviazar, S.; Bhatia,
G. S. Tetrahedron Lett. 1994, 35, 7685–7688. (c) Hale, K. J.; Cai, J.;
Delisser, V.; Manaviazar, S.; Peak, S. A.; Bhatia, G. S.; Collins, T. C.;
Jogiya, N. Tetrahedron 1996, 52, 1047–1068.
(14) Carpino, L. A.; Cohen, B. J.; Stephens, K. E., Jr.; Sadat-Aalaee,
S. Y.; Tien, J. H.; Langridge, D. C. J. Org. Chem. 1986, 51, 3732–3734.
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i
N-Fmoc protecting group of 16 with Pr2NH in acetonitrile
of the protecting groups at both the C-terminus and N-
terminus of 23 led to the final linear precursor for the
macrolactamization. Treatment of this linear head-tail
“amino acid” with PyBOP-HOAt-iPr2NEt in CH2Cl2 and
DMF (10:1) at rt for 4 days successfully afforded the dimeric
cyclopeptide 25 in 45% yield.16 Both two O-TBS protecting
groups of cyclopeptide 25 were then removed with excess
TBAF (20 equiv) and HOAc (22 equiv) in THF.17 Further
deprotection of all four N-Cbz groups of the resultant material
failed either by hydrogenolysis in the presence of 10% Pd/C
in MeOH, or by that using HOAc as an additive. Fortunately,
this problem was resolved by hydrogenolysis (1 atm, rt) in
the presence of 20% Pd(OH)2 in MeOH, providing chloptosin
(1) in 80% yield (2 steps). All characterization data of the
synthetic chloptosin are in good agreement with those
reported for natural sample,3 inclduing the NMR spectra and
optical rotation comparsions. To the best of our knowledge,
few bidirectional total syntheses of cyclopeptides have been
reported in the literature. Our achievement provides a new
successful example to this category of total syntheses.
In summary, the first total synthesis of chloptosin, a potent
apoptosis-inducing cyclopeptide, has been achieved in 16
operations (32 individual reactions) of the longest linear
sequence and 3% overall yield from the readily available
material 4. A Pd-catalyzed tryptophan synthesis, a diaste-
reoselective selenocyclization, and an oxidative deselenation
successfully served as the key steps in the short bidirectional
synthesis of core amino acid in this work. In addition, the
coupling reagent 2-bromo-1-ethyl pyridinium tetraflouorbo-
rate shows great advantages in this synthesis through its
successful application to the peptide couplings with spatial
encumbrance.
gave the free amine 17, which was immediately acylated
with the valine acid chloride 18 in the presence of silver
cyanide15 to afford the tripeptide 19. Deprotection of O-allyl
group of 19 followed by coupling with amine 21 (for the
preparation of 21, see Supporting Information) afforded the
tetrapeptide 22 in a satisfactory yield.
With both dimeric precursor 12 and tetrapeptide 22 in
hand, further total synthesis of chloptosin was then performed
(Scheme 5). Removal of the O-allyl protecting group of 12
Scheme 5
.
Macrocyclization and Completion of the Total
Synthesis
Acknowledgment. Financial supports by MOST
(2010CB833200), MOH (2009ZX09501), NSFC (20621062),
CAS (KJCX2-YW-H08), and SMCST (08JC1422800) are
greatly appreciated. Z.J.Y. is a principle investigator of
e-Institute of Chemical Biology of Shanghai Universities.
Supporting Information Available: Experimental details
and charaterizations of new compounds, NMR copies of new
compounds, and HPLC analysis reports. This material is
available free of charge via the Internet at http://pubs.acs.org.
under mild Pd(PPh3)4-catalyzed conditions in THF afforded
the corresponding peptide acid, which was immediately
coupled with the peptide amine freshly prepared in a parallel
reaction from 22. This coupling successfully provided the
whole linear peptide 23 in 71% yield. Successive removal
OL100135A
(16) Grotenbreg, G. M.; Timmer, M. S.; Llamas-Saiz, A. L.; Verdoes,
M.; van der Marel, G. A.; van Raaij, M. J.; Overkleeft, H. S.; Overhand,
M. J. Am. Chem. Soc. 2004, 126, 3444–3446.
(17) Cao, B.; Park, H.; Joullie, M. M. J. Am. Chem. Soc. 2002, 124,
520–521.
(15) Hale, K. J.; Cai, J.; Williams, G. Synlett. 1998, 149–152.
Org. Lett., Vol. 12, No. 5, 2010
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