A convergent synthesis of the proposed structure of antitumor depsipeptide stereocalpin A

Article abstract of DOI:10.1021/ol900412u

The total synthesis of the proposed structure of anticancer agent stereocalpin A is described. The synthesis features a diastereoselective synthesis of a 5-hydroxy-2,4-dimethyl-3-oxooctanoic acid unit with asymmetric anti- and syn-aldol reactions as the k

Full text of DOI:10.1021/ol900412u

ORGANIC
LETTERS
2009
Vol. 11, No. 9
1963-1966
A Convergent Synthesis of the
Proposed Structure of Antitumor
Depsipeptide Stereocalpin A
Arun K. Ghosh* and Chun-Xiao Xu
Departments of Chemistry and Medicinal Chemistry, Purdue UniVersity, 560 OVal
DriVe, West Lafayette, Indiana 47907
akghosh@purdue.edu
Received February 27, 2009
ABSTRACT
The total synthesis of the proposed structure of anticancer agent stereocalpin A is described. The synthesis features a diastereoselective
synthesis of a 5-hydroxy-2,4-dimethyl-3-oxooctanoic acid unit with asymmetric anti- and syn-aldol reactions as the key steps. Initial cycloamidation
led to complete epimerization at the C-11 stereocenter due to unique steric constraints in the 12-membered depsipeptide ring. A late-stage
methylation strategy led to the synthesis of the proposed structure of stereocalpin A.
A variety of cyanobacterial metabolites have shown
diverse biological properties including antitumor, antibi-
otic, antiviral, antimycobacterial, analgesic, and antipyretic
properties.1 Stereocalpin A (1), a new cyclic depsipeptide,
was isolated from the dry lichen Ramalina terebrata of
Antarctica in 2008, by Oh et al.2 Initial testing for
cytotoxicity against solid tumor cell lines has shown good
activity against human colon carcinoma cell lines (HT-
29, IC50 ) 6.5 µM), human skin carcinoma cell lines
(B16F10, IC50 ) 11.9 µM), and human liver carcinoma
cell lines (HepG2, IC50 ) 13.4 µM). In addition, stereo-
calpin A displayed a protein tyrosine phosphatase 1B
(PTP1B) inhibitory activity in a dose-dependent manner
with an IC50 value of 40 µM. Further biological investiga-
tions could not be carried out because of lack of material.
The structure of stereocalpin A (1, Figure 1) was elucidated
by extensive use of NMR and HPLC analyses of derivatives.
Acid degradation (6 N HCl, 120 °C, 24 h) of 1 followed by
derivatization with Marfey’s reagent,3 and subsequent HPLC
analysis revealed that stereocalpin A contains a L-Phe, a L-N-
Me-Phe, and a 5-hydroxy-2,4-dimethyl-3-oxooctanoic acid
unit, which has not been previously reported as a component
of any natural product. The absolute configuration of the
octanoate derivative was resolved by a thorough analysis of
NOESY data. The unique structure, interesting biological
activity, and our interest in cyclic depsipeptides as antitumor
agents4 led us to explore the chemistry and biology of
stereocalpin A. Herein, we report our preliminary investiga-
tion leading to the stereoselective synthesis of the proposed
structure of stereocalpin A and 11-epi-stereocalpin A. The
present work suggested an incorrect assignment of the
reported structure for stereocalpin A.2 Furthermore, we
(1) (a) Gupta, R. R. In Marine Natural Products; Kiyota, H., Ed.;
Springer: New York, 2006 vol. 5. (b) Burja, A. M.; Banigs, B.; Abou-
Mansour, E.; Burgess, J. G.; Wright, P. C. Tetrahedron 2001, 57, 9347.
(2) Seo, C.; Kim, J. H.; Lee, H. K.; Park, S. M.; Sohn, J.-H.; Oh, H.
Tetrahedron Lett. 2008, 49, 29.
(3) Marfey, P. Carlsberg Res. Commun. 1984, 49, 51.
10.1021/ol900412u CCC: $40.75
Published on Web 04/08/2009
 2009 American Chemical Society

Scheme 1. Synthesis of Cyclization Precursor 12
Figure 1. Retrosynthesis of stereocalpin A.
NMR and 13C NMR analyses determined the anti-aldol
diastereoselectivity to be 37:1 for aldol adduct 7. Protection
of the resulting alcohol as a benzyl ether followed by
reductive cleavage of the chiral auxiliary with LAH afforded
alcohol 8 in 85% yield over two steps.6 We then employed
Evans’ syn-aldol reaction to install the C2 and C3-stereo-
centers of 5. Thus, Swern oxidation of 8 followed by syn-
aldol7 reaction with chiral imide 9 generated aldol product
10 in 89% yield (7:1 dr). Protection of alcohol 10 as its
MOM ether using MOMCl, DIPEA, and DMAP afforded
the corresponding MOM ether in 90% yield. The removal
of chiral imide with LiOOH at 0-23 °C for 12 h provided
the acid 11 in 65% yield. It was coupled with L-phenylalanine
methyl ester to provide the corresponding amide in 90%
yield.8 The benzyl protecting group was removed by catalytic
hydrogenation over Pearlman’s catalyst in a mixture (1:1)
of ethyl acetate and methanol. The resulting alcohol was
esterified with Cbz-N-Me-phenylalanine 4 using DCC and
DMAP to afford the key amino acid derivative 12 in 85%
yield.
As shown in Scheme 2, saponification of amine ester 12
with lithium hydroxide in tert-butyl alcohol/H2O at 0 °C for
2 h followed by exposure of the resulting acid to catalytic
hydrogenation over Pd(OH)2 afforded the corresponding
amino acid precursor for cycloamidation. Treatment of the
resulting amino acid with HATU (1.2 equiv) and HOAt
(2 equiv) in a mixture (5:1) of CH2Cl2 and DMF at 23 °C
for 24 h resulted in the cycloamide 13 as a single product in
95% yield.9 To complete the synthesis, the MOM group in
13 was deprotected using dimethylboron bromide.10 Oxida-
tion of the resulting alcohol with Dess-Martin periodinane11
observed that the proposed structure has an unusual propen-
sity to epimerize at the L-phenylalanine stereocenter.
As illustrated in Figure 1, our initial retrosynthetic analysis
of stereocalpin A led us to disconnect the peptide bond
between L-Phe and N-Me-Phe residues to provide amino acid
derivative 2. The C2 chiral center is located between the C1
amide and C3 ketone, making this chiral center prone to
epimerization. Therefore, we planned to keep the C3 ketone
as a protected alcohol that will be oxidized into a ketone at
the end of the synthesis. The acid 2 can be further
disconnected to provide the octanoic acid subunit 5, Cbz-
N-Me-Phe 4, and Phe-OMe 3. We planned to synthesize
subunit 5 using Ghosh’s TiCl4-promoted anti-aldol reaction
and Evans’ syn-aldol reaction as the key steps.
As depicted in Scheme 1, we utilized an ester-derived
titanium enolate-based highly diastereoselective anti-aldol
reaction to install the C4 and C5 stereocenters of stereocalpin
A.5 In a slightly modified protocol, tosylaminoindanol ester
6 was treated with TiCl4 (1.1 equiv) and diisopropylethy-
lamine (3.8 equiv) in CH2Cl2 at 23 °C for 2 h. Addition of
the resulting titanium enolate to the premixed n-butyralde-
hyde (2 equiv), TiCl4 (2 equiv), and MeCN (2 equiv) at -78
1
°C afforded the anti-aldol product 7 in 65% yield. The H
(4) (a) Ghosh, A. K.; Kulkarni, S. Org. Lett. 2008, 10, 3907. (b) Ghosh,
A. K.; Moon, D. K. Org. Lett. 2007, 9, 2425. (c) Ghosh, A. K.; Xu, X.
Org. Lett. 2004, 6, 2055. (d) Bai, R.; Covell, D. G.; Ghosh, A. K.; Liu, C.;
Hamel, E. J. Biol. Chem. 2002, 277, 32165. (e) Ghosh, A. K.; Liu, C. Org.
Lett. 2001, 3, 635. (f) Ghosh, A. K.; Liu, W.; Xu, Y.; Chen, Z. Angew.
Chem., Int. Ed. 1996, 35, 74.
(5) Ghosh, A. K.; Onishi, M. J. Am. Chem. Soc. 1996, 118, 2527.
1964
Org. Lett., Vol. 11, No. 9, 2009

amino acids during coupling reactions has been reported,13
a complete epimerization during a macrolactamization reac-
tion is unprecendented. The observed facile epimerization
is presumably due to developing steric compression between
the C2 methyl, C3 MOM group, and the C11 benzyl group
in the 12-membered cycloamide.
Scheme 2. Synthesis of 11-epi-Stereocalpin A
Assuming that the C2 methyl and C3 MOM groups may
be involved in facilitating the epimerization of L-phenyla-
lanine during cyclization, we decided to assemble the methyl
group at the C2 position at a later stage after the formation
of the 12-membered ring. As outlined in Scheme 3, Swern
Scheme 3. Synthesis of Stereocalpin A (Proposed Structure)
afforded 14, the presumed structure of stereocalpin A.
1
However, the H NMR and 13C NMR of 14 did not match
the reported data for the natural stereocalpin A.2 To our
surprise, the X-ray crystal structure of 14 (Figure 2)12
Figure 2. ORTEP drawing of compound 14.
revealed that the C11-stereocenter of L-phenylalanine had
completely epimerized to D-phenylalanine, as shown in
Scheme 2. For further confirmation of the structure, cycloa-
mide 14 was hydrolyzed with 6 N HCl at 110 °C for 24 h.
The resulting amino acids were exposed to Marfey’s reagent,3
and the resulting products were analyzed by HPLC which
confirmed the presence of D-phenylalanine and N-Me-L-
phenylalanine. To avoid this epimerization, other coupling
reagents, such as TBTU and BOP reagents, have been
examined. However, in each case, the same epimerization
product was obtained. Although partial epimerization of
oxidation of alcohol 8 followed by Mukaiyama aldol reaction
of the resulting aldehyde with ketene acetal 15 afforded the
corresponding aldol product,14 the ꢀ-hydroxy ester 16 in 72%
yield (5:1 dr). Saponification of methyl ester 16 with aqueous
LiOH and 30% H2O2 afforded the corresponding acid which
was coupled with L-phenylalanine methyl ester to provide
the ꢀ-hydroxy amide derivative. Dess-Martin oxidation11
of the resulting ꢀ-hydroxy amide furnished amide 17 in 71%
yield over two steps. Removal of the benzyl protecting group,
Org. Lett., Vol. 11, No. 9, 2009
1965

followed by esterification of the resulting alcohol with
N-Cbz-N-Me-phenylalanine, afforded amino acid derivative
18 in 89% yield. Saponification of ester 18 with LiOH in
aqueous tert-butyl alcohol, and removal of the Cbz group
by catalytic hydrogenation over Pd(OH)2 in a mixture (1:1)
of EtOAc and MeOH, afforded the corresponding amino
acid. The resulting amino acid was exposed to macrolac-
tamization conditions with HATU and HOAt in a mixture
of CH2Cl2 and DMF (5:1) at 23 °C for 36 h as described
above. The desired cycloamide 19 was formed in 47% yield,
along with 10% of the C11-epimer 20. Methylation of 19
was carried out stereoselectively in the presence of cesium
carbonate and methyl iodide in DMF for 24 h to afford the
proposed stereocalpin A (1) in 50% yield (90% based on
recovered starting material) as a single isomer. It appears
that the alkylation presumably proceeded from the less
hindered face, away from both benzyl and propyl substituents
on the 12-membered ring. Interestingly, however, the spectral
data of synthetic stereocalpin A (1) did not match with the
data reported for the natural stereocalpin A.2 Our detailed
structural analysis using 2D-NMR and NOESY of synthetic
1 fully supported our assignment of 1 as the proposed
structure of stereocalpin A. Therefore, our stereocontrolled
synthesis of the stereocalpin structure (1) now suggested that
the structure of natural stereocalpin A had been assigned
incorrectly. The comparison of NMR data is shown in the
supporting information.
now ascertain that the original assignment of the stereocalpin
A structure is incorrect. Interestingly, a combination of
substituents, their stereochemistry, and the developing steric
strain during the formation of a 12-membered depsipeptide
ring led to an unprecedented complete epimerization at the
C-11 amino acid center. Further investigation leading to the
assignment of stereocalpin A’s structure and structure-activity
studies are in progress.
Acknowledgment. Financial support in part by the
National Institutes of Health is gratefully acknowledged. We
thank Dr. Phillip E. Fanwick (Purdue University) for
assistance with the X-ray crystal structure analysis.
Supporting Information Available: Experimental pro-
cedures and 1H and 13C NMR spectra for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL900412U
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for compound 14. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
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In conclusion, we have developed a short synthesis of the
proposed structure of stereocalpin A. Our synthetic studies
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(7) Evans, D. A.; Fitch, D. M. J. Org. Chem. 1997, 62, 454.
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