Asymmetric total synthesis of apratoxin D

Article abstract of DOI:10.1021/ol302309c

The first asymmetric total synthesis of the marine natural product apratoxin D, a highly potent inhibitor of H-460 human lung cancer cell growth (IC₅₀ value of 2.6 nM), is described. Asymmetric N-amino cyclic carbamate (ACC) α,α-bisalkylation was utilized to establish the isolated C-37 methyl group with excellent selectivity. Other key asymmetric transformations employed were an Evans syn-aldol and a Paterson anti-aldol, both of which also proceeded with excellent stereoselectivity.

Full text of DOI:10.1021/ol302309c

ORGANIC
LETTERS
2012
Vol. 14, No. 20
5192–5195
Asymmetric Total Synthesis of Apratoxin D
Bradley D. Robertson, Sarah E. Wengryniuk, and Don M. Coltart*
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
dcoltart@uh.edu
Received August 18, 2012
ABSTRACT
The first asymmetric total synthesis of the marine natural product apratoxin D, a highly potent inhibitor of H-460 human lung cancer cell growth
(IC50 value of 2.6 nM), is described. Asymmetric N-amino cyclic carbamate (ACC) R,R-bisalkylation was utilized to establish the isolated C-37
methyl group with excellent selectivity. Other key asymmetric transformations employed were an Evans syn-aldol and a Paterson anti-aldol, both
of which also proceeded with excellent stereoselectivity.
Apratoxin D (5) was recently isolated from two
species of cyanobacteria, L. majuscula and L. sordida.1
It exhibits highly potent in vitro cytotoxicity against
H-460 human lung cancer cells with an IC50 value of 2.6
nM. Lung cancer is the deadliest form of cancer for both
men and women and is responsible for 1.3 million
deaths worldwide, annually. In fact, more people die
of lung cancer than breast, colon, and prostate cancers
combined.2 Apratoxin D belongs to a family of cyclic
depsipeptides that also includes compounds 1À4
(Figure 1).3 All known apratoxins are potent inhibitors
of cancer cell growth.4 The potent biological activity
exhibited by the apratoxins in general, combined
with their intriguing molecular architecture, has drawn
attention from the synthetic community. To date, only
apratoxin A (1) has been prepared by total synthesis.
Forsyth was the first to complete the asymmetric total
synthesis of 1,5 and three other syntheses have since been
described.6,7 Herein, we report the first asymmetric total
synthesis of apratoxin D.
Our plan for the synthesis of apratoxin D is shown in
Scheme 1. Macrocyclization would be achieved by cou-
pling between the proline and isoleucine residues, thereby
avoiding late-stage esterification of the sterically congested
C-39 hydroxyl.5a Coupling of tripeptide 6 and carboxylic
acid 7 would set the stage for the macrocyclization event.
Formation of the thiazoline moiety of 7 would be achieved
by using Kelly’s procedure,8 which would require the pre-
paration of D-cysteine-derived intermediate 8and polyketide
fragment 9. Fragment 9would be reached from intermediate
10 utilizing a syn-selective Evans aldol addition to set the
C-39ÀC-40 stereochemistry, followed by a Paterson anti-
aldol addition to establish the C-34 and C-35 stereogenic
centers. The former transformation would also be leveraged
as a means to install the C-41 tert-butyl group, giving rise to
the neopentyl moiety. The neopentyl group of apratoxin D
is unique in comparison to all other known apratoxins.
(1) Gutierrez, M.; Suyama, T. L.; Engene, N.; Wingerd, J. S.;
Matainaho, T.; Gerwick, W. H. J. Nat. Prod. 2008, 71, 1099–1103.
(2) U.S. National Library of Medicine: PubMed Health. http://www.
ncbi.nlm.nih.gov/pubmedhealth/PMH0004529 (accessed Mar 1, 2012).
(3) (a) Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J.;
Corbett, T. H. J. Am. Chem. Soc. 2001, 123, 5418–5423. (b) Luesch,
H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J. Bioorgan. Med. Chem.
2002, 10, 1973–1978. (c) Matthew, S.; Schupp, P. J.; Luesch, H. J. Nat.
Prod. 2008, 71, 1113–1116.
(4) See also: Luesch, H.; Chanda, S. K.; Raya, R. M.; DeJesus, P. D.;
Orth, A. P.; Walker, J. R.; Belmonte, J. C. I.; Shultz, P. G. Nat. Chem.
Biol. 2006, 2, 159–167.
(5) (a) Chen, J.; Forsyth, C. J. J. Am. Chem. Soc. 2003, 125, 8734–
8735. (b) Chen, J.; Forsyth, C. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101,
12067–12072.
(6) (a) Doi, T.; Numajiri, Y.; Munakata, A.; Takahashi, T. Org. Lett.
2006, 8, 531–534. (b) Ma, D. W.; Zou, B.; Cai, G. R.; Hu, X. Y.; Liu, J. O.
Chem.;Eur. J. 2006, 12, 7615–7626. (c) Numajiri, Y.; Takahashi, T.;
Doi, T. Chem.;Asian J. 2009, 4, 111–125.
(7) Some analogues of apratoxin A have been synthesized; see: Chen,
Q.-Y.; Liu, Y.; Luesch, H. ACS Med. Chem. Lett. 2011, 2, 861–865.
(8) You, S. L.; Razavi, H.; Kelly, J. W. Angew. Chem., Int. Ed. 2003,
42, 83–85.
r
10.1021/ol302309c
Published on Web 10/11/2012
2012 American Chemical Society

Scheme 2. Synthesis of Aldehyde 10
The first phase of our synthesis of apratoxin D focused
on the preparation of aldehyde 10. This began with the
acid catalyzed condensation of commercially available
benzyloxyacetone (13) and ACC auxiliary 14 to give
ACC hydrazone 12 (Scheme 2).9,10 To set the required
(R)-configuration at the R-carbon of 15, compound 12 was
subjected to methylation, and the product of that reaction
was then allylated to give 11 as a single diastereomer.
Auxiliary removal provided ketone 15. Transformation of
the ketone function of 15 to the required methylene began
with LiAlH4 reduction, which was followed by conver-
sion of the resulting alcohols to xanthates 16. BartonÀ
McCombie reduction and LemieuxÀJohnson oxidation
then gave aldehyde 10 in very good overall yield.
Figure 1. Apratoxins.
Scheme 1. Retrosynthetic Analysis of Apratoxin D (5)
With an effective route to aldehyde 10 secured, we
undertook the preparation of advanced carboxylic acid 9
(Scheme 3). To do so, N-acyl oxazolidinone 17 and 10 were
engaged in an Evans syn-aldol addition.11 Silica gel chro-
matography of the product mixture gave the desired stereo-
isomer (18) in excellent yield, and this was then converted
to silyl ether 19. Reductive removal of the oxazolidinone
auxiliary provided alcohol 20. With 20 in hand, we at-
tempted to convert it to the tert-butylated compound 22 in
a variety of ways, none of which were suitably successful.
We then tried a new method reported by Terao and Kambe
for the cuprate-based alkylation of various electrophiles
(chlorides, bromides, tosylates).12 We were pleased to find
that these conditions worked remarkably well in the case
of tosylate 21, providing compound 22 reliably and in very
(9) Wengryniuk, S. E.; Lim, D.; Coltart, D. M. J. Am. Chem. Soc.
2011, 133, 8714–8720.
(10) (a) Lim, D.; Coltart, D. M. Angew. Chem., Int. Ed. 2008, 47,
5207–5210. (b) Krenske, E. H.; Houk, K. N.; Lim, D.; Wengryniuk,
S. E.; Coltart, D. M. J. Org. Chem. 2010, 75, 8578–8584. (c) Garnsey,
M. R.; Lim, D.; Yost, J. M.; Coltart, D. M. Org. Lett. 2010, 12, 5234–
5237. (d) Garnsey, M. R.; Matous, J. A.; Kwiek, J. J.; Coltart, D. M.
Bioorg. Med. Chem. Lett. 2011, 21, 2406–2409.
(11) Gage, J. R.; Evans, D. A. Org. Synth. 1990, 68, 83–91.
(12) Terao, J.; Todo, H.; Begum, S. A.; Kuniyasu, H.; Kambe, N.
Angew. Chem., Int. Ed. 2007, 46, 2086–2089.
At this stage, installation of the isolated C-37 methyl group
had to be considered. We decided to approach this using our
recently developed asymmetric N-amino cyclic carbamate
(ACC) R,R-bisalkylation method (12f11).9
Org. Lett., Vol. 14, No. 20, 2012
5193

Scheme 3. Synthesis of Carboxylic Acid 9
Scheme 4. Synthesis of Carboxylic Acid 7
Scheme 5. Completion of the Synthesis of Apratoxin D (5)
good yield. We next removed the TES protecting group
from 22 to enable incorporation of the C-39 proline residue
via the Yamaguchi procedure.13 This was followed by
hydrogenolysis of the benzyl ether to give alcohol 24.
Oxidation of 24 provided aldehyde 25, as required for the
planned anti-selective Paterson aldol using R-benzoyloxy
ketone 26.14 In the event, the aldol addition produced
compound 27 in both excellent yield and diastereoselec-
tivity. A sequence of TBS protection, benzoate hydrolysis,
and oxidation produced carboxylic acid 9.
As indicated above, we planned to generate the required
thiazoline moiety via the Kelly procedure, which, in this
instance, would be conducted on compound 30 (Scheme 4).
Preparation of 30 began with the coupling of carboxylic
acid 9 and compound 8. At this stage the Boc protecting
group on the proline residue was exchanged for an Fmoc
group to simplify the pending macrocyclization event
in a practical sense. In addition, the TBS group on the
C-37 hydroxyl was exchanged for a Troc group,5,6 giving
(13) Inanaga, J.; Hirata, K.; Katsuki, T.; Yamaguchi, M. Bull. Chem.
Soc. Jpn. 1979, 52, 1989–1993.
(14) Paterson, I.; Wallace, D. J.; Cowden, C. J. Synthesis 1998, 639–
652.
5194
Org. Lett., Vol. 14, No. 20, 2012

compound 30. Compound 30 smoothly underwent cycliza-
tion to produce the desired thiazoline (31) in excellent yield.
Removal of the Troc group from 31 was followed by
deprotection of the C-terminal carboxylic acid, providing
advanced intermediate 7.
material against a range of cancer cell lines, as well as
related SAR studies, are currently underway and will be
described in due course.
Acknowledgment. We are grateful to Professor Jeffrey
L. C. Wright and Allison K. Stewart (UNC Wilmington)
for their assistance with the purification of our synthetic
apratoxin D. We thank Dr. Mark C. Kohler (Duke
University) for his assistance in preparing compound 6
and Dr. Emily M. Tarsis, Dr. Guoqiang Zhou, and Insun
Chong (Duke University) for their assistance in conducting
certain experiments. B.D.R. holds a C. R. Hauser Fellow-
ship, and S.E.W., a Kathleen Zielek Fellowship from Duke
University. We also thank the NSF for a Graduate Fellow-
ship for S.E.W. and for additional support (NSF 1012287).
With compound 7 in hand, we embarked on the final
steps of the synthesis of apratoxin D (Scheme 5). Thus, 7
was coupled to the tripeptide 66a using HATU and Hunig’s
€
base, giving 33 in very good yield. At this stage, conversion
of the allyl ester to the corresponding carboxylic acid was
achieved by treatment with Pd(PPh3)4 and N-methylani-
line. Removal of the Fmoc moiety from the proline residue
was followed by HATU-mediated macrolactamization,
which successfully provided apratoxin D. The spectro-
metric data obtained for our synthetic material matched
in all respects with the data reported for its isolation.15
In conclusion, we have achieved the first asymmetric
total synthesis of apratoxin D, a compound that exhibits
highly potent cytotoxicity against H-460 human lung
cancer cells. Evaluation of the cytotoxicity of our synthetic
Supporting Information Available. Experimental pro-
cedures and analytical data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(15) See the Supporting Information for details.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 20, 2012
5195