Total synthesis of ciliatamides A-C: Stereochemical revision and the natural product-guided synthesis of unnatural analogs

Article abstract of DOI:10.1021/ol801842v

(Chemical Equation Presented) The first total synthesis of Ciliatamides A-C was completed, leading to a revision of the reported stereochemistry from (S,S) to the (R,R) enantiomers. Due to the expedited route, a library of over 50 unnatural ciliatamide analogs was also prepared.

Full text of DOI:10.1021/ol801842v

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4545-4548
Total Synthesis of Ciliatamides A-C:
Stereochemical Revision and the Natural
Product-Guided Synthesis of Unnatural
Analogs
Jana A. Lewis, R. Nathan Daniels, and Craig W. Lindsley*
Departments of Chemistry and Pharmacology, Institute of Chemical Biology,
Vanderbilt UniVersity, NashVille, Tennessee 37232
Craig.lindsley@Vanderbilt.edu
Received August 7, 2008
ABSTRACT
The first total synthesis of Ciliatamides A-C was completed, leading to a revision of the reported stereochemistry from (S,S) to the (R,R)
enantiomers. Due to the expedited route, a library of over 50 unnatural ciliatamide analogs was also prepared.
Leishmaniasis is a group of vector-borne diseases, caused
by obligate intramacrophage parasites of the genus Leish-
mania, which is endemic in the tropics.1,2 The disease may
manifest itself as either a cutaneous, a mucosal, a dermal,
or as the deadly visceral variant.3 Second only to malaria,
leishmaniasis afflicts more than 12 million people in 88
countries with an annual death toll exceeding 50,000.1-3 The
current treatments for leishmaniasis are the pentavalent
antimonials 1 and 2, humatin 3, amphotericin B 4, penta-
midine 5, and miltefosine 6 (Figure 1); however, these drugs
possess a number of drawbacks including cardiotoxic effects,
parenteral administration, long treatment regimens, and high
cost.1-3
Earlier this year, Nakao and co-workers reported on the
isolation of three lipopeptides, Ciliatamides A (7), B (8),
and C (9), from the deep sea sponge Aaptos ciliata as the
Figure 1. Structures of current antileishmaniasis treatments.
(1) Pitzer, K. K.; Werbovetz, K. A.; Brendle, J. J.; Scovill, J. P. J. Med.
Chem. 1998, 41, 4885–4889
(2) Sharief, A. H.; Khailil, E. A. G.; Theander, T. G.; Kharazmi, A.;
.
(S,S)-enantiomers (Scheme 1).4 Importantly, Ciliatamides A
and B demonstrated significant antileishmanial activity and,
Omer, S. A.; Ibrahim, M. E. Exp. Parasitol. 2006, 114, 247–252
.
(3) Chappuis, F.; Sundar, S.; Hailu, A.; Ghalib, H.; Rijal, S.; Peeling,
R. W.; Alvar, J.; Boelaert, M. Nat. ReV. Microbiol. 2007, 5, S7–S15.
10.1021/ol801842v CCC: $40.75
Published on Web 09/13/2008
2008 American Chemical Society

Despite this, the NMR spectra obtained were in complete
accord with those reported by Nakao and co-workers;
however, the original spectra were recorded under very dilute
conditions to minimize conformer populations (see Support-
ing Information).4
Scheme 1
.
Structures of Ciliatamides A (7), B (8), and C (9)
and Retrosynthesis
On the basis of these results, we then explored a solution
phase parallel synthesis approach for the synthesis of
Ciliatamides A (7) and C (9) employing only polymer-
supported reagents, scavengers and ion-exchange chroma-
tography to avoid either normal or reverse-phase chroma-
tography.6 This strategy afforded improved results, providing
7 and 9 in overall yields in excess of 75% for the three steps
with >95% purity (Scheme 3).5
Scheme 3
.
Solution Phase Parallel Synthesis of Ciliatamides A
(7) and C (9)
thus, appeared as ideal targets for total synthesis and further
biological evaluation as 7-9 are chemically far less complex
than current antileishmanials 1-6.1-4 The retro-synthesis of
7-9 (Scheme 1) involved cleavage of the two amide bonds
to provide the (S)-3-aminoazepan-2-one 10 (for 7 and 8) or
the (S)-3-aminopiperid-2-one 11 (for 9), L-N-methyl Phe 12
and either decenoic acid 13 (for 7 and 9) or octanoyl chloride
14 (for 8). Surprisingly, all of the requisite precursors were
commercially available.5
In the event, Boc-L-N-MePhe 15 was coupled to 10 with
PS-DCC and HOBt, followed by treatment with HCl in
dioxane to deliver the free base 19 in 59% yield for the two
steps after ion-exchange chromatography. 19 then underwent
a second amide coupling with 13 to afford 7, or with 14 to
afford 8 in 56 and 58% yields, respectively. Following the
same scheme (Scheme 2),5 but substituting (S)-3-aminopi-
Due to the expedited route to 7-9, in parallel, we prepared
a 42-member solution phase library of unnatural ciliatamide
analogs 20a-n, 21a-n and 22a-n. The library employed
three scaffolds 18, 19, and the unnatural (R,S) congener of
19, and a collection of 14 different acid chlorides (Figure
2). All final compounds, including additional copies of 7-9
were purified to >98% by mass-directed preparative HPLC
and afforded yields ranging from <5 to 60% for the three
steps; however, we obtained sufficient quantities for biologi-
cal evaluation in every case.5,7
Scheme 2. Synthesis of Ciliatamides A (7), B (8), and C (9)
As we were compiling the final characterization data for
7-9, a discrepancy was noted with respect to the reported
20
20
optical rotations, [R]D ) +40 (c ) 0.05, MeOH), [R]D
) +55 (c ) 0.1, MeOH), [R]D26 ) +74 (c ) 0.05, MeOH),
for the natural products 7-9, respectively. While the H
and 13C NMR spectra of our synthetic 7-9 overlaid with
the natural products, the optical rotations were of comparable
4
1
20
magnitude, but opposite sign, that is, [R]D ) -35 (c )
20
26
0.05, MeOH), [R]D ) -44 (c ) 0.1, MeOH), [R]D
)
-43 (c ) 0.1, MeOH) for synthetic 7-9, respectively.5 On
the basis of these results, we synthesized the four possible
stereoisomers ((S,S), (S,R), (R,S), and (R,R)) of Ciliatamide
A and Ciliatamide B, employing the route depicted in
Scheme 2, and compared NMR spectra and obtained optical
perid-2-one 10 for 11, provided Ciliatamide C (9) in 44%
yield over the three steps. The overall yields were lower than
anticipated due to the physiochemical properties of 7-9, and
poor chromatographic performance of these lipopeptides.
4546
Org. Lett., Vol. 10, No. 20, 2008

Figure 2
.
Three × 14 library of unnatural ciliatamide analogs.
rotations (Figure 3).5 For Ciliatamide A (7), reported to be
the (S,S)-enantiomer, the NMRs of diasteromeric pairs 23
(S,R) and 24 (R,S), as anticipated, did not match 7; however,
the (R,R)-enantiomer 25 was in complete accord with the
published spectral data and possessed an optical rotation
([R]D20 ) +42 (c ) 0.05, MeOH)) that matched the literature
report as well. Similarly, the (R,R)-enantiomer of Ciliatamide
B (28) and overlaid with the reported NMR spectra of 8 as
well as provided optical rotations in agreement with those
reported by Nakao and co-workers, ([R]D20 ) +49 (c ) 0.1,
MeOH)) for 28.
Figure 3. Library of all possible sterosiomers of Ciliatamides A-C
and the corresonding optical rotations.
utilized Marfey’s analysis8 to establish the L-configuration
Attempts to prepare the (S,R), (R,S) and (R,R) stereoiso-
mers of Ciliatamide C following the route depicted in
Scheme 2 led to significant racemization of the (R)-piperidin-
2-one, which was not observed within the Ciliatamide A and
B series or the (S)-3-aminopiperdin-2-one 11. Therefore, after
several approaches, we developed an alternate route that
avoided racemization and afforded pure stereoisomers (Scheme
4). In this instance, the carbodimide coupling in Scheme 2
was replaced with a HATU/collidine system for the coupling
of pure (S)- or (R)-piperidin-2-one, 10 and 29, respectively,
with either enantiopure Boc-L-N-MePhe 15 or Boc-D-N-
MePhe 30. The milder acidolysis of the Boc deprotection of
31-33 with 5-7% TFA in CH2Cl2 in an ice bath instead of
10 equiv of HCl was used to deliver isomers 34-36. Finally,
a second HATU coupling with decenoic acid provided the
remaining stereoisomers of Ciliatamide C 37 (S,R), 38 (R,S),
and 39 (R,R). As in the case of Ciliatamides A and B, both
of the caprolactams and the MePhe in 7-9; however, the
Scheme 4. Synthesis of the (S,R), (R,S), and (S,S)-stereosiomers
of Ciliatamide C, 37, 38, and 39, respectively
20
NMR spectra and optical rotation ([R]D ) +56 (c ) 0.1,
MeOH)) for 39 confirm a stereochemical reassignment of
the natural product to the (R,R)-enantiomer.
For Ciliatamides A-C, the optical rotations were positive,
and in agreement with the literature report, only when the
unnatural D-MePhe was employed. Nakao and co-workers
data presented herein suggests that either the reported optical
rotations were incorrect, or the stereochemical assignments
Org. Lett., Vol. 10, No. 20, 2008
4547

were incorrect.4 It is also possible that the natural products,
under the forcing acidic conditions of the Marfey’s analysis8
racemized. We have noted that the (R,R)-analogs are prone
to acid catalyzed racemization to the (S,S)-enantiomers. Thus,
based on our data, we propose a stereochemical revision for
Ciliatamides A-C from the (S,S)-7, 8, and 9 to the (R,R)-
25, 28, and 39 (Figure 4).
molecular targets, as we have done previously for other
marine natural products,9 for Ciliatamides A-C that might
afford a mechanisitic understanding of their antileishmanial
activities.
In summary, we have completed the first total synthesis
of Ciliatamides A-C, originally reported as as the (S,S)-
enantiomers 7-9, employing both traditional organic syn-
thesis and solution phase parallel synthesis. Based on spectral
and optical rotation data for all the possible stereoisomers
of 7-9, we propose a stereochemical revision for Ciliata-
mides A-C to the (R,R)-enantiomers 25, 28, and 39 respec-
tively. Due to the expedited route, we also prepared a library
of 42 unnatural cilatamide analogs 20a-n, 21a-n, and
22a-n, and when combined with the unnatural stereosi-
omers, the library exceeds 50 unnatural analogs. Assay
development and biological evaluation for both the natural
products and the unnatural ciliatamides are in progress and
will be reported in due course.
Acknowledgment. We are very grateful to Professor
Nakao, University of Tokyo, for the generous supply of NMR
spectra of the natural products, Ciliataides A-C. This work
was supported, in part, by the Department of Pharamacology,
Vanderbilt University, and J.A.L. is supported by a training
grant from NIH (T90DA022873). Funding for the NMR
instrumentation was provided in part by a grant from NIH
(S10 RR019022).
Figure 4. Revised structures of the natural lipopeptides Ciliatamides
A-C after stereochemical revision.
Supporting Information Available: Experimental pro-
cedures, characterization data, and 1H and 13C NMR spectra
for all new compounds, 7-9, 16-19, 23-28, 31-39, and a
cross section of library members 20a-n, 21a-n, and 22a-n.
This material is available free of charge via the Internet at
http://pubs.acs.org.
We are in the process of developing an antileishmanial
assay, which we hope will further validate the stereochemical
reassignment of Ciliatamides A-C based on biological
activity. However, once a validated assay is in place, we
will evaluate all of the unnatural analog ciliatamide libraries
in an attempt to develop structure-activity relationships
(SAR). We are also in the process of identifying discrete
OL801842V
(6) Kennedy, J. P.; Williams, L.; Bridges, T. M.; Daniels, R. N.; Weaver,
D.; Lindsley, C. W. J. Comb. Chem. 2008, 10, 345–354.
(7) Leister, W. K.; Strauss, K.; Wisnoski, D. D.; Zhao, Z.; Lindsley,
C. W. J. Comb. Chem. 2003, 5, 322–329.
(4) Nakao, Y.; Kawatsu, S.; Okamoto, C.; Okamoto, M.; Matsumoto,
Y.; Matsunaga, S.; van Soest, R. W. M.; Fusetani, N. J. Nat. Prod. 2008,
71, 469–472.
(8) Merfey, P. Carlsberg Res. Commun. 1984, 49, 591–596.
(9) Kennedy, J. P.; Brogan, J. T.; Lindsley, C. W. J. Nat. Prod. In
press.
(5) For full experimental details, see Supporting Information.
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Org. Lett., Vol. 10, No. 20, 2008