Synthesis and Ag(I) complexation studies of tethered westiellamide

Article abstract of DOI:10.1021/ol060677c

A new tethered macrocyclic ring system based on the natural product westiellamide was prepared to increase the affinity and ease of complexation to Ag(I) ions. NMR and fluorescence Ag(I) titrations confirmed that the tethered macrocycles preserve the uniq

Full text of DOI:10.1021/ol060677c

ORGANIC
LETTERS
2006
Vol. 8, No. 11
2381-2384
Synthesis and Ag(I) Complexation
Studies of Tethered Westiellamide
Peter Wipf* and Chenbo Wang
Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
pwipf+@pitt.edu
Received March 18, 2006
ABSTRACT
A new tethered macrocyclic ring system based on the natural product westiellamide was prepared to increase the affinity and ease of complexation
to Ag(I) ions. NMR and fluorescence Ag(I) titrations confirmed that the tethered macrocycles preserve the unique, high-affinity coordination
mode of westiellamide.
The marine genus Lissoclinum has been a rich source of
cyclopeptide alkaloids featuring multiple oxazolines, thia-
zolines, oxazoles, or thiazoles.1 Several of these macrocyclic-
heterocyclic scaffolds have been found to complex to metal
ions,2 in some cases with unique selectivities.2o The direct
biological relevence of these metal ion binding properties,
however, is still unresolved.2k,n,p
Previously, we reported the first example of a silver-ion-
selective Lissoclinum cyclopeptide interaction.2o Westiell-
amide (1) was found to bind to Ag(I) salts in a unique
[(1)2Ag4]4+ complex (Figure 1). An association constant (Ka)
(1) For reviews on the chemistry of Lissoclinum peptides, see: (a) Wipf,
P. In Alkaloids: Chemical and Biological PerspectiVes; Pelletier, S. W.,
Ed.; Elsevier: Amsterdam, 1998; Vol. 12, p 187. (b) Wipf, P. Chem. ReV.
1995, 95, 2115. (c) Hamada, Y.; Shioiri, T. Chem. ReV. 2005, 105,
4441.
(2) (a) van den Brenk, A. L.; Tyndall, J. D. A.; Cusack, R. M.; Jones,
A.; Fairlie, D. P.; Gahan, L. R.; Hanson, G. R. J. Inorg. Biochem. 2004,
98, 1857. (b) Bernhardt, P. V.; Comba, P.; Fairlie, D. P.; Gahan, L. R.;
Hanson, G. R.; Lotzbeyer, L. Chem.-Eur. J. 2002, 8, 1527. (c) Morris, L.
A.; Milne, B. F.; Thompson, G. S.; Jaspars, M. J. Chem. Soc., Perkin Trans.
2 2002, 1072. (d) Cusack, R. M.; Grøndahl, L.; Fairlie, D. P.; Gahan, L.
R.; Hanson, G. R. J. Chem. Soc., Perkin Trans. 2 2002, 556. (e) Morris, L.
A.; Jaspars, M.; Kettenes-van den Bosch, J. J.; Versluis, K.; Heck, A. J.
R.; Kelly, S. M.; Price, N. C. Tetrahedron 2001, 57, 3185. (f) Cusack, R.
M.; Grøndahl, L.; Abbenante, G.; Fairlie, D. P.; Gahan, L. R.; Hanson, G.
R.; Hambley, T. W. J. Chem. Soc., Perkin Trans. 2 2000, 323. (g) Morris,
L. A.; Jaspars, M. Royal Soc. Chem. 2000 257, 140. (h) Blake, A. J.;
Hannam, J. S.; Jolliffee, K. A.; Pattenden, G. Synlett 2000, 1515. (i)
Grøndahl, L.; Sokolenko, N.; Abbenante, G.; Fairlie, D. P.; Hanson, G. R.;
Gahan, L. R. J. Chem. Soc., Dalton Trans. 1999, 1227. (j) Comba, P.;
Cusack, R.; Fairlie, D. P.; Gahan, L. R.; Hanson, G. R.; Kazmaier, U.;
Ramlow, A. Inorg. Chem. 1998, 37, 6721. (k) Freeman, D. J.; Pattenden,
G.; Drake, A. F.; Siligardi, G. J. Chem. Soc., Perkin Trans. 2 1998, 129.
(l) van den Brenk, A. L.; Fairlie, D. P.; Gahan, L. R.; Hanson, G. R.;
Hambley, T. W. Inorg. Chem. 1996, 35, 1095. (m) van den Brenk, A. L.;
Byriel, K. A.; Fairlie, D. P.; Gahan, L. R.; Hanson, G. R.; Hawkins, C. J.;
Jones, A.; Kennard, C. H. L.; Moubaraki, B.; Murray, K. S. Inorg. Chem.
1994, 33, 3549. (n) van den Brenk, A. L.; Fairlie, D. P.; Hanson, G. R.;
Gahan, L. R.; Hawkins, C. J.; Jones, A. Inorg. Chem. 1994, 33, 2280. (o)
Wipf, P.; Venkatraman, S.; Miller, C.; Geib, S. J. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1516. (p) Michael, J. P.; Pattenden, G. Angew. Chem., Int.
Ed. Engl. 1993, 32, 1. (q) van den Brenk, A. L.; Hanson, G. R.; Hawkins,
C. J. J. Inorg. Biochem. 1989, 36, 165.
Figure 1. Westiellamide (1, left) and the crystal structure of the
[(1)2Ag4][ClO4]4 complex.
of g2.8 × 1013 M-5 was determined by NMR titration.
Tethered cyclopeptides, or “peptide cylinders”, have been
(3) (a) van Maarseveen, J. H.; Horne, W. S.; Ghadiri, M. R. Org. Lett.
2005, 7, 4503. (b) Pattenden, G.; Thompson, T. Chem. Commun. 2001,
717. (c) Clark, T. D.; Ghadiri, M. R. J. Am. Chem. Soc. 1995, 117, 12364.
10.1021/ol060677c CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/04/2006

Scheme 1
to the trans-oxazoline configuration.7 Condensation of the
carboxylic acid derived from dipeptide 5 with the primary
amine derived from tetrapeptide 10 in the presence of DPPA
gave the desired hexapeptide 11 in 80% yield. Upon cleavage
of the Cbz group and saponification of the methyl ester,
DPPA-mediated macrolactamization in a Na2CO3-NaHCO3
buffer8 gave the desired cyclopeptide 12 in 60% yield. The
TBDPS group was removed with TBAF to provide alcohol
13. The X-ray analysis of the p-bromobenzoyl ester of 13
confirmed its configuration and demonstrated an aza-crown
ether macrocycle conformation analogous to that of uncom-
plexed westiellamide (Figure 3).9 Acylation of 13 with pent-
4-enoyl chloride gave 4, which was dimerized by alkene
Figure 2. Ligand design and retrosynthetic analysis.
reported as macromolecular devices or scaffolds for protein
mimics.3 We were also encouraged to continue our studies
of the [(1)2Ag4][ClO4]4 system because of the noteworthy
antimicrobial and imaging properties of silver complexes.4
To improve the binding affinity and simplify the stoichiom-
etry of macrocycle-silver chelation, we designed new
tethered cyclopeptide ligands (2, Figure 2). By tethering two
westiellamide molecules, we expected the entropy loss of
the binding process to be greatly reduced, thus increasing
the Ka.
Scheme 2
The westiellamide analogue 4 can be prepared from
dipeptidyl oxazolines 5 and 6.5 The efficient synthesis of
building block 5 is summarized in Scheme 1. TBDPS
protection of the known compound 76 was followed by a
switch of the N-protective group from Boc to Cbz. Saponi-
fication of the methyl ester provided 8, which was subse-
quently coupled to D-threonine methyl ester with isobutyl
chloroformate (IBCF) and N-methylmorpholine (NMM). The
dipeptidyl oxazoline 5 was formed in 79% yield by the
treatment of the dipeptide 9 with Burgess reagent.7
The synthesis of segment 10 was achieved by deprotection
of 68 followed by diphenylphosphoryl azide (DPPA)-
mediated dimerization (Scheme 2). Under the conditions of
methyl ester saponification, R-epimerization converts the cis-
(4) (a) Makarava, N.; Parfenov, A.; Baskakov, I. V. Biophys. J. 2005,
89, 572. (b) Holt, K. B.; Bard, A. J. Biochemistry 2005, 44, 13214. (c)
Melaiye, A.; Simons, R. S.; Milsted, A.; Pingitore, F.; Wesdemiotis, C.;
Tessier, C. A.; Youngs, W. J. J. Med. Chem. 2004, 47, 973. (d) Aymonier,
C.; Schlotterbeck, U.; Antonietti, L.; Zacharias, P.; Thomann, R.; Tiller, J.
C.; Mecking, S. Chem. Commun. 2002, 3018. (e) Balogh, L.; Swanson, D.
R.; Tomalia, D. A.; Hagnauer, G. L.; McManus, A. T. Nano Lett. 2001, 1,
18.
(5) The benzyl side chain, rather than a simple valine-like â-methyl
substituent, was selected to facilitate the synthesis and improve UV
visualization.
(6) Ndungu, J. M.; Gu, X.; Gross, D. E.; Cain, J. P.; Carducci, M. D.;
Hruby, V. J. Tetrahedron Lett. 2004, 45, 4139.
(7) Wipf, P.; Miller, C. P. Tetrahedron Lett. 1992, 33, 907.
(8) Wipf, P.; Miller, C. P.; Grant, C. M. Tetrahedron 2000, 56, 9143.
2382
Org. Lett., Vol. 8, No. 11, 2006

Figure 4. Fluorophore for the competitive binding study.
experiments, westiellamide and 15 exhibited considerably
higher Ka’s of 2.8 ( 1.9 × 1022 M-5 and 3.5 ( 3.9 × 1021
M-4, respectively.14 The competitive binding experiment with
a fluorophore represents a more sensitive tool for the
determination of high association constants. Most impor-
tantly, both analytical methods demonstrate that the tether
considerably improves potential practical applications of the
cyclooxazoline scaffold for silver complexation and delivery
in the low micromolar range. For example, 50% complex-
ation of a 10 µM solution of Ag(I) would require 270 µM
of westiellamide vs 2 µM of 15.
Figure 3. Stereoview of the X-ray structure of the p-bromobenzoyl
ester of 13.
metathesis in the presence of catalyst 1410 to provide the
dimer as a mixture of cis/trans isomers (Scheme 3). Finally,
Scheme 3
Figure 5 shows a stereoview for a model of the silver
complex of 15.15 The overall folding of the macrocyclic aza-
crown ring system and all heteroatom-Ag bond lengths
are close to those in the westiellamide-silver complex, and
the linker chain assumes an extended zigzag conforma-
tion.
the tethered ligand 15 was obtained in quantitative yield by
hydrogenation.
The complexation of 15 with Ag(I) perchlorate was first
studied by NMR titration in a fashion analogous to
westiellamide.2o As expected, a stoichiometry of 4:1 (Ag+/
15) was determined, along with a Ka g 1.8 ( 1.4 × 1011
M-4 in CD3OD and D2O (9:1, v/v). Previously, NMR
titration of the natural product 1, which binds in a 4:2 (Ag+/
1) stoichiometry, provided a Ka g 2.8 × 1013 M-5.2o
Obviously, the silver complexation with 1 is more sensitive
to receptor concentration. However, in both cases, the binding
process is highly cooperative and no intermediate states were
observed during the titration. Moreover, because of the high
silver affinity of both receptors and the relatively high
concentrations required for analyte detection, NMR titration
only provides a lower limit of the association constant.11
Accordingly, the Ka was also determined by a competitive
binding study12 with fluorophore 16 (Figure 4).13 In these
Figure 5. Computer-generated stereomodel of the [(15)Ag4]4+
complex. The model was obtained by a conformational search using
CONFLEX/MM3 followed by AM1/COSMO minimization of the
lowest-energy structure in CaChe 6.1.
We also briefly examined the effect of the tether length
on Ag binding. Acylation of 13 with adipoyl chloride
provided ligand 17, which has two fewer methylene groups
in the tether chain than 15 (Scheme 4). As expected, on the
(12) Connors, K. A. Binding Constants, The Measurement of Molecular
Complex Stabiliy; John Wiley & Sons: New York, 1987; p 175.
(13) Kang, J.; Choi, M.; Kwon, J. Y.; Lee, E. Y.; Yoon, J. J. Org. Chem.
2002, 67, 4384.
(14) The error range is relatively large due to the limited dynamic range
of the fluorescence titration method (see Supporting Information).
(15) CaChe 6.1; Fujitsu Computer Systems, BioSciences Group: West-
wood, MA. For the use of CaChe to model supramolecular clusters, see,
for example: Caulder, D. L.; Brueckner, C.; Powers, R. E.; Koenig, S.;
Parac, T. N.; Leary, J. A.; Raymond, K. N. J. Am. Chem. Soc. 2001, 123,
8923.
(9) Hambley, T. W.; Hawkins, C. J.; Lavin, M. F.; van den Brenk, A.;
Watters, D. J. Tetrahedron 1992, 48, 341.
(10) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18.
(11) See, for example: Shi, Y.; Schneider, H.-J. J. Chem. Soc., Perkin
Trans. 2 1999, 1797.
Org. Lett., Vol. 8, No. 11, 2006
2383

(determined by competitive binding with 16) than 15. In a
similar fashion, 13 was cross-linked with pyridine-2,6-
dicarboxylate to give ligand 18. However, NMR titration of
18 showed a complex binding pattern to Ag(I), presumably
due to the shortness or rigidity of the tether, or additional
silver-pyridine interactions.
Scheme 4
In summary, we have demonstrated the feasibility of im-
proving the unique affinity of the natural product westiell-
amide toward silver ions by structural modifications. The
tethered ligands 15 and 17 show higher affinities toward Ag-
(I) than westiellamide and improve the potential for practical
applications of cyclic oligooxazolines in Ag(I) detection,
transport, detoxification, and targeted delivery.16 The biologi-
cal profile of these complexes is currently under investigation
and will be reported in due course.
Acknowledgment. This work was supported by a grant
from the National Institutes of Health (GM-55433). We thank
Dr. Steve Geib (University of Pittsburgh) for X-ray crystal-
lographic analysis of the p-bromobenzoate of 13.
Supporting Information Available: Experimental pro-
cedures and spectral data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL060677C
basis of the reduction in entropy loss during complexation,
17 exhibited a slightly higher Ka of 3.20 ( 2.98 × 1022 M-4
(16) Silver, S.; Phung, L. T. J. Ind. Microbiol. Biotechnol. 2005, 32,
587.
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Org. Lett., Vol. 8, No. 11, 2006