Efficient route to C2 symmetric heterocyclic backbone modified cyclic peptides

Article abstract of DOI:10.1021/ol0518028

(Chemical Equation Presented) A tandem dimerization-macrocyclization approach using 1,3-dipolar azide-alkyne cycloaddition reactions has been employed in the facile and convergent solution phase syntheses of C₂ symmetric cyclic peptide scaffold

Full text of DOI:10.1021/ol0518028

ORGANIC
LETTERS
2005
Vol. 7, No. 20
4503-4506
Efficient Route to C2 Symmetric
Heterocyclic Backbone Modified Cyclic
Peptides
Jan H. van Maarseveen, W. Seth Horne, and M. Reza Ghadiri*
Departments of Chemistry, Molecular Biology, and The Skaggs Institute for
Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road,
La Jolla, California 92037
ghadiri@scripps.edu
Received July 28, 2005
ABSTRACT
A tandem dimerization
−
macrocyclization approach using 1,3-dipolar azide
−
alkyne cycloaddition reactions has been employed in the facile
and convergent solution phase syntheses of C2 symmetric cyclic peptide scaffolds bearing triazole
E
2-amino acids as dipeptide surrogates.
Hydrogen bond-directed self-assembly of appropriately de-
signed cyclic peptides is an effective method for the prep-
aration of organic nanotubes1,2 for applications in the solid
state, solution phase, membrane environments, and biological
settings.3 Although the structural and functional properties
of self-assembling peptide nanotubes can often be modulated
by varying the cyclic peptide sequence and ring size, their
functional repertoire can be significantly expanded using
cyclic architectures bearing unnatural amino acids or alterna-
tive backbone structures.2 We have recently explored the
utility of 1,4-disubstituted 1,2,3-triazole ꢀ-amino acids as
trans-amide dipeptide surrogates and reported the high-
resolution structural features of these substitutions in the
context of R-helical coiled-coils,4 rigid â-turn scaffolds,5 and
extended â-sheet-like hollow tubular assemblies.2h Consider-
ing that triazole ꢀ-amino acids can be conveniently synthe-
sized via Cu(I)-catalyzed 1,3-dipolar cycloaddition6 between
(1) Peptide nanotubes based on cyclic D,L-R-peptides: (a) Ghadiri, M.
R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich, N. Nature
1993, 366, 324-327. (b) Hartgerink, J. D.; Granja, J. R.; Milligan, R. A.;
Ghadiri, M. R. J. Am. Chem. Soc. 1996, 118, 43-50. (c) Rosenthal-Aizman,
K.; Svensson, G.; Unden, A. J. Am. Chem. Soc. 2004, 126, 3372-3373.
(d) For a review, see: Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M.
R. Angew. Chem., Int. Ed. 2001, 40, 988-1011.
(2) Peptide nanotubes based on other cyclic peptide architectures: (a)
Karle, I. L.; Handa, B. K.; Hassall, C. H. Acta Crystallogr., Sect. B
1975, 31, 555-560. (b) Seebach, D.; Matthews, J. L.; Meden, A.; Wessels,
T.; Baerlocher, C.; McCusker, L. B. HelV. Chim. Acta 1997, 80, 173-
182. (c) Clark, T. D.; Buehler, L. K.; Ghadiri, M. R. J. Am. Chem. Soc.
1998, 120, 651-656. (d) Ranganathan, D.; Lakshmi, C.; Karle, I. L. J.
Am. Chem. Soc. 1999, 121, 6103-6107. (e) Gauthier, D.; Baillargeon,
P.; Drouin, M.; Dory, Y. L. Angew. Chem., Int. Ed. 2001, 40, 4635-4638.
(f) Semetey, V.; Didierjean, C.; Briand, J.-P.; Aubry, A.; Guichard, G.
Angew. Chem., Int. Ed. 2002, 41, 1895-1898. (g) Amorin, M.; Castedo,
L.; Granja, J. R. J. Am. Chem. Soc. 2003, 125, 2844-2845. (h) Horne,
W. S.; Stout, C. D.; Ghadiri, M. R. J. Am. Chem. Soc. 2003, 125, 9372-
9376.
(3) (a) Ghadiri, M. R.; Granja, J. R.; Buehler, L. K. Nature 1994, 369,
301-304. (b) Granja, J. R.; Ghadiri, M. R. J. Am. Chem. Soc. 1994, 116,
10785-10786. (c) Motesharei, K.; Ghadiri, M. R. J. Am. Chem. Soc. 1997,
119, 11306-11312. (d) Fernandez-Lopez, S.; Kim, H.-S.; Choi, E. C.;
Delgado, M.; Granja, J. R.; Khasanov, A.; Kraehenbuehl, K.; Long, G.;
Weinberger, D. A.; Wilcoxen, K. M.; Ghadiri, M. R. Nature 2001, 412,
452-456. (e) Sanchez-Quesada, J.; Kim, H. S.; Ghadiri, M. R. Angew.
Chem., Int. Ed. 2001, 40, 2503-2506. (f) Couet, J.; Jeyaprakash, J. D.;
Samuel, S.; Kopyshev, A.; Santer, S.; Biesalski, M. Angew. Chem., Int.
Ed. 2005, 44, 3297-3301. (g) Horne, W. S.; Wiethoff, C. M.; Cui, C.;
Wilcoxen, K. M.; Amorin, M.; Ghadiri, M. R.; Nemerow, G. R. Bioorg.
Med. Chem. 2005, 13, 5145-5153.
(4) Horne, W. S.; Yadav, M. K.; Stout, C. D.; Ghadiri, M. R. J. Am.
Chem. Soc. 2004, 126, 15366-15367.
(5) Horne, W. S. Ph.D. Thesis, The Scripps Research Institute, La Jolla,
CA, 2005.
10.1021/ol0518028 CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/31/2005

amino acid-derived azides and alkynes, we wished to explore
whether C2 symmetric triazole-substituted cyclic peptides,
useful in the fabrication of self-assembling peptide nanotubes,
could be efficiently synthesized in solution via tandem di-
merization-macrocyclization cycloaddition reactions (Scheme
1).7 We reported recently that peptide 1 has high affinity
that the backbone conformational preference imposed by the
alternating R,S-amino acid chirality would kinetically favor
formation of the desired cyclic structure, which is also the
smallest unconstrained cyclic product possible. This route
was expected to provide a facile entry into a series of self-
assembling pseudo-hexapeptide scaffolds via an expedient
solution phase method.
We set out initially to resynthesize previously characterized
cyclic peptide 1 in order to evaluate the viability of the
proposed route and its efficiency relative to the macrolac-
tamization method (Scheme 1). Addition of copper(I) iodide,
diisopropylethylamine, and 2,6-lutidine to a millimolar
solution of 1d in acetonitrile led to the gradual formation of
a white precipitate from an initially homogeneous reaction
mixture. Analysis of the reaction mixture by RP-HPLC
indicated formation of cyclic pseudo-hexapeptide 1 as the
major product (>90%) together with small amounts of the
cyclic trimer and larger oligomers. Despite the excellent yield
and product selectivity, the insolubility of the reaction
products severely complicated isolation of 1. Self-assembling
cyclic peptides lacking ionizable side chains generally exhibit
low solubility due to their inherent propensity to form large
tubular assemblies. Although this property is desirable from
the nanotube supramolecular design perspective, it neverthe-
less adds practical difficulty to product isolation. To alleviate
this shortcoming, we took advantage of an amide backbone
N-alkylation strategy which has been shown to prevent
formation of nanotubular aggregates by breaking the network
of extended intermolecular backbone-backbone hydrogen
bonding.8 Our synthetic strategy thus was modified to employ
peptide precursors bearing N-(2,4-dimethoxy)benzyl (Dmb)
amide substituents.9
Scheme 1. Alternative Routes to Cyclic Peptide 1
a HPLC yield; bratio of cyclic dimer (1) to cyclic trimer by HPLC.
for self-association in solution and peptide nanotube forma-
tion in the solid state.2h The original synthesis of 1 was
accomplished in 43% overall yield by employing Fmoc-
protected ꢀ2-amino acid 1c in the solid phase peptide syn-
thesis of the linear peptide precursor, which was subsequently
The Dmb backbone modifications were expected to render
the peptides highly soluble in most typical organic solvents
and be readily removed under strongly acidic conditions.
Subjecting N-Dmb-substituted azido-alkyne 1a (Scheme 2)
to a similar reaction condition used in the tandem dimer-
ization-macrocyclization of 1d gave a homogeneous reac-
tion mixture that was >90% cyclic product 1b by HPLC
analysis. The improved solubility of the heterocyclic peptide
allowed facile purification by preparative HPLC providing
80% isolated yield of 1b. The Dmb groups were removed
by treatment with TFA/TMSOTf/anisole (8:1:1) to afford free
peptide 1, which could be isolated quantitatively by trituration
upon addition of diethyl ether to the reaction mixture.
Similarly, azido-alkynes 2a and 3a were used to prepare
heterocyclic products 2 and 3 in good isolated yields and
product selectivity (Scheme 2), suggesting the likely general-
ity of this approach.
Figure 1. Structures of (a) a dipeptide and (b) a triazole ꢀ-amino
acid dipeptide surrogate.
cyclized via macrolactamization to afford the desired ring
structure (Scheme 1, top route). We envisioned an alternative
strategy for the synthesis of macrocycle 1 (Scheme 1, bottom
route) using azido-alkyne functionalized peptide 1d via 1,3-
dipolar cycloaddition reactions involving intermolecular
dimerization followed by an intramolecular ring forming
reaction.7 Although this approach could, in principle, produce
a number of linear and cyclic oligomers, we hypothesized
The use of Dmb substituents and the resulting enhanced
solubility of heterocyclic peptides also enabled facile postsyn-
thetic modifications (Scheme 3). For example, cyclic peptide
(6) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem., Int. Ed. 2002, 41, 2596-2599. (b) Tornoe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057-3064.
(7) (a) Bodine, K. D.; Gin, D. Y.; Gin, M. S. J. Am. Chem. Soc. 2004,
126, 1638-1639. (b) Billing, J. F.; Nilsson, U. J. J. Org. Chem. 2005, 70,
4847-4850. (c) Punna, S.; Kuzelka, J.; Wang, Q.; Finn, M. G. Angew.
Chem., Int. Ed. 2005, 44, 2215-2220.
(8) (a) Ghadiri, M. R.; Kobayashi, K.; Granja, J. R.; Chadha, R. K.;
McRee, D. E. Angew. Chem., Int. Ed. Engl. 1995, 34, 93-95. (b) Clark, T.
D.; Buriak, J. M.; Kobayashi, K.; Isler, M. P.; McRee, D. E.; Ghadiri, M.
R. J. Am. Chem. Soc. 1998, 120, 8949-8962.
(9) (a) Weygand, F.; Steglich, W.; Bjarnaso, J.; Akhtar, R.; Khan, N.
M. Tetrahedron Lett. 1966, 3483-3487. (b) Blaakmeer, J.; Tijsseklasen,
T.; Tesser, G. I. Int. J. Pept. Protein Res. 1991, 37, 556-564.
4504
Org. Lett., Vol. 7, No. 20, 2005

Scheme 2. Synthesis of C2 Symmetric Heterocyclic Peptides
Scheme 3 Postsynthetic Modification of Heterocyclic Peptides
a Ratio of the desired cyclic dimer to cyclic trimer by analytical
HPLC.
4 having p-carboxyphenylalanine side chains could be readily
prepared starting from azido-alkyne 4a bearing a p-iodo-
phenylalanine residue by its conversion to 4b using copper
with a tris(benzyltriazolylmethyl)amine ligand10 and sub-
sequent Pd0-catalyzed carboxylation of the aryl iodides
with acetic anhydride and formate.11 Selective removal of
the side chain protective groups in 2b and 3b by treatment
with TFA in the absence of TMSOTf led to soluble cyclic
architectures that could be further derivatized at their
periphery. Accordingly, the Boc-protected amines in het-
erocyclic peptide 3b were deprotected and then reacted with
4′-substituted terpyridyl carboxylic acid12 to afford, after
Dmb group removal, peptide 5 for potential use in the design
of metallopeptide nanotubular assemblies. In another ex-
ample, the side chains of heterocyclic peptides 2b and 3b
were first selectively deprotected in the presence of TFA
and then condensed by dimerization via macrolactamization
to afford tricyclic peptide 6. Dmb removal yielded pep-
tide 7, which represents a covalently captured minimal
repeating motif of a self-assembling heterocyclic peptide
nanotube.2h,13
a Product isolated as a mixture of dimer and trimer.
It should be noted that the approach of tandem dimeriza-
tion-macrocyclization can also be generalized for the
expedient synthesis of unsymmetrical cyclic peptide se-
quences by taking advantage of the statistical product
distribution arising from the reaction of two different linear
azido-alkynes. As an example, subjecting an equimolar
mixture of peptides 1a and 3a to the standard reaction
conditions gave a 1:1 mixture of homo- and heterodimeric
products from which 8 was readily isolated in 31% yield.
This approach could provide a rapid and convergent route
(10) Chan, T. R.; Hilgraf, R.; Sharpless, K. B.; Fokin, V. V. Org. Lett.
2004, 6, 2853-2855.
(11) Cacchi, S.; Fabrizi, G.; Goggiamani, A. Org. Lett. 2003, 5, 4269-
(13) (a) Clark, T. D.; Ghadiri, M. R. J. Am. Chem. Soc. 1995, 117, 12364-
12365. (b) Clark, T. D.; Kobayashi, K.; Ghadiri, M. R. Chem.sEur. J.
1999, 5, 782-792.
4272.
(12) Fallahpour, R. A. Synthesis 2000, 1138-1142.
Org. Lett., Vol. 7, No. 20, 2005
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heterocyclic pseudo-hexapeptide structures with potential
utility in biological and materials settings.
Scheme 4. Synthesis of a Nonsymmetric Triazole Cyclic
Peptide
Acknowledgment. We thank the National Institutes of
Health (GM52190) and Office of Naval Research (N00014-
03-1-0370) for financial support, The Netherlands Organiza-
tion for Scientific Research for a travel grant to J.v.M., the
National Science Foundation for a Predoctoral Fellowship
to W.S.H., and John M. Beierle for synthesis and charac-
terization of 8.
Supporting Information Available: Scheme for the
synthesis of linear peptides 1a-4a, experimental methods,
and compound characterization data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
for the preparation of combinatorial libraries of heterocyclic
peptide structures from readily available starting materials.
In summary, the approach described here should provide a
convenient entry for the design and synthesis of a variety of
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