Synthesis of cyclic (alpha(2)beta)-tripeptides as potential peptide turn mimetics.

Article abstract of DOI:10.1021/ol025988b

[reaction: see text] The solid-supported synthesis followed by cyclization in solution of cyclic (alpha(2)beta)-tripeptides, potential peptide beta-turn mimetics, is described. The cyclization takes advantage of facilitating the rotation between trans- and cis-rotamers of two amide bonds. The method is amenable to combinatorial approaches as is illustrated by the synthesis of a small array of cyclic (alpha(2)beta)-tripeptides.

Full text of DOI:10.1021/ol025988b

ORGANIC
LETTERS
2002
Vol. 4, No. 13
2173-2176
Synthesis of Cyclic (r2â)-Tripeptides as
Potential Peptide Turn Mimetics
Bas Wels, John A. W. Kruijtzer, and Rob M. J. Liskamp*
Department of Medicinal Chemistry, Utrecht Institute for Pharmaceutical Sciences,
Utrecht UniVersity, P.O. Box 80082, 3508 TB Utrecht, The Netherlands
r.m.j.liskamp@pharm.uu.nl
Received April 10, 2002
ABSTRACT
The solid-supported synthesis followed by cyclization in solution of cyclic (r2â)-tripeptides, potential peptide â-turn mimetics, is described.
The cyclization takes advantage of facilitating the rotation between trans- and cis-rotamers of two amide bonds. The method is amenable to
combinatorial approaches as is illustrated by the synthesis of a small array of cyclic (r2â)-tripeptides.
The â-turn is one of the important secondary structure
As part of a program directed toward covalent control of
shape and folding in peptides, we were interested in
developing a general approach for small molecule â-turn
mimetics, which was also amenable to combinatorial pur-
poses (Figure 1).
elements in proteins. It is defined as any tetrapeptide
i
i+3
sequence in which the CR -CR distance in a nonhelical
region is less than 7 Å and is often stabilized by a
10-membered hydrogen-bonded ring between the amide
carbonyl of residue i and the amide proton of residue i +
3.1 There is a great deal of interest in the synthesis of small
molecules that mimic the â-turn structure, for example, to
mimic or interfere with protein-protein interactions2 or bind
to biological targets.3
Numerous â-turn mimetics have been reported. Many are,
however, not generally applicable and cannot contain any
desired side chain, if any. Furthermore, the resemblance to
a â-turn in solution may be absent.4 The absence of a reliable
solid-phase synthesis route may also be an impediment to
the introduction of diversity. Favorable exceptions are
described, among others,3 by Ellman et al.,5 Kahn et al.,6
Golebiowski et al.,7 and Burgess et al.,8 who have designed
â-turn mimicking ring systems which have also been
successfully used in combinatorial preparations.
Figure 1. Protein â-turn and â-turn mimetic 1.
In this Letter we describe the design and synthesis of a
potential â-turn mimic of entirely tripeptidic nature.9 A
(1) Ball, J. B.; Hughes, R. A.; Alewood, P. F.; Andrews, P. R.
Tetrahedron 1993, 49, 3467.
(2) Burgess, K. Acc. Chem. Res. 2001, 34, 826.
(3) For a recent review, see: Souers, A. J.; Ellman, J. A. Tetrahedron
2001, 57, 7431.
(4) Mu¨ller, G.; Hessler, G.; Decornez, H. Y. Angew. Chem., Int. Ed.
2000, 39, 894.
(5) (a) Virgilio, A. A.; Schu¨rer, S. C.; Ellman, J. A. Tetrahedron Lett.
1996, 37, 6961. (b) Souers, A. J.; Virgilio, A. A.; Schu¨rer, S. C.; Ellman,
J. A. Bioorg. Med. Chem. Lett. 1998, 8, 2297. (c) Haskell-Luevano, C.;
Rosenquist, Å.; Souers, A.; Khong, K. C.; Ellman, J. A.; Cone, R. D. J.
Med. Chem. 1999, 42, 4380. (d) Souers, A. J.; Virgilio, A. A.; Rosenquist,
Å.; Fenuik, W.; Ellman, J. A. J. Am. Chem. Soc. 1999, 121, 1817.
10.1021/ol025988b CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/04/2002

Scheme 1. Retrosynthesis of â-Turn Mimetics with General
Structure 1
Figure 2. Examples of synthesized cyclic tripeptides.
design was the use of substituted amide bonds in the
precursors in order to facilitate rotation and thereby cycliza-
tion.12 The presence of four substituents (Ri-Ri+3) in
principle provides ample opportunity for the generation of
diverse libraries. In 1 the H-bond of the 10-membered ring
of a natural â-turn is replaced by a â-peptoid residue,13
providing covalent control of the structure of the turn as well
as facilitating rotation around the amide bond.
The retrosynthesis of 1 is shown in Scheme 1. Our strategy
for creating the â-turn mimetics was to leave the central (i
+ 1 and i + 2) amino acids of the â-turn largely untouched
with respect to a naturally occurring â-turn. The two amide
substituents Ri and Ri+3 were introduced to facilitate rotation
around the tertiary peptide amide bonds. Moreover, they
provide additional diversity as pharmacophores in the
ultimate tripeptide â-turn mimic. Alternatively, one of the
substituents may be used as a resin attachment site. Ring
closure to 1 should take place in the final coupling step
involving two R-amino acid residues, which should give the
highest coupling efficiency. Nevertheless, because structure
1 is between a tri- and tetrapeptide in size, and therefore
considerably rigid, it was expected that cyclization would
be difficult.
corresponding tripeptidomimetic containing an amino sul-
fonamide will be reported shortly. Examples of cyclic tri-
and tetrapeptides consisting only of (L) amino acids are very
scarce10 and usually contain proline, an N-substituted amino
acid, or the very flexible amino acid glycine, which has no
side chain. The presence of proline or an N-substituted amino
acid is especially noteworthy, since this will facilitate rotation
around the amide bond from the trans- to the cis-rotamer.
This rotation is necessary for the nine-membered ring of the
cyclic tripeptide to form. Clearly, the presence of only trans-
rotamers of the peptide amide bond will hamper the
formation of a small cyclic peptide.11
We designed cyclic (R2â)-tripeptides of general structure
1 as mimics of â-turns (Scheme 1). A crucial feature of our
(6) (a) Eguchi, M.; Lee, M. S.; Nakanishi, H.; Stasiak, M.; Lovell, S.;
Kahn, M. J. Am. Chem. Soc. 1999, 121, 12204. (b) Kim, H.; Nakanishi,
H.; Lee, M. S.; Kahn, M. Org. Lett. 2000, 2, 301. (c) Eguchi, M.; Lee, M.
S.; Stasiak, M.; Kahn, M. Tetrahedron Lett. 2001, 42, 1237.
(7) (a) Golebiowski, A.; Klopfenstein, S. R.; Chen, J. J.; Shao, X.
Tetrahedron Lett. 2000, 41, 4841. (b) Golebiowski, A.; Klopfenstein, S.
R.; Shao, X.; Chen, J. J.; Colson, A.; Grieb, A. L.; Russell, A. F. Org. Lett.
2000, 2, 2615.
Cyclization precursor 2 should be readily accessible using
previously described chemistry. The â-peptoid residue in 2
can be assembled through Michael addition of a primary
amine to a resin-bound acrylate,14 while the N-alkyl sub-
stituent comprising the Ri side chain can be introduced using
(8) (a) Feng, Y.; Burgess, K. Chem. Eur. J. 1999, 5, 3261. (b) Feng, Y.;
Pattarawarapan, M.; Wang, Z.; Burgess, K. Org. Lett. 1999, 1, 121. (c)
Maliartchouk, S.; Feng, Y.; Ivanisevic, L.; Debeir, T.; Cuello, C.; Burgess,
K.; Saragovi, H. U. Mol. Pharmacol. 2000, 57, 385. (d) Zhang, A. J.; Khare,
S.; Gokulan, K.; Linthicum, D. S.; Burgess, K. Bioorg. Med. Chem. Lett.
2001, 11, 207.
(9) A corresponding tripeptidomimetic containing an amino sulfonamide
will be reported shortly.
(10) See, for example: (a) Schmidt, U.; Langner, J. J. Pept. Res. 1997,
49, 67. (b) Ehrlich, A.; Heyne, H. U.; Winter, R.; Beyermann, M.; Haber,
H.; Carpino, L. A.; Bienert, M. J. Org. Chem. 1996, 61, 8831. (c) Ma¨stle,
W.; Link, U.; Witschel, W.; Thewalt, U.; Weber, T.; Rothe, M. Biopolymers
1991, 31, 735. (d) Nishino, N.; Xu, M.; Maihara, H.; Fujimoto, T.
Tetrahedron Lett. 1992, 33, 1479.
(12) The favorable influence on cyclization by an N-substituted amide
containing a Boc-group was described in: Cavalier-Frontin, F.; Achmad,
S.; Verducci, J.; Jacquier, R.; Pe`pe, G. J. Mol. Struct. (THEOCHEM) 1993,
286, 125.
(13) Hamper, B. C.; Kolodziej, S. A.; Scates, A. M.; Smith, R. G.; Cortez,
E. J. Org. Chem. 1998, 63, 708.
(11) Scherer, G.; Kramer, M. L.; Schutkowski.; M. Reimer, U.; Fischer,
G. J. Am. Chem. Soc. 1998, 120, 5568.
(14) Brown, A. R.; Rees, D. C.; Rankovic, Z.; Morphy, J. R. J. Am.
Chem. Soc. 1997, 119, 3288
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Org. Lett., Vol. 4, No. 13, 2002

Scheme 2. Synthesis of a Representative Mimetica
a Reagents and conditions: (a) (i) 20% piperidine, DMF, 30 min; (ii) oNBS-Cl (5 equiv), collidine (10 equiv), 1 h; (b) (i) allyl alcohol
(10 equiv), Ph3P (5 equiv), DIAD (5 equiv), DCE, 1 h; (ii) 0.5 M â-mercaptoethanol, DBU (5 equiv), 30 min; (c) acryloyl chloride (4
equiv), TEA (6 equiv), 16 h; (d) benzylamine (20 equiv), LiCl, DMSO, 3 days; (e) Fmoc-Tyr(tBu)-OH (4 equiv), HBTU (4 equiv), HOAt
(0.5 equiv), collidine (8 equiv), 16 h; (f) (i) 20% piperidine, DMF, 30 min; (ii) 1% TFA/DCM, 10 × 2 min; (g) BOP (1.1 equiv), HOBt
(1.1 equiv), DIEA (2.5 equiv), 16 h; (h) TFA, 30 min.
our “site-specific N-alkylation” method.15 Thus, it should
ultimately be possible to introduce a large amount of
diversity, using an array of alcohols for the Ri side chains
and amines for the Ri+3 side chains. This diversity can be
further increased by accessing the large pool of commercially
available R-amino acids for the Ri+1 and Ri+2 side chains.
As an illustration of its potential, the synthesis of a
representative (1b) of this new class of potential turn
mimetics is shown in Scheme 2. The synthesis of 1b
commenced with resin-bound Fmoc-phenylalanine 3, at-
tached to the resin through the highly acid labile HMPV (4-
(4-hydroxymethyl-3-methoxyphenoxy)-valeric acid) linker.16
After Fmoc deprotection, a solid-supported sulfonamide (4)
having a relatively acidic proton was generated using oNBS-
Cl.17 This sulfonamide was suitable for introduction of an
N-allyl substituent by a Mitsunobu reaction on the solid
phase. Next, the oNBS group was removed using a thiolate
anion, affording the resin-bound deprotected secondary amine
5, which was reacted with acryloyl chloride. The resulting
resin-bound acrylate 6 was then subjected to a Michael
addition yielding secondary amine 7, to which Fmoc-Tyr-
(tBu)-OH was coupled to give 8. After cleavage of the Fmoc
group, the linear RâR (R2â) tripeptide could be selectively
released from the resin by mild acid treatment, leaving the
side chain protection intact. After purification, linear trimer
9 was obtained in 68% yield (average yield 96% per step, 9
steps). Cyclization of 9 was achieved at 1 mM concentration
in DCM using BOP leading to (R2â)-tripeptide 1a in 55%
yield. Because cyclization is relatively slow, the activated
C-terminal residue might have partly racemized.18 Efforts
are underway to clarify the issue of racemization. Finally,
removal of the side chain tBu protecting group was ac-
complished using TFA to yield the desired cyclic (R2â-)
tripeptide 1b in 11% yield after preparative HPLC. Yields
in this last deprotection step turned out to be somewhat
disappointing because of the observed moderate acid stability
of the resulting cyclic tripeptides. Instability of R-amino acid
containing cyclic tripeptides has been noted before.19 De-
(15) (a) Reichwein, J. F.; Liskamp, R. M. J. Tetrahedron Lett. 1998, 39,
1243. (b) Reichwein, J. F.; Wels, B.; Kruijtzer, J. A. W.; Versluis, C.;
Liskamp, R. M. J. Angew. Chem., Int. Ed. 1999, 38, 3684.
(16) The HMPV linker is analogous to the HMPB linker introduced by
Riniker et al. and is available from Senn Chemicals AG. Riniker, B.;
Flo¨rsheimer, A.; Fretz, H.; Sieber, P.; Kamber, B. Tetrahedron 1993, 49,
9307.
(18) The NMR spectra show additional peaks that may be due to the
presence of multiple conformations of the cyclic trimer or may be caused
by the presence of the epimeric product.
(19) (a) Rothe, M.; Faehnle, M.; Wermuth, S. Pept., Proc. Eur. Pept.
Symp., 18th 1984, 573. (b) Rothe, M.; Faehnle, M.; Maestle, W.; Feige, K.
Pept.: Struct. Funct., Proc. Am. Pept. Symp., 9th 1985, 177.
(17) Fukayama, T.; Jow, C.-K.; Cheung, M. Tetrahedron Lett. 1995, 36,
6373.
Org. Lett., Vol. 4, No. 13, 2002
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composition is complete within 2 h upon treatment with 10%
TFA in DCM, so it is essential that the deprotection time is
kept to a minimum. However, this problem can be addressed
by adapting the protecting groups used.
Using this route we were able to synthesize a small array
of other (functionalized) cyclic (R2â) tripeptides in similar
yields (Figure 2). Since this method is clearly amenable to
combinatorial approaches, work is in progress on the
synthesis of libraries of these potential small molecular â-turn
mimetics. Indeed, preliminary modeling studies using Monte
Carlo conformational searching implemented in the Macro-
Model20 program (vs 7.0) showed a clear resemblance of
the global minimum of the R2â-tripeptide backbone with a
type III′ turn (Figure 3).1 The Ri-Ri+1 distance was 6.8 Å,
nicely fitting within the 7 Å limit.1
In conclusion, we have shown the synthesis of putative
small molecule â-turn mimetics comprising a cyclic (R2â)
tripeptide skeleton starting from simple starting materials of
which a large diversity is available. Using this route a variety
of functionalized cyclic tripeptides was accessible. Moreover,
to our knowledge, these are the smallest readily functional-
ized cyclic peptide systems that have been described so far.
A crucial feature of our approach is facilitation of rotation
around amide bonds by using substituted, i.e., tertiary amide
bonds, thereby enabling cyclization. Currently, on-resin
cyclization methods are under investigation. In addition,
modifications of the â-peptoid moiety are being investigated
Figure 3. Global minimum of the R2â-tripeptide backbone. The
calculated torsion angles were Φi+1 ) 59°, ψi+1 ) 44°, Φi+2
66°, ψi+2 ) 30°.
)
in order to further expand the diversity of the cyclic tripeptide
skeleton to tripeptidomimetics.
Acknowledgment. We thank Dr. Johan Kemmink for his
assistance in recording and interpreting the 500 MHz spectra,
Hans Hilbers for his assistance with the modeling studies,
and NDRF-Quadrant for financial assistance.
Supporting Information Available: Experimental pro-
cedures and characterization details (HPLC, 1H, 13C, COSY
and TOCSY spectra) of 1a. This material is available free
of charge via the Internet at http://pubs.acs.org.
(20) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.;
Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput.
Chem. 1990, 11, 440.
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Org. Lett., Vol. 4, No. 13, 2002