Synthesis and circular dichroism spectroscopic investigations of oligomeric β-peptoids with α-chiral side chains

Article abstract of DOI:10.1021/ol061717f

Biomimetic oligomers are of large interest both as targets for combinatorial and parallel synthetic efforts and as foldamers. For example, shorter peptoid derivatives of β-peptides, i.e., oligo-N-substituted β-Ala, have been described as potential lead st

Full text of DOI:10.1021/ol061717f

ORGANIC
LETTERS
2006
Vol. 8, No. 20
4533-4536
Synthesis and Circular Dichroism
Spectroscopic Investigations of
Oligomeric
Side Chains
â-Peptoids with r-Chiral
Anna S. Norgren,† Suode Zhang,† and Per I. Arvidsson*,†,‡
Department of Biochemistry and Organic Chemistry, Uppsala UniVersity,
Box 576, SE-751 23 Uppsala, Sweden, and DiscoVery CNS & Pain Control,
AstraZeneca R&D So¨derta¨lje, SE-151 85 So¨derta¨lje, Sweden
per.arVidsson@astrazeneca.com
Received July 12, 2006
ABSTRACT
Biomimetic oligomers are of large interest both as targets for combinatorial and parallel synthetic efforts and as foldamers. For example,
shorter peptoid derivatives of -peptides, i.e., oligo-N-substituted -Ala, have been described as potential lead structures. Herein, we describe
a solid-phase synthetic route to -peptoids with -chiral aromatic N-substituents up to 11 residues long. Furthermore, the folding propensities
of these oligomers were investigated by circular dichroism (CD) spectroscopy.
â
â
â
r
The design and exploration of novel peptidomimetic oligo-
mers is currently an area of high interest in both medicinal
chemistry and material science.1 Depending on the applica-
tion at hand, it may be desirable to develop oligomers with
an unnatural backbone, e.g., for conformational rigidity and
increased metabolic stability, or with chemically diverse side
chains, e.g., for sampling molecular space and properties.
The discovery of peptidomimetic oligomers capable of
adopting ordered three-dimensional conformations has pro-
moted a rapid growth of the research area devoted to
foldamers.2 Although unstructured oligomers of R-amino
acids and nucleotides may be considered representative
products of combinatorial efforts,3 â- and γ-peptides,4
oligoureas,5 and oligopyrrolinones6 are examples of recently
(3) Peptides: Chemistry and Biology; Sewald, N., Jakubke, H.-D., Eds.;
Wiley-VCH: Weinheim, 2002.
(4) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (b) Appella, D.
H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman, S. H. J. Am.
Chem. Soc. 1996, 118, 13071. (c) Appella, D. H.; Christianson, L. A.; Karle,
I. L.; Powell, D. R.; Gellman, S. H. J. Am. Chem. Soc. 1999, 121, 6206.
(d) Seebach, D.; Abele, S.; Gademann, K.; Guichard, G.; Hintermann, T.;
Jaun, B.; Matthews, J. L.; Schreiber, J. V.; Oberer, L.; Hommel, U.; Widmer,
H. HelV. Chim. Acta 1998, 81, 932. (e) Seebach, D.; Ciceri, P. E.; Overhand,
M.; Jaun, B.; Rigo, D.; Oberer, L.; Hommel, U.; Amstutz, R.; Widmer, H.
HelV. Chim. Acta 1996, 79, 2043. (f) Seebach, D.; Gademann, K.; Schreiber,
J. V.; Matthews, J. L.; Hintermann, T.; Jaun, B.; Oberer, L.; Hommel, U.;
Widmer, H. HelV. Chim. Acta 1997, 80, 2033. (g) Seebach, D.; Overhand,
M.; Ku¨hnle, F. N. M.; Martinoni, B.; Oberer, L.; Hommel, U.; Widmer, H.
HelV. Chim. Acta 1996, 79, 913. (h) Gellman, S. H. Curr. Opin. Chem.
Biol. 1998, 2, 717. (i) Seebach, D.; Beck, A. K.; Bierbaum, D. J. Chem.
BiodiV. 2004, 1, 1111. (j) Hintermann, T.; Gademann, K.; Jaun, B.; Seebach,
D. HelV. Chim. Acta 1998, 81, 983. (k) Hanessian, S.; Luo, X. H.; Schaum,
R. Tetrahedron Lett. 1999, 40, 4925.
† Uppsala University.
‡ AstraZeneca R&D So¨derta¨lje.
(1) (a) Zuckermann, R. N.; Barron, A. E. Curr. Opin. Chem. Biol. 1999,
3, 681. (b) Pseudo-Peptides in Drug DiscoVery; Nielsen, P. E., Ed.; Wiley-
VCH: Weinheim, 2004; pp 1-31.
(2) (a) Cheng, R. P.; Gellman, S. H.; Degrado, W. F. Chem. ReV. 2001,
101, 3219. (b) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore,
J. S. Chem. ReV. 2001, 101, 3893.
(5) (a) Burgess, K.; Linthicum, K. S.; Shin, H. Angew. Chem., Int. Ed.
Engl. 1995, 34, 907. (b) Nowick J. S.; Powell, N. A.; Martinez, E. J.; Smith,
E. M.; Noronha, G. J. Org. Chem. 1992, 57, 3763.
10.1021/ol061717f CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/29/2006

studied foldamers. Of particular interest in this respect are
peptoids, i.e., oligo-N-substituted glycines, as these biomi-
metic oligomers have been widely used both as products in
combinatorial efforts7 and as foldamers. Peptoids with
R-chiral substituents on the nitrogen atom have been shown
to fold into helical structures,8 and several interesting
biological applications have been reported on the basis of
this property.9
â-Peptoids, i.e., oligomers with a backbone composed of
an unsubstituted â-amino acid â-Ala and with a side chain
anchored to the amide nitrogen, were first described by
Hamper et al. in 1998.10 This report described the synthesis
of â-peptoid dimers and trimers with nonchiral substituents
on the nitrogen atom. To the best of our knowledge, no
synthesis or structural investigation of â-peptoids with
R-chiral N-substituents has been reported. Although the
resulting â-peptoid would have three rotatable bonds in the
backbone, and consequently an expected low tendency to
adopt a folded conformation, we found it valuable to
investigate the influence of R-chiral N-substituents on the
structure of â-peptoids, especially considering the large
impact of such groups on the structure of normal R-peptoids
and the large interest in â-peptides.2,4 Herein, we report our
efforts to develop a practical solid-phase synthesis of
â-peptoids with R-chiral aromatic N-substituents (Figure 1)
a high excess of amines (up to 20 equiv) and very long
reaction times (up to 72 h) that preclude synthesis of longer
derivatives in a reasonable time. In contrast, Zuckermann
and co-workers11 invented a submonomer method for syn-
thesis of R-peptoids where each cycle of monomer addition
consists of two steps: acylation of the secondary amine by
an R-halogenated acyl donor followed by nucleophilic dis-
placement of the halogen by the next amine. Both these steps
require short reaction times (90 min), and relatively long
R-peptoid chains can be synthesized by this methodology.
â-Peptoids with R-chiral N-substituents are expected to
be more difficult to synthesize due to low reactivity of the
secondary amine in the acylation step as well as to steric
repulsions during the nucleophilic addition of the R-branched
side chain. In the search of good experimental procedures
to synthesize â-peptoids with R-branched aromatic side
chains, solution-phase chemistry was initially employed.
First, we focused on utilizing the nucleophilic displacement
reaction of Zuckerman in the synthesis of â-peptoids
(Scheme 1, top). Consequently, â-halogenated amides
Scheme 1. Solution-Phase Synthesis of Protected â-Peptoid 5
Figure 1. Structures of â-peptoids 1a-j and N-acetylated monomer
2, all possessing R-chiral aromatic N-substituents.
3a-c12 were treated with (S)-1-phenyl ethylamine in 1 mL
of solvent12 and stirred at 55 °C for 2-4 h. The reactions
were monitored by analytical reversed-phase HPLC-MS, and
the conversions were determined by integration of the UV
trace (Table 1). As expected, the chloride derivative 3a was
less reactive than the bromine and iodine analogues 3b and
3c (which had similar reactivity). Variation of the reaction
media revealed that 20% H2O in THF was one of the best
solvent systems tested. The HPLC-MS analyses also revealed
that the reaction conditions caused major elimination of the
â-halogen to give the R,â-unsaturated amide 412 in situ.
Employing 4 itself as a reactant provided the product 5 in
rates and yields similar to those for the halogen derivatives.
Thus, we next focused our attention on the Hamper method,
using acryloyl chloride as the acylating agent, and tried to
and an investigation concerning the folding propensities of
such molecules by circular dichroism (CD) spectroscopy.
For the synthesis of â-peptoids, Hamper developed a two-
step solid-phase methodology involving first acryloyl chlo-
ride as acylating agents to generate acrylamide resins,
followed by Michael addition of a primary amine to the R,â-
unsaturated system to generate the secondary amine.10 This
method was applied successfully for the synthesis of â-pep-
toids containing 2-3 residues; still, this procedure required
(6) Smith, A. B.; Favor, D. A.; Sprengler, P. A.; Guzman, M. C.; Carroll,
P. J.; Furst, G. T.; Hirschmann, R. Bioorg. Med. Chem. 1999, 7, 9.
(7) (a) Simon, R. J.; Kania, R. S.; Zuckermann, R. N.; Huebner, V. D.;
Jewell, D. A. et al. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 9367. (b)
Zuckermann, R. N.; Martin, E. J.; Spellmeyer, D. C.; Stauber, G. B.;
Shoemaker, K. R. et al. J. Med. Chem. 1994, 37, 2678.
(8) (a) Kirshenbaum, K.; Barron, A. E.; Goldsmith, R. A.; Armand, P.;
Bradley, E. K.; Truong, K. T. V.; Dill, K. A.; Cohen, F. E.; Zuckermann,
R. N. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 4303. (b) Wu, C. W.;
Kirshenbaum, K.; Sanborn, T. J.; Patch, J. A.; Huang, K.; Dill, K. A.;
Zuckermann, R. N.; Barron, A. E. J. Am. Chem. Soc. 2003, 125, 13525.
(9) Patch, J. A.; Barron, A. E. J. Am. Chem. Soc. 2003, 125, 12092.
(10) Hamper, B. C.; Kolodziej, S. A.; Scates, A. M.; Smith, R. G.; Cortez,
E. J. Org. Chem. 1998, 63, 708.
(11) Zuckermann, R. N.; Kerr, J. M.; Kent, S. B. H.; Moos, W. H. J.
Am. Chem. Soc. 1992, 114, 10646.
(12) For further details, see Supporting Information.
(13) Some examples: (a) Bartoli, G.; Bartolacci, M.; Bosco, M.; Foglia,
G.; Giuliani, A.; Marcantoni, E.; Sambri, L.; Torregiani, E. J. Org. Chem.
2003, 68, 4594. (b) Enders, D.; Wallert, S.; Runsink, J. Synthesis 2003, 12,
1856. (c) Yadav, J. S.; Abraham, S.; Reddy, B. V. S.; Sabitha, G. Synthesis
2001, 14, 2165.
4534
Org. Lett., Vol. 8, No. 20, 2006

support, and once again, the conditions needed certain adjust-
ments as the route went from solution to solid phase.12 For
the reaction on solid phase, Tentagel S PHB (0.26 mmol/g)
resin at 55 °C for 9 h was the best choice for the Michael
addition, and the acylation step only required 2 × 30 min.
Table 1. Conversion of Reagents 3a-c and 4 after Reaction
with (S)-1-Phenylethylamine in Various Solvents after 4 h of
Reaction at 25 °C
entry
reagent
solvent
conversion (%)
The optimized conditions were then used as a repetitive
cycle in the synthesis of â-peptoids 1a-j (Scheme 2),
resulting in a new efficient method of synthesizing longer-
chain â-peptoids.
1
2
3
3a
3b
3c
4
DMSO
DMSO
DMSO
DMSO
43
66
66
2
4
5
6
7
8
3c
3c
3c
3c
4
4
4
4
THF
i-PrOH
i-PrOH/THFa
H2O/THFb
THF
i-PrOH
i-PrOH/THFa
H2O/THFb
97
98
96
95
4
95
68
90
Cleavage from the resin using 95% TFA/H2O at room
temperature for 1 h gave crude products which were purified
by preparative reversed-phase HPLC. The dimer 1a and
trimer 1b were isolated in good yields of 70% and 62%,
respectively, and the isolated yields of â-peptoids 1c-i were
determined to be in the range of 50-33%.12 We found these
yields acceptable as the synthesis includes nucleophilic
addition of the sterically demanding R-branched N-substitu-
ent (S)-1-phenylethylamine. Further, shorter and/or longer
oligomers could be collected as byproducts upon purification,
thus increasing the overall yields. As an example, 1j was
collected as a byproduct from the synthesis of 1i, representing
an amount corresponding to a 22% yield of oligomer 1j. The
presence of shorter derivatives may readily be explained by
incomplete coupling; however, the longer derivatives are
more difficult to rationalize. Most likely, these results are
from incomplete washing of the reagents.
9
10
11
12
a 50% v/v. b 20% v/v H2O in THF.
optimize the Michael addition step to allow synthesis of
longer â-peptoid derivatives in a reasonable time.
As the Lewis acid catalyst is known to accelerate the
Michael addition,13 we tested 14 different potential catalysts
[e.g., FeNO3, CF3CO2Ag, and CeCl3] for the conversion of
4 to 5 (see Supporting Information, Table S1). The reaction
temperature was lowered to 35 °C, and all other reaction
conditions were the same as those described previously, using
20% H2O in THF as the reaction solvent. The reactions was
again followed by HPLC-MS. In our system, CF3CO2Ag
proved to be the best catalyst.12 Upon varying the solvent
composition (H2O/THF) in the absence of a catalyst, it was
found that a high percentage of water (>50%) itself
effectively catalyzed the Michael addition (Supporting
Information, Table S2).
Interestingly, a doubling of the peaks was seen for the
shorter-chain peptoids 1a-f when the purified peptoids were
analyzed on analytical HPLC. Analysis by MS, as well as
by NMR spectroscopy, allowed us to establish that the two
peaks originated from the same peptoid and thus represent
conformers undergoing slow cis/trans isomerization around
the tertiary amide linkage; similar observations have been
made for N-alkylated R-peptides.14
The ability of the â-peptoids with the R-chiral side chains
(S)-1-phenylethyl 1a-j to adopt an ordered secondary
structure was evaluated by CD spectroscopy. In previous
studies by Barron and co-workers,8a R-peptoids as short as
five residues in length, possessing chiral aromatic side chains,
were reported to show intense CD, revealing the presence
of a regularly repeating, chiral secondary structure.8a Further,
this class of peptoids has been both predicted and observed
to adopt a helical polyproline type I-like conformation.
In a parallel optimization on solid phase, we established
that a PEGylated resin, i.e., Tentagel, provided superior yields
and reaction times as compared to a polystyren-based resin
(Supporting Information, Table S3).
Therefore, it was concluded that the optimal conditions
for solid-phase synthesis were the use of Tentagel resin in
high water content solution without additional catalysts. In
case a cheaper PS resin needs to be used, we suggest that
the reaction be performed with catalytic CF3CO2Ag in 20%
H2O/THF as solvent.
The folding propensities of the â-peptoids 1a-j were first
analyzed using methanol as the solvent at 25 °C (c ) 0.1
mM) (Figure 2a). All 10 â-peptoid oligomers gave rise to
CD spectra almost identical to those arising from right-
With the optimized procedure at hand, the solid-phase
synthesis of N-chiral â-peptoids 1a-j was initiated. â-Tri-
peptoid 1b was the first sequence to be synthesized on solid
Scheme 2. General Pathway of Solid-Phase Synthesis of â-Peptoids
Org. Lett., Vol. 8, No. 20, 2006
4535

Figure 3. CD spectra of N-acetylated monomer 2 in methanol (a)
and acetonitrile (b).
â-pentapeptoid 1d in trifluoroethanol (TFE) at 25 °C (c )
0.1 mM).12 TFE is known to stabilize helical secondary
structures in polypeptides but to have marginal effects for
R-peptoids.16 Similarly, we did not observe a significant
change in the CD pattern of 1d upon changing the solvent
from methanol and acetonitrile to TFE.
Figure 2. CD spectra of â-peptoids 1a-j in methanol (a) and of
1b,d,f,h,j in acetonitrile (b). Included also are the normalized CD
spectra in methanol (c) and acetonitrile (d), respectively, taking
the number of chromophores into account.
In conclusion, a systematically developed and optimized
strategy for the challenging solid-phase synthesis of â-pep-
toids with R-chiral aromatic side chains has been obtained.
The present methodology uses Tentagel S PHB as a solid
support and 80% H2O/THF as a mixed solvent system.
Although not perfect, these improvements now allow for the
addition of one residue every 12 h, making the synthesis of
longer â-peptoids within a reasonable time span feasible. The
synthetic pathway was demonstrated in the synthesis of nine
novel â-peptoids 1a-j that were also analyzed by CD
spectroscopy. The spectra closely resembled those reported
for R-peptoids with R-chiral ((S)-1-phenylethyl) side chains,
proven to fold into helical secondary structures;8a still, our
results suggest that the homologated â-peptoid analogues
investigated here do not form ordered secondary structures,
although additional studies by high-resolution methods would
be needed to fully ascertain this conclusion. Nonetheless,
longer-chain â-peptoids with R-chiral side chains still
represent an interesting class of biomimetic oligomers that
may find use in biomedical and material science applications
that do not neccessarily require secondary structure formation
function.
handed helical secondary R-peptoids, with the same side
chains, recorded in acetonitrile, both regarding shape and
intensity.8a,15 The CD spectra are characterized by double
minima near 204 and 218 nm. CD spectra of 1b,d,f,h,j in
acetonitrile at 25 °C (c ) 0.1 mM) gave rise to similar CD
spectra (Figure 2b).
Barron15 noted an increase in the per residue CD intensity
up to a certain chain length, after which no further change
was observed. We see no such cooperative effect (Figure 2c
and 2d) with the â-peptoids of chain lengths between 3 and
11 residues investigated here, suggesting that these oligomers
do not adopt helical secondary structures or, alternatively,
that an ordered strucure is present already in the trimer. It
should be noted that cooperative folding is not expected for
conformations resulting exclusively from adjacent interacting
sites;1b thus, â-peptoid folding could still result from
propagation of local conformational preferences.
To further establish if the CD spectra in Figure 2 arose
from a possible secondary structure or if the CD pattern was
inherent of the side chain functionalized amide linkage itself,
we prepared the acetylated “monomer” 2. Analysis by CD
spectroscopy in both methanol and acetonitrile (Figure 3)
showed a CD pattern similar to those originating from the
oligomers. On the basis of this result, it appears most
reasonable to conclude that â-peptoids with R-chiral side
chains lack the high tendency of helical secondary structure
formation characteristic of the analogous R-peptoids, as the
simple monomer would not be expected to adopt an ordered
conformation.
Acknowledgment. This work was supported by Veten-
skapsrådet (the Swedish Research Council), the Carl Trygger
Foundation, and Magnus Bergvall’s Foundation. S.Z. is
grateful to the Wenner-Gren foundation for a fellowship
2003-2004.
Supporting Information Available: Experimental details
and characterization of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
The solvent dependence of the â-peptoid oligomers was
also investigated by running additional CD spectra of
OL061717F
(14) Teixido´, M.; Albericio, F.; Giralt, F. J. Pept. Res. 2005, 65, 153.
(15) Wu, C. W.; Sanborn, T. J.; Zuckermann, R. N.; Barron, A. E. J.
Am. Chem. Soc. 2001, 123, 2958.
(16) Sanborn, T. J.; Wu, C. W.; Zuckermann, R. N.; Barron, A. E.
Biopolymers 2002, 63, 12.
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Org. Lett., Vol. 8, No. 20, 2006