Solid-phase syntheses of peptoids using Fmoc-protected N-substituted glycines: The synthesis of (retro)peptoids of leu-enkephalin and substance P

Article abstract of DOI:10.1002/(SICI)1521-3765(19980807)4:8<1570::AID-CHEM1570>3.0.CO;2-2

A particularly interesting class of oligomeric peptidomimetics is formed by the peptoids, which consist of N-substituted glycine residues. A solid- phase synthesis method for peptoids is presented in which these residues are introduced using their Fmoc derivatives. This 'monomer' method allowed the monitored synthesis of relatively large quantities of pure peptoids as well as the translation of, in principle, any peptide into the corresponding peptoid. The required Fmoc-substituted glycines were accessible by convenient synthesis, and a number of monomers including those containing side chains with functional groups have been synthesized. The use of Fmoc monomers also allowed implementation of a solid-phase synthesis protocol on a commercial peptide synthesizer. The method was exempli- fled by the solid-phase syntheses of the (retro)peptoids of Leu-enkephalin and substance P. Mass spectrometric studies of (retro)peptoids were essential for their characterization, and the presence of the B- and Y'- type ions allows sequence analysis. Substance P (retro)peptoids were biologically active. HPLC analysis showed an increased hydrophobicity, and pepsin treatment resulted in greatly reduced degradation compared with the corresponding peptide.

Full text of DOI:10.1002/(SICI)1521-3765(19980807)4:8<1570::AID-CHEM1570>3.0.CO;2-2

FULL PAPER
Solid-Phase Syntheses of Peptoids using Fmoc-Protected N-Substituted
Glycines: The Synthesis of (Retro)Peptoids of Leu-Enkephalin and
Substance P
John A. W. Kruijtzer, Lovina J. F. Hofmeyer, Wigger Heerma,
Cornelis Versluis, and Rob M. J. Liskamp*
Abstract: A particularly interesting
class of oligomeric peptidomimetics is
formed by the peptoids, which consist of
N-substituted glycine residues. A solid-
phase synthesis method for peptoids is
presented in which these residues are
introduced using their Fmoc derivatives.
This ªmonomerº method allowed the
monitored synthesis of relatively large
quantities of pure peptoids as well as
the translation of, in principle, any pep-
tide into the corresponding peptoid.
The required Fmoc-substituted glycines
were accessible by convenient synthesis,
and a number of monomers including
those containing side chains with func-
tional groups have been synthesized.
The use of Fmoc monomers also allowed
implementation of a solid-phase synthe-
sis protocol on a commercial peptide
synthesizer. The method was exempli-
fied by the solid-phase syntheses of the
(retro)peptoids of Leu-enkephalin and
substance P. Mass spectrometric studies
of (retro)peptoids were essential for
their characterization, and the presence
of the B- and Y''- type ions allows
sequence analysis. Substance P (retro)-
peptoids were biologically active. HPLC
analysis showed an increased hydropho-
bicity, and pepsin treatment resulted in
greatly reduced degradation compared
with the corresponding peptide.
Keywords: mass spectrometry
´
peptidomimetics ´ peptoids ´ solid-
phase synthesis ´ substance P
by variation of the building blocks. The first and perhaps most
well-known class of oligomeric peptidomimetics is composed
of the peptoids, which consist of N-substituted glycine
derivatives.[2, 3] The structural differences between a peptide
and a peptoid are a consequence of the use of these building
blocks and are readily apparent from the general structures of
a peptide, peptoid, and retropeptoid depicted in Figure 1.
Most striking is the absence of chirality in the N-substituted
glycine residues, with the exception of proline. In a manner of
speaking, one might consider peptoids as peptide mimics in
which the side chain has been shifted from the chiral a-carbon
atom in a peptide to the achiral nitrogen. Undoubtedly, this
will have consequences for the biological activity of a
(retro)peptoid compared with that of the parent peptide,
but predictions as to the exact nature of these consequences
will be difficult. For evaluation, several (retro)peptoid
peptidomimetics of biologically active peptides have to be
synthesized. The preparation of several biologically active
peptoids has indeed shown that they are attractive and
promising peptidomimetics in this respect.[2, 4, 5]
Introduction
Peptides and proteins play a crucial role in virtually all
biological processes. In fact it is difficult, if not impossible, to
name any biological process which does not involve these
biopolymers. Peptides especially, which are usually smaller
than proteins, are often important starting structures for the
development of potential therapeutic agents.[1]
Since the application of peptides as drugs has been
hampered, for example, by poor bioavailability and biode-
gradation by proteases, the development of peptidomimetics
aiming at the remedy of these disadvantages while retaining
biological activity has been spectacular. The oligomeric
peptidomimetics are particularly interesting in this respect,
since they provide access to an enormous molecular diversity
[*] Prof. Dr. R. M. J. Liskamp, Dr. Ir. J. A. W. Kruijtzer,
Ing. L. J. F. Hofmeyer
Utrecht Institute for Pharmaceutical Sciences
Department of Medicinal Chemistry, Utrecht University
P.O. Box 80082, 3508 TB Utrecht (The Netherlands)
Fax : (‡ 31)30253-6655
In contrast to the biological activity, direct consequences of
the physical and chemical properties of peptoids can be more
safely predicted. The absence of an amide N ± H, which can
act as a hydrogen-bond donor, is likely to decrease the
solubility in a solvent capable of hydrogen-bond formation
and increase the solubility in a more apolar solvent. The
E-mail: r.m.j.liskamp@pharm.ruu.nl
Dr. W. Heerma, C. Versluis
Bijvoet Center for Biomolecular Research
Utrecht University
P.O. Box 80083, 3508 TB Utrecht (The Netherlands)
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Scheme 1. Submonomer vs. monomer approach to the synthesis of
peptoids.
Figure 1. General structures of a peptide, peptoid, and retropeptoid.
monomer approach, which is analogous to peptide synthesis,
considerable quantities of pure and relatively large peptoids
should be accessible while progress of their synthesis on the
solid phase can be monitored by quantifying the dibenzoful-
vene adduct obtained after cleavage of the Fmoc group
(Scheme 1).[10] A both practical and important advantage of
using the monomer approach is the easy implementation of
the synthesis protocol for peptides on commercial automated
peptide synthesizers. It is also possible to prepare peptide ±
peptoid hybrids as well as complete peptoids on these
apparatus by means of essentially the same synthesis protocol,
but with Fmoc-peptoid monomers and a more powerful
coupling agent.
Our targets were the syntheses of both the peptoids (2 and
5) and retropeptoids (3 and 6) of Leu-enkephalin (1) and
substance P (4; Figure 2). Leu-enkephalin (YGGFL) is
representative of the endogenous opioid peptides, which act
like opiates in biological systems. They display high affinity to
and moderate selectivity for d-opioid receptors.[11] The neuro-
peptide substance P, a member of the tachykinin family,
functions as a neurotransmitter or neuromodulator in the
central and peripheral nervous systems. Substance P (SP) is
generally associated with physiological events leading to pain
and inflammation.[12] These peptides were chosen in view of
our interest in neuropeptides. We considered the relatively
small Leu-enkephalin as the try-out sequence of our solid-
phase synthesis method for the preparation of a (retro)pep-
toid. In contrast, the larger and more complicated sequence of
Substance P was a particular challenge for translation to the
corresponding (retro)peptoid peptidomimetics because it
contains a considerable number of amino acids with side
chains containing a functional group.
presence of tertiary amide bonds, which are also present in
proline-containing peptides, will result in the presence of
(many) cis/trans rotamers due to rotation about each tertiary
amide bond in a peptoid, thereby severely complicating the
NMR spectra.[6]
Finally, it is also realistic to assume that peptoids, because of
the absence of a recurrent peptide ± amide bond, will be less
sensitive to proteolytic degradation. Indeed, this has been
proven to be the case in the examples so far studied.[7]
As part of a program to study the structural characteristics
(by X-ray, NMR, MS, and computer-assisted molecular
modeling) as well as the biological properties of peptoids,[8, 9]
we are interested in the synthesis of relatively large quantities
of pure peptoids. In addition, we want to be able to translate
in principle any peptide into the corresponding peptoid using
a fully automated peptide synthesizer, with the ultimate goal
of creating combinatorial oligomeric libraries for structure ±
activity relationship studies and drug discovery.
Results and Discussion
Zuckermann and colleagues have described the submonomer
approach for the synthesis of peptoids (Scheme 1).[4a] Each
cycle of monomer addition consists of an acylation step (step
1; backbone formation) and a nucleophilic displacement step
(step 2; side-chain introduction). This method is very attrac-
tive because the separate monomers for building the peptoid
do not have to be synthesized, and in principle any arbitrary
amine can be employed in the synthesis of peptoids. While this
approach may be the method of choice when small quantities
of peptoids present in large libraries of relatively small
peptoids (e.g. tripeptoids) are needed, it suffers from the
disadvantage that large excesses of reagents (e.g. the amines)
have to be used and that completion of the acylation and
substitution reactions cannot be determined;[4a, b] this is
essential for preparation of larger peptoids of reasonable
purity.
The required N-substituted glycine derivatives, denoted as
peptoid monomers, for the solid-phase synthesis of Leu-
enkephalin were NTyr, NPhe, and NLeu.[13] The syntheses of
NPhe and NLeu as well as other peptoid monomers contain-
ing nonfunctional side chains are straightforward and are
shown in Scheme 2. The amines which have to be used in the
substitution reaction, involving ethyl bromoacetate, are often
commercially available. Thus, appropriate amines 7a and 7b
were alkylated with ethyl bromoacetate 8 to afford the N-
substituted glycine ethyl esters 9a and 9b. Neither dialkyla-
tion nor aminolysis of the ester in the substitution reaction
was observed. The use of ethyl bromoacetate 8 instead of the
On the basis of present-day peptide synthesis in which
Fmoc-protected amino acids are often used,[10] employing
Fmoc-protected N-substituted glycine derivatives seems a
very attractive approach for synthesizing peptoids both in
solution and on the solid phase (Scheme 1). With this
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FULL PAPER
Figure 2. Acetylated Leu-enkephalin amide (1), its corresponding peptoid 2, and retropeptoid 3; acetylated substance P (4), its corresponding peptoid 5, and
retropeptoid 6.
free acid allowed easy purificationÐif necessaryÐby column
chromatography. Subsequently, esters 9a,b were saponified
with Tesserꢁs base.[14] The amino groups of the
resulting sodium salts were protected with a
Fmoc group to provide the peptoid monomers
10a,b required for the syntheses of the peptoid
and retropeptoid of Leu-enkephalin.[15] How-
ever, the amine 7 f necessary for preparation of
NTyr had to be synthesized (Scheme 2). Start-
ing from 4-hydroxybenzonitrile 11 the OH-
function was protected as a tert-butyl ether
using tert-butyl trichloroacetimidate 12,[16] after
which the nitrile group was reduced to the
desired benzylamine 7 f. Substitution, followed
by saponification, and Fmoc-group attachment
gave the NTyr peptoid monomer 10 f used in
the solid-phase protocol.
cleavage-coupling cycles, the cleavage step of which could be
monitored by UV. A more or less constant high level of Fmoc
In an earlier paper we showed that synthesis
of peptoids in solution went well with PyBroP
as a coupling reagent.[5a] Therefore, this cou-
pling reagent was also used in the solid-phase
syntheses of both the peptoid 2 and retropep-
toid 3 of Leu-enkephalin 1 (Scheme 3). Starting
with TentaGel SRAM resin, containing the
acid-labile Rink-linker to be used for the
preparation of a C-terminal amide,[17, 18] the
Scheme 2. Preparation of monomers required for the syntheses of (retro)peptoids of Leu-
enkephalin and substance P.
peptoid was assembled in six consecutive Fmoc-
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Peptoid Synthesis
1570±1580
cleavage is a good indication for high-yielding coupling steps.
These are shown in Scheme 3, indicating that the solid-phase
peptoid synthesis went very well. For the purpose of
comparison of mass spectra, the retropeptoid 3 was also
synthesized starting from the peptoid monomer on the N-
terminus of the peptoid 2 just synthesized. Peptoid 2 and
retropeptoid 3 were obtained after deprotection and cleavage
from the resin with TFA in the presence of water and
triisopropylsilane (TIS). The mass spectra of both the peptoid
2 and the retropeptoid 3 as well as the parent peptide 1 have
been previously discussed.[8a] Mass spectral studies are
essential in the characterization of peptoids, since interpreta-
tion of NMR spectra is severely hampered by the presence of
many conformations owing to rotation about the tertiary
amide bonds in (retro)peptoids.
Having succeeded in the syntheses of both the peptoid 2
and retropeptoid 3 of Leu-enkephalin 1, we moved on to the
syntheses of both the peptoid 5 and retropeptoid 6 of
Substance P (4). With regard to this, the synthesis of the
peptoid monomer corresponding to the amino acid Arg was a
particular challenge in view of the introduction of the
guanidyl moiety.
In principle, it should have been possible to prepare Fmoc-
NArg(Boc)2-OH (21) in a convenient way by deprotection of
the side chain of Fmoc-NOrn(Boc)-OH and guanylation with
N,N'-bis(Boc)-1-guanylpyrazole (18).[19] Unfortunately, the
yield of this synthetic route was quite low (ca. 30%) and
purification by column chromatography was tedious. There-
fore, Fmoc-NArg(Boc)2-OH (21) was synthesized starting
from H-NOrn(Boc)-OBzl (16), depicted in Scheme 4.
H-NOrn(Boc)-OBzl (16) was prepared by treat-
ment of N-Boc-1,3-diaminopropane (14) with benzyl
bromoacetate (15) in the presence of TEA in THF.
Benzyl bromoacetate was used instead of ethyl
bromoacetate (8) because the a-amino group and
the carboxyl group can then be deprotected simulta-
neously by hydrogenolysis. Treatment of 16 with
benzyl chloroformate gave Cbz-NOrn(Boc)-OBzl
(17). Removal of the Boc group of 17 with a saturated
solution of HCl in ether and subsequent guanidylation
of the free amine group with 18 afforded Cbz-
NArg(Boc)2-OBzl (19).[20] Removal of both the Cbz
group and benzyl ester in 19 by hydrogenolysis over
10% Pd/C gave H-NArg(Boc)2-OH (20). Subsequent
protection of the a-amino group with Fmoc ± OSu in
the presence of TEA provided the protected crystal-
line Fmoc-NArg(Boc)2-OH (21) in 88% yield.
The syntheses of the other required functional-
group containing monomers of Substance P did not
pose any particular problems (Scheme 2). The pre-
viously used Fmoc-NMet-OH (10c) was accessible
using 2-amino ethyl methyl sulfide (7c).[5a, 21] Fmoc-
NLys(Boc)-OH (10d) was accessible using N-Boc-1,4-
diaminobutane (7d), and Fmoc-NGln-OH (10e) was
accessible in a substitution reaction with the amide of
b-alanine (7e) (Scheme 2).
After the preparation of all the required peptoid
monomers, the stage was now set for the preparation
of both the peptoid 5 and retropeptoid 6 of Substance
P. For the purpose of comparison of mass spectra, and
in order to illustrate that the solid-phase synthesis of
(retro)peptoids is virtually identical to that of pep-
tides, the solid-phase synthesis of Substance P is also
shown.
Coupling and deprotection cycles for the solid-
phase synthesis of (retro)peptoids are virtually iden-
tical to those for peptides and therefore can be
implemented on a commercial solid-phase peptide
synthesizer. For synthesis of the substance P peptide 4
(Scheme 5), the first monomer Fmoc-Met-OH was
attached to the same resin used in the synthesis of the
(retro)peptoid of Leu-enkephalin. After the last
coupling cycle, the N-terminal Fmoc group was
removed by a piperidine solution and the N-terminus
Scheme 3. Solid-phase synthesis of the peptoid
2 and retropeptoid 3 of Leu-
enkephalin (1) and the solid-phase synthesis monitoring profiles. i) a) piperidine,
b) Fmoc-NLeu-OH 10b, PyBroP, DiPEA; ii) a) piperidine, b) Fmoc-NPhe-OH 10a,
PyBroP, DiPEA; iii) a) piperidine, b) Fmoc-Gly-OH, PyBroP, DiPEA; iv) a)
piperidine, b) Fmoc-Gly-OH, PyBroP, DiPEA; v) a) piperidine, b) Fmoc-NTyr(tBu)-
OH (10 f), PyBroP, DiPEA; vi) a) piperidine, b) Ac2O, DiPEA, HOBt; vii) TFA/H2O/
TIS, 95/2.5/2.5 (v/v).
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The structures of peptoid 5 and retropeptoid 6 of
substance P were ascertained by FAB mass
spectrometry (Figure 3).[8c]
Both the mass spectra of the peptide and
peptoid exhibit B- and Y''-type sequence ions at
identical m/z values. The presence of N-substi-
tuted glycine residues in a peptoid is further
apparent from the presence of N-substituted
immonium ions, which differ significantly in
their fragmentation behavior from the corre-
sponding immonium ions observed in the spec-
tra of common oligopeptides.[8] Loss of the
ˆ
CH2 NH imine molecule is the dominant frag-
mentation reaction in the collision induced
dissociation (CID) spectra of all peptoid immo-
nium ions investigated in this study. Thus, the
‡
[M‡H] CID spectra of the investigated peptoid
peptidomimetics exhibit characteristic B- and
Y''-type ions, which allow sequence analysis.
The biological activity of substance P (4) and
its corresponding peptoid 5 and retropeptoid 6
Scheme 4. Synthesis of peptoid monomer Fmoc-NArg(Boc)2-OH (22) required for the solid-
phase syntheses of the (retro)peptoids of substance P.
was acetylated.[22] Cleavage from the resin and simultaneous
removal of the protecting groups using TFA with thioanisole,
H2O, ethanedithiol (EDT), and TIS as scavengers gave, after
purification by Sephadex LH-20 and preparative HPLC, pure
substance P (4). Peptoid 5 and retropeptoid 6 (Scheme 5),
attached to the solid support, were prepared according to a
protocol essentially identical to that described above for the
substance P peptide, except that PyBroP was used as a
coupling reagent instead of BOPÐanalogous to the synthesis
of the (retro)peptoid of Leu-enkephalin. After deprotection,
cleavage, and purification, 104 mg (26%) of peptoid 5 and
132 mg (37%) of retropeptoid 6 (Scheme 5) were obtained.
was evaluated on isolated mouse trachea. It was found that 5
and 6 are agonists of substance P, displaying their activity in
micro- to submicromolar concentrations (Scheme 5)![23] This
is remarkable in view of the absence of chirality in most of the
building blocks of the (retro)peptoid. The efficacy of the
peptoid 5 and retropeptoid 6 of substance P is probably even
greater since these peptoids are more lipophilic (Figure 4)
than substance P itself and can therefore cross membrane
barriers more easily. Moreover, degradation of (retro)pep-
toids by proteases is strongly reduced, as was shown by
treatment of substance P with pepsin.[7] After 20 h peptoid 5 of
substance P is still intact, whereas 4 is already largely
Scheme 5. Solid-phase syntheses of substance P (4), its corresponding peptoid 5, and retropeptoid 6, as well as the solid-phase synthesis monitoring profiles.
For 4: i) a) piperidine, b) Fmoc-Met-OH, BOP, HOBt, DiPEA; ii) a) piperidine, b) Fmoc-Leu-OH, BOP, HOBt, DiPEA; iii) a) piperidine, b) Fmoc-Gly-
OH, BOP, HOBt, DiPEA; iv) a) piperidine, b) Fmoc-Phe-OH, BOP, HOBt, DiPEA; v) a) piperidine, b) Fmoc-Phe-OH, BOP, HOBt, DiPEA; vi) a)
piperidine, b) Fmoc-Gln(Trt)-OH, BOP, HOBt, DiPEA; vii) a) piperidine, b) Fmoc-Gln(Trt)-OH, BOP, HOBt, DiPEA; viii) a) piperidine, b) Fmoc-Pro-
OH, BOP, HOBt, DiPEA; ix) a) piperidine, b) Fmoc-Lys(Boc)-OH, BOP, HOBt, DiPEA; x) a) piperidine, b) Fmoc-Pro-OH, BOP, HOBt, DiPEA; xi) a)
piperidine, b) Fmoc-Arg(Pmc)-OH, BOP, HOBt, DiPEA; xii) a) piperidine, b) Ac2O, DiPEA, HOBt; xiii) TFA/thioanisole/H2O/EDT/TIS, 86.5/5/5/2.5/1 (v/
v). For 5 and 6: i) a) piperidine, b) Fmoc-NMet-OH (10c), PyBroP, DiPEA; ii) a) piperidine, b) Fmoc-NLeu-OH (10b), PyBroP, DiPEA; iii) a) piperidine,
b) Fmoc-Gly-OH, PyBroP, DiPEA; iv) a) piperidine, b) Fmoc-NPhe-OH 10a), PyBroP, DiPEA; v) a) piperidine, b) 10a, PyBroP, DiPEA; vi) a) piperidine,
b) Fmoc-NGln-OH (10e), PyBroP, DiPEA; vii) a) piperidine, b) 10e, PyBroP, DiPEA; viii) a) piperidine, b) Fmoc-Pro-OH, PyBroP, DiPEA; ix) a)
piperidine, b) Fmoc-NLys(Boc)-OH (10d), PyBroP, DiPEA; x) a) piperidine, b) Fmoc-Pro-OH, PyBroP, DiPEA; xi) a) piperidine, b) Fmoc-NArg(Boc)2-
OH (21), PyBroP, DiPEA; xii) a) piperidine, b) Ac2O, DiPEA, HOBt; xiii) TFA/thioanisole/H2O/EDT/TIS, 86.5/5/5/2.5/1 (v/v).
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Peptoid Synthesis
1570±1580
Figure 3. FAB mass spectra and mass fragments of substance P (4), its corresponding peptoid 5, and retropeptoid 6.
degraded after 30 min (Figure 5). This reduced degradation
rate will also certainly contribute to a greater effectiveness of
the (retro)peptoid.
be conveniently prepared in high yields using economical
chemistry. Only Fmoc-NGln-OH (10e) was obtained in a
relatively low yield (overall yield of 26%), because poor
solubility of the intermediates hampered the purification.
When suitable protection groups are used, identical or similar
side chains to those found in the proteinogenic amino acids, as
well as other side chains, can be incorporated into peptoid
monomers.
Conclusions
We have developed an efficient synthetic strategy for the
synthesis of peptoid monomers, that is, Fmoc-protected N-
substituted glycines. It was shown that peptoid monomers can
Furthermore, an efficient solid-phase methodology for the
synthesis of peptoids using a fully automated peptide synthe-
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R. M. J. Liskamp et al.
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The synthesized substance P analogues, peptoid 5 and
retropeptoid 6, have been used to evaluate their biological
activity compared with that of substance P derivative 4.
Recent results demonstrate agonist activity for the peptoid 5
and retropeptoid 6 analogues of substance P in vitro. Further
evaluation of the biological properties of these peptoid
peptidomimetics of substance P is currently being undertaken
and will be published elsewhere.[23]
Experimental Section
General: Chemicals were obtained from commercial sources and used
without further purification unless otherwise stated. TEA and DiPEAwere
distilled from ninhydrin and subsequently KOH. Dry solvents were
distilled immediately prior to use from an appropriate drying agent.
THF, ether, and cyclohexane were distilled from LiAlH4. Ethanol-free
DCM was distilled from CaH2. Ethanol and methanol were refluxed over
magnesium for 2 h, distilled, and stored on molecular sieves (3 Š). NMP,
peptide grade, was purchased from Biosolve (The Netherlands). All
protected amino acids were purchased from Advanced Chemtech (Bel-
gium). BOP reagent was purchased from Richelieu Biotechnologies
(Canada). Tentagelꢂ SOH (130 mm) resin and Tentagelꢂ SRAM
(130 mm) resin were purchased from Rapp Polymere, Tübingen (Germa-
ny).
Figure 4. Relative hydrophobicity of substance P (4) and its corresponding
peptoid 5.
Reactions were run at ambient temperature unless otherwise noted.
Reactions were monitored and Rf values were determined by thin-layer
chromatography (TLC) on Merck precoated silica gel 60F-254 (0.25 mm)
plates. Spots were visualized with UV light, ninhydrin, or Cl2-TDM
(N,N,N',N'-tetramethyl-4,4'-diaminodiphenylmethane).[24] Solvents were
evaporated under reduced pressure at 408C. Column chromatography
was performed on Merck Kieselgel 60 (40 ± 63 mm). Flash column
chromatography was performed on Merck Kieselgel 60H (5 ± 40 mm).
Sephadex LH-20 (Pharmacia) was used for gel filtration. Fast atom
bombardment (FAB) mass spectrometry was carried out with a Jeol JMS
SX/SX102A four-sector mass spectrometer coupled with a HP-9000 data
system. 1H NMR spectra were recorded on a Jeol JNM-FX200 (200 MHz)
or a Varian G-300 (300.1 MHz) spectrometer, and chemical shifts are given
in ppm (d) relative to TMS or TSP as internal standard. 13C NMR spectra
were recorded on a Jeol JNM-FX200 spectrometer or a Varian G-300
spectrometer operating at 50.1 and 75.5 MHz, respectively, and the
chemical shifts are given in ppm (d) relative to CDCl3 (d ˆ 77.0) or
(CD3)2SO (d ˆ 39.7) as internal standard. 13C NMR spectra were monitored
by the attached proton test (APT) technique. The compounds were
homogeneous according to NMR and TLC.
Solid-phase syntheses were carried out on an ABI433A Peptide Synthe-
sizer. Deprotection and coupling reactions were monitored at 301 nm.
Analytical HPLC was performed on a semiautomatic HPLC system
(Applied Biosystems) with an analytical reversed-phase column (Altech
Econosphere C8, 5 mm, 250 Â 4.6 mm), an UV detector operating at 214 nm
(peptoids) or 220 nm (peptides), at a flow rate of 1 mLmin 1. Elution of
peptoids was effected using an appropriate gradient from 10mm HClO4/
100mm NaClO4 in water to 10mm HClO4/100mm NaClO4 in acetonitrile/
water (90/10, v/v). Elution of peptides was effected using an appropriate
gradient from 0.1% TFA in water to 0.085% TFA in acetonitrile/water (95/
5, v/v). Preparative HPLC was performed on a semiautomatic HPLC
system (Applied Biosystems) with a preparative reversed-phase column
(Altech Econosphere C8, 10 mm, 250 Â 22 mm), an UV detector operating
at 220 nm, at a flow rate of 11.5 mLmin 1. Elution was effected using an
appropriate gradient from 0.1% TFA in water to 0.085% TFA in
acetonitrile/water (95/5, v/v).
Figure 5. Pepsin treatment of Substance P (4) and its corresponding
peptoid 5.
sizer has been developed. It was shown that peptoids can be
rapidly and conveniently synthesized from peptoid mono-
mers, specifically, Fmoc-protected N-substituted glycines.
With these peptoid monomers it was possible to follow the
progress of the solid-phase synthesis on the ABI433A peptide
synthesizer with the UV monitoring system. This arrangement
provides an adequate representation of the events which have
taken place in the reaction vessel.
H-NPhe-OEt (9a): Ethyl bromoacetate (8, 50 mmol, 5.54 mL) in THF
(25 mL) was added dropwise to a cooled (ice bath) solution of benzylamine
(7a, 110 mmol, 12.0 mL) in THF (25 mL). After stirring for 2.5 h at room
temperature, the reaction mixture was concentrated in vacuo to remove
THF and resuspended in ether. The mixture was filtered to remove
benzylamine hydrobromide, the residue washed with ether, and the filtrate
concentrated in vacuo. Column chromatography (silica, eluent: ether)
1576
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
0947-6539/98/0408-1576 $ 17.50+.50/0
Chem. Eur. J. 1998, 4, No. 8

Peptoid Synthesis
1570±1580
afforded 9a (9.36 g, 48.4 mmol) as an oil in 97% yield.[25] Rf ˆ 0.51 (eluent:
ether); 1H NMR (200 MHz, CDCl3): d ˆ 1.28 (t, 3H, OCH2CH3, J ˆ
7.2 Hz), 1.96 (brs, 1H, NH), 3.41 (s, 2H, NCH2C(O)), 3.81 (s, 2H, CH2Ph),
4.19 (q, 2H, OCH2CH3, J ˆ 7.2 Hz), 7.27 ± 7.35 (m, 5H, Ph); 13C NMR
(50.1 MHz, CDCl3): d ˆ 13.8 (OCH2CH3), 49.6 (NCH2C(O)), 52.8
(CH2Ph), 60.2 (OCH2CH3), 126.6, 127.8, 128.0, 139.2 (Ph), 171.9
(NCH2C(O)).
(t, 2H, CH2SMe, J ˆ 6.2 Hz), 2.84 (t, 2H, HNCH2, J ˆ 6.2 Hz), 3.44 (s, 2H,
NCH2C(O)), 4.20 (q, 2H, OCH2CH3, J ˆ 7.2 Hz); 13C NMR (50.1 MHz,
CDCl3): d ˆ 13.6 (OCH2CH3), 14.4 (SCH3), 33.6 (CH2SMe), 46.8 (HNCH2),
49.9 (NCH2C(O)), 60.0 (OCH2CH3), 171.6 (NCH2C(O)).
Fmoc-NMet-OH (10c): Fmoc-NMet-OH was prepared, analogously to the
preparation of 10a, from H-NMet-OEt (9c, (10 mmol, 1.77 g). Crystal-
lization (EtOAc/hexanes) afforded 10c (3,65 g, 9.82 mmol) as a white solid
in 98% yield. Rf ˆ 0.40 (eluent: DCM/MeOH/HOAc, 90/10/0.5, v/v). H-N-
Met-ONa: 1H NMR (200 MHz, D2O): d ˆ 2.10 (s, 3H, SCH3), 2.62 ± 2.69
(m, 2H, CH2SMe), 2.74 ± 2.81 (m, 2H, HNCH2), 3.18 (s, 2H, NCH2C(O)).
10c: The NMR spectra clearly show the presence of both rotamers; 1H
NMR (300 MHz, CDCl3): d ˆ 1.94, 2.12 (two s, 3H, SCH3), 2.35, 2.66 (two t,
2H, CH2SMe, J ˆ 7.3 Hz), 3.29, 3.51 (two t, 2H, FmocNCH2, J ˆ 7.3 Hz),
3.97, 4.07 (two s, 2H, NCH2C(O)), 4.19 ± 4.27 (m, 1H, CH-Fmoc), 4.48, 4.61
(two d, 2H, CH2-Fmoc, J ˆ 6.2 Hz), 7.29 ± 7.43 (m, 4H, ArH-Fmoc), 7.52 ±
7.59 (4 lines, 2H, ArH-Fmoc), 7.75 (t, 2H, ArH-Fmoc); 13C NMR
(75.5 MHz, CDCl3): d ˆ 15.4 (SCH3), 32.0 (CH2SMe), 47.2 (CH-Fmoc),
48.1, 48.3 (FmocNCH2), 49.2, 49.7 (NCH2C(O)), 67.3, 67.7 (CH2-Fmoc),
119.9, 124.6, 127.1, 127.7, 141.3, 143.6 (ArC-Fmoc), 155.6, 156.2 (Fmoc
Fmoc-NPhe-OH (10a): NaOH (4n, 2.50 mL) was added to a solution of
H-N-Phe-OEt (9a, 10 mmol, 1.93 g) in dioxane (35 mL) and MeOH
(12.5 mL). After stirring for 30 min at room temperature the reaction
mixture was concentrated in vacuo to give H-N-Phe-ONa. 1H NMR
(200 MHz, D2O): d ˆ 3.17 (s, 2H, NCH2C(O)), 3.73 (s, 2H, CH2Ph), 7.37 ±
7.41 (m, 5H, Ph).
The sodium salt was dissolved in water (10 mL) and the pH adjusted to 9 ±
9.5 with concentrated hydrochloric acid. To this mixture a solution of
Fmoc ± OSu (10 mmol, 3.37 g) in acetonitrile (20 mL) was added in one
portion. Stirring was continued for 30 minutes, and the pH was maintained
at pH 8.5 ± 9.0 by the addition of TEA. The reaction mixture was
concentrated in vacuo to remove acetonitrile, and the residue was poured
into 20% citric acid (60 mL). The aqueous layer was extracted with EtOAc
(3 Â 75 mL), and the combined organic layers were washed with water and
brine, dried (Na2SO4), and concentrated in vacuo to give Fmoc-NPhe-OH
(10a) as an oil which was crystallized from EtOAc/hexanes to afford 10a
(3.65 g, 9.42 mmol) as a white solid in 94% yield. Rf ˆ 0.61 (eluent: DCM/
MeOH/HOAc, 90/10/0.5, v/v); the NMR spectra clearly show the presence
ˆ
C O), 174.6, 174.8 (NCH2C(O)OH).
H-NLys(Boc)-OEt (9d): Ethyl bromoacetate (8, 50 mmol, 5.54 mL) in
THF (30 mL) was added dropwise to a solution of N-Boc-1,4-diaminobu-
tane (7d, 50 mmol, 9.41 g) and TEA (100 mmol, 13.94 mL) in THF
(30 mL).[5b, 21] After stirring overnight at room temperature, the reaction
mixture was concentrated in vacuo to remove THF and resuspended in
ether. The reaction mixture was filtered to remove triethylamine hydro-
bromide, the residue was washed with ether and the filtrate concentrated in
vacuo. Column chromatography (silica, eluent: gradient of hexanes/
EtOAc, 1/1 to EtOAc, v/v) gave 9d (10.31 g, 37.6 mmol) as an oil in 75%
yield. Rf ˆ 0.69 (eluent: DCM/MeOH/HOAc, 90/10/0.5, v/v); 1H NMR
(200 MHz, CDCl3): d ˆ 1.29 (t, 3H, OCH2CH3, J ˆ 7.2 Hz), 1.44 (s, 9H,
C(CH3)3), 1.57 (m, 4H, CH2CH2), 2.70 (m, 2H, HNCH2), 3.11 (m, 2H,
BocNCH2), 3.46 (s, 2H, NCH2C(O)), 4.20 (q, 2H, OCH2CH3, J ˆ 7.2 Hz),
4.78 (brs, 1H, NH); 13C NMR (50.1 MHz, CDCl3): d ˆ 14.0 (OCH2CH3),
27.0 (BocNCH2CH2), 27.5 (HNCH2CH2), 28.2 (C(CH3)3), 40.1 (BocNCH2),
48.9 (HNCH2), 50.6 (NCH2C(O)), 60.4 (OCH2CH3), 78.6 (C(CH3)3), 155.8
1
of both rotamers; H NMR (300 MHz, CDCl3): d ˆ 3.76, 4.00 (two s, 2H,
NCH2C(O)), 4.22 ± 4.30 (m, 1H, CH-Fmoc), 4.49, 4.56 (two s, 2H, CH2Ph),
4.57 (d, 2H, CH2-Fmoc, J ˆ 6.2 Hz), 7.07 ± 7.09 (m, 1H, Ph), 7.18 ± 7.40 (m,
8H, ArH-Fmoc, Ph), 7.49 ± 7.56 (4 lines, 2H, ArH-Fmoc), 7.74 (d, 2H, ArH-
Fmoc); 13C NMR (75.5 MHz, CDCl3): d ˆ 46.8, 47.8 (NCH2C(O)), 47.2
(CH-Fmoc), 51.2, 51.4 (CH2Ph), 67.7, 68.0 (CH2-Fmoc), 120.0, 124.9, 127.1,
127.6, 127.7, 128.1, 128.7, 136.3, 141.3, 143.7 (ArC-Fmoc, Ph), 156.2, 156.7
ˆ
(Fmoc C O), 174.9 (NCH2C(O)OH).
H-NLeu-OEt (9b): H-N-Leu-OEt 9b was prepared, analogously to the
preparation of 9a, from isobutylamine (7b, 110 mmol, 10.9 mL) and 8
(50 mmol, 5.54 mL). Column chromatography (silica, eluent: ether) gave
ˆ
(Boc C O), 172.2 (NCH2C(O)).
9b (7.55 g, 47.4 mmol) as an oil in 95% yield. Rf ˆ 0.39 (eluent: ether);[25] 1
H
Fmoc-NLys(Boc)-OH (10d): Fmoc-NLys(Boc)-OH was prepared, anal-
ogously to the preparation of 10a, from 9d (10 mmol, 2.74 g). Crystal-
lization (EtOAc/hexanes) afforded 10d (4.46 g, 9.52 mmol) as a white solid
in 95% yield. Rf ˆ 0.51 (eluent: DCM/MeOH/HOAc, 90/10/0.5, v/v).
H-NLys(Boc)-ONa: 1H NMR (200 MHz, D2O): d ˆ 1.42 (s, 9H, C(CH3)3),
1.55 (m, 4H, CH2CH2), 2.79 (m, 2H, HNCH2), 3.08 (m, 2H, BocNCH2),
3.35 (s, 2H, NCH2C(O)). 10d: The NMR spectra clearly show the presence
NMR (200 MHz, CDCl3): d ˆ 0.93 (d, 6H, CH(CH3)2, J ˆ 6.7 Hz), 1.28 (t,
3H, OCH2CH3, J ˆ 7.2 Hz), 1.56 (brs, 1H, NH), 1.74 (quintet, 1H,
CH2CH(CH3)2), 2.41 (d, 2H, CH2CH(CH3)2, J ˆ 6.7 Hz), 3.39 (s, 2H,
NCH2C(O)), 4.19 (q, 2H, OCH2CH3, J ˆ 7.2 Hz); 13C NMR (50.1 MHz,
CDCl3): d ˆ 13.7 (OCH2CH3), 20.0 (CH2CH(CH3)2), 28.0 (CH2CH(CH3)2),
50.6 (NCH2C(O)), 57.1 (CH2CH(CH3)2), 59.9 (OCH2CH3), 171.9
(NCH2C(O)).
1
of both rotamers; H NMR (300 MHz, CDCl3): d ˆ 1.2 ± 1.4, 1.4 ± 1.6 (two
Fmoc-NLeu-OH (10b): Fmoc-NLeu-OH 10b was prepared, analogous to
the preparation of 10a, from H-NLeu-OEt 9b (10 mmol, 1.59 g). Crystal-
lization (ether/hexanes) afforded Fmoc-NLeu-OH 10b (3,39 g, 9.87 mmol)
as a white solid in 99% yield. Rf ˆ 0.67 (eluent: DCM/MeOH/HOAc,
90/10/0.5, v/v).
H-NLeu-ONa: 1H NMR (200 MHz, D2O): d ˆ 0.89 (d, 6H, CH(CH3)2, J ˆ
6.7 Hz), 1.74 (quintet, 1H, CH2CH(CH3)2), 2.36 (d, 2H, CH2CH(CH3)2,
J ˆ 6.7 Hz), 3.14 (s, 2H, NCH2C(O)).
Fmoc-NLeu-OH 10b: The NMR spectra clearly show the presence of both
rotamers; 1H NMR (300 MHz, CDCl3): d ˆ 0.72, 0.87 (two d, 6H,
CH2CH(CH3)2, J ˆ 6.8 Hz), 1.61, 1.82 (two septets, 1H, CH2CH(CH3)2),
2.93, 3.13 (two d, 2H, CH2CH(CH3)2, J ˆ 7.5 Hz), 3.87, 3.99 (two s, 2H,
NCH2C(O)), 4.16 ± 4.25 (m, 1H, CH-Fmoc), 4.46, 4.54 (two d, 2H, CH2-
Fmoc, J ˆ 6.0 Hz), 7.23 ± 7.41 (m, 4H, ArH-Fmoc), 7.51 ± 7.58 (4 lines, 2H,
ArH-Fmoc), 7.73 (t, 2H, ArH-Fmoc); 13C NMR (75.5 MHz, CDCl3): d ˆ
19.8, 19.9 (CH2CH(CH3)2), 27.2, 27.4 (CH2CH(CH3)2), 47.3 (CH-Fmoc),
48.8, 49.6 (NCH2C(O)), 55.7, 56.1 (CH2CH(CH3)2), 67.4 (CH2-Fmoc), 119.9,
m, 4H, CH2CH2), 1.45 (brs, 9H, C(CH3)3), 2.98 (m, 1H, FmocNCH2), 3.09
(m, 2H, FmocNCH2 and BocNCH2), 3.34 (m, 1H, BocNCH2), 3.89, 3.97
(two brs, 2H, NCH2C(O)), 4.24 (m, 1H, CH-Fmoc), 4.45, 4.58 (two m, 2H,
CH2-Fmoc), 7.25 ± 7.42 (m, 4H, ArH-Fmoc), 7.56 (t, 2H, ArH-Fmoc), 7.75
(m, 2H, ArH-Fmoc); 13C NMR (75.5 MHz, CDCl3): d ˆ 25.0, 25.3
(FmocNCH2CH2), 27.1 (BocNCH2CH2), 28.4 (C(CH3)3), 40.1 (BocNCH2),
47.2 (CH-Fmoc), 48.0 (FmocNCH2), 48.4, 48.9 (NCH2C(O)), 67.2, 67.7
(CH2-Fmoc), 79.3 (C(CH3)3), 119.9, 124.7, 124.9, 127.0, 127.1, 127.6, 141.3,
ˆ
ˆ
141.4, 143.9 (ArC-Fmoc), 156.0, 156.6 (Boc C O and Fmoc C O), 173.4
(broad, NCH2C(O)OH).
H-NGln-OEt (9e): Ethyl bromoacetate (8, 41 mmol, 4.55 mL) in THF
(40 mL) was added dropwise to a solution of H-b-Ala-NH2 (7e, 41 mmol,
3.61 g) and TEA (82 mmol, 11.41 mL) in ethanol (60 mL),. After stirring
overnight at room temperature, the reaction mixture was filtered, the
residue washed with ether, and the filtrate concentrated in vacuo. Flash
column chromatography (silica, eluent: MeOH/DCM, 10/90, v/v) gave 9e
(4.41 g, 25.3 mmol) as a light yellow oil in 62% yield. Rf ˆ 0.37 (eluent:
DCM/MeOH, 80/20, v/v); 1H NMR (300 MHz, CDCl3): d ˆ 1.22 (t, 3H,
OCH2CH3, J ˆ 7.1 Hz), 1.84 (brs, 1H, NH), 2.46 (t, 2H, CH2C(O)NH2, J ˆ
6.6 Hz), 2.87 (t, 2H, HNCH2, J ˆ 6.5 Hz), 3.37 (s, 2H, NCH2C(O)), 4.14 (q,
2H, OCH2CH3, J ˆ 7.1 Hz); 13C NMR (75.5 MHz, CDCl3): d ˆ 14.1
(OCH2CH3), 34.9 (CH2C(O)NH2), 44.7 (HNCH2), 50.8 (NCH2C(O)),
60.6 (OCH2CH3), 172.2 (C(O)NH2), 172.3 (C(O)OEt).
ˆ
124.8, 127.0, 127.6, 141.4, 143.9 (ArC-Fmoc), 156.2, 157.1 (Fmoc C O),
174.5, 174.7 (NCH2C(O)OH).
H-NMet-OEt (9c): H-N-Met-OEt was prepared, analogously to the
preparation of 9a, from 2-aminoethyl methyl sulfide (7c, 30 mmol,
2.74 g) and 8 (15 mmol, 1.66 mL).[5b, 21] Column chromatography (silica,
eluent: gradient of hexanes/EtOAc, 40/60 to hexanes/EtOAc, 20/80, v/v)
gave 9c (2.33 g, 13.2 mmol) as an oil in 88% yield.[25] Rf ˆ 0.56 (eluent:
DCM/MeOH/HOAc, 90/10/0.5, v/v); 1H NMR (200 MHz, CDCl3): d ˆ 1.29
(t, 3H, OCH2CH3, J ˆ 7.2 Hz), 1.90 (brs, 1H, NH), 2.12 (s, 3H, SCH3), 2.66
Fmoc-NGln-OH (10e): NaOH (4n, 2.55 mL) was added to a solution of 9e
(10.2 mmol, 1.78 g) in dioxane (35.7 mL) and MeOH (12.8 mL). After
stirring for 30 min at room temperature the reaction mixture was
Chem. Eur. J. 1998, 4, No. 8
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
0947-6539/98/0408-1577 $ 17.50+.25/0
1577

R. M. J. Liskamp et al.
FULL PAPER
concentrated in vacuo to afford H-NGln-ONa. 1H NMR (300 MHz, D2O):
d ˆ 2.48 (t, 2H, CH2C(O)NH2, J ˆ 6.8 Hz), 2.84 (t, 2H, HNCH2, J ˆ
6.8 Hz), 3.19 (s, 2H, NCH2C(O)).
Fmoc), 6.92 (m, 3H, ArC3H, ArC5H, ArC2H, ArC6H), 7.19 (d, 1H, ArC2H,
ArC6H), 7.29 (m, 2H, ArH-Fmoc), 7.38 (m, 2H, ArH-Fmoc), 7.55 (d, 2H,
ArH-Fmoc), 7.74 (d, 2H, ArH-Fmoc); 13C NMR (75.5 MHz, CDCl3): d ˆ
28.8 (C(CH3)3), 47.2 (CH-Fmoc), 47.5 (NCH2C(O)), 50.6, 50.7 (CH2Ar),
67.9, 68.2 (CH2-Fmoc), 78.6 (C(CH3)3), 120.0, 124.9, 127.1, 127.7, 141.4, 143.7
(ArC-Fmoc), 124.2 (ArC, ArC5), 128.3, 128.8 (ArC2, ArC6), 131.0 (ArC1),
The sodium salt was dissolved in water (50 mL), and the pH adjusted to 9 ±
9.5 using 1n HCl. To this mixture a solution of Fmoc-OSu (11 mmol, 3.71 g)
in acetone (40 mL) was added dropwise. Stirring was continued for 2 h and
the pH was maintained at pH 9.5 by the addition of 1n NaOH. The reaction
mixture was filtered, acidified to pH 2 using 2n HCl, and extracted with
EtOAc (3 Â 100 mL). The combined organic layers were concentrated in
vacuo. Crystallization (EtOAc/hexanes) gave 10e (1.62 g, 4.40 mmol) as a
white solid in 43% yield. Rf ˆ 0.38 (eluent: DCM/MeOH/HOAc, 90/10/0.5,
v/v); the NMR spectra clearly show the presence of both rotamers; 1H
NMR (300 MHz, (CD3)2SO): d ˆ 2.33 (q, 2H, CH2C(O)NH2), 3.46
(quintet, 2H, HNCH2), 3.94, 4.01 (two s, 2H, NCH2C(O)), 4.23 ± 4.29 (m,
3H, CH2-Fmoc, CH-Fmoc), 6.85 (brd, 1H, C(O)NH2), 7.29 ± 7.44 (m, 5H,
ArH-Fmoc, C(O)NH2), 7.61, 7.68 (two d, 2H, ArH-Fmoc), 7.88 ± 7.91 (m,
2H, ArH-Fmoc); 13C NMR (75.5 MHz, (CD3)2SO): d ˆ 33.8, 34.3
(CH2C(O)NH2), 44.3, 44.9 (FmocNCH2), 46.6 (CH-Fmoc), 48.9, 49.1
(NCH2C(O)), 67.1 (CH2-Fmoc), 120.1, 125.1, 126.7, 127.1, 127.7, 140.7, 143.7,
4
ˆ
155.1 (ArC ), 156.6 (Fmoc C O), 174.5 (NCH2C(O)).
Ac-Tyr-Gly-Gly-Phe-Leu-NH2 (Ac-YGGFL-NH2) (1):
Resin preparation: Fmoc-Leu-TentaGel resin: Fmoc-Leu-OH was attached
to the TentaGelꢂ SOH resin (capacity 0.29 mmol/g) by the procedure of
1
Sieber, yielding a resin with a loading of 0.24 mmolg (83%) as was
determined by Fmoc cleavage from a resin sample.[10, 26] With this resin
(1.05 g), peptide 1 was synthesized on an automatic ABI433A Peptide
Synthesizer using the ABI FastMoc 0.25 mmol protocols,[27] except that the
coupling time was 45 min instead of 20 min and BOP/HOBt activation was
used instead of HBTU/HOBt. After cleavage of the Fmoc group by means
of a 20% piperidine solution in NMP (2 Â 3 min and 7.6 min), the resin was
washed with NMP (5 Â ). Subsequently, 4 equiv (1 mmol) of the appro-
priate amino acid were dissolved in NMP (2 mL), and BOP/HOBt (2 mL of
0.45m) in NMP was added. To this mixture DiPEA (1 mL, 2m) in NMP was
added, and the activated amino acid was then transferred to the reaction
vessel. After 45 min, the reaction vessel was drained and the resin was
washed with NMP (3 Â ). The deprotection and coupling reactions were
followed by monitoring the dibenzofulvene ± piperidine adduct at
301 nm.[27] The last coupling cycle was followed by removal of the Fmoc
group by a piperidine solution, washing the resin with NMP, and acetylation
of the N-terminus by treatment with an acetic anhydride capping solution
(0.5m Ac2O, 0.125m DiPEA, and 0.015m HOBt in NMP) for 15 min.
Finally, the resin was washed with NMP (5 Â ) and DCM (6 Â ), removed
from the reaction vessel, washed with ether, and dried in vacuo over P2O5.
The anchored peptide thus obtained was cleaved from the solid support by
treatment with a saturated solution of ammonia (20 mL) in MeOH for 12 h
at room temperature. The mixture was then filtered and the residue washed
thoroughly with MeOH. The total filtrate was concentrated in vacuo.
Purification by flash column chromatography (silica 60H, eluent: MeOH/
DCM, 10/90, v/v) gave a partially protected peptide (140 mg). Rf 0.79
(eluent: MeOH/DCM, 20/80, v/v). The tBu group of Tyr(tBu) in this
peptide was removed by treatment with TFA/H2O (95%, 5 mL) for 2 h at
room temperature. The reaction mixture was concentrated in vacuo to a
volume of approximately 1 to 2 mL, and then cold ether (50 mL) was added
to precipitate the peptide. The precipitated peptide 1 was collected by
filtration through a fritted glass funnel and dried in vacuo over P2O5 to give
96 mg peptide 1 (67%). Rf ˆ 0.56 (eluent: MeOH/DCM, 20/80, v/v). The
peptide 1 was pure according to analytical HPLC. FAB MS: m/z ˆ 597
ˆ
143.8 (ArC-Fmoc), 155.2, 155.5 (Fmoc C O), 171.0, 171.2 (C(O)NH2),
172.4, 172.5 (C(O)OH).
4-tert-Butyloxybenzonitrile (13): 4-Hydroxybenzonitrile (11, 15 mmol,
1.79 g) was dissolved in ether (15 mL) and cyclohexane (30 mL). To this
mixture, tert-butyltrichloroacetimidate 12 (60 mmol,[16] 10 mL) and
a
catalytic amount of BF3 ´ Et2O (300 mL) was added. After stirring for
22 h, solid NaHCO3 was added and stirring was continued for 10 min. The
reaction mixture was filtered and the filtrate concentrated in vacuo.
Column chromatography (silica, eluent: hexanes/ether, 3/1, v/v) afforded
13 (1.66 g, 9.5 mmol) as a syrup in 63% yield. Rf ˆ 0.51 (eluent: ether); 1H
NMR (300 MHz, CDCl3): d ˆ 1.42 (s, 9H, C(CH3)3), 7.0 ± 7.1 (m, 2H,
ArC3H, ArC5H), 7.5 ± 7.6 (m, 2H, ArC2H, ArC6H); 13C NMR (75.5 MHz,
CDCl3): d 28.8 (C(CH3)3), 80.1 (C(CH3)3), 105.7 (CN), 119.0 (ArC1), 122.9
(ArC3, 5), 133.3 (ArC2, 6), 159.9 (ArC4).
4-tert-Butyloxybenzylamine (7 f): A solution of 4-tert-butyloxybenzonitrile
(13, 33.7 mmol, 5.90 g) in ether (20 mL) was added dropwise to
a
suspension of LiAlH4 (51 mmol, 1.94 g) in ether (40 mL). The reaction
mixture was refluxed for 3 h. The reaction mixture was then cooled to room
temperature and water (1.96 mL), 15% NaOH (1.96 mL), and water
(5.90 mL) were subsequently added. The mixture was filtered, the residue
washed twice with ether, and the filtrate concentrated in vacuo. Water
(100 mL) was added to the residue, and the pH adjusted to 2.7 with 1n
KHSO4. The aqueous layer was washed with ether (20 mL), the pH
adjusted to 9 ± 10 with 2n NaOH, and the layer extracted with DCM
(150 mL). The organic layer was washed with brine, dried over MgSO4, and
concentrated in vacuo to obtain 4-tert-butyloxybenzylamine 7 f (5.68 g,
31.7 mmol) in 94% yield. Rf ˆ 0.23 (eluent: DCM/MeOH/TEA, 90/10/1, v/
‡
[M‡H] .
Ac-NTyr-Gly-Gly-NPhe-NLeu-NH2 (peptoid 2): Immobilized pentapep-
toid 2 was assembled on an automatic ABI433A Peptide Synthesizer using
the ABI FastMoc 0.25 mmol protocols,[27] with the exceptions that the
coupling time was 45 min instead of 20 min and PyBroP was used instead of
HBTU/HOBt. TentaGel SRAM resin (1.04 g, capacity 0.24 mmolg 1) was
used, and for each coupling 1 mmol peptoid monomer was activated in the
cartridge, with 1 mmol PyBroP, also present in the cartridge by addition of
2 mmol DiPEA and subsequent transfer to the reaction vessel. The
anchored pentapeptoid 2 thus obtained was deprotected and cleaved from
the solid support by treatment with TFA (10 mL) containing H2O
(0.25 mL) and TIS (0.25 mL) for 2.5 h at room temperature. The reaction
mixture was filtered and the residue was washed thoroughly with TFA (2 Â
3 mL). Work-up, described above for the preparation of 1, and subsequent
purification by flash column chromatography (silica 60H, eluent: MeOH/
DCM, 10/90, v/v) gave peptoid 2 (109 mg, 76%), which was pure according
to analytical HPLC. Rf ˆ 0.11 (eluent: MeOH/DCM, 10/80, v/v); FAB MS:
1
v). H NMR (300 MHz, CDCl3): d ˆ 1.34 (s, 9H, C(CH3)3), 1.82 (brs, 2H,
CH2NH2), 3.84 (s, 2H, CH2NH2), 6.96 (m, 2H, ArC3H, ArC5H), 7.21 (d, 2H,
ArC2H, ArC6H, J ˆ 8.4 Hz); 13C NMR (75.5 MHz, CDCl3): d ˆ 28.7
(C(CH3)3), 45.5 (CH2NH2), 78.2 (C(CH3)3), 124.1, 127.6, 137.1, 154.2 (Ar).
H-NTyr(tBu)-OEt (9 f): H-NTyr(tBu)-OEt was prepared, analogously to
the preparation of 7d, from 7 f (19.5 mmol, 3.50 g), TEA (39 mmol,
5.43 mL), and ethyl bromoacetate (8, 19.5 mmol, 2.16 mL). Column
chromatography (silica, eluent: hexanes/EtOAc, 40/60, v/v) gave 9 f
(4.30 g, 16.2 mmol) as an oil in 83% yield. Rf ˆ 0.51 (eluent: DCM/
MeOH/HOAc, 90/10/0.5, v/v); 1H NMR (300 MHz, CDCl3): d ˆ 1.28 (t,
3H, OCH2CH3, J ˆ 7.2 Hz), 1.33 (s, 9H, C(CH3)3), 1.88 (brs, 1H, NH), 3.41
(s, 2H, NCH2C(O)), 3.74 (s, 2H, CH2Ar), 4.20 (q, 2H, OCH2CH3, J ˆ
7.1 Hz), 6.95 (m, 2H, ArC3H, ArC5H), 7.22 (m, 2H, ArC2H, ArC6H); 13C
NMR (75.5 MHz, CDCl3): d ˆ 14.2 (OCH2CH3), 28.8 (C(CH3)3), 50.1
(NCH2C(O)), 52.8 (CH2Ar), 60.6 (OCH2CH3), 78.2 (C(CH3)3), 124.1
(ArC3, ArC5), 128.7 (ArC2, ArC6), 134.4 (ArC1), 154.5 (ArC4), 172.4
(NCH2C(O)).
‡
m/z ˆ 597 [M‡H] .
Ac-NLeu-NPhe-Gly-Gly-NTyr-NH2 (retropeptoid 3): Synthesis, deprotec-
tion, cleavage from the resin and work-up of the immobilized retropeptoid
3 were carried out as described for the preparation of 2. After purification
by flash column chromatography (silica 60H, eluent: gradient of MeOH/
DCM, 10/90 to MeOH/DCM, 20/80, v/v) and Sephadex LH-20 gel filtration
(eluent: DCM/MeOH, 1/1, v/v), retropeptoid 3 was obtained in a yield of
14% (20 mg) which was pure according to analytical HPLC. Rf ˆ 0.48
Fmoc-NTyr(tBu)-OH (10 f): Fmoc-NTyr(tBu)-OH was prepared, analo-
gously to the preparation of 10a, from 9 f (10 mmol, 2.65 g). Crystallization
(EtOAc/hexanes) afforded 10 f (3.55 g, 7.73 mmol) as a white solid in 77%
yield. Rf ˆ 0.47 (eluent: DCM/MeOH/HOAc, 90/10/0.5, v/v); the NMR
spectra clearly show the presence of both rotamers. 1H NMR (300 MHz,
CDCl3): d ˆ 1.33 (brs, 9H, C(CH3)3), 3.78, 3.97 (two s, 2H, NCH2C(O)),
4.27 (m, 1H, CH-Fmoc), 4.43, 4.52 (two s, 2H, CH2Ar), 4.57 (t, 2H, CH2-
‡
(eluent: MeOH/DCM, 20/80, v/v); FAB MS: m/z ˆ 597 [M‡H] .
1578
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
0947-6539/98/0408-1578 $ 17.50+.50/0
Chem. Eur. J. 1998, 4, No. 8

Peptoid Synthesis
1570±1580
H-NOrn(Boc)-OBzl (16): Benzyl bromoacetate (15, 75 mmol, 11.9 mL) in
THF (75 mL) was added dropwise to a solution of N-Boc-1,3-diaminopro-
pane (14, 75 mmol, 13.07 g) and TEA (112.5 mmol, 15.7 mL) in THF
(75 mL),[5b, 21]. After stirring overnight at room temperature, the reaction
mixture was filtered to remove triethylamine hydrobromide; the residue
was then washed with ether and the filtrate concentrated in vacuo. Column
chromatography (silica, eluent: DCM) gave 16 (22.0 g, 68.3 mmol) as an oil
in 91% yield. Rf ˆ 0.56 (eluent: DCM/MeOH, 90/10, v/v). 1H NMR
(300 MHz, CDCl3): d ˆ 1.44 (s, 9H, C(CH3)3), 1.65 (quintet, 2H,
HNCH2CH2, J ˆ 6.6 Hz), 1.73 (s, 1H, NH), 2.67 (t, 2H, HNCH2, J ˆ
6.7 Hz), 3.20 (brq, 2H, BocNCH2), 3.44 (s, 2H, NCH2C(O)), 4.92 (brs,
1H, NH), 5.17 (s, 2H, CH2Ph), 7.29 ± 7.38 (m, 5H, Ph); 13C NMR
(75.5 MHz, CDCl3): d ˆ 28.0 (C(CH3)3), 29.6 (HNCH2CH2), 38.4
(BocNCH2), 46.7 (HNCH2), 50.4 (NCH2C(O)), 66.0 (CH2Ph), 78.4
10% Pd/C (ca. 300 mg). After stirring for 5 h, the reaction mixture was
filtered through Hyflo, and evaporation of the solvent in vacuo yielded 20
quantitatively (4.50 g, 12 mmol) as a white foam. Rf ˆ 0.30 (DCM/MeOH/
HOAc, 80/20/0.5, v/v). 1H NMR (300 MHz, CDCl3/CD3OD): d ˆ 1.49 (d,
18H, two C(CH3)3), 2.03 (m, 2H, HNCH2CH2), 3.02 (brt, 2H,
HNCH2CH2CH2, J ˆ 6.2 Hz), 3.44 (s, 2H, NCH2C(O)), 3.53 (m, 2H,
HNCH2); 13C NMR (75.5 MHz, CDCl3/CD3OD): d ˆ 26.7 (HNCH2CH2),
27.5 (C(CH3)3), 27.6 (C(CH3)3), 36.5 (HNCH2CH2CH2), 44.1 (HNCH2), 49.4
ˆ
(NCH2C(O)), 80.3 (C(CH3)3), 83.8 (C(CH3)3), 152.4 (C N), 157.2 (Boc
ˆ
ˆ
C O), 161.8 (Boc C O), 168.6 (NCH2C(O)).
Fmoc-NArg(Boc)2-OH (21): H-NArg(Boc)2-OH (20, 10 mmol, 3.74 g) was
dissolved in water (10 mL) and the pH adjusted to 9 ± 9.5 with TEA. To this
mixture a solution of Fmoc-OSu (10 mmol, 3.37 g) in acetonitrile (20 mL)
was added in one portion. Stirring was continued for 30 minutes and the pH
was maintained at pH 8.5 ± 9.0 by the addition of TEA. The reaction
mixture was concentrated in vacuo to remove acetonitrile, and the residue
was poured into citric acid (20%, 60 mL). The aqueous layer was extracted
with EtOAc (3 Â 75 mL) and the combined organic layers were washed
with water and brine, dried over Na2SO4, and concentrated in vacuo to give
an oil which was crystallized from ether/hexanes to afford 21 (5.26 g,
8.82 mmol) as a white solid in 88% yield. Rf ˆ 0.59 (eluent: DCM/MeOH/
HOAc, 90/10/0.5, v/v); the NMR spectra clearly show the presence of both
rotamers; 1H NMR (300 MHz, CDCl3): d ˆ 1.38 ± 1.52 (m, 19H, two
C(CH3)3 and FmocNCH2CH2), 1.81 (m, 1H, FmocNCH2CH2), 3.16 (m, 2H,
FmocNCH2CH2CH2), 3.38 (m, 2H, FmocNCH2), 3.99 (d, 2H, NCH2C(O)),
4.19, 4.26 (two brt, 1H, CH-Fmoc), 4.43, 4.57 (two d, 2H, CH2-Fmoc),
7.25 ± 7.40 (m, 4H, ArH-Fmoc), 7.55 (brt, 2H, ArH-Fmoc), 7.74 (brt, 2H,
ArH-Fmoc); 8.33, 8.54 (two brs, 1H, NH); 13C NMR (75.5 MHz, CDCl3):
d ˆ 27.7 (FmocNCH2CH2), 27.9 (C(CH3)3), 28.2 (C(CH3)3), 38.2
(FmocNCH2CH2CH2), 46.0, 46.4 (FmocNCH2), 47.2 (CH-Fmoc), 49.0,
49.3 (NCH2C(O)), 67.4, 67.7 (CH2-Fmoc), 79.6 (C(CH3)3), 83.2 (C(CH3)3),
119.8, 124.7, 124.9, 127.0, 127.1, 127.6, 141.2, 141.3, 143.8 (ArC-Fmoc), 153.0,
ˆ
(C(CH3)3), 126.4, 126.7 127.9, 128.0, 128.2, 135.3 (Ph), 155.7 (Boc C O),
171.9 (NCH2C(O)).
Cbz-NOrn(Boc)-OBzl (17): Benzyl chloroformate (33 mmol, 4.71 mL) was
added dropwise to a solution of 16 (30 mmol, 9.67 g) and TEA (33 mmol,
4.59 mL) at room temperature. After stirring for 3 h, the reaction mixture
was concentrated in vacuo and dissolved in EtOAc. The organic layer was
washed with 1n KHSO4, water, and brine, dried over MgSO4, and
concentrated in vacuo. Column chromatography (silica, eluent: hexanes/
ether, 1/1, v/v) gave 17 (13.16 g, 28.83 mmol) in 96% yield. Rf ˆ 0.36
(eluent: hexanes/ether, 25/75, v/v); the NMR spectra clearly show the
presence of both rotamers: 1H NMR (300 MHz, CDCl3): d ˆ 1.42, 1.44 (two
s, 9H, C(CH3)3), 1.67 (m, 2H, CbzNCH2CH2), 3.07 ± 3.16 (m, 2H,
CbzNCH2), 3.35 ± 3.45 (m, 2H, BocNCH2), 3.99, 4.06 (two s, 2H,
NCH2C(O)), 5.10 (d, 2H, CH2-Cbz), 5.18 (d, 2H, CH2Ph), 7.28 ± 7.36 (m,
10H, Ph); 13C NMR (75.5 MHz, CDCl3): d ˆ 27.9, 28.5 (CbzNCH2CH2),
28.0 (C(CH3)3), 36.9, 37.4 (BocNCH2), 45.6, 45.9 (CbzNCH2), 49.0, 49.2
(NCH2C(O)), 66.6 (CH2Ph), 67.2, 67.4 (CH2-Cbz), 78.6 (C(CH3)3), 126.6,
127.0, 127.4, 127.7, 127.9, 128.0, 128.1, 128.2, 128.3, 135.1, 135.2, 136.1 (Ph and
ˆ
ˆ
ˆ
ˆ
ˆ
Ph-Cbz), 155.7, 155.8, 156.1 (Boc C O and Cbz C O), 169.3 (NCH2C(O)).
153.1 (C N) 156.1, 156.4 (Boc C O and Fmoc C O), 162.9, 163.1 (Boc
ˆ
C O), 172.9, 173.3 (broad, NCH2C(O)OH).
Cbz-NArg(Boc)2-OBzl (19): A saturated solution of HCl in ether (ca.
20 mL) was added to a stirred solution of 17 (20 mmol, 9.13 g) in
chloroform (150 mL). After stirring for 1.5 h at room temperature, the
reaction mixture was concentrated in vacuo and dried in vacuo over KOH
to yield Cbz-NOrn(HCl)-OBzl (7.8 g, 19.85 mmol) as a white solid in
almost quantitative yield. Rf ˆ 0.27 (eluent: DCM/MeOH/TEA, 80/20/1, v/
v). The NMR spectra clearly show the presence of both rotamers. Cbz-
NOrn(HCl)-OBzl: 1H NMR (300 MHz, (CD3)2SO): d ˆ 1.80 (m, 2H,
CbzNCH2CH2), 2.78 (m, 2H, CbzNCH2), 3.37 (m, 2H, H2NCH2), 4.11, 4.16
(two s, 2H, NCH2C(O)), 5.02 (s, 1H, CH2-Cbz), 5.12 (s, 2H, CH2-Cbz and
CH2Ph), 5.16 (s, 1H, CH2Ph), 7.24 ± 7.37 (m, 10H, Ph), 7.91 (brs, 3H, NH2 ´
HCl); 13C NMR (75.5 MHz, (CD3)2SO): d ˆ 25.6, 26.3 (CbzNCH2CH2),
36.4 (H2NCH2), 45.3, 45.8 (CbzNCH2), 48.9, 49.2 (NCH2C(O)), 65.9, 66.0
(CH2Ph), 66.4, 66.5 (CH2-Cbz), 127.1, 127.4, 127.7, 127.8, 128.0, 128.3, 128.4,
Ac-Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2
(acetylated
substance P, SP1-11, 4): Immobilized undecapeptide 4 was assembled on
an automatic ABI433A Peptide Synthesizer using the ABI FastMoc
0.25 mmol protocols, as described for the preparation of 1. The synthesis
was carried out on TentaGel SRAM resin (1.09 g, capacity 0.23 mmolg 1).
The anchored undecapeptoid 4 thus obtained was deprotected and cleaved
from the solid support by treatment with TFA (10 mL), thioanisole
(0.5 mL), H2O (0.5 mL), EDT (0.25 mL), and TIS (0.1 mL) for 2 h at room
temperature. The reaction mixture was filtered and the residue was washed
thoroughly with TFA (2 Â 3 mL). The reaction mixture was concentrated in
vacuo to a volume of approximately 1 ± 2 mL and the residue was added
dropwise to cold ether (60 mL). The precipitate was collected by
centrifugation (2500 rpm, 10 min), the supernatant was decanted, and the
pellet was resuspended in ether (60 mL) and centrifuged again. This was
repeated twice. The pellet was then dissolved in HOAc (5%, ca. 15 mL)
and lyophilized to give crude undecapeptide 4 (396 mg) as a white fluffy
solid. Gel filtration over Sephadex LH-20 (eluent: MeOH/H2O, 85/15, v/v)
afforded almost pure peptide 4 (352 mg). An aliquot of the obtained
peptide 4 (100 mg) was purified by preparative HPLC to obtain pure
peptide 4 (84 mg) according to analytical HPLC. FAB MS: m/z ˆ 1389.7
ˆ
135.6, 135.7, 136.5, 136.6 (Ph and Ph-Cbz), 155.4, 155.6 (Cbz C O), 169.5,
169.7 (NCH2C(O)).
Subsequently Cbz-NOrn(HCl)-OBzl (15 mmol, 5.89 mmol) was dissolved
in EtOAc (250 mL). This solution was washed with Na2CO3 (5%, 2 Â
100 mL) and brine, dried over MgSO4, and concentrated in vacuo. The
residue, Cbz-NOrn-OBzl, was redissolved in THF (100 mL), and N,N'-
bis(Boc)-1-guanylpyrazole (18, 16.5 mmol, 5.12 g) was added to this
solution.[19] After stirring for 24 h at room temperature, the reaction
mixture was concentrated in vacuo. Column chromatography (silica,
eluent: hexanes/ether, 1/1, v/v) afforded 19 (7.83 g, 13.08 mmol) as an oil
in 87% yield. Rf ˆ 0.19 (eluent: hexanes/ether, 1/1, v/v). The NMR spectra
clearly show the presence of both rotamers; 1H NMR (300 MHz, CDCl3): d
1.48 (brs, 18H, two C(CH3)3), 1.81 (m, 2H, CbzNCH2CH2), 3.42 (m, 4H,
CbzNCH2 and CbzNCH2CH2CH2), 4.07 (d, 2H, NCH2C(O)), 5.09 (s, 2H,
CH2-Cbz), 5.18 (d, 2H, CH2Ph), 7.25 ± 7.35 (m, 10H, Ph and Ph-Cbz), 8.41
(brd, 1H, NH); 13C NMR (75.5 MHz, CDCl3): d ˆ 27.6 (CbzNCH2CH2),
27.8 (C(CH3)3), 28.0 (C(CH3)3), 37.7, 37.8 (CbzNCH2CH2CH2), 45.6, 45.6
(CbzNCH2), 49.2 (NCH2C(O)), 66.6 (CH2Ph), 67.1, 67.3 (CH2-Cbz), 78.8,
82.7, 82.8 (C(CH3)3), 127.4, 127.7, 127.8, 128.0, 128.1, 128.2, 128.3, 128.4,
‡
[M‡H] .
Ac-NArg-Pro-NLys-Pro-NGln-NGln-NPhe-NPhe-Gly-NLeu-NMet-NH2
(peptoid of SP1-11; 5): Synthesis of immobilized undecapeptoid 5 was
carried out using the same protocol as described above for the synthesis of
2. The synthesis was carried out on TentaGel SRAM resin (1.09 g, capacity
0.23 mmolg 1). Deprotection and cleavage from the resin was carried out
as described for the preparation of 4. Work-up was carried out as described
for 4 and gave crude peptoid 5 (275 mg). After Sephadex LH-20 gel
filtration (eluent: MeOH/H2O, 85/15, v/v) and preparative HPLC, peptoid
5 was obtained in a yield of 26% (104 mg). The peptoid 5 was pure
‡
according to analytical HPLC. FAB MS: m/z ˆ 1389.7 [M‡H] .
Ac-NMet-NLeu-Gly-NPhe-NPhe-NGln-NGln-Pro-NLys-Pro-NArg-NH2
(retropeptoid of SP1-11; 6): Synthesis of immobilized retropeptoid 6 was
carried out using the same protocol as described above for the synthesis of
2. Deprotection and cleavage from the resin were carried out as described
for the preparation of 4. Work-up was carried out as described for 4 to
afford crude retropeptoid 6 (335 mg). After Sephadex LH-20 gel filtration
ˆ
135.1, 135.2, 136.1, 136.3 (Ph and Ph-Cbz), 152.9, 153.0 (C N), 155.8, 155.9,
156.0, 156.2 (Boc C O and Cbz C O), 163.3 (Boc C O), 169.4
(NCH2C(O)).
ˆ
ˆ
ˆ
H-NArg(Boc)2-OH (20): Cbz-NArg(Boc)2-OBzl (19, 12 mmol, 7.18 g) was
dissolved in MeOH (150 mL) and hydrogenated (1 atm) in the presence of
Chem. Eur. J. 1998, 4, No. 8
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
0947-6539/98/0408-1579 $ 17.50+.25/0
1579

R. M. J. Liskamp et al.
FULL PAPER
(eluent: MeOH/H2O, 85/15, v/v) and preparative HPLC, retropeptoid 6
was obtained in a yield of 33% (132 mg). The peptoid 6 was pure according
to analytical HPLC. FAB MS: m/z ˆ 1389.7 [M‡H] .
[12] a) K. J. Watling, S. Guard, Neurotransmissions 1992, VIII, 1 ± 6; b) N.
Frossard, C. Advenier, Life Sci. 1991, 49, 1941 ± 1953; c) J. E. Krause,
Y. Takeda, A. D. Hershey, J. Invest. Dermatol. 1992, 98 suppl., 2S ± 7S;
d) J. A. Lowe III, Drugs Future 1992, 17, 1115 ± 1121; e) J. M.
Lundberg, Pharmacol. Rev. 1996, 48, 113 ± 178.
‡
Acknowledgments: We wish to thank Dr. J. Wilting for useful discussions.
[13] The nomenclature is according to the proposed nomenclature by
Simon et al. (ref. [2]). For the one letter code of amino acids, we have
proposed the prefix n; thus for example nY, nF, and nL; see ref. [8a].
[14] G. I. Tesser, I. C. Balvert-Geers, Int. J. Pept. Protein Res. 1975, 7, 295 ±
305.
[15] P. B. W. Ten Kortenaar, B. G. Van Dijk, J. M. Peeters, B. J. Raaben,
P. J. H. M. Adams, G. I. Tesser, Int. J. Pept. Protein Res. 1986, 27, 398 ±
400.
[16] A. Amstrong, I. Brackenridge, R. F. W. Jackson, J. M. Kirk, Tetrahe-
dron Lett. 1988, 29, 2483 ± 2486.
[17] a) H. Rink, Tetrahedron Lett. 1987, 28, 3787 ± 3790; b) M. S. Berna-
towicz, S. B. Daniels, H. Köster, Tetrahedron Lett. 1989, 30, 4645 ±
4648.
[18] It was found that intramolecular aminolysis, that is, diketopiperazine
formation was a serious by-reaction of a dipeptoid connected to
TentaGelꢂ SOH by an ester linkage. The use of Rink amide resin
(TentalGelꢂ SRAM), for the preparation of (peptoid) amides, entirely
circumvented the problem of diketopiperazine formation.
[19] a) Y. Wu, G. R. Matsueda, M. Bernatowicz, Synth. Commun. 1993, 23,
3055 ± 3060; b) M. Bernatowicz, Y. Wu, G. R. Matsueda, Tetrahedron
Lett. 1993, 34, 3389 ± 3392.
[20] a) This protection does appear to be sufficient to mask the
nucleophilicity of the guanidino function. Ornithine formation
through acylation of the unprotected w-nitrogen during coupling
and subsequent intramolecular decomposition during deprotection is
completely avoided; b) A. S. Verdini, P. Lucietto, G. Fossati, C.
Giordani, Tetrahedron Lett. 1992, 33, 6541 ± 6542.
[21] H2N(CH2)4N(H)Boc (7d) and H2N(CH2)3N(H)Boc (14) were pre-
pared according to: A. P. Krapcho, C. S. Kuell, Synth. Commun. 1990,
20, 2559 ± 2564; H2N(CH2)2SCH3 (7c) was prepared by substitution of
2-bromoethylamine hydrobromide in ethanol with 2 equiv of
NaSCH3. The resulting 2-(methylthio)ethylamine was purified by
vacuum distillation, b.p. 56 ± 608C/20 mmHg.
[22] The peptide was acetylated to facilitate purification.
[23] J. F. Westra-de Vlieger, L. J. F. Hofmeyer, J. A. W. Kruijtzer, W.
Gommeren, J. Leysen, D. van Heuven-Nolsen, J. Wilting, F. P.
Nijkamp, R. M. J. Liskamp, Biochem. Pharmacol., submitted.
[24] E. Von Arx, M. Faupel, M. Bruggen, J. Chromatogr. 1976, 120, 224 ±
228.
Received: February 10, 1998 [F998]
[1] A. Giannis, T. Kolter, Angew. Chem. 1993, 105, 1303 ± 1326; Angew.
Chem. Int. Ed. Engl. 1993, 32, 1244 ± 1267.
[2] R. J. Simon, R. S. Kania, R. N. Zuckermann, V. D. Huebner, D. A.
Jewell, S. Banville, S. Ng, L. Wang, S. Rosenberg, C. K. Marlowe, D. C.
Spellmeyer, R. Tan, A. D. Frankel, D. V. Santi, F. E. Cohen, P. A.
Bartlett, Proc. Natl. Acad. Sci. USA 1992, 89, 9367 ± 9371.
[3] H. Kessler, Angew. Chem. 1993, 105, 572 ± 573; Angew. Chem. Int. Ed.
Engl. 1993, 32, 543 ± 544.
[4] a) R. N. Zuckermann, J. M. Kerr, S. B. H. Kent, W. H. Moos, J. Am.
Chem. Soc. 1992, 114, 10646 ± 10647; b) R. N. Zuckermann, E. J.
Martin, D. C. Spellmeyer, G. B. Stauber, K. R. Shoemaker, J. M. Kerr,
G. M. Figliozzi, D. A. Goff, M. A. Siani, R. J. Simon, S. C. Banville,
E. G. Brown, L. Wang, L. S. Richter, W. H. Moos, J. Med. Chem. 1994,
37, 2678 ± 2685; c) R. J. Simon, E. J. Martin, S. M. Miller, R. N.
Zuckermann, J. M. Blaney, W. H. Moos in Techniques in Protein
Chemistry V (Ed.: J. W. Crabb), Academic Press, San Diego, CA,
1994, 533 ± 539; d) J. A. Gibbons, A. A. Hancock, C. R. Vitt, S.
Knepper, S. A. Buckner, M. E. Brune, I. Milicic, J. F. Kerwin Jr., L. S.
Richter, E. W. Taylor, K. L. Spear, R. N. Zuckermann, D. C. Spell-
meyer, R. A. Braeckman, W. H. Moos, J. Pharmacol. Exp. Ther. 1996,
277, 885 ± 899.
[5] a) J. A. W. Kruijtzer, R. M. J. Liskamp, Tetrahedron Lett. 1995, 36,
6969 ± 6972; b) J. A. W. Kruijtzer, Synthesis of Peptoid Peptidomimet-
ics, Thesis, Utrecht University, The Netherlands, 1996.
[6] E. K. Bradley, J. Magn. Reson. B 1996, 110, 195 ± 197.
[7] a) S. M. Miller, R. J. Simon, S. Ng, R. N. Zuckermann, J. M. Kerr,
W. H. Moos, Bioorg. Med. Chem. Lett. 1994, 4, 2657 ± 2662; b) S. M.
Miller, R. J. Simon, S. Ng, R. N. Zuckermann, J. M. Kerr, W. H. Moos,
Drug Dev. Res. 1995, 35, 20 ± 30.
[8] a) W. Heerma, C. Versluis, C. G. de Koster, J. A. W. Kruijtzer, I.
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