Effect of capping groups at the N- and C-termini on the conformational preference of α,β-peptoids

Article abstract of DOI:10.1039/c1ob06386c

A study was conducted to investigate the effect of capping groups at the N- and C-termini on the conformational preference of α, β-peptoids. Capping groups were carefully selected to minimize their influence on the peptoid secondary structure, such as through H-bonding. The effect of protecting group strategy for peptoids was important to consider when investigating conformational preference. The study focused on N spe α, β-peptoids and revealed that the phenylethyl side chain was found to promote helical secondary structure for α-peptoids on the basis of a combination of NMR, molecular modeling, and CD studies. This side chain was selected to facilitate interpretation of results and comparison to CD data of α and β-peptoid homologues.

Full text of DOI:10.1039/c1ob06386c

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PAPER
Effect of capping groups at the N- and C-termini on the conformational
preference of a,b-peptoids†‡
Emiliana De Santis,a Thomas Hjelmgaard,b,c Ce´cile Caumes,b,c Sophie Faure,b,c Bruce D. Alexander,d
Simon J. Holder,e Giuliano Siligardi, f Claude Taillefumier*b,c and Alison A. Edwards*a
Received 13th August 2011, Accepted 27th October 2011
DOI: 10.1039/c1ob06386c
Introduction
Peptoids (N-substituted glycines, i.e. a-peptoids) are artificial
oligoamides which are closely related structurally to a-peptides
with the side chains located on the amide nitrogen rather than
the a-carbon (Fig. 1).1 A major advantage of a-peptoids is
their ease of synthesis, particularly by the so-called ‘submonomer
method’.2 A further benefit is the vast potential for diversity as
the side chains on the amide nitrogens are introduced by primary
amines. Therefore a-peptoids are particularly well suited for the
construction of peptidomimetic libraries in the context of drug
discovery3 as they have been shown to be resistant to proteolytic
enzymes.4
Peptoids occupy a unique position as peptidomimetic foldamers
since their backbones are deprived of free NH amides and
thus have decreased capacity to form intramolecular H-bonding
Fig. 1 Peptidomimetic architectures from a- and b-peptide/peptoid
monomers.
interactions. Although unnatural, a peptoid residue is structurally
related to a proline as the coupling of residues gives tertiary amide
bonds which can populate cis and trans conformations. These
features confer greater flexibility to oligopeptoids but interest-
ingly they maintain the propensity to form ordered secondary
structures. a-Peptoids containing aromatic or aliphatic a-chiral
side chains can fold into stable helical structures.5 For example,
homo-peptoids with (S)-1-cyclohexylethyl or (S)-1-naphthylethyl
side chains (s1npe) were reported to adopt, both in the solid
and solution state, a polyproline type I helix (PPI) featuring
only cis amide bonds,6 whilst a polyproline type II helix (PPII)
was characteristic of N-aryl containing a-peptoids as shown by
molecular modelling.7 A threaded loop conformation was also
reported for an a-peptoid nonamer due to H-bonding at the
N-terminus.8 By contrast to a-peptoids, there are few literature
reports of their b-peptoid homologues (oligomers of N-substituted
b-alanines) (Fig. 1) and no specific secondary structures have been
reported. However they have been shown to be able to adopt
ordered conformations by circular dichroism (CD).9 Furthermore,
the conformational properties of their backbone could be more
important for folding than the possibility of H-bonding as
hypothesized by Hofmann from computation, who showed that
b-peptoids could form helices which resemble that of their parent
b-peptides.10
Early peptidomimetic foldamers focused on the construction
of homogeneous backbones and this has now progressed to
heterogeneous backbones formed by the combination of at least
two different types of building blocks in a defined sequence
pattern.11 This has increased diversity thus allowing greater control
of the spatial positioning of side chain pharmacophores when de-
veloping peptidomimetic foldamers. Heterogeneous a,b-peptides,
with different combinations of a- and b-peptide residues, have
been the most extensively studied of this class and have led to the
design of foldamers able to form quaternary bundle assemblies.12
a-Peptide/b-peptoid chimeras with folding propensities that ex-
hibit antimicrobial properties have also been developed.13 Novel
aMedway School of Pharmacy, Universities of Kent and Greenwich at
Medway, Central Avenue, Chatham Maritime, Kent, United Kingdom, ME4
4TB. E-mail: a.a.edwards@kent.ac.uk; Fax: +44 (0)1634 883927; Tel: +44
(0)1634 202947
bClermont Universite´, Universite´ Blaise Pascal, Laboratoire SEESIB, BP
10448, F-63000, Clermont-Ferrand
cCNRS, UMR 6504, Laboratoire SEESIB, F-63177, AUBIERE cedex,
France. E-mail: claude.taillefumier@univ-bpclermont.fr; Fax: +33 (0)4 73
40 77 17; Tel: +33 (0)4 73 40 54 27
dSchool of Science, University of Greenwich, Central Avenue, Chatham
Maritime, Kent, United Kingdom, ME4 4TB
eSchool of Physical Sciences, University of Kent, Canterbury, United
Kingdom, CN2 7NZ
f
Diamond Light Source Ltd., Diamond House, Harwell Science and Innova-
tion Campus, Didcot, Oxfordshire, United Kingdom, OX11 0DE
† This article is part of an Organic & Biomolecular Chemistry web theme
issue on Foldamer Chemistry.
‡ Electronic supplementary information (ESI) available: Further CD
data and details of IR and molecular modelling. RP-HPLC data of all
compounds and NMR spectra of products. See DOI: 10.1039/c1ob06386c
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linear and cyclic heterogeneous peptoid backbones composed of
alternating a- and b-peptoid residues were first reported by us
in 200914 and also a convenient protocol for gram-scale solution-
phase synthesis of a,b-peptoids.15 Evidence of the conformational
stability of cyclic octamers with this backbone, has been recently
reported on the basis of CD and NMR studies together with the
first X-ray structure for the a,b-peptoid backbone.16
To date suitable crystals of linear a,b-peptoids have not been ob-
tained for X-ray structure determination and their solution-phase
studies by NMR are limited due to amide cis/trans isomerism.
This leads to increased conformational heterogeneity in solution.
Furthermore, the absence of amide protons limits investigation
by infrared spectroscopy. However, there are numerous reports
of solution-phase study of peptoids using CD spectroscopy to
explore their conformational preference.5,6,8,9,16,17a
secondary structure, e.g. via H-bonding. Therefore the effect of
protecting group strategy for peptoids is important to consider
when investigating conformational preference. In the present paper
we report a study aimed to investigate how capping groups at
the N- and C-termini can influence the CD spectral features of
a,b-peptoids and thus conformation. Our study is focused on
Nspe a,b-peptoids. The phenylethyl side chain has been reported
to promote helical secondary structure for a-peptoids, on the
basis of a combination of NMR, molecular modelling and CD
studies.5c,d,17a Therefore, this side chain was selected to facilitate
interpretation of our results and comparison to CD data of a-
and b-peptoid homologues.9
Results and discussion
Despite this, there is only one reported study in which the
CD spectra are associated with conformationally homogeneous
a-peptoids in solution.6b There are no characterised reference
spectra for b- and a,b-peptoid backbones in contrast to those
available for a-peptides and proteins. Therefore interpretation
of peptoid CD spectra has to be undertaken with due caution
and a combination of independent techniques should be used
whenever possible to aid interpretation of spectral features and
assign CD spectra to well defined structures. CD alone can
give information about the presence of secondary structure by
studying the effect of chain length on spectral features. If a chain
length dependence is observed then this typically indicates the
presence of a secondary structure which may arise from long
range interactions. However, the lack of chain length dependence
does not exclude the possibility of conformational order arising
from short range interactions. The system can be perturbed in
different ways to increase information about its conformational
behaviour and the presence of ordered structures. For instance,
CD experiments at different temperatures will give information
about the stability and order of the system. Solvent studies allow
evaluation of the nature of the interactions which promote the
folding, e.g. H-bonding. In addition to this, concentration studies
can give an insight into the inter- or intramolecular nature of
these. When undertaking CD investigation of peptoids, studies
tend to focus on the conformational effect of differing side
chains and/or backbone. Most research groups will maintain a
consistent protecting group strategy for their peptoids but this can
differ between different research groups. The presence, absence
and/or type of protecting groups can have significant effects
upon conformational preference. For example it has been reported
that the threaded loop conformation of the a-peptoid nonamer
with (S)-1-phenylethyl side chains (spe), H-(Nspe)9-NH2, was due
to the protonated N-terminus being involved in H-bonds.8 The
b-peptoids by Olsen9b PhCH2CO-(b-Nrpe)5-NH2 and Norgren9a
H-(b-Nspe)5-OH, for the same backbone and side chains, have
remarkably different conformational preferences. Firstly, a col-
lapse of secondary structure was observed depending upon the
capping groups when in MeCN. A chain length dependence
was also observed in MeOH for H-(b-Nspe)n-NH2 in contrast
to H-(b-Nspe)n-OH.9a,b The importance of capping groups for
folding has been recently highlighted by Blackwell et al. who
reported the conformational investigation of Ns1npe a-peptoids
in comparison to that of Nspe a-peptoids.6b Capping groups
were carefully chosen to minimise their influence on the peptoid
Solution a,b-peptoid synthesis
A family of linear a,b-peptoids up to the decamer length was
prepared, composed of (S)-N-(1-phenylethyl)glycine and (S)-N-
(1-phenylethyl)-b-alanine residues in alternation (Table 1), using
our recently developed methodology.14 All the oligomers are
composed of a,b-dimeric peptoid repeating units resulting from
two coupled ‘submonomer’ synthetic protocols, one for a-peptoid
monomer synthesis and another for b-peptoid monomer synthesis.
Due to the significant effect of capping groups observed for b-
peptoid pentamers with same backbone and side chains,9 we
synthesised a,b-peptoids with varied capping groups to investigate
if this would also be significant in a,b-peptoids. Iteration of
Table 1 a,b-peptoids with different capping groups at the N- and C-
termini studied for conformational preference
Series Peptoids R1
R2
Chain length (n)a Purityb (%)
a
b
c
1
2
3
H
H
H
OtBu
OtBu
OtBu
1
2
3
—
98
94.8
4
5
6
H
H
H
NH2
NH2
NH2
1
2
3
99.9
99.2
96.8
7
Boc
Boc
Boc
Boc
Boc
OtBu
OtBu
OtBu
OtBu
OtBu
1
2
3
4
5
97.9
98.1
94.8
99.9
99.9
8
9
17
18
d
10
11
12
19
20
Boc
Boc
Boc
Boc
Boc
NH2
NH2
NH2
NH2
NH2
1
2
3
4
5
99.2
97.9
97.5
93.1
93.7
e
13
14
15
TFA·H OH
TFA·H OH
TFA·H OH
1
2
3
98.3
96.2
93.6
a n = 1 dimer, n = 2 tetramer, n = 3 hexamer; n = 4 octamer; n = 5 decamer.
b As estimated by analytical reversed-phase HPLC of purified products by
flash chromatography or preparative HPLC (17 and 18) (see ESI).
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Scheme 1 Solution-phase synthesis of a,b-peptoid oligomers with different N- and C-terminal groups.
submonomer methods were used to obtain dimer, tetramer and
hexamer (n = 1, 2, 3; Scheme 1 and Table 1) with two C-terminal
groups (CO2tBu and CONH2). This generated compounds 1–
6 and then subsequently all the modified N- and C-derived
a,b-peptoids 7–16. The longer oligomers 17–20 (n = 4 octamers
and n = 5 decamers) were prepared by peptide-type coupling
(HATU/iPr2NEt) of the free acid tetramer 16 with the amine
tetramers 2 and 5 or amine hexamers 3 and 6. All the synthe-
sised peptoids were obtained in excellent purity, as determined
by analytical reversed-phase HPLC, after purification by flash
chromatography on silica gel or preparative reversed-phase HPLC
(see ESI for details).
CD spectra were processed to include the contribution of all the
chromophores present (see ESI, Fig. S2, Tables S1 and S2). Despite
this, a significant difference was still evident, suggesting that the
spectral differences observed in the MRE spectra (Fig. 2) were not
solely due to the addition of the chromophores at the terminal
capping groups.
Compounds 1 and 4 in Fig. 2 have a positive (I) and two negative
(II and III) maxima at 184, 194 and 219 nm respectively (Table S3).
Very similar maxima were observed for compound 13, however the
maximum at ~219 nm, was blue shifted by 3 nm in comparison to
1 and 4. By contrast, different spectral features were observed for
compounds 7 and 10, where one positive (I) and two negative
maxima (II and III) were observed at 180, 200 and 217 nm,
respectively. The CD spectrum of 4 was found to resemble the
spectral features of an a-peptoid hexamer with same side chains
and capping groups.5c,d
It is of note that compounds 1, 4 and 13, which represent one
set of distinctive CD spectra, have a free N-terminus while 7 and
10 have the N-terminus Boc-protected. This suggested that the
N-terminus is the key structural feature generating the observed
difference in the dimer CD spectra, in particular for the wavelength
shift of positive and negative maxima. By contrast, changing the
terminal groups at the C-terminus seemed to determine a change
in the intensity (MRE).
Conformational studies of dimers
CD in acetonitrile. Comparison between five series (a–e in
Table 1) of a,b-peptoid dimers with differing capping groups at
the N- and C-termini showed significantly different CD spectra in
MeCN (Fig. 2 and S1).
1H NMR analyses of the dimers in CD3CN and CDCl3 also
showed the presence of two sub-sets based on the capping group
at the N-terminus. Peptoids with unprotected N-terminus (1, 4 and
13) gave well-resolved NMR spectra which allowed determination
of the cis/trans ratio by integration of the benzylic protons of
the spe side chain on the tertiary amide (Table S8). By contrast,
the NMR spectra of Boc-protected peptoids 7 and 10 were more
complex and showed significant 1H signal overlap due to the
simultaneous isomerism of the tertiary amide and carbamate. The
values of Kcis/trans were in the range of 0.39–0.89 for 1, 4 and 13 in
both solvents. In particular, Kcis/trans of 0.61 and 0.64 were measured
for dimer 1 in CDCl3 and CD3CN, respectively. These values were
very similar to those measured by Blackwell and co-workers for
a monomeric a-peptoid model bearing the phenylethyl side chain
and capped at the N-terminus by an acetyl group and at the C-
terminus by a methyl ester (Kcis/trans = 0.66–0.67 in CDCl3 and
CD3CN).18
Fig. 2 ECD spectra of dimers in MeCN. All spectra were recorded at
20 ◦C at known concentrations in the range 650–700 mM.
The different intensities of the dimer spectra in the far UV region
may be attributable to the contribution of any chromophore(s)
present in the capping groups. To evaluate this possibility, the
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Table 2 H-bonding potential of solvents used for CD spectroscopy18
which is promoted in protic solvents. Notably, for Boc-protected
oligomers this conformational change was also observed in MeCN
upon increase of chain length (vide infra).
Solvent
a-value
b-value
To investigate the protic/aprotic solvent effect, a solvent titra-
tion from MeCN to TFE was undertaken for compound 10 (Fig.
4). A subtle change to the ensemble of conformations adopted
occurred when 50% of TFE was added to MeCN, as indicated by
the presence of the positive maximum at ~192 nm. This positive
maximum and the negative maxima at ~198 and ~218 nm were
significantly increased upon further addition of TFE. This sug-
gested an increased population of the conformation(s) observed
in MeCN : TFE 1 : 1 and the presence of a conformational
transition. It is noted that TFE is known to be structure inducing
in peptides20 mainly by promoting H-bonds. However, these are
unlikely to be the main driving forces in the peptoid backbone
which has a reduced H-bonding capability. The solvent titration
of 7 in MeCN/TFE underwent similar conformational transitions
(Fig. S3). The presence of the additional positive maximum
at 192 nm clearly relates to the presence of Boc at the N-
terminus for the dimers (vide supra) and is only observed in
protic solvents confirming the presence of a H-bonding interaction
between the peptoid and protic solvents. This is further supported
by the observation that the positive maximum at ~192 nm is
less pronounced in MeOH (Fig. 3), which has less H-bonding
donor potential than TFE and HFIP. a,b-Dipeptoids with an
unprotected N-terminus (1, 4 and 13) did not show the additional
positive maximum at ~192 nm in protic solvents (Fig. 3 and S4) as
their spectra were more comparable to those in MeCN. However,
a significantly reduced intensity (MRE) was observed at 190 nm
for compound 4 in all protic solvents (Fig. 5a). By contrast, this
difference was less pronounced for the spectra of 1 in different
solvents (Fig. 5b). It is of note that the CD spectra of 13 are not
consistent with the trends observed for the other dimers. This is
likely to be due to the presence of the TFA salt at the N-terminus
which will give rise to different interactions compared to other
dimers.
MeCN
MeOH
TFE
0.09
0.35
1.51
1.96
0.31
0.42
0.00
0.00
HFIP
Given the short chain length of the dimers, interpretation of
these observations could be complicated by the possibility that
R1 and R2 may interact with each other. In addition to this,
the difference in the CD spectra may arise from short range
interactions at the N- and C-termini, e.g. H-bonding, rather than
being indicative of secondary structures. Short range interactions
at the termini are generally more significant in short oligomers due
to the smaller contribution from the ‘middle’ (i.e. the backbone
and side chains) – this is known as the ‘end effect’. However, it is
clear that certain combinations of N- and C-terminal groups alter
conformational preference.
CD in protic versus aprotic solvents. CD spectra were also
obtained in protic solvents to explore the potential contribution
of H-bonding from the N- and C-termini. Protic solvents with
different H-bonding potential were selected for the study (Table
2).19 MeOH was selected as a good H-bonding acceptor (b-value)
and TFE and HFIP as H-bonding donors of differing ability (a-
value).
The previously observed trend in MeCN for dimers with
different capping groups (vide supra) was found to be consistent
in all of the solvents investigated (Fig. 3). In each solvent two
distinct sets of CD spectra were observed based on the position of
the positive and negative maxima. This difference again correlated
with the presence of Boc-protected or deprotected N-terminus and
confirmed that, also in protic solvents, the protecting group at the
N-terminus has the greatest effect on the spectral features.
Fig. 3 ECD spectra of dimers in (a) MeCN, (b) TFE, (c) HFIP and (d)
MeOH. All spectra were recorded at 20 ◦C at known concentrations in the
range 650–750 mM.
Fig. 4 MeCN/TFE solvent titration of 10 by ECD. All spectra were
recorded at 20 ◦C at 700 mM or 698 mM.
When the N-terminus was Boc-protected (7 and 10), an
additional positive maximum was observed at ~192 nm in protic
solvents but not in aprotic (MeCN). The presence of this additional
maximum may be indicative of a conformational transition
The difference in the response to solvents can be related to the
H-bonding acceptors and donors strength of the backbone and
terminal capping groups (Table S4). When the peptoid had both
strong H-bonding acceptors and donors (4), a greater intensity
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It is noticeable that compounds 1, 4 and 13 all possess the
potential for intramolecular H-bonding of the amine (1 and 4)
or ammonium group (13) with the adjacent amide carbonyl (C5
H-bonding) in contrast to compound 10.
The N–H and C O stretching spectral regions for these
compounds are shown in Fig. 7. Compound 1 displays a band at
3314 cm-1 which can be attributed to the N–H stretch of an amine
involved in a H-bond (with a significantly smaller peak at 3420
cm-1 attributable to free NH).22 As expected, its counterpart with
Boc-protected N-terminus (7) displayed no N–H stretch from 3600
to 3200 cm-1. The two potential H-bond acceptors in compound
1 are the tertiary amide carbonyl (b = 0.69–0.78) and the ester
(b ~ 0.47). Comparison of the C O stretch of compounds 1 and
that of 7, where no intra- or intermolecular H-bonds are possible
(due to the absence of H-bonding donors), revealed no significant
differences in the C O stretch peak position for the ester function
(1714 cm-1 for 1 and 1721 cm-1 for 7) (Table S6). This suggested
that the ester was not involved in H-bonding for compound 1. In
Fig. 5 ECD spectra of dimers (a) 4 and (b) 1 in different solvents. All
spectra were recorded at 20 ◦C at known concentrations in the range
700–780 mM.
in MeCN compared to protic solvents indicated the presence
of H-bonding in MeCN. However, when only strong H-bonding
acceptors were present in the peptoid structure (1), the decrease
of intensity in protic solvents, relative to aprotic solvents such
as MeCN, was less pronounced (Fig. S4b). The effect of protic
solvents, in combination with the greater intensity observed in
MeCN for compound 4 in contrast to 1, suggested the presence
of stronger and/or more populated H-bonding interactions for
compound 4 in contrast to 1 in MeCN.
Concentration studies were undertaken to establish the nature
of the H-bonding interactions. These showed a significant change
of the intensity (MRE) with increased concentrations in the range
from 5 mM to 1 mM for compound 4 [D(MRE)195nm = 9 ¥ 103 deg
cm2 dmol-1] (Fig. S5a), indicating the presence of intermolecular
H-bonds in MeCN. A less significant change was observed for 1 in
the same concentration range [D(MRE)195nm = 3.75 ¥ 103 deg cm2
dmol-1] (Figure S5b), suggesting that intermolecular H-bonding
does not play a role in its conformational stabilization and that
the decrease of intensity in protic solvent was likely to be due to
intramolecular H-bonds.
contrast, the position of the amide I band (predominantly C
O
stretch) was significantly different (1662 cm-1 for compound 7 and
1641 cm-1 for compound 1). This was indicative of an H-bonded
amide carbonyl.23
Infrared studies. The importance of H-bonding acceptor and
donor strength for the folding of a,b-peptoids dimers was further
investigated by IR in CHCl3. Possible intramolecular H-bonding
relationships for the dimers are shown in Fig. 6. These are
based on the H-bonding acceptor and donor strengths,21 literature
precedents and molecular modelling (PM6) from which the H-
bonds shown are possible and represent local conformational
minima (representative models are shown in Fig. S13). Possible
intermolecular H-bonding relationships are not shown but are
implied.
Fig. 7 (a) N–H bonding spectral region and (b) C O stretching spectral
region for a,b-peptoid dimers in CHCl3. All spectra were recorded at room
temperature at known concentrations in the range 10–20 mM.
However whether the H-bonds are due to intra- or intermolec-
ular H-bonding was uncertain. Typically intermolecular H-bonds
Fig. 6 H-bonding relationship for a,b-peptoid dimers with H-bonding
donor and acceptor ability.
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are concentration dependent and intramolecular H-bonds are
concentration independent. Consequently IR analyses of serial
dilutions of compound 1 were carried out from 23 mM to 1.43 mM
(Fig. S14 and S15). The C O stretch at 1640 cm-1 displayed
almost no change over this concentration and no evidence of
any substantial growth of a C O peak at 1666 cm-1 that might
correspond to the ‘free’ C O stretch. Furthermore no significant
change in the relative proportions of free N–H to H-bonded N–H,
were observed in the N–H region over the concentration range
5.75 mM to 23 mM. Whilst not conclusive, this evidence could
support the occurrence of an intramolecular C5 N–H—O C H-
bond in compound 1. C5 H-bonding between nearest neighbour
amino acids in peptides can be energetically less favoured than
longer H-bonded chains (such as C7) and intermolecular H-
to ascertain the possible H-bonding interactions both inter- and
intramolecular.
These results are consistent with observations by CD in MeCN
suggesting a small degree of intermolecular H-bonding for com-
pound 4 and none for 1 (Fig. S5). However it must be noted that
CH3CN is a substantially stronger H-bond acceptor than CHCl3
(used in the IR studies) and the degree of H-bonding (between
peptoid and solvent) might be expected to differ appreciably.
The combined solvent and concentration studies by CD to-
gether with the IR results showed that H-bonding does play an
important role in the conformational stabilization for a,b-peptoids
and this is closely related to the H-bonding acceptor and donor
strength of the terminal protecting groups and tertiary amides.
Conformational studies of longer oligomers
bonds, due to the parallel alignment of the N–◦H and C
O
vectors and a sharp N—H—O bond angle (~90 ).24 However
they have been documented for peptides25 and recently for
a,b-peptides.26
Tetramers and hexamers. In order to identify if the effect ob-
served in dimers also occurred for longer oligomers, corresponding
a,b-peptoid tetramers and hexamers were investigated in MeCN.
As expected, capping groups were found to affect the CD spectra
to a lesser extent for the tetramers (Fig. 8a and S6a) and hexamers
(Fig. 8b and S6b), as shown by the correlation between the MRE
at 192 nm and the chain length (Fig. 8c).
Compound 4 displayed the peak for the amide I band and
the amine N–H stretch with similar values to those of peptoid
1, though the amine N–H stretch was partly obscured by the
dominant N–H stretches attributable to the primary amide from
3600–3250 cm-1. The sharp peaks at 3527 cm-1 and 3410 cm-1 can
be unequivocally attributed to the anti-symmetric and symmetric
N–H stretches respectively, due to unassociated (i.e. ‘free’) N–
H bonds. The broad accompanying peaks centred at ~3480–
3500 cm-1 and 3329 cm-1 are attributable to anti-symmetric and
symmetric N–H stretches respectively, resulting from associated
H-bonded species.27 Thus intra- or intermolecular H-bonding
is present for 4 in solution. The amide I band of 4 occurred
at 1638 cm-1 which in comparison with 1 and 7 suggests that
the tertiary amide bond is again involved in H-bonding. To
attempt determining the concentration dependence of the H-
bonding, IR analyses of serial dilutions of compound 4, were
carried out from 25 mM to 1.5 mM (Fig. S16 and S17). As with
compound 1, remarkably little change in the relative proportions
of the H-bonded to free N–H or C O bonds was observable
over this concentration range. For comparison, no intermolecular
H-bonding has been observed for acetamide in CHCl3 at a
concentration of 5 mM.27b This strongly suggested that the primary
amides at the C-terminus of compound 4 were likely partaking
in intramolecular H-bonds which presumably must be C8 (Fig.
6). This conclusion however does not preclude the occurrence
of intermolecular species altogether, since small but observable
changes in the N–H region and particularly the tertiary amide
I region were discernible. Given the presence of potentially
competitive H-bonds, the question of whether C5 intramolecular
bonds are present in compound 4 remains open.
Fig. 8 ECD spectra of (a) tetramers and (b) hexamers in MeCN. (c)
Correlation between MRE and chain length at 192 nm. All spectra were
recorded at 20 ◦C at known concentrations in the range 499–500 mM for
tetramers and 249–300 mM for hexamers.
Although the effect of terminal groups was less dramatic for
the tetramers and hexamers, two different sets of spectral features
were evident. This was consistent with observations for the dimers.
Peptoids with Boc-protected N-terminus can be characterised by
the presence of an additional positive maximum at ~193 nm in
contrast to their counterparts with an unprotected N-terminus
(see inset in Fig. 8a and 8b). However, no significant wavelength
shift or change in ellipticity of the negative maxima was observed
for the longer oligomers which was in contrast to the dimers.
IR studies of the hexamers in CHCl3 solutions showed very
similar peak positions to the dimers for the N–H stretching region,
3100–3600 cm-1 (Fig. S18) indicating that similar intramolecular
and intermolecular H-bonding occurs for longer chains. As for
the dimers, this might be responsible for the spectral differences
observed by CD between compounds with Boc-protected and
Similar N–H peaks featured in the N–H stretch region for
compound 10 as for compound 4, however the carbonyl tertiary
amide I band for 10 occurred at an almost identical peak position
as that of compound 7 (1656 cm-1 versus 1662 cm-1), suggesting
that the tertiary amide was not involved in H-bonding at the
concentrations employed or certainly not to the same degree as
implied for 4. However since the N–H stretches of 10 clearly
showed H-bonding it is reasonable to deduce that compound 10 is
undergoing intermolecular H-bonding and exists as equilibrium
of its monomeric form and terminal H-bonded aggregates. It is
apparent that for all of these compounds further studies are needed
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unprotected N-terminus. The carbonyl regions for the hexamers
were too complex to simply interpret. However, it is worth noting
that the relative proportion of H-bonded (~3330 and ~3480 cm-1)
to non H-bonded (~3410 and ~3530 cm-1) primary amide for 6 and
12 are considerably larger than for 4 and 10 (compare Fig. 7(a) with
Fig. S18). The reason for this is not entirely clear but may be related
to the greater intramolecular H-bonding opportunities within the
longer chains due to the larger number of tertiary amide H-bond
acceptors; i.e. between the C-terminal amide N–H and the internal
the presence of conformational stabilization due to H-bonding
interaction with the solvent.
From the data shown it was clear that the conforma-
tional/spectral effects arising from the capping groups were less
pronounced for longer oligomers as there was a greater contri-
bution from the backbone and side chains. This was therefore
consistent with the presence of an end effect for the dimers.
Investigation of cooperative folding within each series. Each
series (a–e in Table 1) was also investigated in MeCN to evaluate
the presence of cooperative folding. When processing the molar
ellipticity data to MRE, no dramatic intensity difference was
observed indicating absence of cooperative folding to increase con-
formational order (Fig. 11 and S9–12). When going from shorter to
longer oligomers, each series had an additional positive maximum
at ~192 nm, suggesting that this spectral feature is related to the
backbone length. However, depending on the capping groups, this
additional maximum was observed at different chain lengths (Fig.
S9–12). In particular this was clearly evident when a tert-butyl
group was present either at the N- (series c and d) or at the C-
(series a) terminus, suggesting that the presence of the tert-butyl
groups promote adoption of a different confomational preference
with increased chain length.
C
O amides. Solvent studies on the hexamers revealed that
capping groups did not dramatically affect the spectral features
of the hexamers in each of the solvents investigated (Fig. 9a, 10
and S7). This was in contrast with results for the dimers and
was consistent with a smaller end effect for longer oligomers
(vide supra). However, a solvent dependence of the conformational
preference was observed for all the compounds investigated where
spectra in protic solvents were significantly different to those in
MeCN (Fig. 9b). The same trend was observed for all the hexamers
regardless of the difference in capping groups (Figure S8). This
suggested that the minor solvent effect observed for the hexamers
was due to the backbone rather than to the presence of differing
terminal groups.
Fig. 11 SRCD spectra of c series in MeCN. (a) Molar ellipticity and (b)
MRE spectra. All spectra were recorded at 20 ◦C at known concentrations
in the range 150 mM–2.3 mM.
Fig. 9 SRCD spectra of (a) hexamers with different capping groups in
TFE and (b) hexamers 9 in different solvents. All spectra were recorded at
20 ◦C at known concentrations in the range 326–420 mM.
It is of note that for the series investigated, the dimers had a
different spectrum relative to the corresponding longer oligomers.
This demonstrated that there was a greater contribution from
terminal protecting groups to dimer CD spectra, which was in
agreement with previous observations.
The lack of increase in ellipticity associated with the presence
of the additional maximum with increased chain length indicates
that the order of the system is not altered but the ensemble of
conformations adopted is changed. This would be consistent with
a change from one ensemble of conformations to another without
increase of conformational order. Conformational heterogeneity
was also observed by NMR studies showing an average Kcis/trans
in CD3CN of 0.92 for tetramer 2 and 1.08 for hexamer 3. It is
important to note that the extent of conformational heterogeneity
that is observed is comparable to Nspe or Nrch pentamers and
hexamers for which the major conformers adopt a PPI like
conformation.6a,17a
The NMR spectra of compounds 1–3 in CD3CN were also
recorded at 45 ◦C and 65 ◦C and the average Kcis/trans were
measured and compared to the values at room temperature. No
collapse of the cis/trans signals and no change of the amide
cis and trans populations could be observed (Table S9) which
indicates slow cis/trans equilibria on the NMR time scale. On the
whole, the cis/trans conformational heterogeneity (Kcis/trans) and
Fig. 10 Correlation between MRE intensity by SRCD and a-values at
218 nm for hexamers with different N- and C-capping groups.
The correlation between the intensity at 218 nm for all the
hexamers investigated and the a-value of the solvents showed an
increase of the intensity of the negative maxima with increased H-
bonding donor potential of the solvent (Fig. 10). This suggested
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cis/trans dynamic behaviour of Nspe a,b-peptoids backbones is
comparable to that of Nspe a-peptoids.
The absence of increased conformational order from coop-
erative folding for a,b-peptoids with spe side chains was also
consistent with literature data on b-peptoids by CD.9a This
difference certainly arises from a greater flexibility of b-peptoids
residues in a,b-peptoids compared to pure a-peptoids.
and fitted with a bespoke thermostated cell holder.28 A quartz
Suprasil cylindrical cell with a path length of 0.01 cm was used.
Spectra were recorded at 20 ◦C in acetonitrile (MeCN), methanol
(MeOH) or 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) supplied
by Romil at super-purity grade and 2,2,2-trifluoroethanol (TFE)
supplied by Sigma Aldrich at spectroscopic grade. The following
parameters were used: 4 acquisitions, 10 nm min-1 scan speed,
0.1 nm data interval, 1.0 nm spectral band width, 4 s time constant,
260 nm–175 nm scan range (depending on solvent transparency).
Solvent baseline spectra (4 accumulations) were recorded in the
same cell at proximal time averaged to give the baseline spectrum.
The spectra were averaged and the solvent baseline subtracted
from the resulting spectrum which was normalised for path length
and concentration to give the molar ellipticity ([q], deg cm2
dmol-1). Correction of molar ellipticity spectra by the number
or residues provided the mean residue ellipticity spectra (MRE).
Conclusions
A family of a,b-alternating peptoids with different chain length
and bearing exclusively chiral side chains (spe) was efficiently
synthesised by submonomer and block coupling methods. Based
on the collapse of secondary structure observed for b-peptoid
pentamers with same backbone and side chains but different
capping groups,9 we synthesised five series of a,b-peptoids with
different capping groups. This allowed us to investigate if this effect
would also occur in a,b-peptoids. No change in conformational
order and/or collapse of secondary structure was observed for
these systems. Despite this, we were able to ascertain that the
presence of different capping groups at the N- and C-termini
has a significant effect on the spectral features, which is more
pronounced for shorter oligomers and is also solvent dependent.
When studying longer oligomers the contribution of capping
groups decreased with increased chain length. However, this is not
always the case as reported in literature for b-peptoid pentamers. It
is therefore important to compare systems with the same capping
groups when undertaking conformational investigation by CD.
We hope that this work will raise a greater awareness of the
importance of adopting a consistent protecting group strategy
for spectroscopic investigation. This is particularly important for
more flexible systems, such as peptoids, for which the lack of
consistency at the capping groups can lead to false interpretation
of their spectral features.
Infrared
Infrared (IR) spectra were recorded on a Perkin-Elmer Spectrum
One IR Fourier Transform spectrophotometer in chloroform
solution (CHCl3) as stated. Spectroscopic grade CHCl3 was dried
˚
over 4 A molecular sieves before use. Solution spectra were
recorded at room temperature by using cells equipped of KBr
windows and pathlength of 0.1 cm. Spectra were the result of 12
scans obtained with 4 cm-1 resolution when recording the entire
spectral range (5000–370 cm-1) and 1 cm-1 when recording spectra
in the N–H (2000–4000 cm-1) and C O (2000–1200 cm-1) regions.
Baseline spectra were recorded at proximal time and subtracted
from the peptoid spectrum. Concentrations used are reported in
SI (Table S7). For concentration studies spectra were the result
of 64 scans obtained with 8 cm-1 resolution and recorded in the
N–H (2000–4000 cm-1) and C O (2000–1200 cm-1) regions. The
concentrations used are reported in the ESI (Fig. S14–S17).
Molecular modelling
Experimental section
Molecular modelling was performed using version 12 of the
MOPAC Interface for ChemDraw3D (CambridgeSoft). A PM6
method was employed to minimise energy/geometry using an
Open Shell (Unrestricted) wavefunction, an EF optimizer, mini-
mum RMS of 0.1000 and a dieletric constant of 37.5 (acetonitrile).
Circular dichroism
CD samples were prepared by weighing accurately small amounts
of compound and adding the solvent volumetrically to achieve a
sample solution of appropriate concentration (Table S5). For the
solvent titrations, 5 samples (for each compound) were prepared
at similar concentrations with different solvent combinations and
an appropriate solvent background recorded at proximal time for
each sample. For concentration studies, a stock solution of peptoid
was prepared from which samples of appropriate concentrations
were obtained by volumetric dilution and recorded by using a
quartz cell as follows: 1 cm path length was used for samples
at 1 mM and 500 mM, 0.1 cm path length for 100 mM and
50 mM and 0.01 cm pathlength for 10 mM and 5 mM. Circular
dichroism analyses were performed either by ECD or SRCD
as specified. The effect of radiation for SRCD versus ECD has
been previously reported and found to be neglegible.16 ECD
spectra were recorded using a Chirascan instrument (Applied
Photophysics Ltd.) equipped with a Peltier temperature control
system. SRCD spectra were recorded at beamline B23 of the
Diamond Light Source, UK using an Olis CD spectropolarimeter
(Module B) equipped with a Peltier temperature control system
Synthesis – general information
Chemicals obtained from commercial sources were used without
further purification, unless stated otherwise. THF was distilled
˚
from potassium/benzophenone and stored over 4 A molecular
sieves. CH2Cl2 and MeOH were distilled from CaH2 and stored
˚
over 4 A molecular sieves. DMF, Et2O, Et3N and iPr2NEt were
˚
dried over 4 A molecular sieves. (S)-1-phenylethylamine 99% (99%
ee) was purchased from Acros. Melting points were determined
on a Reichert microscope apparatus and are uncorrected. Specific
rotations were measured on a Jasco DIP-370 polarimeter using
a 10 cm cell. IR spectra were recorded on a Shimadzu FTIR-
8400S spectrometer equipped with a Pike Technologies MIRacleTM
ATR and wavenumbers (n) are expressed in cm-1. NMR spectra
were recorded on a Bruker AC 400 spectrometer. Chemical shifts
are referenced to the residual solvent peak and J values are
given in Hz. The following multiplicity abbreviations are used:
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(s) singlet, (d) doublet, (t) triplet, (q) quartet, (m) multiplet, and
(br) broad. Where applicable, assignments were based on COSY,
HMBC, HSQC and J-mod-experiments. Thin layer chromatogra-
phy (TLC) was performed on Merck TLC aluminum sheets, silica
gel 60, F254. Flash chromatography was performed with Merck
silica gel 60, 40–63 mm. HRMS was recorded on a Micromass
Q-Tof Micro (3000 V) apparatus.
The synthesis and characterisation of compound 1 from tert-
butyl acrylate is described in a previous publication.16 Chemical
structures of intermediates 1a, 2a, 4a and 5a are shown in ESI
(Figure S19).
several times with CH2Cl2. The residue was dried in vacuo, yielding
the desired TFA salt.
General procedure for coupling of peptoid oligomers using HATU
(method E). To a solution of the peptoid with a free N-terminus
(2, 3, 5 or 6: 1.0 equiv, 50 mM) and the peptoid 16 with a free C-
terminus (1.0 equiv) in CH2Cl2/DMF 4 : 1 at 0 ◦C under Ar was
added iPr2NEt (1.2 equiv.) and HATU (1.2 equiv.) The resulting
mixture was stirred overnight while allowing to warm slowly to
rt. The solvents were evaporated under reduced pressure and the
residue was taken up in EtOAc. The organic layer was washed with
an equal amount of water and the aqueous layer was extracted
with EtOAc (¥1). The combined organic layers were dried
over MgSO4, filtered and concentrated under reduced pressure.
Flash chromatography of the residue yielded the desired product
(17–20).
General procedure for chain elongation of one a,b-dimeric peptoid
unit using two consecutive submonomer methods (A and B).
b-Peptoid submonomer synthesis (method A). To a solution of
the secondary amine (N-terminus a-peptoidic residue: 1.0 equiv,
0.2 M) in THF at 0 ◦C under Ar was added Et3N (1.4 equiv) and
then acryloyl chloride (1.2 equiv). After stirring for 1 h at 0 ◦C
the resulting mixture was filtered, washing the solids with THF.
The filtrate was then concentrated and dried in vacuo, yielding the
crude acrylamide. To a solution of the crude acrylamide (0.4 M)
in MeOH at rt under Ar was a◦dded (S)-1-phenylethylamine (2.0
equiv). After stirring 6 h at 50 C, the mixture was concentrated
under reduced pressure. Flash chromatography of the crude
yielded the desired secondary amine which was engaged in the
a-peptoid submonomer synthesis.
a-Peptoid submonomer synthesis (method B). To a solution of
the previously obtained secondary amine (1.0 equiv, 0.2 M) in
THF at 0 ◦C under Ar was added Et3N (1.2 equiv) and then
bromoacetyl bromide (1.2 equiv). After stirring for 1 h at 0 ◦C the
resulting mixture was filtered, washing the solids with THF. The
filtrate was then concentrated and dried in vacuo, yielding the crude
bromoacetyl amide. To a solution of the crude bromoacetyl amide
(0.2 M) in THF at 0 ◦C under Ar was added Et3N (2.0 equiv)
followed by (S)-1-phenylethylamine (2.0 equiv). After stirring
overnight at rt, further Et3N (3.0 equiv) was added and stirring was
continued for 10 min. The resulting mixture was filtered, washing
the solids with THF. The filtrate was then concentrated under
reduced pressure. Flash chromatography of the residue yielded
the desired product (1–6).
a,b-Peptoid 2. Reaction of 1 (8.41 g, 20.5 mmol) following
method A yielded 1a (10.8 g, 90%) as a pale yellowish oil. Then
reaction of 1a (5.27 g, 9.00 mmol) following method B and
performing the flash column chromatography with EtOAc/MeOH
97 : 3 as eluent until the impurities had passed followed by
change to CH2Cl2/MeOH 97 : 3 yielded 2 (5.33 g, 79%) as a
pale yellowish solid: Rf (CH2Cl2/MeOH 97 : 3) = 0.23; [a]D21
◦
1
-118.8 (c 0.74 in CHCl3); mp 36–38 C; H NMR (400 MHz,
CDCl3): d 7.38–6.94 (20H, m, PhH), 6.09–5.69 (1.24H, m,
NCHCH3), 5.34–5.05 (0.30H, m, NCHCH3), 4.98–4.50 (1.46H,
m, NCHCH3), 4.42–2.98 (9H, m, 2¥NCH2C O, HNCHCH3
and 2¥NCH2CH2C O), 2.83–1.75 (4H, m, 2¥NCH2CH2C O),
1.64–1.26 (21H, m, 4¥NCHCH3 and tBu); 13C NMR (100 MHz,
CDCl3): d 172.3, 172.1, 171.3, 171.2, 171.1, 171.0, 170.9, 170.8,
170.6, 170.3, 170.2, 170.0, 170.0, 169.7, 168.3, 168.0, 167.9, 167.8,
167.6, 167.4, 167.0, 166.9 (4C, 4¥C O), 144.8 (C, Ph), 140.6,
140.5, 140.3, 140.2, 140.0, 139.9, 139.7, 139.4, 139.2, 139.0 (3C,
Ph), 128.9, 128.6, 128.6, 128.4, 128.0, 127.8, 127.5, 127.4, 127.3,
126.9, 126.8, 126.7, 126.5, 126.4, 126.0, 125.9 (20CH, Ph), 80.4,
t
80.2 (C, Bu), 58.4, 58.4, 58.0, 57.8, 57.8 (CH, HNCHCH3),
55.1, 55.0, 54.5, 54.4, 54.2, 54.1, 54.0 (1.76CH, NCHCH3), 52.0,
51.8, 51.4, 51.3, 51.2, 50.7 (1.24CH, NCHCH3), 49.1, 48.9, 48.7,
48.5 (CH2, HNCH2C O), 45.1, 44.9, 44.6, 44.0, 43.9 (CH2,
NCH2C O), 40.3, 40.2, 39.9, 39.8, 39.6, 39.2, 39.1, 38.8, 38.5,
38.4, 38.3 (2CH2, 2¥NCH2CH2C O), 36.2, 36.1, 35.6, 35.6, 34.1,
34.0, 33.7, 33.6, 33.4, 33.1, 32.7, 32.5 (2CH2, 2¥NCH2CH2C O),
27.9, 27.9 (3CH3, tBu), 24.6, 24.6, 24.4 (CH3, HNCHCH3), 18.6,
18.5, 18.4, 18.3, 18.3, 18.2, 17.8, 16.5, 16.4, 16.2, 16.2, 16.0, 15.8
(3CH3, 3¥NCHCH3); nmax/cm-1 2975, 1724 (C O, ester), 1642
(C O, amide), 1496, 1451, 1415, 1393, 1369, 1290, 1257, 1207,
1151, 1074, 1051, 1033, 1007, 991, 914, 846, 788, 755, 700; HRMS
(TOF MS ES+) calcd for C46H59N4O5 [M + H]+ m/z 747.4485,
found 747.4493. HPLC (Water (0.1% TFA)/MeCN 40 : 60, flow =
0.80): tr = 22.42 min; HPLC (Water (0.1% TFA)/MeOH 20 : 80,
flow = 0.60): tr = 13.57 min, purity = 98.0%.
General procedure for N-Boc protection of peptoids (method C).
To a solution of the peptoid with a free N-terminus (1.0 equiv, 0.2
M) in dioxane at 0 ◦C was added 0.5 M aq. NaOH (1 mL per 1 mL
dioxane) and the resulting mixture was stirred for 5 min. Boc2O
(2 equiv) was then added and stirring was continued overnight
while allowing the reaction to warm to rt. The dioxane was then
evaporated under reduced pressure and the residue was diluted
with water (approx. twice the volume used for the reaction). The
aqueous layer was extracted with EtOAc (2¥) and the combined
organic layers were dried over MgSO4, filtered and concentrated
under reduced pressure. Flash chromatography yielded the desired
product (7–12).
a,b-peptoid 3. Reaction of 2 (1.50 g, 2.01 mmol) follow-
ing method A yielded 2a (1.45 g, 79%) as a pale yellow-
ish solid. Then reaction of 2a (1.29 g, 1.40 mmol) following
method B and performing the flash column chromatography in
EtOAc/MeOH 95 : 5 to EtOAc/MeOH 90 : 10 yielded 3 (1.14 g,
75%) as a colorless solid: Rf (EtOAc/MeOH 90 : 10) = 0.64; [a]D21
General procedure for TFA t-butyl ester deprotection and isola-
tion of compounds 13–15 as TFA salts (method D). To a solution
of the tButyl ester (1, 2 or 3: 1.0 equiv, 0.10 M) in CH2Cl2 at
0
◦C under Ar was added TFA (1 mL per 1 mL CH2Cl2) and
the resulting mixture was stirred for 4 h at 0 ◦C. The solvents were
evaporated under reduced pressure and the residue was evaporated
◦
1
-120.0 (c 0.69 in CHCl3); mp 62–64 C; H NMR (400 MHz,
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CDCl3): d 7.42–6.92 (30H, m, PhH), 6.10–5.61 (2.23H, m,
NCHCH3), 5.33–4.53 (2.77H, m, NCHCH3), 4.48–2.95 (13H, m,
3¥NCH2C O, HNCHCH3 and 3¥NCH2CH2C O), 2.84–1.84
(6H, m, 3¥NCH2CH2C O), 1.70–1.20 (27H, m, 6¥NCHCH3
1157, 1121, 1074, 1028, 1005, 914, 762, 700; HRMS (TOF MS ES+)
calcd for C21H28N3O2 [M + H]+ m/z 354.2182, found 354.2192.
HPLC (Water (0.1% TFA)/MeOH 40 : 60, flow = 0.40): tr = 11.17
min, purity = 99.9%.
t
and Bu); 13C NMR (100 MHz, CDCl3): d 172.2, 172.2, 172.0,
a,b-Peptoid 5. Reaction of 4 (2.19 g, 6.20 mmol) fol-
lowing method A yielded 4a (2.92 g, 89%) as a colorless
solid. Then reaction of 4a (2.00 g, 5.30 mmol) following
method B and performing the flash column chromatography
in EtOAc/MeOH 95 : 5 until the impurities had passed fol-
lowed by change to CH2Cl2/MeOH 95 : 5 yielded 5 (1.22 g,
33%) as a pale yellowish solid: Rf (CH2Cl2/MeOH 95 : 5) =
171.5, 171.3, 171.1, 171.0, 170.9, 170.8, 170.6, 170.2, 170.1, 170.0,
168.5, 168.4, 168.1, 168.1, 167.8, 167.8, 167.2, 167.1 (6C, 6¥C O),
144.8 (C, Ph), 141.0, 140.9, 140.7, 140.6, 140.5, 140.4, 140.3,
140.2, 140.1, 139.9, 139.8, 139.7, 139.4, 139.4, 139.2, 139.1, 139.1
(5C, Ph), 128.6, 128.3, 127.5, 127.3, 126.9, 126.8, 126.6, 126.5,
126.4, 126.3 (30CH, Ph), 80.4, 80.3 (C, tBu), 58.4, 57.9, 57.8 (CH,
HNCHCH3), 55.4, 55.3, 55.2, 55.0, 54.9, 54.7, 54.6, 54.5, 54.4,
54.4, 54.3, 54.0 (2.77CH, NCHCH3), 51.8, 51.7, 51.6, 51.6, 51.4,
51.4, 51.2, 51.1, 50.9, 50.8, 50.6, 50.6 (2.23CH, NCHCH3), 49.0,
48.9, 48.6, 48.4 (CH2, HNCH2C O), 45.1, 45.0, 44.9, 44.8, 44.7,
44.6, 44.5, 44.3, 44.3, 44.2, 44.1, 44.0, 43.9 (2CH2, 2NCH2C O),
40.6, 40.2, 40.1, 39.8, 39.7, 39.5, 39.4, 39.3, 39.2, 39.1, 38.8,
38.8, 38.6, 38.4, 38.2 (3CH2, 3¥NCH2CH2C O), 36.1, 36.1, 35.6,
34.1, 33.9, 33.9, 33.8, 33.6, 33.5, 33.4, 33.3, 33.2, 32.5, 32.4, 32.2
◦
1
0.28; [a]2D1 -133.1 (c 0.58 in CHCl3); mp 46–48 C; H NMR
(400 MHz, CDCl3): d 7.38–6.91 (20H, m, PhH), 6.86–6.70
(0.55H, m, NH2), 6.44–6.35 (0.19H, br s, NH2), 6.29–6.20
(0.08H, br s, NH2), 6.05–5.80 (1.20H, m, NCHCH3), 5.74–5.40
(1.25H, m, 0.07¥NCHCH3 and 0.59¥NH2), 5.30–5.16 (0.29H, m,
NCHCH3), 4.92–4.76 (1.19H, m, NCHCH3), 4.68–4.56 (0.25H,
m, NCHCH3), 4.32–4.12 (0.84H, m, 0.42¥NCH2C O), 3.91–
3.10 (8.16H, m, 0.58¥NCH2C O, HNCH2C O, HNCHCH3
and 2¥NCH2CH2C O), 2.81–1.95 (4H, m, 2¥NCH2CH2C O),
1.64–1.29 (12H, m, 4¥NCHCH3); 13C NMR (100 MHz, CDCl3):
d 173.8, 173.7, 173.0, 172.6, 172.1, 171.9, 171.4, 171.2, 171.0,
170.9, 170.8, 170.4, 170.3, 168.6, 168.2, 168.1, 167.7, 167.5,
167.4 (4C, 4¥C O), 144.7, 144.7, 144.6, 144.1 (C, Ph), 140.7,
140.6, 140.6, 140.5, 140.5, 140.1, 139.9, 139.7, 139.6, 139.2, 139.1,
139.0 (3C, Ph), 128.9, 128.7, 128.6, 128.6, 128.5, 128.4, 128.1,
127.8, 127.7, 127.7, 127.6, 127.5, 127.5, 127.4, 127.3, 127.1, 127.0,
126.9, 126.7, 126.6, 126.5, 126.3, 126.2 (20CH, Ph), 58.4, 58.4,
58.2, 58.0, 57.8, 57.8 (CH, HNCHCH3), 55.3, 55.1, 54.8, 54.6,
54.4, 54.3, 54.1 (1.73CH, NCHCH3), 52.9, 52.0, 51.8, 51.5, 51.4,
51.1, 51.0, 50.9 (1.27CH, NCHCH3), 49.0, 48.9, 48.6, 48.3, 47.9
(CH2, HNCH2C O), 45.3, 45.1, 45.0, 44.7, 44.4, 44.3, 44.1,
44.0 (CH2, NCH2C O), 40.5, 40.1, 39.8, 39.0, 38.9, 38.9 (2CH2,
2¥NCH2CH2C O), 36.1, 35.9, 35.7, 35.4, 35.2, 34.9, 34.2, 34.0,
33.9, 33.4, 32.5 (2CH2, 2¥NCH2CH2C O), 24.5, 24.4, 24.3, 24.3,
23.9, 23.5 (CH3, HNCHCH3), 18.7, 18.5, 18.3, 18.2, 18.1, 17.9,
17.7, 16.9, 16.7, 16.5, 16.4, 16.2, 16.1, 16.0 (3CH3, 3¥NCHCH3);
t
(3CH2, 3¥NCH2CH2C O), 27.9, 27.8 (3CH3, Bu), 24.6, 24.4,
24.2, 24.0 (CH3, HNCHCH3), 18.9, 18.7, 18.5, 18.4, 18.2, 18.1,
18.1, 18.0, 17.9, 17.7, 17.6, 16.5, 16.4, 16.3, 16.1, 16.0, 15.8 (5CH3,
5¥NCHCH3); nmax/cm-1 1724 (C O, ester), 1643 (C O, amide),
1495, 1449, 1418, 1370, 1288, 1206, 1150, 1074, 1042, 1028, 993,
916, 845, 818, 808, 785, 760, 752, 700; HRMS (TOF MS ES+)
calcd for C67H83N6O7 [M + H]+ m/z 1083.6318, found 1083.6309.
HPLC (Water (0.1% TFA)/MeCN 30 : 70, flow = 0.80): tr = 26.12
min; HPLC (Water (0.1% TFA)/MeOH 20 : 80, flow = 0.60): tr =
35.18 min, purity = 94.8%.
3-{((S)-1-Phenyl-ethyl)-[2-((S)-1-phenyl-ethylamino)-acetyl]-
amino}-propionamide (4). To a solution of acrylamide (2.13 g,
30.0 mmol) in MeOH (75 mL) at rt under Ar was added (S)-1-
phenylethylamine (7.65 mL, 60.1 mmol). After stirring overnight,
the mixture was concentrated under reduced pressure. Flash
chromatography of the crude yielded 3-((S)-1-phenyl-ethylamino)-
propionamide (5.19 g, 90%) as a colorless oil. It (2.50 g, 13.0 mmol)
was then reacted following method B to yield 4 (2.45 g, 53%) as a
colorless oil after flash column chromatography in EtOAc/MeOH
95 : 5 until the impurities had passed followed by change to
CH2Cl2/MeOH 90 : 10: Rf (CH2Cl2/MeOH 90 : 10) = 0.53; [a]D22
-135.3 (c 0.55 in CHCl3); 1H NMR (400 MHz, CDCl3): d 7.37–7.09
(10H, m, PhH), 6.35–6.22 (0.70H, br s, NH2), 5.96 (0.30H, q, J =
7.0 Hz, (c)-NCHCH3), 5.80–5.75 (1.00H, 2¥br s, NH2), 5.25–5.14
(0.30H, br s, NH2), 4.83 (0.70H, q, J = 6.9 Hz, (t)-NCHCH3), 3.86–
3.75 (1H, m, HNCHCH3), 3.52–3.16 (4H, m, HNCH2C O and
NCH2CH2C O), 2.65–2.56 (1H, br s, NH), 2.39–2.29 (0.70H,
m, NCH2CH2C O), 2.25–2.15 (0.70H, m, NCH2CH2C O),
2.06–1.95 (0.30H, m, NCH2CH2C O), 1.68–1.59 (0.30H, m,
NCH2CH2C O), 1.52–1.47 (3H, m, NCHCH3), 1.40 (3H, d, J =
6.6 Hz, HNCHCH3); 13C NMR (100 MHz, CDCl3): d 173.7, 172.1,
171.2, 171.0 (2C, 2¥C O), 144.6 (C, Ph), 140.3, 139.4 (C, Ph),
128.7, 128.5, 127.8, 127.5, 127.1, 126.9, 126.7, 126.6 (10CH, Ph),
58.3, 58.0 (CH, HNCHCH3), 54.1 (0.70CH, (t)-NCHCH3), 51.4
(0.30CH, (c)-NCHCH3), 48.9, 48.5 (CH2, HNCH2C O), 39.8,
38.6 (CH2, NCH2CH2C O), 35.9, 35.6 (CH2, NCH2CH2C O),
24.5, 24.4 (CH3, HNCHCH3), 17.9 (0.70CH3, (t)-NCHCH3), 16.4
(0.30CH3, (c)-NCHCH3); nmax/cm-1 2972, 1670 (C O, amide),
1636 (C O, amide), 1495, 1451, 1416, 1370, 1283, 1252, 1204,
n
max/cm-1 1653 (C O, amide), 1647 (C O, amide), 1640 (C O,
amide), 1494, 1450, 1415, 1293, 1208, 1164, 1071, 1050, 1029,
987, 913, 789, 753; HRMS (TOF MS ES+) calcd for C42H52N5O4
[M + H]+ m/z 690.4019, found 690.4016. HPLC (Water (0.1%
TFA)/MeOH 30 : 70, flow = 0.50): tr = 14.49 min, purity = 99.2%.
a,b-Peptoid 6. Reaction of 5 (828 mg, 1.20 mmol) following
method A yielded 5a (975 g, 94%) as a pale yellowish solid. Then
reaction of 5a (813 mg, 0.94 mmol) following method B and
performing the flash column chromatography in EtOAc/MeOH
95 : 5 until the impurities had passed followed by change to
CH2Cl2/MeOH 95 : 5 to 90 : 10 yielded 6 (577 mg, 60%) as a yel-
lowish solid: Rf (CH2Cl2/MeOH 90 : 10) = 0.54; [a]2D1 -127.0 (c 0.61
in CHCl3); mp 65–67 ◦C; 1H NMR (400 MHz, CDCl3): d 7.40–6.80
(30H, m, PhH), 6.52–6.10 (0.68H, m, NH2), 6.06–5.64 (2.29H,
m, NCHCH3), 5.60–5.30 (1.32H, m, NH2), 5.30–5.04 (0.53H, m,
NCHCH3), 4.96–4.56 (2.18H, m, NCHCH3), 4.52–2.90 (13H, m,
3¥NCH2C O, HNCHCH3 and 3¥NCH2CH2C O), 2.86–1.94
(6H, m, 3¥NCH2CH2C O), 1.70–1.24 (18H, m, 6¥NCHCH3);
13C NMR (100 MHz, CDCl3): d 173.5, 173.4, 172.7, 172.2, 172.1,
172.0, 171.8, 171.6, 171.6, 171.3, 171.2, 170.7, 170.6, 170.4, 170.3,
168.9, 168.6, 168.3, 168.1, 167.9, 167.7, 167.6, 167.4 (6C, 6¥C O),
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144.5, 144.2 (C, Ph), 140.8, 140.6, 140.5, 140.4, 140.3, 140.2,
140.1, 140.0, 139.9, 139.7, 139.6, 139.4, 139.4, 139.2, 139.2, 139.1,
139.0, 138.6 (5C, Ph), 128.8, 128.6, 128.4, 128.1, 127.7, 127.6,
127.5, 127.4, 127.3, 127.2, 127.1, 126.8, 126.7, 126.6, 126.5, 126.4
(30CH, Ph), 58.5, 58.4, 58.2, 58.0 (CH, HNCHCH3), 55.6, 55.5,
55.2, 55.0, 54.9, 54.8, 54.6, 54.6, 54.4, 54.3, 54.3, 54.0, 54.0, 53.8,
53.8 (2.71CH, NCHCH3), 52.3, 52.0, 51.9, 51.7, 51.6, 51.6, 51.4,
51.3, 51.0, 50.9, 50.7 (2.29CH, NCHCH3), 48.8, 48.8, 48.5, 48.5
(CH2, HNCH2C O), 45.6, 45.3, 45.1, 45.1, 44.9, 44.8, 44.6, 44.3,
44.2, 43.9, 43.8 (2CH2, 2NCH2C O), 41.2, 41.1, 41.0, 40.7, 40.6,
40.3, 40.3, 40.1, 39.9, 39.7, 39.3, 39.2, 39.1, 38.9, 38.8 (3CH2,
3¥NCH2CH2C O), 36.5, 36.3, 36.2, 35.9, 35.8, 35.5, 35.4, 35.2,
35.0, 34.9, 34.8, 34.4, 34.2, 34.1, 33.9, 33.8, 33.7, 33.5, 33.4, 33.3,
32.5, 32.3, 32.2, 32.0 (3CH2, 3¥NCH2CH2C O), 24.4, 24.3, 24.2,
24.0, 23.9, 23.7 (CH3, HNCHCH3), 18.7, 18.6, 18.4, 18.3, 18.2,
18.1, 18.0, 18.0, 17.9, 17.7, 17.4, 17.3, 16.9, 16.9, 16.8, 16.7, 16.6,
16.3, 16.2, 16.1, 16.0, 15.9 (5CH3, 5¥NCHCH3); nmax/cm-1 1654
(C O, amide), 1647 (C O, amide), 1638 (C O, amide), 1495,
1449, 1421, 1414, 1289, 1205, 1187, 1159, 1054, 1033, 789, 748;
HRMS (TOF MS ES+) calcd for C63H76N7O6Na [M + H + Na]2+
m/z 524.7877, found 524.7899. HPLC (Water (0.1% TFA)/MeOH
20 : 80, flow = 0.60): tr = 11.92 min, purity = 96.8%.
155.7, 155.6, 155.5 (C, Boc), 141.4, 141.3, 140.9, 140.8, 140.7,
140.6, 140.3, 139.9, 139.7, 139.4, 139.2, 139.0 (4C, Ph), 128.9,
128.8, 128.6, 128.4, 128.2, 128.0, 127.6, 127.5, 127.5, 127.3, 127.1,
127.1, 126.9, 126.8, 126.7, 126.4, 126.2 (20CH, Ph), 80.3, 79.9,
t
79.8 (2C, Boc and Bu), 55.4, 55.1, 54.7, 54.5, 54.2, 54.1, 54.0,
53.9, 53.2, 52.8, 52.8, 51.9, 51.7, 51.5, 51.3, 51.2, 50.8 (4CH,
4¥NCHCH3), 45.6, 45.5, 45.2, 45.0, 44.9, 44.6, 44.1, 44.0 (2CH2,
2¥NCH2C O), 40.6, 40.1, 39.6, 39.5, 39.3, 39.1, 38.5, 38.3 (2CH2,
2¥NCH2CH2C O), 36.2, 36.2, 35.8, 35.6, 34.2, 34.1, 33.9, 33.6,
33.3, 33.3 (2CH2, 2¥NCH2CH2C O), 28.3, 28.0, 27.9 (6CH3,
t
Boc and Bu), 18.6, 18.5, 18.4, 18.3, 18.1, 17.9, 17.8, 17.2, 17.1,
16.4, 16.2, 16.0, 15.9, 15.8 (4CH3, 4¥NCHCH3); nmax/cm-1 2976,
1724 (C O, ester), 1692 (C O, Boc), 1659 (C O, amide), 1497,
1451, 1418, 1393, 1368, 1319, 1287, 1277, 1254, 1209, 1165, 1072,
1042, 1028, 993, 864, 847, 783, 752; HRMS (TOF MS ES+) calcd
for C51H67N4O7 [M + H]+ m/z 847.5010, found 847.5024. HPLC
(Water (0.1% TFA)/MeCN 20 : 80, flow = 0.80): tr = 29.42 min,
purity = 98.1%.
a,b-Peptoid 9. N-Boc protection of 3 (108 mg, 0.10 mmol)
following method C yielded 9 (110 mg, 93%) as a colorless solid:
Rf (EtOAc) = 0.82; [a]D21 -90.3 (c 0.65 in CHCl3); mp 71–74 ◦C; 1H
NMR (400 MHz, CDCl3): d 7.45–6.88 (30H, m, PhH), 6.10–5.52
(2.88H, m, NCHCH3), 5.35–4.50 (3.12H, m, NCHCH3), 4.45–
2.95 (12H, m, 3¥NCH2C O and 3¥NCH2CH2C O), 2.77–1.85
(6H, m, 3¥NCH2CH2C O), 1.71–1.21 (36H, m, 6¥NCHCH3,
Boc and tBu); 13C NMR (100 MHz, CDCl3): d 172.4, 171.5, 171.3,
171.2, 171.1, 170.7, 170.7, 170.3, 170.0, 169.6, 168.9, 168.9, 168.5,
168.4, 168.1, 167.9, 167.7, 167.3, 167.1, 167.0 (6C, 6¥C O), 155.7,
155.6, 155.5 (C, Boc), 141.4, 141.3, 141.2, 140.8, 140.6, 140.3,
140.1, 139.8, 139.4, 139.2, 138.9 (6C, Ph), 128.8, 128.8, 128.6,
128.4, 128.2, 128.2, 127.6, 127.5, 127.4, 127.0, 126.9, 126.7, 126.7,
126.5, 126.4, 126.3 (30CH, Ph), 79.9, 79.7, 79.6 (2C, Boc and
tBu), 55.7, 55.5, 55.3, 55.0, 54.5, 54.5, 54.2, 53.3, 53.2 (3.12CH,
NCHCH3), 51.8, 51.6, 51.5, 51.4, 51.2, 51.1, 50.9, 50.8, 50.7
(2.88CH, NCHCH3), 45.5, 44.9, 44.8, 44.7, 44.5, 44.2, 44.0, 43.9
(3CH2, 3¥NCH2C O), 40.7, 40.6, 40.3, 40.1, 39.4, 39.2, 39.1,
39.0, 38.5, 38.3 (3CH2, 3¥NCH2CH2C O), 36.2, 35.6, 34.2, 34.0,
33.5, 33.4, 33.3, 33.2 (3CH2, 3¥NCH2CH2C O), 28.3, 27.9, 27.9,
27.3 (6CH3, Boc and tBu), 18.8, 18.7, 18.6, 18.4, 18.3, 18.2, 18.1,
18.0, 17.9, 17.7, 17.2, 17.1, 16.4, 16.3, 16.2, 16.0, 15.9, 15.8 (6CH3,
6¥NCHCH3); nmax/cm-1 2978, 1722 (C O, ester), 1690 (C O,
Boc), 1657 (C O, amide), 1649 (C O, amide), 1497, 1449, 1418,
1368, 1281, 1256, 1206, 1165, 1119, 1072, 1028, 993, 914, 845,
783, 752, 700; HRMS (TOF MS ES+) calcd for C72H91N6O9
[M + H]+ m/z 1183.6848, found 1183.6898. HPLC (Water (0.1%
TFA)/MeCN 10 : 90, flow = 0.80): tr = 17.82 min, purity = 94.8%.
3-[{2-[tert-Butoxycarbonyl-((S)-1-phenyl-ethyl)-amino]-acetyl}-
((S)-1-phenyl-ethyl)-amino]-propionic acid tert-butyl ester (7).
N-Boc protection of 1 (41 mg, 0.10 mmol) following method C
yielded 7 (44 mg, 86%) as a colorless oil: Rf (cyclohexane/EtOAc
70 : 30) = 0.60; [a]2D1 -58.1 (c 0.52 in CHCl3); 1H NMR (400 MHz,
CDCl3): d 7.37–7.07 (10H, m, PhH), 6.02–5.83 (0.43H, m,
NCHCH3), 5.70–5.53 (0.55H, m, NCHCH3), 5.37–5.21 (0.37H,
m, NCHCH3), 5.01–4.67 (0.65H, m, NCHCH3), 4.30–3.10
(4H, m, NCH2C O and NCH2CH2C O), 2.44–1.76 (2H, m,
NCH2CH2C O), 1.60–1.46 and 1.40–1.32 (15H and 9H, 2¥m,
t
2¥NCHCH3, Boc and Bu); 13C NMR (100 MHz, CDCl3): d
171.1, 169.9, 168.2 (2C, 2¥C O), 155.7 (C, Boc), 139.6 (2C,
Ph), 128.7, 128.5, 128.3, 127.6, 127.5, 127.1, 126.9, 126.5 (10CH,
t
Ph), 80.4, 80.0 (2C, Boc and Bu), 54.9, 54.1, 53.1, 53.0, 52.9,
51.7, 51.6, 51.4 (2CH, 2¥NCHCH3), 45.3, 45.1, 44.8 (CH2,
NCH2C O), 39.2, 38.4 (CH2, NCH2CH2C O), 36.3, 34.2
t
(CH2, NCH2CH2C O), 28.4, 28.0, 27.9 (6CH3, Boc and Bu),
18.0, 16.9, 16.5 (2CH3, 2¥NCHCH3); nmax/cm-1 2976, 1726
(C O, ester), 1695 (C O, Boc), 1667 (C O, amide), 1497,
1472, 1451, 1418, 1393, 1368, 1325, 1277, 1254, 1213, 1163, 1072,
1042, 866, 847, 783, 764, 748, 700; HRMS (TOF MS ES+) calcd
for C30H43N2O5 [M + H]+ m/z 511.3166, found 511.3172. HPLC
(Water (0.1% TFA)/MeCN 20 : 80, flow = 0.80): tr = 16.18 min,
purity = 97.9%.
a,b-Peptoid 8. N-Boc protection of 2 (112 mg, 0.15 mmol)
following method C yielded 8 (106 mg, 83%) as a colorless solid:
Rf (cyclohexane/EtOAc 60 : 40) = 0.28; [a]2D1 -86.9 (c 0.62 in
CHCl3); mp 57–59 C; H NMR (400 MHz, CDCl3): d 7.41–
6.90 (20H, m, PhH), 6.07–5.82 (1.34H, m, NCHCH3), 5.78–5.52
(0.66H, m, NCHCH3), 5.40–5.13 (0.66H, m, NCHCH3), 5.00–
3-[{2-[tert-Butoxycarbonyl-((S)-1-phenyl-ethyl)-amino]-acetyl}-
((S)-1-phenyl-ethyl)-amino]-propionamide (10). N-Boc pro-
tection of 4 (88 mg, 0.249 mmol) following method C yielded
10 (111 mg, 98%) as a colorless solid: Rf (CH2Cl2/MeOH
90 : 10) = 0.57; [a]2D3 -73.0 (c 0.64 in CHCl3); mp 44–46 ◦C;
1H NMR (400 MHz, CDCl3): d 7.36–7.04 (10H, m, PhH),
6.60–6.48 (0.62H, m, NH2), 6.01–5.06 (2.51H, m, 0.69¥NH2 and
1.13¥NCHCH3), 5.00–4.73 (0.87H, m, NCHCH3), 4.22–3.16
(4H, m, NCH2C O and NCH2CH2C O), 2.62–1.90 (2H, m,
NCH2CH2C O), 1.62–1.36 (15H, m, 2¥NCHCH3 and Boc);
13C NMR (100 MHz, CDCl3): d 173.9, 173.7, 169.1, 168.7 (2C,
2¥C O), 155.8, 155.6 (C, Boc), 141.6, 141.4, 141.4, 141.3, 139.3
◦
1
4.55 (1.34H, m, NCHCH3), 4.41–2.99 (8H, m, 2¥NCH2C
O
and 2¥NCH2CH2C O), 2.78–1.82 (4H, m, 2¥NCH2CH2C O),
1.61–1.29 (30H, m, 4¥NCHCH3, Boc and tBu); 13C NMR
(100 MHz, CDCl3): d 172.3, 172.3, 171.6, 171.4, 171.2, 171.1,
171.0, 170.8, 170.7, 170.6, 170.4, 170.4, 170.1, 170.0, 169.6, 169.2,
168.6, 168.4, 168.0, 167.9, 167.3, 167.2, 167.0 (4C, 4¥C O),
1118 | Org. Biomol. Chem., 2012, 10, 1108–1122
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(2C, Ph), 128.7, 128.5, 128.3, 127.8, 127.5, 127.2, 126.8, 126.7,
126.6 (10CH, Ph), 80.5, 80.3, 80.1, 80.0, 79.9, 79.8 (2C, Boc),
55.1, 55.0, 54.4, 54.2, 54.0, 53.2, 52.9, 52.8, 52.7, 51.6, 51.5, 51.4
(2CH, 2¥NCHCH3), 45.7, 45.5, 45.4, 45.3, 45.0, 44.9, 44.8 (CH2,
NCH2C O), 40.1, 38.8 (CH2, NCH2CH2C O), 36.1, 35.9,
35.9, 35.8, 35.7, 35.6, 35.5 (CH2, NCH2CH2C O), 28.4 (3CH3,
Boc), 18.1, 17.8, 16.3 (2CH3, 2¥NCHCH3); nmax/cm-1 2979, 1700
(C O, Boc), 1665 (C O, amide), 1654 (C O, amide), 1496,
1449, 1399, 1365, 1331, 1276, 1254, 1215, 1166, 1136, 1071,
1043, 1028, 997, 953, 918, 864, 811, 787, 762, 748; HRMS (TOF
MS ES+) calcd for C26H36N3O4 [M + H]+ m/z 454.2706, found
454.2726. HPLC (Water (0.1% TFA)/MeOH 20 : 80, flow = 0.60):
tr = 11.12 min, purity = 99.2%.
45.5, 45.3, 45.2, 45.2, 45.1, 45.0, 44.9, 44.8, 44.7, 44.6, 44.5, 44.4,
44.3, 44.1, 44.0 (3CH2, 3¥NCH2C O), 40.9, 40.9, 40.7, 40.6,
40.2, 40.0, 39.9, 39.4, 39.2, 39.0, 38.9, 38.8, 38.7, 38.4 (3CH2,
3¥NCH2CH2C O), 36.3, 36.1, 35.9, 35.8, 35.7, 35.6, 35.4, 35.2,
35.0, 34.4, 34.2, 34.0, 33.8, 33.7, 33.6, 33.6, 33.5, 33.3, 32.4, 32.3,
32.1, 31.9, 31.8, 29.2 (3CH2, 3¥NCH2CH2C O), 28.3 (3CH3,
Boc), 18.8, 18.7, 18.5, 18.3, 18.1, 17.9, 17.8, 17.7, 17.4, 17.0,
16.9, 16.8, 16.6, 16.3, 16.1 (6CH3, 6¥NCHCH3); nmax/cm-1 2976,
1683, 1655 (C O, amide), 1648 (C O, amide), 1496, 1449, 1423,
1366, 1331, 1293, 1257, 1207, 1165, 1072, 1063, 1056, 1045,
1028, 996, 918, 786, 757, 750; HRMS (TOF MS ES+) calcd
for C68H83N7O8Na2 [M + 2Na]2+ m/z 585.8049, found 585.8057.
HPLC (Water (0.1% TFA)/MeOH 10 : 90, flow = 0.75): tr = 10.25
min, purity = 97.5%.
a,b-Peptoid 11. N-Boc protection of 5 (106 mg, 0.154 mmol)
following method C yielded 11 (118 mg, 97%) as a colorless
solid: Rf (CH2Cl2/MeOH 95 : 5) = 0.42; [a]2D3 -105.8 (c 0.62
in CHCl3); mp 56–58 ◦C; 1H NMR (400 MHz, CDCl3): d
7.38–6.87 (20H, m, PhH), 6.64–6.08 (0.22H, m, NH2), 6.08–
5.70 (1.49H, m, NCHCH3), 5.70–5.10 (2.75H, m, 0.89¥NH2 and
0.97¥NCHCH3), 5.00–4.58 (1.54H, m, NCHCH3), 4.48–3.02 (8H,
m, 2¥NCH2C O and 2¥NCH2CH2C O), 2.86–2.00 (4H, m,
2¥NCH2CH2C O), 1.70–1.23 (21H, m, 4¥NCHCH3 and Boc);
13C NMR (100 MHz, CDCl3): d 174.3, 174.0, 172.2, 171.8, 171.5,
171.4, 169.2, 168.9, 168.7, 168.6, 168.0, 167.8, 167.6 (4C, 4¥C O),
155.9, 155.7, 155.5 (C, Boc), 141.3, 141.1, 141.0, 140.7, 140.5,
139.6, 139.5, 139.2 (4C, Ph), 128.9, 128.8, 128.7, 128.6, 128.5,
128.4, 128.3, 128.2, 128.1, 127.8, 127.7, 127.5, 127.4, 127.3, 127.3,
127.1, 127.0, 126.8, 126.6, 126.4 (20CH, Ph), 80.6, 80.1, 80.0,
79.8 (C, Boc), 55.2, 55.0, 54.9, 54.6, 54.5, 54.4, 54.3, 52.0, 51.5,
51.4, 51.0, 50.9 (4CH, 4¥NCHCH3), 45.3, 45.2, 45.1, 45.0, 44.5,
44.2, 44.0 (2CH2, 2¥NCH2C O), 41.1, 40.9, 40.1, 40.0, 39.0,
38.8 (2CH2, 2¥NCH2CH2C O), 36.3, 35.9, 35.4, 35.3, 35.0, 33.6,
33.4 (2CH2, 2¥NCH2CH2C O), 28.4, 28.3 (3CH3, Boc), 18.3,
17.9, 17.7, 16.7, 16.2, 16.0 (4CH3, 4¥NCHCH3); nmax/cm-1 2978,
1683, 1653 (C O, amide), 1648 (C O, amide), 1496, 1449, 1412,
1399, 1366, 1334, 1284, 1276, 1254, 1205, 1166, 1128, 1072, 1028,
993, 918, 858, 787, 758, 474; HRMS (TOF MS ES+) calcd for
C47H60N5O6 [M + H]+ m/z 790.4538, found 790.4520. HPLC
(Water (0.1% TFA)/MeOH 20 : 80, flow = 0.60): tr = 26.56 min,
purity = 97.9%.
{[(2-Carboxy-ethyl)-((S)-1-phenyl-ethyl)-carbamoyl]-methyl}-
((S)-1-phenyl-ethyl)-ammonium trifluoro-acetate (13·TFA). Re-
action of 1 (41 mg, 0.100 mmol) following method D yielded
13·TFA (47 mg, 100%) as a colorless solid: [a]2D2 -91.3 (c 0.61
in CHCl3); mp 27–29 ◦C; 1H NMR (400 MHz, CDCl3): d
7.58–7.06 (10H, m, PhH), 5.77 (0.04H, q, J = 6.9 Hz, (c)-
NCHCH3), 5.58 (0.37H, q, J = 6.9 Hz, (c)-NCHCH3), 4.85–
4.80 (0.07H, m, (t)-NCHCH3), 4.67 (0.52H, q, J = 6.7 Hz,
(t)-NCHCH3), 4.58–4.44 (1H, m, HNCHCH3), 4.11–3.78 (2H,
m, HNCH2C O), 3.51–3.10 (2H, m, NCH2CH2C O), 2.34–
2.98 (2H, m, NCH2CH2C O), 1.81–1.73 (3H, m, HNCHCH3),
1.64–1.45 (3H, m, NCHCH3); 13C NMR (100 MHz, CDCl3):
d 174.9, 173.5, 165.5, 165.1 (2C, 2¥C O), 161.6 (C, q, J =
37.0 Hz, CF3C O), 138.7, 138.1, 135.1, 135.0 (2C, Ph), 129.7,
129.7, 129.4, 129.3, 128.9, 128.7, 128.3, 128.0, 128.0, 127.2,
126.6 (10CH, Ph), 116.0 (C, q, J = 290.9 Hz, CF3C O),
59.2, 59.1 (CH, HNCHCH3), 55.1 (0.59CH, NCHCH3), 53.1
(0.41CH, NCHCH3), 46.0 (CH2, HNCH2C O), 39.4, 39.0 (CH2,
NCH2CH2C O), 33.8, 33.4 (CH2, NCH2CH2C O), 19.6, 19.3,
17.5, 16.3 (2CH3, 2¥NCHCH3); nmax/cm-1 2983, 1721 (C O,
acid), 1654 (C O, amide), 1498, 1457, 1426, 1387, 1311, 1193,
1137, 1071, 1033, 916, 835, 798, 765, 720; HRMS (TOF MS
ES+) calcd for C21H27N2O3 [M–TFA + H]+ m/z 355.2016, found
355.2017. HPLC (Water (0.1% TFA)/MeOH 30 : 70, flow = 0.50):
tr = 7.23 min, purity = 98.3%.
a,b-Peptoid (14·TFA). Reaction of 2 (37 mg, 0.050 mmol)
following method D yielded 14·TFA (40 mg, 100%) as a
pale yellowish solid: [a]2D1 -101.7 (c 0.53 in CHCl3); mp 57–
60 ◦C; 1H NMR (400 MHz, CDCl3): d 7.52–6.85 (20H,
m, PhH), 6.04–5.54 (1.49H, m, NCHCH3), 5.08–4.60 (1.51H,
a,b-Peptoid 12. N-Boc protection of 6 (103 mg, 0.100 mmol)
following method C yielded 12 (108 mg, 96%) as a colorless
solid: Rf (CH2Cl2/MeOH 95 : 5) = 0.26; [a]2D2 -100.0 (c 0.65 in
CHCl3); mp 66–68 ◦C; 1H NMR (400 MHz, CDCl3): d 7.60–6.82
(30H, m, PhH), 6.73–6.12 (0.22H, m, NH2), 6.07–4.54 (7.78H, m,
m, NCHCH3), 4.56–2.90 (9H, m, HNCHCH3, 2¥NCH2C
O
0.89¥NH2 and 6¥NCHCH3), 4.47–3.06 (12H, m, 3¥NCH2C
O
and 2¥NCH2CH2C O), 2.64–2.10 (4H, m, 2¥NCH2CH2C O),
1.81–1.20 (12H, m, 4¥NCHCH3); 13C NMR (100 MHz, CDCl3):
d 174.7, 174.6, 173.5, 173.2, 172.6, 172.2, 171.8, 171.5, 170.9,
168.5, 168.3, 168.2, 167.9, 165.5, 165.4, 164.8 (4C, 4¥C O), 161.3
(C, q, J = 37.4 Hz, CF3C O), 140.5, 140.4, 140.1, 140.0, 139.6,
139.4, 139.3, 139.1, 138.7, 135.2, 135.0, 135.0, 134.9, 134.8 (4C,
Ph), 130.0, 129.8, 129.8, 129.7, 129.6, 129.4, 129.4, 129.0, 128.9,
128.8, 128.7, 128.6, 128.4, 128.3, 128.2, 128.0, 127.9, 127.8, 127.7,
127.6, 127.5, 127.4, 127.4, 127.1, 126.7, 126.6, 126.5, 126.4, 126.2
(20CH, Ph), 115.9 (C, q, J = 289.2 Hz, CF3C O), 59.2, 59.1,
59.0 (CH, HNCHCH3), 56.0, 55.5, 55.4, 55.3, 55.2, 55.1, 55.0,
54.5 (1.51CH, NCHCH3), 53.5, 53.2, 53.0, 52.9, 51.5, 51.4, 51.3
(1.49CH, NCHCH3), 46.1, 46.0, 45.9, 45.8, 45.7, 45.6, 45.5, 45.3,
and 3¥NCH2CH2C O), 3.00–1.85 (6H, m, 3¥NCH2CH2C O),
1.70–1.20 (27H, m, 6¥NCHCH3 and Boc); 13C NMR (100 MHz,
CDCl3): d 172.7, 172.6, 172.3, 171.9, 171.8, 171.7, 171.6, 171.4,
171.3, 168.9, 168.6, 168.4, 168.1, 167.9, 167.7, 167.5 (6C, 6¥C O),
155.6 (C, Boc), 141.4, 141.0, 140.8, 140.7, 140.5, 140.3, 140.2,
139.9, 139.5, 139.4, 139.1, 139.0 (6C, Ph), 128.7, 128.6, 128.5,
128.4, 128.4, 128.2, 127.9, 127.7, 127.6, 127.5, 127.4, 127.3, 127.1,
127.1, 127.0, 126.8, 126.7, 126.6, 126.5, 126.4 (30CH, Ph), 80.1,
80.0, 79.9, 79.9, 79.8 (C, Boc), 55.6, 55.5, 55.4, 55.3, 55.1, 55.1,
54.9, 54.8, 54.7, 54.5, 54.4, 54.4, 54.2, 54.0, 53.9, 53.8, 53.8, 53.7
(3.27CH, NCHCH3), 53.3, 53.3, 53.2, 53.1, 52.4, 52.3, 52.1, 52.0,
51.7, 51.5, 51.5, 51.1, 51.0, 50.8, 50.6, 50.5 (2.73CH, NCHCH3),
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45.2, 45.1, 45.0, 44.7, 44.5, 44.4, 44.2, 44.0 (CH2, HNCH2C O),
41.3, 41.1, 40.7, 40.6, 40.4, 40.2, 40.1, 40.0, 39.8, 39.4, 39.3, 39.3,
39.1, 39.0, 38.9, 38.7 (2CH2, 2¥NCH2CH2C O), 34.4, 34.0, 33.9,
33.8, 33.6, 32.6, 32.3, 31.8 (2CH2, 2¥NCH2CH2C O), 19.9, 19.6,
19.5, 19.4, 19.3, 18.5, 18.4, 18.1, 18.1, 17.8, 17.7, 16.9, 16.5, 16.3,
16.1, 16.0 (4CH3, 4¥NCHCH3); nmax/cm-1 1728 (C O, acid), 1654
(C O, amide), 1647 (C O, amide), 1497, 1453, 1420, 1383, 1299,
1200, 1183, 1176, 1136, 1073, 1042, 1029, 983, 916, 833, 798, 749;
HRMS (TOF MS ES+) calcd for C42H51N4O5 [M–TFA + H]+
m/z 691.3859, found 691.3871. HPLC (Water (0.1% TFA)/MeOH
20 : 80, flow = 0.60): tr = 7.06 min, purity = 96.2%.
18.3, 18.3, 18.0, 17.9, 17.7, 17.6, 17.4, 17.2, 17.1, 16.9, 16.6, 16.4,
16.4, 16.2, 16.0 (4CH3, 4¥NCHCH3); nmax/cm-1 2978, 1730 (C O,
acid), 1692 (C O, Boc), 1654 (C O, amide), 1496, 1451, 1399,
1367, 1332, 1284, 1254, 1206, 1164, 1071, 1044, 1028, 994, 916,
862, 793, 751, 700; HRMS (TOF MS ES+) calcd for C47H59N4O7
[M + H]+ m/z 791.4384, found 791.4392. HPLC (Water (0.1%
TFA)/MeCN 30 : 70, flow = 0.80): tr = 17.46 min, purity = 99.9%
a,b-Peptoid 17. Coupling of 16 (317 mg, 0.401 mmol) with 2
(299 mg, 0.400 mmol) following method E and performing the
flash column chromatography in EtOAc to EtOAc/MeOH 97 : 3
yielded 17 (536 mg, 88%) as a colorless solid: Rf (EtOAc/MeOH
97 : 3) = 0.86. The following analytical data were obtained
using a sample purified by prep-HPLC: [a]2D1 -94.5 (c 0.48 in
a,b-Peptoid (15·TFA). Reaction of 3 (54 mg, 0.050 mmol)
following method D yielded 15·TFA (57 mg, 100%) as a colorless
◦
◦
1
1
solid: [a]2D1 -102.4 (c 0.83 in CHCl3); mp 63–66 C; H NMR
(400 MHz, CDCl3): d 7.52–6.75 (30H, m, PhH), 6.05–5.55 (2.23H,
m, NCHCH3), 5.16–4.54 (2.77H, m, NCHCH3), 4.53–3.04
(13H, m, HNCHCH3, 3¥NCH2C O and 3¥NCH2CH2C O),
2.83–1.86 (6H, m, 3¥NCH2CH2C O), 1.84–1.20 (18H, m, m,
6¥NCHCH3); Due to the limited amount of product available
satisfactory 13C NMR spectra could not be obtained; nmax/cm-1
1728 (C O, acid), 1654 (C O, amide), 1651 (C O, amide), 1646
(C O, amide), 1497, 1449, 1420, 1380, 1295, 1171, 1076, 1043,
1029, 983, 913, 843, 814, 793, 762; HRMS (TOF MS ES+) calcd
for C63H75N6O7Na [M–TFA + Na + H]2+ m/z 525.2797, found
525.2808. HPLC (Water (0.1% TFA)/MeOH 20 : 80, flow = 0.60):
tr = 13.55 min, purity = 93.6%.
CHCl3); mp 75–78 C; H NMR (400 MHz, CDCl3): d 7.42–
6.88 (40H, m, PhH), 6.10–5.52 (3.75H, m, NCHCH3), 5.34–
4.52 (4.25H, m, NCHCH3), 4.44–2.98 (16H, m, 4¥NCH2C
O
and 4¥NCH2CH2C O), 2.74–1.79 (8H, m, 4¥NCH2CH2C O),
1.71–1.14 (42H, m, 8¥NCHCH3, Boc and tBu); 13C NMR
(100 MHz, CDCl3): d 172.2, 172.1, 172.0, 171.8, 171.5, 171.5,
171.2, 171.0, 170.9, 170.8, 170.5, 170.2, 169.9, 169.5, 169.4, 169.3,
168.8, 168.8, 168.7, 168.6, 168.4, 168.3, 168.2, 168.1, 168.0, 167.9,
167.8, 167.6, 167.3, 167.2, 167.1, 166.9, 166.8 (8C, 8¥C O), 155.5,
155.4 (C, Boc), 141.2, 141.1, 140.9, 140.7, 140.6, 140.4, 140.3,
140.2, 140.0, 139.9, 139.8, 139.6, 139.6, 139.5, 139.4, 139.3, 139.2,
139.0 (8C, Ph), 128.5, 128.3, 128.1, 127.8, 127.4, 127.3, 127.3,
127.2, 127.0, 126.9, 126.6, 126.3, 126.2 (40CH, Ph), 79.7, 79.6 (2C,
t
Boc and Bu), 55.6, 55.5, 55.4, 55.3, 55.2, 54.9, 54.8, 54.5, 54.3,
a,b-Peptoid 16. To a solution of 2 (1.12 g, 1.50 mmol) in
CH2Cl2 (15 mL) at rt under Ar was added TFA (15 mL) and
the resulting mixture was stirred for 1.5 h at rt. The solvents were
evaporated under reduced pressure and the residue was dried in
vacuo. The residue was dissolved in dioxane (8.25 mL)) and 0.5 M
aq. NaOH (8.25 mL) at 0 ◦C. 20% aq. NaOH was added dropwise
until the mixture turned basic. Boc2O (360 mg, 1.65 mmol) was
added and stirring was continued overnight while allowing to
warm slowly to rt. The dioxane was then evaporated under reduced
pressure and 1 M HCl (35 mL) and CH2Cl2 (80 mL) were added.
The organic layer was isolated and the aqueous layer was extracted
with CH2Cl2 (2 ¥ 35 mL). The combined organic layers were dried
over MgSO4, filtered and concentrated under reduced pressure.
Flash chromatography of the residue yielded 16 (901 mg, 76%) as a
colorless solid: Rf (CH2Cl2/MeOH 95 : 5) = 0.41; [a]2D1 -97.6 (c 0.68
in CHCl3); mp 71–74 C; H NMR (400 MHz, CDCl3): d 7.42–
6.85 (20H, m, PhH), 6.07–5.77 (1.44H, m, NCHCH3), 5.71–5.54
(0.59H, m, NCHCH3), 5.37–5.18 (0.65H, m, NCHCH3), 4.95–
4.47 (1.58H, m, 1.32¥NCHCH3 and 0.13¥NCH2C O), 4.37–3.12
(7.74H, m, 1.87¥NCH2C O and 2¥NCH2CH2C O), 2.90–1.95
(4H, m, 2¥NCH2CH2C O), 1.66–1.26 (21H, m, 4¥NCHCH3 and
Boc); 13C NMR (100 MHz, CDCl3): d 174.6, 174.5, 173.7, 173.3,
172.5, 172.4, 171.7, 169.0, 168.9, 168.5, 168.1, 167.9 (4C, 4¥C O),
155.8, 155.6 (C, Boc), 141.9, 141.4, 141.1, 140.5, 140.1, 139.7,
139.6, 139.3, 139.2, 139.0, 138.8 (4C, Ph), 128.9, 128.7, 128.7,
128.4, 128.4, 128.3, 127.7, 127.6, 127.4, 127.3, 127.1, 127.0, 126.8,
126.7, 126.5 (20CH, Ph), 81.2, 80.1 (C, Boc), 55.3, 55.2, 55.0,
54.8, 54.8, 54.7, 54.6, 54.4, 54.1, 53.2, 53.1, 52.5, 51.7, 51.5, 51.4,
51.3, 51.0, 50.8 (4CH, 4¥NCHCH3), 45.6, 45.5, 45.3, 45.2, 45.0,
44.6, 44.2, 43.9, 43.7 (2CH2, 2¥NCH2C O), 40.8, 40.0, 39.7, 39.0,
38.8, 38.4 (2CH2, 2¥NCH2CH2C O), 34.8, 34.6, 34.4, 33.8, 33.5,
33.2 (2CH2, 2¥NCH2CH2C O), 28.3 (3CH3, Boc), 18.7, 18.5,
54.2, 54.2, 54.1, 53.3, 53.2, 53.0 (4.25CH, NCHCH3), 51.6, 51.5,
51.3, 51.2, 51.2, 51.1, 51.0, 50.9, 50.7, 50.6 (3.75CH, NCHCH3),
45.4, 44.8, 44.7, 44.5, 44.4, 44.3, 44.2, 44.2, 44.1, 44.0, 43.8 (4CH2,
4¥NCH2C O), 40.6, 40.4, 40.1, 40.1, 39.9, 39.7, 39.5, 39.3, 39.1,
39.0, 38.9, 38.8, 38.5, 38.3, 38.1, 38.1 (4CH2, 4¥NCH2CH2C O),
36.0, 35.9, 35.8, 35.5, 34.0, 33.8, 33.7, 33.4, 33.3, 33.2, 33.0,
32.9, 32.4, 32.2, 31.9 (4CH2, 4¥NCH2CH2C O), 28.2, 27.8, 27.8
(6CH3, Boc and tBu), 19.1, 19.1, 19.0, 18.9, 18.7, 18.6, 18.5, 18.4,
18.2, 18.1, 18.0, 17.9, 17.7, 17.6, 17.1, 17.0, 16.7, 16.6, 16.3, 16.2,
16.1, 15.9 (8CH3, 8¥NCHCH3); nmax/cm-1 1724 (C O, ester),
1718 (C O, ester), 1691 (C O, Boc), 1654 (C O, amide), 1646
(C O, amide), 1496, 1450, 1420, 1413, 1407, 1367, 1332, 1285,
1253, 1205, 1201, 1164, 1159, 1075, 1045, 1029, 993, 916, 845, 788,
752; HRMS (TOF MS ES+) calcd for C93H114N8O11 [M + 2H]2+
m/z 760.4377, found 760.4440. Original sample: HPLC (Water
(0.1% TFA)/MeCN 10 : 90, flow = 0.80): tr = 28.61 min; HPLC
(Water (0.1% TFA)/MeOH 5 : 95, flow = 0.80): tr = 12.65 min,
purity = 81.8%. Sample purified by prep-HPLC: purity = 99.9%.
◦
1
a,b-Peptoid 18. Coupling of 16 (198 mg, 0.250 mmol) with 3
(271 mg, 0.250 mmol) following method E and performing the
flash column chromatography in EtOAc to EtOAc/MeOH 97 : 3
yielded 18 (389 mg, 84%) as a colorless solid: Rf (EtOAc/MeOH
97 : 3) = 0.89. The following analytical data were obtained
using a sample purified by prep-HPLC: [a]2D1 -98.5 (c 0.46 in
◦
1
CHCl3); mp 80–83 C; H NMR (400 MHz, CDCl3): d 7.41–
6.88 (50H, m, PhH), 6.02–5.52 (4.77H, m, NCHCH3), 5.35–
4.50 (5.23H, m, NCHCH3), 4.43–2.95 (20H, m, 5¥NCH2C
O
and 5¥NCH2CH2C O), 2.76–1.30 (10H, m, 5¥NCH2CH2C O),
1.71–1.14 (48H, m, 10¥NCHCH3, Boc and tBu); 13C NMR
(100 MHz, CDCl3): d 172.4, 172.3, 172.2, 172.1, 172.0, 171.6,
171.5, 171.4, 171.3, 171.1, 171.0, 170.9, 170.7, 170.7, 170.6, 170.5,
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170.3, 170.0, 169.5, 168.6, 168.5, 168.2, 168.1, 167.9, 167.8, 167.7,
167.5, 167.4, 167.3, 167.2, 167.1, 167.0, 166.9 (10C, 10¥C O),
155.7, 155.6, 155.5 (C, Boc), 141.5, 141.3, 141.2, 141.1, 140.8,
140.6, 140.5, 140.5, 140.3, 140.2, 140.0, 139.7, 139.6, 139.4, 139.3,
139.1, 139.0 (10C, Ph), 128.6, 128.4, 127.9, 127.5, 127.5, 127.3,
127.0, 126.7, 126.7, 126.4, 126.3 (50CH, Ph), 79.9, 79.7, 79.6 (2C,
5¥NCH2CH2C O), 2.78–1.78 (10H, m, 5¥NCH2CH2C O),
1.72–1.12 (39H, m, 10¥NCHCH3 and Boc); 13C NMR (100 MHz,
CDCl3): d 172.3, 172.1, 172.0, 171.7, 171.4, 171.3, 171.1, 171.0,
170.7, 170.6, 170.4, 170.3, 170.2, 170.0, 169.2, 169.0, 168.9, 168.7,
168.7, 168.5, 168.5, 168.3, 168.2, 168.1, 167.9, 167.9, 167.8, 167.7,
167.4, 167.3, 167.2, 167.0 (10C, 10¥C O), 155.6, 155.6, 155.5
(C, Boc), 141.3, 141.0, 140.9, 140.9, 140.8, 140.7, 140.6, 140.5,
140.4, 140.4, 140.2, 140.1, 139.9, 139.8, 139.7, 139.6, 139.4, 139.3,
139.1, 139.0 (10C, Ph), 128.6, 128.4, 128.2, 128.2, 127.6, 127.4,
127.3, 126.8, 126.7, 126.6, 126.4 (50CH, Ph), 79.9, 79.8, 79.7,
79.7, 79.6 (C, Boc), 55.5, 55.4, 55.3, 55.1, 55.0, 55.0, 54.9, 54.7,
54.6, 54.6, 54.3, 54.2, 54.0 (5.30CH, NCHCH3), 53.5, 53.1, 51.9,
51.8, 51.4, 51.3, 51.1, 51.0, 50.9, 50.8 (4.70CH, NCHCH3), 45.5,
45.3, 45.2, 44.9, 44.8, 44.5, 44.4, 44.3, 44.3, 44.1, 43.9, 43.8, 43.7
(5CH2, 5¥NCH2C O), 41.1, 41.0, 40.9, 40.8, 40.7, 40.6, 40.5,
40.4, 40.0, 39.3, 39.3, 39.2, 39.0, 38.9, 38.8, 38.7, 38.6 (5CH2,
5¥NCH2CH2C O), 34.0, 33.9, 33.8, 33.8, 33.7, 33.5, 33.4, 33.3,
33.2, 33.1, 32.4, 32.2, 32.2, 31.9, 31.8 (5CH2, 5¥NCH2CH2C O),
28.3 (3CH3, Boc), 19.1, 19.0, 18.9, 18.8, 18.6, 18.5, 18.4, 18.3,
18.2, 18.1, 18.1, 18.0, 17.9, 17.8, 17.7, 17.4, 17.1, 17.0, 16.9, 16.7,
16.7, 16.5, 16.3, 16.3, 16.2, 16.1 (10CH3, 10¥NCHCH3); nmax/cm-1
2977, 1685, 1647 (C O, amide), 1642 (C O, amide), 1638 (C O,
amide), 1495, 1449, 1421, 1405, 1364, 1288, 1207, 1182, 1167, 1071,
1056, 1045, 1028, 997, 784, 746; HRMS (TOF MS ES+) calcd for
C110H131N11O12Na2 [M + 2Na]2+ m/z 921.9887, found 921.9893.
HPLC (Water (0.1% TFA)/MeOH 10 : 90, flow = 0.75): tr = 25.03
min, purity = 93.7%.
t
Boc and Bu), 55.5, 55.3, 55.1, 55.0, 54.9, 54.7, 54.5, 54.4, 54.1,
53.9, 53.3, 53.2 (5.23CH, NCHCH3), 51.8, 51.6, 51.4, 51.3, 51.1,
51.0, 50.8, 50.7 (4.77CH, NCHCH3), 45.5, 45.2, 44.9, 44.8, 44.6,
44.5, 44.4, 44.2, 44.1, 43.9 (5CH2, 5¥NCH2C O), 40.8, 40.6,
40.1, 40.0, 39.9, 39.4, 39.2, 39.1, 39.0, 38.8, 38.5, 38.2 (5CH2,
5¥NCH2CH2C O), 36.2, 36.1, 35.6, 34.1, 33.9, 33.4, 33.3, 32.4,
32.3, 32.2 (5CH2, 5¥NCH2CH2C O), 28.3, 27.9, 27.9 (6CH3, Boc
t
and Bu), 19.2, 19.1, 19.0, 18.9, 18.7, 18.6, 18.4, 18.3, 18.2, 18.1,
17.9, 17.8, 17.7, 17.5, 17.2, 17.1, 16.4, 16.3, 16.1, 16.0 (10CH3,
10¥NCHCH3); nmax/cm-1 1692 (C O, Boc), 1653 (C O, amide),
1647 (C O, amide), 1496, 1451, 1412, 1367, 1285, 1251, 1204,
1162, 1099, 1073, 1045, 1032, 1029, 953, 912, 842, 809, 788, 746,
734; HRMS (TOF MS ES+) calcd for C114H139N10O13 [M + 2H]2+
m/z 928.5295, found 928.5291. Original sample: HPLC (Water
(0.1% TFA)/MeCN 10 : 90, flow = 0.80): tr = 42.16 min; HPLC
(Water (0.1% TFA)/MeOH 5 : 95, flow = 0.80): tr = 17.05 min,
purity = 85.6%. Sample purified by prep-HPLC: purity = 99.9%.
a,b-Peptoid 19. Coupling of 16 (158 mg, 0.200 mmol) with
5 (139 mg, 0.201 mmol) following method E yielded 19 (277
mg, 95%) as a colorless solid: Rf (CH2Cl2/MeOH 95 : 5) =
◦
1
0.43; [a]2D2 -101.0 (c 0.74 in CHCl3); mp 68–71 C; H NMR
(400 MHz, CDCl3): d 7.46–6.80 (40H, m, PhH), 6.06–4.50 (10H,
Acknowledgements
m, NH2 and 8¥NCHCH3), 4.42–3.00 (16H, m, 4¥NCH2C
O
and 4¥NCH2CH2C O), 2.75–1.85 (8H, m, 4¥NCH2CH2C O),
1.70–1.18 (33H, m, 8¥NCHCH3 and Boc); 13C NMR (100 MHz,
CDCl3): d 172.4, 172.2, 171.7, 171.4, 171.2, 170.7, 168.7, 168.5,
168.1, 167.9, 167.7, 167.4, 167.0 (8C, 8¥C O), 155.6 (C, Boc),
141.0, 140.9, 140.7, 140.7, 140.5, 140.3, 140.1, 140.0, 139.7, 139.5,
139.4, 139.1, 139.0 (8C, Ph), 128.6, 128.5, 128.4, 128.4, 128.2,
127.6, 127.4, 127.1, 127.0, 126.9, 126.7, 126.6, 126.4 (40CH, Ph),
79.9, 79.8, 79.7 (C, Boc), 55.6, 55.5, 55.4, 55.2, 55.1, 55.0, 54.9,
54.8, 54.7, 54.5, 54.4, 54.3, 54.1, 53.9 (4.32CH, NCHCH3), 52.0,
51.9, 51.8, 51.6, 51.5, 51.4, 51.3, 51.2, 51.2, 51.0, 50.8 (3.68CH,
NCHCH3), 45.6, 45.5, 45.4, 45.3, 45.3, 45.1, 44.9, 44.9, 44.6, 44.5,
44.4, 44.3, 44.2, 44.1, 44.0, 43.8 (4CH2, 4¥NCH2C O), 41.2, 41.0,
40.9, 40.7, 40.6, 40.6,40.4, 40.1, 39.9, 39.9, 39.4, 39.2, 39.1, 39.0,
39.0, 38.9, 38.8, 38.7 (4CH2, 4¥NCH2CH2C O), 34.0, 33.8, 33.8,
33.6, 33.5, 33.3, 32.2, 32.1, 31.9, 31.8 (4CH2, 4¥NCH2CH2C O),
28.3 (3CH3, Boc), 18.3, 18.2, 18.1, 18.0, 17.9, 17.7, 17.3, 17.1,
17.0, 16.9, 16.8, 16.7, 16.5, 16.2, 16.1, 16.0 (8CH3, 8¥NCHCH3);
We acknowledge the beam time awarded from the Diamond
Light Source, UK for the circular dichroism beamline B23. We
gratefully thank the Carlsberg Foundation for a grant to T.H.
and Aure´lie Job and Bertrand Le´geret (Clermont-Universite´,
Laboratoire SEESIB) for HPLC and mass spectrometry analysis.
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