Chiral 1,5-disubstituted 1,2,3-triazoles-versatile tools for foldamers and peptidomimetic applications

Article abstract of DOI:10.1039/d0ob00168f

1,4- A nd 1,5-Disubstituted triazole amino acid monomers have gained increasing interest among peptidic foldamers, as they are easily prepared via Cu- A nd Ru-catalyzed click reactions, with the potential for side chain variation. While the latter is key to their applicability, the synthesis and structural properties of the chiral mono-or disubstituted triazole amino acids have only been partially addressed. We here present the synthesis of all eight possible chiral derivatives of a triazole monomer prepared via a ruthenium-catalyzed azide alkyne cycloaddition (RuAAC). To evaluate the conformational properties of the individual building units, a systematic quantum chemical study was performed on all monomers, indicating their capacity to form several low energy conformers. This feature may be used to effect structural diversity when the monomers are inserted into various peptide sequences. We envisage that these results will facilitate new applications for these artificial oligomeric compounds in diverse areas, ranging from pharmaceutics to biotechnology.

Full text of DOI:10.1039/d0ob00168f

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Org. Biomol. Chem., 2020, DOI: 10.1039/D0OB00168F.
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DOI: 10.1039/D0OB00168F
Organic & Biomolecular Chemistry
PAPER
Chiral 1,5-disubstituted 1,2,3-triazoles − versatile tools for
foldamers and peptidomimetic applications†
Anna Said Stålsmeden,a Andrew J. Paterson,a Imola Cs. Szigyártó,b Linda Thunberg,c Johan R.
Received 00th January 20xx,
Accepted 00th January 20xx
Johansson,*d Tamás Beke-Somfai,*b and Nina Kann*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
1,4- and 1,5-Disubstitued triazole amino acid monomers have gained increasing interest among peptidic
foldamers, as they are easily prepared via Cu- and Ru-catalyzed click reactions, with the potential for side chain
variation. While the latter is key to their applicability, the synthesis and structural properties of the chiral mono-
or disubstituted triazole amino acids have only been partially addressed. We here present the synthesis of all
eight possible chiral derivatives of a triazole monomer prepared via a ruthenium-catalyzed azide alkyne
cycloaddition (RuAAC). To evaluate the conformational properties of the individual building units, a systematic
quantum chemical study was performed on all monomers, indicating their capacity to form several low energy
conformers. This feature may be used to effect structural diversity when the monomers are inserted into
various peptide sequences. We envisage that these results will facilitate new applications for these artificial
oligomeric compounds in diverse areas, ranging from pharmaceutics to biotechnology.
acids,2, 5 a wide variety of compounds have been investigated in
this direction.3c Numerous reports have also pointed out that
these compounds will have a propensity to fold into secondary
structures. Furthermore they can provide antibacterial,6
antiviral,7 and cell-penetrating activity,8 as well as having self-
assembling properties.9 This may lead to applications in
pharmaceutics or bionanotechnology.
Introduction
The structural and functional properties of natural biomolecules
can be mimicked by synthetic compounds. Such man-made
molecules can retain the beneficial properties of their natural
counterparts while also benefiting from additional useful
attributes. They can provide increased stability against
enzymatic degradation,1 better designability, and diverse side-
chain chemistry,2 characteristics that are often desired for novel
bioactive molecules. Consequently, peptidomimetics and
protein engineering often employ synthetic, non-natural, amino
acid-derived insertions to improve characteristics of natural
compounds.3
In the last two decades, it has become apparent that
peptidic oligomers containing, or composed almost entirely of,
non-natural amino acids can fold into well-defined secondary
structures, leading to the emerging field of peptidic foldamers,
or foldamers.3c, 4 Since the initial studies demonstrating helical
oligomers containing homologated derivatives of natural amino
One advantage of constructing foldamers from non-natural
amino acids is that standard techniques for peptide coupling
can be applied for assembling these monomers into a
sequence.3c, 10 Such monomers may include synthetic amino
acids based on a cycloalkyl, aromatic or heteroaromatic
scaffold.11 1,4-Disubstitued 1,2,3-triazoles, easily accessible via
the copper-catalyzed azide alkyne cycloaddition (CuAAC)
reaction,12 have also been applied in this context. Examples
include oligo(phenyl-amide-triazole)-type foldamers that
function as halide ion receptors,13 oxazolidinone-triazole units
incorporated into dipeptide mimics for use as foldamer building
blocks.14 as well as other examples of peptide backbone
modifications using 1,4-disubstitued 1,2,3-triazoles.15 In
contrast to CuAAC, the ruthenium-catalyzed cycloaddition
reaction (RuAAC) instead provides access to the 1,5-
disubstituted 1,2,3-triazole with a different spatial arrangement
of the carboxylic acid and amino functionality used for linking
the monomeric units.16 Homo- and heterochiral
peptidomimetics have been developed from enantiomerically
pure propargylamines and -azido acids using the RuAAC
reaction.17 NMR and MD simulations have shown the presence
of a β-turn-like structure in homochiral peptido-triazolamers,
while a heterochiral triazolamer shows a polyproline-like helix.
a. Department of Chemistry and Chemical Engineering, Chalmers University of
Technology, SE-41296 Göteborg, Sweden. E-mail: kann@chalmers.se.
b. Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar
túdosok krt., H-1117 Budapest, Hungary. E-mail: beke-somfai.tamas@ttk.mta.hu.
c. Early Chemical Development, Pharmaceutical Sciences, BioPharmaceuticals R&D,
AstraZeneca, Gothenburg, Sweden.
d. Medicinal Chemistry, Research and Early Development Cardiovascular, Renal and
Metabolism, BioPharmaceuticals R&D, AstraZeneca, Gothenburg, Sweden. E-mail:
johan.x.johansson@astrazeneca.com.
†
Electronic Supplementary Information (ESI) available: computational data,
determination of enantiomeric excess for monomers, 1H and 13C NMR spectra. See
DOI: 10.1039/x0xx00000x
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Moreover, a well defined protocol for the synthesis of chiral followed by a Bestmann-Ohira reaction of the formed aldehyde
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propargylamines, with a side chain in the -position, is feasible in situ, and this may be a better optioDnOtI:o10a.1v0o3i9d/Dp0rOoBlo00n1g6e8dF
using Ellman’s tert-butylsulfinamide as a chiral auxiliary.18 The handling of the sensitive aldehyde.20b We instead opted for
RuAAC reaction has been frequently utilized in medicinal using commercially available (R)-1 and (S)-1 for the continued
chemistry applications,19 including the synthesis of studies. For compound 2, both enantiomers were prepared via
peptidomimetics,20 but its use in the area of foldamers is as yet diazo transfer from the corresponding amine by using 2-azido-
very limited. In earlier reports from our group, we have 1,3-dimethylimidazolinium hexafluoro-phosphate (ADMP),
developed microwave-assisted methodology for the synthesis following a report by Kitamura et al.26 The non-chiral derivatives
of 1,5-disubstituted 1,2,3-triazole monomers via RuAAC and 3 and 4 are both commercially available. With all starting
also investigated the structures and conformations of - materials in place, reaction conditions for the RuAAC coupling
peptides prepared from these monomers.21 In addition, we were then investigated. This involved screening various
have compared the conformational properties of triazole catalysts, including the two most commonly used for RuAAC, i.e.
monomers prepared via CuAAC and RuAAC.22 In this study, we Cp*RuCl(PPh3)2 and Cp*RuCl(COD), as well as [Cp*RuCl2]n and
instead focus on the conformational effects of introducing [Cp*RuCl]4. A solution of (S)-1 and azido ester 4 in THF was
chiral substituents, on either side or both sides of the triazole. heated to 60 or 80 °C for 20 minutes under microwave
During the preparation of this manuscript, a related report, conditions, and products were purified by HPLC or flash
concerning the synthesis of some 1,5-disubstituted chromatography. The results are shown in Table 1. All the
peptidotriazolomers, was published by Sewald and co- catalysts screened, with the exception of [Cp*RuCl]4, provided
workers.17 While there are some touching points between the triazole (S)-5 in moderate to good yields, and showed no
two studies, the present study describes the complete set of significant differences in enantiomeric excess. The preparation
eight possible combinations for mono- and disubstituted chiral of the full set of chiral triazoles was then pursued using either
derivatives. Thus, these studies are different both in terms of Cp*RuCl(COD) or [Cp*RuCl2]n as the catalyst.
the triazoles prepared, as well as with respect to the the
computational studies performed. Our results from these
Table 1 Catalyst evaluation in the RuAAC reaction between (S)-1 and 4.
investigations are described herein.
O
N
N
O
Ru-cat.
THF
microwave
60 or 80 °C
20 min.
NHBoc
+
N
N3
O
O
Results and discussion
(S)-1
4
(S)-5
NHBoc
Monomer synthesis
Entry
Catalyst
Temp. (°C)
Yield (%)a
ee (%)b
97.7
98.4
97.3
97.5
The chiral alkyne 1 and azide 2 (Fig. 1) used in this study
originate from the amino acid alanine. Employing both
enantiomers, in combination with the corresponding non-chiral
non-methylated N-Boc propargylamine 3 and methyl 2-
azidoacetate 4, allowed the synthesis of eight different
stereoisomers of the triazole derivatives (Fig. 2). The chiral
alkynes (R)-1 and (S)-1 were initially prepared via a Bestmann-
Ohira reaction23 from Boc-protected (R)- and (S)-alaninal, where
the (R)-enantiomer was obtained via Dess-Martin oxidation24 of
Boc-(R)-alaninol, and the (S)-form was commercially available.
1
2
3
4
Cp*RuCl(PPh3)3
Cp*RuCl(COD)
[Cp*RuCl2]n
80
60
80
80
42
73
57
5
[Cp*RuCl]4
a Isolated yield. bDetermined by chiral HPLC (see ESI for details).
NHBoc
NHBoc
NHBoc
(R)-1
(S)-1
3
O
O
O
N3
N3
N3
O
O
O
(R)-2
(S)-2
4
Fig. 1
Chiral and non-chiral alkynes and azides used in this study.
While both reactions are documented to proceed without
25
causing racemisation,23,
we found that erosion of the
stereochemistry had taken place for both enantiomers of 1 in
our case. Raines has reported the preparation of (S)-1 via
reduction of the corresponding Weinreb amide of alanine,
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of alkyne 1 to produce the diastereomers (R,R)- and (R,S)-7 in
similar yields as for compounds 6. The same reaction starting
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DOI: 10.1039/D0OB00168F
O
O
N
N
N
N
N
N
O
O
instead from (S)-2 afforded the second set of diastereomers
(S,R)- and (S,S)-7 in 95% and 77% yield, respectively.
Interestingly, all these compounds were obtained with excellent
enantioselectivity and good to excellent diastereoselectivity.
The loss of stereochemical integrity for compounds 6 may thus
be a result of the lower degree of sterical hindrance in these
compounds, allowing racemisation via enolisation to take place,
while the extra methyl group in compounds 7 may in some
manner block this process. More detailed studies are needed,
however, to elucidate the mechanism for this racemisation.
NHBoc
(R)-5 79% (ee 99%)
NHBoc
(S)-5 73% (ee 98%)
O
O
N
N
N
N
N
N
O
O
NHBoc
(R)-6 84% (ee 82%)
NHBoc
(S)-6 87% (ee 79%)
O
O
N
N
N
N
NHBoc
N
N
O
O
(R)-1
NHBoc
(R,R)-7 84% (dr >99/1, ee >99%)
NHBoc
(R,S)-7 87% (dr 97/3, ee >99%)
CuSO4·5H2O (2 mol%)
sodium ascorbate
H2O:t-BuOH (1:1)
O
O
RT, 20 h
N3
N3
O
O
N
N
N
N
O
O
N
N
(S)-2
(R)-2
O
O
NHBoc
NHBoc
O
O
N
N
N
N
N
N
(S,R)-7 95% (dr 96/4, ee >99%)
(S,S)-7 77% (dr 99/1, ee 99%)
O
O
Fig. 2 Chiral 1,5-subsituted triazoles prepared via RuAAC. See experimental section
for method used.
NHBoc
(R,R)-8 92% (dr >99/1, ee 90%)
Scheme 1 Chiral 1,4-substituted triazoles prepared via CuAAC.
NHBoc
(S,R)-8, 87% (dr >99/1, ee >99%)
Alkyne (R)-1 was reacted with 4 using 4 mol% [Cp*RuCl2]n as the
catalyst, affording (R)-5, i.e. the enantiomer of the earlier
prepared triazole (S)-5, in a somewhat lower yield (52%).
Conventional heating at 100 °C, using Cp*RuCl(COD) as the
catalyst, was found to be more successful in this case, affording
(R)-5 in 79% yield (Fig. 2) and high enantiomeric excess (>99%).
Subsequently, the (R)- and (S)-enantiomers of chiral azide 2
were applied and combined with N-Boc-propargylamine 3,
yielding the two enantiomers of triazole 6, with the methyl
substituent in the -position relative to the ester, in 84 and 87%
yield respectively. 2D NOESY NMR confirmed the 1,5-
regioselectivity of the 1,2,3-triazoles (see ESI). However, some
loss of stereochemistry was found for compounds (R)-6 and (S)-
6, with approximately 10% of the other enantiomer formed
alongside the desired product. As the chiral centre next to the
alkyne 1 remains intact during the reaction, as seen in Table 1,
the cause of this partial racemisation was pinpointed to the
methyl-substituted carbon of azides 2. To determine if this
racemisation occurs during the synthesis of the chiral precursor
azides 2, or during the RuAAC reaction itself, azides 2 were
subjected to a CuAAC reaction at room temperature, using
CuSO4 as the catalyst (Scheme 1). Both 1,4-triazoles (R,R)-8 and
(S,R)-8 were formed in good yields (92% and 87% respectively)
and good to excellent stereoselectivity, indicating that the
minor loss of stereochemistry is most likely occuring during the
triazole formation itself, rather than in the preparation of 2.
With these results in hand, all four stereoisomers of the
dimethylated triazole 7 (Fig. 2) were prepared in the same
fashion. Azide (R)-2 was first combined with both enantiomers
Trimer synthesis
To verify that the chiral triazoles prepared could be used for
foldamer synthesis, a trimer was prepared from triazole (S)-5
(Scheme 2). N-Methyl amide (S)-9 was first synthesized via
hydrolysis of (S)-5 with LiOH to the corresponding carboxylic
acid, followed by HATU-mediated amide formation, affording
(S)-9 in 65% yield. A dimer of (S)-5 was then prepared. In one
experiment, (S)-5 was Boc-deprotected with trifluoroacetic
acid, and in a second experiment, (S)-5 was again hydrolyzed
with LiOH. The two parts were then coupled directly without
further purification, using HATU as the coupling agent and
DIPEA as the base, affording dimer (S,S)-10 in 78% yield. (S)-9
was then deprotected and subsequently coupled with (S,S)-10,
affording trimer (S,S,S)-11 in 46% yield as a white solid. 2D
NOESY NMR was performed on (S,S,S)-11 to study the
conformational properties of the trimer. The 1H-1H NOESY was
recorded at 24 mM concentration in DMSO-d6, using a mixing
time of 500 ms. In comparison to a corresponding non-chiral
trimer reported by us earlier,21 with the N-methyl amide
replaced by a methyl ester, the results are much less conclusive.
Where the non-chiral structure displays a long range coupling
between the Boc tert-butyl group and the terminal methyl
group, i.e. between the two ends of the oligomer, indicating a
turn-like conformer, no such correlation can be seen for (S,S,S)-
11. This is perhaps not surprising, as a greater backbone
flexibility would be expected for the corresponding non-chiral
trimer.
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DOI: 10.1039/D0OB00168F
O
N
N
TFACH2Cl2 1:2
O
N
rt, 1.5 h
NH3 TFA
O
LiOH
(3 equiv.)
MeOH
O
N
N
N
N
HATU (1.05 equiv.)
DIPEA (4 equiv.)
O
O
O
N
HO
N
N
N
N
N
N
O
N
N
O
N
N
rt, 1.5 h
CH2Cl2, 15 h, rt
N
H
N
H
(S)-5
NHBoc
NHBoc
HATU
(1.05 equiv.)
DIPEA
NHBoc
(S,S)-10 78%
O
N
N
N
O
HO
N
N
N
LiOH (3 equiv.)
MeOH, rt, 1.5 h
TFACH2Cl2 1:2
N
H
rt, 1.5 h
NHBoc
(4 equiv.)
DMF, 15 h, rt
NH3 TFA
HATU (1.05 equiv.)
DIPEA (6 equiv.)
MeNH3Cl
O
N
N
N
N
H
O
N
N
CH2Cl2, 15 h, rt
N
H
N
N
N
NHBoc
O
N
(S)-9 65%
N
N
N
O
N
H
N
H
NHBoc
(S,S,S)-11 46%
Scheme 2
Synthesis of trimer (S,S,S)-11.
Of more interest is a weak short-range coupling between the and conformational enantiomers were not considered. This way
triazole CH signals and the methyl substituents on the chiral we obtained the relative energies for only one side of the
carbons in (S,S,S)-11, indicating a twisted conformer that allows asymmetric potential energy hypersurface (PEHS) for each
these groups to come into proximity, rather than a structure model, where those of the enantiomer pairs can be converted
with a straighter elongated backbone chain. Excluding expected into each other by their inversion center. Therefore the
correlations, no other interactions of interest could be obtained results are discussed for the enantiomer pairs
discerned.
together. Considering the (R)-5 and (S)-5 models, respectively,
9 and 7 out of the located 18 and 17 conformers are within 3
kcal/mol in relative energy, counted from the most stable
conformer 2’ and 2 (see SI). This indicates that in principle both
derivatives have over ten conformers which have low relative
energies and thus could be the structural building units of
several low energy secondary structures. For (R)-6 and (S)-6,
there are 6 and 11, respectively, low energy conformers, i.e.
below 3 kcal/mol relative energy, with the most stable
conformers being 2 and 1 (see SI), respectively. These results
indicate that the single substituted models closely resemble to
energetic properties of the achiral triazole derivative where
numerous low-energy secondary structures can be found. In
contrast, for the (R,R)-7 and (S,S)-7 enantiomer pair, there are
only 3 and 5 low energy conformers, with 6 and 1 (see SI),
respectively, being the most stable. The (R,S)-7 and (S,R)-7 have
3 and 7 low energy conformers, respectively, with conformers 2
and 2’ as most stable ones (see SI). The lower number of stable
conformers for the disubstituted models most likely originate
from steric interactions.
Molecular modelling
With the full set of chiral triazoles at hand, our attention was
then directed towards the conformational properties of these
compounds. To explore the conformational properties of the
eight chiral derivatives prepared, quantum chemical (QM)
calculations were performed employing three different levels of
theory. QM methods are regularly employed in understanding
structural and energetic properties of non-natural amino acid
compounds. In contrast to natural amino acids and proteins,
proper parametrization of these exotic residues is often
missing, making extensive use of molecular dynamics (MD)
simulations less reliable.27 Previously, the potential energy
hypersurface of an achiral 1,5-disubstituted 1,2,3-triazole
amino acid was explored in an exhaustive scan.22 In the present
study, 20 of these previously found achiral conformers were
employed to construct the eight chiral models, which resulted
the general structure of Ac-5Tzl-NHMe (for more details see
Computational methods, and Supporting Information, Tables
S1-S8). Since the R and S conformers could be considered
enantiomer pairs, these were tested against the same initial
torsional angles from results of the achiral triazole calculations,
Overall, all chiral compounds have several low energy
conformers, which may result in conformational diversity when
homo-oligomers are synthesized from these monomers.
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Structure and energetics of heptamers
ARTICLE
form, H16M. For these compounds, the VaiebwoAvrteicle OHnl1in6e
DOI: 10.1039/D0OB00168F
The eight sets of secondary structures were investigated in conformations have a high relative stability compared to the
order to predict the potentially most preferred structure for other investigated secondary structures, as their energy is
each substitution pattern. To allow formation of defined nearly 9 kcal/mol lower than that of the second most stable
secondary structures, longer peptide sequences, heptamers, conformer, H14. Note, that for (R)-5 and (R,R)-7 the H10P is
were employed. All these sets were investigated in an aqueous quite close in relative free energy to H14P. Thus, for these
environment, as detailed in the experimental section under compounds it is very likely that an oligomer with the above
Computational methods. These conformations have chiral substitution patterns could fold into a helical structure,
intramolecular hydrogen bonds formed between relevant parts such as H16, H14 or H10. In general, when investigating
of the amide groups. Our nomenclature for the conformations handedness there is a strong preference towards either P or M,
is based on the pseudo-cycles formed by hydrogen bonds, using as for (R)-6, and (S)-6, respectively, where the least stable
the number of atoms in the peptide backbone involved in conformer is H14 for both of them, but with the inverse
forming the H-bond. Thus, conformers H8, H10, H14, and H16 handedness. For the (R)-5 and (S)-5 substitution patterns we see
were investigated for both right (P) and left handed (M) a very similar relative energy distribution, and preference of
structures.
handedness is also pronounced because all conformers
When using water as an environment, for (R)-6, (R)-5, and belonging to the preferred P, or M subsets have higher or lower
(R,R)-7, the most preferred secondary structure is the right- relative energies than its inverse conformer, respectively. For
handed H16 conformation, H16P (Fig. 3, Table S9) while the (R,R)-7 and (S,S)-7, H16 is again the most stable conformer with
second most stable conformation is H14P. Similarly, due to H14 being the second most stable one, however, here these
being conformational enantiomers, for (S)-6, (S)-5, and (S,S)-7, conformers have a somewhat higher stability relative to the
the most preferred structure is also H16, but its left handed other conformers investigated (Fig. 3).
Fig. 3. The relative free energies of the investigated secondary structures for hepta-homooligomers using all 8 possible substitution patterns. Values were obtained at the B97X-
D/6-31+G(d,p)//B97X-D/6-31G(d) level of theory using water as solvent and involving entropy contributions as well as zero point and thermal energy corrections. Relative values
were determined for each peptide using the average energy value of all secondary structures as reference point. For more details, see section Computational methods.
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Fig.
4
Conclusions
The complete set of possible chiral combinations for 1,5-
disubstituted 1,2,3-triazole units have been prepared via a
ruthenium-catalyzed azide alkyne cycloaddition (RuAAC), in
moderate to good yields. These chiral monomeric triazole
building blocks are intended for applications in the area of
foldamer synthesis. The precursor alkynes and azides used for
the RuAAC reaction all originate from the amino acids glycine
and alanine. To demonstrated the applicability of the 1,5-
triazole monomer building blocks in the synthesis of foldamers,
a trimer structure was also prepared. A conformational and
structural investigation of these monomers was then carried
out to predict the effect of introducing these units into a
foldamer. Computational studies on the monomers indicate
that several low energy conformers are present for all eight
compounds, where several secondary structures may be built
with low relative energies for peptidic molecules containing
these compounds as single insertions. This is especially
indicated for the monosubstituted compounds, where the
number of lower energy conformers exceeds ten. However,
when considering homooligomer foldamers built from these
compounds, relative energies show a single most stable
conformer for 6 out of the 8 substitution patterns. The only two
exceptions are (R,S) and (S,R), where several secondary
structures result in relative energies very close to each other,
indicating that structural diversity of the backbone, as a key
feature for fine-tuning structural properties with various side
chain patterns, is present. Overall, we anticipate that the
outcome of these studies can be of use in the design and
synthesis of new classes of foldamers from non-natural amino
acids based on a 1,5-disubstituted 1,2,3-triazole scaffold.
The three most stable conformation of (S,R)-7 heptahomooligomer. Calculations
were performed at the B97X-D/6-31+G(d,p) //B97X-D/6-31G(d) level of theory.
In contrast to the above six substitution patterns, from a
structural point of view the oligomers built from (S,R)-7 and
(R,S)-7 monomers seem to be the most interesting. In the case
of these two oligomers, the relative energy of the investigated
conformers have values much closer to each other. For (R,S)-7,
the most stable conformer is H10M, however its relative energy
is only ~3 kcal/mol from the second most stable conformer,
H14M (Fig. 4). Furthermore, the third most stable conformer is
H16P, which has the opposite handedness as the above two
conformers, yet its energy is still within 7 kcal/mol to that of
H10M. Similarly, in case of (S,R)-7, the same energy distribution
can be observed. Based on these results for (S,R)-7, (R,S)-7
systems, there is probably a steric factor introduced which
renders various secondary structures to comparable stability.
This will probably give rise to a conformational diversity that is
comparable to that observed for natural peptidic compounds in
living systems. Note, that for homooligomers, the above results
are in contrast to the conformational diversity observed for
monomeric compounds. Once the monosubstituted Tzl amino
acids are oligomerized, there is a clear preference for one
secondary structure among the ones investigated. This
observation is also valid for disubstituted compounds with both
substitutions having the same chirality, (S,S)-7 and (R,R)-7. In
contrast to the above, the mixed substitution patterns, (S,R)-7
and (R,S)-7, may result in conformational diversity for
homooligomeric foldamers as several secondary structure
elements have close relative energies, irrespective of
handedness.
Experimental
General experimental methods
Starting materials were purchased from commercial sources
unless otherwise stated. All reactions were performed under an
inert atmosphere (argon or nitrogen). Chiral alkynes 1 were
initially synthesized from commercial Boc-(R)-alaninol and Boc-
(S)-alaninal in one or two steps, but were subsequently
purchased from Ark Pharm Inc. (tert-butyl (S)-but-3-yn-2-yl-
carbamate, 99.5% ee) and J&W PharmLab LLC (tert-butyl (R)-
but-3-yn-2-ylcarbamate, 98.8% ee). THF was purchased as
anhydrous grade. Automated flash chromatography was
performed on a Biotage Isolera One. 1H, 13C and 2D NOESY NMR
were acquired on a Varian MR 400 MHZ instrument or on a
Bruker Avance III 500 MHz spectrometer fitted with a cryoprobe
in CDCl3 or DMSO, using residual solvent peaks for reference.
FTIR was performed on
a PerkinElmer Frontier FTIR
Spectrometer with an ATR device using Spectrum software. The
enantiomeric excess was determined by chiral HPLC, using a
Chiralpak IC column (25% EtOH in CO2) or a Lux Cellulose-5
column (25% EtOH in CO2) or a Lux Amylose-1 column (10%
6 | J. Name., 2012, 00, 1-3
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Organic & Biomolecular Chemistry
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EtOH in CO2) at 120 bar, 40 °C, flow rate 3.5 mL/min, 220 nm quenched with saturated NaHCO3 (15 mL) and stirred for 10
View Article Online
detection (see SI for HPLC data for each compound).
min. CH2Cl2 (20 mL) and water (5 mL) wDaOs Ia: d10d.1e0d3.9/ADf0tOerB0p0h1a6s8eF
separation, the aqueous layer was extracted with CH2Cl2 (3 x 5
mL). The combined organic layers were filtered through a phase
separator (Telos) and concentrated under reduced pressure.
The crude product was purified by gel filtration through silica
Experimental procedures
Synthesis of chiral propargylic amines
tert-Butyl (R)-but-3-yn-2-ylcarbamate ((R)-1) Alkyne (R)-1 was gel (25 g), eluting with CH2Cl2. Concentration of this solution
prepared via oxidation of N-Boc-D-alaninol ((R)-2-(Boc-amino)- under reduced pressure afforded the product as a colourless oil
1-propanol) to N-Boc-D-alaninal, followed by a Bestmann-Ohira (351 mg, 45%). 1H NMR (500 MHz, DMSO): δ 4.33 (q, J = 7.1 Hz,
reaction to afford (R)-1. Oxidation: To a solution of N-Boc-D- 1H), 3.72 (s, 3H), 1.34 (d, J = 7.1 Hz, 3H); 13C NMR (126 MHz,
alaninol (146 mg, 0.833 mmol) in CH2Cl2 (15 mL) was added DMSO) δ 171.20, 56.64, 52.52, 16.48. This compound has been
Dess-Martin periodinane (429 mg, 1.01 mmol) in one portion. reported earlier,31 using a different preparative procedure. Due
The reaction mixture was stirred at ambient temperature for to the sensitivity of this azide, the ee was not determined at this
2.5 h and then extracted with EtOAc (20 mL). After phase stage.
Methyl (S)-2-azidopropanoate ((S)-2) Procedure as for (R)-2 (340
mg, 43%). 1H NMR (500 MHz, DMSO) δ 4.33 (q, J = 7.1 Hz, 1H),
3.72 (s, 3H), 1.34 (d, J = 7.1 Hz, 3H); 13C NMR (126 MHz, DMSO)
δ 171.20, 56.64, 52.52, 16.48. This compound has been
reported earlier, using a different preparative procedure.32 Due
to the sensitivity of this azide, the ee was not determined at this
stage.
separation, the organic phase was washed with 2M NaHSO3 (5
mL) and the aqueous phase was extracted with EtOAc (3x10
mL). The combined organic phases were washed with 1M NaOH
(15 mL) and brine (10 mL), then dried (Na2SO4) and
concentrated to afford N-Boc-D-alaninal (138 mg, 91%) as a
1
white solid of approximately 95% purity. H and 13C NMR data
were in accordance with published data for this compound.28
This aldehyde was used directly in the next step without further
purification. Bestmann-Ohira reaction: To a solution of N-Boc-
D-alaninal (253 mg, 1.46 mmol) in MeOH (25 mL) was added
K2CO3 (404 mg, 2.92 mmol) in one portion. The reaction was
stirred at ambient temperature for 4 h. The solution was
concentrated by rotary evaporation and water (25 mL) and
EtOAc (25 mL) were added. After phase separation, the aqueous
phase was further extracted with EtOAc (3x25 mL) and the
combined organic phases washed with brine (25 mL), dried
(Na2SO4) and concentrated. Flash chromatography (silica gel,
eluent: CH2Cl2) afforded 143 mg of (R)-1 as a white solid. 1H and
13C NMR data were in accordance with published data for this
compound.29 However, the ee of (R)-1 was found to be 13.4%.
For further studies, compound (R)-1 was instead purchased
from J&W PharmLab (ee 98.8%).
General procedure for the RuAAC reaction
RuAAC reactions were performed using microwave heating
(Method A) or conventional oil bath heating (Method B).
Method A: The corresponding alkyne (0.44 mmol) and azide
(0.44 mmol) were charged in a Biotage microwave vial with a
stirrer bar and dissolved in anhydrous THF (3 mL).
Cp*RuCl(COD) (8.4 mg, 0.02 mmol) was added and the vial was
sealed, evacuated and refilled with nitrogen three times. The
reaction mixture was then heated to 60 °C for 20 min in a
microwave reactor or heated in an oil bath at 100 °C for 15 h.
The resulting solution was filtered through a syringe filter and
concentrated. The crude product was purified by preparative
HPLC on a Kromasil C8 column (10 μm 250x20 ID mm), using a
gradient of 5-45% acetonitrile in a H2O/acetonitrile/formic acid
(95/5/0.2) buffer, over 20 minutes with a flow rate of 19
mL/min. The compounds were detected by UV at 225 nm. Pure
fractions were combined and lyophilized to obtain the product.
Method B, exemplified by the formation of (S)-6: To a
microwave vial equipped with a stirred bar was added methyl
(S)-2-azidopropanoate (26 mg, 0.2 mmol) and N-Boc-
propargylamine (34 mg, 0.2 mmol) along with Cp*RuCl(COD)
(3.5 mg, 0.009 mmol) and THF (1 mL). The vessel was sealed in
an argon atmosphere and stirred at 100 °C for 15 h or until crude
TLC analysis indicated no more starting materials were being
consumed (visualized by staining with ninhydrin solution and
heating). The crude product was purified by automated flash
chromatography on a Biotage Isolera One system, using a
gradient of 30-60% EtOAc in petroleum ether. Pure fractions
were combined and concentrated by rotary evaporation to
afford the product.
tert-Butyl (S)-but-3-yn-2-ylcarbamate ((S)-1) Alkyne (S)-1 was
prepared via a Bestmann-Ohira reaction of N-Boc-D-alaninal
1
using the same procedure as for (R)-1. H and 13C NMR data
were in accordance with published data for this compound.30
However, the ee of (S)-1 was found to be 25.8%. For further
studies, compound (S)-1 was thus instead purchased from Ark
Pharm Inc. (ee 98.5%).
Synthesis of chiral azides
Methyl (R)-2-azidopropanoate ((R)-2) NOTE: This is a small-
molecule azide and should be handled with care. This reaction
should not be scaled up. Do not apply heat to the neat azide
product and any reaction with it should be diluted at least 10
times. (R)-Methyl 2-aminopropanoate·HCl (855 mg, 6.0 mmol)
was suspended in anhydrous acetonitrile (10 mL) and
diethylamine (3.0 mL, 28.7 mmol) was added. A suspension of
2-azido-1,3-dimethyl-4,5-dihydro-1H-imidazol-3-ium
hexa-
Methyl (R)-2-(5-(1-((tert-butoxycarbonyl)amino)ethyl)-1H-1,2,3-
triazol-1-yl)acetate ((R)-5) Prepared from (R)-1 and 4, affording
(R)-5 as a brown solid (method A: 66 mg, 52%, ee 99.1%;
method B: 45 mg, 79%; ee 99.1%); []D +28.1 (c 1.1 in CH2Cl2);
fluorophosphate (ADMP) (2.07 g, 7.26 mmol) in anhydrous
acetonitrile (10 mL) was added dropwise at 23 °C over 10 min
(during the addition, the temperature increased to 32 °C). The
reaction mixture was stirred ambient temperature for 1 h, then
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J. Name., 2013, 00, 1-3 | 7
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Organic & Biomolecular Chemistry
Page 8 of 11
ARTICLE
max (neat) 2979, 1753, 1698, 1517, 1453, 1367, 1304, 1243,
Journal Name
53.15, 40.20, 28.40, 20.56, 17.60; HRMS (ESI+): found 299.1732
View Article Online
1167, 1058, 986, 859, 706; 1H NMR (400 MHz, CDCl3): δ 7.53 (s, [M+H]+; C13H23N4O4 requires 299.1719.
DOI: 10.1039/D0OB00168F
Methyl (R)-2-(5-((S)-1-((tert-butoxycarbonyl)amino)ethyl)-1H-
1H, triazole-CH), 5.32-5.19 (m, 2H, CH2), 4.92-4.80 (m, 1H,
CHCH3), 4.88 (br s, 1H, NH), 3.74 (s, 3H, OCH3), 1.53 (d, J = 7.0
Hz, 3H, CHCH3), 1.37 (s, 9H, C(CH3)3); 13C NMR (100 MHz, CDCl3):
δ 167.20, 154.91, 140.09, 131.53, 80.33, 52.84, 49.06, 40.07,
28.19, 20.27; HRMS (ESI+): found 285.1578 [M+H]+; C12H21N4O4
requires 285.1563.
1,2,3-triazol-1-yl)propanoate ((R,S)-7) Prepared from (S)-1 and
(R)-2, using method B, affording (R,S)-7 as a pale brown solid (52
mg; 87%; dr >93/7, ee >99%); []D −0.22 (c 0.8 in CH2Cl2; max
(neat) 3322, 2979, 1750, 1696, 1514, 1451, 1367, 1305, 1246,
1213, 1162, 1118, 1079, 1057, 1024, 976, 859, 761, 705, 659; 1H
NMR (400 MHz, CDCl3): 7.57 (s, 1H, triazole-CH), 5.55 (q, J = 5.3
Hz, 1H, COCHCH3), 5.00 – 4.89 (m, 1H, NHCH), 4.78 (d, J = 7.8 Hz,
1H, NH), 3.72 (s, 3H, -OCH3), 1.91 (d, J = 7.1 Hz, 3H) and 1.57 (d,
J = 6.9 Hz, 3H, COCHCH3), 1.41 (s, 9H, C(CH3)3); 13C NMR (100
MHz, CDCl3): 169.60, 155.00, 140.22, 131.29, 80.65, 56.04,
53.17, 40.08, 28.39, 20.53, 16.83; HRMS (ESI+): found 299.1735
[M+H]+; C13H23N4O4 requires 299.1719.
Methyl
(S)-2-(5-(1-((tert-butoxycarbonyl)amino)ethyl)-1H-1,2,3-
triazol-1-yl)acetate ((S)-5) Prepared from (S)-1 and 4, affording
(S)-5 as a brown solid (method A: 92 mg, 73%, ee 98.4%; method
B: 42 mg, 74%, ee 98.4%); []D −28.6 (c 0.925 in CH2Cl2); max
(neat) 2980, 1752, 1698, 1513, 1453, 1392, 1304, 1239, 1160,
1
1058, 986, 859, 704; H NMR (400 MHz, CDCl3): δ 7.58 (s, 1H,
triazole-CH), 5.24-5.37 (m, 2H, CH2), 4.87-4.95 (m, 1H, CHCH3),
4.76 (br s, 1H, NH), 3.78 (s, 3H, OCH3), 1.58 (d, J = 6.9 Hz, 3H,
CHCH3), 1.41 (s, 9H, C(CH3)3); 13C NMR (100 MHz, CDCl3): δ
167.22, 154.91, 140.08, 131.56, 80.42, 52.87, 49.10, 40.09,
28.22, 20.33; HRMS (ESI+): found 285.1581 [M+H]+; C12H21N4O4
requires 285.1563.
Methyl (S)-2-(5-((R)-1-((tert-butoxycarbonyl)amino)ethyl)-1H-
1,2,3-triazol-1-yl)propanoate ((S,R)-7)) Prepared from (R)-1
and (S)-2, using method B, affording (S,R)-7 as a pale brown
solid (56 mg; 95%; dr 96/4, ee >99%); []D +0.23 (c 1.05 in
CH2Cl2; max (neat) 3329, 2979, 1750, 1699, 1517, 1452, 1367, 1306,
1247, 1166, 1118, 1079, 1058, 1025, 976, 859, 761, 660; 1H NMR
(400 MHz, CDCl3): δ 7.55 (s, 1H, triazole-CH), 5.69 – 5.40 (m, 1H,
COCHCH3), 4.92 (bra app s, 2H, NHCH and NH), 3.70 (s, 3H,
OCH3), 1.89 (d, J = 7.2 Hz, 3H) and 1.62 – 1.50 (m, 3H, COCHCH3),
1.40 (s, 9H, C(CH3)3); 13C NMR (100 MHz, CDCl3): 169.57, 155.01,
140.21, 131.24, 80.53, 55.98, 53.12, 40.03, 28.33, 20.46, 16.79;
HRMS (ESI+): found 299.1728 [M+H]+; C13H23N4O4 requires
299.1719.
Methyl (S)-2-(5-((S)-1-((tert-butoxycarbonyl)amino)ethyl)-1H-
1,2,3-triazol-1-yl)propanoate ((S,S)-7) Prepared from (S)-1 and
(S)-2, using method B, affording (S,S)-7 as a light brown solid
(46 mg; 77%; dr > 99/1; ee 98.8%); []D −0.71 (c 0.925 in CH2Cl2);
max (neat) 3055, 2983, 1751, 1702, 1499, 1451, 1382, 1368,
1306, 1265, 1160, 1118, 1079, 1058, 1021, 975, 896, 858, 835,
731, 702, 555; 1H NMR (400 MHz, CDCl3): δ 7.57 (s, 1H, triazole-
CH), 5.31 (q, J = 7.0 Hz, 1H, COCHCH3), 4.97 (dt, J = 13.3, 6.2 Hz,
1H, NHCH), 4.67 (br s, 1H, NH), 3.72 (s, 3H, OCH3), 1.95 (d, J =
7.3 Hz, 3H) and 1.58 (d, J = 6.8 Hz, 3H, COCHCH3), 1.42 (s, 9H,
C(CH3)3); 13C NMR (100 MHz, CDCl3): δ 169.97, 154.87, 138.98,
131.69, 80.54, 56.19, 53.16, 40.22, 28.40, 20.56, 17.60; HRMS
(ESI+): found 299.1725 [M+H]+; C13H23N4O4 requires 299.1719.
General procedure for the CuAAC reaction
Methyl
(R)-2-(5-(((tert-butoxycarbonyl)amino)methyl)-1H-
1,2,3-triazol-1-yl)propanoate ((R)-6) Prepared from (R)-2 and 3,
using method A, affording (R)-6 as a pale yellow oil (186 mg;
84%; ee 81.8%); []D +12.2 (c 0.975 in CH2Cl2); max (neat) 2978,
1749, 1749, 1703, 1514, 1449, 1392, 1366, 1247, 1165, 1110,
1079, 1019, 978, 930, 858, 757, 658; 1H NMR (500 MHz, DMSO-
d): δ 7.54 (s, 1H, triazole-CH), 7.44 (br s, 1H, triazole-CH), 5.61
(q, J = 7.2 Hz, 1H, CHCH3), 4.18 - 4.28 (m, 2H, CH2), 3.66 (s, 3H,
OCH3), 1.76 (d, J = 7.2 Hz, 3H, CHCH3), 1.37 (s, 9H, C(CH3)3); 13
C
NMR (126 MHz, DMSO-d): δ 169.45, 155.55, 136.37, 132.39,
78.44, 55.10, 52.77, 32.55, 28.09, 16.90; HRMS (ESI+): found
285.1557 [M+H]+; C12H21N4O4 requires 285.1563.
Methyl
(S)-2-(5-(((tert-butoxycarbonyl)amino)methyl)-1H-
1,2,3-triazol-1-yl)propanoate ((S)-6) Prepared from (S)-2 and 3,
using method A, affording (S)-6 as a pale yellow oil (173 mg;
87%; ee 79.2%); []D = −14 (c 1.1 in CH2Cl2); max (neat) 2978,
1748, 1698, 1506, 1449, 1367, 1246, 1165, 1079, 1020; 1H NMR
(400 MHz, DMSO-d): δ 7.57 (s, 1H, triazole-CH), 5.54-5.35 (m,
1H, CHCH3), 5.11 (s, 1H, NH), 4.54-4.27 (m, 2H, CH2), 3.72 (s, 3H,
OCH3), 1.91 (d, J = 7.3 Hz, 3H, CHCH3), 1.42 (s, 9H, C(CH3)3); 13
C
NMR (100 MHz, CDCl3): δ 169.66, 155.70, 135.28, 133.51, 80.57,
56.31, 53.21, 33.17, 28.36, 28.36, 17.07; HRMS (ESI+): found
285.1556 [M+H]+; C12H21N4O4 requires 285.1563.
To a Biotage microwave vial equipped with a stirred bar was
added the corresponding azide (26 mg, 0.2 mmol) and alkyne
(34 mg, 0.2 mmol) along with CuSO4.5H2O (1 mg, 0.004 mmol),
sodium ascorbate (2.5 mg, 0.012 mmol) and a 1:1 mixture of
H2O:tBuOH (1 mL). The vessel was sealed in an air atmosphere
and stirred at room temperature for 20 h at which point TLC
analysis indicated the reaction was complete. The crude
reaction mixture was then diluted in CH2Cl2 (10 mL) and water
(10 mL) and the aqueous layer was extracted a further two
times with CH2Cl2 (2 x 10 mL). The combined organic fractions
were then washed with brine (10 mL) and dried using MgSO4.
The MgSO4 was then removed from the organic fractions by
filtration, concentrated, and purified by flash column
chromatography (40-60% EtOAC/hexane) to afford the pure
products.
Methyl (R)-2-(5-((R)-1-((tert-butoxycarbonyl)amino)ethyl)-1H-
1,2,3-triazol-1-yl)propanoate ((R,R)-7) Prepared from (R)-1 and
(R)-2, using method B, affording (R,R)-7 as a brown solid (50 mg;
84% yield; dr >99/1; ee >99%); []D +0.59 (c 1.3 in CH2Cl2); max
(neat) 3206, 2982, 1753, 1698, 1500, 1448, 1436, 1383, 1367,
1344, 1307, 1265, 1249, 1160, 1135, 1117, 1081, 1060, 1021,
976, 856, 834, 732, 703, 663, 636, 556, 513, 471, 418; 1H NMR
(400 MHz, CDCl3): 7.57 (s, 1H, triazole CH), 5.31 (q, J = 5.7 Hz,
1H, COCHCH3), 5.02 – 4.90 (m, 1H, NHCH), 4.69 (d, J = 8.2 Hz,
1H, NH), 3.72 (s, 3H, OCH3), 1.95 (d, J = 7.3 Hz, 3H) and 1.58 (d,
J = 6.8 Hz, 3H, COCHCH3), 1.41 (s, 9H, C(CH3)3); 13C NMR (100
MHz, CDCl3): δ 169.97, 154.89, 138.99, 131.69, 80.54, 56.19,
8 | J. Name., 2012, 00, 1-3
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Organic & Biomolecular Chemistry
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ARTICLE
Methyl (R)-2-(4-((R)-1-((tert-butoxycarbonyl)amino)ethyl)-1H- reaction was stirred at rt for 1.5 h. The reactionVmiewixAtruticrleeOwnlianes
DOI: 10.1039/D0OB00168F
1,2,3-triazol-1-yl)propanoate ((R,R)-8) Prepared from (R)-1 and then concentrated under a stream of nitrogen and the salt was
(R)-2, affording (R,R)-8 as an oil (55 mg; 92%; dr >99/1; ee used directly for the amide coupling reaction.
89.8%); []D +20.2 (c 1.0 in CH2Cl2); max (neat) 2978, 2155, Hydrolysis of (S)-5: To a microwave vial equipped with a stirrer
1749, 1699, 1515, 1450, 1366, 1244, 1166, 1095, 1047, 860, bar was added (S)-5 (56 mg, 0.2 mmol) and MeOH (2 mL). 1M
780; 1H NMR (400 MHz, CDCl3) δ 7.59 (s, 1H), 5.42 (q, J = 7.4 Hz, LiOH (0.6 mL, 0.6 mmol) was then added dropwise and the
1H), 5.13 (s, 1H), 4.93 (dq, J = 7.5, 6.2 Hz, 1H), 3.74 (s, 3H), 1.79 solution was stirred at rt for 1.5 h. Deionised water (5 mL) was
(d, J = 7.3 Hz, 3H), 1.54 (d, J = 6.9 Hz, 3H), 1.41 (s, 9H); 13C NMR then added and 1M HCl (0.6 mL, 0.6 mmol) was added dropwise
(101 MHz, cdcl3) δ 169.80, 155.20, 150.10, 119.93, 79.58, 58.11, with stirring. The mixture was transferred to a separating funnel
53.14, 43.01, 28.43, 21.35, 18.25. HRMS (ESI+): found 299.1725 and extracted with CH2Cl2 (4 x 5 mL). The combined organic
[M+H]+; C13H23N4O4 requires 299.1719.
phases were passed through a TELOS phase separator. The
Methyl (S)-2-(4-((R)-1-((tert-butoxycarbonyl)amino)ethyl)-1H- solvent was then removed under reduced pressure and the
1,2,3-triazol-1-yl)propanoate ((S,R)-8) Prepared from (R)-1 and resulting carboxylic acid was used directly in the amide coupling
(S)-2, affording (R,S)-8 as an oil (52 mg; 87%; dr >99/1; ee reaction.
99.4%); []D +50.4 (c 0.5 in CH2Cl2); max (neat) 2978, 1749, 1702, Amide coupling reaction: To a microwave vial equipped with a
1516, 1454, 1366, 1246, 1169, 1048; 1H NMR (400 MHz, CDCl3) stirrer bar was added the Boc-deprotected (S)-5 and the
δ 7.58 (s, 1H), 5.42 (q, J = 7.4 Hz, 1H), 5.13 (br s, 1H), 4.92 (dq, J carboxylic acid formed by hydrolysis of (S)-5, together with
= 8.1, 7.6 Hz, 1H), 3.73 (s, 3H), 1.79 (d, J = 7.3 Hz, 3H), 1.54 (d, J CH2Cl2 (4 mL). DIPEA (140 µL, 0.8 mmol) was then added
= 6.9 Hz, 3H), 1.41 (s, 9H); 13C NMR (101 MHz, CDCl3) δ 169.79, followed by HATU (80 mg, 0.21 mmol). The vessel was sealed
155.22, 150.22, 119.93, 119.90, 79.60, 58.12, 53.14, 43.01, and allowed to react overnight at rt. The reaction mixture was
28.44, 21.32, 18.26. HRMS (ESI+): found 299.1713 [M+H]+; then transferred to a separating funnel along with aqueous
C13H23N4O4 requires 299.1719.
tert-Butyl (S)-(1-(1-(2-(methylamino)-2-oxoethyl)-1H-1,2,3- combined organic phases were passed through a TELOS phase
triazol-5-yl)ethyl)carbamate ((S)-9) separator, concentrated, and purified by flash column
saturated NH4Cl (5 mL) and extracted with EtOAc (4 x 5 mL). The
To a microwave vial equipped with a stirrer bar was added (S)- chromatography eluting with 80-100% EtOAc/hexane. The
5 (56 mg, 0.2 mmol) and MeOH (2 mL). 1M LiOH (0.6 mL, 0.6 crude white solid was then further purified by trituration using
mmol) was then added dropwise and the solution was stirred at hexane and dried to yield dimer as a white solid (68 mg, 78%
rt for 1.5 h. Water (5 mL) was then added and 1M HCl (0.6 mL, yield). []D −33.0 (c 0.1 in CH2Cl2); max (neat) 3311 (br), 2986,
1
0.6 mmol) was added dropwise with stirring. The mixture was 1749, 1672, 1535, 1523; H NMR (400 MHz, CDCl3) δ 7.60 (s,
transferred to a separating funnel and extracted with CH2Cl2 (4 1H), 7.56 (s, 1H), 7.41 (d, J = 8.2 Hz, 1H), 5.43 (d, J = 17.7 Hz, 1H),
x 5 mL). The combined organic phases were passed through a 5.33 (d, J = 17.6 Hz, 1H), 5.20 – 5.07 (m, 2H), 5.07 (d, J = 7.2 Hz,
TELOS phase separator. The solvent was then removed under a 1H), 4.90 (d, J = 16.6 Hz, 1H), 4.73 (p, J = 7.0 Hz, 1H), 3.77 (s, 3H),
reduced pressure and the resulting carboxylic acid was 1.57 (d, J = 7.0 Hz, 3H), 1.47 (d, J = 6.9 Hz, 3H), 1.41 (s, 9H); 13C
transferred to a microwave vial equipped with stirrer bar and NMR (101 MHz, CDCl3) δ 167.66, 165.27, 155.49, 141.10,
dissolved in CH2Cl2 (2 mL). Methylammonium chloride (54 mg, 140.05, 132.04, 131.47, 81.02, 53.13, 50.79, 49.41, 41.13, 39.20,
0.8 mmol) was then added followed by DIPEA (209 µL, 1.2 28.43, 20.76, 19.78. HRMS (ESI+): found 437.2248 [M+H]+;
mmol). HATU (80 mg, 0.21 mmol) was then added, and the C18H29N8O5 requires 437.2261.
Synthesis of trimer (S,S,S)-11
vessel was sealed and allowed to react over night at rt. The
reaction mixture was then transferred to a separating funnel
along with aq sat NH4Cl (5 mL) and was extracted with EtOAc (4
x 5 mL). The combined organic phases were dried using MgSO4,
concentrated, and purified by flash column chromatography
eluting with 80-100% EtOAc/hexane to afford (S)-9 as a white
solid (37 mg, 65% yield). A second chromatography step was
sometimes necessary to remove impurities deriving from the
coupling reagent. []D −2.1 (c 0.75 in CH2Cl2); max (neat) 3340,
3318, 2986, 2937, 1676, 1643; 1H NMR (400 MHz, CDCl3) δ 7.62
(s, 1H), 6.15 (s, 1H), 5.21 (d, J = 16.7 Hz, 1H), 5.03 (d, J = 16.5 Hz,
1H), 4.97 – 4.81 (m, 2H), 2.81 (d, J = 4.8 Hz, 3H), 1.55 (d, J = 6.7
Hz, 3H), 1.41 (s, 9H); 13C NMR (101 MHz, CDCl3) δ 165.74,
154.95, 140.70, 131.60, 80.65, 51.02, 40.66, 28.24, 26.46, 20.65.
HRMS (ESI+): found 284.1729 [M+H]+; C12H22N5O3 requires
284.1722.
Boc-deprotection of (S)-9: To a microwave vial equipped with a
stirrer bar was added (S)-9 (28mg, 0.1 mmol) and CH2Cl2 (2 mL).
TFA (1 mL) was then added and the reaction mixture was stirred
at rt for 1.5 h. The reaction mixture was then condensed under
a stream of nitrogen and the salt was used directly in the amide
coupling reaction.
Hydrolysis of (S,S)-10: To a microwave vial equipped with stirrer
bar was added (S,S)-10 (44 mg, 0.1 mmol) and MeOH (1 mL). 1M
LiOH (0.3 mL, 0.3 mmol) was then added dropwise and the
solution was stirred at rt for 1.5 h. Deionised water (5 mL) was
then added and 1M HCl (0.3 mL, 0.3 mmol) was added dropwise
with stirring. The mixture was transferred to a separating funnel
and extracted with EtOAc (4 x 10 mL). The combined organic
layers were passed through a TELOS phase separator. The
solvent was then removed under a reduced pressure and the
resulting carboxylic acid was used directly.
Synthesis of dimer (S,S)-10
Boc-deprotection of (S)-5: To a microwave vial equipped with a Amide coupling reaction: To a microwave vial equipped with
stirrer bar was added (S)-5 (56 mg, 0.2 mmol) and CH2Cl2 (0.2 stirrer bar was added the Boc-deprotected (S)-9 and the
mL). Trifluoracetic acid (TFA)(0.1 mL) was then added and the carboxylic acid formed by hydrolysis of (S,S)-10, together with
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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Organic & Biomolecular Chemistry
Page 10 of 11
ARTICLE
Journal Name
DMF (1 mL). DIPEA (70 µL, 0.4 mmol) was then added, followed Considering the large size and the unfavorable nature of the
View Article Online
DOI: 10.1039/D0OB00168F
by HATU (39 mg, 0.105 mmol). The vessel was sealed and the extended conformation, for many substitution patterns this
reaction was allowed to react overnight at rt. The DMF was conformer experienced convergence problems during the
removed and the crude reaction mixture was purified directly calculations, therefore this conformer was excluded from
by flash column chromatography eluting with a gradient of 5- further investigation. To validate the critical points, i.e. to
15% MeOH/CH2Cl2. The crude white solid was then further confirm that they are local minima, as well as to obtain zero
purified by trituration using CHCl3, followed by 1M NaOH and point and thermal corrections to the energy, and entropy
H2O, before being dried, affording trimer (S,S,S)-11 as a white contributions to provide Gibbs free energy values, normal mode
solid (27 mg, 46% yield). []D –9.8 (c 0.22 in CH2Cl2); max (neat) analysis was performed on the obtained conformers at the
3303 (br), 2983, 1749, 1669 (br), 1537; 1H NMR (400 MHz, B97x-D/6-31G(d) level of theory. These optimised conformers
DMSO-d6) δ 8.93 (d, J = 7.6 Hz, 2H), 8.20 (q, J = 4.6 Hz, 1H), 7.73 were also submitted to point energy calculations employing the
(s, 1H), 7.72 (s, 1H), 7.57 (s, 1H), 7.45 (d, J = 8.0 Hz, 1H), 5.20 – same functional and the 6-31+G(d,p) basis set. Further on,
4.96 (m, 7H), 4.92 (p, J = 7.0 Hz, 1H), 4.62 (dt, J = 15.3, 6.6 Hz, environmental effects of water were considered also by single
1H), 2.60 (d, J = 4.6 Hz, 3H), 1.45 (d, J = 6.9 Hz, 3H), 1.41 (d, J = point energy calculations at the samel level of theory, using the
7.0 Hz, 3H), 1.32 (s, 9H), 1.27 (d, J = 7.0 Hz, 3H); 13C NMR (101 implicit solvent model integral equation formalism for
MHz, DMSO-d6) δ 165.71, 165.13, 165.05, 154.98, 141.60, polarizable continuum model (IEFPCM) (Table S1-S9).
140.52, 140.29, 131.8, 131.05, 130.70, 78.36, 49.83, 49.65,
49.53, 40.30 (overlaps with DMSO), 38.82, 38.75, 28.10, 25.59,
20.17, 19.77, 19.70; HRMS (ESI+): found 588.3096 [M+H]+;
Conflicts of interest
C24H38N13O5 requires 588.3119.
Computational methods
There are no conflicts of interest to declare.
All computations were carried out using the Gaussian 09
software package.33 In contrast to experiments, for the
calculations the monomer amino acids had N-methyl and
acetamide protecting groups at their C- and N-terminals,
respectively, abbreviated as Ac-5Tzl-NHMe. In case of the
longer models the C-terminal protecting group was NH2,
resulting the general Ac-(5Tzl)7-NH2 form. Nevertheless, the
same nomenclature was applied for the substitution patterns as
for the experiments (Fig. 2). For the monomers, the 27 minima
obtained previously in an exhaustive conformational search for
the achiral models at the RHF/3-21G level of theory22, were
employed in the current study as initial conformations. Out of
these 7 starting conformers were completely planar, with
relative energies of their achiral version higher than 20
kcal/mol. Consequently, these were omitted for the current
chiral models, leaving 20 starting test conformers for all eight
models. The total of 160 optimisations were primarily
performed at the RHF/3-21G level of theory. Further on, the
optimised structures were submitted to calculations using
Becke’s three parameter functional with the Lee-Yang-Parr
exchange functional (B3LYP),34 the B97X-D functional by Chai
and Head-Gordon,35 which included dispersion correction and
long-range electron correlation corrections, as well as M06-2X
functional developed by Zhao and Truhlar.36 For all functionals,
the 6-311++G(2d,2p) basis set was employed. The obtained
results for all the conformers are found in the Supporting
Information.
Acknowledgements
The following sources of funding are gratefully acknowledged:
the Swedish Research Council (N.K., 2015-05360), the Swedish
Research Council Formas (N.K., 942-2015-1106), the Carl
Trygger Foundation (A.P., CTS 16:235), the Hungarian Scientific
Research Fund (T.B.-S., OTKA-109588), the Momentum
program of the Hungarian Academy of Sciences (T.B.-S., LP2016-
2), the National Competitiveness and Excellence Program
(NVKP_16-1-2016-0007),
BIONANO_GINOP-2.3.2-15-2016-00017, and a Marie Curie
fellowship (T.B.-S., MSCA-IF BARREL 660030).
the
Excellence
program
References
1.
(a) J. V. Schreiber, J. Frackenpohl, F. Moser, T. Fleischmann,
H. P. E. Kohler and D. Seebach, ChemBioChem, 2002, 3,
424-432; (b) D. F. Hook, F. Gessier, C. Noti, P. Kast and D.
Seebach, ChemBioChem, 2004, 5, 691-706.
2.
3.
D. Seebach, A. K. Beck and D. J. Bierbaum, Chem. Biodivers.,
2004, 1, 1111-1239.
(a) T. Kimmerlin, D. Seebach and D. Hilvert, Helv. Chim.
Acta, 2002, 85, 1812-1826; (b) G. V. M. Sharma, B. S. Babu,
K. V. S. Ramakrishna, P. Nagendar, A. C. Kunwar, P.
Schramm, C. Baldauf and H. J. Hofmann, Chem. Eur. J.,
2009, 15, 5552-5566; (c) T. A. Martinek and F. Fulop, Chem.
Soc. Rev., 2012, 41, 687-702.
To estimate secondary structure preference of longer
4.
5.
6.
7.
K. Gademann, T. Hintermann and J. V. Schreiber, Curr.
Med. Chem., 1999, 6, 905-925.
W. S. Horne and S. H. Gellman, Acc. Chem. Res., 2008, 41,
1399-1408.
G. N. Tew, R. W. Scott, M. L. Klein and W. F. Degrado, Acc.
Chem. Res., 2010, 43, 30-39.
W. S. Horne, L. M. Johnson, T. J. Ketas, P. J. Klasse, M. Lu, J.
P. Moore and S. H. Gellman, Proc. Natl. Acad. Sci. USA,
2009, 106, 14751-14756.
oligomers composed of the investigated
8 monomers,
previously defined secondary structures were employed.21
Initially, five secondary structures, four helical, and one
extended, were modified with the 8 chiral substitution patterns
and optimised at the B97x-D/6-31G(d) level of theory. To
predict preference of helicity handedness, both P and M helices
were optimised resulting a total of 9 secondary structures.
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 11
Organic & Biomolecular Chemistry
Please do not adjust margins
Journal Name
ARTICLE
8.
9.
N. Umezawa, M. A. Gelman, M. C. Haigis, R. T. Raines and
S. H. Gellman, J. Am. Chem. Soc., 2002, 124, 368-369.
J. Kim, S. Kwon, S. H. Kim, C. K. Lee, J. H. Lee, S. J. Cho, H. S.
Lee and H. Ihee, J. Am. Chem. Soc., 2012, 134, 20573- 21.
20576.
(a) R. V. Nair, K. N. Vijayadas, A. Roy and G. J. Sanjayan, Eur. 22.
J. Org. Chem., 2014, 7763-7780; (b) J. S. Laursen, J. Engel-
Andreasen and C. A. Olsen, Acc. Chem. Res., 2015, 48, 23.
2696-2704.
Org. Biomol. Chem., 2017, 15, 4704-4710; (Vj)iewE.AErtniclgehOonllmine,
N. Stuhr-Hansen and O. Blixt, TetrDahOeI:d1r0o.1n0L3e9t/tD.0, O20B1070,16588F,
2272-2275.
J. R. Johansson, E. Hermansson, B. Norden, N. Kann and T.
Beke-Somfai, Eur. J. Org. Chem., 2014, 2014, 2703-2713.
N. Kann, J. R. Johansson and T. Beke-Somfai, Org. Biomol.
Chem., 2015, 13, 2776-2785.
10.
D. Habrant, V. Rauhala and A. M. P. Koskinen, Chem. Soc.
Rev., 2010, 39, 2007-2017.
11.
12.
R. Gopalakrishnan, A. I. Frolov, L. Knerr, W. J. Drury and E. 24.
Valeur, J. Med. Chem., 2016, 59, 9599-9621.
(a) D. B. Dess and J. C. Martin, J. Org. Chem., 1983, 48,
4155-4156; (b) A. Yoshimura and V. V. Zhdankin, Chem.
Rev., 2016, 116, 3328-3435.
H. Tohma and Y. Kita, Adv. Synth. Catal., 2004, 346, 111-
124.
M. Kitamura, S. Kato, M. Yano, N. Tashiro, Y. Shiratake, M.
Sando and T. Okauchi, Org. Biomol. Chem., 2014, 12, 4397-
4406.
(a) C. W. Tornoe, C. Christensen and M. Meldal, J. Org.
Chem., 2002, 67, 3057-3064; (b) V. V. Rostovtsev, L. G. 25.
Green, V. V. Fokin and K. B. Sharpless, Angew. Chem. Int.
Ed., 2002, 41, 2596-2599; (c) E. Haldon, M. C. Nicasio and 26.
P. J. Perez, Org. Biomol. Chem., 2015, 13, 9528-9550; (d) C.
L. Wang, D. Ikhlef, S. Kahlal, J. Y. Saillard and D. Astruc,
Coord. Chem. Rev., 2016, 316, 1-20.
Y. Wang, J. F. Xiang and H. Jiang, Chem. Eur. J., 2011, 17,
613-619.
N. Castellucci and C. Tomasini, Eur. J. Org. Chem., 2013,
3567-3573.
27.
28.
29.
C. Baldauf and H. J. Hofmann, Helv. Chim. Acta, 2012, 95,
2348-2383.
13.
14.
15.
J. Ryan, M. Siauciulis, A. Gomm, B. Macia, E. O'Reilly and V.
Caprio, J. Am. Chem. Soc., 2016, 138, 15798-15800.
B. Rajagopal, Y. Y. Chen, C. C. Chen, X. Y. Liu, H. R. Wang
and P. C. Lin, J. Org. Chem., 2014, 79, 1254-1264.
J. A. Fehrentz and B. Castro, Synthesis (Stuttg), 1983, 676-
678.
R. V. Hoffman and H. O. Kim, Tetrahedron, 1992, 48, 3007-
3020.
A. Paul, H. Bittermann and P. Gmeiner, Tetrahedron, 2006,
62, 8919-8927.
(a) W. S. Horne, M. K. Yadav, C. D. Stout and M. R. Ghadiri,
J. Am. Chem. Soc., 2004, 126, 15366-15367; (b) T. 30.
Boddaert, J. Sola, M. Helliwell and J. Clayden, Chem.
Commun,, 2012, 48, 3397-3399; (c) K. B. Salah, S. Das, N. 31.
Ruiz, V. Andreu, J. Martinez, E. Wenger, M. Amblard, C.
Didierjean, B. Legrand and N. Inguimbert, Org. Biomol. 32.
Chem., 2018, 16, 3576-3583.
16.
(a) L. Zhang, X. G. Chen, P. Xue, H. H. Y. Sun, I. D. Williams, 33.
K. B. Sharpless, V. V. Fokin and G. C. Jia, J. Am. Chem. Soc.,
2005, 127, 15998-15999; (b) B. C. Boren, S. Narayan, L. K.
Rasmussen, L. Zhang, H. T. Zhao, Z. Y. Lin, G. C. Jia and V. V.
Fokin, J. Am. Chem. Soc., 2008, 130, 8923-8930.
O. Kracker, J. Gora, J. Krzciuk-Gula, A. Marion, B. Neumann,
H. G. Stammler, A. Niess, I. Antes, R. Latajka and N. Sewald,
Chem. Eur. J., 2018, 24, 953-961.
M. Wunsch, D. Schroder, T. Frohr, L. Teichmann, S. Hedwig,
N. Janson, C. Belu, J. Simon, S. Heidemeyer, P. Holtkamp, J.
Rudlof, L. Klemme, A. Hinzmann, B. Neumann, H. G.
Stammler and N. Sewald, Beilstein J. Org. Chem., 2017, 13,
2428-2441.
J. R. Johansson, T. Beke-Somfai, A. S. Stålsmeden and N.
Kann, Chem. Rev., 2016, 116, 14726-14768.
(a) D. S. Pedersen and A. Abell, Eur. J. Org. Chem., 2011,
2399-2411; (b) A. Tam, U. Arnold, M. B. Soellner and R. T.
Raines, J. Am. Chem. Soc., 2007, 129, 12670-12671; (c) J. K.
Pokorski, L. M. M. Jenkins, H. Q. Feng, S. R. Durell, Y. W. Bai 34.
and D. H. Appella, Org. Lett., 2007, 9, 2381-2383; (d) D.
Tietze, M. Tischler, S. Voigt, D. Imhof, O. Ohlenschlager, M.
Gorlach and G. Buntkowsky, Chem. Eur. J., 2010, 16, 7572- 35.
7578; (e) M. Empting, O. Avrutina, R. Meusinger, S. Fabritz,
M. Reinwarth, M. Biesalski, S. Voigt, G. Buntkowsky and H. 36.
Kolmar, Angew. Chem. Int. Ed., 2011, 50, 5207-5211; (f) J.
Q. Zhang, J. Kemmink, D. T. S. Rijkers and R. M. J. Liskamp,
Chem. Commun,, 2013, 49, 4498-4500; (g) S. Ferrini, J. Z.
Chandanshive, S. Lena, M. C. Franchini, G. Giannini, A. Tafi
and M. Taddei, J. Org. Chem., 2015, 80, 2562-2572; (h) X.
Yang, L. P. Beroske, J. Kemmink, D. T. S. Rijkers and R. M. J.
Liskamp, Tetrahedron Lett., 2017, 58, 4542-4546; (i) S.
Pacifico, A. Kerckhoffs, A. J. Fallow, R. E. Foreman, R.
Guerrini, J. McDonald, D. G. Lambert and A. G. Jamieson,
G. W. T. M. J. Frisch, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery Jr., J. Peralta, F. Oglario,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.
Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, A. V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J.
W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D.
Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski
and D. J. Fox, Gaussian 09 (Revision A.1), Gaussian, Inc.,
Wallingford CT, 2009.
17.
18.
19.
20.
(a) C. T. Lee, W. T. Yang and R. G. Parr, Phys. Rev. B, 1988,
37, 785-789; (b) A. D. Becke, J. Chem. Phys., 1993, 98, 1372-
1377.
J. D. Chai and M. Head-Gordon, Phys. Chem. Chem. Phys.,
2008, 10, 6615-6620.
Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2006, 120,
215-241.
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