Conformational behavior of 1:1 [α/α-Hydrazino]mer, 1:1 [α/Aza-β3-amino]mer and 1:1 [Aza-β3-amino/ α]mer series: Three series of foldamers

Article abstract of DOI:10.1002/ejoc.201300567

Analysis of ¹H NMR spectroscopic data allowed us to assign the secondary structure of three new series of mixed foldamers. 1:1 [α/aza-β³-amino]mers, 1:1 [aza-β³-amino/ α]mers and 1:1 [α/α-hydrazino]mers are self-organized in solution by a hydrogen bond network of alternating a γ-turn and a hydrazinoturn. Nevertheless, owing to the donor and acceptor H-bond characters of the amidic NH, 1:1 [α/α-hydrazino]mers can exist as an equilibrium between pseudospiranic and hydrazinoturn conformations. We describe the synthesis of 1:1 [α/α-hydrazino]mer, 1:1 [α/aza- β³-amino]mer and 1:1 [aza-β³-amino/α]mer series and analyze their ¹H NMR spectroscopic data to understand their conformational behavior. This study shows that all series are self-organized as γ-turn and hydrazinoturns. For 1:1 [α/α- hydrazino]mers the sp³ N^α allowed an equilibrium between this conformation and a pseudospiranic one. Copyright

Full text of DOI:10.1002/ejoc.201300567

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FULL PAPER
Foldamers
We describe the synthesis of 1:1 [α/α-hy-
drazino]mer, 1:1 [α/aza-β3-amino]mer and
1:1 [aza-β3-amino/α]mer series and analyze
their 1H NMR spectroscopic data to
understand their conformational behavior.
This study shows that all series are self-or-
ganized as γ-turn and hydrazinoturns. For
1:1 [α/α-hydrazino]mers the sp3 Nα allowed
an equilibrium between this conformation
and a pseudospiranic one.
S. Acherar, A. Salaün, P. Le Grel,
B. Le Grel, B. Jamart-Grégoire* ..... 1–12
Conformational Behavior of 1:1 [α/α-Hy-
drazino]mer, 1:1 [α/Aza-β3-amino]mer and
1:1 [Aza-β3-amino/α]mer Series: Three
Series of Foldamers
Keywords: Protein folding / Conformation
analysis / Hydrogen bonds / Peptidomimet-
ics
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S. Acherar, A. Salaün, P. Le Grel, B. Le Grel, B. Jamart-Grégoire
FULL PAPER
DOI: 10.1002/ejoc.201300567
Conformational Behavior of 1:1 [α/α-Hydrazino]mer, 1:1 [α/Aza-β3-amino]mer
and 1:1 [Aza-β3-amino/α]mer Series: Three Series of Foldamers
Samir Acherar,[a] Arnaud Salaün,[a] Philippe Le Grel,[b] Barbara Le Grel,[b] and
Brigitte Jamart-Grégoire*[a]
Keywords: Protein folding / Conformation analysis / Hydrogen bonds / Peptidomimetics
1
Analysis of H NMR spectroscopic data allowed us to assign
the secondary structure of three new series of mixed folda-
mers. 1:1 [α/aza-β3-amino]mers, 1:1 [aza-β3-amino/α]mers
and 1:1 [α/α-hydrazino]mers are self-organized in solution by
a hydrogen bond network of alternating a γ-turn and a
hydrazinoturn. Nevertheless, owing to the donor and ac-
ceptor H-bond characters of the amidic NH, 1:1 [α/α-hydraz-
ino]mers can exist as an equilibrium between pseudospiranic
and hydrazinoturn conformations.
formational analysis of pure α-hydrazinopeptides is difficult
and their conformational behavior remains undeter-
mined.[5b] Relative to other pseudopeptides, hydrazinopept-
ides have not received much attention, because generating
enantiomerically pure α-hydrazino acids directly suitable for
oligomerization remains synthetically difficult, in spite of
the numerous methods that have been developed.[6]
Introduction
Since the middle of the 1990s, the field of foldamers has
gained respectability. The pioneering contributions of Gell-
man[1] and Seebach,[2] with β-peptidic foldamers based on
remote intrastrand interactions, have undoubtedly played a
crucial role in the current dynamism of this rapidly evolving
field. Objectives include, in particular, designing biolo-
gically active molecules with increased resistance to enzy-
matic proteolysis, and also developing a better understand-
ing of the phenomena that govern the three-dimensional
organization of the naturally occurring peptides. The ever-
growing number of papers concerning β-peptides[3] and bio-
polymer mimetics is indisputable proof of the attraction
and the potential of the foldamer concept.
In this context, an original synthesis of Nα-Z-protected
α-hydrazino acids through a Mitsunobu reaction has been
developed in our laboratory.[6t] These units could have been
incorporated with natural α-amino acids leading to the for-
mation of mixed 1:1 [α/Nα-Z-α-hydrazino]mers[7] (Figure 1,
1
a) but H NMR spectra of these compounds show broad
and poorly dispersed signals, indicative of conformational
heterogeneity.
In the case of aza-analogues of β-peptides, alternative H-
bond networks may occur because the nitrogen atom can
act both as H-bond acceptor and/or H-bond donor, ex-
plaining that oligomers of α-hydrazino acids arise from a
logical extension of the β-peptide concept.[4] Yet, few in-
vestigations concerning the structural analysis of foldamers
built from α-hydrazino acids have been performed. Mar-
raud’s team showed that the presence of α-hydrazino acid
residues in pseudopeptidic constructions leads to new intra-
molecular structural patterns.[5a] This was confirmed for
aza-β3-peptides by the work of Le Grel and co-
workers.[5c,5d] However, Seebach’s work shows that the con-
Figure 1. (a) 1:1 [α/Nα-Z-α-hydrazino]mers, (b) 1:1 [α/α-hydrazino]-
mers, (c) 1:1 [α/aza-β3-amino]mers and (d) 1:1 [aza-β3-amino/α]-
mers.
[a] Laboratoire de Chimie Physique Macromoléculaire, FRE 3564,
ENSIC, Université de Lorraine,
1, rue Grandville BP 451, 54001 Nancy Cedex, France
E-mail: brigitte.jamart@univ-lorraine.fr
Homepage: www.univ-lorraine.fr
Results and Discussion
Because the presence of a Z protecting group should par-
ticipate in this conformational dispersion, we turned to the
synthesis of a closely related series of oligomers to try to
identify new foldamers that contain hydrazino acid deriva-
[b] ICMV, UMR CNRS 6226, Université de Rennes I
263 avenue du Général Leclerc 35042 Rennes Cedex, France
Homepage: http://www.univ-rennes1.fr/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300567.
2
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Conformational Behavior of Three Series of Foldamers
tives. Oligomers of type 1:1 [α/α-hydrazino]mers, 1:1 [α/aza- reaction between 5 and 6. By using the same procedure,
β3-amino]mers, and 1:1 [aza-β3-amino/α]mers have been corresponding octamer 10 can be obtained in 30% yield
prepared (Figure 1, parts b–d, respectively).
starting from 7 (Scheme 1).
Synthesis of 1:1 [α/α-Hydrazino]mers
Synthesis of 1:1 [α/Aza-β3-amino]mers
Dimer tert-Butyloxycarbonyl(Boc)-(Phe-α-hAla)-OMe 4
was obtained through a coupling reaction between HCl·H-
(α-hAla)-OMe 3 and Boc-Phe-OH (Scheme 1). Previous
work in our group showed that the presence of an acyl,
aryloxy or alkyloxy group on the Nα-nitrogen atom drasti-
cally decreased the nucleophilic character of the adjacent
Nβ-nitrogen atom of hydrazino esters and necessitated the
use of acyl fluoride in the coupling reaction.[7] To avoid this
problem we directly used non-protected HCl·H-(α-hAla)-
OMe 3 in the coupling reaction. Compound 3 was obtained
by using the method described first by Hoffman[6p] and im-
proved for large-scale preparation by our team.[6u,6w] The
saponification of 1 let us obtain corresponding free C-ter-
minal compound 2. The use of thionyl chloride in meth-
anol[8] allowed the removal of the Boc protection and esteri-
fication of the carboxylic group leading to desired com-
pound 3 in quantitative yield. The coupling reaction of
methyl ester 3 and Boc-Phe-OH was performed by using
To get information on the conformational behavior of
the mixed 1:1 [α/α-hydrazino]mer series, we first decided to
1
analyze their H NMR spectroscopic data obtained for the
1:1 [α/aza-β3-amino]mers which differ from each other by
replacement of the α-hydrazino acid units by an aza-β3-
amino acid one (Figure 1, c). The synthesis of correspond-
ing tetra-15, hexa-17, octa-19 and decamer 21 is described
in Scheme 2. Compounds 15, 17, 19 and 21 were obtained
in very good yields ranged from 94 to 96% by an iterative
sequence of deprotection and coupling reaction starting
from 12.
Synthesis of 1:1 [Aza-β3-amino/α]mers
We also decided to use the mixed 1:1 [α/aza-β3-amino]-
classical conditions[9] and led to corresponding dimer 4 mer series in which the α-amino acid and aza-β3-amino acid
(Scheme 1) in 53% yield.
units are reversed (Figure 1, d), because this series shows a
1
Saponification and protonolysis of dimer 4 led, in quan- single set of signals in the H NMR spectra in contrast to
titative yield, to corresponding unprotected dimers 5 and 6 the two other series. The synthesis path, reagents, and yields
bearing, respectively, a free C- and N-terminal. Tetramer 7 leading to tetramer 25 and hexamer 27 are summarized in
was then obtained in 70% yield as a solid, after a coupling Scheme 3.
Scheme 1. Synthesis of tetramer Boc-(Phe-α-hAla)2-OMe 7 and octamer Boc-(Phe-α-hAla)4-OMe 10.
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Scheme 2. Synthesis of 1:1 [α/aza-β3-amino]mers 12, 15, 17, 19 and 21.
Scheme 3. Synthesis of tetramer Boc-(aza-β3-Phe-Ala)2-OMe 25 and hexamer Boc-(aza-β3-Phe-Ala)3-OMe 27.
Comparative Studies of the Conformational Behaviour of
formational preferences. These three types of oligomers
the 1:1 [α/α-Hydrazino]mer, 1:1 [α/Aza-β3-amino]mer and
contain amidic (NHa), hydrazidic (NHh), and carbamidic
(NHcb), or carbazidic (NHcz), NH protons. Le Grel and co-
workers showed that non-bonded (free NH) aza-β3-hy-
drazidic NHs appears around δ = 6–6.5 ppm, as in model
compound 28 (Figure 2), whereas they can be shifted 2–
3 ppm to lower fields when involved in a hydrazinoturn. By
analogy, amidic NHs, for which the free NH chemical shift
is somewhat lower (around δ = 5.5 ppm; 10 mm, CDCl3) as
in commercially available model compound 29, should be
observed around ca. 8 ppm when involved in the hydraz-
inoturn motif (Figure 2). To get a reference value for non-
bonded carbamidic and carbazidic NHs, we prepared com-
pounds 30 and 31 (Figure 2) which show δ = 4.7 and
5.5 ppm, respectively, at 10 mm in CDCl3.
1
1:1 [Aza-β3-amino/α]mer Series by H NMR Studies in
CDCl3 Solution
Considering that hydrazinoturns have been defined by Le
Grel et al.[5c,5d] as the main structural element responsible
for the self-organization of aza-β3-oligomers, we anticipated
that the presence of aza-β3-amino acids in 1:1 [α/aza-β3-
amino]mers, and 1:1 [aza-β3-amino/α]mers (Figure 1, c, d)
could induce the amidic NHs to form such a motif. This
could promote further intramolecular H-bonding contact.
Actually, Wang and co-workers showed that the amidoxy
NH of pseudopeptides with alternating α-aminoxy acid res-
idues and α-amino acids forms a γ-turn with the carbonyl
of the i-2 residue.[10]
In 1:1 [α/α-hydrazino]mers, (Figure 1, b) the shift of the
The 1H NMR spectra of 1:1 [aza-β3-amino/α]tetramer 25
side chain from the Nα to the Cα atom makes it possible to (Figure 3, d), 1:1 [α/α-hydrazino]tetramer 7 (Figure 3, a),
obtain alternative H-bonding patterns by promoting a new and 1:1 [α/aza-β3-amino]tetramer 15 (Figure 3, b) show
H-bond donor on the chiral nitrogen centers. Analyzing the good signal dispersion and appreciable signal resolution.
three kinds of oligomers will make it easier to identify anal- The spectra of 1:1 [aza-β3-amino/α]mers 25 and 27 show a
ogies or divergences that could help understand their con- single set of signals, whereas one major conformer predomi-
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Figure 2. H NMR chemical shifts of free hydrazidic (NHh), amidic (NHa), carbamidic (NHcb), and carbazidic NH (NHcz) protons in
reference compounds 28, 29, 30 and 31 (300 MHz, CDCl3/10 mm).
Figure 3. 1H NMR spectra (300 MHz, CDCl3/10 mm) of (a) tetramer Boc-(Phe-α-hAla)2-OMe 7, (b) tetramer Boc-(Phe-aza-β3-Ala)2-
OMe 15, (c) octamer Boc-(Phe-aza-β3-Ala)4-OMe 19, (d) tetramer Boc-(aza-β3-Phe-Ala)2-OMe 25 and (e) hexamer Boc-(aza-β3-Phe-
Ala)3-OMe 27.
nates for compounds 7 and 15. The second set of signals, by Le Grel et al.[11] who also observed that the percentage
which is more intense for compound 15, probably results of the E-isomer at the C terminus of aza-β3-peptides is
from residual E/Z isomerism of the hydrazidic bond at the higher when the chiral nitrogen atom bears a side chain
C terminus. This phenomenon has recently been identified with low steric demand. The lower occurrence of the minor
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isomer in compound 7, seems surprising at the first glance
For tetramer Boc-(Phe-aza-β3-Ala)2-OMe 15 (Figure 5,
because an H atom is smaller than a methyl group, will a), the chemical shift of the amidic NHa (δ = 8.03 ppm) is
1
be discussed in the next section. H NMR signals of the little affected by the addition of DMSO (Δδ = +0.31 ppm).
predominant conformation of tetramers 7 and 15 and sig- Together with its high chemical shift, this strongly supports
nals of tetramer 25 were assigned through a combination that NHa is probably involved in a hydrazinoturn. The
of 2D-COSY and 2D-ROESY experiments. In the case of chemical shift of the most deshielded hydrazidic NHh1 at δ
upper homologues, this cannot be done because of prohibi- = 7.76 ppm, which corresponds to the hydrazidic NH lo-
tive signal overlap. Nevertheless, the general trend of signal cated at the C-terminus, is also little affected by the addition
distributions remains the same, as exemplified for hexamer of DMSO (Δδ = +0.33 ppm). This suggests H-bond interac-
27 (Figure 3, e) and octamer 19 (Figure 3, c).
tion with the carbonyl group of the i-1 residue, closing a γ-
The analysis of the δ values for the NH protons in the turn. An additional strengthening of this interaction by the
series of oligomers reveals several points. The chemical shift contribution of a six-membered pseudocycle with the carb-
of carbazidic NHs protons (NHcz) in 1:1 [aza-β3-amino/α]- onyl of the C-terminus ester group is also possible, as ob-
tetramer 25 is unaffected relative to the reference value in served for the C-terminus of aza-β3-peptides.[12] The chemi-
compound 31, whereas it is slightly higher (+ 0.62 ppm) in cal shift of the hydrazidic NH protons at the N terminus (δ
1:1 [aza-β3-amino/α]hexamer 27. In [α/α-hydrazino]mer and = 7.30 ppm), is higher than for reference compound 28 but
1:1 [α/aza-β3-amino]mers, the chemical shifts of the carb- reveals much more sensitive to the addition of DMSO (Δδ
amidic NHs (NHcb) range between 5–5.5 ppm. These values = +1.20 ppm). A weak γ-turn could form transiently with
remain close (although also slightly higher) to the reference the carbonyl of the Boc group. Concomitantly, the carb-
value in compound 30. On the contrary, the chemical shifts amidic NH (NHcb) has a chemical shift close to that of a
of all the amidic and hydrazidic NHs protons (NHa and free carbamidic NH, is very resistant to addition of DMSO
NHh, respectively) are all shifted downfield (δ =7.30 ppm Յ up to 5% revealing a possible contact with an H-bonding
δNH Յ 8.49 ppm) relative to the canonical values of free partner inside the oligomer, in relation with the weakness
amidic NHa and free hydrazidic NHb as defined above. Rel- of the γ-turn at the N-terminus. It should be noted that
ative to references values, the effect of downfield shifting is the presence of the pyramidal nitrogen centers along the
more pronounced for amidic NHs relative to hydrazidic oligomer, like in aza-β3-peptides themselves,[5c] gives the
NHs. Amidic NHs are shifted downfield around 2–3 ppm, backbone the flexibility to transiently bring the N terminus
whereas the downfield shifts of hydrazidic NHs range 0.8– close to the C-terminus and to give rise to such an intramo-
1.8 ppm. To get more information on the structure of 1:1 lecular contact. Figure 4b shows the major conformation of
[α/aza-β3-amino]mers, we monitored the effect of solvent tetramer 15.
composition in the mixed solvent CDCl3/[D6]DMSO (from
When applied to tetramer Boc-(Phe-α-hAla)2-OMe 7,
0 up to 5%) on the chemical shifts of the NH protons. Sol- [D6]DMSO titration gives similar results (Figure 5, b). The
vents with hydrogen-bond acceptor atoms, such as [D6]- amidic (NHaЈ) proton at δ = 7.33 ppm is affected in the
DMSO, are able to form intermolecular hydrogen bonds same way (Δδ = +0.31 ppm) as its homologue in compound
producing a downfield displacement, which is less signifi- 15. The difference in chemical shifts between (NHaЈ) and
cant when the hydrogen atom participates in an intra- (NHa) in CDCl3 (7.33 ppm versus 8.03 ppm) could be ow-
molecular hydrogen bond.
ing to the presence of the H-bond donor (NαH) on the pre-
Figure 4. (a) Conformational equilibrium for the major corformer of compound Boc-(Phe-α-hAla)2-OMe 7 in CDCl3, (b) major confor-
mation of the Boc-(Phe-aza-β3-Ala)2-OMe 15 in CDCl3 and (c) conformation of compound Boc-(aza-β3-Phe-Ala)3-OMe 27 in CDCl3.
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Figure 5. H NMR (300 MHz, 10 mm) chemical shift values of NH protons in (a) 15, (b) 7, (c) 25 and (d) 27 and the effect of solvent in
mixed CDCl3/[D6]DMSO.
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FULL PAPER
ceding α-hydrazino acid unit. Actually, it has been showed erate downfield shift relative to free hydrazidic NH, and to
that when the side chain is replaced by a hydrogen atom, the their moderate sensitivity to DMSO. This is in agreement
hydrazinoturn motif is in equilibrium with the energetically with the formation of a γ-turn and is consistent with the
similar pseudospirannic conformation.[12]
work of Yang.[10] The alternation of these two hydrogen
The equilibrium between these two conformations is bonding motifs gives rise to greater conformational homo-
formed by 180° rotation around the central N–N bond. In geneity in the case of 1:1 [aza-β3-amino/α]mers, in which
both conformations, short-range interaction (C5) with the the more stable hydrazinoturn motif locks the conformation
Nα-nitrogen lone pair contributes to the folding process, but at the N-terminus, whereas no residual E/Z isomerism is
the transient loss of NH···OC interaction lowers the down- observed at the C-terminus owing to the presence of the
field shift. In the pseudospirannic conformer, the central amide bond. Nevertheless, we note that δNHa and δNHa1 are
NαH is H-bonded with the CO group of residue i-1. This less sensitive to the addition of DMSO for compound 27
could explain the very weak sensitivity of (NαH1) and (hexamer) than for corresponding tetramer 25. This obser-
(NαH2) to the addition of DMSO (Δδ = +0.10 and vation led us to conclude that the increase of the chain
+0.21 ppm, respectively) although the effect of DMSO on length contributes to the stabilization of the hydrazinoturn
the less “acidic” NαH could be intrinsically weaker. How- and, in turn, of the whole structure. In the 1:1 [α/aza-β3-
ever, the NαH···OC H-bonding, prone to stabilize the Z- amino]mer and 1:1 [α/α-hydrazino]mer series, E/Z isomer-
rotamer, is also consistent with the lower percentage of re- ism of the hydrazide bond occurs at the C-terminus, justify-
sidual E-isomer at the C terminus observed for tetramer 7, ing the existence of a minor conformer. The percentage of
as discussed above. The (NHhЈ2) and (NHhЈ1) protons be- the minor isomer is lower in the case of 1:1 [α/α-hydrazino]-
have similarly to the NHh2 and NHh1 homologues. Fig- mers owing to the pseudospirannic alternative conforma-
ure 4, a shows this fast equilibrium in the case of the major tion.
conformer of compound 7.
In all cases, additional capping involving the C-terminal
For tetramer 25 (Figure 5, c), a single set of signals is ester group is likely to occur as suggested by the higher
observed. The presence of only one rotamer at the level of chemical shifts of the NH protons at the terminus in all
the two peptidic bonds is not surprising, but E/Z isomerism series of oligomers.
is expected for the central hydrazide bond.[13] Both NHa1
and NHa2 amidic protons are strongly deshielded (δ = 8.45
and 8.16 ppm, respectively) and resistant to the addition of
DMSO (Δδ = +0.18 and + 0.05 ppm, respectively), which
Conclusions
supports their involvement in the hydrazinoturn motif. The
hydrazidic NHh is deshielded δ = 1.38 ppm relative to refer-
ence compound 28 and is weakly sensitive to the addition
The occurrence of a γ-turn is lower than for a β-turn in
of DMSO (Δδ = +0.58 ppm). Combined with the absence proteinic matter.[14] The exceptional stability of the γ-turn
of E/Z isomerism at the level of the hydrazide bond, this in oligomers alternating α-amino acids and α-aminoxyacids,
supports its inclusion in a γ-turn. The carbazidic NHcz at δ demonstrated by the Yang’s team, is owing to the high acid-
= 5.80 ppm is more sensitive to DMSO (Δδ = +0.98 ppm). ity of the amidoxydic NHs resulting from the presence of
Although the set of NHs could not be fully assigned for the strongly electronegative neighboring oxygen. The differ-
hexamer 27, the same behavior is observed (Figure 5, d). ence in electronegativity between the nitrogen atom and the
All amidic NHas are strongly deshielded (δ = 8.49 ppm for oxygen atom results in the acidity of the hydrazidic NH
the amidic NHa1 close to the C-terminus, 8.31, and being lowered. Nevertheless, the data collected through
8.25 ppm for the two other NHa) and very poorly sensitive NMR analysis show that the structures of the pseudo-
to the addition of DMSO (Δδ = +0.10 ppm, + 0.05 ppm, peptidic backbones depicted here are also able to sustain
and + 0.06 ppm, respectively). The hydrazidic NHhs at δ = the γ-turn motif, although its stability is certainly weakened
8.27 and 7.88 ppm are weakly sensitive to DMSO titration by the difference in nature of the H-bond donor. The
(Δδ = +0.27 and +0.62 ppm, respectively). The carbazidic hydrazino acetic residue, introduced here under a different
NHcz at δ = 6.12 ppm is less sensitive to DMSO than the structural variation, is prone to induce folding when hy-
corresponding tetramer with a Δδ value of 0.69 ppm. This bridized with natural α-amino acids. Fülöp’s team recently
set of data is consistent with the conformation depicted on established that hybrid pseudopeptides alternating ACPC
Figure 4, c for hexamer 27.
β-amino acid and aza-ACPC (a hydrazinoacetic derivative)
To sum up the data gathered from this study, we make retains a stable fold in water.[15] Preliminary results seem to
the following comments. In all series of oligomers, the demonstrate that the secondary structure of all compounds
amidic NHs are strongly H-bonded, as demonstrated by is retained in protic solvent. More experiments have to be
their high chemical-shift relative to free amidic NH, and performed to confirm this result and this work is under
their poor response to DMSO titration. They are frequently active investigation. At the moment, and in contrast to pure
involved in the hydrazinoturn motif as a H-bond donor. As aza-β3-peptides, no X-ray data was obtained for oligomers
best observed in the 1:1 [α/aza-β3-amino]mer series, which alternating α-aminoxy acid or aza-β3-amino acid with α-
are conformationally more homogeneous, hydrazidic NHs amino acid, preventing a deeper conformational analysis of
are more weakly H-bonded as demonstrated by their mod- their interesting folding properties.
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1
obtained as a white solid, m.p. 188 °C. H NMR (300 MHz, [D6]-
DMSO): δ = 9.53 (s, 1 H), 9.40 (s, 1 H), 8.21–8.14 (m, 1 H), 7.38–
7.09 (m, 10 H), 7.01 (d, J = 8.11 Hz, 1 H), 4.55–4.37 (m, 1 H),
4.19–3.98 (m, 1 H), 3.71–3.12 (m, 4 H), 3.08–2.67 (m, 4 H), 1.30
(s, 9 H), 1.12–0.81 (m, 6 H) ppm. 13C NMR (75 MHz, [D6]DMSO):
δ = 174.1 (C=O), 172.2 (C=O), 170.7 (C=O), 169.9 (C=O), 155.1
(C=O), 137.9 (C Ar), 137.3 (C Ar), 129.2 (4 CH Ar), 128.0 (4 CH
Ar), 126.3 (CH Ar), 126.2 (CH Ar), 78.0 (C), 58.6 (CH), 57.1 (CH),
54.5 (CH), 52.2 (CH), 38.0 (CH2), 37.5 (CH2), 28.1 (3 CH3), 16.5
(CH3), 15.9 (CH3) ppm.
Experimental Section
1. General: Unless otherwise stated, reagents were purchased from
chemical companies and used without purification. Reactions were
monitored with TLC by using aluminium-backed silica gel plates
(Macherey–Nagel ALUGRAM® SIL G/UV254). TLC spots were
viewed under ultraviolet light and by heating the plate after treat-
ment with a staining solution of phosphomolybdic acid. Product
purifications were performed with Geduran 60 H Silica Gel (63–
200 mesh). Reagent grade solvents were used as received. 1H and
13C NMR spectra were recorded with a Bruker Advance 300 spec-
trometer. Multiplicities are reported as follow: s = singlet, d =
doublet, q = quadruplet, m = multiplet, br. = broad, Ar = aro-
matic. IR spectra were recorded with a Bruker Tensor 27. Melting
points were obtained on a hot-stage apparatus. Electron spray ion-
ization mass spectra (ESI-MS) were recorded with a Bruker Micro-
Tof-Q HR spectrometer in the “Service commun de Spectrométrie
de Masse”, Faculté des Sciences et Techniques, Vandoeuvre-lès-
Nancy, France.
2.2.4. Boc-(Phe-aza-β3-Ala)-OH (13): From the reaction on the
6.20 g scale of 12 in CH3CN (60 mL) at 0 °C for 4 h, 13 (5.96 g,
100%) was obtained as a yellow oil. H NMR (300 MHz, CDCl3):
δ = 8.07 (s, 1 H), 7.45–7.08 (m, 5 H), 5.47 (br. d, J = 7.89 Hz, 1
H), 4.73–4.41 (m, 1 H), 3.67–3.22 (m, 2 H), 3.11–2.78 (m, 2 H),
2.56 (s, 3 H), 1.42 (s, 9 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
173.3 (C=O), 170.6 (C=O), 156.5 (C=O), 136.8 (C Ar), 129.9 (2
CH Ar), 129.1 (2 CH Ar), 127.6 (CH Ar), 81.5 (C), 58.2 (CH2),
54.9 (CH), 44.4 (CH3), 40.2 (CH2), 28.8 (3 CH3) ppm.
1
2. Synthesis
2.2.5. Boc-(Phe-aza-β3-Ala)2-OH (18): From the reaction on the
1.90 g scale of 15 in CH3CN (15 mL) at 0 °C for 4 h, 18 (1.85 g,
100%) was obtained as a white solid, m.p. 110 °C. 1H NMR
(300 MHz, CDCl3): δ = 8.35 (br. s, 1 H), 8.28 (d, J = 7.53 Hz, 1
H), 7.97 (br. s, 1 H), 7.47–7.03 (m, 10 H), 5.44 (br. s, 1 H), 4.65–
4.47 (m, 1 H), 4.38–4.17 (m, 1 H), 3.61–2.79 (m, 8 H), 2.58 (s, 3 H),
2.47 (s, 3 H), 1.40 (s, 9 H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ
= 170.9 (C=O), 169.9 (C=O), 169.0 (C=O), 168.6 (C=O), 155.1
(C=O), 137.8 (C Ar), 137.2 (C Ar), 129.2 (4 CH Ar), 128.0 (4 CH
Ar), 126.3 (2 CH Ar), 78.1 (C), 61.3 (CH2), 58.2 (CH2), 54.7 (CH),
52.4 (CH), 45.2 (CH3), 43.6 (CH3), 37.9 (CH2), 37.4 (CH2), 28.1 (3
CH3) ppm.
2.1. Boc-(α-hAla)-OMe (1): A solution of (S)-methyl lactate (3.07 g,
29.5 mmol) and 2,6-lutidine (7.5 mL, 64.8 mmol) in CH2Cl2
(40 mL) at 0 °C was treated with triflic anhydride (5.4 mL,
32.4 mmol). After 15 min, a solution of BocNHNH2 (3.90 g,
29.5 mmol) in CH2Cl2 (30 mL) was added dropwise over 30 min
and the mixture was stirred for 4 h at 0 °C. After evaporation of
the solvent, Et2O (50 mL) was added and 2,6-lutidium salt was
precipitated by storing the reaction mixture overnight in the fridge.
The 2,6-lutidinium salt was filtered and the filtrate was diluted with
Et2O (100 mL) and washed with water (20 mL), dried with MgSO4,
filtered, and evaporated in vacuo. The residue was purified by flash
column chromatography (20:80, AcOEt/petroleum ether) to afford
2.2.6. Boc-(Phe-aza-β3-Ala)3-OH (20): From the reaction on the
2.33 g scale of 17 in CH3CN/MeOH (40 mL/10 mL) at 0 °C for 6 h,
1
1 (5.78 g, 90%) as a yellow oil. H NMR (300 MHz, CDCl3): δ =
6.64 (br. s, 1 H), 4.26 (br. s, 1 H), 3.92–3.70 (m, 1 H), 3.74 (s, 3 H),
1.45 (s, 9 H), 1.32 (d, J = 7.04 Hz, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 174.7 (C=O), 157.0 (C=O), 81.0 (C), 58.9 (CH), 52.5
(CH3), 28.8 (3 CH3), 16.4 (CH3) ppm.
1
20 (2.28 g, 100%) was obtained as a white solid, m.p. 132 °C. H
NMR (300 MHz, CDCl3): δ = 8.70–87.57 (m, 5 H), 7.47–6.95 (m,
15 H), 5.39 (br. s, 1 H), 4.72–4.15 (m, 3 H), 3.73–2.89 (m, 12 H),
2.85–2.35 (m, 9 H), 1.38 (s, 9 H) ppm. 13C NMR (75 MHz, [D6]-
DMSO): δ = 170.9 (C=O), 170.8 (C=O), 170.0 (C=O), 169.1
(C=O), 169.0 (C=O), 168.5 (C=O), 155.1 (C=O), 137.8 (C Ar),
137.2 (2 C Ar), 129.1 (6 CH Ar), 128.0 (6 CH Ar), 126.3 (3 CH
Ar), 78.1 (C), 61.3 (2 CH2), 58.3 (CH2), 54.6 (CH), 52.5 (2 CH),
45.2 (CH3), 45.0 (CH3), 43.7 (CH3), 37.8 (2 CH2), 37.4 (CH2), 28.1
(3 CH3) ppm.
2.2. General Procedure for the Preparation of Compounds 2, 5, 8,
13, 18 and 20
2.2.1. Boc-(α-hAla)-OH (2): To a solution of 1 (5.78 g, 26.5 mmol)
in MeOH (100 mL) was added a NaOH solution (1 n, 2.12 g/50 mL
H2O, 53.0 mmol) and the mixture was stirred for 2 d at room tem-
perature. After evaporation of MeOH, the aqueous layer was
washed with Et2O (30 mL), acidified with HCl (2 n) until pH = 1
and extracted with CH2Cl2 (2ϫ 50 mL). The residue was dried with
MgSO4, filtered, and evaporated in vacuo to give 2 (5.40 g, 100%)
2.3. HCl·H-(α-hAla)-OMe (3): To a solution of crude 2 (5.40 g,
26.5 mmol) in MeOH (50 mL) was added SOCl2 (3.8 mL,
53.0 mmol) dropwise at 0 °C and the mixture was stirred overnight
at room temperature. Evaporation of MeOH in vacuo gives crude
3 (4.09 g, 100%) as an oil. Compound 3 was used immediately.
1
as a yellow oil. H NMR (300 MHz, CDCl3): δ = 8.35 (br. s, 2 H),
7.18 (br. s, 1 H), 3.82–3.65 (m, 1 H), 1.40 (s, 9 H), 1.30 (d, J =
1
7.07 Hz, 3 H) ppm. The H NMR spectroscopic data are in agree-
ment with those previously reported.[16]
2.4. General Procedure for the Preparation of Compounds 4, 7 and
10
2.2.2. Boc-(Phe-α-hAla)-OH (5): From the reaction on the 2.50 g
scale of 4 in CH3CN (30 mL) at 0 °C for 4 h, 5 (2.40 g, 100%) was
obtained as a yellow amorphous solid. 1H NMR (300 MHz,
CDCl3): δ = 8.35 (br. s, 1 H), 7.62 (br. s, 1 H), 7.27–7.12 (m, 5 H),
5.66 (s, 1 H), 5.48–5.32 (m, 1 H), 4.69–4.21 (m, 1 H), 3.75–3.59 (m,
1 H), 4.28–2.90 (m, 2 H), 1.48–1.20 (m, 12 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 176.6 (C=O), 172.0 (C=O), 156.4 (C=O),
136.9 (C Ar), 129.9 (2 CH Ar), 129.2 (2 CH Ar), 127.5 (CH Ar),
81.2 (C), 58.6 (CH), 54.8 (CH), 39.5 (CH2), 28.3 (3 CH3), 16.3
(CH3) ppm.
2.4.1. Boc-(Phe-α-hAla)-OMe Dimer (4): Compound 3 (4.09 g,
26.5 mmol) was immediately dissolved in a mixture of CH2Cl2
(100 mL) and NEt3 (5.33 g, 52.7 mmol). HOBt (4.28 g, 31.7 mmol),
Boc-Phe-OH (7.03 g, 26.5 mmol) and 1-ethyl-3-(3-dimethylamino-
propyl)carbodiimide (EDC, 6.08 g, 31.7 mmol) were added. The
reaction was stirred for 1 d. The addition of HCl (1 n 60 mL) whilst
stirring gave rise to the apparition of a white solid (HCl·HOBt),
which was filtered by suction. The solution was then washed suc-
cessively with water (60 mL), NaHCO3 (1 n, 60 mL), and dried
with Na2SO4. After evaporation of solvent, Et2O (100 mL) was
added and 4 was precipitated by storing the reaction mixture over-
2.2.3. Boc-(Phe-α-hAla)2-OH (8): From the reaction on the 1.40 g
scale of 7 in MeOH (30 mL) overnight, 8 (1.36 g, 100%) of was
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
9

Job/Unit: O30567
/KAP1
Date: 15-07-13 12:21:52
Pages: 12
S. Acherar, A. Salaün, P. Le Grel, B. Le Grel, B. Jamart-Grégoire
FULL PAPER
night in the fridge (5.12 g, 53%), m.p. 95 °C. IR (ATR): ν
=
to give 11 as a yellow oil (4.73 g, 79%). 1H NMR (300 MHz,
CDCl3): δ = 3.74 (s, 3 H), 3.42 (s, 2 H), 3.33–3.08 (br. s, 2 H), 2.64
˜
max
3342, 3286, 2983, 1741, 1685, 1648, 1519, 1451, 1374, 1293, 1241,
1
1168 cm–1. H NMR (300 MHz, CDCl3): δ = 7.46 (s, 1 H), 7.32– (s, 3 H) ppm. Compound 11 was used immediately in the next step.
7.10 (m, 5 H), 4.95 (br. s, 1 H), 4.59 (br. s, 1 H), 4.33–4.18 (m, 1
2.7. Boc-(Phe-aza-β3-Ala)-OMe Dimer (12): To a solution of 11
(2.30 g, 19.5 mmol) in CH2Cl2 (70 mL) was added Boc-Phe-OH
(4.50 g, 17.0 mmol), EDC (3.90 g, 20.3 mmol) and the reaction was
stirred overnight. After the addition of H2O (20 mL) whilst stirring
for 1 h, the organic layer was recovered and dried with MgSO4.
The evaporation of solvent in vacuo gave 12 as a yellow oil (5.48 g,
H), 3.70 (s, 3 H), 3.76–3.59 (m, 1 H), 3.12–2.98 (m, 2 H), 1.41 (s,
9 H), 1.23 (d, J = 7.00 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 174.2 (C=O), 171.6 (C=O), 155.8 (C=O), 137.0 (C Ar), 129.8
(2 CH Ar), 129.1 (2 CH Ar), 127.5 (CH Ar), 80.8 (C), 58.8 (CH),
55.0 (CH), 52.6 (CH3), 39.4 (CH2), 28.8 (3 CH3), 16.4 (CH3) ppm.
HRMS: calcd. for C18H27N3NaO5 [M + Na]+ 388.1849; found
77%). IR (ATR): νmax = 3435, 3346, 2987, 2935, 1737, 1692, 1495,
˜
388.1843.
1446, 1366, 1293, 1245, 1204, 1168 cm–1. 1H NMR (300 MHz,
CDCl3): δ = 7.56 (s, 1 H), 7.35–7.12 (m, 5 H), 5.16 (br. s, 1 H),
4.27–4.06 (m, 1 H), 3.66 (s, 3 H), 3.75–3.32 (m, 2 H), 3.21–2.85 (m,
2 H), 2.60 (s, 3 H), 1.44 (s, 9 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 171.1 (C=O), 169.8 (C=O), 155.7 (C=O), 137.1 (C Ar), 129.9
(2 CH Ar), 129.1 (2 CH Ar), 127.3 (CH Ar), 80.5 (C), 57.9 (CH2),
55.4 (CH), 52.1 (CH3), 44.3 (CH3), 39.6 (CH2), 28.8 (3 CH3) ppm.
HRMS: calcd. for C18H27N3NaO5 [M + Na]+ 388.1849; found
388.1843.
2.4.2. Boc-(Phe-α-hAla)2-OMe Tetramer (7): From the reaction on
the 2.59 g scale of 6 in CH2Cl2 (20 mL) for 1 d, 7 (2.86 g, 70%)
was obtained as a white powder, m.p. 202 °C. IR (ATR): ν
=
˜
max
3318, 3294, 2979, 1753, 1680, 1644, 1531, 1451, 1370, 1326, 1277,
1233, 1172 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.85 (br. s, 1
H), 7.40 (br. s, 1 H), 7.32–7.16 (m, 11 H), 5.07 (br. s, 1 H), 4.72
(br. s, 1 H), 4.59–4.37 (m, 1 H), 4.28–4.13 (m, 2 H), 3.70 (s, 3 H),
3.68–3.53 (m, 1 H), 3.45–3.29 (m, 1 H), 3.25–2.83 (m, 4 H), 1.40
(s, 9 H), 1.24 (d, J = 7.08 Hz, 3 H), 1.19 (d, J = 6.96 Hz, 3 H) ppm.
13C NMR (75 MHz, [D6]DMSO): δ = 172.9 (C=O), 172.4 (C=O),
170.7 (C=O), 170.0 (C=O), 155.1 (C=O), 137.9 (C Ar), 137.4 (C
Ar), 129.2 (4 CH Ar), 128.0 (4 CH Ar), 126.3 (2 CH Ar), 78.0 (C),
58.7 (CH), 57.1 (CH), 54.6 (CH), 52.2 (CH), 51.6 (CH3), 38.0
(CH2), 37.6 (CH2), 28.1 (3 CH3), 16.5 (CH3), 15.8 (CH3) ppm.
HRMS: calcd. for C30H42N6NaO7 [M + Na]+ 621.3013; found
621.3007.
2.8. General Procedure for the Preparation of Compounds 14 and 16
2.8.1. H-(Phe-aza-β3-Ala)-OMe (14): Compound 12 (3.34 g,
9.15 mmol) was treated with a mixture of CH2Cl2/TFA (6 mL/
4 mL) for 6 h. The reaction mixture was diluted in CH2Cl2 (20 mL)
and H2O (10 mL), treated whilst stirring with NaHCO3 until basic
pH was reached and extracted with CH2Cl2 (3ϫ 50 mL). The or-
ganic layer was dried with MgSO4 and the solvent was evaporated
in vacuo to afford 14 as a yellow oil (2.42 g, 100%). Compound 14
was used immediately in the next step.
2.4.3. Boc-(Phe-α-hAla)4-OMe Octamer (10): From the reaction on
the 1.43 g scale of 9 in CH2Cl2/CH3CN (20 mL/5 mL) overnight,
10 (747 mg, 30%) was obtained as a white solid, m.p. 252 °C. IR
2.8.2. H-(Phe-aza-β3-Ala)2-OMe (16): From the reaction on the
2.38 g scale of 6 for 6 h, 16 (1.98 g, 100%) was obtained as a yellow
oil. Compound 16 was used immediately in the next step.
(ATR): ν
= 3278, 3249, 3213, 3100, 2979, 2935, 1745, 1684,
˜
max
1640, 1543, 1527, 1454, 1438, 1370, 1317, 1241, 1172 cm–1. 1H
NMR (300 MHz, [D6]DMSO): δ = 9.55–9.32 (m, 4 H), 8.28–8.11
(m, 3 H), 7.33–7.13 (m, 20 H), 6.99 (d, J = 8.09 Hz, 1 H), 5.21–
4.92 (m, 4 H), 4.53–4.37 (m, 3 H), 4.18–4.02 (m, 1 H), 3.60 (s, 3
H), 3.42–3.15 (m, 4 H), 3.08–2.67 (m, 8 H), 1.29 (s, 9 H), 1.13–0.82
(m, 12 H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 173.0 (2
C=O), 172.5 (2 C=O), 170.8 (2 C=O), 170.0 (2 C=O), 155.1 (C=O),
137.9 (C Ar), 137.5 (2 C Ar), 137.4 (C Ar), 129.2 (8 CH Ar), 128.0
(8 CH Ar), 126.3 (4 CH Ar), 78.0 (C), 58.6 (2 CH), 57.2 (2 CH),
54.5 (2 CH), 52.3 (2 CH), 51.7 (CH3), 37.9 (2 CH2), 37.4 (2 CH2),
28.1 (3 CH3), 16.6 (2 CH3), 15.8 (2 CH3) ppm. HRMS: calcd. for
C54H72N12NaO11 [M + Na]+ 1087.5342; found 1087.5336.
2.9. General Procedure for the Preparation of Compounds 15, 17, 19
and 21
2.9.1. Boc-(Phe-aza-β3-Ala)2-OMe Tetramer (15): To a solution of
13 (3.21 g, 9.15 mmol) in CH2Cl2 (20 mL) was added HOBt (1.48 g,
11.0 mmol), EDC (2.10 g, 11.0 mmol) and 14 (2.42 g, 9.15 mmol).
The reaction was stirred overnight. The addition of HCl (1 n,
30 mL) whilst stirring gave rise to a white solid (HCl·HOBt) that
was filtered by suction. The solution was then washed successively
with water (40 mL), NaHCO3 (1 n, 40 mL), and dried with
Na2SO4. After evaporation of solvent, the residual solid was tritu-
rated successively with Et2O and a mixture of AcOEt/Et2O and
filtered to afford 15 (5.19 g, 95%) as a white powder, m.p. 138 °C.
2.5. General Procedure for the Preparation of Compounds 6 and 9
IR (ATR): ν
= 3322, 3286, 3225, 3064, 2979, 2858, 1753, 1668,
˜
max
2.5.1. TFA·H-(Phe-α-hAla)-OMe (6): Compound
4 (2.50 g,
1531, 1503, 1446, 1394, 1370, 1293, 1249, 1209, 1168, 1128 cm–1.
1H NMR (300 MHz, CDCl3): δ = 8.06 (d, J = 7.26 Hz, 1 H), 7.75
(s, 1 H), 7.38–7.09 (m, 11 H), 5.47 (br. s, 1 H), 4.48–4.35 (m, 1 H),
4.31–4.16 (m, 1 H), 3.68 (s, 3 H), 3.63–3.44 (m, 2 H), 3.33–2.92 (m,
6 H), 2.63 (s, 3 H), 2.52 (s, 3 H), 1.41 (s, 9 H) ppm. 13C NMR
(75 MHz, [D6]DMSO): δ = 169.8 (C=O), 169.5 (C=O), 168.8
(C=O), 168.5 (C=O), 155.1 (C=O), 137.8 (C Ar), 137.2 (C Ar),
129.2 (4 CH Ar), 128.0 (4 CH Ar), 126.2 (2 CH Ar), 78.1 (C), 61.3
(CH2), 57.8 (CH2), 54.7 (CH), 52.3 (CH), 51.4 (CH3), 45.2 (CH3),
43.5 (CH3), 37.9 (CH2), 37.4 (CH2), 28.1 (3 CH3) ppm. HRMS:
calcd. for C30H42N6NaO7 [M + Na]+ 621.3013; found 621.3007.
6.85 mmol) was treated with a mixture of CH2Cl2/trifluoroacetic
acid (TFA, 6 mL/4 mL) overnight. Excess of TFA was co-evapo-
rated with toluene, CH2Cl2 and Et2O until 6 was obtained as a
white amorphous solid (2.59 g, 100%). Compound 6 was used im-
mediately in the next step.
2.5.2. TFA·H-(Phe-α-hAla)2-OMe (9): From the reaction on the
1.40 g scale of 7 overnight, 9 (1.43 g, 100%) was obtained as a
white amorphous solid. Compound 9 was used immediately in the
next step.
2.6. H-(aza-β3-Ala)-OMe Monomer (11): To a solution of methyl
hydrazine (2.34 g, 50.8 mmol) and NEt3 (6.16 g, 61.0 mmol) in 2.9.2. Boc-(Phe-aza-β3-Ala)3-OMe Hexamer (17): From the reac-
CH2Cl2 (110 mL) at 0 °C was added a solution of methyl 2-bromo-
tion on the 1.98 g scale of 16 in CH2Cl2 (10 mL) overnight, 17
acetate (7.78 g, 50.9 mmol) in CH2Cl2 (150 mL) dropwise over 2.5 h (3.10 g, 94%) was obtained as a white powder, m.p. 168 °C. IR
and the mixture was stirred for 3 h at room temperature. After
evaporation of solvent, Et2O (50 mL) was added and triethylam-
monium salt was precipitated. The filtrate was dried with MgSO4
(ATR): ν
= 3314, 3278, 3221, 3068, 2971, 2850, 1753, 1660,
˜
max
1535, 1503, 1446, 1390, 1374, 1293, 1253, 1208, 1172, 1128 cm–1.
1H NMR (300 MHz, CDCl3): δ = 8.21–7.97 (m, 2 H), 7.88–7.57
10
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Job/Unit: O30567
/KAP1
Date: 15-07-13 12:21:52
Pages: 12
Conformational Behavior of Three Series of Foldamers
(m, 2 H), 7.43–6.95 (m, 16 H), 5.62–5.25 (m, 1 H), 4.57–4.19 (m,
3 H), 3.66 (s, 3 H), 3.62–2.89 (m, 12 H), 2.58 (s, 3 H), 2.53 (s, 3
2.11. Boc-aza-β3-Phe-Ala-OH (23): To a solution of 22 (5.78 g,
26.5 mmol) in CH3CN (50 mL) was added a solution NaOH (1 n,
H), 2.50 (s, 3 H), 1.38 (s, 9 H) ppm. 13C NMR (75 MHz, CDCl3): 2.12 g/50 mL H2O, 53.0 mmol) and the mixture was stirred vigor-
δ = 171.2 (C=O), 171.0 (C=O), 170.5 (C=O), 170.3 (C=O), 170.1
(C=O), 169.7 (C=O), 156.2 (C=O), 137.5 (C Ar), 137.4 (C Ar),
137.3 (C Ar), 130.2 (6 CH Ar), 129.1 (6 CH Ar), 127.3 (3 CH Ar),
80.6 (C), 62.3 (2 CH2), 58.3 (CH2), 55.1 (CH), 54.4 (2 CH), 52.2
ously for 5 h. After evaporation of CH3CN, the aqueous layer was
washed with Et2O (50 mL), acidified with HCl (1 n) until pH 1 and
extracted with CH2Cl2 (2ϫ 50 mL). The residue was dried with
MgSO4, filtered, and evaporated in vacuo to give 23 (9.26 g, 97%)
(CH3), 46.4 (CH3), 45.6 (CH3), 44.4 (CH3), 39.2 (CH2), 38.5 (CH2), as a white foam. 1H NMR (300 MHz, CDCl3): δ = 8.57 (d, J =
38.1 (CH2), 28.8 (3 CH3) ppm. HRMS: calcd. for C42H57N9NaO9
6 Hz, 1 H), 7.42–7.28 (m, 5 H), 5.81 (br. s, 1 H), 4.49 (q, J = 7 Hz,
1 H), 3.94 (d, J = 13 Hz, 1 H), 3.88 (d, J = 13 Hz, 1 H), 3.49 (d, J
= 17 Hz, 1 H), 3.42 (d, J = 17 Hz, 1 H), 1.48 (d, J = 7 Hz, 3 H),
1.35 (s, 9 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 175.5 (C=O),
170.8 (C=O), 155.9 (C=O), 135.1 (C Ar), 129.7 (CH Ar), 128.5
(CH Ar), 128.0 (CH Ar), 80.8 (C), 63.0 (CH2), 59.9 (CH2), 48.2
(CH), 28.2 (3 CH3), 17.1 (CH3) ppm.
[M + Na]+ 854.4177; found 854.4171.
2.9.3. Boc-(Phe-aza-β3-Ala)4-OMe Octamer (19): From the reaction
on the 1.85 g scale of 18 in CH2Cl2 (10 mL) overnight, 19 (3.20 g,
94%) was obtained as a white powder, m.p. 172 °C. IR (ATR): ν
˜
max
= 3265, 3068, 1733, 1656, 1543, 1527, 1503, 1454, 1394, 1374, 1297,
1249, 1221, 1172, 1132 cm–1. 1H NMR (300 MHz, CDCl3): δ =
8.39–7.45 (m, 7 H), 7.39–7.02 (m, 20 H), 5.57 (br. s, 1 H), 4.62–
4.21 (m, 4 H), 3.64 (s, 3 H), 3.58–2.75 (m, 16 H), 2.65–2.38 (m, 12
H), 1.37 (s, 9 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 171.3
(C=O), 171.0 (C=O), 170.8 (C=O), 170.5 (3 C=O), 170.2 (C=O),
169.9 (C=O), 156.3 (C=O), 137.6 (2 C Ar), 137.4 (2 C Ar), 130.2
(4 CH Ar), 130.0 (4 CH Ar), 129.2 (8 CH Ar), 127.5 (4 CH Ar),
80.8 (C), 62.4 (CH2), 62.2 (CH2), 61.9 (CH2), 58.1 (CH2), 55.1
(CH), 54.8 (CH), 54.6 (CH), 54.3 (CH), 52.2 (CH3), 46.4 (3 CH3),
44.5 (CH3), 39.2 (CH2), 38.7 (CH2), 38.0 (2 CH2), 28.9 (3
2.12. General Procedure for the Preparation of Compounds 24 and
26
2.12.1. H-aza-β3-Phe-Ala-OMe (24): Compound 22 (5 g,
13.7 mmol) was treated with a mixture of CH2Cl2/TFA (10 mL/
6 mL) for 12 h. The reaction mixture was diluted in CH2Cl2
(60 mL) and H2O (60 mL), treated carefully whilst stirring with
solid NaHCO3 until basic pH 8–9. The organic layer was isolated
and dried with Na2SO4 and evaporated in vacuo to afford 24
1
+
Na]+
(3.60 g, 99%) as a colorless oil. H NMR (300 MHz, CDCl3): δ =
CH3) ppm. HRMS: calcd. for C54H72N12NaO11 [M
1087.5342; found 1087.5336.
7.60 (d, J = 7 Hz, 1 H), 7.48–7.28 (m, 5 H), 4.67 (q, J = 7 Hz, 1
H), 3.86 (d, J = 13 Hz, 1 H), 3.76 (d, J = 13 Hz, 1 H), 3.78 (s, 3
H), 3.35 (d, J = 16 Hz, 1 H), 3.24 (d, J = 16 Hz, 1 H), 3.15 (br. s,
2 H), 1.45 (d, J = 7 Hz, 3 H) ppm. Compound 24 was used immedi-
ately in the next step.
2.9.4. Boc-(Phe-aza-β3-Ala)5-OMe Decamer (21): From the reac-
tion on the 2.28 g scale of 20 in CH2Cl2 (10 mL) overnight, 21
(3.48 g, 96%) was obtained as a white powder, m.p. 226 °C. IR
(ATR): ν
= 3266, 3217, 3064, 1741, 1656, 1539, 1503, 1450,
˜
max
1394, 1374, 1293, 1249, 1229, 1172, 1132 cm–1. 1H NMR
(300 MHz, [D6]DMSO): δ = 9.41–9.07 (m, 3 H), 8.51–7.89 (m, 5
H), 7.39–7.08 (m, 25 H), 7.05–6.83 (m, 1 H), 5.23 (br. s, 1 H), 4.52–
4.27 (m, 3 H), 4.12–3.95 (m, 1 H), 3.59 (s, 3 H), 3.56–3.42 (m, 1
H), 3.40–3.09 (m, 20 H), 3.05–2.63 (m, 11 H), 2.61–2.22 (m, 4 H),
1.30 (s, 9 H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 172.8
(C=O), 170.0 (C=O), 169.8 (C=O), 169.5 (C=O), 169.1 (2 C=O),
168.8 (2 C=O), 168.7 (C=O), 168.5 (C=O), 155.1 (C=O), 137.8 (2
C Ar), 137.2 (3 C Ar), 129.2 (10 CH Ar), 128.2 (10 CH Ar), 126.4
(5 CH Ar), 78.1 (C), 61.4 (3 CH2), 57.8 (2 CH2), 54.7 (2 CH), 52.7
(3 CH), 51.3 (CH3), 45.3 (3 CH3), 43.6 (2 CH3), 37.8 (3 CH2), 37.4
(2 CH2), 28.1 (3 CH3) ppm. HRMS: calcd. for C66H87N15NaO13
[M + Na]+ 1320.6506; found 1320.6500.
2.12.2. H-(aza-β3-Phe-Ala)2-OMe (26): From the reaction on the
4.8 g scale (8.03 mmol) of 25, compound 26 (4 g, 8.03 mmol) was
obtained as a colorless glassy compound. 1H NMR (300 MHz,
CDCl3): δ = 8.47 (s, 1 H), 8.03 (d, J = 7 Hz, 1 H), 7.33–7.11 (m,
11 H), 4.54 (q, J = 7 Hz, 1 H), 3.87 (q, J = 7 Hz, 1 H), 3.86 (d, J
= 13 Hz, 1 H), 3.80 (d, J = 13 Hz, 1 H), 3.72 (d, J = 13 Hz, 1 H),
3.64 (s, 3 H), 3.56 (d, J = 13 Hz, 1 H), 3.34 (d, J = 16.5 Hz, 1 H),
3.27 (d, J = 15.5 Hz, 1 H), 3.12 (d, J = 16.5 Hz, 1 H), 2.865 (d, J
= 15.5 Hz, 1 H), 2.35 (br. s, 2 H), 1.36 (d, J = 7 Hz, 3 H), 1.26 (d,
J = 7 Hz, 3 H) ppm. Compound 26 was used immediately in the
next step.
2.13. General Procedure for the Preparation of Compounds 25 and
27
2.10. Boc-aza-β3-Phe-Ala-OMe Dimer (22): To a suspension of
HCl·H-Ala-OMe (7 g, 50.0 mmol) under magnetic stirring in
CH2Cl2 (120 mL), was added successively NEt3 (15 g, 147 mmol),
HOBt (9.18 g, 60 mmol), Boc-aza-β3-Phe-OH (14 g, 50 mmol), and
finally EDC (11.5 g, 60 mmol). The mixture homogenized progress-
ively and was stirred for a further 12 h at room temperature. HCl
(1 n, 100 mL) was added and stirring was prolonged until a white
solid precipitated. The solid was filtered and the biphasic liquid
residue was transferred into a funnel. The organic layer was then
washed successively with HCl (1 n, 2ϫ 100 mL), with water
(100 mL), and NaHCO3 (2ϫ 100 mL). Drying over Na2SO4 fol-
lowed by evaporation under reduced pressure gives dimer 22 quan-
titatively (18.2 g) as a pale yellow oil. 1H NMR (300 MHz, CDCl3):
δ = 8.33 (br. s, 1 H), 7.44–7.28 (m, 5 H), 4.54 (q, J = 7 Hz, 1 H),
2.13.1. Boc-(aza-β3-Phe-Ala)2-OMe Tetramer (25): To a solution of
23 (5.5 g, 15.7 mmol) and 24 (3.6 g, 13.6 mmol) in CH2Cl2 (60 mL)
was added HOBt (2.88 g, 18.8 mmol) and EDC (3.61 g,
18.8 mmol). The reaction mixture was stirred 12 h and treated as
depicted above for 22. The residue was purified by column
chromatography (CH2Cl2, then AcOEt, then AcOEt/EtOH) to af-
ford tetramer 25 (5.7 g, 70%) as a white glassy compound. 1H
NMR (500 MHz, CDCl3): δ = 8.44 (s, 1 H), 8.16 (s, 1 H), 7.88 (s,
1 H), 7.41–7.32 (m, 10 H), 5.80 (s, 1 H), 4.53 (q, J = 7 Hz, 1 H),
4.27 (q, J = 7 Hz, 1 H), 4.01 (d, J = 13 Hz, 1 H), 3.99 (d, J =
13 Hz, 1 H), 3.96 (d, J = 13 Hz, 1 H), 3.93 (d, J = 13 Hz, 1 H),
3.70 (s, 3 H), 3.49 (d, J = 17 Hz, 1 H), 3.45 (d, J = 17 Hz, 1 H),
3.39 (d, J = 17 Hz, 1 H), 3.35 (d, J = 16.8 Hz, 1 H), 1.45 (d, J =
3.98 (d, J = 12 Hz, 1 H), 3.94 (d, J = 12 Hz, 1 H), 3.72 (s, 3 H), 7 Hz, 3 H), 1.37 (s, 9 H), 1.25 (d, J = 7 Hz, 3 H) ppm. 13C NMR
3.67 (d, J = 18 Hz, 1 H), 3.46 (d, J = 18 Hz, 1 H), 1.43 (d, J = (125 MHz, CDCl3): δ = 173.4 (C=O), 171.3 (C=O), 170.1 (C=O),
7 Hz, 3 H), 1.36 (s, 9 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 169.1 (C=O), 155.5 (C=O), 135.4 (C Ar), 134.9 (C Ar), 129.6 (2
173.0 (C=O), 169.8 (C=O), 155.8 (C=O), 135.4 (C Ar), 129.6 (CH
Ar), 128.4 (CH Ar), 127.9 (CH Ar), 80.3 (C), 62.9 (CH2), 60.0
(CH2), 52.2 (CH3), 47.8 (CH), 28.1 (3 CH3), 17.3 (CH3) ppm.
CH Ar), 129.3 (2 CH Ar), 128.7 (2 CH Ar), 128.5 (2 CH Ar), 128.2
(CH Ar), 127.9 (CH Ar), 81.1 (C), 62.9 (CH2), 62.4 (CH2), 60.3
(CH2), 60.0 (CH2), 52.2 (CH3), 47.9 (CH), 47.7 (CH), 28.2 (3 CH3),
Eur. J. Org. Chem. 0000, 0–0
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11

Job/Unit: O30567
/KAP1
Date: 15-07-13 12:21:52
Pages: 12
S. Acherar, A. Salaün, P. Le Grel, B. Le Grel, B. Jamart-Grégoire
FULL PAPER
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17.4 (CH3), 16.1 (CH3) ppm. HRMS: calcd. for C30H42N6NaO7 [M
+ Na]+ 621.3013; found 621.3013.
2.13.2. Boc-(aza-β3-Phe-Ala)3-OMe Hexamer (27): From the reac-
tion on the 3.21 g scale (9.16 mmol) of 23 and 3.8 g scale
(7.63 mmol) of 26, hexamer 27 (3.85 g, 61%) was obtained as a
colorless glassy compound. 1H NMR (500 MHz, CDCl3): δ = 8.46
(s, 1 H), 8.30 (s, 1 H), 8.23 (s, 2 H), 7.84 (br. s, 1 H), 7.40–7.24 (m,
15 H), 6.08 (br. s, 1 H), 4.48 (q, J = 7 Hz, 1 H), 4.19 (q, J = 7 Hz,
1 H), 4.11 (q, J = 7 Hz, 1 H), 4.02–3.89 (m, 6 H), 3.70 (s, 3 H),
3.51–3.31 (m, 6 H), 1.43 (d, J = 7 Hz, 3 H), 1.36 (s, 9 H), 1.20 (d,
J = 7 Hz, 3 H), 1.19 (d, J = 7 Hz, 3 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 173.4 (C=O), 171.7 (C=O), 170.5 (2 C=O), 169.9
(C=O), 169.8 (C=O), 155.9 (C=O), 135.8 (C Ar), 135.5 (C Ar),
135.2 (C Ar), 129.7 (2 CH Ar), 129.5 (2 CH Ar), 129.3 (2 CH Ar),
128.5 (2 CH Ar), 128.4 (2 CH Ar), 128.3 (2 CH Ar), 128.0 (2 CH
Ar), 127.8 (CH Ar), 80.8 (C), 62.9 (CH2), 62.5 (CH2), 62.2 (CH2),
60.1 (2 CH2), 59.8 (CH2), 52.3 (CH3), 48.5 (CH), 48.0 (2 CH), 28.2
(3 CH3), 17.2 (CH3), 17.1 (CH3), 16.6 (CH3) ppm. HRMS: calcd.
for C42H57N9NaO9 [M + Na]+ 854.4177; found 854.4171.
Supporting Information (see footnote on the first page of this arti-
1
cle): H and 13C NMR spectra.
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Received: April 18, 2013
Published Online: ᭿
12
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