Reactivity of 1-aminoazetidine-2-carboxylic acid during peptide forming procedures: Observation of an unusual variant of the hydrazino turn

Article abstract of DOI:10.1016/j.tetlet.2012.11.112

Peptide formation on the N-terminal of 1-aminoazetidine-2-carboxylic acid is rendered problematic due to a ring opening reaction. However C-terminal development is possible and two diastereomeric mixed hydrazino dipeptides were prepared. Solution-state studies of these compounds suggest the presence of intramolecular hydrogen bonding, consistent with a hydrazino turn, and the crystal structure of one of these compounds shows a horse-shoe conformation, centered around what appears to be a hydrazino turn involving three hydrogen bond acceptors.

Full text of DOI:10.1016/j.tetlet.2012.11.112

Tetrahedron Letters 54 (2013) 802–805
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reactivity of 1-aminoazetidine-2-carboxylic acid during peptide forming
procedures: observation of an unusual variant of the hydrazino turn
⇑
Amandine Altmayer-Henzien, Valérie Declerck, Régis Guillot, David J. Aitken
Université Paris-Sud, ICMMO, CNRS UMR 8182, 15 rue Georges Clemenceau, 91405 Orsay cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Peptide formation on the N-terminal of 1-aminoazetidine-2-carboxylic acid is rendered problematic due
to a ring opening reaction. However C-terminal development is possible and two diastereomeric mixed
hydrazino dipeptides were prepared. Solution-state studies of these compounds suggest the presence of
intramolecular hydrogen bonding, consistent with a hydrazino turn, and the crystal structure of one of
these compounds shows a horse-shoe conformation, centered around what appears to be a hydrazino
turn involving three hydrogen bond acceptors.
Received 11 September 2012
Revised 8 November 2012
Accepted 14 November 2012
Available online 2 December 2012
Keywords:
Ó 2012 Elsevier Ltd. All rights reserved.
Hydrazino acid
Hydrazino turn
Peptides
Azetidine
Ring opening reaction
Hydrogen bonding
There is an increasing general interest in
a
-hydrazino acids.1
These compounds are of notable potential in the peptidomimetic
field, and the hydrazidic bond is resistant to proteases.2 Modifica-
tion of peptides by inclusion of an
a-hydrazino acid was consid-
ered some time ago,3 although synthetic difficulties have
hampered progress. More recently, a variety of biologically and/
or structurally interesting peptides containing one or several
Figure 1. An a-hydrazino acid (left) and the hydrazino turn feature of a hydrazino
peptide (right). The R groups may be H, alkyl, aryl, or other.
a
-hydrazino acid residues have become more accessible,4 although
selective N-functionalization remains something of a challenge.
Hydrazino peptides can be considered as aza-analogs of b-
peptides, and the presence of the extra nitrogen can give rise to a
structural feature known as a ‘hydrazino turn’ implicating a bifur-
cated intramolecular hydrogen bond (Fig. 1).5
hydrogen bonding was implicated, although no direct evidence
for a hydrazino turn emerged. Indeed, the ring nitrogen atoms of
the aza-ACPC residues appeared to be solvent accessible and could
be protonated without disruption to the secondary structures. Pre-
viously, aza-ACPC had displayed hydrazino turn behavior in crystal
structures of smaller peptides.10
The hydrazino turn dictates conformational preferences in
mixed a-hydrazino/a
-amino acid oligomers4f and in homo hydra-
zino peptides,6 and controls nitrogen chirality in aza-b3-peptides.7
The analogy with b-peptides has attracted further attention from
the foldamer community, and pioneering theoretical studies on
hydrazino peptides suggest that a variety of low-energy regular
helical structures may be accessible.8 As part of a program which
examines oligomers of 2-aminocyclopentane-1-carboxylic acid
(ACPC), oligomers composed of alternating sequences of ACPC
and aza-ACPC were studied, and it was found that the latter resi-
dues easily accommodated the secondary structure type induced
by the former.9 In some sequences, enhanced intramolecular
The lower ring-size homolog of ACPC, 2-aminocyclobutane-1-
carboxylic acid (ACBC),11 also has the ability to promote particular
conformations in dipeptides12 and in longer oligomers.13 We re-
cently prepared the aza-analog of ACBC, 1-aminoazetidine-2-car-
boxylic acid (AAzC), (1), a novel small-ring
a
-hydrazino acid.14
Here, we disclose our findings on the particular reactivity of this
compound as regards hydrazino peptide construction.
In initial studies to determine appropriate conditions for pep-
tide coupling, we used racemic material (Scheme 1). ( )-N-Boc-
AAzC (2) was prepared according to the literature.14 Methyl ester
formation was achieved uneventfully via EDCI/DMAP activation,
to give compound 3 in 83% yield. All attempts to liberate the N-ter-
minal nitrogen, however, were unsuccessful. The numerous efforts
using varied acidic conditions either failed to transform 3 or
⇑
Corresponding author. Tel.: +33 1 6915 3238; fax: +33 1 6915 6278.
E-mail address: david.aitken@u-psud.fr (D.J. Aitken).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.112

A. Altmayer-Henzien et al. / Tetrahedron Letters 54 (2013) 802–805
803
Scheme 2.
The nucleophilic component in each of the above coupling reac-
tions was clearly not the anticipated hydrazino ester 4. The hydro-
gen chloride generated by the reaction of TMSCl with methanol not
only catalyzes the esterification, but presumably also protonates
the more basic ring nitrogen (Scheme 3). By analogy with azetidine
chemistry,15 this quaternarization activates the four-membered
cycle for nucleophilic ring opening by chloride ion. It is possible
that the quaternarization process may be enhanced by a donating
effect from the adjacent nitrogen atom, in analogy with the so-
Scheme 1.
induced an extensive degradation thereof. TLC and NMR spectral
analysis of the crude reaction product indicated several products
were present, and efforts to isolate the desired hydrazine 4 (or
its salt) were fruitless. We decided to obviate the use of acidic con-
ditions to liberate the amine, and decided to study the Cbz-group
protected AAzC ester as an alternative (Scheme 1). In stark contrast
to the behavior of 3, TFA-mediated Boc-cleavage of 2 proceeded
smoothly to furnish ( )-AAzC (1) in near quantitative yield!14
Cbz-group introduction followed by esterification as before pro-
ceeded without incident to provide 6 via 5. Here again, however,
deprotection was problematic. Hydrogenolysis of 6 over palla-
dium-on-charcoal was investigated under various conditions
(catalyst loading, time, solvent). In all cases, complete conversion
was achieved, although the reaction product profile was capricious,
varying considerably from one run to the next. NMR spectral
analysis suggested that the requisite free hydrazine 4 was present
(in one non reproducible case, the yield was around 90%) although
other components were always in evidence. Chromatographic
separation of crude mixtures simply produced further degradation,
so we used crude samples of 4 to attempt peptide coupling
reaction using N-Boc-b-alanine as a model partner (Scheme 1).
Numerous activating reagents were investigated (DCC, EDCI, FDPP,
HATU, HBTU) without success. Finally, the use of IBCF/NMM in THF
provided dipeptide 7 in 28% isolated yield, after repeated chro-
matographic separation from other components.
called
a-effect which is sometimes invoked in connection with
the nucleophilicity of hydrazines.16 Thus the hydrazino ester
component present at the moment when the peptide coupling
reactions are initiated is in fact the hCBA derivative 10, presumably
as its monohydrochloride. Retrospectively, the acid-promoted ring
opening of the AAzC skeleton is a plausible explanation for the
failure to obtain 4 from 3 in Scheme 1.
Hydrazino dipeptide 9 gave crystals amenable to X-ray diffrac-
tion analysis.17 The solid state conformation of the molecule is
illustrated in Figure 2. The pyramidal chiral ring nitrogen adopts
an R configuration, which means the four-membered ring adopts
a trans-like relative configuration with a backbone torsion angle
a
of À96.3°. The 5-atom chain from the ring N to the quaternary
carbon of the t-butyl group is completely extended and occupies
a single plane. The carbonyl group of the AAzC residue is a part
of a regular planar trans amide bond with the Nb of the hCBA
residue (torsion angle 177.8°). The torsion angles of 174.7° for
a
a
a
a
N –C –C@O and À59.9° for H–Nb–N –C of the AAzC residue allow
a network of strong intermolecular hydrogen bonds to link up the
Pursuing our efforts towards dipeptide coupling, we next con-
sidered a procedure intended to provide the hydrochloride salt of
hydrazino ester 4 directly from 1 (Scheme 2). For these studies,
we used enantiomerically pure samples of 1 and 2, obtained
according to the literature.14 Treatment of (R)-1 with TMSCl
(3 equiv) in methanol produced a material which was difficult to
characterize and was used directly in a coupling reaction with
(R)-2 using IBCF/Et3N as the activating system. To our surprise,
the main component, isolated pure in 28% yield after chromatogra-
phy, was hydrazino dipeptide 8, in which the C-terminal residue
was the previously unknown 2-hydrazino-4-chlorobutanoic acid
(hCBA). A repeat of the above procedure using (S)-2 as the intended
N-terminal residue provided the diastereomeric hydrazino
dipeptide 9 in 34% yield.
Scheme 3.

804
A. Altmayer-Henzien et al. / Tetrahedron Letters 54 (2013) 802–805
provide the hCBA residue suggests that other side-chain substi-
tuted -hydrazino butanoic acids might be available via 1.
a
Nonetheless, the preparation of dipeptides 8 and 9 shows that
C-terminal coupling of AAzC can be achieved without difficulty,
and conformational studies suggest an interesting folding potential
for hydrazino peptides bearing AAzC at the C-terminal. Future
work will be directed by this premise.
Acknowledgments
A. A.-H. is grateful to the French Ministry of Higher Education
and Research for an Allocation Spécifique pour Normalien doctoral
research Grant. We thank COST (Action CM 0803) for travel funds
allowing constructive networking in relation to this work. We also
wish to acknowledge the constructive comments made by the
referees.
Supplementary data
Supplementary data (experimental procedures, characteriza-
tion data for new compounds) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2012.11.112.
Figure 2. X-ray structure of hydrazino dipeptide 9.
References and notes
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molecules in the lattice (C@OÁ Á ÁH–N distance 2.04 Å), and facilitate
an 8-membered turn feature linking the AAzC Boc C@O and the
hCBA Nb–H (C@OÁ Á ÁH–N distance 2.52 Å) within each molecule
(Fig. 2). The AAzC ring nitrogen is at a distance of 2.27 Å from
the hCBA Nb–H and, although the geometrical alignment is not
optimal due to ring constraints, a hydrazino turn would appear
to be in evidence. What is intriguing is that the hydrazine moiety
of the hCBA residue is twisted so that the C-terminal can bend back
and present the sp3 oxygen of the methyl ester towards Nb–H. The
OÁ Á ÁH–N distance (2.52 Å) and the geometry (OÁ Á ÁH–N bond angle
111.1°) are at the acceptable upper limit for constituting a hydro-
gen bond. There are no other significant close contacts (hydrogen
bonds or packing effects) in evidence which might induce this
horse-shoe conformation for the molecule. The hCBA Nb–H thus
appears to be involved in a modified hydrazino turn in which three
hydrogen bond acceptors are implicated. On one previous occasion,
the carbonyl oxygen (as opposed to the alkoxy oxygen) of a methyl
ester-capped hydrazino peptide was observed to make an intra-
residue H-bond contact with the hydrazidic NH in the solid state.6b
We obtained some support for the existence of the hydrazino
turn feature for each of the dipeptide 8 and 9 in solution. When
a 60 mM CDCl3 solution of either peptide was treated with metha-
nol-d4 (30 equiv) the change in chemical shifts in the 1H NMR spec-
5. Aubry, A.; Bayeul, D.; Mangeot, J.-P.; Vidal, J.; Sterin, S.; Collet, A.; Lecoq, A.;
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Protein Res. 1994, 43, 305–311.
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7097; (b) Fernandes, C.; Gauzy, C.; Yang, Y.; Roy, O.; Pereira, E.; Faure, S.;
Aitken, D. J. Synthesis 2007, 2222–2232; (c) Fernandes, C.; Pereira, E.; Faure, S.;
Aitken, D. J. J. Org. Chem. 2009, 74, 3217–3220; (d) Declerck, V.; Aitken, D. J.
Amino Acids 2011, 41, 587–595.
tra was significant for the AAzC hydrazidic protons (
Dd = 0.84 ppm
for 8; 0.79 ppm for 9) but not for the hCBA hydrazidic protons
(Dd = 0.08 ppm for 8; 0.07 ppm for 9), suggesting that the latter
are involved in intramolecular hydrogen bonds.18 In the N–H
stretching frequency range of the solution state IR spectra of pep-
tides 8 and 9 (10 mM solutions in CHCl3), two absorptions were ob-
12. Torres, E.; Gorrea, E.; Da Silva, E.; Nolis, P.; Branchadell, V.; Ortuño, R. M. Org.
Lett. 2009, 11, 2301–2304.
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Aitken, D. J. Org. Lett. 2010, 12, 3606–3609; (b) Torres, E.; Gorrea, E.; Burusco, K.
K.; DaSilva, E.; Nolis, P.; Rúa, F.; Boussert, S.; Díez-Pérez, I.; Dannenberg, S.;
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served in each case, at around 3360 and 3300 cmÀ1
. The
appearance of the latter band provides strong evidence for an
intramolecularly hydrogen-bonded hydrazidic NH, consistent with
the hydrazino turn.
In summary, the synthetic work described here underlines the
particular difficulties which may be encountered in the prepara-
tion of hydrazino peptides of AAzC by coupling at the N-terminal.
This objective looks likely to remain a considerable challenge,
although the ring opening during the liberation of the amine to
14. Declerck, V.; Aitken, D. J. J. Org. Chem. 2011, 76, 708–711.
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Dechoux, L.; Hamon, L.; Kadouri-Puchot, C. Eur. J. Org. Chem. 2007, 4517–4524;
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De Kimpe, N. J. Org. Chem. 2012, 77, 5982–5992.
16. For a review and discussion, see: (a) Buncel, E.; Um, I.-H. Tetrahedron 2004, 60,
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805
17. CCDC 893998 contains the crystallographic data for compound 9. These data
can be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
each of the peptides were also difficult to interpret: in the range 300–325 K,
d/
D
D
T values were about À6 ppb/K for hCBA residues and À4 ppb/K for AAzC
residues. Care in interpreting such data has been advocated before (Ref. 9):
relatively quick proton exchange in azapeptides is not necessarily an indication
of conformational inhomogeneity (see also Ref. 2), and temperature gradient
values cannot be directly compared to values observed for amide hydrogens
due to the increased H-bond acidity.
18. Other 1H NMR data obtained for hydrazidic NH atoms were inconclusive.
Extensive H/D isotopic exchange in the methanol-d4 dilution experiment was
observed after only 5 min for both the AAzC (22% H signal remaining in 8, 12%
in 9) and the hCBA (10% H signal remaining in both 8 and 9) hydrazidic NH
atoms. The chemical shift temperature gradients of 60 mM CDCl3 solutions of