Inverse peptide synthesis via activated α-aminoesters

Article abstract of DOI:10.1002/anie.201402147

A mild, practical, and simple procedure for peptide-bond formation is reported. Instead of activation of the carboxylic acid functionality, the reaction involves an unprecedented use of activated α-aminoesters. The method provides a straightforward entry to dipeptides and was effective when a sensitive cysteine residue was used, as no epimerization was detected in this case. The applicability of this method to iterative peptide synthesis was illustrated by the synthesis of a model tetrapeptide in the challenging reverse N→C direction. How to advance by going into reverse: In a mild and practical procedure for peptide-bond formation, free α-aminoesters were activated by treatment with N,N′-carbonyldiimidazole, instead of activating the carboxylic acid functionality (see scheme). The method provided a straightforward route to dipeptides, and its applicability to iterative peptide synthesis was illustrated by the synthesis of a tetrapeptide in the challenging reverse N→C direction.

Full text of DOI:10.1002/anie.201402147

Angewandte
Chemie
DOI: 10.1002/anie.201402147
Peptide Synthesis
Inverse Peptide Synthesis via Activated a-Aminoesters**
Jean-Simon Suppo, Gilles Subra, Matthieu Bergꢀs, Renata Marcia de Figueiredo,* and Jean-
Marc Campagne*
Dedicated to Professor Jacques Coste
Abstract: A mild, practical, and simple procedure for peptide-
bond formation is reported. Instead of activation of the
carboxylic acid functionality, the reaction involves an unpre-
cedented use of activated a-aminoesters. The method provides
a straightforward entry to dipeptides and was effective when
a sensitive cysteine residue was used, as no epimerization was
detected in this case. The applicability of this method to
iterative peptide synthesis was illustrated by the synthesis of
a model tetrapeptide in the challenging reverse N!C direction.
an amino acid is transformed into a good leaving group (X =
ꢀ
O acyl, urea, oxyphosphonium, guanidinium/uronium, F,
B
esides their crucial physiological life functions, peptides
are attracting increasing attention as drugs[1] owing to their
associated low toxicity and high specificity, but also as
valuable auxiliaries for targeting,[2] vecto-
rization,[3] and diagnostic purposes. Fur-
thermore, as a result of their very nearly
unlimited diversity of activity, structure,
and functions, they are also now studied
for other promising applications in mate-
rials science, such as artificial silks,[4]
hydrogels,[5] supported catalysts, and bio-
compatible matrices for cell growth.
Scheme 1. Ribosomal and chemical peptide synthesis.
Whereas peptides are naturally built
up in the N!C direction[6] (ribosomal
synthesis), they are chemically synthe-
sized from the C-terminal to the N-
terminal end (Scheme 1) either by
means of solid-phase peptide synthesis
(SPPS) or by classical solution synthesis.[7]
Traditionally, peptides are assembled
through the use of coupling reagents,[8]
whereby the carboxylic acid moiety of
Scheme 2. a) Conventional peptide-bond formation through carboxylic activation. b) This
study: peptide-bond formation via activated a-aminoesters. P=amine-protecting group, R1
and R2 =amino acid side chains, R=acid-protecting group, LG=leaving group, Act=activa-
tion agent.
etc.), thus enabling the amine functionality of a second
[*] J.-S. Suppo, M. Bergꢀs, Dr. R. Marcia de Figueiredo,
Prof. Dr. J.-M. Campagne
amino acid to react with it to form the amide bond
(Scheme 2a). Although highly efficient, these coupling tech-
niques suffer from some drawbacks. For example, epimeriza-
tion could be observed during the introduction of sensitive
amino acids (histidine and cysteine) and, more importantly,
these conditions force peptide synthesis in the C!N direc-
tion. In fact, the synthesis of a whole peptide in the reverse
way (i.e. the N!C direction) would imply the C-terminal
activation of a peptide fragment highly prone to epimeriza-
tion (through the formation of oxazolone[9] intermediates)
and thus the formation of an intractable diastereoisomeric
mixture.
Institut Charles Gerhardt Montpellier (ICGM), UMR 5253 CNRS-
UM2-UM1-ENSCM, Ecole Nationale Supꢁrieure de Chimie
8 Rue de l’Ecole Normale, 34296 Montpellier Cedex 5 (France)
E-mail: renata.marcia_de_figueiredo@enscm.fr
jean.marc-campagne@enscm.fr
Prof. Dr. G. Subra
Institut des Biomolꢁcules Max Mousseron (IBMM)
UMR 5257 CNRS-UM2-UM1, LAPP-Facultꢁ de Pharmacie
Avenue Charles Flahault, Bat E, 34093 Montpellier Cedex 5 (France)
[**] We gratefully thank the Agence Nationale de la Recherche for
financial support (NIPS project ANR JCJC): ANR-12-JS07-0008-01
and Jonathan Brand for preliminary studies on this project.
A number of innovative strategies for the synthesis of
peptides have been reported: decarboxylative condensation
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402147.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
Table 1: Peptide-bond formation by the use of activated a-aminoesters.[a]
of N-alkyl hydroxylamines and a-ketoacids,[10] the use of
nitroalkanes as acyl-anion equivalents,[11] and various strat-
egies involving amino thioacids as the acyl donor with
isonitriles,[12] with azides,[13] and very recently, with dithiocar-
bamate terminal amines.[14,15] However, all these methods
require the prior modification of both amino acids, which
considerably limits their attractiveness. The reaction of
carboxylic acids with isocyanates also constitutes an interest-
ing alternative,[16] although the need to prepare the sensitive
isocyanate reaction partners hinders the application of this
methodology in peptide synthesis.[17]
Herein, we describe our preliminary results towards a new
route for the preparation of peptides from readily available,
bench-stable, activated a-aminoesters under mild and neutral
conditions (Scheme 2b). In contrast to all previously reported
methodologies, we first activate the amino group, instead of
the classical carboxylic activation; therefore, we expect to
palliate the epimerization process that is generally observed
in conventional coupling methods.
The starting point of our study was the synthesis of
a suitable (simple, high-yielding, and stable) activated a-
aminoester. After some unfruitful experiments involving p-
nitrophenol derivatives,[18] we turned to commercially avail-
able N,N’-carbonyldiimidazole (CDI),[19,20] which has been
broadly employed as a carboxyl-activating agent, and has
proved its efficiency in peptide coupling over conventional
peptide synthesis.
We prepared intermediates 1 in high yield by the treat-
ment of free a-aminoesters with CDI and triethylamine in
a CH2Cl2/THF mixture (see the Supporting Information for
details).[21] Small amounts (< 7%) of symmetrical ureas were
occasionally observed as side products in the crude material.
Compounds 1 were readily purified by a simple filtration
through a short plug of silica gel. Moreover, they are stable for
months when stored at 48C.
Entry
Additive
Reaction time [h]
Yield [%][b]
1
2
3
4
5
6
7
8
9
Et3N (1.0 equiv)
–
–
–
imidazole (10 mol%)
HOBt (10 mol%)
PTSA (10 mol%)
CuBr2 (10 mol%)
CuBr2/HOBt (10 mol%)
14
14
20
20
20
20
20
20
20
NR
63
23[c]
62[d]
53
75
77
77
90
[a] Reactions were performed on a 0.51 mmol scale and proceeded to full
conversion as judged by TLC. [b] Yield of the isolated product after
purification by column chromatography. [c] DMF was used as the solvent
(1.0m). [d] CH3NO2 was used as the solvent (1.0m). DMF=dimethyl-
formamide, NR=no reaction, HOBt=1-hydroxybenzotriazole,
PTSA=4-methylbenzenesulfonic acid.
of the expected dipeptide 2a was improved to 90% (Table 1,
entry 9).
Having established that amide-bond formation can take
place when activated a-aminoesters are used instead of
activation of the acid functionality, we next focused on the
scope of this reaction (Table 2) and attempted the synthesis of
a range of dipeptides. Under the optimized conditions with
CuBr2/HOBt, all reactions proceeded cleanly, and the
expected dipeptides were isolated in moderate-to-good
yields. Coupling efficiency was not significantly affected by
the different side chains (Table 2, entries 4, 6, 8, and 11).
Interestingly, the reaction proved to be fully compatible with
With these compounds in hand, we carried out a first
experiment with compound 1a and Boc-Phe-OH to validate
our approach (Table 1). In the presence of triethylamine in
CH2Cl2, no dipeptide formation was observed (Table 1,
entry 1). However, and quite intriguingly, when the reaction
was carried out in the absence of a base, we observed the
formation of the expected dipeptide 2a in 63% yield (Table 1,
entry 2). Brief solvent screening revealed that the reaction
proceeded smoothly in CH2Cl2 as well as in CH3NO2, another
polar aprotic solvent, whereas the solvent DMF, which is
traditionally suitable for supported peptide synthesis, gave
lower yields (Table 1, entries 3 and 4). Next, to improve the
yield, we evaluated the addition of some additives
(10 mol%). The same reaction profile was observed when
imidazole was added to the reaction mixture (Table 1,
entry 5). However, in the presence of HOBt or PTSA, we
observed higher yields (Table 1, entries 6 and 7). After
evaluating the use of Brønsted acids, we also tested the
viability of the reaction in the presence of Lewis acid
additives. Among many Lewis acids tested, CuBr2 proved to
be as efficient as HOBt or PTSA (Table 1, entry 8). Interest-
ingly, when used together as additives, CuBr2 and HOBt had
a synergic effect on the condensation reaction, and the yield
Table 2: Scope of the reaction for the synthesis of dipeptides.
Entry Imi-AA-OR 1
Dipeptide 2
Yield [%][a]
1
2
3
4
5
Imi-Ala-OMe (1a) Boc-Phe-Ala-OMe (2a)
90
65
61
67
84
1a
1a
1a
1a
1a
Fmoc-Phe-Ala-OMe (2b)
Boc-Pro-Ala-OMe (2c)
Boc-Cys(Bn)-Ala-OMe (2d)
Cbz-Met-Ala-OMe (2e)
6
Fmoc-Lys(Alloc)-Ala-OMe (2 f) 54
7
8
9
10
11
12
Imi-Val-OMe (1b) Boc-Phe-Val-OMe (2g)
62
74
66
68
70
52
Imi-Gly-OEt (1c)
1c
Boc-Asp(OBn)-Gly-OEt (2h)
Fmoc-Ala-Gly-OEt (2i)
Imi-Met-OMe (1d) Cbz-Phe-Met-OMe (2j)
1d
1d
Boc-Trp-Met-OMe (2k)
Fmoc-Ala-Met-OMe (2l)
[a] Isolated yield after purification by column chromatography. Boc=tert-
butoxycarbonyl, Fmoc=9-fluorenylmethoxycarbonyl, Bn=benzyl,
Cbz=benzyloxycarbonyl, Alloc=allyloxycarbonyl.
2
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
Boc-, Fmoc- and Cbz-protected amino acids,[22] an important
issue for the method to be applied in routine peptide
synthesis. Slightly lower, but still respectable, yields were
observed with Fmoc derivatives, mainly owing to their lower
solubility under these reaction conditions (Table 2, entries 6
and 12).
Having developed a straightforward route to dipeptides,
we next investigated the use of our methodology in cases in
which classical coupling reagents suffer from some limita-
tions. If our hypothesis is right, and our method does not
imply an activation of the carboxylate functionality, some of
the side reactions commonly observed with classical peptide
reagents should be alleviated. For example, it is well-known
that the activation of cysteine residues may occur with some
degree of epimerization as a result of the transient formation
of a stabilized enol derivative.[23] Indeed, in our hands, when
Boc-Cys(Bn)-OH was coupled to H-Ala-OMe (3) after
preactivation for 1 min in CH2Cl2 with BOP,[24] 12% epime-
rization was observed (35% epimerization after preactivation
for 5 min; Table 3, entries 2 and 3).[25] As suggested by one
functionality, 8% epimerization was observed (Table 3,
entry 6). This observation further corroborates our mecha-
nistic proposal (Scheme 2b), as no cysteine epimerization was
observed under our reaction conditions.
We next checked whether this protocol could be extended
to longer peptides. As a proof of concept, the synthesis of
Cbz-Trp-Met-Val-Phe-OtBu (4) tetrapeptide was performed
in the N!C direction. As mentioned above, N!C peptide
synthesis is not recommended, since the activation of the C-
terminal carboxylic acid of an elongating peptide fragment is
a source of potential epimerization through oxazolone
formation (see below). For the sake of comparison, the
same tetrapeptide 4 was assembled by PyBOP[26] activation in
the same N!C direction (Scheme 3).
Table 3: Racemization study on the synthesis of Boc-Cys(Bn)-Ala-OMe
dipeptide.
Entry Conditions
Yield
[%][a]
Epimerization
[%][b]
1
2
3
4
5
this study[c]
67
87
80
41
0
35
BOP, DIPEA (4.0 equiv)[d]
BOP, DIPEA (4.0 equiv)[d,e]
BOP, DIPEA (2.0 equiv)[d]
12
7[f]
BOP, DIPEA (1.0 equiv), H2N-Ala- 27
1.5[f]
OMe[d]
6
CDI carboxylic acid activation[g]
80
8
[a] Yield of the isolated product after column chromatography. [b] The
degree of racemization of the crude material was determined by HPLC on
a chiral stationary phase (for details and chromatograms, see the
Supporting Information). [c] Reaction conditions: 1a, Boc-Cys(Bn)-OH,
CuBr2/HOBt (10 mol%), 20 h. [d] Reaction conditions: BOP (1.5 equiv),
DIPEA, preactivation for 5 min, 3, 2 h. [e] Preactivation for 1 min. [f] The
epimerization level was estimated owing to the complexity of the HPLC
chromatogram of the crude product. [g] Reaction conditions: Boc-
Cys(Bn)-OH, CDI (1.0 equiv), preactivation for 5 min, 3, 20 h. BOP=
(benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluoro-
phosphate.
Scheme 3. Iterative N!C synthesis of tetrapetide 4. a,b) HPLC profiles
of the tetrapeptides synthesized by the PyBOP strategy (carboxylate
activation; a) and by CDI-mediated a-aminoester activation (b). For
details, see the Supporting Information. PyBOP=(benzotriazol-1-yl-
oxy)tripyrrolidinophosphonium hexafluorophosphate.
The synthesis proceeded in five linear steps in a straight-
forward manner (Scheme 3). Indeed, the reactions were very
easy to set up and, conveniently, after each peptide coupling,
a classical workup of the crude material, followed by filtration
through a short plug of silica gel, was carried out to eliminate
excess reagents and by-products. The crude compounds were
clean enough to be used in the next step (i.e. TFA-mediated
tBu deprotection; see the Supporting Information for details).
At the end of the synthesis, comparable yields and HPLC
profiles of the crude product (see the Supporting Informa-
tion) were observed for the two methods, carboxylic acid
activation with PyBOP and CDI-mediated a-aminoester
referee, several amounts of base were surveyed, and indeed,
lowering the amount of the base was very effective in
minimizing epimerization, but also deleterious to the chem-
ical yield of the BOP-mediated coupling reactions (Table 3,
entries 4 and 5). Alternatively, the condensation of Imi-Ala-
OMe (1a) with Boc-Cys(Bn)-OH in CH2Cl2 gave the
expected dipeptide in 67% yield with no detectable epime-
rization in the crude product (Table 3, entry 1). Notably, when
CDI was used as the activating group of the carboxyl
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
MgSO4, and the solvents were evaporated. Purification by column
chromatography on silica gel afforded the corresponding dipeptide 2.
activation. Purification of the product by preparative HPLC
gave 4 in 35 and 25% overall yield for the PyBOP and CDI
strategies, respectively. The HPLC profiles were similar and
very clean (Scheme 3). A minor product (probably resulting
from epimerization) was observed in the PyBOP synthesis,
whereas this product was absent in the CDI synthesis.[27]
In summary, we have presented herein a practical and
simple procedure for the formation of peptide bonds on the
basis of a new mode of activation of amino acids (Scheme 4).
A prominent feature of this strategy is the unprecedented use
Received: February 6, 2014
Published online: && &&, &&&&
Keywords: amides · amino acids · epimerization · peptides ·
.
synthetic methods
[1] a) “Therapeutic Peptides under the Spotlight”: C. Pichereau, C.
Allary in European Biopharmaceutical Review (EBR), January
2005; b) V. Marx, Chem. Eng. News 2005, 83, 17; c) L. M. Jarvis,
Chem. Eng. News 2006, 84, 23; d) P. Vlieghe, V. Lisowski, J.
Martinez, M. Khrestchatisky, Drug Discovery Today 2010, 15,
40; e) A. Sharman, S. Arora, Res. J. Pharm. Biol. Chem. Sci.
2011, 2, 778.
[2] N. Svensen, J. J. Dꢁaz-Mochꢂn, K. Dhaliwal, S. Planonth, M.
Dewar, J. D. Armstrong, M. Bradley, Angew. Chem. 2011, 123,
6257; Angew. Chem. Int. Ed. 2011, 50, 6133.
[3] C. M. Yamazaki, I. Nakase, H. Endo, S. Kishimoto, Y.
Mashiyama, R. Masuda, S. Futaki, T. Koide, Angew. Chem.
2013, 125, 5607; Angew. Chem. Int. Ed. 2013, 52, 5497.
[4] Y. Hidala, K.-I. Kontani, R. Taniguchi, M. Saiki, S. Yokoi, K.
Yukuhiro, H. Yamaguchi, M. Miyazawa, Biopolymers 2011, 96,
222.
[5] R. P. Nagarkar, J. P. Schneider, Methods Mol. Biol. 2008, 474, 61.
[6] For selected references on inverse SPPS, see: a) A. Johansson, E.
ꢃkerblom, K. Ersmark, G. Linderberg, A. Hallberg, J. Comb.
Chem. 2000, 2, 496; b) B. H. Lipshutz, Y.-J. Shin, Tetrahedron
Lett. 2001, 42, 5629; c) N. Thieriet, F. Guibꢄ, F. Albericio, Org.
Lett. 2000, 2, 1815; d) W. G. Gutheil, Q. Xu, Chem. Pharm. Bull.
2002, 50, 688; e) R. Sasubilli, W. G. Gutheil, J. Comb. Chem.
2004, 6, 911; f) J. Martinez, P. Zajdel, M. Pawlowski, G. Subra,
PCT Int. Appl., WO2010023295 (A1) 20100304, 2010.
[7] a) J. Jones, The Chemical Synthesis of Peptides, Oxford Science
Publications, Oxford, 1991; b) “Synthesis of Peptides and
Peptidomimetics”: M. Goodman, A. Felix, L. Moroder, C.
Toniolo, Houben-Weyl, Methods in Organic Chemistry,
Vol E22a, Thieme, Stuttgart, New York, 2002; c) N. Sewald,
H.-D. Jakubke, Peptides: Chemistry and Biology, Wiley-VCH,
Weinheim, 2002; d) F. Guzmꢅn, S. Barberis, A. Illanes, Electron.
J. Biotechnol. 2007, 10, 279.
[8] For selected reviews, see: a) F. Albericio, R. Chinchilla, D. J.
Dodsworth, C. Nꢅjera, Org. Prep. Proced. Int. 2001, 33, 203;
b) S.-Y. Han, Y.-A. Kim, Tetrahedron 2004, 60, 2447;
c) C. A. G. N. Montalbetti, V. Falque, Tetrahedron 2005, 61,
10827; d) E. Valeur, M. Bradley, Chem. Soc. Rev. 2009, 38, 606;
e) A. El-Faham, F. Albericio, Chem. Rev. 2011, 111, 6557;
f) M. M. Joulliꢄ, K. M. Lassen, ARKIVOC 2010, 189; g) M.
Amblard, J.-A. Fehrentz, J. Martinez, G. Subra, Mol. Biotechnol.
2006, 33, 239; h) R. M. Lanigan, T. D. Sheppard, Eur. J. Org.
Chem. 2013, 7453.
Scheme 4. CDI-mediated activation of a-aminoester derivatives.
of activated a-aminoesters instead of classical carboxylic
activation. Our activated amino acid residues (i.e. Imi-AA-
OR) are readily obtained from a-aminoesters and readily
available and inexpensive CDI. Additionally, they are stable
and can be stored at 48C for several months. Peptide-bond
formation proceeds under mild conditions in the absence of
a base and is compatible with common N-urethane protecting
groups (i.e. Fmoc, Boc and Cbz). In this study, preliminary
investigations on the susceptibility to racemization were
carried out with the sensitive amino acid residue cysteine [that
is, with Boc-Cys(Bn)-OH]. The promising results obtained
prompted us to further evaluate our method with the
synthesis of a tetrapeptide in the reverse N!C direction.
The desired tetrapeptide was isolated in good overall yield
and purity. The extension of this new strategy to the synthesis
of longer peptide targets and to solid-phase peptide synthesis
(SPPS) is currently under investigation.
[9] M. Goodman, L. Levine, J. Am. Chem. Soc. 1964, 86, 2918, and
references therein.
[10] J. W. Bode, R. M. Fox, K. D. Baucom, Angew. Chem. 2006, 118,
1270; Angew. Chem. Int. Ed. 2006, 45, 1248.
Experimental Section
[11] B. Shen, D. M. Makley, J. N. Johnston, Nature 2010, 465, 1027.
[12] T. Wang, S. J. Danishefsky, J. Am. Chem. Soc. 2012, 134, 13244.
[13] a) R. V. Kolakowski, N. Shangguan, R. R. Sauers, L. J. Williams,
J. Am. Chem. Soc. 2006, 128, 5695; b) G. H. Hakimelahi, G. Just,
Tetrahedron Lett. 1980, 21, 2119.
[14] W. Chen, J. Shao, M. Hu, W. Yu, M. A. Giulianotti, R. A.
Houghten, Y. Yu, Chem. Sci. 2013, 4, 970.
[15] For strategies involving the synthesis of nonpeptidic carbox-
amides, see: a) S. H. Cho, E. J. Yoo, I. Bae, S. Chang, J. Am.
Chem. Soc. 2005, 127, 16046; b) M. P. Cassidy, J. Raushel, V. V.
Preparation of dipeptides 2: Copper bromide (10 mol%), 1-hydrox-
ybenzotriazole hydrate (10 mol%), and P-AA2-OH (1.5 equiv) were
successively added to a solution of the Imi-AA1-OR derivative
(1.0 equiv) in CH2Cl2 (1.0m) at room temperature (228C), and the
resulting blue suspension was stirred at this temperature for 20 h.
After the completion of the reaction (TLC monitoring), the mixture
was washed once with aqueous HCl (0.5n). The aqueous phase was
then extracted with CH2Cl2 (2 ꢀ). The organic layers were combined,
washed with saturated aqueous NaHCO3 and brine, and dried over
4
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
Fokin, Angew. Chem. 2006, 118, 3226; Angew. Chem. Int. Ed.
2006, 45, 3154; c) J. Gao, G.-W. Wang, J. Org. Chem. 2008, 73,
2955; d) W.-J. Yoo, C.-J. Li, J. Am. Chem. Soc. 2006, 128, 13064;
e) L. U. Nordstrøm, H. Vogt, R. Madsen, J. Am. Chem. Soc.
2008, 130, 17672; f) N. Gernigon, R. M. Al-Zoubi, D. G. Hall, J.
Org. Chem. 2012, 77, 8386.
Yoshina-Ishii, S. D. Taylor, V. Santhakumar, Tetrahedron Lett.
1999, 40, 2669; c) J. A. Grzyb, R. A. Batey, Tetrahedron Lett.
2003, 44, 7485; d) J. A. Grzyb, M. Shen, C. Yoshina-Ishii, W. Chi,
R. S. Brown, R. A. Batey, Tetrahedron 2005, 61, 7153.
[20] R. Paul, G. W. Anderson, J. Am. Chem. Soc. 1960, 82, 4596.
[21] a) S. T. Heller, R. Sarpong, Org. Lett. 2010, 12, 4572; b) S. T.
Heller, T. Fu, R. Sarpong, Org. Lett. 2012, 14, 1970.
[22] A. Isidro-Llobet, M. ꢉlvarez, F. Albericio, Chem. Rev. 2009, 109,
2455.
[23] H. Li, X. Jiang, Y.-h. Ye, C. Fan, T. Romoff, M. Goodman, Org.
Lett. 1999, 1, 91.
[24] B. Castro, J. R. Dormoy, G. Evin, C. Selve, Tetrahedron Lett.
1975, 16, 1219.
[25] For additional details, NMR spectra, and HPLC chromatograms
for all reactions in the racemization study on the synthesis of
Boc-Cys(Bn)-Ala-OMe dipeptide, see the Supporting Informa-
tion.
[26] a) J. Coste, D. Le-Nguyen, B. Castro, Tetrahedron Lett. 1990, 31,
205; b) E. Frꢄrot, J. Coste, A. Pantaloni, M.-N. Dufour, P. Jouin,
Tetrahedron 1991, 47, 259.
[16] a) F. Jꢆger, P. Stelzel, P. Thamm, G. Wendlberger, Houben-Weyl,
Vol. 15/2, Thieme, Stuttgart, 1974, p. 184;b) A. C. Schuemacher,
R. W. Hoffmann, Synthesis 2001, 243; c) K. Sasaki, D. Crich,
Org. Lett. 2011, 13, 2256; d) D. Crich, K. Sasaki, Org. Lett. 2009,
11, 3514; e) I. S. Blagbrough, N. E. Mackenzie, C. Ortiz, A. I.
Scott, Tetrahedron Lett. 1986, 27, 1251.
[17] For a few examples, see: a) S. Goldschmidt, M. Wick, Justus
Liebigs Ann. Chem. 1952, 575, 217; b) S. Goldschmidt, H.
Lautenschlager, Justus Liebigs Ann. Chem. 1953, 580, 68; c) G.
Losse, H. Vietmeyer, J. Prakt. Chem. 1966, 32, 204.
[18] Matthieu Bergꢇs, Masterꢈs Thesis, Universitꢄ Montpellier 2,
2010.
[19] For the use of N-methylimidazolium salt derivatives in the
synthesis of ureas, thioureas, carbamates, and thiocarbamates,
see: a) R. A. Batey, V. Santhakumar, C. Yoshina-Ishii, S. D.
Taylor, Tetrahedron Lett. 1998, 39, 6267; b) R. A. Batey, C.
[27] LC/MS chromatograms showed an isobaric profile for tetrapep-
tide 4 derived from the PyBOP reaction and its major impurity.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Peptide Synthesis
J.-S. Suppo, G. Subra, M. Bergꢁs,
R. Marcia de Figueiredo,*
J.-M. Campagne*
&&&& — &&&&
Inverse Peptide Synthesis via Activated a-
Aminoesters
How to advance by going into reverse: In
a mild and practical procedure for pep-
tide-bond formation, free a-aminoesters
were activated by treatment with N,N’-
carbonyldiimidazole, instead of activating
the carboxylic acid functionality (see
scheme). The method provided
a straightforward route to dipeptides, and
its applicability to iterative peptide syn-
thesis was illustrated by the synthesis of
a tetrapeptide in the challenging reverse
N!C direction.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!