Peptide synthesis in room temperature ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2003.12.124

We demonstrate that chemical peptide coupling using modern coupling agents is efficient in rt ionic liquids. This new approach presents some advantages, especially in the case of hindered amino acids, which are not easy to couple under standard conditions

Full text of DOI:10.1016/j.tetlet.2003.12.124

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1617–1619
Peptide synthesis in room temperature ionic liquids
a
b
b
c
ꢀ ꢁ
ꢀ
Helene Vallette, Laurent Ferron, Gerard Coquerel, Annie-Claude Gaumont
and Jean-Christophe Plaqueventa,*
aUMR-CNRS 6014 de l’IRCOF, Universitꢀe de Rouen, rue Tesniꢁere, F-76821 Mont Saint Aignan Cedex, France
bUPRES-EA 2659, SMS, IRCOF, Universitꢀe de Rouen, rue Tesniꢁere, F-76821 Mont-Saint Aignan Cedex, France
cUMR-CNRS 6507 LCMT, ENSICAEN-ISMRA, Universitꢀe de Caen, 6 bd du Marꢀechal Juin, F-14050 Caen, France
Received 28 November 2003; revised 19 December 2003; accepted 23 December 2003
Abstract—We demonstrate that chemical peptide coupling using modern coupling agents is efficient in rt ionic liquids. This new
approach presents some advantages, especially in the case of hindered amino acids, which are not easy to couple under standard
conditions, since high purities for the crude peptides were observed with respect to coupling in classical solvents.
Ó 2004 Elsevier Ltd. All rights reserved.
The unique properties of room temperature ionic liquids
(RTILÕs) present many advantages in the context of
green chemistry.1 As part of our program focusing on
chemistry in ionic liquids,2 including asymmetric syn-
thesis and chirality studies,3 we describe herein pre-
liminary results on peptide synthesis. As outlined in
recent reviews,1 a number of classical organic reactions
have been examined with success in these new media. To
the best of our knowledge (and rather curiously with
regard to the importance of this topic) no report
describes chemical peptide synthesis in ionic liquids;
only a study from Erbeldinger et al. was devoted to
enzymatic access to (Z)-aspartame.4 We decided to
embark on this approach by considering ÔmodernÕ cou-
pling agents such as HATU and BOP as reagents of
choice for this chemistry in ionic liquids, since they
present structural similarities with neoteric solvents,
such as [bmim]PF6 (Fig. 1). Thus we expected these
coupling agents to be easily dissolved in ionic liquids,
and sufficiently stabilized by the solvent to give slow and
selective reactions.
As a first approach, we chose to examine the coupling of
nonproteinogenic quaternary a-amino acids, which are
known to be more difficult to couple than their tertiary
congeners.5 The coupling reactions were also carried out
in the usual solvents, showing the necessity of using
charged coupling agents for efficient reaction. The
+
Me N
2
NMe
2
N
N
-
-
N
N
PF
PF
6
6
-
N
O
N
N
+
PF
6
+
N
O
NMe
2
N
P
NMe
2
NMe
2
[bmim]PF
6
HATU
BOP
Coupling Agents
Solvent
Figure 1. Comparison of the structures of coupling agents and the ionic liquid.
Keywords: Ionic liquids; Peptide synthesis; Aminoacids.
* Corresponding author. Tel.: +33-235522464; fax: +33-235522971; e-mail: jean-christophe.plaquevent@univ-rouen.fr
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.12.124

1618
H. Vallette et al. / Tetrahedron Letters 45 (2004) 1617–1619
structures of amino acids studied are presented in Fig-
ure 2.
generally more tedious than those in a classical solvent,
an observation, which can be explained by the expected
stabilization of the charged species in the ionic solvent.
On the other hand, a real advantage, which was
observed in this set of experiments was the higher purity
of the crude product when the reaction was performed
in an ionic liquid with respect to reactions in dichlo-
romethane or THF. This could be explained by a more
selective chemical pathway due to the stabilization effect
of the ionic liquid as the coupling reagents. Indeed, we
observed that extraction of the peptide from the ionic
solvent was not easy, and thus assumed that the modest
yields observed were mostly due to this extraction step.
We therefore decided to examine procedures other than
ÔmanualÕ diethyl ether extraction (Table 2).
All the amino acids studied are commercially available
except for MPG, which was prepared according to a
literature procedure.6 The usual Z and Boc protecting
groups were used in the first set of experiments. The
carboxyl function of the second amino acid was pro-
tected as its methyl ester (Scheme 1).
The main results for different coupling reactions are
given in Table 1, along with a comparison with analo-
gous reactions carried out in classical solvents (dichloro-
methane or THF).
Interestingly, fair conversions were observed under both
sets of conditions. The reactions in the ionic liquid seem
According to our hypothesis, we were delighted to
obtain substantially better results using toluene as the
extraction solvent, as well as when using a liquid–liquid
extractor for an overnight continuous extraction. These
rather good yields, along with the excellent purity of the
crude products encouraged us to consider ionic liquids
as interesting solvents for peptide coupling. With this
procedure in hand, we carried out a second set of cou-
plings, which are summarized in Table 3. Warming the
reaction mixture up to 65 °C accelerated the coupling.
H N
COOH
Gly
H N
2
COOH
H N
2
COOH
MPG
2
Aib
In addition, we carried out the synthesis of a tetrapep-
tide under the same conditions in 83% overall yield
(Scheme 2).
H N
2
COOH
Phe
H N COOH
2
c-Leu
In this study, we have demonstrated that ÔchemicalÕ
peptide coupling using ÔmodernÕ coupling agents is effi-
cient in ionic liquids. Even if numerous controls and
optimizations remain to be done, this new approach
could be powerful, especially for hindered amino acids,
which are not easy to couple under standard conditions.
Interestingly, the crude peptides were obtained with
higher purities than those from coupling in classical
solvents. Our ongoing studies focus on the possibility of
coupling the salts of the parent amino acids directly,
since these salts can be solubilized into ionic liquids. If
Figure 2. Structures of the amino acids studied (Gly ¼ glycine,
Aib ¼ aminoisobutyric acid, MPG ¼ 2-methyl-2-(p-tolyl)-glycine,
c-Leu ¼ cycloleucine, Phe ¼ phenylalanine).
coupling agent
PAA1 + AA2OMe
PAA1-AA2OMe
DIEA, solvent
Scheme 1. Synthesis of dipeptides in ionic liquids.
Table 1. First set of peptide couplings in ionic liquids (extraction with diethyl ether)
Dipeptide formed
Coupling agenta
T (°C)
Time (d)
Yield in [bmim]PF6 (%)
Yield in CH2Cl2 (%)
ZGly-MPGOMe
ZGly-MPGOMe
HATU
BOP
rt
rt
rt
rt
50
rt
rt
1.5
4
66
62
60
82
43
52
49
93
67
BocMPG-GlyOMe
BocMPG-GlyOMe
BocMPG-MPGOMe
BocAib-MPGOMe
BocMPG-AibOMe
HATU
BOP
1.5
1.5
8
92
66
HATU
HATU
HATU
43b
59b
58
4
4
a Experiments without any coupling agent, or using DCC led to no conversion.
b Experiment carried out in THF.
Table 2. Role of the extraction procedure
Dipeptide formed
Coupling agent
T (°C)
Time (d)
Yield in [bmim]PF6 (%)
Extraction
ZGly-MPGOMe
ZGly-MPGOMe
ZGly-MPGOMe
HATU
HATU
HATU
rt
rt
rt
4
4
4
66
77
87
Et2O manual extraction
Toluene manual extraction
Et2O liquid–liquid extraction

H. Vallette et al. / Tetrahedron Letters 45 (2004) 1617–1619
1619
Table 3. Peptide synthesis under optimized conditions (Et2O liquid–liquid continuous extraction)
Dipeptide formed
Coupling agent
T (°C)
Time (d)
Yield in [bmim]PF6 (%)
ZGly-GlyOMe
HATU
HATU
HATU
HATU
HATU
HATU
HATU
65
65
65
65
65
65
65
3
3
3
3
3
3
3
93
Quantitative
ZGly-MPGOMe
ZGly-cLeuOMe
BocAib-cLeuOMe
BocPhe-PheOMe
BocPhe-GlyOMe
BocPhe-cLeuOMe
93
43
Quantitative
96
Quantitative
HATU, 65˚C, 4.5 d
DIEA, [bmim]PF6
ZGly-MPG-Gly-MPGOMe
83%
ZGly-MPG + Gly-MPGOMe
Scheme 2. Synthesis of a tetrapeptide in an ionic liquid.
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usual protection/deprotection procedures.7
Acknowledgements
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ꢀ
ꢀ
The authors thank the Reseau Interregional de
Recherche ÔPunchÕOrgaÕ for financial support and a
research grant given to H.V., as well as Rhodia Orga-
nique Fine for a research grant given to L.F.
4. Erbeldinger, M.; Mesiano, A. J.; Russel, A. J. Biotechnol.
Prog. 2000, 16, 1129–1131.
References and notes
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€
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