Toward waste-free peptide synthesis using ionic reagents and ionic liquids as solvents

Article abstract of DOI:10.1016/j.tetlet.2013.03.072

CBEIT 1, an ionic coupling reagent, gives access to amino acid carbamates, and allows waste-free peptide coupling. The only by-products are carbon dioxide and ethylimidazolium triflate that is an ionic liquid playing the role of the solvent.

Full text of DOI:10.1016/j.tetlet.2013.03.072

Tetrahedron Letters 54 (2013) 2703–2705
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Toward waste-free peptide synthesis using ionic reagents and ionic liquids
as solvents
⇑
Nicolas Galy, Marie-Rose Mazières, Jean-Christophe Plaquevent
CNRS-UMR 5068, SPCMIB, Université Paul Sabatier, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history:
CBEIT 1, an ionic coupling reagent, gives access to amino acid carbamates, and allows waste-free peptide
coupling. The only by-products are carbon dioxide and ethylimidazolium triflate that is an ionic liquid
playing the role of the solvent.
Received 29 January 2013
Revised 7 March 2013
Accepted 13 March 2013
Available online 24 March 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Ionic liquids
Imidazolium triflate
Peptide coupling
Coupling reagent
Carbonyldiimidazole
Owing to their unique properties, ionic liquids (ILs) nowadays
are widely examined as eco-compatible solvents for organic chem-
istry.1 In recent years, many modern applications have been re-
ported, such as enantioselective,2 catalytic,3 or multicomponent
reactions.4
Peptide synthesis remains a challenging area in organic synthe-
sis, both for the wide applications of peptides in bioorganic and
pharmaceutical fields, and for the rather sophisticated methods
needed for the control of chain elongation, good efficiency, limita-
tion of racemization, and applicability to unnatural aminoacids.5 A
shining series of excellent reviews has been published in recent
years, disclosing the various issues in this area.6
Some years ago, we disclosed our first approach about peptide
synthesis in ionic liquids, in which we described that these uncon-
ventional solvents were quite convenient for this process. Indeed,
we observed high conversions even for the coupling of hindered
amino acids, as well as purification simplified thanks to the rather
pure crude material obtained.7 Most of these results, along with
other aspects of ionic liquids related to peptide chemistry, have re-
cently been reviewed.8 However our previous findings did not af-
ford significant changes in the fundamental paradigm of peptide
synthesis, and we remained convinced that ionic liquid chemistry
may afford fundamentally new insights into the field.
the use of ionic coupling reagent with ionic solvents in a method
in which the only by-products are carbon dioxide and ethylimi-
dazolium triflate that is an ionic liquid playing the role of the solvent!
In other words, this process, belonging to our current studies about
synthesis in ionic liquid only (SILO),9 can be considered as a waste-
free peptide synthesis. Indeed, such an approach is of real interest
in the context of sustainable chemistry, since classical peptide cou-
plings require highly expensive protecting groups and coupling
reagents.5,6
The basic idea of this new approach was inspired by results
published by Rapoport group some years ago.10 The authors de-
scribed 1,10-carbonylbis(3-methylimidazolium) triflate (CBMIT,
Fig. 1) as an efficient reagent for aminoacylations, including
O
O
TfO
Me
TfO
Et
TfO
Me
TfO
Et
N
N
N
N
N
N
N
N
N
CBMIT
1, CBEIT
H
N
H
N
Herein we wish to disclose our recent research, which deeply
improves the previous results and opens the route for new devel-
opments in peptide coupling reactions: we are now combining
Me
N
Et
O
O
O
O
S
S
O
CF3
O
CF3
[H-MIM][TfO]
2, [H-EIM][TfO]
mp: ca 100°C (ref 11, 12)
mp: 8°C (ref 12)
viscosity: > 1000 cp (ref 12b)
viscosity: 58 cp (ref 12b)
⇑
Corresponding author.
E-mail address: plaquevent@chimie.ups-tlse.fr (J.-C. Plaquevent).
Figure 1. Ionic coupling reagents and their corresponding by-products.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.072

2704
N. Galy et al. / Tetrahedron Letters 54 (2013) 2703–2705
O
O
TfO
Et
MeNO2
0°C, 1h
TfO
Et
O
O
S
N
N
N
N
EtO
CF3
N
N
N
N
(2 eq)
1, CBEIT, 99%
Scheme 1. Synthesis of CBEIT 1.
We then engaged both CBMIT and CBEIT 1 in the same model
peptide coupling, without any added solvent, in order to compare
their reactivity and to ensure that both could be efficient under sol-
vent-free conditions (Scheme 2).
coupling reagent
COOMe
Et3N
H
N
Z
Z
COOMe
H3N
Cl
N
COOH
N
H
H
O
Although obtained in moderate yield,13 the model peptide 3a
could be synthesized under these conditions using both coupling
reagents. As anticipated, CBEIT 1 appeared to be of more friendly
use since the liberated ionic liquid [H-EIM][TfO] 2 fluxed the reac-
tion mixture, thus allowing working at rt.
3a, 65%
coupling reagent: CBMIT, 60°C, 24h
CBEIT 1, 25°C, 24h
Scheme 2. Model peptide couplings and comparison of CBMIT and CBEIT 1.
Encouragedby thisfirstresult, wedecidedtofocusona moregen-
eral method in which all the partners (i.e. protected amino acids)
would be prepared in a way that gives access to a full step peptide
synthesis with only carbon dioxide and [H-EIM][TfO] 2 as by-prod-
ucts. As depicted in Scheme 3, a series of unprecedented methyl ami-
no ester triflates 5 was directly obtained by alkylation of the starting
amino acid with methyl triflate. As shown later (Scheme 4), we ex-
pected these salts to be liberated by means of ethyl imidazole acting
as the base, thus liberating [H-EIM][TfO] 2 as well as the subsequent
peptidecoupling. CBEIT1 was alsoreacted withbenzylalcohol and t-
butanol, then with amino ester salts 5 to give the corresponding car-
bamates 4 in good yields (Scheme 3).
The last and most exciting step consisted in the coupling of both
partners using CBEIT 1 as the coupling agent and ethyl imidazole as
the base able to liberate the amino ester salt 5 (Scheme 4).
After optimization of experimental procedure (see Ref. 13),
peptides 3 were obtained in yields as high as 75%. We engaged
peptide couplings. The resulting by-products were only carbon
dioxide and methylimidazolium triflate ([H-MIM][TfO]).
Because of its high melting point and viscosity (see Fig. 1),
[H-MIM][TfO] could not easily be considered as an ionic liquid
acting as a potential solvent.11 Thus, we searched for a congener
that could be both liquid and fluid enough at low temperature.
Literature data gave us access to numerous options,12 among
which we chose to examine the ethyl homolog. Indeed, the corre-
sponding CBEIT 1 appeared easy to synthesize from commercially
available materials; also, the resulting salt ([H-EIM][TfO]) 2 was
reported to behave as a true ionic liquid with low melting point
and acceptable viscosity (Fig. 1).
As expected, the synthesis of CBEIT 1 was realized by direct
reaction of CDI with ethyl triflate, in quantitative yield and no need
for further purification before its use as the coupling reagent
(Scheme 1).
N-protection:
TfO
R1
TfO
N
O
R1
Et
COOMe
N
O
1.
CBEIT 1
CH3NO2
0°C, 1 h
H3N
R2O
85%
R2O
N
H
COOH
80%
N
N
Et
R2OH
2. aq. NaOH then aq. HCl
R1 = H (Gly, 4a) or i-Pr (Val, 4b)
R
2 = CH2Ph or t-Bu (Z ou BOC)
C-protection:
quantitative
R3
R3
R4
O
O
R4
TfO
S
MeO
CF3
H3N
COOMe
H2N
COOH
R3 = R4 = H (Gly, 5a)
R
3 = R4 = Me (Aib, 5b)
R3 = i-Pr, R4 = H (Val, 5c)
R3 = i-Bu, R4 = H (Leu, 5d)
Scheme 3. N- and C-protection of starting amino acids.
N
Et
N
TfO
H3N
H
CBEIT 1
Z
N
COOMe
Z
N
COOMe
N
H
COOH
H
25°C, 24h
O
4a
5d
5a
3a, 75%
TfO
H3N
same reagents
and conditions
H
Z
Z
N
COOMe
3b, 55%
COOMe
N
H
COOH
N
H
O
4b
Scheme 4. ‘Waste-free’ peptide couplings.

N. Galy et al. / Tetrahedron Letters 54 (2013) 2703–2705
2705
[H-EIM][TfO], Ionic Liquid
O
TfO
N Et
TfO
Et
H
N
N
N
Et N
N
O
O
O
R3
N
TfO
5
S
CF3
2
CBEIT 1
H3N
Et
COOMe
R1
N
O
R1
O
R1
TfO
O
H2N
Et
[H-EIM][TfO]
COOH
CBEIT
H
N
CBEIT 1
R2OH
COOMe
R2O
N
H
COOH
R2O
N
R2O
N
N
H
O
R3
4, R2 = Ph or t-Bu
(Z or BOC)
3
[H-EIM][TfO]
3 [H-EIM][TfO]
Figure 2. ‘Waste-free’ peptide synthesis.
V., Plaquevent, J.-C., Gruttadauria, M., Giacalone, F., Eds.; Wiley, 2011. Chapter
7.
enantiopure amino acids both in C- and N-terminal positions, and
checked that no racemization occurred (as controlled by optical
rotation and chiral HPLC, see Supplementary data). Of course,
[H-EIM][TfO] 2 was recovered and purified by standard procedure
(60–70% unoptimized yield): it can be used either as an ionic
solvent for further applications or recycled as starting material
for recovery of ethyl imidazole or CBEIT 1.
3. Durand, J.; Teuma, E.; Gomez, M. C. R. Chimie 2007, 152–177.
4. Isambert, N.; del Mar Sanchez Duque, M.; Plaquevent, J.-C.; Génisson, Y.;
Rodriguez, J.; Constantieux, T. Chem. Soc. Rev. 2011, 40, 1347–1357.
5. For a pertinent and stimulating discussion, see: Kent, S. Angew. Chem., Int. Ed.
2006, 45, 4234 (About: Benoiton, N. L. Chemistry of Peptide Synthesis, CRC
Taylor & Francis Group: Andover, 2005).
6. Overview of modern approaches and prospects in peptide synthesis: Albericio,
F. Curr. Opin. Chem. Biol. 2004, 8, 211–221; Review on protease-catalyzed
peptide synthesis: Lombard, C.; Saulnier, J.; Wallach, J. M. Protein Pept. Lett.
2005, 12, 621–629; Review about amino acid-protecting groups: Isidro-Llobet,
A.; Alvarez, M.; Albericio, F. Chem. Rev. 2009, 109, 2455–2504; Review about
peptide coupling reagents: Ayman El-Faham, A.; Albericio, F. Chem. Rev. 2011,
111, 6557–6602; Chemical modifications of peptides: Van Lancker, F.; Adams,
A.; De Kimpe, N. Chem. Rev. 2011, 111, 7876–7903; Total synthesis of proteins:
Kent, S. B. H. Chem. Soc. Rev. 2009, 38, 338–351.
7. (a) Vallette, H.; Ferron, L.; Coquerel, G.; Gaumont, A.-C.; Plaquevent, J.-C.
Tetrahedron Lett. 2004, 45, 1617–1619; (b) Vallette, H.; Ferron, L.; Coquerel, G.;
Guillen, F.; Plaquevent, J.-C. ARKIVOC 2006, 200–211; (c) Guillen, F.; Brégeon,
D.; Plaquevent, J.-C. Tetrahedron Lett. 2006, 47, 1245–1248; (d) Ternois, J.;
Ferron, L.; Coquerel, G.; Guillen, F.; Plaquevent, J.-C. ACS Symp. Ser. 2010, 1038,
13–24.
8. (a) Plaquevent, J.-C.; Levillain, J.; Guillen, F.; Malhiac, C.; Gaumont, A.-C. Chem.
Rev. 2008, 108, 5035–5060; (b) Tietze, A. A.; Heimer, P.; Stark, A.; Imhof, D.
Molecules 2012, 17, 4158–4185.
In the following Figure 2 is summarized the full strategy for
waste-free peptide coupling.
Finally, we show that ionic coupling reagents such as CBEIT 1
could be perfect tools for efficient and ‘waste-free’ peptide synthe-
sis, since the only by-products are carbon dioxide and an ionic li-
quid that acts as the solvent. Not only the peptide coupling
reaction itself is realized under such conditions, but also the re-
quired protections of the starting amino acids. Our current studies
focus on the direct coupling of unprotected amino acids, which we
expect to be possible thanks to the liberated ionic liquid able to
dissolve amino acids.
Supplementary data
9. For part 2 of this series, see Jebri, K.; Mazières, M.-R.; Ballereau, S.; Ben Ayed, T.;
Baltas, M.; Plaquevent, J.-C., ‘Mukaiyama coupling in ionic liquids: from
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
03.072.
ulosonic series to
liquids only’, in press.
a-oxo c-thio-esters via unprecedented synthesis in ionic
10. (a) Saha, A. K.; Schultz, P.; Rapoport, H. J. Am. Chem. Soc. 1989, 111, 4856–4859;
(b) Gibson, F. S.; Rapoport, H. J. Org. Chem. 1995, 60, 2615–2617.
11. Zhao, G.; Jiang, T.; Gao, H.; Han, B.; Huang, J.; Sun, D. Green Chem. 2004, 6, 75–
77.
12. (a) Zhang, S.; Sun, N.; He, X.; Lu, X.; Zhang, X. J. Phys. Chem. Ref. Data 2006, 35,
1475–1517; (b) Ohno, H.; Yoshikawa, M. Solid State Ionics 2002, 154–155, 303–
309.
13. The moderate yield observed in the first set of experiments was due to the
in situ formation of glycine anhydride. Although this intermediate was also
coupled with leucine ester, yield was hampered because of the loss of one
glycine unit during this process.
References and notes
1. (a)Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds., 2nd ed.; Wiley-
VCH: Weinheim, 2008; (b) Hallet, J. P.; Welton, T. Chem. Rev. 2011, 111, 3508–
3576.
2. (a) Baudequin, C.; Plaquevent, J.-C.; Audouard, C.; Cahard, D. Green Chem. 2002,
4, 584–586; (b)Catalytic Methods in Asymmetric Synthesis: Advanced Materials,
Techniques, and Applications; Gaumont, A.-C., Génisson, Y., Guillen, F., Zgonnik,