Ethyl propiolate: A simple and convenient peptide coupling reagent

Article abstract of DOI:10.1055/s-2004-829553

Ethyl propiolate has been used to activate N-protected amino acids to form a moderately activated vinyl-ester which aminolysis requires a catalytic amount of sodium bisulfite.

Full text of DOI:10.1055/s-2004-829553

1826
LETTER
Ethyl Propiolate: A Simple and Convenient Peptide Coupling Reagent
E
thyl Propiolate:
o
A
S
imple an
g
d
Convenien
d
t
Peptide Co
a
upling Rea
n
gent Iorga, Jean-Marc Campagne*
ICSN-CNRS, Avenue de la Terrasse, 91198 Gif sur Yvette, France
Fax +33(1)69077247; E-mail: campagne@icsn.cnrs-gif.fr
Received 26 May 2004
of alkynes as coupling reagents in peptide synthesis have
been reported. The activation of carboxyl groups by
‘push-pull’ alkynes9 or the Ru-catalysed addition of car-
boxyl groups to aliphatic alkynes10 give the desired dipep-
tides. However, these procedures are hampered by the
instability of the coupling reagent and the long reaction
time (1–4 d);9 the harsh conditions (autoclave, 100 °C)
and the need of KCN.10
Abstract: Ethyl propiolate has been used to activate N-protected
amino acids to form a moderately activated vinyl-ester which ami-
nolysis requires a catalytic amount of sodium bisulfite.
Key words: peptide, coupling reagent, sodium bisulfite
Proteins, peptides and pseudo-peptides are an important
class of bioactive molecules.1 The development of new
peptide coupling reagents thus represents a continuous
challenge, culminating in the recent years by the develop-
ment of chemically efficient peptide coupling reagents (in
terms of yield, mild conditions, racemisation) such as
EDC/HOBt,2 PyBOP (1),3 PyBroP (2),4 HATU (3),5
FDPP6 (4) and congeners.1,7 However, from an atom-
economy8 point of view, these reagents are poorly effi-
cient. Indeed, these compounds are heavy, complex and
expensive molecules (1–4, Figure 1) and usually require
an excess of a tertiary amine as a base. Thus, large
amounts of non-recycled by-products, often toxic, are
produced. An ideal reagent would be general, atom eco-
nomical, catalytic, and tolerant of a wide range of func-
tional/protecting groups and would not require/produce
toxic reagents/waste.
Our initial strategy is outlined in Scheme 1. The reaction
of a catalytic amount of a nucleophile R3Y 7 with ethyl
propiolate 5 in the presence of a N-protected amino acid 6
was expected to lead to the amino carboxylate and activat-
ed propiolate 9. The amino carboxylate could then react
with 9 to form the active ester 10 and regenerate R3Y. The
activated ester 10 will then be able to be aminolysed by
amino-esters 11 to yield the dipeptides 12 and easily elim-
inated (bp 35–50 °C, 17 torr)11 ethyl formylacetate as the
sole by-product.
Scheme 1
The formation of the active ester 10 (P = Boc, R1 = Me)
was first investigated using tertiary phosphines as cata-
lysts.12 However, the reaction stops at the stage of phos-
phonium salt 8 (R3Y = Ph3P, Bu3P). Various attempts to
use amines such as DMAP gave the same disappointing
results. Gratifyingly, the use of sub-stoichiometric (20%)
amounts of tertiary aliphatic amines [Et3N, N-Methyl
Morpholine (NMM)] lead to the clean and quantitative
formation of active ester 10 (CHCl3, r.t., 12 h).13
Figure 1 Peptide coupling reagents
Herein, we would like to describe the use of commercially
available ethyl propiolate 5 as a simple, cheap and conve-
nient peptide-coupling reagent. A few examples of the use
SYNLETT 2004, No. 10, pp 1826–1828
Advanced online publication: 15.07.2004
0
9
.0
8
.2
0
0
4
DOI: 10.1055/s-2004-829553; Art ID: G50204ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Ethyl Propiolate: A Simple and Convenient Peptide Coupling Reagent
1827
The aminolysis of active ester 10 was then investigated: found that environmental benign sodium bisulfite could
when all reagents (N-protected amino acid, amino ester, act as an efficient co-catalyst in these reactions (Table 1,
ethyl propiolate and NMM) were mixed together, only entries 4–6).
complex reaction mixtures were obtained, due to the com-
petitive addition of the amino-ester 11 on ethyl propiolate.
Consequently, a one-pot two-step sequence was investi-
gated (Scheme 2).
The concentration of the reaction mixture is another im-
portant parameter. A drastic increase of the reaction speed
and yield is observed by changing the concentration from
0.06 M to 0.5 M (Table 1, entry 6) leading to a 82% yield
Since amino-esters are usually sold as hydrochloride salts, after one hour at room temperature.
the addition of a stoichiometric amount of base is neces-
Under these optimised conditions, the procedure could
sary to free the amine in the second step of the process
then be applied to the synthesis of various dipeptides
(Scheme 2, procedure a). It was thus found more practical
(Table 2). The usual protecting groups (Boc, Fmoc, Cbz)
to use 1.5 equivalents of NMM from the beginning: in
are tolerated, as well as functional amino acids (entries 4
these conditions (Scheme 2, procedure b), intermediate 10
and 8). Compared to other modern peptide-coupling re-
is formed quantitatively after two hours at room tempera-
agents, the yields are somehow lower.14 However, we
ture.
believe that a simple and affordable reagent (1 g, 3 €),
such as ethylpropiolate 5, will be of substantial interest for
research groups that could not afford more sophisticated
(and expensive!) peptide-coupling reagents in particular
for large scale synthesis. Simple amines such as benzyl-
amine are also reactive leading to the corresponding
amide in a quantitative yield (entry 11). The reactions
with hydroxylamine and Weinreb amine (entries 9 and 10)
were also successful. Alcohols are non-reactive compo-
nents in these reactions (entry 12).
Table 2 Dipeptides, Amides and Hydroxamates 12 Prepareda
Entry
P-AA1-OH
H-AA2-OR
Yield (%)
Scheme 2
1
2
Boc-Ala-OH
Boc-Ala-OH
Boc-Ala-OH
Boc-Tyr(Bn)-OH
Fmoc-Ala-OH
Cbz-Val-OH
Cbz-Phe-OH
Cbz-Met-OH
Boc-Ala-OH
Cbz-Phe-OH
Boc-Ala-OH
Boc-Ala-OH
H-Val-OEt
H-Gly-OEt
H-Pro-Ot-Bu
H-Pro-Ot-Bu
H-Val-OEt
H-Ala-OMe
H-Gly-OEt
H-Gly-OEt
HN(Me)OMe
H2N-OH
82
80
90
77
65
67
75
83
74
50
99
–
As previously observed,10 vinyl esters are moderately re-
active towards aminolysis. In a model reaction (P = Boc,
R1 = Me, R2 = i-Pr, R3 = Et) the dipeptide 12 is isolated in
useful yield (60%) only after five days at room tempera-
ture (Table 1, entry 1). Dixneuf10 has described the use of
KCN as a useful co-catalyst in these reactions: indeed in
the presence of KCN (10%) the dipeptide was isolated in
64–68% yield (Table 1, entries 2 and 3).
3
4
5
6
However from a ‘green chemistry’ point of view the use
of KCN should be better avoided. To our delight, we
7
8
Table 1 Aminolysis of Ester 10 (P = Boc, R1 = Me, R2 = i-Pr,
9
R3 = Et) Synthesised Using Procedure B.
10
11
12
Entry Conditions
Co-catalyst
–
Yield 12 (%)
H2N-Bn
1
2
CHCl3, 0.06 M, 5 d
60
68
EtOH or BnOH
CHCl3, 0.06 M,
overnight
KCN (10%)
a See typical experimental procedure.
3
4
5
6
CH2Cl2, 0.06 M,
overnight
KCN (10%)
64
50
Racemisation tests (Scheme 3) were finally carried out
using Anteunis’ test.15
CHCl3, 0.06 M,
overnight
NaHSO3 (10%)
CHCl3, 0.06 M,
overnight
NaHSO3 (1 equiv) 62
CHCl3, 0.5 M, 3 H
NaHSO3 (10%)
82
Scheme 3 Anteunis’ racemisation test
Synlett 2004, No. 10, 1826–1828 © Thieme Stuttgart · New York

1828
B. Iorga, J.-M. Campagne
LETTER
The efficiency of our procedure was compared with those
of PyBOP,3 one of the most efficient coupling reagents
available (Table 3, entry 1). Using procedure b (Table 3,
entry 3), an important level of racemisation (30%) was ob-
served, precluding the use of this procedure for fragment
couplings. Gratifyingly, using the ‘catalytic’ procedure a
(Table 3, entry 2), a low level of racemisation (3%) was
measured. Our procedure favourably compares with the
PyBOP procedure (Table 3, entry 1).
Acknowledgment
We thank the CNRS for financial support.
References
(1) Nájera, C. Synlett 2002, 1388.
(2) (a) Sheehan, J. C.; Cruickshank, P. A.; Boshart, G. L. J. Org.
Chem. 1961, 26, 2525. (b) Sheehan, J. C.; Preston, J.;
Cruickshank, P. A. J. Am. Chem. Soc. 1965, 87, 2492.
(c) Williams, A.; Ibrahim, I. T. J. Am. Chem. Soc. 1981, 103,
7090.
Table 3 Epimerisation Levels Obtained in Anteunis’ Test15
(3) Coste, J.; Le-Nguyen, D.; Castro, B. Tetrahedron Lett. 1990,
31, 205.
(4) Coste, J.; Frérot, E.; Castro, B. J. Org. Chem. 1994, 59,
2437.
(5) (a) Carpino, L. A. J. Am. Chem. Soc. 1993, 115, 4397.
(b) Carpino, L. A.; El-Faham, A.; Minor, C. A.; Albericio, F.
J. Chem. Soc., Chem. Commun. 1994, 201.
(6) (a) Chen, S.; Xu, J. Tetrahedron Lett. 1991, 32, 6711.
(b) Dudash, J.; Jiang, J.; Mayer, S. C.; Joullié, M. M. Synth.
Commun. 1993, 23, 349.
Entry
Conditions
Epimerisation
(%)a
1
2
PyBOP, Et3N, CH2Cl2, r.t.
3b
3
HC≡C-COOEt, NMM cat. (12 h),
CHCl3, r.t.
3
HC≡C-COOEt, NMM (2 h),
CHCl3, r.t.
30
(7) Reviews: (a) Albericio, F.; Chinchilla, R.; Dodsworth, D. J.;
Nájera, C. Org. Prep. Proc. Int. 2001, 33, 203. (b) Methods
in Organic Chemistry (Houben-Weyl), Vol. E22a;
Goodman, M., Ed.; Georg Thieme Verlag: Stuttgart, 2002.
(8) (a) Trost, B. M. Science 1991, 254, 1471. (b) Trost, B. M.
Acc. Chem. Res. 2002, 35, 1471.
(9) (a) Gais, H. J. Angew. Chem., Int. Ed. Engl. 1978, 17, 597.
(b) Neuenschwander, M.; Fahmi, H.-P.; Lienhard, U. Helv.
Chim. Acta 1978, 61, 2437.
(10) (a) Ruppin, C.; Dixneuf, P.; Lecolier, S. Tetrahedron Lett.
1988, 29, 5365. (b) Kabouche, Z.; Bruneau, C.; Dixneuf, P.
Tetrahedron Lett. 1991, 32, 5359.
(11) Sato, M.; Yoneda, N.; Katagiri, N.; Watanabe, H.; Kaneko,
C. Synthesis 1986, 672.
a Determined by HPLC and 1H NMR.
b 3.5% in the original publication.3
In conclusion, we have described the use of ethyl propi-
olate as a new peptide-coupling reagent. This reagent is a
simple, low weighing, and non-expensive commercial
compound. Moreover reduced amounts of base are re-
quired in this process which constitutes a first step to-
wards the development of a truly atom economic peptide-
coupling reagent.
(12) For a review on the activation of conjugated alkynes with
phosphines, see: Lu, X.; Zhang, C.; Xu, Z. Acc. Chem. Res.
2001, 34, 535; and references cited therein.
(13) The mechanism is believed to imply the catalysis of the
tertiary amine (Scheme 1) and not a direct Michael addition
of the carboxylate since no reaction is observed in the
presence of various inorganic bases.
(14) One referee has suggested that lower yields could be
explained by the formation of a Schiff base from by product
13 and the amino ester. However, this kind of by-product
was never observed either by TLC or by 1H NMR on the
crude mixture.
Typical Experimental Pocedure
Ethyl propiolate (33 mL, 0.325 mmol) and N-methylmorpholine (41
mL, 0.375 mmol) are successively added to a solution of 0.25 mmol
of N-protected amino acid 6 in CHCl3 (0.5 mL) at r.t. The reaction
mixture is stirred for 2 h, then the hydrochloride salt of the O-pro-
tected amino acid 11 (0.325 mmol) and NaHSO3 (2.9 mg, 0.028
mmol) are successively added. The reaction is usually finished after
1 h, but for sterically hindered amino acids stirring was continued
overnight. The solvent is then evaporated and the mixture purified
by flash chromatography on silica gel (60 Å, 70–200 mesh, hep-
tane–EtOAc 70:30) to give the dipeptide 12.
(15) Van der Auwera, C.; Vandamme, S.; Anteunis, M. J. O. Int.
J. Pept. Protein Res. 1987, 29, 464.
Synlett 2004, No. 10, 1826–1828 © Thieme Stuttgart · New York