LETTER
2525
Direct Carbodiimide-Mediated Conjugation of Carboxylates Using
Pyridinium p-Toluenesulfonate and Tertiary Amines as Additives
D
S
irec
t
Carbod
i
iimide
-
m
Mediated Conjuga
t
o
nx ylates Ficht, Lars Röglin, Matthias Ziehe, David Breyer, Oliver Seitz*
Institut für Chemie der Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Str. 2, 12489 Berlin, Germany
Fax +49(30)20937266; E-mail: oliver.seitz@chemie.hu-berlin.de
Received 4 August 2004
O
Abstract: The use of carboxylates in the carbodiimide-mediated
coupling to amines was investigated. The addition of pyridinium p-
toluenesulfonate (PPTS) and a tertiary amine was found to signifi-
cantly improve acylation yields by up to 70%.
H
H2N
TATACG
N
CONH2
1
CH2
DABCYL
N
N
H
4
O
O
O
N
Key words: amides, coupling, carbodiimides, carboxylates, solid-
phase synthesis
N
N
2
3
DABCYL-OSu
O
O
O
Carbodiimides are among the most popular activating
reagents for carboxylic acids. The formed active esters
N
1
N
N
Na
can react with nucleophiles leading to a large variety of
2
possible products. Despite the development of new cou-
pling reagents, e.g. of the aminium or phoshonium type,
Figure 1 DABCYL-labeled PNA-peptide conjugate and structures
of the DABYL succinimidyl ester 2 and methyl red (para) 3.
3
carbodiimide activation continues to be one of the most
commonly used synthetic methods. Carbodiimides are in
active use in the field of natural product synthesis and of-
4
mation has been achieved can be liberated by treatment
with sodium- or potassium hydroxide solution. The con-
version of the resulting sodium or potassium carboxylate
to the free acid can be problematic due to acid labile pro-
tecting groups in other parts of the molecule. Apparently,
the direct applicability of carboxylates in subsequent
amidation or esterification reactions would be desirable.
fer interesting opportunities for chemoselective coupling
5
6
reactions, polymer grafting or in reactions under the in-
7
fluence of microwaves. In most cases amines are used as
nucleophiles resulting in the formation of amides. Often
carbodiimide activation serves as a practical means of
accessing anhydrides and a variety of active esters.
The carbodiimide-mediated activation of carboxylic acids
For the described activation, the free acids are mandatory.
Carboxylates, the salts of carboxylic acids, are unable to
form the active esters in a completely aprotic environ-
ment. Nevertheless, there is interest in the direct activa-
tion of such salts. Many common chromophores are only
commercially available as carboxylates or as preformed
active esters, normally the hydroxysuccinimidyl esters.
Usually, these active esters are by far more expensive than
the corresponding carboxylates. For example, in our re-
search efforts towards the development of biosensors we
were in need of PNA-peptide conjugate 1 labeled with the
DABCYL chromophore (Figure 1).
is commenced by a protonation of the carbodiimide 4
8
(
Scheme 1). In a second step, the carboxylate 6 reacts
with the protonated carbodiimide 5 to give an O-acyli-
8
sourea 7. To avoid the rearrangement to N-acylurea 8,
usually HOBt is added, which by a transesterification fi-
9
nally leads to the desired active ester 9. Thus, the activa-
tion of carboxylic acid salts requires the presence of at
least one equivalent of protons. However, addition of
strong inorganic acids reduces the nucleophilicity of
amines and carboxylate 6 and shall hence be avoided.
R
R
The introduction of DABCYL groups is commonly per-
formed by using the commercially available N-hydrox-
ysuccinimidyl ester 2. This active ester is, however, 300-
fold higher in price than the DABCYL sodium carboxy-
late 3 sold as methyl red (para). A direct use of carboxy-
lates in coupling reactions would not only allow the use of
inexpensive starting materials but would also offer advan-
tages in natural product synthesis. Carboxylic acids are
often protected as esters, which after a particular transfor-
N
C
N
H+
HN
C
O
O
+ H+
- H+
+
-
H+
O
R''
HO
R''
N
R'
O
4
R'
5
6
R'
O
R
N
N
HN
O
HN
C
HOBt
O
N
R''
N
R
R''
O
N
R''
O
R'
8
7
9
SYNLETT 2004, No. 14, pp 2525–2528
Advanced online publication: 20.10.2004
2
2
.1
1
.2
0
0
4
Scheme 1 Carbodiimide-mediated activation of carboxylic acids.
DOI: 10.1055/s-2004-834819; Art ID: G31104ST
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Georg Thieme Verlag Stuttgart · New York