New observations on peptide bond formation using CDMT

Article abstract of DOI:10.1016/S0040-4039(02)00754-2

The optimized formation of the peptide bond by means of 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) has been found to occur rapidly and essentially quantitatively in a one-pot, one-step procedure. This new method is effective for the coupling of a variety of reactive partners, including chiral amino acids (e.g. N-acetyl-L-leucine) without significant loss of configuration. Significant racemization was observed when the typical literature conditions were used, due to the formation of an azlactone intermediate which is configurationally unstable under the reaction conditions. A simpler, precipitative workup procedure is also disclosed in this report.

Full text of DOI:10.1016/S0040-4039(02)00754-2

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TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4161–4165
New observations on peptide bond formation using CDMT
Christine E. Garrett,* Xinglong Jiang,* Kapa Prasad and Oljan Repicˇ
Process Research & Development, Novartis Institute for Biomedical Research, One Health Plaza, East Hanover, NJ 07936, USA
Received 2 April 2002; revised 16 April 2002; accepted 17 April 2002
Abstract—The optimized formation of the peptide bond by means of 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) has been
found to occur rapidly and essentially quantitatively in a one-pot, one-step procedure. This new method is effective for the
coupling of a variety of reactive partners, including chiral amino acids (e.g. N-acetyl-L-leucine) without significant loss of
configuration. Significant racemization was observed when the typical literature conditions were used, due to the formation of an
azlactone intermediate which is configurationally unstable under the reaction conditions. A simpler, precipitative workup
procedure is also disclosed in this report. © 2002 Elsevier Science Ltd. All rights reserved.
Due to the need for a variety of diverse peptides, many
coupling agents have been developed that facilitate
formation of the peptide bond. The desired peptide
often contains stereogenic centers and therefore reten-
tion of the optical purity of the compound is also of
great importance for any method. The use of CDMT as
a coupling agent has been investigated fairly exten-
sively, and it demonstrates several advantages over
other coupling agents. It is a stable, crystalline com-
pound, with good solubility in organic solvents, and is
commercially available in large quantities.1 These quali-
ties made CDMT attractive for process development in
our laboratories. The present communication describes
the results of these investigations.
amine coupling partner in the same pot. The reaction
typically proceeds to completion in a matter of 8–14 h.1
This method is effective for the formation of a variety
of compounds, including esters3 and Weinreb amides.4
Workup of the products is typically afforded by extrac-
tion with dilute acid, as the CDMT and its triazine
by-products are typically weakly basic and easily
removed extractively. Analytically pure material is gen-
erally isolated via column chromatography,1 although
the crude material is often pure enough to proceed with
subsequent reactions.4
When we attempted to synthesize 5 (Scheme 2) via the
standard two-step CDMT coupling conditions, we
observed complete conversion of the amine (4) over
about 22 h. Surprisingly, the intermediate observed
after the pre-activation step was not the expected active
ester, but an azlactone (3).5 We theorized that the
azlactone could be generated via the intermediacy of
The standard method for making amides using CDMT
is to activate the acid using CDMT and a base such as
N-methylmorpholine. This generates an active ester,1,2
1 (Scheme 1), which is subsequently reacted with the
O
Cl
O
R
O
R2NH2
O
N
N
NMM
R2
+
N
N
R
N
H
R
OH
O
N
O
O
N
O
CH3
CH3
CH3
CH3
1
Scheme 1. Typical procedure for two-step peptide bond formation using CDMT.
Keywords: CDMT; amides; chiral amides.
* Corresponding authors. E-mail: christine.garrett@pharma.novartis.com; xinglong.jiang@pharma.novartis.com
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00754-2

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4162 C. E. Garrett et al. / Tetrahedron Letters 43 (2002) 4161–4165
Ph
Ph
H3C
Br
CH3
H3C
Br
CH3
CDMT,
NMM
O
HCl-H2N
CO2Et
H
N
H
N
CO2H
4
N
H
CO2Et
Ph
CH3CN
O
O
5
2
H3C
CH3
CDMT,
NMM,
CH3CN
HCl-H2N
CO2Et
N
Br
4
O
O
3
Scheme 2. Synthesis of 5 under standard CDMT coupling conditions.
To determine if this one-step method really resulted in
a significantly different pathway (i.e. active ester versus
azlactone intermediate), we applied this one-step
the desired active ester. In addition, we thought that the
presence of the amine from the outset of the reaction
might intercept the active ester as it is formed and
provide for a faster and more direct route to the amide.
method to a chiral amino acid, e.g. N-acetyl-L-leucine,
which is typically prone to racemization under standard
coupling conditions. As shown (Fig. 1), the coupling of
Therefore the coupling of 2 and 4 was attempted in
one-step instead of two; the CDMT, 2, 4, and solvent
were mixed together and then the NMM was added.
We observed complete conversion of the amine in less
than 1 h. A trace of 3 was still observed after all the
amine (4) had reacted due to the fact that the amine is
partially consumed by the formation of a by-product, 6.
The product was purified by extraction and used for the
next step of the process. The product is essentially
indistinguishable from that produced via the two-step
process.
the acid with
D
-4 under the one-step conditions gener-
ated a product with a diastereomeric ratio of 94:6. The
product generated via the two-step method had a
diastereomeric ratio of 44:56.6
While optimizing the reaction in Fig. 1 using the Sur-
veyor parallel reactor system, we noted that there was a
significant concentration, solvent and temperature
effect. The reactions were found to generate less impu-
rities when run at higher dilution (<0.10 M of 4). This
was adopted as the standard condition for concentra-
tion of the reaction. Retention of stereochemistry is
more pronounced in the alkylacetate solvents (Table 1,
compare entry 1 with entries 2–4), although there is
little difference between the three alkylacetate solvents.
Additionally, while there seems to be no effect of the
temperature on the amount of racemization observed
(compare entries 2, 5, and 6), fewer side-products are
observed when the reaction is run at 25°C, and there-
fore a higher yield is possible.7
Ph
HN
CO2Et
N
N
H3C
O
N
O CH3
6
Ph
Ph
O
O
H
N
O
H
N
H3C
Method A or B
OH
H3C
OEt
+
N
CH3CN,
20-25 °C
OEt
O
CH3
H
HCl-H2N
O
CH3
O
CH3
CH3
D-4
syn : anti
94 :
44 : 56
Method A:
Method B:
6
Method A = CDMT mixed with acid and amine and then NMM added.
Method B = CDMT mixed with acid and NMM, stirred for 1 h, then amine added.
Figure 1. One-step versus two-step coupling.

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C. E. Garrett et al. / Tetrahedron Letters 43 (2002) 4161–4165
4163
Table 1. Optimization of conditions for one-step coupling
generated under either method in comparable
diastereomer ratio, but here the stereocenter is exo to
the azlactone intermediate so racemization would be
more difficult.
with CDMT (Fig. 1)7
Entry
Solvent
Temp. (°C)
Diastereomeric ratio
1
2
3
4
5
6
CH3CN
EtOAc
i-PrOAc
n-BuOAc
EtOAc
15
15
35
35
35
25
92:8
98:2
99:1
99:1
98:2
99:1
While optimizing the coupling of non-cyclizable sub-
strates, we developed a very simple workup for this
reaction.9 When the one-step coupling of trans-cin-
namic acid and 2-phenylethyleneamine was run in ace-
tonitrile,10 it was observed that the product could be
precipitated out of the reaction solution simply by the
addition of water. The product was isolated in 97%
yield and was analytically pure. This one-step coupling
method and workup11 were then applied to the prepara-
tion of a number of amides,12 as shown in Table 3.13
EtOAc
A variety of amino acids were then coupled with
L
D- or
-4 under the optimized conditions8 (See Table 2). For
comparison, the diastereomer ratio achieved under the
two-step conditions (Method B) is also shown. All of
the amides generated by the one-step method (Method
A) were isolated in >84% yield and with >97% purity.
The retention of stereochemistry for the N-acetyl-
leucine and N-acetyl- -phenylalanine is significantly
improved with the one-step conditions (compare entries
1 and 2; 3 and 4). The Boc-protected amino acid (entry
5) should not racemize under the conditions employed
in either method. Compound 5 (entries 7 and 8) is
Both simple and sterically hindered primary and sec-
ondary amines, either aryl or alkyl, couple with acids
effectively under these conditions providing good
(entries 2, 3, 5, 7 and 8) to high (1 and 6) yields of
amide. One notable exception is entry 4, which couples
very slowly due to the presence of two sterically hin-
dered groups in close proximity to the coupling site.
a,b-Unsaturated acids couple effectively (entry 6) as
L-
L
Table 2. Coupling of amino acids and 4

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C. E. Garrett et al. / Tetrahedron Letters 43 (2002) 4161–4165
Table 3. Amides prepared by a one-step coupling (in CH3CN) with precipitative workup
well as amine hydrochloride salts (compare entries 9
and 5), although the salts require additional NMM to
neutralize the HCl.11 The coupling reaction where both
partners are chiral (entry 11) also proceeds quite well,
with retention of configuration at both stereogenic sites.
Interestingly, when the amine contains both a hydroxyl
and an amine functional group (entry 12), no ester
by-product is observed demonstrating that the amine
couples exclusively and there is no competition between
the two functional groups. The reaction proceeds to
completion, but the low yield here is attributed to the
solubility of the product in CH3CN/water.
In conclusion, we have demonstrated that a one-pot,
one-step procedure for the coupling of acids and
amines proceeds effectively with CDMT as the coupling
agent. Chiral amino acids with unhindered N-sub-
stituents can be effectively coupled without loss of
configuration. Additionally, when the product is a non-
water soluble solid, the reaction can be run in acetoni-