Convenient primary amidation of N-protected phenylglycine and dipeptides without racemization or epimerization

Article abstract of DOI:10.1016/j.tetlet.2013.11.042

Primary amidation of N-protected phenylglycine and dipeptide proceeded easily to afford the corresponding amides in 57-95% yields with 99% ee and 81-99% de, respectively. The procedure is very easy to avoid racemization and epimerization of the products in the reactions by keeping exactly the reaction temperature at -15 C when the activation of carboxylic acids, followed by the reaction of the mixed carbonic carboxylic anhydride with NH₄Cl.

Full text of DOI:10.1016/j.tetlet.2013.11.042

Tetrahedron Letters 55 (2014) 394–396
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Convenient primary amidation of N-protected phenylglycine
and dipeptides without racemization or epimerization
⇑
Takuya Noguchi, Seunghee Jung, Nobuyuki Imai
Faculty of Pharmacy, Chiba Institute of Science, 15-8 Shiomi-cho, Choshi, Chiba 288-0025, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Primary amidation of N-protected phenylglycine and dipeptide proceeded easily to afford the corre-
sponding amides in 57–95% yields with 99% ee and 81–99% de, respectively. The procedure is very easy
to avoid racemization and epimerization of the products in the reactions by keeping exactly the reaction
temperature at ꢀ15 °C when the activation of carboxylic acids, followed by the reaction of the mixed car-
bonic carboxylic anhydride with NH4Cl.
Received 12 September 2013
Revised 1 November 2013
Accepted 11 November 2013
Available online 19 November 2013
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords:
Phenylglycine
Dipeptide
Ethyl chloroformate
Without racemization
Without epimerization
Recently, we have developed that the amidation of N-protected
-amino acids with unprotected -amino acids, NH4Cl, and aniline
ClCO2Et
Et3N
O
R'
*
O
R'
O
R'
*
NH4Cl
a
a
*
R
*
O
R
*
O
R
O
*
N
H
N
H
N
H
derivatives via the mixed carbonic carboxylic anhydrides provided
the corresponding dipeptides, primary amides, and anilides in
good to excellent yields.1 Unfortunately, it was found that the pro-
duced primary amides of phenylglycine and dipeptides were race-
mized or epimerized2 under the reaction conditions in our
developed method.
THF-H2O
P-HN
NH2
P-HN
O
O
THF
P-HN
OH
OEt
P = protecting group
Scheme 1.
Herein, we describe preparation of primary amides from the
corresponding phenylglycine (Phg-OH) and dipeptides with NH4Cl
via activation by ClCO2Et and Et3N without racemization or epi-
merization (Scheme 1).
Ph
H N
O
O
O
Ph
OH
O
Ph
H N
O
O
O
O
5 o
C
O
O
H N
O
OEt
O
OEt
O
OEt
racemization
O
Ph
Ph
Ph
1-OCO2Et
In a preliminary investigation, the reaction of Cbz-L-Phg-OH (1)
with 1.5 equiv of NH4Cl in the presence of 1.4 equiv of ClCO2Et and
3.0 equiv of Et3N in aqueous tetrahydrofuran (THF) at ice-cooled
Scheme 2. Possible pathway for racemization of 1-OCO2Et.
temperature (5 °C) afforded Cbz-L-Phg-NH2 (1) in 43% yield with
46% ee as indicated in entry 1 of Table 1. When the reaction was
carried out for 0.5 h at 0 °C on the step (2), the ee was improved
to 67% (entry 3). When the reaction temperature was decreased
to ꢀ5 °C on both steps (1) and (2), the corresponding primary
amide 2 was obtained with 83% ee as shown in entry 6. Effect of
decreasing the reaction temperature until ꢀ15 °C was amazingly
well and an excellent enantioselectivity (93% ee) was observed.
The reaction conditions were further optimized by using precooled
reagents on both steps (1) and (2) and the best results (83% yield,
99% ee) were obtained when the reaction was carried out for
10 min at ꢀ15 °C on the step (1) and for 24 h at ꢀ15 °C on the step
(2) as described in entry 11. In order to make sure that the opti-
mized reaction conditions are best, the reaction time was checked
on 15, 60, and 120 min for the step (1) and on 0.5, 3, and 48 h for
the step (2) as indicated in entries 12–17. Longer reaction times of
the step (1) decreased the enantiomer excesses (97–26% ee, entries
12, 13, and 14). Then, shorter reaction times of the step (2) de-
creased the reaction yields (77–73% yields, entries 16 and 15)
and longer reaction time of the step (2) did not give any change
in both the yield and the enantiomer excess (entry 17). It is well
known that Cbz-L-Phg-OH (1) is easily racemized under various
kinds of reaction conditions because of the reactivity of the benzyl
position on it as shown in Scheme 2.
⇑
Corresponding author. Tel./fax: +81 479 30 4610.
E-mail address: nimai@cis.ac.jp (N. Imai).
0040-4039/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.11.042

T. Noguchi et al. / Tetrahedron Letters 55 (2014) 394–396
395
Table 1
Table 2
Primary amidation of Cbz-L-Phg-OH (1)a
Optimization of the reaction conditions on the primary amidation of Cbz-Phe-Phe-OH
(3aa)a
Ph
O
Ph
O
1) ClCO2Et, Et3N, THF
2) NH4Cl, H2O
Ph
Ph
Cbz-HN
NH2
O
Cbz-HN
OH
O
O
1) ClCO2Et, Et3N, THF
*
O
*
Ph
P-HN
N
H
1
2
*
Ph
P-HN
N
H
*
NH2
OH 2) NH4Cl, H2O
Entry
Step (1)
Step (2)
Yieldd (%) % eee
3aa
4aa
Temp (°C) Time (min) Temp (°C) Time (h)
Entry 3aa
Yieldd (%) % dee % def Ratio of productsg (%)
1
5
5
5
5
0
30
30
30
30
30
30
30
30
30
5
10
15
60
120
10
10
10
5
5
0
0
0
0.5
0.5
0.5
43
44
47
58
59
63
63
68
79
73
83
89
58
32
73
77
82
46
47
67
51
59
83
86
93
92
99
99
97
75
26
99
99
98
LL
DL
DD LD
2b
3
1
2
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
L
L
L
L
-Phe-
-Phe-
-Phe-
-Phe-
-Phe-
-Phe-
-Phe-
-Phe-
-Phe-
L
-Phe-OH 63
-Phe-OH 66
52
88
13
67
59
23
48
83
15
68
58
25
73.8
8.6
57.7
16.1
0
0
0
0
0
0
0
0
0
26.2
4
5
6
7
24
D
91.4
42.3
83.9
0
24
24
24
24
24
24
24
24
24
24
3b
4b
5b
6b
7c
L-Phe-OH 94
D
ꢀ5
ꢀ5
-Phe-OH 86
-Phe-OH 90
-Phe-OH 86
ꢀ10
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ10
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
ꢀ15
D
L
79.0 21.0
37.7 62.3
0
0
>99
0
8
D
D
0
0
9c
L
L
L-Phe-OH 72
>99 >99 >99
0
0
10c
11c
12c
13c
14c
15c
16c
17c
8c
D
-Phe-OH 84
-Phe-OH 81
-Phe-OH 74
>99 >99
>99 >99
>99 >99
0
0
0
0
0
>99
0
9c
D
L
10c
D
-Phe-
D
>99
0
a
All reactions were carried out with 0.30 mmol of 3aa, 0.42 mmol of ClCO2Et, and
0.90 mmol of Et3N in 6 mL of THF. After stirring for 30 min at 5 °C, 0.45 mmol of
1.0 M aqueous solution of NH4Cl was added at 5 °C to the reaction mixture.
0.5
3
48
b
The reaction time on the step (2) was 24 h.
c
ClCO2Et and 1.0 M aqueous solution of NH4Cl precooled at ꢀ15 °C were added
a
All reactions were carried out with 0.5 mmol of 1, 0.7 mmol of ClCO2Et, and
1.5 mmol of Et3N in 10 mL of THF. After stirring for 30 min, 0.75 mmol of 1.0 M
aqueous solution of NH4Cl was added to the reaction mixture.
at ꢀ15 °C to the prepared solutions on the steps (1) and (2), respectively. Then, the
reaction times on the steps (1) and (2) were 10 min and 24 h, respectively.
d
Isolated yield.
b
e
The D-form was used instead of Cbz-
L
-Phg-OH.
Determined by 1H NMR analysis.
c
f
ClCO2Et and 1.0 M aqueous solution of NH4Cl precooled at ꢀ15 °C were added
Determined by HPLC analysis with a 95:5:0.05 mixture of hexane, ethanol, and
diethylamine as an eluent using Chiralcel OD (1.0 mL/min).
at ꢀ15 °C to the prepared solutions on the steps (1) and (2), respectively.
d
g
Isolated yield.
Retention times of LL-, DL-, DD-, and LD-forms on HPLC analysis were 34.5, 41.2,
e
Determined by HPLC analysis with 4:1 mixture of hexane and 2-propanol as an
49.5, and 53.6 min, respectively.
eluent using Chiralcel AD.
Ph
Ph
Next, the results of the primary amidation of Cbz-Phe-Phe-OH
(3aa) using NH4Cl via the corresponding mixed carbonic carboxylic
O
O
O
1) ClCO2Et, Et3N, THF, -15 oC, 10 min
2) NH4Cl, H2O, -15 oC, 24 h
O
O
Ph
N
H
Ph
Ph
N
H
NH2
OH
anhydride are collected in Table 2. The reactions of Cbz-
L-Phe-L-
6 (76% yield, 94% ee)
5
Phe-OH (3aLaL) and Cbz- -Phe- -Phe-OH (3aLaD) in our original
L
D
conditions (for 30 min at 5 °C on the step (1) then for 30 min at
5 °C on the step (2)) afforded the corresponding primary amides
(4aLaL and 4aLaD) in 63% and 66% yields, respectively (entries 1
and 2). Unfortunately, these diastereomer excesses were not good
(52% de and 88% de, respectively), by epimerization. The reactions
of 3aLaL and 3aDaD for 24 h at 5 °C on the step (2) proceeded fur-
ther epimerization to afford worse diastereomer excesses (13% de
and 23% de, respectively), as shown in entries 3 and 6. The reaction
conditions were optimized well by using precooled reagents at
ꢀ15 °C on both steps (1) and (2) and the results (72–84% yields,
>99% de) were drastically improved as described in entries 7–10.
Furthermore, the primary amidations of N-protected dipeptides
3 with NH4Cl were carried out using the precooled dropping meth-
Ph
O
Ph
1) ClCO2Et, Et3N, THF, 5 oC, 30 min
2) NH4Cl, H2O, 5 oC, 30 min
O
O
Ph
N
H
N
H
NH2
OH
6 (29% yield, 67% ee)
5
Scheme 3. The amidation of 5 with NH4Cl at 5 °C or ꢀ15 °C.
7–14). The reactions of Cbz-
using ClCO2Et and Et3N afforded Cbz-
57% yield with 92% de by 1H NMR analysis (entry 15). Unfortu-
nately, the reaction of Cbz- -Phe- -Cys-OH (3aLgD) with NH4Cl
using ClCO2Et and Et3N gave a complex mixture (entry 16).
As shown in Scheme 3, the reaction of N-phenethyl- -Phe-OH
L
-Phe-
D
-Ser-OH (3aLfD) with NH4Cl
L
-Phe- -Ser-NH2 (4aLfD) in
D
L
D
od and the results are collected in Table 3. The reaction of Cbz-
L
-
L
Phe-
L
-Phe-OH (3aLaL) with NH4Cl using ClCO2Et and Et3N afforded
-Phe-
-Phe-NH2 (4aLaL) in 72% yield with >99% de by 1H NMR
-Phe- -Phe-OH (3aLaD) reacted with NH4Cl in the
similar conditions (entry 1) to afford Cbz- -Phe- -Phe-NH2 (4aLaD)
(5) with NH4Cl in the presence of ClCO2Et at 5 °C proceeded with
racemization via the easier formation of the corresponding 1,3-
oxazol-5-one and 2,4-disubstituted 1,3-oxazolin-5-ol.4 In addition,
the unprotected side of dipeptides 3 is epimerized on the basis of
the similar reason as indicated in Scheme 4. On the other hand,
Cbz-
L
L
analysis.3 Cbz-
L
D
L
D
in 84% with >99% de as the diastereoiomer (entry 2). The reactions
of the dipeptides 30aLaL, 30aLaD and 300aLaL, 300aLaD protected
other conventional groups such as tert-butoxycarbonyl (Boc) and
9-fluorenylmethoxycarbonyl (Fmoc) with NH4Cl afforded the cor-
responding primary amides 40aLaL, 40aLaD and 400aLaL, 400aLaD,
respectively, in 85–95% yields with 81–99% de by HPLC analysis
using Chiralcel OD (entries 3–6), and slight epimerization was
observed in the cases of entries 3 and 5. Finally, the reactions of
several kinds of dipeptides 3aLbL–3aLeL and 3aLbD–3aLeD with
NH4Cl gave the corresponding primary amides 4aLbL–4aLeL and
4aLbD–4aLeD, respectively, in 75–93% yields with >99% de (entries
Cbz-L-Phe-OCO2Et was not converted to the corresponding
1,3-oxazol-5-one and was not racemized by the stronger
electron-donating effect of the benzyloxy group compared with
the phenethyl group of 5-OCO2Et.
In conclusion, we have found that primary amides 2 and 4 were
easily prepared in 83% yield with 99% ee and 57–95% yields with
81–99% de from the corresponding phenylglycine 1 and dipeptides
3 using NH4Cl under extremely mild conditions by the precooled
dropping method. Particularly, it is amazing that racemization or
epimerization does not proceed in the reactions of Cbz-protected

396
T. Noguchi et al. / Tetrahedron Letters 55 (2014) 394–396
R
O
O
R
O
R
OH
R
O
O
5oC
H N
*
N
O
N
O
N
O
O
OEt
epimerization
EtOCOO-
*
*
*
Ph
Ph
Ph
Ph
Ph
P-HN
P-HN
P-HN
Ph
NH-P
3-OCO2Et
Ph
H N
O
O
O
Ph
O
Ph
OH
Ph
O
5oC
N
O
N
O
N
O
O
OEt
racemization
Ph
EtOCOO-
Ph
5-OCO2Et
Ph
H N
O
O
O
5oC
no formation of 1,3-oxazolin-5-one
O
OEt
O
Ph
Cbz-L-Phe-OCO2Et
Scheme 4. Possible pathway for epimerization of 3-OCO2Et and for racemization of 5-OCO2Et.
Table 3
dipeptides 3aLaL–3aLeL and 3aLaD–3aLfD using our developed
method. Further investigations about this type of reaction are
under way in our group.
Preparation of primary amides derived from various dipeptides (3)a
O
R
*
O
R
*
1) ClCO2Et, Et3N, THF, -15 oC, 10 min
2) NH4Cl, H2O, -15 oC, 24 h
O
O
*
*
Ph
P-HN
N
H
Ph
P-HN
N
H
NH2
OH
References and notes
3
4
1. (a) Jung, S.; Tsukuda, Y.; Kawashima, R.; Ishiki, T.; Matsumoto, A.; Nakaniwa, A.;
Takagi, M.; Noguchi, T.; Imai, N. Tetrahedron Lett. 2013, 54, 5718–5720; (b)
Noguchi, T.; Sekine, M.; Yokoo, Y.; Jung, S.; Imai, N. Chem. Lett. 2013, 42, 580–
582; (c) Noguchi, T.; Jung, S.; Imai, N. Chem. Lett. 2012, 41, 577–579; (d) Noguchi,
T.; Tehara, N.; Uesugi, Y.; Jung, S.; Imai, N. Chem. Lett. 2012, 41, 42–43.
2. (a) Carpino, L. A.; El-Faham, A. J. Org. Chem. 1994, 59, 695–698; (b) El-Faham, A.;
Albericio, F. J. Org. Chem. 2008, 73, 2731–2737; (c) El-Faham, A.; Funosas, R. S.;
Prohens, R.; Albericio, F. Chem. Eur. J. 2009, 15, 9404–9416. and the references
cited therein.
Entry
Dipeptide
3
Yieldb (%)
% dec
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Cbz-
Cbz-
L
-Phe-
-Phe-
L
-Phe-OH
-Phe-OH
3aLaL
3aLaD
30aLaL
30aLaD
300aLaL
300aLaD
3aLbL
3aLbD
3aLcL
3aLcD
3aLdL
3aLdD
3aLeL
3aLeD
3aLfL
72 (63)
84
>99 (52)
>99
L
D
Boc-
Boc-
L
-Phe-
-Phe-
L
-Phe-OH
85
94
90
95
81d
L
D
-Phe-OH
>99d
90d
Fmoc-
Fmoc-
L
L
-Phe-
-Phe-
L
-Phe-OH
-Phe-OH
D
98d
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
Cbz-
L-Phe-
L-Phe-
L-Phe-
L-Phe-
L-Phe-
L-Phe-
L-Phe-
L-Phe-
L-Phe-
L-Phe-
L
-Ala-OH
-Ala-OH
-Val-OH
-Val-OH
-Met-OH
-Met-OH
-Phg-OH
-Phg-OH
-Ser-OH
-Cys-OH
89 (83)
76
93 (69)
90
78 (70)
85
79 (0)
75
>99 (43)e
>99e
3. A typical procedure of the primary amidation of 3aLaL using ClCO2Et is as
D
follows. To a solution of 134 mg (0.30 mmol) of Cbz-
L
-Phe-
L-Phe-OH (3aLaL) and
L
>99 (88)
>99
>99 (56)
>99
>99 (-)
>99
92
40 lL (0.42 mmol, 1.4 equiv) of ClCO2Et in 6 mL of THF, 125
l
L (0.90 mmol,
D
3.0 equiv) of Et3N precooled at ꢀ15 °C was added at ꢀ15 °C. After stirring for
30 min at ꢀ15 °C, 0.45 mL of 1.0 M aqueous solution of NH4Cl (0.45 mmol,
1.5 equiv) precooled at ꢀ15 °C was added at ꢀ15 °C to the colorless suspension.
The mixture was stirred for 24 h at ꢀ15 °C, and 15 mL of water was added at
ꢀ15 °C to the mixture. The resulted colorless clear solution was extracted with
100 mL of EtOAc, washed with 10 mL of brine, and dried over anhydrous MgSO4.
The crude product was chromatographed on silica gel with a 1:1 mixture of
L
D
L
D
D
D
57
0
3aLfD
—
hexane and EtOAc to afford 97.2 mg (72% yield) of 4aLaL (Cbz-
L
-Phe-
L-Phe-NH2).
Compound 4aLaL: colorless powder; 1H NMR (DMSO-d6):
d
2.67 (1H, dd,
a
All reactions were carried out with 0.30 mmol of 3, 0.42 mmol of ClCO2Et, and
0
J = 10.7, 13.7 Hz, CHCHAPh), 2.84 (1H, dd, J = 8.7, 13.5 Hz, CHCHA Ph), 2.92 (1H,
dd, J = 3.7, 13.7 Hz, CHCHBPh), 3.02 (1H, dd, J = 5.0, 13.5 Hz, CHCHB Ph), 4.20–
0.90 mmol of Et3N in 6 mL of THF. After stirring for 10 min at ꢀ15 °C, 0.45 mmol of
1.0 M aqueous solution of NH4Cl was added at ꢀ15 °C to the reaction mixture.
ClCO2Et and 1.0 M solution of NH4Cl precooled at ꢀ15 °C were added at ꢀ15 °C to
the prepared solutions on the steps (1) and (2), respectively.
0
4.25 (1H, m, NHCH), 4.44–4.49 (1H, m, NHCH), 4.94 (2H, s, OCH2Ph), 7.05–7.39
(17H, m, C6H5ꢁ3, NH2), 7.48 (1H, d, J = 8.6 Hz, NH), 8.02 (1H, d, J = 8.3 Hz, NH).
4. Montalbetti, C. A. G. N.; Falque, V. Tetrahedron 2005, 61, 10827–10852. and the
references cited therein.
b
Isolated yield. The yields of the products obtained in the reactions at 5 °C are
described in parentheses.
Determined by 1H NMR analysis. The diastereomer excesses of the products
c
obtained in the reactions at 5 °C are described in parentheses.
d
Determined by HPLC analysis with a 4:1 mixture of hexane and 2-propanol as
an eluent using Chiralcel OD (1.0 mL/min).
e
Determined by HPLC analysis with a 9:1 mixture of hexane and 2-propanol as
an eluent using Chiralcel OD (1.0 mL/min).