Convenient green preparation of dipeptides using unprotected α-amino acids

Article abstract of DOI:10.1016/j.tetasy.2016.11.009

Dipeptides and amides were obtained in high yields from N-carbobenzyloxy α-amino acids and 3-phenylpropanoic acid with unprotected α-amino acids via the corresponding mixed carbonic carboxylic anhydrides using ethyl chloroformate and triethylamine by an ecological and convenient method in which the protection of C-terminals is not needed.

Full text of DOI:10.1016/j.tetasy.2016.11.009

Tetrahedron: Asymmetry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Convenient green preparation of dipeptides using unprotected
a-amino acids
⇑
Tetsuya Ezawa, Seunghee Jung, Yuya Kawashima, Takuya Noguchi, 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:
Dipeptides and amides were obtained in high yields from N-carbobenzyloxy
a-amino acids and 3-phenyl-
Received 14 September 2016
Revised 8 November 2016
Accepted 14 November 2016
Available online xxxx
propanoic acid with unprotected -amino acids via the corresponding mixed carbonic carboxylic anhy-
a
drides using ethyl chloroformate and triethylamine by an ecological and convenient method in which the
protection of C-terminals is not needed.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
carbonic carboxylic anhydrides activated with alkyl chloroformate
in the presence of triethylamine as shown in Scheme 2.12e
Peptides are one of the most important human components and
induce various physiological activities as an element of proteins.1–3
Therefore, various types of convenient peptide syntheses have
been developed in chemical and biomedical research.4–6 Recently,
solvent-free syntheses of peptide using ball-milling were reported
by Lamaty et al.7 and Juaristi et al.8 Generally, condensations
between carboxylic acids and amines need activation of the corre-
sponding carbonyl group with an activating reagent such as thionyl
chloride, phosphoryl chloride, oxalyl chloride, diphenylphosphoryl
azide, carbonyldiimidazole, or alkyl chloroformate.9 Among them,
the mixed carbonic carboxylic anhydrides prepared from the corre-
sponding carboxylic acids are relatively stable and efficiently reac-
tive with nucleophiles.10 They have been commonly used as a
reactive intermediate for amidation of carboxylic acids with
amines. Furthermore, it is well known that the functional groups
2. Results and discussion
In
a
preliminary investigation, the reaction of 3-phenyl-
-phenylalanine 2a in the pres-
propanoic acid 1 with 1.5 equiv of
L
ence of 1.4 equiv of ethyl chloroformate and 3.0 equiv of
triethylamine in tetrahydrofuran (THF)-H2O afforded the corre-
sponding amide 3a in 97% yield as indicated in entry 2 of Table 1.
The solvent effect of the reaction was examined as shown in
Table 1. The reaction of 1 and 2a in ether was monitored by TLC
in both steps; the second step did not proceed at 0 °C. Therefore,
the second step was performed at rt to afford the corresponding
amide 3a in 87% yield (see entry 1). In the case of using dimethyl
sulfoxide (DMSO) and N,N-dimethylformamide (DMF) as the sol-
vent, the yield of 3a decreased to 4% and 59%, respectively as
shown in entries 4 and 5. The reactions using 1,4-dioxane, DMSO,
and DMF as the solvent were carried out at rt in order to avoid
freezing. Amidation of 1 with 2a proceeded in THF, 1,4-dioxane,
acetone, and acetonitrile to give 3a in 93–97% yields. A small
such as hydroxy, amino, and carboxy group on
a-amino acids
should be protected in order to avoid production of undesired
by-products in the synthesis of peptides.11 However, the methods
using protecting groups are not environmentally friendly (see
Scheme 1).
amount of N-ethoxycarbonyl-L-phenylalanine 4a was detected as
a by-product on the basis of 1H NMR analysis in all entries of
Table 1. It is assumed that 4a is mainly produced by the reaction
of 2a with the remaining ethyl chloroformate (see Scheme 2).
Next, the effect of the quantity of ethyl chloroformate on the
amidation was examined and the results are summarized in
Table 2. Carboxylic acid 1 was efficiently coupled with 2a to afford
3a in 94–97% yields, and a small amount of the by-product 4a was
detected by 1H NMR analysis in all entries of Table 2. The opti-
mized conditions for preparing 3a were the treatment of 1 with
1.4 equiv of ethyl chloroformate and 3.0 equiv of triethylamine in
Recently, we reported the convenient amidation of the mixed
carbonic carboxylic anhydrides with unprotected
a-amino acids
under neutral or basic conditions to afford the corresponding
dipeptides easily by crystallization.12d,e In addition, we have just
developed an amidation using ammonium chloride or aniline
derivatives as a nucleophile in the presence of water.12a–c
Herein, we describe in detail the condensation of carboxylic
acids 1 and 5 with
a-amino acids 2 via the corresponding mixed
⇑
Corresponding author. Tel.: +81 479 30 4610.
E-mail address: nimai@cis.ac.jp (N. Imai).
http://dx.doi.org/10.1016/j.tetasy.2016.11.009
0957-4166/Ó 2016 Elsevier Ltd. All rights reserved.
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2
T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
R2
O
O
R2
R1
O
R1
O
X
R1
O
H2N
OP2
N
H
P1-HN
P1-HN
P1-HN
OP2
OH
activation
Scheme 1. General synthetic methods of dipeptides.
R2
O
O
R2
O
R2
O
O
2
ClCO2R3
Et3N
R1
O
R1
O
O
R1
H2N
OH
N
H
R3O
N
H
O
OR3
+
in THF-H2O
P-HN
OH
in THF
P-HN
OH
OH
P-HN
5
6
4
Scheme 2. Convenient synthetic method of dipeptides via the mixed carbonic carboxylic anhydrides.
Table 1
Solvent effect on the amidation of 3-phenylpropanoic acid 1 with L-phenylalanine 2a in the presence of ethyl chloroformatea
Ph
O
O
1) ClCO2Et, Et3N, 30 min
2) -Phe-OH 2a in H2O, 30 min
O
Ph
OH
Ph
N
H
L
OH
1
3a
Entry
Solvent
Reaction Temp.
Yieldb/%
Step 1)
Step 2)
rtc
1
2
3
4
5
6
7
Ether
THF
1,4-Dioxane
DMSO
DMF
0 °C
0 °C
rt
87
97
95
4
59
96
93
0 °C
rt
rt
rt
0 °C
0 °C
0 °C to rt
0 °C to rt
0 °C
Acetone
Acetonitrile
0 °C
a
All reactions were carried out with 0.5 mmol of 1, 0.7 mmol of ethyl chloroformate, and 1.5 mmol of triethylamine in 10 mL of the solvent. After stirring for 30 min,
0.75 mmol of 2a in 10 mL of H2O was added to the reaction mixtures.
b
Isolated yields.
The reaction time was 3 h.
c
Table 2
Effect of the quantity of ethyl chloroformate on the amidation of 3-phenylpropanoic acid 1 with L-phenylalanine 2aa
Ph
O
O
1) ClCO2Et, Et3N, THF, 30 min
2) -Phe-OH 2a in H2O, 30 min
O
Ph
N
H
Ph
OH
L
OH
3a
1
Entry
ClCO2Et
Reaction Temp.
Yieldb/%
Step 1)
0 °C
Step 2)
0 °C
0 °C
0 °C
0 °C to rt
0 °C to rt
1
2
3
4
5
1.1 eq
1.4 eq
2.0 eq
1.4 eq
1.4 eq
96
97
97
94
95
0 °C
0 °C
0 °C
0 °C to rt
a
All reactions were carried out with 0.5 mmol of 1, ethyl chloroformate, and 1.5 mmol of triethylamine in 10 mL of THF. After stirring for 30 min, 0.75 mmol of 2a in 10 mL
of H2O was added to the reaction mixtures.
b
Isolated yields.
THF at 0 °C, followed by the addition of 1.5 equiv of 2a in H2O at
0 °C (see entry 2 of Table 2).
give the corresponding amide 3a in 97%, 96%, and 94% yields,
respectively (see entries 2–4). We decided to use ethyl chlorofor-
mate as the activating reagent because the yield of 3a in the reac-
tion with ethyl chloroformate was similar to those with isopropyl
and isobutyl chloroformates, while ethyl chloroformate is less
expensive than isopropyl and isobutyl chloroformates.
Subsequently, we checked the effect of various activating
reagents on the amidation of 1 with 2a and the results are shown
in Table 3. The reaction of 1 with 2a using methyl chloroformate
afforded the corresponding amide 3a in 82% yield (see entry 1). In
contrast, 2a reacted with the mixed carbonic carboxylic anhydrides
prepared from 1 and ethyl, isopropyl or isobutyl chloroformate to
Then, Table 4 shows the results of the reactions of 1 with
several types of
a-amino acids 2a–2j. We selected L-Phe-OH 2a,
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T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
3
Table 3
tions. Hence, we decided to check the variety of the reaction using
Cbz group.
Effect of activating reagents on the amidation of 3-phenylpropanoic acid 1 with
L-phenylalanine 2aa
Ph
O
1) ClCO2R3, Et3N, THF, 0 oC, 30 min
O
O
Table 4
L
-Phe-OH 2a in H2O, 0 oC, 30 min
Ph
N
H
Ph
OH
2)
Amidation of 3-phenylpropanoic acid 1 with a-amino acids 2 without protection of C-
OH
3a
1
terminalsa
Entry
R3
Yieldb/%
R2
O
O
1) ClCO2Et, Et3N, THF, 0 oC, 30 min
1
2
3
4
Me
Et
i-Pr
i-Bu
82
97
96
94
O
Ph
OH
R2
O
2
Ph
N
H
2)
in H2O, 0 oC, 30 min
OH
1
3
H2N
OH
a
All reactions were carried out with 0.5 mmol of 1, 0.7 mmol of alkyl chloro-
Entry
Amide 3
Yieldb/%
97
formate, and 1.5 mmol of triethylamine in 10 mL of THF. After stirring for 30 min,
0.75 mmol of 2a in 10 mL of H2O was added to the reaction mixtures.
Ph
O
b
O
O
Isolated yields.
1
3a
Ph
N
H
OH
L
-Phg-OH 2b,
acids, -Val-OH 2c and
-amino acids, and -Glu-OH 2e,
-Met-OH 2h as -amino acids containing another functional
L
-Trp-OH 2i and
-tert-Leu-OH 2d as chain-branched
-Gln-OH 2f, -Thr-OH 2g and
L-His-OH 2j as aromatic a-amino
Ph
L
L
O
2
3
3b
3c
89
89
Ph
Ph
N
H
a
L
L
L
L
a
OH
group. The amidation of 1 with 2a proceeded efficiently to give
the corresponding amide 3a in excellent yield (see entry 1). Amide
3b was obtained in 89% yield by the reaction of 1 with 2b (see
entry 2). Amino acids 2c and 2d were reacted with the activated
1 to afford the corresponding amides 3c and 3d in excellent yields
in spite of sterically hindered side-chain such as isopropyl and tert-
butyl groups (see entries 3 and 4). The coupling reaction of 1 and
2e containing the carboxy group gave the corresponding amide
3e in lower yield with the recovered 1 because of the solubility
in the reaction solution (see entry 5). In contrast, the amidation
of 1 with 2f containing the amide group proceeded smoothly to
afford the corresponding amide 3f in 85% yield because 2f is highly
soluble in water (see entry 6). Amides 3g and 3h were prepared in
85% and 87% yields from 2g containing a hydroxy group and 2h
containing a sulfide group, respectively (see entries 7 and 8).
Amino acid 2i possessing indole moiety was converted into the
corresponding amide 3i in 86% yield (see entry 9). The reaction
with 2j gave N-ethoxycarbonylimidazolylamide 3j in 19% yield
due to the nucleophilic nitrogen atom on the imidazole moiety
(see entry 10).
O
O
N
H
OH
O
O
O
4
3d
Quant.
O
Ph
Ph
Ph
N
H
OH
COOH
O
5
6
3e
3f
59
85
N
H
OH
CONH2
O
N
H
OH
OH
O
O
O
7
8
3g
3h
85
87
Ph
Ph
N
H
OH
Next, the results of the condensation of N-protected
L-pheny-
lalanines 5a–5c with several types of unprotected -amino acids
a
SMe
2a and 2c–2j via the corresponding mixed carbonic carboxylic
anhydrides are shown in Table 5. We examined the effect of the
protecting group on the N-terminal of the starting a-amino acids.
We selected benzyl carbamate (Cbz), tert-butyl carbamate (Boc),
O
N
H
OH
H
N
and 9-fluorenylmethyl carbamate (Fmoc) as protecting groups for
the starting
tion of Cbz-
a
-amino acids 5a, 5a0, and 5a00, respectively. The reac-
O
9
3i
86
L
-Phe-OH 5a with L-Val-OH 2c afforded the correspond-
O
ing dipeptide 6ac in 75% yield (see entry 1), and Boc-L
-Phe-OH 5a0
Ph
Ph
N
H
was reacted with 2c to give the corresponding dipeptide 6a0c in
82% yield (see entry 2). The dipeptide 6a00c was obtained by the
OH
O
reaction of Fmoc-L
-Phe-OH 5a00 with 2c in 77% yield (see entry 3).
EtO
Generally, the Boc group was cleaved by strong acids such as
HCl, trifluoroacetic acid (TFA), or p-toluenesulfonic acid (TsOH),
providing t-BuOH or isobutylene and CO2 as the by-products. It is
well known that tert-butylation generated from deprotection of
N
N
10
3j
19c
O
O
N
H
Boc group react readily with electron rich side-chain of
a-amino
OH
acids (Cys, Met, Thr, Ser, and Trp).13 We were concerned about
racemization under basic conditions although the Fmoc group is
cleaved by weak bases such as NaHCO3, piperidine, and morpho-
line. On the other hand, deprotection of the Cbz group is usually
carried out with hydrogenation using H2/Pd-C under mild condi-
a
All reactions were carried out with 0.5 mmol of 1, 0.7 mmol of ethyl chloro-
formate, and 1.5 mmol of triethylamine in 10 mL of THF. After stirring for 30 min,
0.75 mmol of 2 in 10 mL of H2O was added to the reaction mixtures.
b
Isolated yields.
The N-ethoxycarbonyl derivative was obtained.
c
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4
T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
Table 5
Synthesis of dipeptides 6 without protection of C-terminals in
a
-amino acids 2a
R2
Ph
O
1) ClCO2Et, Et3N, THF, 0 oC, 30 min
Ph
P-HN
O
O
N
H
6
2)
in H2O, 0 oC, 30 min
R2
O
2
OH
P-HN
OH
5
H2N
OH
Entry
1
P
5
Dipeptide 6
Yieldb/%
75
Ph
O
O
Cbz
5a
6ac
N
H
Cbz-HN
Ph
OH
O
O
2
3
Boc
5a0
5a00
6a0c
6a00c
82
77
N
H
Boc-HN
Ph
OH
O
O
Fmoc
N
H
Fmoc-HN
Ph
OH
O
O
O
O
4
Cbz
5a
6ad
68
N
H
Cbz-HN
Ph
OH
COOH
O
5
6
Cbz
Cbz
5a
5a
6ae
6af
36
87
N
H
Cbz-HN
Ph
OH
CONH2
O
N
H
Cbz-HN
Ph
OH
Ph
O
O
O
7
8
Cbz
Cbz
5a
5a
6aa
80
81
N
H
Cbz-HN
Ph
OH
Ph
O
6aa0
N
H
Cbz-HN
Ph
OH
OH
O
O
O
9
Cbz
Cbz
5a
5a
6ag
6ah
81
65
N
H
Cbz-HN
Ph
OH
SMe
O
10
N
H
Cbz-HN
OH
H
N
Ph
O
11
Cbz
5a
6ai
78
O
N
H
Cbz-HN
OH
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T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
5
Table 5 (continued)
Entry
P
5
Dipeptide 6
Yieldb/%
O
EtO
N
N
21c
Ph
O
12
Cbz
5a
6aj
O
N
H
Cbz-HN
OH
a
All reactions were carried out with 0.5 mmol of 5, 0.7 mmol of ethyl chloroformate, and 1.5 mmol of triethylamine in 10 mL of THF. After stirring for 30 min, 0.75 mmol of
2 in 10 mL of H2O was added to the reaction mixtures.
b
Isolated yields.
The N-ethoxycarbonyl derivative was obtained.
c
L
-Trp-OH 2i. The amino acid 5a reacted with 2g and 2i to afford
The yield of dipeptide 6ad was slightly lower due to bulky side-
chain on -tert-Leu 2d as indicated in entry 4. The reaction of 5a
with -Glu-OH 2e did not proceed well because of the solubility
(see entry 5); while the dipeptide 6af was synthesized from 5a
and
-Gln-OH 2f in 87% yield (see entry 6). The two 1H NMR spectra
of the diastereomers 6aa and 6aa0 prepared from the reactions of
5a with the enantiomers -Phe-OH 2a and
-Phe-OH 2a0 in 80%
and 81% yield, respectively, showed no epimerization under the
optimized reaction conditions (see entries 7 and 8). Subsequently,
we tried to run the reactions of L-Thr-OH 2g, L-Met-OH 2h, and
the corresponding dipeptides 6ag and 6ai in 81% and 78% yields,
respectively (see entries 9 and 11). The yield of the dipeptide
6ah synthesized from 5a and 2h containing sulfide group was
65% (see entry 10). In the case of L-His-OH 2j containing the imida-
zole moiety, the reaction gave N-ethoxycarbonyl dipeptide 6aj in
L
L
L
21% yield as indicated in entry 12 of Table 5.
L
D
Finally, we applied this method to various N-Cbz
a-amino acids
5b–5g, and the results are shown in Table 6. The reaction of Cbz-
Gly-OH 5b with
L-Phe-OH 2a proceeded to afford the correspond-
Table 6
Application to synthesis of various dipeptides 6 with N-Cbz
a
-amino acids 5 and L-phenylalanine 2aa
Ph
O
R1
O
1) ClCO2Et, Et3N, THF, 0 oC, 30 min
R1
Cbz-HN
O
N
in H2O, 0 oC, 30 min
H
2a
2) L-Phe-OH
Cbz-HN
OH
OH
6
5
Entry
1
5
Dipeptide 6
Yield b/%
Ph
O
O
5b
6ba
6ca
6da
6ea
6fa
75
79
71
76
70
N
H
Cbz-HN
O
OH
Ph
O
2
3
4
5
5c
5d
5e
5f
N
H
Cbz-HN
O
OH
Ph
O
N
H
Cbz-HN
O
OH
Ph
O
N
H
Cbz-HN
MeS
OH
Ph
O
O
N
H
Cbz-HN
OH
H
N
Ph
O
6
5g
6ga
85
O
N
H
Cbz-HN
OH
a
All reactions were carried out with 0.5 mmol of 5, 0.7 mmol of ethyl chloroformate, and 1.5 mmol of triethylamine in 10 mL of THF. After stirring for 30 min, 0.75 mmol of
2a in 10 mL of H2O was added to the reaction mixtures.
b
Isolated yields.
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6
T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
ing dipeptide 6ba in 75% yield (see entry 1), and Cbz-
reacted with 2a to provide the corresponding dipeptide 6ca in
79% yield (see entry 2). Cbz- -Val-OH 5d and Cbz- -Leu 5e as
chain-branched -amino acids were converted to the correspond-
ing dipeptides 6da and 6ea in 71% and 76% yields, respectively (see
entries 3 and 4). Dipeptide 6fa was synthesized from Cbz- -Met-
OH 5f and 2a in 70% yield (see entry 5). The activated Cbz- -Trp-
OH 5g coupled easily with 2a to produce the dipeptide 6ga in
85% yield because of the intramolecular
stacking interaction14
L
-Ala-OH 5c
J = 1.0, 7.6, 7.6 Hz, 1H, CHACO), 2.45 (ddd, J = 1.0, 8.0, 8.0 Hz, 1H,
CHBCO), 2.79 (dd, J = 7.6, 8.0 Hz, 2H, CH2CH2C6H5), 2.91 (dd,
J = 8.9, 13.8 Hz, 1H, CHCHAC6H5), 3.15 (dd, J = 5.0, 13.8 Hz, 1H,
CHCHBC6H5), 4.64 (dd, J = 5.0, 8.9 Hz, 1H, CHCO), 7.13–7.17, 7.18–
7.26 (m, m, 5H, 5H, C6H5 ꢀ2); 13C NMR (100 MHz, MeOD-d4): d
30.5, 36.2, 36.4, 52.9, 124.9, 125.4, 127.1, 127.1, 127.2, 128.0,
136.3, 139.9, 172.7, 172.8; HRMS (ESI-TOF): Calcd for C18H19NO3Na
L
L
a
L
L
(M+Na)+: 320.1257, found: 320.1283; IR (KBr,
(OH), 1708 (CO2), 1600 (CON).
m
max/cmꢁ1) = 3307
p-p
between phenyl and indole moieties in 5g (see entry 6).
4.2.2. N-(3-Phenylpropanoyl)-
L-Phg-OH 3b
27
126 mg (89%), colorless powder; mp: 152–153 °C;
[a
]
=
D
3. Conclusions
+108.4 (c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 2.57
(ddd, J = 1.5, 7.6, 7.6 Hz, 1H, CHACO), 2.57 (ddd, J = 1.5, 8.0,
8.0 Hz, 1H, CHBCO), 2.90 (dd, J = 7.6, 8.0 Hz, 2H, CH2C6H5), 5.41 (s,
1H, CHCO), 7.12–7.24, 7.30–7.34 (m, m, 5H, 5H, C6H5 ꢀ2); 13C
NMR (100 MHz, MeOD-d4):d 32.8, 38.5, 58.2, 127.2, 128.8, 129.3,
129.5, 129.5, 129.8, 138.2, 142.2, 173.8, 174.8; HRMS (ESI-TOF):
Calcd for C17H17NO3Na (M+Na)+: 306.1101, found: 306.1086; IR
We have developed an environmentally friendly and conve-
nient synthetic method to dipeptides without protection of C-ter-
minals, in which the dipeptides 6aa, 6aa0, 6a0c, 6a00c, and 6ac–6aj
were obtained in 36–81% yields from the corresponding
acids. Additionally, we succeeded to synthesizing various dipep-
a-amino
tides 6ba–6ga in 70–85% yields using the corresponding
acids 5a–5g protected by Cbz group on N-terminal and
a-amino
(KBr, m
max/cmꢁ1) = 3338 (OH), 1698 (CO2), 1616 (CON).
L
-Phe-OH
2a. The main by-products are triethylamine hydrochloride, carbon
dioxide, and the corresponding alcohols and racemization was not
observed in our method.
4.2.3. N-(3-Phenylpropanoyl)-
L-Val-OH 3c
111 mg (89%), colorless powder; mp: 141–143 °C; [
a
]
21 = ꢁ22.9
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 0.87 (d, J = 6.8 Hz,
6H, (CH3)2), 2.06–2.14 (m, 1H, CH(CH3)2), 2.57 (ddd, J = 3.9, 7.6,
7.6 Hz, 1H, CHACO), 2.58 (ddd, J = 3.9, 7.7, 7.7 Hz, 1H, CHBCO),
2.91 (dd, J = 7.6, 7.7 Hz, 2H, CH2C6H5), 4.28 (d, J = 5.7 Hz, 1H,
CHCO), 7.13–7.17, 7.20–7.27 (m, m, 1H, 4H, C6H5); 13C NMR
(100 MHz, MeOD-d4):d 18.3, 19.6, 31.7, 32.9, 59.1, 127.2, 129.5,
129.5, 142.2, 175.1, 175.4; HRMS (ESI-TOF): Calcd for C14H19NO3Na
4. Experimental
4.1. General
All reagents were used without further purification. The 1H
NMR and 13C NMR spectra were measured with a Bruker Ultra-
shieldTM 400 Plus spectrometer. The chemical shifts are expressed
in parts per million downfield from tetramethylsilane (d = 0.00)
as an internal standard. Chemical shifts (d) are reported in ppm,
and spin–spin coupling constants (J) are given in hertz. Abbrevia-
tions to denote the multiplicity of a particular signal are s (singlet),
d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad).
The high-resolution mass spectra (HRMS) of the compounds with
a high molecular weight were recorded using a Waters LCT Premier
(ESI-TOF-MS) spectrometer. Reactions were monitored using thin-
layer chromatography with silica gel 60 F254. Purification of the
reaction products was carried out by column chromatography
using silica gel (64–210 mesh). Melting points were determined
with a hot plate apparatus. Optical rotations were measured on a
digital polarimeter with a sodium lamp at room temperature.
Infrared (IR) spectra were recorded on HORIBA FT-IR Fourier trans-
form infrared spectrophotometer.
(M+Na)+: 272.1257, found: 272.1243; IR (KBr,
(OH), 1699 (CO2), 1616 (CON).
m
max/cmꢁ1) = 3338
4.2.4. N-(3-Phenylpropanoyl)-L-tert-Leu-OH 3d
132 mg (Quant), colorless powder; mp: 183–185 °C;
[
a
]
26 = ꢁ9.7 (c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 0.94
D
(s, 9H, (CH3)3C), 2.59 (ddd, J = 6.5, 7.7, 7.7 Hz, 1H, CHACO), 2.59
(ddd, J = 6.5, 7.7, 7.7 Hz, 1H, CHBCO), 2.91 (dd, J = 7.7, 7.7 Hz, 2H,
CH2C6H5), 4.25 (s, 1H, CHCO), 7.13–7.17, 7.20–7.26 (m, m, 1H,
4H, C6H5); 13C NMR (100 MHz, MeOD-d4):d 27.2, 32.9, 34.8, 38.5,
62.2, 127.2, 129.5, 142.2, 174.4, 175.2; HRMS (ESI-TOF): Calcd for
C
m
15H21NO3Na (M+Na)+: 286.1414, found: 286.1386; IR (KBr,
max/cmꢁ1) = 3348 (OH), 1699 (CO2), 1610 (CON).
4.2.5. N-(3-Phenylpropanoyl)-L-Glu-OH 3e
83 mg (59%), colorless powder; mp: 131–134 °C; [
a]
23 = ꢁ28.4
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.80–1.90 (m,
1H, CHACH), 2.07–2.15 (m, 1H, CHBCH), 2.22–2.26 (m, 2H, CH2CH2-
CH), 2.54 (dd, J = 7.4, 8.0 Hz, 2H, CH2CO), 2.91 (dd, J = 7.4, 8.0 Hz,
2H, CH2C6H5), 4.40 (dd, J = 4.8, 9.8 Hz, 1H, CHCO), 7.15–7.18,
7.20–7.27 (m, m, 1H, 4H, C6H5); 13C NMR (100 MHz, MeOD-d4):d
27.9, 31.2, 32.8, 38.7, 53.0, 127.3, 129.5, 129.5, 142.2, 175.0,
175.4, 176.4; HRMS (ESI-TOF): Calcd for C14H17NO5Na (M+Na)+:
4.2. Typical procedure for amidation of 3-phenylpropanoic acid
1 with L-phenylalanine 2a
To a colorless solution of 75 mg (0.50 mmol) of 1 in 10 mL of
THF were added at 0 °C 67 L (0.70 mmol, 1.4 equiv) of ethyl chlo-
roformate and 209 L (1.5 mmol, 3.0 equiv) of triethylamine. After
l
l
stirring at 0 °C for 30 min, a solution of 124 mg (0.75 mmol,
1.5 equiv) of 2a in 10 mL of H2O was added at 0 °C to the colorless
suspension. The mixture was stirred at 0 °C for 30 min and concen-
trated in vacuo. To the residue was added a 1.0 M aqueous HCl
solution to adjust to pH 2. The resulting suspension was extracted
with 50 mL of EtOAc, washed with 10 mL of brine, and dried over
MgSO4. The crude product was chromatographed on silica gel with
a 2:1 mixture of hexane and EtOAc containing 1% AcOH to afford
302.0999, found: 302.1010; IR (KBr,
1716 (CO2), 1653 (CON).
m
max/cmꢁ1) = 3276 (OH),
4.2.6. N-(3-Phenylpropanoyl)-L-Gln-OH 3f
74 mg (53%), colorless powder; mp: 141–144 °C; [
a]
22 = ꢁ19.2
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.83–1.93 (m,
1H, CHACH), 2.07–2.24 (m, 3H, CHBCH, CH2CH2CH), 2.55 (dd,
J = 7.5, 8.1 Hz, 2H, CH2CO), 2.92 (dd, J = 7.5, 8.1 Hz, 2H, CH2C6H5),
4.37 (dd, J = 4.5, 9.4 Hz, 1H, CHCO), 7.14–7.18, 7.21–7.28 (m, m,
1H, 4H, C6H5); 13C NMR (100 MHz, MeOD-d4):d 28.6, 32.7, 32.8,
38.7, 53.3, 127.3, 129.5, 129.5, 142.2, 175.0, 175.4, 177.7; HRMS
(ESI-TOF): Calcd for C14H18N2O4Na (M+Na)+: 301.1159, found:
145 mg (97% yield) of N-(3-phenylpropanoyl)-L-Phe-OH 3a.
4.2.1. N-(3-Phenylpropanoyl)-
145 mg (97%), colorless powder; mp: 167–169 °C; [
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4): d 2.45 (ddd,
L-Phe-OH 3a
a]
23 = +1.4
D
Please cite this article in press as: Ezawa, T.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.11.009

T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
7
301.1184; IR (KBr,
(CON).
m
max/cmꢁ1) = 3292 (OH), 1724 (CO2), 1646
for C22H27N2O5 (M+H)+: 399.1914, found: 399.1918; IR (KBr, mmax
cmꢁ1) = 3307 (OH), 1697 (CO2), 1635 (CON).
/
4.2.7. N-(3-Phenylpropanoyl)-
L
-Thr-OH 3g
4.3.2. Boc-
149 mg (82%), colorless powder; mp; 64–67 °C; [
L-Phe-L
-Val-OH 6a0c
107 mg (85%), colorless oil; [
a
]
D
22 = ꢁ7.0 (c 1.00, MeOH); 1H
a]
27 = ꢁ5.3 (c
D
NMR (400 MHz, MeOD-d4):d 1.06 (d, J = 6.4 Hz, 3H, CH3), 2.61
(dd, J = 7.4, 8.2 Hz, 2H, CH2CO), 2.94 (dd, J = 7.4, 8.2 Hz, 2H, CH2C6-
H5), 4.24–4.29 (m, 1H, CHCH3), 4.41 (d, J = 3.0 Hz, 1H, CHCO), 7.13–
7.20, 7.21–7.27 (m, m, 1H, 4H, C6H5); 13C NMR (100 MHz, MeOD-
d4):d 20.4, 32.8, 38.7, 59.1, 68.4, 127.2, 129.5, 129.5, 142.2, 173.9,
1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 0.96 (d, J = 6.8 Hz,
6H, (CH3)2), 1.34 (s, 9H, (CH3)3C), 2.13–2.21 (m, 1H, CH(CH3)2),
2.80 (dd, J = 4.2, 13.9 Hz, 1H, CHAC6H5), 3.12 (dd, J = 4.9, 13.9 Hz,
1H, CHBC6H5), 4.32–4.37 (m, 2H, CHCO ꢀ2), 7.17–7.26 (m, 5H,
C6H5); 13C NMR (100 MHz, MeOD-d4):d 18.3, 19.6, 28.7, 32.1,
39.0, 57.3, 58.9, 80.7, 127.7, 129.4, 130.4, 138.7, 157.7, 174.4,
175.7; HRMS (ESI-TOF): Calcd for
274.1050, found: 274.1040; IR (NaCl,
1732 (CO2), 1653 (CON).
C
m
13H17NO4Na (M+Na)+:
max/cmꢁ1) = 3325 (OH),
174.5; HRMS (ESI-TOF): Calcd for
387.1890, found: 387.1901; IR (KBr,
1716 (CO2), 1652 (CON).
C
m
19H28N2O5Na (M+Na)+:
max/cmꢁ1) = 3309 (OH),
4.2.8. N-(3-Phenylpropanoyl)-
L-Met-OH 3h
123 mg (87%), yellow powder; mp: 73–75 °C; [
a]
21 = ꢁ28.9 (c
4.3.3. Fmoc-
187 mg (77%), colorless powder; mp; 93–95 °C; [
L-Phe-L
-Val-OH 6a00c
D
0.98, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.80–1.89 (m, 1H,
CHACH2S), 2.01–2.10 (m, 1H, CHBCH2S), 2.03 (s, 3H, CH3), 2.27–
2.41 (m, 2H, CH2CH2S), 2.54 (dd, J = 7.4, 7.9 Hz, 2H, CH2CO), 2.91
(dd, J = 7.4, 7.7 Hz, 2H, CH2C6H5), 4.48–4.51 (m, 1H, CHCO), 7.14–
7.27 (m, 5H, C6H5); 13C NMR (100 MHz, MeOD-d4):d 15.2, 31.2,
32.2, 32.8, 38.7, 52.6, 127.3, 129.5, 129.5, 142.2, 175.2, 175.4;
HRMS (ESI-TOF): Calcd for C14H19NO3SNa (M+Na)+: 304.0978,
a]
25 = ꢁ19.6 (c
D
1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 0.96 (d, J = 6.8 Hz,
6H, CH3 ꢀ2), 2.13–2.21 (m, 1H, CH(CH3)2), 2.85 (dd, J = 9.8,
13.9 Hz, 1H, CHAC6H5), 3.12 (dd, J = 4.9, 13.9 Hz, 1H, CHBC6H5),
4.11–4.36 (m, 4H, CHCH(CH3)2, CH2O, CHCH2O), 4.45–4.51 (m,
1H, CHCH2C6H5), 7.16–7.30, 7.37, 7.55–7.58, 7.78 (m, t, m, d,
J = 7.6, 7.6 Hz, 7H, 2H, 2H, 2H, C6H5, Fmoc); 13C NMR (100 MHz,
MeOD-d4):d 18.3, 19.6, 32.0, 39.0, 57.7, 59.0, 59.1, 68.1, 121.0,
126.3, 126.3, 127.8, 128.2, 128.8, 129.5, 130.4, 138.7, 142.6,
145.2, 158.3, 174.3, 174.5; HRMS (ESI-TOF): Calcd for C29H30N2O5-
found: 304.0953; IR (KBr,
1647 (CON).
m
max/cmꢁ1) = 3302 (OH), 1705 (CO2),
4.2.9. N-(3-Phenylpropanoyl)-
111 mg (89%), colorless powder; mp: 160–162 °C; [a]
27 = +1.3 (c
D
L
-Trp-OH 3i
Na (M+Na)+: 509.2047, found: 509.2039; IR (KBr,
= 3301 (OH), 1718 (CO2), 1691 (CON), 1656 (CON).
m )
max/cmꢁ1
1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 2.44 (dd, J = 7.6,
8.0 Hz, 2H, CH2CO), 2.78 (ddd, J = 3.1, 7.6, 7.6 Hz, 1H, CHAC6H5),
2.78 (ddd, J = 3.1, 8.0, 8.0 Hz, 1H, CHBC6H5), 3.11 (dd, J = 8.0,
14.8 Hz, 1H, CHACH), 3.30 (dd, J = 5.0, 14.8 Hz, 1H, CHBCH), 4.71
(dd, J = 5.0, 8.0 Hz, 1H, CHCO), 6.96, 7.00, 7.06–7.21, 7.32, 7.53 (s,
t, m, d, d, J = 7.4, 7.8, 7.8 Hz, 1H, 1H, 6H, 1H, 1H, indole, C6H5);
13C NMR (100 MHz, MeOD-d4):d 28.6, 32.7, 38.8, 54.7, 111.1,
112.3, 119.3, 119.8, 122.4, 124.4, 127.2, 128.9, 129.4, 129.5,
138.1, 142.2, 175.1, 175.3; HRMS (ESI-TOF): Calcd for C20H20N2O3-
4.3.4. Cbz-
L-Phe-L-tert-Leu-OH 6ad
140 mg (68%), colorless powder; mp; 164–167 °C; [
a
]
25 = ꢁ14.3
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.00 (s, 9H, CH3
ꢀ3), 2.83 (dd, J = 9.6, 13.8 Hz, 1H, CHAC6H5), 3.11 (dd, J = 5.3,
13.8 Hz, 1H, CHBC6H5), 4.29 (s, 1H, CHC(CH3)3), 4.48 (dd, J = 5.3,
9.6 Hz, 1H, CHCH2C6H5), 5.09 (s, 2H, OCH2C6H5), 7.17–7.32 (m,
10H, C6H5 ꢀ2); 13C NMR (100 MHz, MeOD-d4):d 27.1, 35.2, 38.9,
57.8, 61.9, 67.6, 127.6, 128.7, 129.0, 129.5, 130.4, 138.2, 138.5,
158.3, 173.8, 174.0; HRMS (ESI-TOF): Calcd for C23H28N2O5Na (M
Na (M+Na)+: 359.1366, found: 359.1348; IR (KBr,
= 3410 (OH), 1734 (CO2), 1655 (CON).
m )
max/cmꢁ1
+Na)+: 435.1890, found: 435.1890; IR (KBr,
(OH), 1699 (CO2), 1635 (CON).
m
max/cmꢁ1) = 3327
4.3. Typical procedure for amidation of Cbz-L-phenylalanine 5a
with -valine 2c
L
4.3.5. Cbz-
L-Phe-L-Glu-OH 6ae
78 mg (36%), colorless powder; mp; 149–152 °C; [
a]
27 = ꢁ12.1
D
To a colorless solution of 150 mg (0.50 mmol) of 5a in 10 mL of
THF were added at 0 °C 67 L (0.70 mmol, 1.4 equiv) of ethyl chlo-
roformate and 209 L (1.5 mmol, 3.0 equiv) of triethylamine. After
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.89–1.99 (m,
1H, CHACH2CO), 2.15–2.23 (m, 1H, CHBCH2CO), 2.39 (dd, J = 7.6,
7.8 Hz, 2H, CH2CO), 2.84 (dd, J = 9.6, 14.0 Hz, 1H, CHAC6H5), 3.15
(dd, J = 5.0, 14.0 Hz, 1H, CHBC6H5), 4.40–4.47 (m, 2H, CH ꢀ 2),
4.99 (d, J = 13.0 Hz, 1H, OCHAC6H5), 5.03 (d, J = 13.0 Hz, 1H, OCHB-
C6H5), 7.18–7.33 (m, 10H, C6H5 ꢀ2); 13C NMR (100 MHz, MeOD-
d4):d 28.0, 31.1, 39.0, 53.1, 57.8, 67.6, 127.8, 128.8, 129.0, 129.5,
130.4, 138.2, 138.6, 158.3, 174.2, 174.6, 176.4; HRMS (ESI-TOF):
Calcd for C22H25N2O7 (M+H)+: 429.1656, found: 429.1672; IR
l
l
stirring at 0 °C for 30 min, a solution of 88 mg (0.75 mmol,
1.5 equiv) of 2c in 10 mL of H2O was added at 0 °C to the colorless
suspension. The mixture was stirred at 0 °C for 30 min, and con-
centrated in vacuo. To the residue was added a 1.0 M aqueous
HCl solution to adjust to pH 2. The resulted suspension was
extracted with 50 mL of EtOAc, washed with 10 mL of brine, and
dried over MgSO4. The crude product was chromatographed on sil-
ica gel with a 1:1 mixture of hexane and EtOAc containing 1% AcOH
(KBr, m
max/cmꢁ1) = 3286 (OH), 1716 (CO2), 1660 (CON).
to afford 149 mg (75% yield) of Cbz-
L
-Phe-
L
-Val-OH 6ac.
4.3.6. Cbz-
L-Phe-L-Gln-OH 6af
187 mg (87%), colorless powder; mp; 179–182 °C; [
a
]
28 = ꢁ10.2
D
4.3.1. Cbz-
L
-Phe-L-Val-OH 6ac
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.91–1.98 (m, 1H,
CHACH2CO), 2.16–2.35 (m, 3H, CHBCH2CO, CH2CO), 2.85 (dd, J = 9.7,
14.0 Hz, 1H, CHAC6H5), 3.16 (dd, J = 4.9, 14.0 Hz, 1H, CHBC6H5),
4.38–4.43 (m, 2H, CH ꢀ 2), 4.99 (d, J = 12.7 Hz, 1H, OCHAC6H5),
5.03 (d, J = 12.7 Hz, 1H, OCHBC6H5), 7.17–7.33 (m, 10H, C6H5 ꢀ2);
13C NMR (100 MHz, MeOD-d4):d 28.8, 32.7, 39.0, 53.2, 57.8, 67.6,
127.8, 128.7, 129.0, 129.5, 129.5, 130.4, 138.2, 138.6, 158.3,
174.3, 174.6, 177.8; HRMS (ESI-TOF): Calcd for C22H25N3O6Na (M
149 mg (75%), colorless powder; mp; 140–143 °C; [
a
]
25 = ꢁ7.7
D
(c 1.00, MeOH); 1H NMR (400 MHz, CDCl3):d 0.84 (d, J = 6.8 Hz,
3H, CH3), 0.89 (d, J = 6.8 Hz, 3H, CH3), 2.13–2.21 (m, 1H, CH
(CH3)2), 3.08 (dd, J = 7.0, 7.3 Hz, 2H, CHCH2C6H5), 4.46–4.49 (m,
2H, CHCO ꢀ2), 5.07 (d, J = 12.3 Hz, 1H, OCHAC6H5), 5.11 (d,
J = 12.3 Hz, 1H, OCHBC6H5), 5.45 (br s, 1H, NH), 6.36 (br s, 1H,
NH), 7.18–7.37 (m, 10H, C6H5 ꢀ2); 13C NMR (100 MHz, CDCl3):d
17.6, 18.8, 30.9, 38.2, 56.3, 57.2, 67.2, 127.1, 128.1, 128.3, 128.6,
128.8, 129.3, 136.1, 156.1, 171.2, 174.7; HRMS (ESI-TOF): Calcd
+Na)+: 450.1636, found: 450.1606; IR (KBr,
(OH), 1716 (CO2), 1654 (CON).
m
max/cmꢁ1) = 3309
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8
T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
4.3.7. Cbz-
L
-Phe-
L
-Phe-OH 6aa
508.1843, found: 508.1836; IR (KBr,
3313 (OH), 1712 (CO2), 1627 (CON).
m
max/cmꢁ1) = 3408 (NH),
178 mg (80%), colorless powder; mp; 148–151 °C; [
a]
24 = ꢁ10.5
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 2.76 (dd, J = 9.7,
0
14.0 Hz, 1H, CHAC6H5), 3.00 (dd, J = 8.2, 13.8 Hz, 1H, CHA C6H5),
4.3.12. Cbz-Gly-
133 mg (75%), colorless sticky oil; [a]
28 = +25.1 (c 0.98, MeOH);
D
L-Phe-OH 6ba
3.08 (dd, J = 5.0, 14.0 Hz, 1H, CHBC6H5), 3.19 (dd, J = 5.2, 13.8 Hz,
1H, CHB’C6H5), 4.36 (dd, J = 5.0, 9.7 Hz, 1H, CH), 4.65 (dd, J = 5.2,
8.2 Hz, 1H, CH0), 4.97 (d, J = 12.6 Hz, 1H, OCHAC6H5), 5.01 (d,
J = 12.6 Hz, 1H, OCHBC6H5), 7.15–7.33 (m, 15H, C6H5 ꢀ3); 13C
NMR (100 MHz, MeOD-d4):d 38.5, 39.1, 55.0, 57.7, 67.6, 127.7,
127.8, 128.7, 128.9, 129.4, 129.5, 130.4, 130.5, 138.2, 138.6,
158.2, 173.9, 174.2; HRMS (ESI-TOF): Calcd for C26H27N2O5 (M
1H NMR (400 MHz, MeOD-d4):d 3.00 (dd, J = 7.6, 14.2 Hz, 1H, CHA-
C6H5), 3.18 (dd, J = 5.4, 14.2 Hz, 1H, CHBC6H5), 3.72 (d, J = 17.0 Hz,
1H, CHACONH), 3.79 (d, J = 17.0 Hz, 1H, CHBCONH), 4.67 (dd,
J = 5.4, 7.6 Hz, 1H, CHCH2C6H5), 5.09 (s, 2H, OCH2C6H5), 7.16–
7.27, 7.28–7.38 (m, m, 5H, 5H, C6H5 ꢀ2); 13C NMR (100 MHz,
MeOD-d4):d 38.4, 44.8, 54.9, 67.9, 127.9, 128.9, 129.1, 129.5,
+H)+: 447.1914, found: 447.1890; IR (KBr,
(OH), 1716 (CO2), 1695 (CON), 1660 (CON).
m
max/cmꢁ1) = 3307
130.4, 138.1, 159.0, 171.9, 174.4; HRMS (ESI-TOF): Calcd for C19-
H
20N2O5Na (M+Na)+: 379.1264, found: 379.1250; IR (KBr, mmax
/
cmꢁ1) = 3326 (OH), 1732 (CO2), 1662 (CON).
4.3.8. Cbz-
181 mg (81%), colorless powder; mp; 109–112 °C; [
L-Phe-D
-Phe-OH 6aa0
a]
27 = ꢁ19.2
4.3.13. Cbz-
L-Ala-L-Phe-OH 6ca
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 2.67 (dd, J = 9.5,
14.0 Hz, 1H, CHAC6H5), 2.92–2.98 (m, 2H, CHA’C6H5, CHBC6H5),
3.16 (dd, J = 5.0, 13.6 Hz, 1H, CHB’C6H5), 4.37 (dd, J = 5.4, 9.5 Hz,
1H, CH), 4.66 (dd, J = 5.0, 8.4 Hz, 1H, CH0), 4.96 (d, J = 12.8 Hz, 1H,
OCHAC6H5), 5.02 (d, J = 12.8 Hz, 1H, OCHBC6H5), 7.08–7.33 (m,
15H, C6H5 ꢀ3); 13C NMR (100 MHz, MeOD-d4):d 38.4, 39.3, 54.9,
57.7, 67.6, 127.7, 127.9, 128.7, 129.0, 129.4, 129.5, 129.6, 130.4,
130.4, 138.2, 138.5, 158.1, 173.8, 174.3; HRMS (ESI-TOF): Calcd
for C26H26N2O5Na (M+Na)+: 469.1734, found: 469.1726; IR (KBr,
144 mg (79%), colorless powder; mp; 125–127 °C; [
a
]
D = +1.0
27
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.27 (d, J = 7.1 Hz,
3H, CH3), 3.01 (dd, J = 7.8, 13.9 Hz, 1H, CHAC6H5), 3.18 (dd, J = 5.2,
13.9 Hz, 1H, CHBC6H5), 4.13 (q, J = 7.1 Hz, 1H, CH CH3), 4.64 (dd,
J = 5.2, 7.8 Hz, 1H, CHCH2C6H5), 5.06 (s, 2H, OCH2C6H5), 7.16–
7.26, 7.28–7.38 (m, m, 5H, 5H, C6H5 ꢀ2); 13C NMR (100 MHz,
MeOD-d4):d 18.3, 38.4, 51.9, 54.9, 67.7, 127.8, 128.9, 129.0, 129.5,
129.5, 130.5, 138.2, 158.2, 174.3, 175.3; HRMS (ESI-TOF): Calcd
for C20H22N2O5Na (M+Na)+: 393.1421, found: 393.1449; IR (KBr,
m
max/cmꢁ1) = 3307 (OH), 1716 (CO2), 1652 (CON).
m
max/cmꢁ1) = 3330 (OH), 1736 (CO2), 1714 (CON), 1691 (CON).
4.3.9. Cbz-
L
-Phe-
L
-Thr-OH 6ag
4.3.14. Cbz-
L-Val-L-Phe-OH 6da
162 mg (81%), colorless powder; mp; 141–143 °C; [
a]
25 = ꢁ7.7
142 mg (71%), colorless powder; mp; 174–177 °C; [
a
]
27 = ꢁ0.8
D
D
(c 1.00, MeOH); 1H NMR (400 MHz, CDCl3):d 1.17 (d, J = 6.4 Hz,
3H, CH3), 2.95 (dd, J = 8.0, 14.0 Hz, 1H, CHAC6H5), 3.11 (dd, J = 5.5,
14.0 Hz, 1H, CHBC6H5), 4.40–4.48 (m, 1H, CHOH), 4.60–4.69 (m,
2H, CHCH2, CHCHOH), 4.92 (d, J = 12.2 Hz, 1H, OCHAC6H5), 5.04
(d, J = 12.2 Hz, 1H, OCHBC6H5), 5.46 (br s, 1H, NH), 7.09–7.11,
7.19–7.21, 7.28–7.31 (m, m, m, 11H, NH, C6H5 ꢀ2); 13C NMR
(100 MHz, CDCl3):d 19.2, 38.6, 55.8, 57.3, 67.3, 68.6, 127.1, 128.0,
128.2, 128.5, 128.7, 129.3, 135.9, 156.4, 172.0, 172.8; HRMS (ESI-
TOF): Calcd for C21H25N2O6 (M+H)+: 401.1707, found: 401.1708;
(c 1.00, DMSO); 1H NMR (400 MHz, MeOD-d4):d 0.87 (d,
J = 6.8 Hz, 3H, CH3), 0.89 (d, J = 6.8 Hz, 3H, CH3), 1.93–2.00 (m,
1H, CH(CH3)2), 2.97 (dd, J = 8.6, 13.9 Hz, 1H, CHAC6H5), 3.18 (dd,
J = 5.2, 13.9 Hz, 1H, CHBC6H5), 3.90 (d, J = 7.3 Hz, 1H, CHCH
(CH3)2), 4.66 (dd, J = 5.2, 8.6 Hz, 1H, CHCH2C6H5), 5.08 (s, 2H, OCH2-
C6H5), 7.13–7.22, 7.28–7.35 (m, m, 5H, 5H, C6H5 ꢀ2); 13C NMR
(100 MHz, MeOD-d4):d 18.6, 19.7, 32.1, 38.5, 54.9, 62.1, 67.7,
127.8, 128.9, 129.1, 129.5, 129.5, 130.4, 138.3, 158.5, 174.0,
174.4; HRMS (ESI-TOF): Calcd for
421.1734, found: 421.1757; IR (KBr,
1734 (CO2), 1635 (CON).
C
22H26N2O5Na (M+Na)+:
m
max/cmꢁ1) = 3342 (OH),
IR (KBr,
m
max/cmꢁ1) = 3294 (OH), 1701 (CO2), 1658 (CON).
4.3.10. Cbz-
L-Phe-L-Met-OH 6ah
140 mg (65%), colorless powder; mp; 103–104 °C; [
a
]
26 = ꢁ16.1
4.3.15. Cbz-
L-Leu-L-Phe-OH 6ea
D
(c 1.00, MeOH); 1H NMR (400 MHz, CDCl3):d 1.92–2.04 (m, 1H,
CHACH2S), 2.04 (s, 3H, CH3), 2.10–2.19 (m, 1H, CHBCH2S), 2.37–
2.48 (m, 2H, CH2S), 3.03–3.15 (m, 2H, CH2C6H5), 4.43–4.55 (m,
1H, CHCH2CH2), 4.61–4.66 (m, 1H, CHCH2C6H5), 5.08 (s, 2H, OCH2-
C6H5), 5.39 (br s, 1H, NH), 6.72 (br s, 1H, NH), 7.17–7.38 (m, 10H,
C6H5 ꢀ2); 13C NMR (100 MHz, CDCl3):d 15.3, 29.8, 30.8, 38.2,
51.7, 56.1, 67.3, 127.2, 128.1, 128.3, 128.6, 128.8, 129.3, 136.0,
156.2, 171.5, 174.4; HRMS (ESI-TOF): Calcd for C22H26N2O5SNa
156 mg (76%), colorless powder; mp; 81–83 °C; [
a]
27 = ꢁ7.4 (c
D
1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 0.89 (d, J = 6.6 Hz,
3H, CH3), 0.92 (d, J = 6.6 Hz, 3H, CH3), 1.44–1.48 (m, 2H, CHACH
(CH3)2, CH(CH3)2), 1.58–1.66 (m, 1H, CHBCH(CH3)2), 3.00 (dd,
J = 8.4, 13.7 Hz, 1H, CHAC6H5), 3.19 (dd, J = 5.2, 13.7 Hz, 1H, CHBC6-
H5), 4.14 (dd, J = 6.3, 6.3 Hz, 1H, CHCH2CH), 4.65 (dd, J = 5.2, 8.3 Hz,
1H, CHCH2C6H5), 5.07 (s, 2H, OCH2C6H5), 7.15–7.24, 7.27–7.35 (m,
m, 5H, 5H, C6H5 ꢀ2); 13C NMR (100 MHz, MeOD-d4):d 22.0, 23.4,
25.9, 38.4, 42.1, 54.9, 54.9, 67.7, 127.8, 128.8, 129.0, 129.5, 129.5,
(M+Na)+: 453.1455, found: 453.1440; IR (KBr,
(OH), 1712 (CO2), 1689 (CON), 1652 (CON).
m
max/cmꢁ1) = 3301
130.5, 138.2, 158.4, 174.3, 175.1; HRMS (ESI-TOF): Calcd for C23
-
/
H
28N2O5Na (M+Na)+: 435.1890, found: 435.1909; IR (KBr, mmax
4.3.11. Cbz-
L
-Phe-
L
-Trp-OH 6ai
cmꢁ1) = 3319 (OH), 1716 (CO2), 1697 (CON), 1664 (CON).
189 mg (78%), colorless powder; mp; 141–144 °C; [
a
]
27 = ꢁ3.3
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 2.74 (dd, J = 9.7,
14.0 Hz, 1H, CHAC6H5), 3.07 (dd, J = 5.0, 14.0 Hz, 1H, CHBC6H5),
3.22 (dd, J = 7.2, 14.7 Hz, 1H, CHA-indole), 3.34 (dd, J = 5.2,
14.7 Hz, 1H, CHB-indole), 4.37 (dd, J = 5.0, 9.7 Hz, 1H, CHCH2C6H5),
4.71 (dd, J = 5.2, 7.2 Hz, 1H, CHCH2-indole), 4.94 (d, J = 12.8 Hz, 1H,
OCHAC6H5), 5.00 (d, J = 12.8 Hz, 1H, OCHBC6H5), 6.99, 7.05–7.09,
7.14–7.32, 7.55 (t, m, m, d, J = 7.0, 7.9 Hz, 1H, 2H, 11H, 1H, C6H5
ꢀ2, indole); 13C NMR (100 MHz, MeOD-d4):d 28.5, 39.0, 54.7,
57.8, 67.6, 110.7, 112.3, 119.4, 119.9, 122.4, 124.6, 127.7, 128.7,
129.0, 129.4, 129.5, 130.4, 138.0, 138.2, 138.6, 158.2, 173.9,
4.3.16. Cbz-
L-Met-L-Phe-OH 6fa
150 mg (70%), colorless powder; mp; 125–126 °C; [
a]
27 = ꢁ7.0
D
(c 1.00, MeOH); 1H NMR (400 MHz, MeOD-d4):d 1.77–1.86 (m,
1H, CHACH2S), 1.92–1.99 (m, 1H, CHBCH2S), 2.04 (s, 3H, CH3),
2.40–2.53 (m, 2H, CH2S), 3.00 (dd, J = 8.3, 13.9 Hz, 1H, CHAC6H5),
3.19 (dd, J = 5.2, 13.9 Hz, 1H, CHBC6H5), 4.22 (dd, J = 5.6, 8.5 Hz,
1H, CHCH2CH2), 4.66 (dd, J = 5.2, 8.3 Hz, 1H, CHCH2C6H5), 5.07 (s,
2H, OCH2C6H5), 7.15–7.26, 7.28–7.35 (m, m, 5H, 5H, C6H5 ꢀ2);
13C NMR (100 MHz, MeOD-d4):d 15.2, 31.0, 32.9, 38.3, 54.9, 55.5,
67.8, 127.9, 128.9, 129.1, 129.5, 129.5, 130.4, 138.2, 158.4, 174.1,
174.9; HRMS (ESI-TOF): Calcd for
C
28H27N3O5Na (M+Na)+:
174.3; HRMS (ESI-TOF): Calcd for C
22H26N2O5SNa (M+Na)+:
Please cite this article in press as: Ezawa, T.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.11.009

T. Ezawa et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
9
453.1455, found: 453.1454; IR (KBr,
(CO2), 1695 (CON), 1660 (CON).
m
max/cmꢁ1) = 3311 (OH), 1722
of Peptides: A Brief History of Peptide Chemistry; Springer-Verlag: New York,
1991; pp 77–102; For selected reports for peptide syntheses using unprotected
a-amino acids. (d) Gagnon, P.; Huang, X.; Therrien, E.; Keillor, J. W. Tetrahedron
Lett. 2002, 43, 7717–7719. and the references cited therein; (e) Anuradha, M.
V.; Ravindranath, B. Tetrahedron 1997, 53, 1123–1130; (f) Anderson, G. W.;
Zimmerman, J. E.; Callahan, F. M. J. Am. Chem. Soc. 1964, 86, 1839–1842.
5. Furka, A.; Sebestyen, F.; Asgedom, M.; Dibo, G. Int. J. Pept. Protein Res. 1991, 37,
487–493.
4.3.17. Cbz-
L-Trp-L-Phe-OH 6ga
207 mg (85%), colorless powder; mp; 70–72 °C; [
a]
28 = ꢁ18.1 (c
D
0.99, MeOH); 1H NMR (400 MHz, MeOD-d4):d 2.96 (dd, J = 7.8,
13.9 Hz, 1H, CHAC6H5), 3.01 (dd, J = 8.4, 14.8 Hz, 1H, CHA-indole),
3.14 (dd, J = 5.4, 13.9 Hz, 1H, CHBC6H5), 3.20 (dd, J = 5.4, 14.8 Hz,
1H, CHB-indole), 4.43 (dd, J = 5.4, 8.4 Hz, 1H, CHCH2-indole), 4.64
(dd, J = 5.4, 7.8 Hz, 1H, CHCH2C6H5), 5.00 (s, 2H, OCH2C6H5), 6.96–
7.33, 7.57 (m, d, J = 7.9 Hz, 14H, 1H, C6H5 ꢀ2, indole); 13C NMR
(100 MHz, MeOD-d4):d 29.1, 38.4, 55.0, 57.2, 67.7, 111.0, 112.3,
119.4, 119.9, 122.4, 124.7, 127.8, 128.7, 128.8, 128.9, 129.4,
6. Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149–2154.
7. Hernandez, J. G.; Juaristi, E. J. Org. Chem. 2010, 75, 7107–7111.
8. Declerck, V.; Nun, P.; Martinez, J.; Lamaty, F. Angew. Chem., Int. Ed. 2009, 48,
9318–9321.
9. Faham, A. E.; Albericio, F. Chem. Rev. 2011, 111, 6557–6602.
10. (a) Chu, W.; Tu, Z.; McElveen, E.; Xu, J.; Taylor, M.; Luedtke, R. R.; Mach, R. H.
Bioorg. Med. Chem. 2005, 13, 77–87; (b) Chaudhary, A.; Girgis, M.; Prashad, M.;
Hu, B.; Har, D.; Repic, O.; Blacklock, Y. J. Tetrahedron Lett. 2003, 44, 5543–5546;
(c) Shieh, W.-C.; Carlson, J. A.; Shore, M. E. Tetrahedron Lett. 1999, 40, 7167–
7170; (d) Wenger, R. M. Helv. Chim. Acta 1983, 66, 2672–2702; (e) Meienhofer,
J. In Major Methods of Peptide Bond Formation in the Peptides: Analysis, Synthesis,
Biology; Gross, E., Meienhofer, J., Eds.; Academic Press: New York, 1979; Vol. 1,
pp 263–314.
129.5, 130.5, 138.1, 158.2, 174.2; HRMS (ESI-TOF): Calcd for C28
-
H
27N3O5Na (M+Na)+: 508.1843, found: 508.1820; IR (KBr, mmax
/
cmꢁ1) = 3402 (NH), 3313 (OH), 1716 (CO2), 1662 (CON).
11. Llobet, A. I.; Alvarez, M.; Albericio, F. Chem. Rev. 2009, 109, 2455–2504.
12. (a) Jung, S.; Noguchi, T.; Imai, N. Tetrahedron Lett. 2014, 55, 394–396; (b) Jung,
S.; Tsukuda, Y.; Kawashima, R.; Ishiki, T.; Matsumoto, A.; Nakaniwa, A.; Takagi,
M.; Noguchi, T.; Imai, N. Tetrahedron Lett. 2013, 54, 5718–5720; (c) Noguchi, T.;
Sekine, M.; Yokoo, Y.; Jung, S.; Imai, N. Chem. Lett. 2013, 42, 580–582; (d)
Noguchi, T.; Jung, S.; Imai, N. Chem. Lett. 2012, 41, 577–579; (e) Noguchi, T.;
Tehara, N.; Uesugi, Y.; Jung, S.; Imai, N. Chem. Lett. 2012, 41, 42–43.
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239–254.
References
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3. Puccetti, A.; Lunardi, C. Discov. Med. 2010, 9, 224–228.
4. For selected reviews on peptide syntheses: (a) Blaskovich, M. A. Handbook on
Syntheses of Amino Acids: General Routes for the Syntheses of Amino Acids; Oxford
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Please cite this article in press as: Ezawa, T.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.11.009